Mechanism of Development of EPSP
The neurotransmitter receptor complex causes opening of ligand gated sodium channels. Now, sodium ions from extracellular fluid enter the synapse, i.e. soma. As sodium ions are positively charged, resting membrane potential inside soma is altered and mild depolarization develops. This mild depolarization is called EPSP. It is a local response in the synapse.
Properties of EPSP
EPSP is confined only to synapse. It differs from action potential and is similar to receptor potential and endplate potential. EPSP has such properties:
1. it is non-propagated (local);
2. it does not work according to law “everything or nothing”.
Significance of EPSP
EPSP is not transmitted into axon of postsynaptic neuron. It causes development of action potential in axon because of opening of voltage gated sodium channels in initial segment of axon. Now due to entrance of sodium ions, depolarization occurs in initial segment of axon and thus, action potential develops. From here action potential spreads to other segment of axon.
INHIBITORY FUNCTION
Inhibition of synaptic transmission is classified into three types.
1. Postsynaptic inhibition
2. Presynaptic inhibition and
3. Renshaw cell inhibition.
Postsynaptic Inhibition
This is also called direct inhibition. This occurs due to release of an inhibitory neurotransmitter from presynaptic terminal instead of excitatory neurotransmitter substance. The most important inhibitory neurotransmitter is gamma-amino butyric acid (GABA).
Action of GABA—IPSP
Inhibitory neurotransmitter substance acts on postsynaptic membrane by binding with receptor. Transmitter receptor complex opens ligand gated potassium channels instead of opening sodium channels. Now, potassium ions pass out of synapse into extracellular fluid. Chloride channels also open followed by influx of chloride ions inside. Exit of potassium ions and influx of chloride ions cause more negativity inside, leading to hyperpolarization. This is called inhibitory postsynaptic potential (IPSP). With other words, IPSP is numeral to which postsynaptic membrane membrane potential is increased (hyperpolarization) at mediator action to it.
Presynaptic Inhibition
This is also known as indirect inhibition and it occurs because of failure of presynaptic axon terminal to release excitatory neurotransmitter substance. Inhibitory neurotransmitter substance is glycine.
Renshaw Cell Inhibition
This occurs in spinal cord. Anterior nerve root consists of nerve fibers leaving spinal cord. These nerve fibers arise from alpha motor neurons in anterior grey horn of spinal cord and reach effector organ, muscles. Some of fibers called collaterals end in Renshaw cells instead of leaving spinal cord. Renshaw cells are situated in between motor neurons.
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When motor neurons send motor impulses, some of impulses reach the Renshaw cell by passing via collaterals. Now the Renshaw cell is stimulated. In turn, it sends inhibitory impulses to alpha motor neurons so that the discharge from motor neurons is reduced.
Significance of Synaptic Inhibition
It helps to select exact number of impulses and to block the excess ones.
PROPERTIES OF SYNAPSE
1. ONE WAY CONDUCTION (BELL-MAGENDIE LAW)
According to Bell-Magendie law, impulses are transmitted only in one direction in synapse, i.e. from presynaptic neuron to postsynaptic neuron.
2. SYNAPTIC DELAY
During transmission of impulses via the synapse, there is a little delay in the transmission. This is called synaptic delay. This is due to the time taken for:
a) Release of neurotransmitter
b) Movement of neurotransmitter from axon terminal to postsynaptic membrane for mediator interaction with receptor
c) Action of the neurotransmitter to open the ionic channels in postsynaptic membrane
d) Mediator restoration.
The synaptic delay is one of the causes for the latent period of the reflex activity.
3. FATIGUE
The fatigue at the synapse is due to neuro-tansmitter substance, acetylcholine, level decreasing and mediator exhausting. After producing the action, this neurotransmitter is destroyed by acetylcholinesterase. Synapse is the mostly-fatigable structure in nervous system (compare: nervous fiber is practically non-fatigable).
4. SUMMATION
When many presynaptic excitatory terminals are stimulated simultaneously or when single presynaptic terminal is stimulated repeatedly, there is summation in postsynaptic neuron. This is called summation. Summation is of two types.
1) Spatial Summation
This occurs when many presynaptic terminals are stimulated simultaneously (for example, in central synapses).
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2) Temporal Summation
It occurs when one presynaptic terminal is stimulated repeatedly. Also subliminal stimuli summation (3-5) to form EPSP can be proper example.
Thus, both spatial summation and temporal summation play an important role in the facilitation of response.
5. ELECTRICAL PROPERTIES
The electrical properties of the synapse are the EPSP and IPSP.
6. DECREASING TRANSFORMATION OF RHYTHM INTO FREQUENCY. 3-5 subliminal stimuli on entrance will give 1 action potential on exit.
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