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Synapse and Neurotransmitter

بسم الله الرحمن الرحيم

Dr. Shaikh Mujeeb AhmedAssistant prof. Physiology

Al Maarefa College

Lecture slides are prepared byDr.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

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

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

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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SynapsesSignal 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

‘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

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 post-synaptic 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.

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

‘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. 25

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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