Synapse

As the neuromuscular junction is a junction between a nerve and a muscle, synapse is the junction between two neurons. Here too there is no anatomical connectivity but physiological connectivity exists because of neurotransmitters.

Synapse is formed by a presynaptic neuron, synaptic cleft and postsynaptic neuron. The electrical impluse is transmitted from one neuron to another through synapse with the help of neurotransmitters.

The impulse transmission can either be exitatory or inhibitory to the post synaptic neuron, depending upon the type of neurotransmitter.

Anatomical types of synapse

Axoaxonic: 

• Axon of one neuron makes synaptic connection with the axon of another neuron.

Axodendritic: 

• Axon of one neuron makes connection with the dendrite of another neuron

Axosomatic:

• Axon of one neuron makes synaptic connection with cell body (soma) of another neuron

Axodendritic

• Dendrites of two neurons makes synaptic connection with each other

Functional types of synapse

• Electrical synapse
• Chemical synapse

Electrical synapse have open channels through which the impluses are transmitted. The open channels are low resistance bridges through which ions can pass through easily

In chemical synapse the transmission of impluse occurs through neurotransmitters.

Structure of a Synapse

Synpase has following parts:

• Presynaptic terminal with neurotransmitters in the vesicles
• Synaptic cleft or gutter is the gap between the two neurons
• Postsynpatic terminal with receptors for neurotransmitters

Transmission of impulse across a Synapse

Impulse arrives at the nerve terminal

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Opening of Voltage Gated Calcium channel

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Influx of Calcium in to the nerve terminal

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Calcium helps in docking of the vesicles containing Ach to the inner side of the presynaptic terminal

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Release of Acetylcholine into the synaptic cleft by Exocytosis

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Acetylcholine binds to the Ligand Gated Sodium channel on the motor end plate

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Opening of Ligand Gated Sodium channel

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Influx of Sodium into the muscle

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Generation of Post synaptic Potential

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Summation of Post synaptic Potential produces Action Potential

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After the impulse transmission is completed, the neurotransmitter is broken down

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The broken down products are taken up by the nerve terminal for the resynthesis of the neurotransmitter

Excitatory Postsynaptic Potentail

When the neurotransmitter is exitatory, it produces an Excitatory Postsynaptic Potentail (EPSP). As the neurotransmitter binds to its receptor on the postsynaptic membrane there is opening of sodium channels causing influx of sodium. This generates Exitatory Postsynaptic Potential (EPSP). Summation of EPSP produces Action Potnetial which is propogative.

Inhibitory Postsynaptic Potential

When the neurotransmitter is inhibitory, it produces an Inhibitory Postsynaptic Potentail (EPSP). As the neurotransmitter binds to its receptor on the postsynaptic membrane there is opening of chloride channels causing efflux of chloride. This  causes hyperpolarisation producing Inhibitory Postsynaptic Potential (IPSP).

Properties of Synapse

Synaptic delay:

• There is a delay between thea application of stimulus and onset of response. This is due to delay in transmission of impulse from presynaptic terminal to postsynaptic terminal via the synapse. 

Convergence & Divergence:

• Many number of presynaptic terials can end on a single postsynaptic terminal which is called Convergence and conversly a single presynaptic terminal can end on many number of postsynaptic terminal which is called Divergence.

Oneway Conduction

• There is a oneway conduction happening in the synapse. It is always from presynaptic terminal to postsynaptic terminal via synapse and not vice-versa.

Fatique

• A phenomenon of fatique is seen in the synapse due to depletion of nerurotransmitter, accumalation of metabolic byprocducts and lack of oxygen supply. Fatique delays the tramssion of impulse.

Summation

• When two stimuli are applied simultaneously the response is higher than the single stimulation which is due to the effect of summation. It has two types Spatial summation & Temporal summation

Occlusion

• This property is oppostite to that of summation. When many stimuli is applied the net effect could be less than the effect produced by a single stimuli.

Other properties include

• Habituation and sensitization
• Post-tetanic potentiation
• Subliminal fringe

Synaptic inhibition

Synaptic inhibition can occur in two ways

1. Presynaptic inhibition
2. Postsynaptic inhibition

Presynaptic inhibition

• This type of inhibition occurs when an inhibitory neuron ends on the presynaptic terminal. The inhibitory neuron inhibits the action of presynaptic neuron by causing hyperpolarisation in it.

Postsynaptic inhibition

• This type of inhibition is seen in the process of reciprocal inhibition. 
• When muscle contracts its antagonistic muscle relaxes, this is by reciprocal inhibition.
• A special type of postsynaptic inhibition is the Renshaw cell inhibition where the same neuron which causes excitation also cause its inhibition via an interneuron called Renshaw cell.