Synapse is the junction between two neurons. It is not an anatomical continuation. But, it is only a physiological continuity between two nerve cells.
CLASSIFICATION OF SYNAPSE
Synapse is
classified by two methods:
A.
Anatomical classification
B.
Functional classification.
ANATOMICAL
CLASSIFICATION
Usually
synapse is formed by axon of one neuron ending on the cell body, dendrite
or axon of the next
neuron.
Depending upon ending of axon, synapse is classified
into three types:
1. Axoaxonic
synapse in which axon of one neuron terminates on axon of another neuron 2.
Axodendritic synapse in which the axon of one neuron
terminates on dendrite of another neuron
3. Axosomatic
synapse in which axon of one neuron ends on soma (cell body) of another
neuron.
FUNCTIONAL
CLASSIFICATION
Functional
classification of synapse is on the basis of mode of
impulse transmission. According to this,
synapse is
classified into two categories:
1.
Electrical synapse
2.
Chemical synapse.
However,
generally the word synapse refers to a chemical synapse.
1. Electrical
Synapse
Electrical
synapse is the synapse in which the physiological continuity between the
presynaptic and the post synaptic neurons is provided by gap junction between the two neurons. There is direct exchange
of ions between the
two neurons through the gap junction. Because of this reason, the action
potential
reaching the terminal
portion of presynaptic neuron directly enters the postsynaptic neuron.
Important feature of
electrical synapse is that the synaptic delay is very less because of the
direct flow of
current. Moreover, the
impulse is transmitted in either direction through the electrical synapse.
This type of impulse
transmission occurs in some tissues like the cardiac muscle fibers, smooth
muscle
fibers of intestine
and the epithelial cells of lens in the eye.
2. Chemical Synapse
Chemical
synapse is the junction between a nerve fiber and a muscle fiber or between two
nerve fibers, through which the signals are transmitted by the release of chemical transmitter. In the chemical synapse, there is no
continuity between the two neurons because of the presence
of a space called synaptic cleft between the two neurons. Action potential reaching the presynaptic terminal
causes release of neurotransmitter substance from the
vesicles of this terminal. Neurotransmitter reaches the
postsynaptic neuron through synaptic cleft and causes the production of
potential change. Structure and functions of the chemical synapse are given
here.
FUNCTIONAL ANATOMY OF CHEMICAL SYNAPSE
Neuron from which the axon arises is called the presynaptic neuron and the
neuron on
which the axon ends is
called postsynaptic neuron. Axon of the
presynaptic neuron divides into many small branches before forming the synapse. These branches are known as presynaptic axon terminals.
Types of Axon
Terminals
1. Terminal knobs
Some of the terminals are
enlarged slightly like knobs called terminal knobs. Terminal knobs are concerned with
excitatory function of the synapse.
2. Terminal coils or
free endings
Other terminals are wavy
or coiled with free ending with out the knob. These
terminals are concerned with
inhibitory function.
Structures of Axon
Terminals and Presynaptic Membrane
Presynaptic
axon terminal has a definite intact membrane known as presynaptic
membrane.
Axon
terminal has two important structures:
i. Mitochondria,
which help in the synthesis of neurotransmitter
substance
ii. Synaptic
vesicles, which store neurotransmitter substance.
Synaptic Cleft and
Postsynaptic Membrane
Membrane
of the postsynaptic neuron is called postsynaptic membrane. It contains some receptor
proteins. Small space in between the presynaptic membrane and the postsynaptic
membrane is called
synaptic
cleft. The basal lamina of this
cleft contains cholinesterase, which destroys acetylcholine.
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