L3, Synapse and synaptic transmission, physio

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It is the area of contact between two neurons (presynaptic and

postsynaptic neurons) without cytoplasmic continuity.


1) According to method of transmission:

a-Electrical synapse: the electrical current is transmitted directly

from cell to cell through direct channels called Gap Junction as

seen in the smooth and cardiac muscles. It is very few in CNS and

its significance in general is not known.

B-Chemical synapse: in which the activity is transmitted from the

presynaptic to the postsynaptic neuron through release of chemical

transmitter (Neurotransmitter, Synaptic transmitter] from vesicles

found in presynaptic knobs. These chemical transmitters act on

receptor protein to excite, inhibit or modify the sensitivity of

postsynaptic neuron.

2) According to the area of postsynaptic neuron involved:

a- Axo-dendritic: least excitable but most common (80-95%), i.e., 80-

95% of presynaptic knobs end on the dendrites.


C-Axo-axonic: most excitable (initial segment of the axon, first 20-30 u

is the most excitable part of the neuron) but least common.


1) Synaptic knobs (Terminal knobs):

They are the ends of the terminal branches of the axon of pre-synapti neuron,

and contain:

• In its cytosol

1. Multiple transmitter vesicles that contain the neurotransmitter.

2. Multiple mitochondria that supply energy for formation of the


• In its membrane (Presynaptic membrane)

1. Large number of voltage gated Ca2+ channels in contrast to other areas of the

nerve fibers.

2. Multiple protein molecules on the inner surface called release sites

(2) Synaptic cleft:

Contains enzymes for destruction of some neurotransmitters after their release.

(3) Synaptic gutter:

The part of the postsynaptic neuron involved in the synapse. It contains large 

number of protein receptor that has two components:

a. Binding component: 

for binding of neurotransmitter. It protrudes to the

synaptic cleft.

B. Intracellular component: 

passes all the way through the membrane to the

interior of the postsynaptic neuron. It transmits the activity to postsynaptic

neuron by one of two ways:

1- Gating lon channels directly and allowing passage of specified ions:

These ion channels open and close within a fraction of a millisecond

thus it provides a means for rapid changes in post-synaptic neuron

(depolarization or hyperpolarization in post-synap. N). In this case the

receptor is called ionotropic receptor. These ion channels may be:

• Na channels ««« Na entry and activation (Depolarization).

• K channels «««K+ exit and inhibition (Hyperpolarization).

• Cl channels «««Cl entry and inhibition (Hyperpolarization).

2- Activation of second messenger system:

• It provides a means for slow and prolonged changes in


neurons for seconds, minutes, days, or months after initial

transmitter is gone, i.e. Is no longer present. In this case the

receptor is called metabotropic receptor.

• The most common type of receptor in the synapse that

activates second messenger system is G-protein linked

receptor (G-protein coupled receptor) (GPCR).

• This receptor when activated leads to activation of a group of protein linked

to guanosine triphosphate (GTP) hence the name G-proteins.

Activated G-proteins in turn triggers the production of number of second


• G-protein consists of 3 components:

a-Alpha component: it is the activator part of the G-protein.



• The inactive G-protein is free in the cytoplasm and its alpha component binds

a molecule of GDP.

• When the receptor is activated by the neurotransmitter conformational

change occur in the receptor resulting in binding of the G-protein to the

intracellular component of the receptor.

• This process permits the alpha subunit to release GDP and bind to

GTP and separate from the B and y portions of the complex. The

separated alpha-GTP portion moves free within the cytoplasm to

perform one or more of the following effects depending on the type

of the neuron:

a. Opening of specific ion channelsthrough the postsynaptic neuron.

This channel often stays open for a prolonged time, in contrast to the

rapid closure of directly activated ion channel.

b. Activation of membrane enzymes → activation of CAMP or CGMP

in the neuronal cell → change in the metabolic machinery in the neuron → alteration in the long-term excitability of the neuron 

c. Activation of one or more intracellular enzymes change in the metabolic machinery of the neuron.

d. Activation of gene transcription → formation of new protein →change in cell structure or change in the metabolic machinery of the cell. This is the most important effect of the second messenger system.

• Inactivation of the G protein occur when the GTP bound to the a subunit is hydrolyzed to GDP. This action causes the a subunit to release from its target protein,and then to combine again with the ß

and y subunits, returning the G protein complex to its inactive state .


1) Arrival of action potential to the synaptic knobs.

2) Entry of Ca2+ to the synaptic knobs from ECF through the voltage- gated Ca2+ channels, which is abundant in this area of the membrane.

The amount of neurotransmitter released to the synaptic cleft is

proportionate to the amount of Ca2+ entered the synaptic knob.

3) Release of chemical transmitter by exocytosis: Ca2+ binds to the release sites on the inner surface of the presynaptic membrane →near vesicles fuse with the mambrane open – release of chemical transmitter to the synaptic cleft