The synapse is an area of functional contact between one neuron and another for the purpose of transferring information.
Synapses are usually found between the fine terminal branches of the axon of one neuron and the dendrites or cell body of another This type of neuron is called axo-dendrite synapse. Sir Charles Sherrington (1861-1954) was the first person who used the term ‘synapse’ to the unctional points between two neurons.
Synapses are usually found between the fine terminal branches of the axon of one neuron and the dendrites or cell body of another This type of neuron is called axo-dendrite synapse. Sir Charles Sherrington (1861-1954) was the first person who used the term ‘synapse’ to the unctional points between two neurons.
Structure of synapse:
A typical (generalized ) synapse consists of a bulbous expansion of a nerve terminal called a pre-synaptic know lying close to the membrane of dendrite;
The cytoplasm of the synaptic knob contains mitochondria, smooth endoplasmic reticulum, microfilaments and numerous synaptic vesciles.
Each vesicle contains neurotransmitter (chemical substance) responsible for the transmission of the nerve impulse across the synapse.
The membrane of the synaptic known nearest the synapse is thickened and forms the presynaptic membrane.
The membrane of the dendrite is also thickened and is called the post synaptic membrane
These membranes are separated by a gap, the synaptic cleft. It is about 200 A across. The post synaptic membrane contains large protein molecules which act as receptor sites for neurotransmitter and numerous channels and pores
The two main neurotransmitters in vertebrate nervous system are acetylcholine (Ach) and noradrenaline although other neurotransmitters also exist.
Acetylocholine (Ach) was the first neurotransmitter to be isolated and obtained by Otto Lowei I 1920 from the endings of parasympathetic neurons of the vagus nerve in frog heart.
Neurons releasing acetylcholine are described as cholinergic neurons and those releasing noradrenaline are described as adrenergic neurons.
The cytoplasm of the synaptic knob contains mitochondria, smooth endoplasmic reticulum, microfilaments and numerous synaptic vesciles.
Each vesicle contains neurotransmitter (chemical substance) responsible for the transmission of the nerve impulse across the synapse.
The membrane of the synaptic known nearest the synapse is thickened and forms the presynaptic membrane.
The membrane of the dendrite is also thickened and is called the post synaptic membrane
These membranes are separated by a gap, the synaptic cleft. It is about 200 A across. The post synaptic membrane contains large protein molecules which act as receptor sites for neurotransmitter and numerous channels and pores
The two main neurotransmitters in vertebrate nervous system are acetylcholine (Ach) and noradrenaline although other neurotransmitters also exist.
Acetylocholine (Ach) was the first neurotransmitter to be isolated and obtained by Otto Lowei I 1920 from the endings of parasympathetic neurons of the vagus nerve in frog heart.
Neurons releasing acetylcholine are described as cholinergic neurons and those releasing noradrenaline are described as adrenergic neurons.
Mechanism of transmission of nerve impulse at a synapse:
The process of chemical transmission across synapses was discovered by Henry Dale (1936). The physiological importance of synapse for the transmission of nerve impulses was established by McLennan in 1963. A brief description of the mechanism of synaptic transmission is given below.
When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob
The calcium ions cause the movement of the synaptic vesicles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured (exocytosis) to discharge their contents (neurotransmitter) into the synaptic cleft.
The synaptic vesicles then return to the cytoplasm of the synaptic know where they are refilled with neurotransmitter
The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post synaptic membrane. This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell. This causes the depolarization and generation of action potential in the post-synaptic membrane. Thus the impulse is transferred to the next neuron.
Having produced a change in the permeability of the postsynaptic membrane the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine (Ach) is hydorlysed by an enzyme acetylcholinesterase (AChE) which is present in high concentration at the synapse
The products of the hydrolysis are acetate and choline which are reabsorbed into the synaptic know were they are resynthesized into acetylcholine, using energy from ATP
The process of chemical transmission across synapses was discovered by Henry Dale (1936). The physiological importance of synapse for the transmission of nerve impulses was established by McLennan in 1963. A brief description of the mechanism of synaptic transmission is given below.
When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob
The calcium ions cause the movement of the synaptic vesicles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured (exocytosis) to discharge their contents (neurotransmitter) into the synaptic cleft.
The synaptic vesicles then return to the cytoplasm of the synaptic know where they are refilled with neurotransmitter
The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post synaptic membrane. This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell. This causes the depolarization and generation of action potential in the post-synaptic membrane. Thus the impulse is transferred to the next neuron.
Having produced a change in the permeability of the postsynaptic membrane the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine (Ach) is hydorlysed by an enzyme acetylcholinesterase (AChE) which is present in high concentration at the synapse
The products of the hydrolysis are acetate and choline which are reabsorbed into the synaptic know were they are resynthesized into acetylcholine, using energy from ATP
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