· Nervous system is derived from embryonic Ectoderm.
· Nerve cells do not divide because they do not have Centrosome.
· Neurons are the longest cell of the human body and have the least power of regeneration.
· Nissl’s granules are the irregular masses of the ribosomes and RER and are the sites of protein synthesis.
· Cell body of neurons is called Perikaryon or soma or Cyton.
· Axons are of two types: myelinated and non-myelinated.
· The myelinated or medullated nerve fibres are enveloped with Schwann cells which form or medullary sheath around the axon.
· The gaps of two myelin sheaths are called Nodes of Ranvier.
· Unmyelinated or non-medullated nerve fibres are enclosed by the Schwann cell that does not form a myelin sheath around the axon.
· Myelinated nerve fibres appear white and non-myelinated grey.
· The immediate covering of a neuron is Endoneurium.
· The outermost covering of nerve: epineurium
· Outermost covering of nerve fasciculus: perineurium
· Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.
· Myelinated nerve fibres appear white and non-myelinated grey.
· The immediate covering of a neuron is Endoneurium.
· The outermost covering of nerve: epineurium
· Outermost covering of nerve fasciculus: perineurium
· Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.
· An important exception is the basal ganglia (nuclei) situated within the cerebrum.
· Groups of axons are called tracts at the periphery of the spinal cord and nerves or nerve fibres outside the brain and spinal cord.
· Meninges are the connective tissue coverings of the brain.
· They are dura mater (tough or hard, mater-mother), arachnoid mater (Arachnoid-spider web-like) and pia mater (pia-soft).
· Dura mater and arachnoid mater are separated by a space called subdural space whereas arachnoid mater and pia mater are separated by subarachnoid space, containing cerebrospinal fluid.
· Inflammation of meninx is called meningitis. It may be caused by bacteria, viruses or protozoa.
· CSF (Cerebrospinal fluid) is produced continuously by the choroid plexuses within the ventricles of the brain.
· Two Lateral ventricles (Paracoels), a third ventricle (diocoel) and a fourth ventricle (metacoel).
· Three openings in the roof of the fourth ventricle from which CSF can leave the ventricle and enter the sub-arachnoid space-two lateral apertures (foramina of Luschka) and a median aperture (foramen of Magendie)
· Choroid plexus secretes CSF; arachnoid granulations (villi) absorb CSF.
· CSF functions- Mechanical protection, circulation and maintenance of the intracranial pressure (ICP).
· Grey matter contains cell bodies, unmyelinated axons and dendrites. White matter contains myelinated axons (Myelin is white).
· CNS axons are myelinated by oligodendrocytes (KUMET 2001) whereas PNS axons are myelinated by Schwann cells (MOE 2059).
· The region where the nerve endings of one neuron come in contact with another neuron is called a synapse. Hence, called a junction between 2 neurons.
· Cerebrum of the human brain is more developed in comparison to others.
· Mammalian brain is different from frogs brain due to the presence of corpus callosum.
· Brain stem contains midbrain, pons Varolii and medulla oblongata.
A. CEREBRUM (TELENCEPHALON)
· The largest part of the brain, the most superior part of the brain and consists of two ovoid cerebral hemispheres partly separated from each other by a deep longitudinal groove called a longitudinal cerebral fissure.
Functional areas of the Cerebrum
FUNCTIONS OF CEREBELLUM [MOE 2011, IOM 2008, MOE, 2003, BPKIHS 2005]
· Cerebellum is also called the little brain and is well developed in man.
SPINAL CORD (MYELON)
· Caudal (lower) end of the spinal cord gradually tapers in the form of a cone and is referred to as Conus medullaris.
PERIPHERAL NERVOUS SYSTEM
· It consists of nerves arising from the brain called cranial nerves and from the spinal cord called spinal nerves.
A. Cranial nerves:
AUTONOMIC NERVOUS SYSTEM
· It is called autonomic because it is partly independent and not under voluntary control.
· The nerve impulse conduction is faster in the myelinated neurone (due to saltatory conduction) and in a neurone with more diameter. The fastest fibres can conduct impulses to, e.g. skeletal muscles at a rate of 130m/s while the slowest impulses travel at 0.5 m/s. The insulating properties of the myelin sheath prevent the movement of ions so electrical changes across the membrane can only occur at the nodes of Ranvier. The neurotransmitter is secreted only by the axon terminals so the axon is not excited anywhere and nerve impulse always travels in a single direction.
Various types of synaptic contacts between neurons have been observed viz.
· Axo-dendritic synapse – between an axon and a dendrite.
· Axo-somatic synapse – between an axon and muscles/glands/organs
· Axo-axonic synapse – between an axon and another axon
· Dendro-dendritic synapse – between a dendrite and another dendrite
B. Lateral zone: The lateral zone contains the following nuclei: the supraoptic nuclei, lateral nucleus, tuberomammillary nucleus and the lateral mammillary nuclei overlaps both the zones. The small supra-chiasmatic nucleus, which lies over the optic chiasma, overlaps both the zones.
CONNECTIONS
· Hypothalamus is an integral part of the limbic system. The connections of the hypothalamus are numerous and very complex.
2. Stria terminalis: This tract interconnects the amygdala with many hypothalamic nuclei.
3. Fornix: This complex fibre tract arises from the hippocampal formation and reciprocally connects with the mammillary bodies.
4. Hypothalamus receives visceral, gustatory, and somatic sensory information from the spinal cord and the brainstem. These inputs reach the hypothalamus through multisynaptic pathways via the mammillary peduncle, the dorsal longitudinal fasciculus and the medial forebrain bundle.
5. Olfactory afferents reach the hypothalamus through the medial forebrain bundle.
6. Visual afferents are projected bilaterally to the suprachiasmatic nuclei via the optic nerve and tract.
2. Control of the endocrine system
· The central nervous system controls the activities of the endocrine system. The brain contains neurons, which synthesize and secrete peptides, a process referred to as neurosecretion.
· Hypothalamus contains the densest concentration of neurosecretory neurons. The neurosecretory neurons of the hypothalamus have been categorized into—(1) parvocellular and (2) magnocellular neurons.
· The parvocellular neurons lie within the periventricular zone and the arcuate (infundibular) nucleus. The magnocellular neurons are located in the supraoptic and paraventricular nuclei.
·The axons of the parvocellular neurons converge on the infundibulum and end on the capillary loops, which form the hypophyseal portal vessels.
· The parvocellular neurons secrete - (1) Growth hormone-releasing hormone (GHRH), Somatostatin, Gonadotropin-releasing hormone (GnRH), Corticotropin-releasing hormone (CRH), Thyrotropin-releasing hormone (TRH) and Dopamine.
· The hypothalamic hormones either stimulate or inhibit the secretion of their respective trophic hormones secreted by the anterior pituitary gland. Dopamine acts as a prolactin release-inhibiting factor.
· Through its effects on the anterior pituitary, the hypothalamus controls the activities of the thyroid gland, the adrenal cortex, gonad, and influences the growth and metabolic activities. The hormones or the controlled variable exert feedforward and feedback control at every level.
· The axons of the magnocellular neurons in the supraoptic and paraventricular nuclei descend down the infundibulum and end in the posterior pituitary around the capillaries. The magnocellular neurons secrete vasopressin (ADH) and oxytocin. Vasopressin and oxytocin are secreted by separate neurons.
· Vasopressin neurons are osmosensitive and receive osmosensory afferents from the median preoptic nucleus and subfornical organ. ADH is primarily concerned with osmotic homeostasis by its effects on renal tubular water reabsorption.
· Oxytocin is reflexly secreted in women through polysynaptic afferents from the nipple and the uterine cervix. It causes post-partum milk- ejection and contraction of the uterus during childbirth.
· Moreover, the hypothalamus can influence the activities of other peripheral endocrine organs through their autonomic innervation. For example, the endocrine pancreas is partly under neural control through its autonomic nerves.
3. Control of body temperature
· Homeothermic animals, including man, maintain a constant body temperature. In human beings, the body temperature is maintained around 3 7°C, by a balance between heat loss and heat production mechanisms.
· A raised temperature activates the heat loss mechanisms and depresses heat production mechanisms. A fall in temperature produces reverse effects.
· The environmental temperature is sensed by peripherally located cutaneous thermoreceptors.
· The core temperature is monitored by temperature-sensitive neurons in the preoptic area of the hypothalamus. The peripheral thermal afferents and limbic afferents are projected to the preoptic area.
· Stimulation of the anterior hypothalamus produces heat loss responses - sweating, vasodilatation and panting. Damage to this region can lead to a rise in body temperature.
