Synaptic Transmission

Synaptic transmission is the process by which a neuron communicates with another neuron, a muscle fibre, or a gland by converting an electrical signal into a chemical (and back to electrical) signal across the synapse. For NEET PG this topic anchors both Physiology (nerve–muscle) and clinical correlates (myasthenia gravis, Lambert–Eaton syndrome, botulism, organophosphate poisoning).

Definition and Classification of Synapses

A synapse is a specialised junction where a presynaptic terminal transfers information to a postsynaptic cell. Transmission can be electrical or chemical.

Feature	Electrical synapse	Chemical synapse
Structure	Gap junction (connexons)	Synaptic cleft (~20–40 nm)
Delay	Almost none (~0.1 ms)	Synaptic delay ~0.5 ms
Direction	Usually bidirectional	Unidirectional
Neurotransmitter	None	Required
Fatigability	Resistant	Fatigable
Examples	Cardiac muscle, smooth muscle, retina, some CNS	NMJ, most CNS synapses
Plasticity	Limited	High (basis of learning/memory)

High-yield: Synaptic delay (~0.5 ms) at a chemical synapse is mostly due to the time taken for Ca²⁺ entry and neurotransmitter release, not diffusion across the cleft.

Chemical synapses are further classified by location (axodendritic, axosomatic, axoaxonic) and by effect (excitatory vs inhibitory).

Neuromuscular Junction (NMJ) — Anatomy

The NMJ is the prototype chemical synapse and the most heavily tested.

Components:

1. Presynaptic terminal of a motor neuron containing synaptic vesicles loaded with acetylcholine (ACh).
2. Synaptic cleft containing acetylcholinesterase (AChE) anchored to the basal lamina.
3. Postsynaptic membrane (motor end plate) thrown into junctional folds; the crests are densely packed with nicotinic (N_M) ACh receptors, and the depths of the folds contain voltage-gated Na⁺ channels.

• Each vesicle (quantum) contains ~5,000–10,000 ACh molecules.
• A single action potential releases ~125–300 quanta, producing far more depolarisation than threshold — this safety margin is the "safety factor" of the NMJ.

High-yield: The adult nicotinic NMJ receptor has the subunit composition α₂βδε (the foetal/extrajunctional form has a γ subunit instead of ε). Two ACh molecules must bind the two α subunits to open the channel, which is a non-selective cation channel (Na⁺ in, K⁺ out; reversal potential ~0 mV).

Steps of Neuromuscular Transmission (Stepwise Flow)

Action potential reaches terminal → voltage-gated Ca²⁺ channels open (P/Q-type at NMJ) → Ca²⁺ influx → vesicle docking & fusion via SNARE proteins → exocytosis of ACh → ACh diffuses across cleft → binds 2 α-subunits of N_M receptor → cation channel opens → end-plate potential (EPP) → EPP opens voltage-gated Na⁺ channels → muscle action potential → excitation–contraction coupling → ACh hydrolysed by AChE → choline reuptake.

High-yield: The calcium channel at the motor nerve terminal is the P/Q (Cav2.1) type. Antibodies against this channel cause Lambert–Eaton myasthenic syndrome (LEMS).

SNARE complex (vesicle fusion machinery)

• v-SNAREs (on vesicle): synaptobrevin (VAMP).
• t-SNAREs (on target membrane): syntaxin and SNAP-25.
• Synaptotagmin is the Ca²⁺ sensor that triggers fusion.

Toxin	Target	Effect
Botulinum toxin	Cleaves SNAP-25 / synaptobrevin / syntaxin (serotype-dependent)	Blocks ACh release → flaccid paralysis
Tetanus toxin	Cleaves synaptobrevin in inhibitory (Renshaw) interneurons	Blocks glycine/GABA release → spastic paralysis
α-Latrotoxin (black widow)	Triggers massive vesicle release	Sustained ACh discharge

High-yield mnemonic — "BOTOX Blocks, TETanus Tightens": Botulinum = flaccid (peripheral, blocks ACh); Tetanus = spastic/rigid (central, blocks inhibitory transmitters).

End-Plate Potential (EPP) and MEPP

• MEPP (miniature end-plate potential): spontaneous ~0.5 mV depolarisation from release of a single quantum (one vesicle). Basis of the quantal hypothesis of Katz (Nobel-winning eponym).
• EPP: the summed, graded depolarisation from many quanta after a nerve impulse. Unlike an action potential, the EPP is local, graded, non-propagating, and not all-or-none; it has no refractory period and cannot be summated into a tetanus at the membrane itself.

High-yield: The EPP at the NMJ normally always reaches threshold because of the large safety factor — this distinguishes it from a CNS EPSP, which is usually subthreshold and requires summation.

EPSP and IPSP in the CNS

Unlike the one-to-one NMJ, CNS neurons integrate thousands of inputs.

Feature	EPSP	IPSP
Direction	Depolarising (toward threshold)	Hyperpolarising (away from threshold) / shunting
Typical transmitter	Glutamate	GABA, glycine
Receptor/ion	Na⁺ (and Ca²⁺) influx via AMPA/NMDA	Cl⁻ influx (GABA_A, glycine) or K⁺ efflux (GABA_B)
Net effect	Excitation	Inhibition

• Glutamate is the main excitatory transmitter; AMPA receptors mediate fast EPSPs, NMDA receptors (Ca²⁺-permeable, blocked by Mg²⁺ at rest, require depolarisation + glycine co-agonist) underlie long-term potentiation (LTP) and memory.
• GABA is the main inhibitory transmitter in the brain; glycine dominates in the spinal cord (e.g., Renshaw cell inhibition).

