Animal Tissues and their Types: Nervous tissues

Structure of a typical neurone
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Nervous tissues make the nervous system in animals. It consists of the nerve cells (neurons), the processes of the nerve cells (nerve fibres), packing cells (neuroglia), ependymal cells, and neurosecretory cells.

I. Neurone

The neuron or nerve cell is a unit of the nervous system. It includes a bundle of nerve fibres. It is the longest cells in the body. Some of the neurons are as long as one meter. The neurons have the important properties – excitability and conductivity. Excitability is the phase in which the nerve fibres are excited upon receiving any stimulus. Conductivity is the phase in which the message (impulse) is transmitted along with an axon. Neurons form a nerve. A nerve is covered by a sheath of dense connective tissue, the epineurium. The nerve contains a number of bundles, called the fasciculi.

Structure of neuron or nerve cell

A neuron consists of two parts – cell body and nerve processes.

1. Cell body

The Cell body is the main body of a neuron. It is also called cyton or soma or perikaryon. Its size ranges from 4 – 135 μm. Its cytoplasm contains a centrally placed nucleus, mitochondria, Golgi body, and endoplasmic reticulum with ribosomes. Besides these organelles, the characteristic granules, called Nissl’s bodies are present. Once the neurons fully formed, they never divide.

2. Nerve processes

Nerve processes arise from soma. They are dendrites and axons.

Dendrites

Dendrites are numerous small, branched nerve processes. They have Nissl’s granules and neurofibrils. They are non-myelinated. They are afferent in nature i.e. conducting the nerve impulse towards the cyton. 

Axon

Axon is the long and unbranched nerve processes arising from the cyton of a neuron.  It has neurofibrils but no Nissl’s granules. In general, it is covered by a myelinated sheath. This sheath is broken at short distances forming the nodes of Ranvier. At its terminal end, it forms many branches, called terminal aborization. Each branch bears a small swelling, called synaptic knob. Synaptic knob is attached to the dendrites of another nerve fibres.

Axon is enveloped by axolemma. Its cytoplasm is called axoplasm, which contains neurofibrils and mitochondria. Axons are efferent in nature i.e. they conduct nerve impulse away from the cyton. Nerve impulses are transmitted from axon to dendrite.

Types of nerve fibres and neurones

Nerve fibres and neurons are classified on the basis of structure, function of nerve impulse, and the number of nerve processes.

a. Based on structure

On the basis of the presence and absence of the myelin sheath, nerve fibres are myelinated or medullated nerve fibres and non-myelinated or non-medullated nerve.

i. Myelinated nerve fibres

Myelin sheath encloses these myelinated nerve fibres. These are found in white matter of the brain and spinal cord, cranial and spinal nerves. These are involved in conduction of nerve impulses.

ii. Non-myelinated nerve fibres

These nerve fibres do not bear the myelin sheath. These are found in the grey matter of the brain and spinal cord. They are involved in integration of nerve impulses.

The central axis of non-myelinated nerve fibres is covered by neurilemma formed by the sheath of the Schwann cells. They do not have the medullary sheath and the nodes of Ranvier.   

b. Based on function of nerve impulse

On the basis of the conduction of nerve impulses, nerve fibres are afferent nerve fibres and efferent nerve fibres.

i. Afferent nerve fibres

These nerves conduct nerve impulses from the sense organs (receptors) to the central nervous system (CNS). They are also called sensory nerve fibres, e.g. optic nerve.

ii. Efferent nerve fibres

These nerves conduct nerve impulses from CNS to the body (sense organs). They are also called motor nerve fibres, e.g. oculomotor nerve.

c. Based on the number of nerve processes

On the basis of the number of nerve processes, neurons are unipolar, bipolar, and multipolar neurones.

i. Unipolar neurones

Unipolar neurons have only one axon with dendrites. They are found in the dorsal root ganglia of the spinal nerve and in the roots of V, XI, and X cranial nerves.

ii. Bipolar neurones

These neurons have two cell processes arising from the cell body. Among these two processes, one may be a dendron and other may be an axon. They are found in the retina of the eyes and in the olfactory epithelium of the nasal chamber.

iii. Multipolar neurones

These neurons have many cell processes. At least one of these processes is an axon. They are found in the ganglia of the autonomic nervous system.

II. Neuroglia or glial cells

Neuroglia or glial cells are non-excitable cells that support neurons of the central nervous system. They are of three types – microgliocytes, astrocytes, and oligodendrocytes. These cells are ten times more numerous than the neurons. These cells are packed between the nerve cells of CNS, ganglia and retina of the eye.

Microgliocytes are small spindle shaped cells.

Astrocytes are large sized cells with several highly branched processes.

Oligodendrocytes are small sized few branched processes similar to the dendrons of neurons.

III. Ependymal cells

These are small cells that form a thin membranous lining of the ventricles (cavities) in the brain and spinal cord.

Ependymal cells help in the formation of cerebrospinal fluid.

IV. Neurosecretory cells

These cells are found in the posterior lobe of hypothalamus of brain. These cells produce hormones, called releasing factors.

The releasing factors produced by these cells direct pituitary gland to secrete stimulating hormones accordingly.

Figure 20 Different types of nervous cells