· Stimulation of the posterior hypothalamus produces heat production responses vasoconstriction, decreased sweating, increased visceral activity, increased metabolism and shivering.
· Lesions of the posterior hypothalamus make the animal poikilothermic (coldblooded). The posterior hypothalamic region does not contain any thermoreceptors.
· The heat loss and heat production responses seem to be controlled by the anterior hypothalamus. Deviations from the body temperature are sensed by the thermosensitive neurons in the anterior hypothalamus and evoke responses that restore the body temperature. The hypothalamus, thus, functions as the temperature servomechanism.
4. Regulation of fluid balance
· Hypothalamus is involved in the maintenance of fluid balance by its involvement in the control of water intake and in the control of water loss from the body. Fluid intake is influenced by osmolality and volume of ECF.
· In conditions of water deprivation, the ECF becomes hyperosmotic. Hyperosmolaltiy is detected by osmoreceptors located in the anterior hypothalamus and the circumventricular organs. The osmoreceptors can detect a 1% increase in osmolality evoke the thirst sensation and cause drinking behaviour.
· Water deprivation or fluid loss will produce a decrease in blood volume. A fall in blood volume by 5 to 10% is sensed by receptors in the great veins, atria and the baroreceptors in the carotid sinus. This causes the release of renin by the kidney. Renin breaks down angiotensinogen and forms angiotensin II. Angiotensin II acts on the subfornical organ and stimulates drinking.
· Water loss is regulated by stimulation or inhibition of release of ADH from the supraoptic and paraventricular neurons at their terminals in the posterior pituitary.
· Hyperosmolality or hypovolaemia stimulates the release of ADH from the ADH- secreting neurons in the supraoptic and paraventricular nuclei and produces increased reabsorption of water in the renal tubules.
· On the other hand, hyperosmolality or hypervolaemia inhibits the release of ADH and decreases the reabsorption of water in the renal tubules.
· Supraoptic and paraventricular nuclei of the hypothalamus are important in the maintenance of fluid balance. Lesions of these nuclei produce diabetes insipidus with excessive loss of fluids through polyuria
5. Regulation of food intake and metabolism
· The source of energy in humans and animals are the nutrients derived from ingested food. Food intake is regulated to maintain a balance between body weight and energy expenditure. The balance between energy intake and energy expenditure depends on many factors, like feeding behaviour, neural control of the GI tract and pancreas, and hormonal control by GH, thyroid hormones, and glucocorticoids. The hypothalamus plays an important role in the maintenance of energy balance.
· The ventromedial nucleus receives visceral afferents from the nucleus tractus solitarius and the lateral hypothalamus receives the olfactory afferents. Both these nuclei receive extensive limbic afferents.
· The ventromedial nucleus functions as a satiety centre which generates a sense of satisfaction and inhibits feeding behaviour. Bilateral lesions in the ventromedial nucleus cause hyperphagia and obesity. Stimulation of this nucleus stops feeding behaviour.
· The lateral hypothalamus functions as the feeding centre, and activates the animal to search for food. Lesions in the lateral hypothalamus cause aphagia, starvation and eventual death of the animal. Stimulation of the lateral hypothalamus causes eating, even in a fully satiated animal.
· Besides, lesions in the paraventricular nucleus produce hyperphagia and lesions in the dorsomedial nucleus depress feeding behaviour. These hypothalamic nuclei together, coordinate the feeding behaviour and perception of satiety.
· Hypothalamic neurons contain receptors for neurotransmitters and hormones directly concerned with food intake. Serotonin, CCK, GLP-1 and Leptin inhibit food intake, while
· Neuropeptide-Y (NPY) increases food intake. The receptors for these regulatory factors have been identified in the hypothalamic neurons.
· Neuropeptide-Y secreting neurons are present in the arcuate nucleus. The ventromedial nucleus contains neurons that are sensitive to glucose and other nutrients.
· Hypothalamus regulates the activities of the endocrines involved in the energy balance and metabolism through its humoral effects on the anterior pituitary and via the projections to the autonomic efferents.
· The ventromedial nucleus, the lateral hypothalamus and the paraventricular nucleus influence the intermediary metabolism and complement their effects on feeding behaviour.
· Stimulation of ventromedial nucleus produces glucagon release, increased gluconeogenesis, glycogenolysis and glycolysis.
· On the other hand, stimulation of the lateral hypothalamus causes insulin release and the opposite metabolic effects.
6. Regulation of circadian rhythms
· Body temperature, the plasma concentration of many hormones, renal secretion and sleep-wake cycle show an approximately 24 hours cycle pattern in tune with the light-dark cycle. Such a type of cyclic pattern is called circadian rhythms. The suprachiasmatic nucleus (SCN) is concerned with the regulation of circadian rhythms.
· SCN receives visual afferents from the retina and influences the activity of the pineal gland. SCN has interconnections with other hypothalamic nuclei. SCN neurons discharge rhythmically even when cultured in vitro.
· Lesions in the SCN disrupt the normal circadian rhythms and can be restored by transplantation of suprachiasmatic tissue.
· The mechanism of rhythm generation in SCN is not known and is found to be inherent. SCN is now considered to be the body's 'biological clock'.
7. Emotion and behaviour
· Emotion and behaviour involve the limbic system and the prefrontal cortex. The hypothalamus is the principal centre for emotional expressions. The physical expression of emotion concerns the sympathetic and parasympathetic nervous systems. The hypothalamus, by virtue of its projections to the autonomic efferents, activates the autonomic manifestations that are part of emotions.
· Decorticate animals (cats) with intact hypothalamus show 'sham rage'. These animals exhibit all the physical aspects of anger - like hissing, growling, baring of claws and fangs - when they are subject to provocation. Such aggressive behaviour can be abolished in decorticate animals by destroying the posterior hypothalamus.
· Stimulation of the ventromedial nucleus produces a similar aggressive pattern and stimulation of the lateral region of the anterior hypothalamus produces the 'flight' response in animals. Similarly, stimulation or ablation of other hypothalamic nuclei produces behavioural patterns associated with fear, punishment, sexual drive and passivity in animals.
· The hypothalamus is thought to integrate the afferent inputs from limbic structures and produce the physical expression of emotions and associated behavioural patterns.
8. Sleep-wake cycle
· The phenomenon of sleep depends on a balance between the activity of the ascending reticular activating system and certain sleep zones situated in the brainstem. The ascending reticular activating system projects diffusely to the cerebral cortex.
· This system causes arousal, wakefulness and desynchronization of the EEG. The sleep zones are thought to induce sleep, by active inhibition of the ascending reticular activating system.
· Stimulation of the rostral part in the suprachiasmatic area of the hypothalamus promotes sleep.
· Bilateral lesions in the medial rostral part of the anterior hypothalamus cause a state of wakefulness.
· Damage to the preoptic nucleus of the hypothalamus produces insomnia whereas stimulation of the nucleus induces the state of slow-wave sleep.
· Damage to neurons in the ventrolateral posterior hypothalamus produces transient sleep.
· Neurons from the posterior hypothalamic area have arousal properties through both direct excitatory histaminergic projections to the cerebral cortex, and by indirect excitatory projections to the brainstem reticular formation.
· The preoptic nucleus induces sleep indirectly through inhibition of the excitatory neurons of the posterior hypothalamus.
· A hypothalamic sleep-inducing peptide has been isolated and may have a role in the sleep mechanism.
9. Sexual behaviour and reproduction
· Dura mater and arachnoid mater are separated by a space called subdural space whereas arachnoid mater and pia mater are separated by subarachnoid space, containing cerebrospinal fluid.
· Inflammation of meninx is called meningitis. It may be caused by bacteria, viruses or protozoa.
· CSF (Cerebrospinal fluid) is produced continuously by the choroid plexuses within the ventricles of the brain.
· Two Lateral ventricles (Paracoels), a third ventricle (diocoel) and a fourth ventricle (metacoel).
· Three openings in the roof of the fourth ventricle from which CSF can leave the ventricle and enter the sub-arachnoid space-two lateral apertures (foramina of Luschka) and a median aperture (foramen of Magendie)
· Choroid plexus secretes CSF; arachnoid granulations (villi) absorb CSF.
· CSF functions- Mechanical protection, circulation and maintenance of the intracranial pressure (ICP).
· Grey matter contains cell bodies, unmyelinated axons and dendrites. White matter contains myelinated axons (Myelin is white).
