SYNAPSE
A synapse is defined as the functional connection between two neurons. It permits a neuron to pass an electrical or chemical signal to another neuron the term “synapse” was given by Charles Sherrington in 1887.The word synapse is derived from the Greek word synapsis meaning conjugation.
Structure of Synapse
At a synapse the plasma membrane of the signal passing neuron (presynaptic neuron) comes into close contact with the plasma membrane of the target (postsynaptic neuron) cell but does not fuse with it. Inside the pre-synaptic membrane there are several vesicles filled with neurotransmitters and numerous mitochondria necessary for active synthetic processes occurring in the terminals and at the post-synaptic membrane there are receptor proteins which respond to chemical stimulation and inhibition. The gap between pre-synaptic and post- synaptic membranes is known as synaptic cleft and it is about 20-50nm.
Types of Synapses
Synapses are of two types: electrical synapses and chemical synapses. Depending upon mode of transmission across the synapse.✅Electrical synapse: In electrical synapse, synaptic cleft is only 0.2 mm, so action potential can directly be transmitted to the next neuron. Electric synapse is quite rare and occurs in neural system that requires fastest possible response
✅Chemical synapse: in chemical synapses, neurotransmitters are present. Each synaptic vesicle is of about 50 mm. diameters and stores about 10,000 molecules of a neurotransmitter. It is the most common type of synapse and allows the nervous system to connect and control other systems of the body. Acetylcholine, adrenaline and noradrenaline are chemical transmitters, released at synapses. Dopamine, serotonin and sympathy are some other excitatory neuron transmitters. Most synapses are chemical synapse.
✅Conjoint synapse: the synapses where transmission of nerve impulse is both chemical and electrical are, called conjoint synapse. According to the nature of connections the synapse is of the following types: -
1. Axo-axonic synapse: A synapse between axon of one neuron and axon of another neuron is Axo-axonic synapse.
2. Axo-dendritic synapse: A synapse between axon of one neuron and dendrite of another neuron is known as Axo-dendritic Synapse. It is found in the cerebellum where the climbing fibers form connections with dendrite of Purkinje cells.
3. Axo-dendrosomatic synapse: A synapse between axon of one neuron and dendrites and cell body of other neuron is known as axo-dendrosomatic synapse.
4. Axo somatic synapse: A synapse between axon of one neuron and cell body of another neuron is axosomatic synapse.
5. Dendro-dendritic synapse: A synapse between dendrites of two different neurons is known as dendro-dendritic synapse.
Properties of a Synapse: -
✔An impulse can be transmitted only in one direction across the synapse i.e. from presynaptic neuron to post synaptic neuron. This is known as the law of forward conduction.
✔The minimum time required for the transmission of impulse from one neuron to next is known as synaptic delay and it is about 0.5miilisecond.
✔Synapse is a site where impulses are received and discharged, it is therefore, regarded as relay station.
✔ Summation is an important characteristic of synapse. It means adding up of the effects of multiple impulses at the synapses. It is of two types: Spatial and temporal summation.
✔ Synapses bring about convergence and divergence of nerve impulses. Suppose many neurons synapse with a common post synaptic membrane, then impulses coming from various directions get converged at the synapse. All the impulses are further transmitted in a single uniform direction. This is known as convergence.
Suppose a neuron makes synaptic contact with many postsynaptic neurons, the impulse coming through the first neuron in a single direction get diverted at the level of synapse and transmitted further in different directions. This is known as divergence.
✔Sometimes when impulses are transmitted repeatedly across a synapse it stops transmitting impulses after sometimes. This is fatigue and it is due to neurotransmitter. However, it is a temporary phenomenon.
✔The phenomenon of passage of impulses from presynaptic to post synaptic neuron and back to presynaptic neuron is known as reverberation .As a result of continous transmission of impulses, a circuit is maintained.
Mechanism of synaptic transmission: -
When a nerve impulse travelling along an efferent peripheral nerve reaches the synaptic terminals it produces a characteristic response in the effector tissue (increased or decreased activity of smooth muscle or cardiac muscle, secretion of glands, contraction of skeletal muscles). The current in the presynaptic membrane is too weak to excite the post synaptic membrane directly and there is evidence that the activity is now conveyed to the effector organs by the release of chemical transmitters into the synaptic cleft.
The process of chemical transmission across the synapse was first revealed by Loewi in 1921.Later Henry Dale (1936) worked out the chemical nature of these neurotransmitters and their mode of action is mentioned below:
✔ When a nerve impulse reaches the interneural or the neuromuscular junction, the weak action potential that it has carried to the synapse causes the calcium ions to move from the extracellular fluid into the membranes of the axon terminals.
✔The calcium ions in turn, cause the synaptic vesicle to rupture through the membrane and release the neurotransmitter.
✔Within approx.2-3 milliseconds after the chemical transmitter (which is either excitatory or inhibitory in nature) is released by the axon terminal, it traverses the synaptic gap, combines with a specific receptor on the post synaptic membrane and causes a local depolarization.
✔The local depolarization creates a synaptic potential across the synapse and when this reaches a certain magnitude it fires off an action potential in the next neuron or in the effecter cell.
✔The weak presynaptic potential is thus sufficient to release the transmitter which then greatly lowers the resistance of the post synaptic membrane by increasing the membrane permeability to Na+ and K+.The post-synaptic membrane gets depolarized. A fresh action potential is generated in the post synaptic neuron and is propagated further. Transmission of impulses across the synapse is chemical in nature.
Factors affecting synaptic transmission: -
✔Hypoxia: As synaptic transmission requires energy expenditure, so oxygen deficiency stops it.
✔ Acidosis: It depresses neuronal activity. A fall in pH from 7.4-7 may cause comma stage.
✔ Alkalosis: It increases neuronal activity
✔ Drugs: Like caffeine, theophylline and theobromine found in coffee, tea and cocoa increases neuronal activity
✔Hypocalcemia: or lack of calcium increase synaptic transmission while hypercalcemia retards synaptic transmission.
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