Synapse: the junction between the axon terminal of one neuron and dendrites, cell body or axon of another neuron is called synapse. The neuromuscular junction is also known as synapse.

Synaptic knob: the swelling terminal of axon or dendrites is known as synaptic knob.

Pre-synaptic neuron: the neuron carrying impulse toward synapse

Post synaptic neuron: the neuron which receive the impulse and carry the impulse away from the synapse.

Two theories have been put forward to explain the conduction of nerve impulse across the synapse. They are;

1. Electrical transmission theory
2. Chemical transmission theory

Electrical transmission theory:

Impulse transmission through synapse is accomplished by electric current. When the impulse reaches the pre synaptic knob, the impulse itself act as stimulus for the post synaptic neuron causing depolarization. Now the action potential generate in second neuron.

Chemical transmission theory:

Nerve impulse are conducted across the synapse with the help of chemical substances called neurotransmitter. The process of chemical transmission was discovered by Henry (1936).

Mechanism

Figure: Nerve impulse transmission through Synapse

• When nerve impulse reaches the pre-synaptic knob, it depolarized the presynaptic membrane and causes the opening of voltage gated calcium channel.
• Diffusion of Ca++ ion in the presynaptic knob causes movement of synaptic vesicle to the surface of the knob. Synaptic vesicle carries the neurotransmitter.
• Synaptic vesicles then fused with the presynaptic membrane and get rupture to discharge its content ie. Neurotransmitter (Acetlcholine) into synaptic cleft.
• Synaptic vesicles then return to the cytoplasm of pre-synaptic knob for refilling.
• Some of the released neurotransmitter binds with the protein receptor present on the post synaptic membrane of another neuron and change the membrane potential.
• Other unbound neurotransmitter immediately get lost from the synaptic cleft.
• The depolarization of the post synaptic membrane opens the Sodium channel causing influx of Na+ ion. Thus causing depolarization and generate action potential. In this way, the impulse get transmitted to next neuron along the synapse.

Nerve Impulse Transmission across Synapse

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• Nerve impulse: Nerve impulse is an overall physiological changes that occur in a neuron due mechanical, chemical or electrical disturbance created by a stimulus. It propagation through axon, synapse and neuromuscular junction is called Nerve Impulse conduction.

Nerve Impulse transmission along Neuron

Transmission of nerve impulse along nerve fibre can be summarized in three steps

1. Polarization (Resting Potential)
2. Depolarization (Action Potential)
3. Repolarization

                               Figure: nerve impulse transmission along neuron

Polarization (Resting potetential):

• A neuron at resting is electrically charged but not conducting.
• The Axoplasm or plasma membrane of a resting neuron is negatively charged as compared to the interstitial fluid.
• The potential difference measured at this stage is called resting potential which is about -70mV. The interstitial fluid has high concentration of Na+ ion which is about 16 times higher outside the neuron than inside neuron. Similarly, the axoplasm has high concentration of K+ ion which is about 25 times higher inside than in outer interstitial fluids.
• Due to difference in concentration of ions, Na+ ion tends to diffuse into the axoplasm and K+ ion tends to diffuse outside the axoplasm.
• The membrane of neuron at resting is more permeable to K+ ion than Na+ ion. So, K+ leaves the neuron faster than Na+ enter the neuron.
• The difference in permeability results in accumulation of high concentration of cation (+ve charged ion) outside the neuron compared to the concentration of cation inside.
• This state of resting neuron is called Polarized state and it is electro-negatively charged.

Depolarization (Action Potential):

• Any stimulus beyond the threshold can initiate an impulse.
• When such stimulus is applied in the resting neuron, it opens the sodium channel. Now the permeability of Na+ ion suddenly increases at the point of stimulus causing depolarization.
• The diffusion of Na+ ion increases by 10 times from outside to inside. As a result the axoplasm become positively charges, which is exact opposite to polarized state, so called as depolarized state or reverse polarized state.
• The depolarization of the membrane stimulates the adjacent voltage channel, so the action potential passes as a wave along the length of neuron.

Repolarization:

• When the concentration of Na+ ion inside axoplasm increases, the permeability to Na+ decreases and the sodium channel starts to close.
• The Na-K pump activates, so that Na+ are pumped out and K+ inside until the original resting potential is restored. The process is known as repolarization and it starts from the same point from where depolarization starts.
• The entire process of polarization, depolarization and repolarization occur within fraction of seconds. Now, again the neuron is read for another impulse.

Saltatory conduction:

• Transmission of nerve impulses is very rapid. However, nerve impulse conduction along unmyelinated neuron is slow than that of myelinated neuron. It is because, the myelin sheath act as insulator, so that the impulse have to jump from one node of Raniver to another.
• This speed up the conduction process, and this type of conduction is known as Saltatory conduction. 

Nerve Impulse Conduction

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