· CNS axons are myelinated by oligodendrocytes (KUMET 2001) whereas PNS axons are myelinated by Schwann cells (MOE 2059).
· The region where the nerve endings of one neuron come in contact with another neuron is called a synapse. Hence, called a junction between 2 neurons.
· Cerebrum of the human brain is more developed in comparison to others.
· Mammalian brain is different from frogs brain due to the presence of corpus callosum.
· Brain stem contains midbrain, pons Varolii and medulla oblongata.
 |
| Parts of brain |
· A band of nerve fibres that joins cerebral hemispheres in man is the corpus callosum.
· In the human brain, the central sulcus is found between the frontal and parietal lobes.
· In the brain, 3rd ventricle is connected to 4th ventricle by the cerebral aqueduct or Iter or Aqueduct of Sylvius.
· The thermoregulatory centre in the body of man is found in the hypothalamus.
· Foramen of Monro communicates lateral ventricles with the third ventricles.
· Equilibrium (balancing) is maintained by the cerebellum.
· Vestibular nerve supplies the semicircular canals of the inner ear.
· Spinal cord comes out through Foramen magnum.
· Smallest cranial nerve is the Trochlear nerve.
· Saltatory conduction occurs in myelinated nerve fibres.
· The number of spinal nerves in men is 31 pairs.
· Parasympathetic nerve fibres arise from the craniosacral region.
· Vertebral column develops from embryonic Notochord.
· Spinal cord comes out through the foramen magnum.
· Active transport is maintained by the use of ATP of the ions of the Na-K pump which transports 3Na+ outwards for 2K+ into the cell. [IOM 1996]
· Auditory nerve carries the impulses towards the CNS but not Oculomotor and Abducens. [MOE, 2071/8/20]
· Potassium causes the repolarization in the action potential in a nerve fibre. [IOM 2014]
· Action potential obeys all or none law i.e. it either happens completely in the case of a threshold stimulus or does not happen at all when the stimulus is of sub-threshold strength.
· Synapse occurs between two neurons or a neuron and an effector cells (Muscle cells of glandular cells) [MOE 2070, Ind. Emb. 2013]
· Central nervous system consists of the brain and spinal cord.
· Brain is the site of processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
· The brain constitutes about 2% of the body weight and in adults; it weighs about 1.2 – 1.4 kg.
· In the human brain, the central sulcus is found between the frontal and parietal lobes.
· In the brain, 3rd ventricle is connected to 4th ventricle by the cerebral aqueduct or Iter or Aqueduct of Sylvius.
· The thermoregulatory centre in the body of man is found in the hypothalamus.
· Foramen of Monro communicates lateral ventricles with the third ventricles.
· Equilibrium (balancing) is maintained by the cerebellum.
· Vestibular nerve supplies the semicircular canals of the inner ear.
· Spinal cord comes out through Foramen magnum.
· Smallest cranial nerve is the Trochlear nerve.
· Saltatory conduction occurs in myelinated nerve fibres.
· The number of spinal nerves in men is 31 pairs.
· Parasympathetic nerve fibres arise from the craniosacral region.
· Vertebral column develops from embryonic Notochord.
· Spinal cord comes out through the foramen magnum.
· Active transport is maintained by the use of ATP of the ions of the Na-K pump which transports 3Na+ outwards for 2K+ into the cell. [IOM 1996]
· Auditory nerve carries the impulses towards the CNS but not Oculomotor and Abducens. [MOE, 2071/8/20]
· Potassium causes the repolarization in the action potential in a nerve fibre. [IOM 2014]
· Action potential obeys all or none law i.e. it either happens completely in the case of a threshold stimulus or does not happen at all when the stimulus is of sub-threshold strength.
· Synapse occurs between two neurons or a neuron and an effector cells (Muscle cells of glandular cells) [MOE 2070, Ind. Emb. 2013]
· Central nervous system consists of the brain and spinal cord.
· Brain is the site of processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
· The brain constitutes about 2% of the body weight and in adults; it weighs about 1.2 – 1.4 kg.
· It receives 20% of cardiac output.
· Forebrain (Prosencephalon) consists of telencephalon and diencephalon.
· Forebrain (Prosencephalon) consists of telencephalon and diencephalon.
A. CEREBRUM (TELENCEPHALON)
· The largest part of the brain, the most superior part of the brain and consists of two ovoid cerebral hemispheres partly separated from each other by a deep longitudinal groove called a longitudinal cerebral fissure.
· Cerebral hemispheres are connected by a mass of nerve fibre (bundles of axons) called the corpus callosum. It is an important feature of the mammalian brain.
· The upper layer of the cerebrum is made up of grey matter and deeper layers of white matter.
 |
| Structure of cerebrum |
· Cerebral cortex has many infoldings to increase the surface area.
· The exposed areas of the folds are the gyri and depressions between them are sulci (sing. sulcus).
· Deep sulci are called fissures.
· Each cerebral hemisphere has a lateral ventricle (paracoel) that communicates with the 3rd ventricle (diocoel) present in the diencephalon by means of a small opening called “Foramen of Monro”.
· Each cerebral hemisphere is divided into four lobes- frontal, parietal, temporal and occipital on the basis of names of the cranium.
· Each cerebral hemisphere has a lateral ventricle (paracoel) that communicates with the 3rd ventricle (diocoel) present in the diencephalon by means of a small opening called “Foramen of Monro”.
· Each cerebral hemisphere is divided into four lobes- frontal, parietal, temporal and occipital on the basis of names of the cranium.
· They are marked by three deep sulci (fissures). They are central, lateral and parieto-occipital sulcus.
· The central sulcus separates the frontal lobe from the parietal lobe, the lateral sulcus separates the frontal with the temporal lobe and the parieto-occipital sulcus separates the parietal with the occipital lobe.
· Cerebrum governs all mental activities like memory, intelligence, thinking, reasoning, moral sense and learning.
· Cerebrum governs all mental activities like memory, intelligence, thinking, reasoning, moral sense and learning.
· It is also associated with sensory perceptions including the perception of pain, touch, temperature, sight, hearing, taste and smell.
· It initiates and controls voluntary muscle contraction.
· The right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa.
Hippocampus
· It is the portion of the cerebral hemispheres in the basal medial part of the temporal lobe.
· It is important for learning and for converting short term memory to more permanent memory, and for recalling spatial relations in the world around us.
· It is the portion of the cerebral hemispheres in the basal medial part of the temporal lobe.
· It is important for learning and for converting short term memory to more permanent memory, and for recalling spatial relations in the world around us.
Amygdala
· It is the part of the telencephalon, located in the temporal lobe, involved in memory, emotion and fear.
· It is a component of the limbic system.
· It is the part of the telencephalon, located in the temporal lobe, involved in memory, emotion and fear.
· It is a component of the limbic system.
Functional areas of the Cerebrum
· Motor area: It controls voluntary muscle movement.
· Premotor area: It controls the motor area.
· Frontal area: The communication between this and other regions of the cerebrum is responsible for the behaviour, character and emotional state of an individual.
· Sensory area: It perceives sensations of pain, temperature, pressure, touch, knowledge of muscular movement. The sensory area of the right hemisphere receives impulses from the left side of the body and vice versa.
· Parietal area: It is believed to be associated with obtaining and retaining accurate knowledge of the objects. Objects can be recognized by touch alone due to the knowledge from the past experience retained in this area.
· Sensory speech area: Spoken word is perceived.
· Auditory area: It receives and interprets impulses transmitted from the inner ear by the cochlear part of the vestibulocochlear nerve (8th cranial nerve).
· Olfactory area: It receives and interprets the impulses from the nose via the olfactory (1st cranial nerve).
· Taste area: Impulses from the taste buds in the tongue and in the lining of cheeks, palate and pharynx are perceived as taste.
· Visual area: The optic nerves (2nd cranial nerves) pass from the eye to this area which receives and interpret the impulses as visual impressions.
· Size and complexity of the cerebrum are indicative of progressive development of intelligence of vertebrates from fishes to mammals.
· Olfactory lobe is distinct in rabbits and is attached to the anterior end of the cerebrum and contains rhinocoel but it is indistinct in a human occurs as the olfactory bulb. [MOE2005]
· Diencephalon consists of the thalamus, hypothalamus, epithalamus and subthalamus.
· Premotor area: It controls the motor area.
· Frontal area: The communication between this and other regions of the cerebrum is responsible for the behaviour, character and emotional state of an individual.
· Sensory area: It perceives sensations of pain, temperature, pressure, touch, knowledge of muscular movement. The sensory area of the right hemisphere receives impulses from the left side of the body and vice versa.
