In this presentation I have described about motor synapse and the mechanism of excitation.

1. DONE BY
KRISHNAMOORTHI BHARANI
TOPIC : MOTOR SYNAPSE AND
MECHANISM OF EXCITATION

2. CONTENTS
MOTOR SYNAPSE
STRUCTURE OF SYNAPSE
PROPERTIES OF SYNAPSE
MECHANISM OF EXCITATION CONDUCTION

3. MOTOR SYNAPES
• Motor neuron is a neuron whose cell body is
located in the motor cell cortex or brain stem or
spinal cord whose axon fiber projects to the
spinal cord or outside the spinal cord to directly
or indirectly control effector organ mainly
muscles and glands.
• Synapse can be defined as functional junction
between parts of two different neurons. There is
no anatomical continuity formation between two
neurons involved in the formation of synapse.

4. STRUCTURE OF SYNAPSE
• Synapse consists of three elements , they are :
• presynaptic membrane
• Postsynaptic membrane
• Synaptic cleft
• Presynaptic membrane – formed by the
terminal button of axon .
• Postsynaptic membrane – is composed of a
segment of dendrite or cell body.
• Synaptic cleft – the space between
presynaptic and postsynaptic membrane .

6. PROPERTIES OF SYNAPSE
• ONE WAY CONDUCTION (UNIDIRECTIONAL
CONDUCTION)
In chemical synapse , since neurotransmitter
is present only in presynaptic region ,
impulse gets conducted from pre-synaptic
region to post-synaptic region only and not
vice versa.

7. PROPERTIES OF SYNAPSE
• Synaptic delay is for neurotransmitter to:
»Get released from synaptic vesicles when
action potential has reached presynaptic
region.
»Pass through synaptic cleft
»Act on postsynaptic region to bring about
production of action potential in
postsynaptic region.

8. PROPERTIES OF SYNAPSE
• FATIGABILITY
»When synapses are continuously
stimulated, after some time due to
exhaustion of neurotransmitter at
presynaptic terminal , impulses fail to get
conducted . This results in fatigue
occurring at level of synapse. Fatigue is a
temporary phenomenon. If some rest is
given to neurons, resting facilitates
resynthesis of neurotransmitter for further
conduction of impulse across synapse.

9. PROPERTIES OF SYNAPSE
• CONVERGENCE AND DIVERGENCY
»Impulses from one presynaptic nerve fiber
may end on postsynaptic region of large
number neurons and this is called as
divergence .
»When nerve fibers of different presynaptic
neurons ends on a common postsynaptic
neuron, this is known as convergence .
»In CNS , on an average about 10000
synapses are found on any one neuron.

10. PROPERTIES OF SYNAPSE
• Summation
»When a stimulus of sub threshold strength
is applied , there will not be development
of action potential in postsynaptic region .
But if many sub threshold stimuli are
applied at presynaptic region , effects of
these stimuli can get added up and lead to
action potential development in
postsynaptic region . This is known as
summation .

11. MECHANISM OF EXCITATION
CONDUCTION
• When an impulse arrives at a presynaptic knob,
calcium ions from the synaptic cleft enter the
cytoplasm of the presynaptic cleft.
• 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 .

12. MECHANISM OF EXCITATION
CONDUCTION
• The synaptic vesicles then return to the cytoplasm of
the synaptic knob 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 post synaptic membrane ,
opening the channels in the membrane and allowing
sodium ions to enter the cell. This causes the
depolarisation and generation of action potential in the
post synaptic membrane . Thus the impulse is
transferred to the next neuron.

13. MECHANISM OF EXCITATION
CONDUCTION
• Having produced a change in permeability in the
post synaptic membrane the neurotransmitter is
immediately lost from the synaptic cleft . In case
of cholinergic synapses, acetylcholine (ACh) s
hydrolysed by an enzyme acetylcholinestrase
(AChE) which is present in high concentration at
the synapse.
• The products of hydrolysis are acetate and
choline which are reabsorbed into the synaptic
knob where they are resynthesized into
acetylcholine , using energy from ATP.