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law of convergence and divergence
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Properties of the Synapse

The properties of the synapse or synaptic transmissions are the following:

  • Law of convergence and divergence
  • Law of forward conduction
  • Synaptic delay
  • Summation
  • Occlusion
  • Subliminal Fringe
  • Synaptic inhibition
  • Feedback inhibition/ Renshaw cell inhibition
  • Synaptic facilitation
  • Synaptic plasticity

 

Law of convergence and divergence

Convergence: input signals from multiple sources (presynaptic membrane) terminate on a single neuron(postsynaptic membrane) called the law of convergence.

law of convergence and divergence

Divergence: an axon of presynaptic neuron divides into many branches that are diverse to end on many post-synaptic neurons in the body.

Law of forward (one way) conduction

Through a synapse, impulse can travel in one direction only i.e from the presynaptic neuron to the postsynaptic neuron (Except electrical synapse).

Chemical synapses always transmit the signals in one direction i.e. from the neuron that secretes the neurotransmitter (transmitter) substance, called the presynaptic neuron, to the neuron on which the transmitter acts, called the postsynaptic neuron.

Synaptic delay

It is another property of the synapse. The minimal time required for the impulse to be transmitted through the synapse (junction between the two neurons) is synaptic delay ( 0.5 milliseconds). Time is consumed in the process of :
Discharge of the neurotransmitter substance by the presynaptic terminal
Diffusion of the transmitter (neurotransmitter) to the postsynaptic neuronal membrane
The action of the neurotransmitter (transmitter) on the membrane receptor
The action of the receptor to increase the permeability of the membrane
Inward diffusion of sodium ions to raise the excitatory postsynaptic potential to a high enough level to elicit an action potential.

Summation

The postsynaptic neuron can be brought to a threshold in two ways: temporal Summation and spatial Summation.

Temporal Summation

This is the summing of several EPSPs occurring very close together in time because of the successful firing of a single presynaptic neuron.

If excitatory presynaptic input is stimulated a second time before the first excitatory postsynaptic potential (or EPSP) has died off, the second excitatory postsynaptic potential (or EPSP) will add onto, or sum with the first excitatory postsynaptic potential (or EPSP) resulting in temporal summation.

When a presynaptic terminal fires, it releases a neurotransmitter (transmitter) substance that opens the membrane channels for a millisecond.

But the changed postsynaptic potential lasts up to 15 milliseconds after the channels of the synaptic membrane have already closed.

So, a second opening of the same channels can increase their postsynaptic potential to a still greater level, and the more rapid the rate of stimulation occurs, the greater the postsynaptic potential.

so, if successive discharges from a single presynaptic terminal occur rapidly then can add to one another i.e. “summate.” This type of summation is known as temporal summation.

Spatial Summation

This summation of EPSPs originating simultaneously from several different presynaptic inputs or, summing simultaneous postsynaptic potentials by activating multiple terminals on widely spaced areas of the neuronal membrane is called spatial summation.

Spatial summation of EPSPs is initiated by simultaneous activation of two or more excitatory presynaptic inputs.

Subliminal Fringe

properties of synapse
Subliminal Fringe

The effect of stimulating two nerve fibers simultaneously may turn out to be greater than the sum of stimulating either of them separately.

This is called subliminal fringe/facilitation. It is seen because combined stimulation leads to the excitation of synapses in the body.

Occlusion

Properties of the Synapse
occlusion

When stronger stimuli are applied, the effect of stimulating two nerve fibers simultaneously may turn out to be less than the sum of stimulating either of them separately, this is called occlusion.

It occurs due to the overlapping nerve fiber during distribution in the body.

Synaptic inhibition (properties of the synapse)

Synaptic inhibition is of two types such as presynaptic inhibition and postsynaptic inhibition.

Presynaptic inhibition

It mainly occurs in axo-axonal synapses. The activity at synapses reduces the magnitude of the action potential at the presynaptic membrane.

presynaptic and postsynaptic inhibition

And it is caused by a release of inhibitory substances onto the presynaptic nerve fibrils before its own ending terminates on the postsynaptic neuron.

Postsynaptic Inhibition

    • Direct inhibition: postsynaptic inhibition during the course of IPSP
    • Indirect inhibition: inhibition is due to the effects of previous postsynaptic discharge

Feedback inhibition/ Renshaw cell inhibition

Neurons may also inhibit themselves in a negative feedback fashion (a mechanism) called Renshaw cell inhibition.

For example, impulses generated in motor neurons activate inhibitory interneurons to secrete inhibitory neurotransmitter glycine, this reduces or stops the discharge of motor neurons.

 

Presynaptic Facilitation (properties of the synapse)

It is the process by which transmission through a synapse increases and increases in the release of neurotransmitter (transmitter) from the presynaptic terminal of the neuron.

It is produced when an action potential is prolonged and the Ca2+ channel is open for a longer period of time and it is mediated by serotonin.

Synaptic plasticity (properties of the synapse)

Plasticity refers to the capability of being easily molded or changed in shape. Synaptic conduction thus can be decreased or increased on the basis of their past experience.

There are different forms of synaptic plasticity including;

    • Post-tetanic potentiation
    • Long term potentiation
    • Habituation
    • Sensitization

Post-tetanic Potentiation

If tetanic stimulation is followed by a brief pause, the response to subsequent stimulation is frequently enhanced, this is called post-tetanic potentiation or post-tetanic facilitation.

It may be due to the accumulation of calcium ions in the presynaptic terminal during tetanic stimulation. The brief pause gives time for the neurotransmitter to be replenished.

Long term potentiation

If post-tetanic potentiation gets much more prolonged and lasts for days then it is known as long-term potentiation.

It increases in intracellular ca2+ in the postsynaptic neuron in the body. It is most commonly seen in the hippocampus.

Habituation

The repeated stimulation of the presynaptic neuron leads to a gradual decrease and finally the disappearance of the postsynaptic response that is called habituation.

It occurs due to the gradual inactivation of calcium channels in the body.

Sensitization

It is the prolonged occurrence of augmented postsynaptic responses after a stimulus (that has become habituated) is paired once or several times with a noxious stimulus.

In Aplysia, sensitization has been shown to be due to the activity of serotonergic neurons acting presynaptically on the sensory neurons.

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