· Parietal area: It is believed to be associated with obtaining and retaining accurate knowledge of the objects. Objects can be recognized by touch alone due to the knowledge from the past experience retained in this area.
· Sensory speech area: Spoken word is perceived.
· Auditory area: It receives and interprets impulses transmitted from the inner ear by the cochlear part of the vestibulocochlear nerve (8th cranial nerve).
· Olfactory area: It receives and interprets the impulses from the nose via the olfactory (1st cranial nerve).
· Taste area: Impulses from the taste buds in the tongue and in the lining of cheeks, palate and pharynx are perceived as taste.
· Visual area: The optic nerves (2nd cranial nerves) pass from the eye to this area which receives and interpret the impulses as visual impressions.
· Size and complexity of the cerebrum are indicative of progressive development of intelligence of vertebrates from fishes to mammals.
· Olfactory lobe is distinct in rabbits and is attached to the anterior end of the cerebrum and contains rhinocoel but it is indistinct in a human occurs as the olfactory bulb. [MOE2005]
· Diencephalon consists of the thalamus, hypothalamus, epithalamus and subthalamus.
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| Structure of diencephalon |
· Thalamus is the major sensory relay for many sensory systems.
· Hypothalamus helps in homeostasis, has roles on Autonomic Nervous System (ANS), endocrine and limbic systems.
· Hypothalamus has osmoregulatory and thermoregulatory centres.
· Hypothalamus is referred to as “master of the master endocrine gland”.
· Hypothalamus is the link between the nervous system and endocrine system.
· Hypothalamus helps in homeostasis, has roles on Autonomic Nervous System (ANS), endocrine and limbic systems.
· Hypothalamus has osmoregulatory and thermoregulatory centres.
· Hypothalamus is referred to as “master of the master endocrine gland”.
· Hypothalamus is the link between the nervous system and endocrine system.
Functions of Hypothalamus (SEAT)
S- Sexual behaviour and sleeping pattern
E- Emotions, endocrine functions
A- Appetite, ANS [BPKIHS 2001]
T- Thirst, Temperature [IOM 1999, 2009, KU 2012]
· Epithalamus consists of the pineal body and the habenular nuclei.
S- Sexual behaviour and sleeping pattern
E- Emotions, endocrine functions
A- Appetite, ANS [BPKIHS 2001]
T- Thirst, Temperature [IOM 1999, 2009, KU 2012]
· Epithalamus consists of the pineal body and the habenular nuclei.
· The pineal gland (epiphyseal cerebri) secretes the melatonin hormone, which plays a role in the regulation of circadian rhythms (sleep-wake patterns).
· Midbrain (Mesencephalon) consists of cerebral aqueduct or iter or aqueduct of Sylvius, superior and inferior colliculi, cerebral peduncles or crus cerebri or Crura cerebri [MOE 2069]
· Substangia nigra: It is the largest nucleus of the mid-brain. It appears black to dark down in the freshly cut brain because the nigral cells contain melanin pigments.
· Neurons in the substantia nigra utilize Dopamine and GABA as neurotransmitters.
· Loss of the pigmented dopaminergic neurons from substantia nigra results in Parkinson’s disease.
· Mid-brain of frog has two large and oval sacs of optic lobes (corpora bigemina), each enclosing a cavity called optic ventricle or optocoel.
· Substangia nigra: It is the largest nucleus of the mid-brain. It appears black to dark down in the freshly cut brain because the nigral cells contain melanin pigments.
· Neurons in the substantia nigra utilize Dopamine and GABA as neurotransmitters.
· Loss of the pigmented dopaminergic neurons from substantia nigra results in Parkinson’s disease.
· Mid-brain of frog has two large and oval sacs of optic lobes (corpora bigemina), each enclosing a cavity called optic ventricle or optocoel.
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| Diagrammatic representation of ventricles of the brain |
· Apneustic and Pneumotaxic centres in the pons help to control respiration. [IOM 1996]
· Medulla oblongata has cardiovascular centre, respiratory and vasomotor centres [Ind. Emb. 2066], Vomiting, coughing and sneezing, salivation, swallowing, yawning reflexes etc.
· Medulla oblongata has cardiovascular centre, respiratory and vasomotor centres [Ind. Emb. 2066], Vomiting, coughing and sneezing, salivation, swallowing, yawning reflexes etc.
FUNCTIONS OF CEREBELLUM [MOE 2011, IOM 2008, MOE, 2003, BPKIHS 2005]
· Cerebellum is also called the little brain and is well developed in man.
· It is situated behind pons and below the posterior region of the cerebrum.
· Cerebrum consists of a pair of lateral parts; the cerebellar hemispheres are separated by a narrow, median-strip, the vermis. Grey matter forms the surface of the cerebellum and white matter lies deeply.
· A cross-section of cerebellar hemispheres shows branching tree-like arrangements of grey and white matter called arborvitae, “the tree of life”.
· Pons Varolii is situated in front of the cerebellum, above the medulla oblongata and below the midbrain. It consists of nerve fibres that interconnect the two cerebellar hemispheres and also join the medulla with higher brain centres, hence its name (pons means bridge).
· In pons Varolii, the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface. It is named Pons varolii after the Italian Anatomist Costanzo Varolio (1543-1575 AD).
· It relays impulses between the medulla oblongata and the superior part of the brain, between the hemispheres of the cerebellum and between the cerebrum and cerebellum.
· Cerebrum consists of a pair of lateral parts; the cerebellar hemispheres are separated by a narrow, median-strip, the vermis. Grey matter forms the surface of the cerebellum and white matter lies deeply.
· A cross-section of cerebellar hemispheres shows branching tree-like arrangements of grey and white matter called arborvitae, “the tree of life”.
· Pons Varolii is situated in front of the cerebellum, above the medulla oblongata and below the midbrain. It consists of nerve fibres that interconnect the two cerebellar hemispheres and also join the medulla with higher brain centres, hence its name (pons means bridge).
· In pons Varolii, the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface. It is named Pons varolii after the Italian Anatomist Costanzo Varolio (1543-1575 AD).
· It relays impulses between the medulla oblongata and the superior part of the brain, between the hemispheres of the cerebellum and between the cerebrum and cerebellum.
SPINAL CORD (MYELON)
· Caudal (lower) end of the spinal cord gradually tapers in the form of a cone and is referred to as Conus medullaris.
 |
| Structure of spinal cord |
· Filum terminale is the delicate fibrous cord (thread) like structure extending from the apex of the conus medullaris to the bottom of the vertebral column. It is composed of Pia mater.
· Cauda equina is a collection of spinal nerve roots below the conus medullaris.
· The cavity of the spinal cord is called neurocoel or central canal lined by ependymal cells.
· Cauda equina is a collection of spinal nerve roots below the conus medullaris.
· The cavity of the spinal cord is called neurocoel or central canal lined by ependymal cells.
PERIPHERAL NERVOUS SYSTEM
· It consists of nerves arising from the brain called cranial nerves and from the spinal cord called spinal nerves.
A. Cranial nerves:
· There are 12 pairs of cranial nerves arising from the ventral side of the brain.
· Each is given a name according to its distribution.
· Cranial nerves may be sensory, motor or both.
· There are 12 pairs of cranial nerves in mammals (human and rabbit) and 10 pairs of cranial nerves in fishes and amphibians (frog). [IOM 2012, KUMET 2008]
· The origin, distribution, nature and functions of cranial nerves are listed below.
· There are 12 pairs of cranial nerves in mammals (human and rabbit) and 10 pairs of cranial nerves in fishes and amphibians (frog). [IOM 2012, KUMET 2008]
· The origin, distribution, nature and functions of cranial nerves are listed below.
| Name | Origin | Distribution | Nature | Function |
|---|---|---|---|---|
| Olfactory | Olfactory Lobe | Olfactory epithelium | Sensory | Smell, Shortest cranial nerve |
| Optic | Optic chiasma | Retinae of eyes | Sensory | Sight |
| Oculomotor | Cerebral aqueduct | Four muscles of the eyeball | Motor | Eye movement |
| Trochlear (Pathetic) | Cerebral aqueduct | Superior oblique muscles of the eyes | Motor | Eye movement, Smallest and thinnest cranial nerve |
| Trigeminal i. Ophthalmic nerve ii. Maxillary nerve iii. Mandibular nerve |
Pons varolii Pons varolii Pons varolii Pons varolii | Eye, Eyelids Cheeks, Upper teeth Lower jaw, Pinna, Tongue | Both Sensory Sensory Both (sensory and motor) | Sensation Motor- Mastication Largest cranial nerve |
| Abducent | Pons Varolii | Lateral rectal muscles of eyeballs | Motor | Eye movement |
| Facial | Lower part of Pons varolii | Motor- muscles of facial expression Sensory- convey impulses from taste buds to cerebral cortex. | Both | Facial expression Taste reception |
| Auditory i.Vestibular nerve ii.Cochlear nerve | Cerebellum Cerebral cortex | Semicircular canal of inner ear Spiral organ of Corti of inner ear | Sensory Sensory | Equilibrium Hearing |
| Glossopharyngeal | Medulla oblongata | Tongue, Pharynx, Salivary glands | Both | Swallowing, Salivation, Taste |
| Vagus | Medulla oblongata | Pharynx, Larynx, Trachea, Heart, Exocrine pancreas, Oesophagus, Stomach, Gall bladder, Bile ducts, Spleen, Kidney, Ureter | Both Motor-to distribution Sensory-from distribution to brain | Movement Longest cranial nerve It has maximum branches and is also called the wandering nerve. |
| Accessory | Medulla oblongata | Pharynx, Larynx, Neck | Motor | Movement |
| Hypoglossal | Medulla oblongata | Tongue, Hyoid bone | Motor | Swallowing and speech |
B. Spinal nerves
· They originate from the lateral sides of the spinal cord.
· There are 31 pairs of spinal nerves that leave the vertebral canal through intervertebral foramina formed by the adjacent vertebrae.
· They are named and grouped on the basis of vertebrae with which they are associated. They are:
i) Cervical – 8 pairs (in the neck region)
ii) Thoracic – 12 pairs ( in the thorax)
iii) Lumbar – 5 pairs (in the upper part of the abdomen)
iv) Sacral – 5 pairs ( in the lower part of the abdomen)
v) Coccygeal – 1 pair ( in the tail region)
· Number of spinal nerves in humans is 31 pairs, in rabbits is 37 pairs and in frogs is 10 pairs.
· They originate from the lateral sides of the spinal cord.
· There are 31 pairs of spinal nerves that leave the vertebral canal through intervertebral foramina formed by the adjacent vertebrae.
· They are named and grouped on the basis of vertebrae with which they are associated. They are:
i) Cervical – 8 pairs (in the neck region)
ii) Thoracic – 12 pairs ( in the thorax)
iii) Lumbar – 5 pairs (in the upper part of the abdomen)
iv) Sacral – 5 pairs ( in the lower part of the abdomen)
v) Coccygeal – 1 pair ( in the tail region)
· Number of spinal nerves in humans is 31 pairs, in rabbits is 37 pairs and in frogs is 10 pairs.
AUTONOMIC NERVOUS SYSTEM
· It is called autonomic because it is partly independent and not under voluntary control.
Differences between the sympathetic and parasympathetic nervous system
| Characteristics | Sympathetic Nervous System (Thoracolumbar outflow) | Parasympathetic Nervous System (Craniosacral outflow) |
|---|---|---|
| Origin | It originates from the thoracic and lumbar regions of the spinal cord. | It originates from the cranial region of the brain and sacral region of the spinal cord. |
| Ganglia | Its ganglia are linked to form chains. | Its ganglia remain isolated. |
| Pre-ganglionic fibres | Short | Long |
| Post-ganglionic fibres | Long | Short |
| Effects | They produce the widespread and diffused effect | They produce a limited but localized effect. |
| Neurotransmitter | Nor-adrenaline is produced at the terminal end of post-ganglionic fibres at the effector organ. | Acetylcholine is produced at the terminal end of post-ganglionic fibres at the effector organ. |
| Activity | Active during stressful conditions, so preparing the body to face them | Active during relaxing times thus restoring normal activity after stress. |
| In Penis | It induces ejaculation. | Stimulates erection. |
| Rate of heartbeat | Increases | Decreases |
| Blood vessels/ (Arterial B.P.) | Constrict/Increases | Dilate/Decreases |
| Pupil of eye | Dilates | Constricts |
| Ciliary muscle | Relax | Contract |
| Gastrointestinal secretion | Slows down | Speeds up |
| Urinary bladder | Contracts | Relaxes |
| Lung | Bronchodilation | Bronchoconstriction |
| Overall activity | Excitatory and stimulating | Inhibitory |
CONDUCTION OF NERVE IMPULSE
· Nerve impulse is defined as an electrochemical event that changes the membrane potential of a nerve fibre that causes transmission of stimulus from receptor to central nervous system (CNS) and from CNS to effectors.
· The covering of an axon is axolemma or plasma membrane which has some minute pores through which water and other substances can pass in and out of it i.e. it is semipermeable in nature. Axon membrane is made up of lipoproteins and is about 100 A° in thickness. It has some minute pores or channels of 7–10 A° in diameter. Axolemma exhibits resting potential due to differences in concentrations of various ions inside and outside the axon due to the semi-permeability of the axon membrane.
· In the resting state, sodium ions are transported from inside to outside. So the concentration of Na+ inside axoplasm is almost negligible and that of K+ ions is more.
· Nerve impulse is defined as an electrochemical event that changes the membrane potential of a nerve fibre that causes transmission of stimulus from receptor to central nervous system (CNS) and from CNS to effectors.
· The covering of an axon is axolemma or plasma membrane which has some minute pores through which water and other substances can pass in and out of it i.e. it is semipermeable in nature. Axon membrane is made up of lipoproteins and is about 100 A° in thickness. It has some minute pores or channels of 7–10 A° in diameter. Axolemma exhibits resting potential due to differences in concentrations of various ions inside and outside the axon due to the semi-permeability of the axon membrane.
· In the resting state, sodium ions are transported from inside to outside. So the concentration of Na+ inside axoplasm is almost negligible and that of K+ ions is more.
· The outward transport of sodium ions is called sodium pump. Due to sodium pump, axon membrane towards its inner side becomes electronegative while towards outside electropositive. This state of nerve fibre is called a polarized state.
· When an axon is excited by an adequate stimulus, axolemma gets depolarized and sodium pump stops. So, the permeability of axolemma to Na increases and sodium ions diffuse from interstitial fluid to inside. Now, the inside of an axolemma gets depolarized and becomes positive to that of the outside. It is the opposite of resting-state and is called action potential or depolarization.
· As soon as the wave of depolarization has travelled the entire length of a nerve fibre, axolemma becomes totally impermeable to Na ions and permeable to K+ ions so no more sodium ions can enter from outside. Now, K+ ions diffuse inside and sodium ions outside. So, action potential disappears and resting potential appears. This process is called repolarization. Now, once again sodium pump starts functioning.
· When an axon is excited by an adequate stimulus, axolemma gets depolarized and sodium pump stops. So, the permeability of axolemma to Na increases and sodium ions diffuse from interstitial fluid to inside. Now, the inside of an axolemma gets depolarized and becomes positive to that of the outside. It is the opposite of resting-state and is called action potential or depolarization.
· As soon as the wave of depolarization has travelled the entire length of a nerve fibre, axolemma becomes totally impermeable to Na ions and permeable to K+ ions so no more sodium ions can enter from outside. Now, K+ ions diffuse inside and sodium ions outside. So, action potential disappears and resting potential appears. This process is called repolarization. Now, once again sodium pump starts functioning.
 |
| Nerve impulse conduction |
SYNAPSES
· Synapses are the sites of impulse transmission between the pre-synaptic and post-synaptic neurons. The neuron contributing the source of the incomings action potential is the pre-synaptic neuron while the cell contributing the dendritic sites is the postsynaptic neuron.
· At the free end of the axon of the pre-synaptic neuron, it breaks up into minute branches that terminate into small swellings called synaptic knobs or terminal boutons.
· Synaptic knobs are in close proximity to the dendrites and the cell body of the post-synaptic neurons. The gap between the pre and post-synaptic neuron is called the synaptic cleft.
· The synaptic knob of a pre-synaptic neuron contains synaptic vesicles, containing neurotransmitters that are released into the synaptic cleft.
· Neurotransmitters are synthesized by the nerve cells, actively transported along the axons and stored in synaptic vesicles and are released by exocytosis.
· The neurotransmitters in the brain and spinal cord are adrenaline, nor-adrenaline, dopamine, histamine, serotonin, gamma-aminobutyric acid (GABA) and acetylcholine.
· Synapses are the sites of impulse transmission between the pre-synaptic and post-synaptic neurons. The neuron contributing the source of the incomings action potential is the pre-synaptic neuron while the cell contributing the dendritic sites is the postsynaptic neuron.
