21. NEURAL CONTROL AND CO-ORDINATION
Impulse transmission is electrochemical. It has 3 steps:
1. Maintenance of resting membrane potential
Neural membrane contains various selectively permeable ion channels.
In a resting neuron (neuron not conducting impulse), the axonal membrane is more permeable to K+ ions and nearly impermeable to Na+ ions. Also, the membrane is impermeable to negatively charged proteins in axoplasm.
Therefore, concentration of K+ and –vely charged proteins in axoplasm is high and concentration of Na+ is low.
The fluid outside the axon contains low concentration of K+ and high concentration of Na+. This forms an ionic or concentration gradient across resting membrane.
The ionic gradients are maintained by the active transport of ions by the Na-K pump. It transports 3 Na+ outwards for 2 K+ into the cell. As a result, the outer surface becomes positively charged and inner surface becomes negatively charged (i.e, polarized).
The electrical potential difference across the resting plasma membrane is called as the resting potential.
2. Action Potential
When a stimulus is applied, the membrane at the site A becomes permeable to Na+. This causes rapid influx of Na+ and reversal of the polarity at that site (outer negative and inner positive). It is called depolarization.
The electrical potential difference during depolarization across the plasma membrane is called action potential (a nerve impulse).
3. Propagation of action potential
At sites ahead (site B), outer surface is positive and inner surface is negative. As a result, a current flows on the inner surface from site A to site B.
On the outer surface, current flows from site B to site A to complete the circuit. Hence, the polarity is reversed and action potential is generated at site B. i.e., action potential at site A arrives at site B.
The sequence is repeated along the axon and the impulse is conducted.
The rise in permeability to Na+ is extremely short lived. It is quickly followed by a rise in permeability to K+.
Immediately, K+ diffuses outside the membrane and restores the resting membrane. Thus the fibre becomes ready for further stimulation.
Synaptic transmission of impulses
Synapse is a functional junction between two neurons.
It is 2 types: Electrical & Chemical.
1. Electrical synapses
In this, the membranes of pre- and post-synaptic neurons are in close proximity. So impulse transmission is similar to the transmission along an axon.
Impulse transmission is faster than in chemical synapse.
Electrical synapses are very rare in human system.
2. Chemical synapses
In this, there is a fluid filled space (synaptic cleft) between the presynaptic neuron and postsynaptic neuron.
The presynaptic regions have swellings called Synaptic knob (buttons). They contain synaptic vesicles filled with neurotransmitters (acetylcholine or adrenaline).
Impulse transmission through chemical synapse:
Impulse reaches at axon terminal → synaptic vesicles bind on plasma membrane → release of neurotransmitter → It diffuses across synaptic cleft → combine with receptors on the post synaptic membrane → opening of ion channels allowing entry of ions → generates action potential.
This action potential may be excitatory or inhibitory.
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