TRANSMISSION ACROSS SYNAPSE

Synapse: It is a point or site communication ( or gap) between the plasma membranes of two nerurons or between a neuron and an effector cell. If the effector cell is a skeletal muscle fibre, it forms a neirpmuscular junction.

         The mechanism of transmission of nerve impulse across a synapse occurs in the following steps.

(1) The tips of some axon terminals swell into bulf-shaped structures called synaptic end bulbs which contain many small membrane enclosed sacs called synaptic vesicles that store a chemical neurotransmitter.

(2) A nerve impulse arrives at synaptic end bulb of a pre-synaptic axon ("pre"=before; synape, i.e., axon of a neuron which is present before synapse).

(3) This causes exocytosis of some of the synaptic vesicles, because of increase in calcium iron concentration inside the pre-synaptic neuron, due to the opening of calcium channels.

(4) There is release of neurotransmitter molecules in the synaptic cleft.

(5)the neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors present in the plasma membrane of the post-synaptic neuron. ("Post" means after, therefore it refers to a neuron present after the synapse).

(6) this binding leads to the opening of sodium channels.

(7)Inflow of sodium ions, causes depolarisation.This also causes a change in membrane voltage, called as post-synaptic potential. This is excitatory in nature therefore also known as excitatory post-synaptic potential or EPSP.

(Note: If instead of sodium channels, chloride channels open, then it causes inflow of chloride ions resulting in hyper -polarisation, i.e., the inside of the membrane becomes more negative than the resting membrane potential. This is known as inhibitory post-synaptic potential or IPSP).

(8) When a depolarsing post-synaptic potential reaches sufficiently high or critical levels known as threshold, an action potential is generated and a nerve impluse passes across the synapse.

(9) After the passage of nerve impulse across s the synapse, the neurotransmitter either diffuses away from the site or is taken up again by the synaptic vesicles or undergoes enzymatic degradation (e.g., Accetylcholine is degraded by the enzyme acetycholinestrase).