Synapse and Neurotransmitter
Dr. Shaikh Mujeeb Ahmed
Assistant prof. Physiology
Al Maarefa College
Lecture slides are prepared by
Dr.Mohammed Sharique Ahmed Quadri
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Objectives
• Define synaptic transmission
• List the types of synapses
• Describe the mechanism of action of chemical
and electrical synapse.
• Explain the mechanisms of excitatory and
inhibitory post-synaptic potentials.
• Compare EPSP &IPSP
• Describe the properties of synaptic
transmission
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SNAPSES AND NEURONAL
INTEGRATION
• A Neuron may terminate on one of THREE
structures:
1) MUSCLE
2) GLAND
3) ON ANOTHER NEURON –
JUNCTION BETWEEN TWO NEURON IS CALLED
SYNAPSE
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Synapses
• Junction between two presynaptic and
postsynaptic neurons
• There are two types of Synapse:
• 1. Electrical Synapses: Two neurons connected by
gap junctions
• 2. Chemical Synapses: Chemical messenger is
transmitted across the junction separating the
two neurons
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ELECTRICAL SYNAPSES
• In electrical synapses, two
neuron are connected by
Gap Junction which allow
ions (charged particles)
to flow between the two
cells . Therefore Action
Potential travels from one
cell to another.
• Electrical signals are rare
in Human Nervous
System.
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ELECTRICAL SYNAPSES [cont]
• Example of Electrical Synapses
--- Pulp of tooth
--- Retina of the Eye
--- Cardiac Muscle
--- Smooth Muscle
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CHEMICAL SYNAPSES
• In human CNS , most of the synapses are
Chemical, where chemical messenger
transmits information from one neuron to
another .
• WE WILL DISCUSS CHEMICAL SYNAPSE
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synapse
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Synapse
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Synaptic Delay
• Transmission of electrical impulse [AP] by
chemical means from presynaptic neuron to
post-synaptic neuron takes time. It is called
‘Synaptic Delay’. It is 0.5 to 1 millisecond.
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Synapses
Signal at synapse either
excites or inhibits the
postsynaptic neuron
• Two types of
synapses
– Excitatory synapses
– Inhibitory synapses
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Neurotransmitters
• Vary from synapse to
synapse
• Same neurotransmitter
is always released at a
particular synapse
• Quickly removed from
the synaptic cleft
• Some common
neurotransmitters
– Acetylcholine
– Dopamine
– Norepinephrine
– Epinephrine
– Serotonin
– Histamine
– Glycine
– Glutamate
– Aspartate
– Gamma-aminobutyric acid
(GABA
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‘Important’
• Glutamate – Most common excitatory
neurotransmitter in the brain. It causes EPSP.
• GABA [Gamma amino butyric acid] – Most
common inhibitory neurotransmitter in brain. It
causes IPSP.
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Neurotransmitter Removal From The
Synaptic Cleft
• Neurotransmitters are quickly removed from
the synaptic cleft by various ways:
1. Inactivated by enzymes present on post
synaptic membrane.
2. Diffuse away from synaptic cleft.
3. Actively taken back into axon terminal.
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Neuropeptides
• Large molecules consisting of from 2 to 40 amino
acids
• Synthesized in neuronal cell body in the
endoplasmic reticulum and Golgi complex
• Packaged in large, dense-core vesicles present in
axon terminal
• Neuropeptides are considered neuromodulators
don’t cause the formation of EPSP or IPSP, but
bring about long term changes that subtly
modulate, depress or enhance the action of the
synapse
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Comparison of Classical Neurotransmitters and Neuropeptides
Post-Synaptic Potential
• EPSPs and IPSPs are graded potential [local].
They can be summated [added].
• Types of Summation
1. Temporal Summation
2. Spatial Summation
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Temporal Summation
• When single presynaptic neuron is stimulated
many times after short intervals, to summate
several EPSP, it is called ‘Temporal
Summation’.
• Up to 50 EPSPs might be needed to bring postsynaptic membrane to threshold level.
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Spatial Summation
• When two or more excitatory neurons are
stimulated together at the same time, to get
the action potential.
It is called Spatial Summation.
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Presynaptic inhibition or facilitation can
selectively alter the effectiveness of a
presynaptic input.
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Convergence And Divergence
 Convergence:
On a given neuron, many other
neurons come and synapse on it. It is called
‘Convergence’.
• Due to convergence input, a single neuron is
influenced by thousands of other cells.
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Convergence And Divergence [cont]
 Divergence
It refers to the branching of axon
terminals so that single cell synapses with
many other cells.
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Convergence And Divergence
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‘Important Information’
• There are about 100 billion neurons in the
brain.
• A single neuron maybe connected to 5000 to
10,000 other neurons.
• Brain is responsible for different activities like
sensations, movements of muscle, thought,
emotion, memory – all these depend on
electrical and chemical signaling between
neurons along wired neural pathways.
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Synaptic Drug Interactions
• Possible drug actions
1. Altering the synthesis, axonal transport, storage,
or release of a neurotransmitter
2. Modifying neurotransmitter interaction with the
postsynaptic receptor
3. Influencing neurotransmitter reuptake or
destruction
4. Replacing a deficient neurotransmitter with a
substitute transmitter
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References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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