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Human Body
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are not directly connected, The terminal branches of an axon are separated from the deadrites or cell body of the next neuron in the sequence by a small gap (200 to 300 X ) called the synapse. Nerve impulses are transmitted across the synapse by the diffusion of chemical transmitters. The most common neurotransmitter is acetylcholine.
The Nerve : Like a muscle, a peripheral nerve is a bundle of bundles. Each individual nerve fibre is enclosed in a connective tissue sheath called the endoneurium. A bundle of nerve fibres is wrapped in a sheath called perineurium, These bundles, in turn, are grouped into a larger bundle ensheathed in the epineurium. Each nerve fibre runs the full length of the nerve and retains its own distinct identity. In the CNS, there is also some grouping of nerve fibres, fibres transmitting impulses associated with a specific type of information (e.g. pain-sensation) lie together in a common pathway called a tract. But the fibres of a tract are not ensheathed. If the cell body of a neuron is damaged, the entire cell dies and cannot be regenerated. But if the axon is severed, leaving the cell-body intact, a new axon may be regenerated. Voluntary Movement and Reflex Action
Willed or voluntary movement begins when the neurons of the cerebral cortex send nerve impulses along nerve fibres to the motor neurons of the spinal cord. From here the impulses are conveyed to neuro-muscular junctions. When the impulse arrives at such a special junction, it induces contraction of the muscle fibre. Reflexes, on the other hand, are involuntary actions, designed to obtain the quickest possible motor response. The withdrawal of a hand from something very hot that has been touched inadvertently is an example of reflex action. It is a protective mechanism of the central nervous system having an incoming sensory and an outgoing motor pathway in common i.e the incoming message is carried directly to the motor-neuron in the spinal cord. The spinal cord and the brain stem are responsible for most of the reflex movements and the use of cerebral cortex is not involved. They are inborn and involuntary actions and they may still be present even after a great deal of brain damage has occurred.
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