· Definition: A group of specialized cells similar in embryonic origin, form and function is called tissue.
· But the term histology was given by a German histologist Mayer 1819.
· Study of tissue is called histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Marcello Malpighi, an Italian scientist established a separate branch for the study of tissues so he is the founder of histology.
· Types of Animal Tissues:
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
· Depending upon the location and function, animal tissue is divided into the following 4 types:
A. EPITHELIAL TISSUE
B. MUSCULAR TISSUE
C. CONNECTIVE TISSUE
D. NERVOUS TISSUE
Nervous Tissue
· They are generally ectodermal in origin except for the microgliocytes which are mesodermal in origin.· They have excitability and conductivity.
· Neuron is the structural and functional unit of nervous tissue.
· They are the longest cells of the animal body.
· Human nervous system has 100 billion neurons.
· They are mainly confined in the brain.
· Neuron consists of mainly cell body and cytoplasmic processes arising from it.
a. Cell body or Soma or Cyton
· It varies in size and shape.
· It varies in size and shape.
· It may be up to 13.5 µm in diameter and maybe irregular, oval, rounded, star-shaped or pyramidal.
· It consists of the nucleus, cytoplasm and cell membrane.
· It consists of the nucleus, cytoplasm and cell membrane.
· The nucleus is a large oval-shaped with distinctive Nucleolus.
· Cytoplasm contains Mitochondria, Golgi apparatus, Ribosomes, Endoplasmic reticulum, Fat globules, Neurofibrils and Nissl's granules.
· Nissl's granules or bodies are the irregular masses of the rough endoplasmic reticulum with numerous attached ribosomes for synthesizing the protein for the cell.
· Cyton is concerned with metabolic maintenance and growth.
· Nissl's granules or bodies are the irregular masses of the rough endoplasmic reticulum with numerous attached ribosomes for synthesizing the protein for the cell.
· Cyton is concerned with metabolic maintenance and growth.
b. Neuritis or Cytoplasmic processes
These are of 2 types: Dendron & Axon.
1. Dendrites
· These are short, much-branched and tapering cytoplasmic processes of the neuron which may be one or several in a neuron.
· It contains Neurofibrils, Nissl's granules and Golgi apparatus.
· It conducts the nerve impulse towards the cell body and so they are called afferent processes.
2. Axon
· It is a large single very long process of the neuron that arises from the axon hillock.
· It contains Neurofibrils, Neurotubules but does not contain Nissl's granules, Golgi apparatus, Ribosomes.
· It is a large single very long process of the neuron that arises from the axon hillock.
· It contains Neurofibrils, Neurotubules but does not contain Nissl's granules, Golgi apparatus, Ribosomes.
· Axon ends in many terminal arborizations or axon terminals boutons or endbulb.
· Each axon terminal or Telodenria has a synaptic knob.
· Its membrane is called axolemma and cytoplasm is called axoplasm.
· It conducts the nerve impulse away from the neuron to another neuron so it acts as an efferent nerve fibre.
· Axon may have side branches called collateral neurons which are finer than the main axon.
· Its membrane is called axolemma and cytoplasm is called axoplasm.
· It conducts the nerve impulse away from the neuron to another neuron so it acts as an efferent nerve fibre.
· Axon may have side branches called collateral neurons which are finer than the main axon.
VARIOUS TYPES OF NEURONS
A. Based on the functions (Sensory & Motor)1. Sensory or Afferent neuron or Receptor neuron
· They connect the sense organs with the central nervous system.
· They conduct the nerve impulse from the sense organs to the central nervous system.
2. Motor or Efferent neuron or Effector neuron
· They connect the central nervous system to the effector organs that is muscle or glands.
· They connect the central nervous system to the effector organs that is muscle or glands.
· So they conduct the impulses to the effector organs from the central nervous system.
3. Mixed neurons
· They act as both sensory and motor neuron. Eg – spinal nerves.
B. Based on their Polarity
1. Non-polar Neuron
· There is no functional difference between dendrites and axons.
1. Non-polar Neuron
· There is no functional difference between dendrites and axons.
· Each process can bring an impulse or conduct away from cyton.
· They are found in coelenterates or in Hydra.
2. Unipolar Neuron
· It has a single process.
· It has a single process.
3. Bipolar Neuron
· It has one axon and one dendrite at another end.
· It has one axon and one dendrite at another end.
· These types of neurons are found in the retinal of eye olfactory epithelium, the Cochlea and vestibules of the internal ear.
4. Multipolar Neuron
· It has several dendrites and an axon.
