Q1 LONG ANSWER 10M ★ High-yield

Define synapse. Classify synapses and describe the mechanism and properties of synaptic transmission. Add a note on synaptic inhibition.

Physiology · 1st Professional — Pre-Clinical

Definition

Synapse = the junction between two neurones, or between a neurone and an effector cell, across which the nerve impulse is transmitted.

Classification

A. By structure

Type	Gap	Transmission	Example
Chemical	20–40 nm	By neurotransmitter	Most synapses in the CNS
Electrical	3 nm (gap junction)	Direct ionic current	Cardiac and smooth muscle, some brainstem nuclei

B. By site of contact

Axodendritic — the commonest
Axosomatic — usually inhibitory, near the axon hillock
Axoaxonic — the basis of presynaptic inhibition
Dendrodendritic

Structure of a Chemical Synapse

Presynaptic terminal (synaptic knob) — contains synaptic vesicles and numerous mitochondria
Synaptic cleft — 20–40 nm
Postsynaptic membrane — bears the receptors; thickened

Mechanism of Transmission

Action potential reaches the presynaptic terminal → Voltage-gated Ca2+ channels open → Ca2+ influx → vesicles fuse with membrane → Transmitter released by exocytosis → Diffuses across the cleft; binds postsynaptic receptor → Ion channels open → EPSP or IPSP

Postsynaptic Potentials

Feature	EPSP	IPSP
Change	Depolarisation	Hyperpolarisation
Ion movement	Na+ influx (and Ca2+)	Cl− influx or K+ efflux
Effect	Brings the membrane toward firing level	Takes it away from firing level
Transmitter	Glutamate, ACh	GABA, glycine
Magnitude	About 0.5–1 mV per synapse	Similar
Nature	Graded, non-propagated, can summate	Graded, non-propagated

A single EPSP is far too small to fire the neurone — about 40–80 must summate
The axon hillock has the lowest threshold and is where the action potential is finally generated — the neurone acts as an integrator of all EPSPs and IPSPs

Properties of Synaptic Transmission

One-way (unidirectional) conduction — Bell–Magendie law; transmitter is present only on the presynaptic side
Synaptic delay — 0.5 ms; used to count the number of synapses in a pathway
Summation — temporal (repeated impulses down one fibre) and spatial (simultaneous impulses down many fibres)
Fatigue — on repeated stimulation; transmitter stores deplete. The synapse is the first structure to fatigue in a reflex arc
Convergence and divergence
Occlusion and subliminal fringe
After-discharge — response outlasts the stimulus, due to reverberating circuits
Susceptibility to hypoxia, drugs and pH — far more than nerve fibre

Inhibition

Type	Site	Mechanism
Postsynaptic (direct)	Postsynaptic membrane	IPSP → hyperpolarisation
Presynaptic	Axoaxonic synapse	↓ Ca2+ entry → less transmitter released; mediated by GABA. No IPSP is produced
Renshaw cell (recurrent)	Anterior horn	Collateral from motor neurone excites a Renshaw cell, which inhibits the same motor neurone — negative feedback
Reciprocal	Spinal cord	Agonist contracts while antagonist relaxes

Neurotransmitters

Transmitter	Chief role
Glutamate	Main excitatory transmitter of the CNS
GABA	Main inhibitory transmitter of the brain
Glycine	Main inhibitory transmitter of the spinal cord
Acetylcholine	NMJ, autonomic ganglia, memory (cortex)
Dopamine	Basal ganglia, reward, prolactin inhibition
Serotonin	Mood, sleep, pain modulation
Noradrenaline	Arousal, attention, sympathetic

Applied Aspects

Tetanus toxin blocks the release of glycine and GABA → loss of inhibition → spastic paralysis, trismus, risus sardonicus
Strychnine blocks glycine receptors → convulsions
Benzodiazepines and barbiturates enhance GABA action → sedation and anticonvulsant effect
Parkinson disease — dopamine deficiency in the nigrostriatal pathway
Myasthenia gravis — antibodies against the postsynaptic nicotinic receptor

All questions in Nervous System (neurophysiology)

Every Physiology chapter →