Nervous system.

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NERVOUS SYSTEM

Nerve tissue of vertebrates consists of:-

(a)

Three types of cell:-

(i) Neurons, many million, function transmit messages (impulses).

(ii) Schwann cells, associate with neurons in PNS

(iii) Neurological cells, found within CNS

(b)

Connective tissue and blood vessels.

Neurons

Described as unipolar, bipolar and multipolar, according to how many processes project from the cell body.

Three types:-

(i)

Motor

(ii)

Sensory

(iii)

Connector (intermediate, relay or inter)

1

Motor (efferent) neurons

Transmits impulses from CNS to effectors e.g. muscles. Cell body located CNS. Axon enters a peripheral nerve and terminates in a muscle. May be over a metre long. A peripheral nerve may contain several thousand axons. Axon enclosed within a fatty myelin sheath. At about 1mm intervals are constrictions called nodes of Ranvier. Function of sheath is protection, insulates axon and speeds up transmissions of impulses. Nodes allow exchange of materials between axoplasm and surrounding tissue.

Some vertebrates have axons which are non-myelated, however majority are myelated.

Both myelated and non myelated neurons associated with Schwann cells, which produces the myelin sheath in the