· It has several dendrites and an axon.
· They are the most common type of neuron which occurs in the grey matter of the brain and spinal cord.
5. Pseudo- unipolar Neuron
· It has a single dendrite and axon arising from the common stem of the cell body.
· It has a single dendrite and axon arising from the common stem of the cell body.
· It is found in embryo and dorsal root ganglia of adult vertebrates.
Various Types of Nerve Fibers
1. Myelinated or Medullated nerve fibre
· The axon of this neuron is surrounded by many satellite cells called Schwann cells which secrete a fatty insulating layer called myelin or medullary sheath around the axon.
· The sheath is interrupted by constrictions at regular intervals called Nodes of Ranvier.
1. Myelinated or Medullated nerve fibre
· The axon of this neuron is surrounded by many satellite cells called Schwann cells which secrete a fatty insulating layer called myelin or medullary sheath around the axon.
· The sheath is interrupted by constrictions at regular intervals called Nodes of Ranvier.
· The outermost thin and continuous layer is called the neurilemma.
· These nerve fibres help to conduct the nerve impulse at a greater speed because they have salutatory conduction.
· They are generally thicker than the non-myelinated neuron.
· They are generally thicker than the non-myelinated neuron.
· They are present in cranial and spinal nerves of central and peripheral nervous systems.
· It has no myelin sheath around the axon.
· There are no Nodes of Ranvier's but the Schwann cells are present around the axon which don't secrete the myelin sheath.
· It conducts the nerve impulse at a slower speed than myelinated nerve fibre.
· These are comparatively thinner or less in diameter.
· They are present in the autonomic nervous system.
Neuroglia or Glial cells
· These are the specialised cells that support the neuron and their fibres.
· These are the specialised cells that support the neuron and their fibres.
· About 50% of the brain is formed by the neuroglial cell
· They are divided into 2 types: Macroglia & Microglia.
A. Macroglia or Large glial cells:
· They are ectodermal in origin and are of 2 types: Astrocytes & Oligodendrocytes.
1. Astrocytes
· They are star-shaped cells with a number of processes.
· They are of 2 types
a. Fibrous astrocytes – they have thin and asymmetrical processes, found in the white matter of CNS.
b. Protoplasmic astrocytes- they have thick and symmetrical processes and are found in the grey matter of CNS.
· Functions of Astrocytes: They provide mechanical support to neurons, insulate the adjacent neuron which allows the nerve impulse conduction only from one to another neuron, they maintain the blood-brain barrier and are also responsible for the repairment of damaged areas of nervous tissue.
· They are star-shaped cells with a number of processes.
· They are of 2 types
a. Fibrous astrocytes – they have thin and asymmetrical processes, found in the white matter of CNS.
b. Protoplasmic astrocytes- they have thick and symmetrical processes and are found in the grey matter of CNS.
· Functions of Astrocytes: They provide mechanical support to neurons, insulate the adjacent neuron which allows the nerve impulse conduction only from one to another neuron, they maintain the blood-brain barrier and are also responsible for the repairment of damaged areas of nervous tissue.
2. Oligodendrocytes
· They have fewer and short cells processes, have rounded nucleus, many mitochondria and glycogen.
· They are found near the myelinated neuron and soma of the neuron.
· They are found near the myelinated neuron and soma of the neuron.
· They form myelin sheath of neurons around the axon that lies with the central nervous system because the Schwann,s cells are absent in CNS.
B. Microglial cells or Microglia
· They are mesodermal in origin.
· They are mesodermal in origin.
· They are the smallest neuroglial cells.
· They are numerous in the grey matter than white matter.
· They have short and fine cytoplasmic processes.
· They engulf and destroy the microbes and cellular debris.
· So they are phagocytic and scavenger cells.
· They migrate to the injured tissue and function as small macrophages.
Ependymal cells
· These are squamous or columnar cells.
· These are squamous or columnar cells.
· They are found in the ventricles of the brain and spinal canal.
· Their free surface bears numerous microvilli and cilia.
· Microvilli help to absorb the cerebrospinal fluid whereas cilia help in its movement.
· They have many long processes on the opposite side which penetrate the nervous tissue.
Neurosecretary cells
· They are specialized nerve cells that function as endocrine organs.
· They release chemicals from their axon directly into the blood.
· They are present in the hypothalamus of the vertebrate brain which secretes the neurohormone which stimulates the pituitary to secrete the pituitary hormones such as ACTH, TSH, and LH OR FSH.
Also, Read our Other Notes Related to Animal Tissues: