Nervous system transmits information as a series of nerve impulses. A nerve impulse, is the movement of an action potential as a wave through a nerve fibre. A wave of negative charge on the surface of an axon marks the position, at any moment, of the action potential. Action potentials are propagated, that is, self-generated along the axons.
Synapse is the close proximity of the axon of one neuron and the dendrite or cyton of another neuron with a gap of just about 200Ã… in between. The term synapse was first used by Sir Charles Sherrington, a British Physiologist and Nobel Laureate, 1857-1952, for neuron-neuron contact.
A synapse between an axon and a muscle fibre is called neuromuscular junction. A synapse between an axon and a glandular cell is termed neuroglandular junction. Structure of these junctions and the transmission of nerve impulse across them are similar to those of neuron-neuron synapse.
Mechanism of Transmission
Transmission of nerve impulse across a synapse by chemical means was discovered in 1936 by Sir Henry Hallet Dale, a British Pharmacologist and Nobel laureate, 1875-1968. It occurs as under-
i) When an impulse arrives at the synaptic knob of the axon, it depolarizes the presynaptic membrane and increases its permeability to calcium ions (Ca+).
ii) Entry of Ca2+ ions from the synaptic cleft into the synaptic knob causes the release of a chemical, called neurotransmitter substance, from small synaptic vesicles present there into the synaptic cleft by exocytosis through the presynaptic membrane, i.e., membrane of axon terminal. A common neurotransmitter substance is acetylcholine.
iii) This chemical diffuses across the synaptic cleft and attaches to special molecular sites, the acetylcholine receptors, on the membrane of the dendrite of the next neuron.
iv) The combination of the chemical with the chemoreceptors opens channels present in some receptors to let the ionic flow through the channels. The Na+ ions enter and K+ ions leave the dendrite down their concentration gradients. This causes depolarization of the post-synaptic membrane and initiates a new action potential.
v) The new action potential passes as a wave (nerve impulse) along the new neuron.
vi) The acetylcholine is inactivated by an enzyme, acetylcholinesterase, present in the post-synaptic (dendrite) membrane. The enzyme splits acetylcholine by hydrolysis into its components, acetic acid and choline, and this allows the membrane to repolarize. The constituents of acetylcholine are inactive. Thus, continued stimulation of the dendrite is avoided.
vii) The constituents return by diffusion to the axon where these are recombined into acetylcholine with the help of necessary synthesizing enzymes.
The synapse, thus, cannot transmit an impulse in the reverse direction as the dendrite cannot secrete acetylcholine or any other chemical transmitter. The nerve fibres which release acetylcholine are referred to as cholinergic.
Certain neurons of the sympathetic nervous system secrete a neurotransmitter called norepinephrine (noradrenaline) from their axon endings for transmitting the nerve impulse across a synaptic cleft. Such neurons are said to be adrenergic. Norepinephrine is inactivated by an enzyme monoamine oxidase.
Chemical transmission at the synapse involves two processes: neurosecretion by axon ending, and chemoreception by dendrites or muscle fibres.
Synapse, A One-way Valve
The synapse cannot transmit an impulse in the reverse direction as the dendrites cannot secrete a neurotransmitter. Thus, the synapse acts as a one-way valve, allowing the conduct of impulse from axon to dendron only.
Transmission of an impulse across a synapse is slower than its conduction along a neuron. This is because of the time needed for the release of a neurotransmitter, its diffusion through the synaptic cleft, and its action on the post-synaptic membrane. The difference in the rate is called synaptic delay. It amounts to about half a milesecond at body temperature (37ÂºC).
Repeated stimulation of the presynaptic knob may deplete the neurotransmitter, and this may fail to stimulate the post-synaptic membrane. This condition of the synapse is termed synaptic fatigue. It lasts for several seconds during which the neurotransmitter is resynthesized. Synaptic fatigue is the only fatigue that affects the nervous tissue. Conduction of the nerve impulse along the neurons is not subject to fatigue.
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