Neural Control & Coordination


Mechanism of Impulse Transmission Across a Chemical Synapse
The cytoplasm of the synaptic knob has round, a tiny sac called synaptic vesicles which contain neurotransmitter.
Neurotransmitters are the chemical which is involved in the transmission of impulses across chemical synapses but not in electrical synapses. 
                 Image result for image of impulse transmission across chemical synapses

When a wave of depolarization reaches the pre-synaptic membrane, the calcium channel open. Due to the opening of calcium channel.  Ca²+ diffuse inside the cytoplasm of the synaptic knob. As a result, the synaptic vesicles move towards the pre-synaptic membrane and fuses with the plasma membrane of the synaptic knob. It leads to the rupture of synaptic vesicle and releases neurotransmitter in the synaptic cleft. This releases neurotransmitter binds to the specific receptors present on the post-synaptic membrane.
This binding opens the ion channel in the post-synaptic neuron,     allowing the entry of the ion. The entry of the ion generates a new potential in the post-synaptic neuron. The newly generated potential could be excitatory or inhibitory.
Excitatory neurotransmitter: Acetylcholine, Epinephrine, Norepinephrine, Glutamate
Inhibitory neurotransmitter: GABA, Serotonin, Dopamine, Glycine.
Neuromuscular junction: The junction b/w the neuron and the sarcolemma. The impulse reaches the junction creating an action potential in the sarcolemma.

One Way Conduction of Impulse
The transmission of nerve impulse takes place b/w axon of one neuron to the dendrites/cell body of another neuron but never in reverse direction. The neurotransmitter is released by the axon terminal and not from the dendrites/cell body. This explains the one-way conduction of impulse.

Synaptic delay:
There is a delay in nerve impulse transmission at each synapse.

Synaptic fatigue:
Due to the repeated transmission of a nerve impulse across the synapse, there occurs a temporary suspension of transmission of impulse at the synapse. This occurs due to the exhaustion of the neurotransmitter in the synaptic vesicles. When the neurotransmitter accumulates, it gains its ability to transmit the impulse.

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