• This tissue is considered as the second specialized tissue with the property of excitability and conductivity i.e., cells of this tissue, are specialized for receiving stimuli and transmitting messages (the first being the muscular tissue).


• This tissue forms the nervous system of the body and consists of following parts -

• neurons are considered as the longest cells of the body.


• Each neuron arises in the embryo, from a single neuroblast cell. Its main cell body is called Cyton, Perikaryon or Soma.


• Cytons of most neurons occur in the Grey matter of central nervous system particularly in brain. A few cytons are present outside the CNS also in clusters, called ganglia.


• The cyton contains neuroplasm, a prominent spherical nucleus, mitochondria, golgi bodies, ER, ribosomes, lysosomes, fat globules, pigment granules etc. Besides the presence of Nissl Bodies and Neurofibrils are characteristic of all neurons. Nissl bodies are comparatively large masses of ribosomes and RER. Neurons are without centrosome. Once fully formed, these never divide and remain in the G0 phase of interphase throughout the life. These, however, grow to a larger size with growth of the body.


• The processes of neurons are called neurites and are of two types - Dendrites or dendrons and an axon or axis cylinder (= neuroaxon). Dendrites may be one to several but axon is always one. Nissl bodies, neurofibrils and mitochondria are present in dendrites.


• Axon is usually a long process of uniform thickness and branched only terminally (terminal arborization) in to slender branches called telodendria having knobbed ends called buttons or end bulbs or terminals. Certain axons also give out twig like thin branches at right angles called colletral fibres. Axoplasm contains neurofibrils and mitochondria, but Nissil bodies, golgi bodies, ribosomes, pigment granules, fat globules etc. are said to be absent. The part of cyton from which axon arises is called axon hillock. The complete neuron looks like -
  
  
  
  Fig. 1.21 A neuron

TYPES OF NEURONS

A. On the basis of functional nature in relation to structure -

The neurons are of following 4 types i.e.,

[i] Apolar neurons i.e., neurons without polarity. Here the fibres of neuron are not differentiated in to axon and dendrites. All the fibres are of same nature and can carry information towards or away from the cell body, eg., neurons of Hydra.

[ii] Unipolar neurons i.e., neurons with unidirectional flow of information. These have one axon or one dendrite only. Most sensory neurons are unipolar. These are common in invertebrate and vertebrate embryos.

[iii] Bipolar neurons i.e., neurons with unidirectional flow of information but with one dendron and one axon at opposite poles. These occur in the Retina of eyes, olfactory epithelium etc.

[iv] Multipolar Neurons i.e., neurons with unidirectional flow of information but with one axon and many dendrites. They occur in the nervous system of adult vertebrates.

• Neurons can also be classified according to their functions only as:

[i]

or

neurons i.e., these connect sensory or receptor cells or organs to the CNS and conduct sensory impulses. Branched or unbranched, and naked or encapsulated free endings of numerous sensory neurons found scattered in skin epidermis. These serve as cutaneous sense organs or

. Similar endings scattered in skeletal muscles, bone joints, ligaments and tendons serve as

.

[ii]

or

neurons i.e., these connect the CNS to effectors (muscles and glands) and conduct motor impulses.

[iii]

or

or

-These occur only in the CNS and serve to connect two or more neurons for distant transmission of impulses.

or

• These are supporting cells, which form the packing around the neurons. There present three types of neuroglia i.e., astrocytes, oligodendrocytes and microgliocytes. Out of these the first two are ectodermal in origin while the third one is mesodermal in origin.

3.

cells form an epithelium that lines the ventricle [cavities of brain] and the central canal of spinal cord. Here the cells are generally ciliated.

4.

cells release neuroharmones [releasing factors] in to the blood stream.

• A nerve fibre is the axon together with its envelops.


• At regular intervals the neurilemma is constricted and its myelin sheath is interrupted forming the so-called Nodes of Ranvier. Certain axons such as those of sympathetic nervous system do not have the Myelin Sheath [nonmyelinated or nonmedullated nerve fibres].


• Electron microscopy has revealed that the myelin sheath consists mainly of the plasma membrane of the Schwann cells [in peripheral nervous system] or of oligodendrocytes wrapped around the axon several times in a spiral fashion. Each internode or its myelin sheath is the product of a single Schwann cell or oligodendrocyte. At each internode therefore, the myelin is discontinued but the two adjacent Schwann cells / oligodendrocytes profusely interdigitate to maintain a functional continuity of neurilemma.


• Myelin appears glisting white, hence myelinated fibres appear white, while nonmyelinated fibres appear grey. In the CNS the myelinated fibres form the white matter while the nonmyelinated fibres form the grey matter.


• The functioning of nerves is given in the unit nerve physiology and sensory organs.