· At the free end of the axon of the pre-synaptic neuron, it breaks up into minute branches that terminate into small swellings called synaptic knobs or terminal boutons.
· Synaptic knobs are in close proximity to the dendrites and the cell body of the post-synaptic neurons. The gap between the pre and post-synaptic neuron is called the synaptic cleft.
· The synaptic knob of a pre-synaptic neuron contains synaptic vesicles, containing neurotransmitters that are released into the synaptic cleft.
· Neurotransmitters are synthesized by the nerve cells, actively transported along the axons and stored in synaptic vesicles and are released by exocytosis.
· The neurotransmitters in the brain and spinal cord are adrenaline, nor-adrenaline, dopamine, histamine, serotonin, gamma-aminobutyric acid (GABA) and acetylcholine.
Various types of synaptic contacts between neurons have been observed viz.
· Axo-dendritic synapse – between an axon and a dendrite.
· Axo-somatic synapse – between an axon and muscles/glands/organs
· Axo-axonic synapse – between an axon and another axon
· Dendro-dendritic synapse – between a dendrite and another dendrite
HYPOTHALAMUS OF BRAIN
· Hypothalamus is a part of the diencephalon and lies below the thalamus.
· Hypothalamus is a part of the diencephalon and lies below the thalamus.
· It extends from the optic chiasma to the mammillary bodies and includes the nuclei below the hypothalamic sulcus.
· It occupies a volume of 4 cubic meters and forms about 0.3% of the brain tissue.
· Hypothalamus consists of a collection of nuclei and fibre tracts that form the lateral walls and floor of the III ventricle.
NUCLEI OF HYPOTHALAMUS
· Two fibre tracts – fornix and mammillothalamic tract- divide the hypothalamus into a medial zone and a lateral zone.
A. Medial zone: The medial zone contains the following nuclei: preoptic nucleus, paraventricular nucleus, dorsomedial nucleus, ventromedial nucleus, infundibular nucleus and the posterior nucleus.
· Two fibre tracts – fornix and mammillothalamic tract- divide the hypothalamus into a medial zone and a lateral zone.
A. Medial zone: The medial zone contains the following nuclei: preoptic nucleus, paraventricular nucleus, dorsomedial nucleus, ventromedial nucleus, infundibular nucleus and the posterior nucleus.
B. Lateral zone: The lateral zone contains the following nuclei: the supraoptic nuclei, lateral nucleus, tuberomammillary nucleus and the lateral mammillary nuclei overlaps both the zones. The small supra-chiasmatic nucleus, which lies over the optic chiasma, overlaps both the zones.
CONNECTIONS
· Hypothalamus is an integral part of the limbic system. The connections of the hypothalamus are numerous and very complex.
· The following characteristics should be kept in mind regarding the connections of hypothalamus:
A. The neural afferent inputs are derived from ascending visceral, sensory, visual, and olfactory pathways, and from the brainstem, thalamus, and limbic structures.
B. The efferent neural connections are reciprocal and converge to control the autonomic nerve fibres.
C. Hypothalamus exerts control over the endocrine system through its vascular and neural connections with the pituitary gland.
A. The neural afferent inputs are derived from ascending visceral, sensory, visual, and olfactory pathways, and from the brainstem, thalamus, and limbic structures.
B. The efferent neural connections are reciprocal and converge to control the autonomic nerve fibres.
C. Hypothalamus exerts control over the endocrine system through its vascular and neural connections with the pituitary gland.
Afferents
1. Median forebrain bundle: This is a complex fibre pathway, which passes through the lateral hypothalamic zone. It interconnects the forebrain limbic structures with the hypothalamus and the brainstem. This pathway interconnects the hypothalamus with the rest of the brain.
1. Median forebrain bundle: This is a complex fibre pathway, which passes through the lateral hypothalamic zone. It interconnects the forebrain limbic structures with the hypothalamus and the brainstem. This pathway interconnects the hypothalamus with the rest of the brain.
2. Stria terminalis: This tract interconnects the amygdala with many hypothalamic nuclei.
3. Fornix: This complex fibre tract arises from the hippocampal formation and reciprocally connects with the mammillary bodies.
4. Hypothalamus receives visceral, gustatory, and somatic sensory information from the spinal cord and the brainstem. These inputs reach the hypothalamus through multisynaptic pathways via the mammillary peduncle, the dorsal longitudinal fasciculus and the medial forebrain bundle.
5. Olfactory afferents reach the hypothalamus through the medial forebrain bundle.
6. Visual afferents are projected bilaterally to the suprachiasmatic nuclei via the optic nerve and tract.
Efferents
· The efferent connections of the hypothalamus are largely reciprocal to the afferents. These include reciprocal projections to:
1. the limbic system
2. descending polysynaptic paths to autonomic and somatic motor neurons and
3. neural and neurovascular link with the pituitary gland.
· The median forebrain bundle reciprocally interconnects the hypothalamus with the limbic structures and the brainstem. The fornix and the stria terminalis contain reciprocal connections to the hippocampal formation and the amygdala respectively.
· The mammillothalamic tract arises from the mammillary bodies and projects to the anterior thalamic nucleus.
· The efferent connections of the hypothalamus are largely reciprocal to the afferents. These include reciprocal projections to:
1. the limbic system
2. descending polysynaptic paths to autonomic and somatic motor neurons and
3. neural and neurovascular link with the pituitary gland.
· The median forebrain bundle reciprocally interconnects the hypothalamus with the limbic structures and the brainstem. The fornix and the stria terminalis contain reciprocal connections to the hippocampal formation and the amygdala respectively.
· The mammillothalamic tract arises from the mammillary bodies and projects to the anterior thalamic nucleus.
HYPOTHALAMUS THE MAJOR AFFERENT AND EFFERENT CONNECTIONS
| Afferents | Afferents |
|---|---|
| Median forebrain bundle | Median forebrain bundle |
| Fornix | Dorsal longitudinal fasciculus |
| Stria terminalis | Mammillo-thalamic tract |
| Brainstem reticular afferents via the mammillary peduncle and the dorsal longitudinal fasciculus Retino-hypothalamic fibres | Mammillo-tegmental tract Descending hypothalamic projections to the brainstem and the spinal cord Neural and vascular link to the pituitary gland |
· The mammillotegmental tract projects to the dorsal and ventral tegmental nuclei of the midbrain.
· The descending hypothalamic fibres are conveyed through the medial forebrain bundle to the tegmentum of the midbrain. These fibres are relayed to the sympathetic and parasympathetic preganglionic neurons in the intermediolateral column of the spinal cord. In their course, they innervate several brainstem nuclei, which control visceral functions.
· Hypothalamus is connected to the pituitary gland by two routes - (1) by nerve fibres arising from the supraoptic and paraventricular nuclei, ending in the posterior pituitary gland i.e., the hypothalamic- hypophyseal tract, and (2) the hypothalamic- hypophyseal portal system vascularly links the median eminence and the infundibulum, with the capillary plexuses of the anterior lobe of the pituitary gland.
· The descending hypothalamic fibres are conveyed through the medial forebrain bundle to the tegmentum of the midbrain. These fibres are relayed to the sympathetic and parasympathetic preganglionic neurons in the intermediolateral column of the spinal cord. In their course, they innervate several brainstem nuclei, which control visceral functions.
· Hypothalamus is connected to the pituitary gland by two routes - (1) by nerve fibres arising from the supraoptic and paraventricular nuclei, ending in the posterior pituitary gland i.e., the hypothalamic- hypophyseal tract, and (2) the hypothalamic- hypophyseal portal system vascularly links the median eminence and the infundibulum, with the capillary plexuses of the anterior lobe of the pituitary gland.
FUNCTIONS
· The nervous system is directly concerned with several homeostatic mechanisms that enable an individual to survive in a changing environment.
· Transection studies, in animals, have revealed that transactions below the midbrain level affect many vital functions of the body, besides affecting motor activity.
· A midbrain animal lacks thermoregulation and the animal cannot survive without proper care. However, animals with transactions above the hypothalamus separating it from the limbic system but preserving its connections with the pituitary, brainstem and spinal cord, are able to survive and exhibit several of the homeostatic responses seen in an intact animal.
· For example, such animals possess thermoregulation, moderately withstand stress, can feed and drink, and display visceromotor and endocrine responses.
· The animals survive even in adverse conditions if the limbic connections are not affected. Such experimental studies have shown that the hypothalamus is an important neural structure involved in the regulation of several homeostatic mechanisms.
· Clinically, hypothalamic lesions have been associated with a wide range of endocrine disorders with abnormal metabolic, visceral, motor and emotional manifestations. Sherrington considered the hypothalamus as the ‘head ganglion' of the autonomic nervous system in view of the many autonomic abnormalities seen in such conditions.
· Subsequent investigations revealed different 'centres' concerned with feeding, drinking and other visceral functions. Hypothalamus is now considered to be a region for integrating the external and internal afferent inputs with cortically generated activities.
· The hypothalamus is a part of a hierarchy that controls the various visceral and somatic mechanisms that maintain homeostasis.
· Hypothalamus has reciprocal connections with the forebrain autonomic and limbic structures and regulates the visceral and somatic activities by its - (1) descending neural projections to the autonomic efferent neurons and (2) by its neural and vascular connections with the pituitary gland.
· The nervous system is directly concerned with several homeostatic mechanisms that enable an individual to survive in a changing environment.
· Transection studies, in animals, have revealed that transactions below the midbrain level affect many vital functions of the body, besides affecting motor activity.
· A midbrain animal lacks thermoregulation and the animal cannot survive without proper care. However, animals with transactions above the hypothalamus separating it from the limbic system but preserving its connections with the pituitary, brainstem and spinal cord, are able to survive and exhibit several of the homeostatic responses seen in an intact animal.
· For example, such animals possess thermoregulation, moderately withstand stress, can feed and drink, and display visceromotor and endocrine responses.
· The animals survive even in adverse conditions if the limbic connections are not affected. Such experimental studies have shown that the hypothalamus is an important neural structure involved in the regulation of several homeostatic mechanisms.
· Clinically, hypothalamic lesions have been associated with a wide range of endocrine disorders with abnormal metabolic, visceral, motor and emotional manifestations. Sherrington considered the hypothalamus as the ‘head ganglion' of the autonomic nervous system in view of the many autonomic abnormalities seen in such conditions.
· Subsequent investigations revealed different 'centres' concerned with feeding, drinking and other visceral functions. Hypothalamus is now considered to be a region for integrating the external and internal afferent inputs with cortically generated activities.
· The hypothalamus is a part of a hierarchy that controls the various visceral and somatic mechanisms that maintain homeostasis.
· Hypothalamus has reciprocal connections with the forebrain autonomic and limbic structures and regulates the visceral and somatic activities by its - (1) descending neural projections to the autonomic efferent neurons and (2) by its neural and vascular connections with the pituitary gland.
The specific functions involving the hypothalamus are:
1. General autonomic control (autonomic nervous system)
2. Endocrine control
3. Control of body temperature
4. Regulation of fluid balance
5. Appetite and satiety
6. Regulation of circadian rhythms
7. Emotion and behaviour
8. Sleep-wake cycle
9. Sexual behaviour and reproduction.
1. General autonomic control (autonomic nervous system)
2. Endocrine control
3. Control of body temperature
4. Regulation of fluid balance
5. Appetite and satiety
6. Regulation of circadian rhythms
7. Emotion and behaviour
8. Sleep-wake cycle
9. Sexual behaviour and reproduction.
1. General autonomic control
· Hypothalamus influences the autonomic nervous system. Stimulation of anterior and medial hypothalamic area, in animals, evoke parasympathetic effects - fall in blood pressure, a fall in heart rate, contraction of the urinary bladder, increased gastrointestinal motility, increased gastric acid secretion, salivation and pupillary constriction.
· Stimulation of the posterior hypothalamic area produces the sympathetic effects - a rise in blood pressure, an increase in heart rate, decreased gastrointestinal motility, pupillary dilatation, hyperglycaemia, the general increase in metabolic and somatic activity.
· However, there are no discrete parasympathetic or sympathetic centres in the hypothalamus. The paraventricular nucleus is an important integrative centre for autonomic effects. The autonomic effects caused by the hypothalamus are part of the complex survival mechanisms and come into play during the various homeostatic responses mediated by the hypothalamus.
· Hypothalamus influences the autonomic nervous system. Stimulation of anterior and medial hypothalamic area, in animals, evoke parasympathetic effects - fall in blood pressure, a fall in heart rate, contraction of the urinary bladder, increased gastrointestinal motility, increased gastric acid secretion, salivation and pupillary constriction.
· Stimulation of the posterior hypothalamic area produces the sympathetic effects - a rise in blood pressure, an increase in heart rate, decreased gastrointestinal motility, pupillary dilatation, hyperglycaemia, the general increase in metabolic and somatic activity.
· However, there are no discrete parasympathetic or sympathetic centres in the hypothalamus. The paraventricular nucleus is an important integrative centre for autonomic effects. The autonomic effects caused by the hypothalamus are part of the complex survival mechanisms and come into play during the various homeostatic responses mediated by the hypothalamus.
2. Control of the endocrine system
· The central nervous system controls the activities of the endocrine system. The brain contains neurons, which synthesize and secrete peptides, a process referred to as neurosecretion.
· Hypothalamus contains the densest concentration of neurosecretory neurons. The neurosecretory neurons of the hypothalamus have been categorized into—(1) parvocellular and (2) magnocellular neurons.
· The parvocellular neurons lie within the periventricular zone and the arcuate (infundibular) nucleus. The magnocellular neurons are located in the supraoptic and paraventricular nuclei.
·The axons of the parvocellular neurons converge on the infundibulum and end on the capillary loops, which form the hypophyseal portal vessels.
· The parvocellular neurons secrete - (1) Growth hormone-releasing hormone (GHRH), Somatostatin, Gonadotropin-releasing hormone (GnRH), Corticotropin-releasing hormone (CRH), Thyrotropin-releasing hormone (TRH) and Dopamine.
· The hypothalamic hormones either stimulate or inhibit the secretion of their respective trophic hormones secreted by the anterior pituitary gland. Dopamine acts as a prolactin release-inhibiting factor.
· Through its effects on the anterior pituitary, the hypothalamus controls the activities of the thyroid gland, the adrenal cortex, gonad, and influences the growth and metabolic activities. The hormones or the controlled variable exert feedforward and feedback control at every level.
· The axons of the magnocellular neurons in the supraoptic and paraventricular nuclei descend down the infundibulum and end in the posterior pituitary around the capillaries. The magnocellular neurons secrete vasopressin (ADH) and oxytocin. Vasopressin and oxytocin are secreted by separate neurons.
· Vasopressin neurons are osmosensitive and receive osmosensory afferents from the median preoptic nucleus and subfornical organ. ADH is primarily concerned with osmotic homeostasis by its effects on renal tubular water reabsorption.
· Oxytocin is reflexly secreted in women through polysynaptic afferents from the nipple and the uterine cervix. It causes post-partum milk- ejection and contraction of the uterus during childbirth.
· Moreover, the hypothalamus can influence the activities of other peripheral endocrine organs through their autonomic innervation. For example, the endocrine pancreas is partly under neural control through its autonomic nerves.
3. Control of body temperature
· Homeothermic animals, including man, maintain a constant body temperature. In human beings, the body temperature is maintained around 3 7°C, by a balance between heat loss and heat production mechanisms.
· A raised temperature activates the heat loss mechanisms and depresses heat production mechanisms. A fall in temperature produces reverse effects.
· The environmental temperature is sensed by peripherally located cutaneous thermoreceptors.
· The core temperature is monitored by temperature-sensitive neurons in the preoptic area of the hypothalamus. The peripheral thermal afferents and limbic afferents are projected to the preoptic area.
· Stimulation of the anterior hypothalamus produces heat loss responses - sweating, vasodilatation and panting. Damage to this region can lead to a rise in body temperature.
· Stimulation of the posterior hypothalamus produces heat production responses vasoconstriction, decreased sweating, increased visceral activity, increased metabolism and shivering.
· Lesions of the posterior hypothalamus make the animal poikilothermic (coldblooded). The posterior hypothalamic region does not contain any thermoreceptors.
· The heat loss and heat production responses seem to be controlled by the anterior hypothalamus. Deviations from the body temperature are sensed by the thermosensitive neurons in the anterior hypothalamus and evoke responses that restore the body temperature. The hypothalamus, thus, functions as the temperature servomechanism.
4. Regulation of fluid balance
· Hypothalamus is involved in the maintenance of fluid balance by its involvement in the control of water intake and in the control of water loss from the body. Fluid intake is influenced by osmolality and volume of ECF.
· In conditions of water deprivation, the ECF becomes hyperosmotic. Hyperosmolaltiy is detected by osmoreceptors located in the anterior hypothalamus and the circumventricular organs. The osmoreceptors can detect a 1% increase in osmolality evoke the thirst sensation and cause drinking behaviour.
· Water deprivation or fluid loss will produce a decrease in blood volume. A fall in blood volume by 5 to 10% is sensed by receptors in the great veins, atria and the baroreceptors in the carotid sinus. This causes the release of renin by the kidney. Renin breaks down angiotensinogen and forms angiotensin II. Angiotensin II acts on the subfornical organ and stimulates drinking.
· Water loss is regulated by stimulation or inhibition of release of ADH from the supraoptic and paraventricular neurons at their terminals in the posterior pituitary.
· Hyperosmolality or hypovolaemia stimulates the release of ADH from the ADH- secreting neurons in the supraoptic and paraventricular nuclei and produces increased reabsorption of water in the renal tubules.
· On the other hand, hyperosmolality or hypervolaemia inhibits the release of ADH and decreases the reabsorption of water in the renal tubules.
· Supraoptic and paraventricular nuclei of the hypothalamus are important in the maintenance of fluid balance. Lesions of these nuclei produce diabetes insipidus with excessive loss of fluids through polyuria
5. Regulation of food intake and metabolism
· The source of energy in humans and animals are the nutrients derived from ingested food. Food intake is regulated to maintain a balance between body weight and energy expenditure. The balance between energy intake and energy expenditure depends on many factors, like feeding behaviour, neural control of the GI tract and pancreas, and hormonal control by GH, thyroid hormones, and glucocorticoids. The hypothalamus plays an important role in the maintenance of energy balance.
· The ventromedial nucleus receives visceral afferents from the nucleus tractus solitarius and the lateral hypothalamus receives the olfactory afferents. Both these nuclei receive extensive limbic afferents.
· The ventromedial nucleus functions as a satiety centre which generates a sense of satisfaction and inhibits feeding behaviour. Bilateral lesions in the ventromedial nucleus cause hyperphagia and obesity. Stimulation of this nucleus stops feeding behaviour.
· The lateral hypothalamus functions as the feeding centre, and activates the animal to search for food. Lesions in the lateral hypothalamus cause aphagia, starvation and eventual death of the animal. Stimulation of the lateral hypothalamus causes eating, even in a fully satiated animal.
· Besides, lesions in the paraventricular nucleus produce hyperphagia and lesions in the dorsomedial nucleus depress feeding behaviour. These hypothalamic nuclei together, coordinate the feeding behaviour and perception of satiety.
· Hypothalamic neurons contain receptors for neurotransmitters and hormones directly concerned with food intake. Serotonin, CCK, GLP-1 and Leptin inhibit food intake, while
· Neuropeptide-Y (NPY) increases food intake. The receptors for these regulatory factors have been identified in the hypothalamic neurons.
· Neuropeptide-Y secreting neurons are present in the arcuate nucleus. The ventromedial nucleus contains neurons that are sensitive to glucose and other nutrients.
· Hypothalamus regulates the activities of the endocrines involved in the energy balance and metabolism through its humoral effects on the anterior pituitary and via the projections to the autonomic efferents.
· The ventromedial nucleus, the lateral hypothalamus and the paraventricular nucleus influence the intermediary metabolism and complement their effects on feeding behaviour.
· Stimulation of ventromedial nucleus produces glucagon release, increased gluconeogenesis, glycogenolysis and glycolysis.
· On the other hand, stimulation of the lateral hypothalamus causes insulin release and the opposite metabolic effects.
6. Regulation of circadian rhythms
· Body temperature, the plasma concentration of many hormones, renal secretion and sleep-wake cycle show an approximately 24 hours cycle pattern in tune with the light-dark cycle. Such a type of cyclic pattern is called circadian rhythms. The suprachiasmatic nucleus (SCN) is concerned with the regulation of circadian rhythms.
· SCN receives visual afferents from the retina and influences the activity of the pineal gland. SCN has interconnections with other hypothalamic nuclei. SCN neurons discharge rhythmically even when cultured in vitro.
· Lesions in the SCN disrupt the normal circadian rhythms and can be restored by transplantation of suprachiasmatic tissue.
· The mechanism of rhythm generation in SCN is not known and is found to be inherent. SCN is now considered to be the body's 'biological clock'.
7. Emotion and behaviour
· Emotion and behaviour involve the limbic system and the prefrontal cortex. The hypothalamus is the principal centre for emotional expressions. The physical expression of emotion concerns the sympathetic and parasympathetic nervous systems. The hypothalamus, by virtue of its projections to the autonomic efferents, activates the autonomic manifestations that are part of emotions.
· Decorticate animals (cats) with intact hypothalamus show 'sham rage'. These animals exhibit all the physical aspects of anger - like hissing, growling, baring of claws and fangs - when they are subject to provocation. Such aggressive behaviour can be abolished in decorticate animals by destroying the posterior hypothalamus.
· Stimulation of the ventromedial nucleus produces a similar aggressive pattern and stimulation of the lateral region of the anterior hypothalamus produces the 'flight' response in animals. Similarly, stimulation or ablation of other hypothalamic nuclei produces behavioural patterns associated with fear, punishment, sexual drive and passivity in animals.
· The hypothalamus is thought to integrate the afferent inputs from limbic structures and produce the physical expression of emotions and associated behavioural patterns.
8. Sleep-wake cycle
· The phenomenon of sleep depends on a balance between the activity of the ascending reticular activating system and certain sleep zones situated in the brainstem. The ascending reticular activating system projects diffusely to the cerebral cortex.
· This system causes arousal, wakefulness and desynchronization of the EEG. The sleep zones are thought to induce sleep, by active inhibition of the ascending reticular activating system.
· Stimulation of the rostral part in the suprachiasmatic area of the hypothalamus promotes sleep.
· Bilateral lesions in the medial rostral part of the anterior hypothalamus cause a state of wakefulness.
· Damage to the preoptic nucleus of the hypothalamus produces insomnia whereas stimulation of the nucleus induces the state of slow-wave sleep.
· Damage to neurons in the ventrolateral posterior hypothalamus produces transient sleep.
· Neurons from the posterior hypothalamic area have arousal properties through both direct excitatory histaminergic projections to the cerebral cortex, and by indirect excitatory projections to the brainstem reticular formation.
· The preoptic nucleus induces sleep indirectly through inhibition of the excitatory neurons of the posterior hypothalamus.
· A hypothalamic sleep-inducing peptide has been isolated and may have a role in the sleep mechanism.
9. Sexual behaviour and reproduction
· Initiation and coordination of sexual behaviour and reproductive function require an intact hypothalamus. Hypothalamus regulates sexual behaviour by its control over the secretion of gonadotropins, prolactin and oxytocin.
· The secretion of the gonadotropins in both sexes is controlled by the hypothalamic releasing hormones.
· Pulsatile release of GnRH at puberty initiates the changes in the reproductive system and the accompanying physical and mental changes.
· Hypothalamic diseases can either inhibit reproductive development and function or lead to precocious puberty depending on the nature of the lesion.
· The tuberal region of the hypothalamus is essential for the maintenance of basal levels of gonadotropins.
· The preoptic area is concerned with a cyclic surge of gonadotropins prior to ovulation. Stimulation of the preoptic area produces ovulation in animals.
· The sexually dimorphic nucleus in the preoptic area (SDN-POA) seems to be involved in sex differentiation. In animals, it has been shown that in early postnatal life under the influence of the circulating androgens, the SDN-POA nucleus becomes larger in the male than in the female. It has been suggested that an interaction between the developing brain and the sex hormones influence the sexual orientation of the individual.
· The secretion of the gonadotropins in both sexes is controlled by the hypothalamic releasing hormones.
· Pulsatile release of GnRH at puberty initiates the changes in the reproductive system and the accompanying physical and mental changes.
· Hypothalamic diseases can either inhibit reproductive development and function or lead to precocious puberty depending on the nature of the lesion.
· The tuberal region of the hypothalamus is essential for the maintenance of basal levels of gonadotropins.
· The preoptic area is concerned with a cyclic surge of gonadotropins prior to ovulation. Stimulation of the preoptic area produces ovulation in animals.
· The sexually dimorphic nucleus in the preoptic area (SDN-POA) seems to be involved in sex differentiation. In animals, it has been shown that in early postnatal life under the influence of the circulating androgens, the SDN-POA nucleus becomes larger in the male than in the female. It has been suggested that an interaction between the developing brain and the sex hormones influence the sexual orientation of the individual.