In this article we will discuss about:- 1. Definition of Synapse 2. Mechanism of Synaptic Transmission 3. Properties.
Definition of Synapse:
Synapse can be defined as functional junction between parts of two different neurons. There is no anatomical continuity between two neurons involved in the formation of synapse.
At level of synapse, impulse gets conducted from one neuron to another due to release of neurotransmitters, like ACh, noradrenaline, serotonin, etc.
The synapses, which require release of some chemical substance (neurotransmitter) during synaptic transmission, are termed as chemical synapses. In human body, almost all synapses are chemical type. Parts involved in a synapse are given in Fig. 9.5.
Presynaptic region is mostly contributed by axon and postsynaptic region may be contributed by dendrite or soma (cell body) or axon of another neuron.
Accordingly, synapses can be of following types based on different parts of neuron involved information of synapse:
Mechanism of Synaptic Transmission:
a. Arrival of impulse
b. Depolarization of pre-synaptic region
c. Influx of calcium ions from ECF into presynaptic region
d. Release of neurotransmitter
e. Passage of neurotransmitter through synaptic cleft.
f. Binding of neurotransmitter to receptors on postsynaptic region.
g. Change in electrical activity of postsynaptic region. Depending on transmitter substance released, there can be generation of EPSP or IPSP (EPSP or excitatory postsynaptic potential or IPSP or inhibitory postsynaptic potential). If EPSP is produced, postsynaptic region becomes less negative and if IPSP is produced, postsynaptic region becomes more negative.
h. When EPSP reaches firing level, there will be generation of action potential in postsynaptic region. EPSP is due to influx of sodium ion. If IPSP is produced, postsynaptic region becomes hyperpolarized and hence there will not be development of action potential in postsynaptic region. IPSP will be due to efflux of potassium ions or influx of chloride ions at postsynaptic regions.
Properties of Synapse:
1. One-way conduction (unidirectional conduction):
In chemical synapse, since neurotransmitter is present only in presynaptic region, impulse gets conducted from pre- to postsynaptic region only and not vice versa.
2. Synaptic delay is for neurotransmitter to:
a. Get released from synaptic vesicles when action potential has reached presynaptic region.
b. Pass through synaptic cleft.
c. Act on postsynaptic region to bring about production of action potential in postsynaptic region.
For all the above events to be brought about, sometime is required. This is known as synaptic delay, which is normally about 0.5 msec at every synapse.
When synapses are continuously stimulated, after some time, due to exhaustion of neurotransmitter at presynaptic terminals, impulses fail to get conducted. This results in fatigue occurring at level of synapse. Fatigue is a temporary phenomenon. If some rest is given to neurons, resting facilitates resynthesis of neurotransmitter for further conduction of impulse across synapse.
4. Convergence and divergence:
Impulses from one presynaptic nerve fiber may end on postsynaptic region of large number neurons and this is called as divergence. When nerve fibers of different presynaptic neurons end on a common postsynaptic neuron, this is known as convergence. In CNS, on an average about 10000 synapses are found on any one neuron.
When a stimulus of subthreshold strength is applied, there will not be development of action potential in postsynaptic region. But if many subthreshold stimuli are applied at presynaptic region, effects of these stimuli can get added up and lead to action potential development in postsynaptic region. This is known as summation.
There are two types of summation namely spatial and temporal. In temporal summation, presynaptic neuron stimulated will be same, but many stimuli are applied in rapid succession (timing of stimuli will be different, but place of stimulation will be same).
In spatial summation, presynaptic neurons stimulated will be different but stimuli will be applied simultaneously (time of stimulation shall be same, but places of stimulation will be different). This is possible because of the property of convergence.
6. Excitation or inhibition:
The impulse conduction across a synapse may either stimulate or inhibit activity of postsynaptic region. If there is stimulatory influence, then there will be production of action potential in postsynaptic neuron and if it has an inhibitory influence, then there is no action potential generation in postsynaptic region.
Examples of inhibition are:
i. Postsynaptic inhibition (direct)
ii. Presynaptic inhibition
iii. Renshaw cell inhibition (feedback or recurrent)
iv. Reciprocal inhibition (feed forward)
v. Lateral inhibition
Events in postsynaptic inhibition:
i. Arrival of impulse at presynaptic region
ii. Release of neurotransmitter
iii. Stimulation of internuncial neuron
iv. Action potential production in internuncial neuron
v. Release of neurotransmitter from internuncial neuron
vi. Binding of neurotransmitter to receptors on postsynaptic region
vii. Influx of chloride ions into postsynaptic region
viii. Postsynaptic membrane becoming more negative (development of inhibitory postsynaptic potential or also known as IPSP)
ix. Hyperpolarization of postsynaptic region
x. No action potential production in postsynaptic region.
Glycine substance is a classical example of inhibitory neurotransmitter at postsynaptic region. For example, when biceps muscle is contracting; there will be associated relaxation of triceps muscle because of postsynaptic inhibition.
The mechanism of inhibition in all other types namely Renshaw cell (Fig. 9.6), lateral (Fig. 9.7) and reciprocal inhibitions (Fig. 9.8), will be like what has been explained for postsynaptic inhibition but orientation of neuron involved in inhibition will be different.
Presynaptic Inhibition (Fig. 9.9):
In presynaptic inhibition, the events occurring are as follows:
i. The neuron ending on presynaptic terminal liberates neurotransmitter.
ii. Because of this, presynaptic terminal becomes less negative (because of influx of potassium ions)
iii. So the presynaptic terminal fails to remain in resting state.
Now when action potential reaches this presynaptic region, depolarization of presynaptic terminal will not be to the extent it normally occurs.
The amplitude of spike potential will be less than normal. When presynaptic neuron is kept in normal resting, during development of action potential, membrane potential will reach plus 35 mV from resting state of minus 70 mV. In which case, net change in potential will be about 105 mV.
When action potential reaches a neuron which has been kept in a partially depolarized state (when membrane potential is made to be less negative), the net change in potential will be less than usual (will be less than 105 mV).
i. This leads to release of less than normal amount of neurotransmitter from presynaptic terminals.
ii. Neurotransmitter on binding to receptors on postsynaptic region brings about development of EPSP.
iii. But the amplitude of EPSP will be less than normal and hence it will not be able to bring postsynaptic region to threshold state of stimulation.
iv. Because of this, there will not be production of action potential in postsynaptic region.
In reciprocal inhibition (Fig. 9.10), impulse from presynaptic terminal, will stimulate motor neuron supplying agonist muscle and through an intern-uncial neuron inhibits motor neuron supplying antagonist muscle.