Page 7 - LN-NEURAL CONTROL & COORDINATION

flow of electrical current from one neuron into the other across these

synapses.
Thus, impulse transmission across an electrical synapses is always faster than
that across a chemical synapse. In such synapses, transmission of impulse is
very similar to impulse conduction along a single axon.
(iii) Electrical synapses are rarely found in our system. It is found in cardiac
muscle fibres, smooth muscle fibres of intestine and the epithelial cells of lens.
Chemical Synapses
The membranes of pre and post-synaptic neurons are separated by a fluid-

filled space called synaptic cleft.
A brief description of the mechanism of synaptic transmission is given below
(i) When an impulse (action potential) arrives at a pre-synaptic knob, calcium
ions from the synaptic cleft enter the cytoplasm of the pre-synaptic knob.
(it) The calcium ions cause the movement of the synaptic vesicles to the
surface of the knob.
The synaptic vesicles are fused with the pre-synaptic (plasma membrane and

get ruptured (exocytosis) to discharge their contents (neurotransmitter) into
the synaptic cleft.
(iii) The neurotransmitter of the synaptic cleft binds with specific protein
receptor molecules, present on the post-synaptic membrane.
(iv) This binding action changes the membrane potential of the post-synaptic
membrane, opening channels in the membrane and sodium ions to enter the
cell. This causes the depolarisation and generation of action potential in the
post-synaptic membrane. Thus, the impulse is transferred to the next neuron.

(v) The new potential developed may be either excitatory or inhibitory.

Topic 2 Human Nervous System