What is synapse?
In a synapse one neuron ends and other neuron begins and they are held very closely to permit transmission of nerve impulse from one neuron to the next one. So the junction region between two neurons is called synapse.
Types of synapses
As per the structure synapses are mainly of two types. Those are :Axo-somatic and Axo-dendritic.
Axo-somatic: in which the axon terminal of one neuron makes synapse with the cell body of the next neuron is called axo-somatic synapse.
Axo-dendritic: where the axon of one neuron terminates on the dendrite of the next neuron. A third variety of synapse has been observed in which the axon terminals of two neurons come in close contact which are called axo-axonic synapse.
Structure of synapse: -
Structure of synapse has been exposed clearly with the help of electronic microscope. The synapse is an oval shaped structure in which the axon terminal expands to form a swollen end termed as terminal button or synaptic knob. The synaptic knob is held very close to the dendrite or soma of the next neuron, but there is no protoplasmic continuity between the two neurons. At the synaptic region, both the neurons have intact membranes. The membrane of the synaptic knob is called pre-synaptic membrane whereas that of the dendrite or soma of the next neuron is called post-synaptic membrane. These two membranes are separated by a clear gap which is termed as synaptic cleft. Te gap is measured about 200Å. The synaptic knob contains mitochondria and numerous small vesicles called synaptic vesicles that are filled with transmitter materials. Some synapses of the CNS contain a few parallel filaments, about 50Å in diameter, across the synaptic cleft. These are named inter-synaptic filaments that connect the pre- and post-synaptic membranes. moreover, there is a network of filaments in the postsynaptic membrane extending into the cytoplasm of the postsynaptic neuron. This is known as Subsynaptic web.
A single presynaptic axon may divide into a number of branches to form multiple synapses with several post-synaptic neurons ; this is called divergence. Conversely, a single postsynaptic neuron may receive a number of presynaptic terminals ; this is known as convergence. In the CNS, usually both divergence and convergence occur concurrently to form a diffuse connection between several pre- and post-synaptic neurons.
Transmission by Synapse
Transmission of nerve impulse (information) across a synapse (i.e., from presynaptic to postsynaptic membrane) is called synaptic transmission. This mediated by chemical transmitters stored within the synaptic vesicles. Transm materials are synthesized in the cell bodies of the neurons and become memb bound to form small vesicles (the synaptic vesicles) within which the mate remain stored. The vesicles are then carried by the axoplasmic flow and accumul in the synaptic knob region.
When a nerve impulse arrives at the presynaptic membrane, its permeabilitCa ++is increased leading to entry of Ca ++ within the synaptic knob. This is results in rupture of synaptic vesicles and release of transmitter material into synaptic cleft. The transmitter material then diffuses through the cleft and binds ' certain specific receptor sites present on the postsynaptic membrane.
Several transmitter substances have been identified among which acetylcho (Ach) is the most important because it is found in majority of the synapses. 0 transmitters include noradrenalin (NA), serotonin or 5-hydroxy tryptamine HT), gamma amino butyric acid (GABA), glycine etc. . A transmitter substance present in a particular synapse may be either excitatory or inhibitory in nature, example, Ach, NA, 5-IIT are excitatory whereas glycine and GABA are inhibit Binding of an excitatory transmitter with the receptor sites on the postsynaptic membrane causes activation of the postsynaptic membrane increasing its permeability to Na + and K+. As a result of this, an excitatory post synaptic potential (or EPSP) is developed. As soon as the EPSP reaches a critical (threshold) level, a nerve impulse is generated in the postsynaptic neuron conducted through its axon.
Thus, the transmitters do not carry the impulse arrr at the presynaptic side to the postsynaptic side like a ferry boat across the synacleft, rather they generate a new impulse in the second neuron. On the other hand binding of an inhibitory transmitter with the postsynaptic membrane receptor al fhp permeability of the postsynaptic membrane in such a way that it becomes iaac (hyperpolarized) and an inhibitory post synaptic potential (IPSP) is developed a result of this, a new impulse cannot be generated in the postsynaptic neu Immediately after their action, the transmitters are enzymatically hydrolyzed removed so that the postsynaptic membrane returns to original resting state, sequence of events in synaptic transmission is summarized below :
[i] Arrival of the, nerve impulse at the presynaptic terminal (synaptic knob)
[ii] Release of transmitter from the synaptic vesicles.
[iii] Diffusion of the transmitter across the synaptic cleft and its binding with receptor site on the postsynaptic membrane.
[iv] Change of permeability in the postsynaptic membrane.
[v] Excitation (depolarization and formation of EPSP) or inhibit (hyperpolarization and formation of IPSP) of the postsynaptic membrane.
[vi] Formation and propagation of impulse or blockage of impulse formatioi the postsynaptic neuron.
[viij Removal of the transmitter to restore the original resting state.