Page 64 - Physiology and Pathophysiology MNU 2024-2025 نظرى

Clinical pharmacy 2024/2025 Level 2 Pharm D Physiology and Pathophysiology (MD303)

2) Chemical synapse

presynaptic neuron and the postsynaptic neuron are not in direct contact ➔
Synaptic cleft
but separated by a narrow (0.01 to 0.02 mm) space called synaptic cleft.

Chemical substance synthesized in the nerve ending in nerve knob, stored in
Neurotransmitter nerve ending & released from presynaptic neuron➔ diffuse across synaptic

cleft➔ bind to specific receptors on post synaptic nerves

1- Passive diffusion of neurotransmitter away from the synaptic cleft.
2- Destruction of neurotransmitter by enzymes located in the synaptic

Inactivation cleft or in the plasma membrane of presynaptic or postsynaptic

/Removal neurons.
3- Active reuptake of neurotransmitter into the synaptic knob of

presynaptic neuron for reuse or enzymatic destruction.

➢ The arrival of an action potential at the axon terminal causes voltage-
gated Ca++ channels to open.

➢  in conc. Of Ca++ ions in the intracellular fluid facilitates exocytosis of
Mechanism of action
the neurotransmitter into the synaptic cleft.
at a chemical
➢ Binding of the neurotransmitter to its specific receptor on the
synapse
postsynaptic neuron alters the permeability of the membrane to one or
more ions ➔ causing a change in the membrane potential and generation

of a graded potential in this neuron.

➢ neurotransmission across chemical synapses is unidirectional (from
In contrast to presynaptic cell to postsynaptic cell).

electrical synapses ➢ The synaptic delay is the time required for the multiple steps in
chemical neurotransmission to occur.

➢ Synapses may be excitatory or inhibitory.
Type of synapses

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