Monday, 30 September 2019

SYNAPSE

SYNAPSE

SYNAPSE

Synapse is an anatomically specialized junction between two neurons, where the axon (or some other portion) of one cell (neuron) terminates on the dendrites or some other portion of another cell. The term 'synapse' of first introduced by Charles Sherrington (1924). Transmission of nerve impulse takes to place across a synapse between neurons or neurons and an effector. The neuron which sends messages is called the presynaptic cell whereas the neuron which receives messages is a postsynaptic neuron.

STRUCTURE OF SYNAPSE

Most of the synapses comprise the following structures:
(i) Synaptic knob – The Terminal bulbous ending of presynaptic axon which is devoid of neurofilaments but its cytoplasm contains:

(a) Synaptic vesicles – Small vesicles present in presynaptic cytoplasm that contain neurotransmitters (for excitation or inhibition), like acetylcholine, GABA, etc.

(b) Mitochondria, ER and microtubules.

(c) Presynaptic membrane – Nerve membrane which is in close approximation with membrane of the postsynaptic cell.

(ii) Sub-synaptic and postsynaptic membrane – The surface of the cell membrane involved in the synapse is called the sub-synaptic membrane and the remaining of the motor neuron cell membrane is called the postsynaptic membrane. Receptor sites for neurotransmitters are usually located on the sub-synaptic membrane.

TYPES OF SYNAPSE

(1) On the basis of proximity and location within the nervous system

proximity

Axo-dendritic Synapse
The synapse between fine terminal branches of the axon of one neuron and dendrites or cell body of another neuron. It is located in motor neurons in the spinal cord, excitatory synapse in the cerebral cortex, etc.

Axo-somatic Synapse
The synapse between the axon of one neuron and soma of another neuron. It is present in motor neurons in the spinal cord and autonomic ganglia.

Dendro-dendritic Synapse
The synapse between dendrites of two neurons, but is rare. It is present between mitral and
granule cell in the bulb.

Axo-axonal Synapse
The synapse between axons of two neurons. It is present in the spinal cord.

(2) On the basis of physiology

Chemical Synapse Electrical Synapse

Chemical Synapse
Signals are transmitted across a synaptic cleft in the form of the chemical messenger – a neurotransmitter, released from the presynaptic axon terminal. 

Chemical synapse operates only in one direction, as a neurotransmitter is stored on the presynaptic side of the synaptic cleft, whereas receptors for neurotransmitters are on postsynaptic side.

Electrical Synapse
Here pre-and postsynaptic membrane is joined by gap junctions, through which ions can ass easily.

Impulse transmission across the electrical synapse is faster than chemical synapse because of the direct flow of electrical current from one neuron to another through gap junction.

MECHANISM OF IMPULSE CONDUCTION

At Chemical Synapse
Mechanism of chemical transmission across a
synapse is as follows:
Action potential arrives at the axon terminal
Voltage-gated Ca²⁺ ion channels open and electrochemical
gradient favors the influx of Ca²⁺ and Ca²⁺flows into the axon terminal
Ca²⁺ ions cause synaptic vesicles to move to the
the surface of the knob and fuse with synaptic membrane terminal
Vesicles released neurotransmitters by exocytosis
Neurotransmitters diffuse across the synaptic cleft and bind to
receptors on the postsynaptic membrane
This causes depolarization and generation of
action potential in the postsynaptic
membrane.

At Electrical Synapse
1. Gap junctions the electrical synapse allow the local currents resulting from arriving action potentials to flow directly across the junction from one neuron to the other.

2. This depolarises the membrane of the second neuron to the threshold, continuing the propagation of the action potential.

PROPERTIES OF SYNAPSE

1.Convergence and Divergence: Many presynaptic neurons converge on any single postsynaptic neuron, e.g., in spinal motor neurons, some inputs come from the dorsal root, some from long descending spinal tracts and many from interconnecting neurons. The axons of most presynaptic neurons divided into many branches that diverge to end on many postsynaptic neurons.

2.Fatigue: Repeated stimulation of the presynaptic neuron leads to a gradual decrease and finally the disappearance of the postsynaptic response. This is due to the exhaustion of a chemical transmitter, as its synthesis is not as rapid as the release.

3.Synaptic Delay: When an impulse reaches the presynaptic terminal, there is a gap of about 0.5 msec., before a response is obtained in the postsynaptic neuron. This is due to the time taken by the synaptic mediator to be released and to act on the postsynaptic membrane.

4.Synaptic Plasticity: Plasticity implies the capability of being easily molded or changed. Synaptic conduction thus can be increased or decreased on the basis of past experience. These changes can be presynaptic or postsynaptic in location and play an important role in
learning and memory.

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