The synapse

CENTRAL NERVOUS

SYSTEM SYNAPSES

DR.AIESHA DURREBAR

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Physiological anatomy

1. Structure of neuron

2. Structure of synapse

3. Types of synapses

4. Classification of synapse

Electrical events at a synapse

Inhibition at synapse

Properties of synapse

OVERVIEW

STRUCTURE OF A NEURON

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STRUCTURE OF SYNAPSE

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TYPE OF

SYNAPSE

SITE OF CONTIGUITY

PART OF PRESYNAPTIC

CELL

PART OF POST SYNAPTIC

CELL

AXO-

DENDRITIC

AXON DENDRITE

AXO- SOMATIC AXON SOMA

AXO-AXONAL AXON AXON

DENDRO-

DENDRITIC

DENDRITE DENDRITE

TYPES OF SYNAPSES

CLASSIFICATION OF SYNAPSES

ANATOMICAL CLASSIFICATION

TYPE I TYPE II

Axodendritic Axosomatic

Marked thickening of subsynaptic

membrane

Non continuous slight thickening in

subsynaptic membrane

Wide synaptic cleft=30nm filled with dense

ECF

Narrow synaptic cleft=20nm with not such

dense ECF

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PHYSIOLOGICAL CLASSIFICATION

CHEMICAL SYNAPSE ELECTRICAL SYNAPSE

Transmission of impulse occurs due to liberation of

chemical mediator

Pre and post synaptic membranes come very close

to form gap junctions which act as low resistance

bridges through which ions pass with ease.

Most of the synaptic junctions Some of the junctions in lateral vestibular nucleus,

hippocampus and cerebral cortex

Signals transmitted only in one direction Signals transmitted in either directions

Sensitive to hypoxia Insensitive to hypoxia

ELECTRICAL EVENTS AT A SYNAPSE

Resting membrane potential

Release of neurotransmitters

Development of postsynaptic potential

Removal of neurotransmitter

Genesis of action potential in post synaptic neuron

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RESTING MEMBRANE POTENTIAL

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• For Na+ = -61mV

• For K+ =-87mV

• For Cl- = +70mV

RELEASE OF NEUROTRANSMITTER

Release sites

Kiss and run discharge

Dales phenomenon

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DEVELOPMENT OF POST SYNAPTIC

POTENTIAL

EXCITATORY POSTSYNAPTIC

POTENTIAL (EPSP)

INHIBITORY POST SYNAPTIC

POTENTIAL(IPSP)

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SPATIAL SUMMATION TEMPORAL SUMMATION

REMOVAL OF TRANSMITTER

SUBSTANCE

Diffusion of transmitter substance outside the cleft.

or

Enzymatic degradation of transmitter in the cleft. Eg:ACh

or

Active reuptake of neurotransmitter back into presynaptic terminal.

Eg:NE

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GENESIS OF ACTION POTENTIAL

Soma as an integrator

Initial segment spike

Bell Magendie law; law

of dynamic polarity.

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SYNAPTIC INHIBITION

They are of two types

1. Postsynaptic inhibition

2. Presynaptic inhibition

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POSTSYNAPTIC INHIBITION

1. Direct inhibition

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2. Indirect inhibition

Refractory period

Renshaw cell/negative feedback

inhibition-limit excitability.

APPLIED: Strychnine decreases

postsynaptic inhibition

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Ventromedial part of

anterior horn

PRESYNAPTIC INHIBITION

a. Reduces opening of Ca+ channels

directly.

b. Increases Cl- and/or K+ permeability

so that size of action potential

reaching excitatory ending is reduced

c. Direct inhibition of neurotransmitter

release.

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19Example=Reciprocal inhibition:

APPLIED:

1. Convulsant drug picrotoxin-

decrease presynaptic inhibition

2. Barbiturates-increase presynaptic

inhibition

PROPERTIES OF SYNAPSE

1. Convergence and divergence

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3. Occlusion 4. Subliminal fringe

Significance:

1. Pattern in peripheral nerves are usually altered as they pass through synapses on the way to brain.

E.g.: referred pain.

1. The interaction between excitatory and inhibitory influences at the synapse required for integrating

and modulating activity of nervous system.

4. Reverberation

5. After discharge

6. Acidosis and hypoxia

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7. Synaptic plasticity

Post tetanic potentiation

Long term potentiation

Low frequency depression

Sensitization/ facilitation

Habituation/ synaptic fatigue

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REFERENCES

1. TEXTBOOK OF PHYSIOLOGY,11TH EDITION, GUYTON AND

HALL

2. TEXTBOOK OF MEDICAL PHYSIOLOGY, INDU KHURANA

3. GANONG’S REVIEW OF MEDICAL PHYSIOLGY. 23RD

EDITION.

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THANK YOU

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