High-yield: GABA_A and glycine receptors are ligand-gated Cl⁻ channels (ionotropic, fast IPSP). GABA_B is a G-protein–coupled receptor opening K⁺ channels (slow IPSP). Strychnine blocks glycine receptors → spinal disinhibition → convulsions; tetanus toxin does the same by blocking glycine release.

Summation: Temporal and Spatial

A single EPSP (~0.5–1 mV) is far below threshold, so the postsynaptic neuron must summate inputs at the axon hillock / initial segment (the trigger zone with the lowest threshold and highest Na⁺ channel density).

• Temporal summation: Successive EPSPs from the same presynaptic fibre arriving in rapid succession add up before the first decays.
• Spatial summation: EPSPs from several different synapses arriving simultaneously add up.

Many subthreshold EPSPs → temporal + spatial summation → depolarisation reaches threshold at axon hillock → action potential fires.

High-yield: The initial segment / axon hillock has the lowest threshold and is the site where the decision to fire an action potential is made.

Presynaptic Inhibition and Facilitation

• Presynaptic inhibition acts via axoaxonic synapses that reduce Ca²⁺ entry into the terminal → fewer quanta released → smaller EPSP without changing the postsynaptic membrane potential. Mediated largely by GABA (GABA_B) in the spinal cord; important in sensory pathway modulation.
• Presynaptic facilitation increases Ca²⁺ entry/residual Ca²⁺ → more transmitter (basis of short-term potentiation, e.g., Aplysia sensitisation).
• Post-tetanic potentiation: after a high-frequency train, residual Ca²⁺ in the terminal enhances release transiently.

High-yield: Presynaptic inhibition decreases transmitter release (presynaptic Ca²⁺) whereas postsynaptic inhibition hyperpolarises the postsynaptic cell (IPSP).

Neurotransmitter Recycling and Termination

ACh action is terminated by AChE, which hydrolyses ACh into choline + acetate. Choline is taken back into the presynaptic terminal by a high-affinity Na⁺-dependent transporter (the rate-limiting step of ACh synthesis) and reacetylated by choline acetyltransferase (ChAT) using acetyl-CoA. ACh is then loaded into vesicles by VAChT.

Other transmitters are terminated by reuptake transporters: dopamine (DAT), serotonin (SERT), noradrenaline (NET), GABA (GAT), glutamate (EAAT). Monoamines are then degraded by MAO and COMT.

Transmitter	Synthesised from	Terminated mainly by
ACh	Choline + acetyl-CoA (via ChAT)	Enzymatic (AChE)
Glutamate	Glutamine / α-ketoglutarate	Reuptake (EAAT into glia → glutamine cycle)
GABA	Glutamate (via GAD, needs vit B6)	Reuptake (GAT) + GABA-transaminase
Dopamine/NA	Tyrosine → DOPA → DA → NA	Reuptake (DAT/NET) + MAO/COMT
Serotonin	Tryptophan → 5-HTP → 5-HT	Reuptake (SERT) + MAO

High-yield: ACh is the only major neurotransmitter terminated principally by enzymatic degradation in the cleft; most others are terminated by reuptake. GABA synthesis requires glutamate decarboxylase (GAD) and pyridoxine (B6) — hence B6 deficiency (or INH toxicity) causes seizures.

Pharmacology of the NMJ

Drug class	Example	Mechanism	Clinical note
Non-depolarising blocker	Vecuronium, rocuronium, atracurium	Competitive antagonist at N_M receptor	Reversed by neostigmine (AChE inhibitor) + sugammadex (for aminosteroids)
Depolarising blocker	Succinylcholine	Persistent depolarisation (Phase I block)	NOT reversed by neostigmine; risks: hyperkalaemia, malignant hyperthermia, scoline apnoea (pseudocholinesterase deficiency)
AChE inhibitor	Neostigmine, edrophonium, pyridostigmine	↑ ACh in cleft	Treat myasthenia; reverse non-depolarising block
Hemicholinium	—	Blocks choline uptake	Depletes ACh
Vesamicol	—	Blocks VAChT (vesicular loading)	Experimental
Botulinum toxin	—	Blocks ACh release	Dystonia, spasticity, cosmetic

High-yield: Edrophonium (Tensilon) test — short-acting AChE inhibitor; improves strength in myasthenia gravis but worsens a cholinergic crisis (helps differentiate the two).

Clinical Correlates Arising From NMJ Mechanics

Myasthenia Gravis (MG)

• Postsynaptic disorder: autoantibodies (mostly anti-AChR, some anti-MuSK) destroy and block nicotinic receptors and complement-mediated end-plate damage.
• Features: fatigable weakness, ptosis, diplopia, bulbar weakness, worse with activity/end of day; proximal weakness.
• Associations: thymic hyperplasia/thymoma (CT chest mandatory).
• Investigation of choice: anti-AChR antibody (most specific); repetitive nerve stimulation shows decremental response; single-fibre EMG is the most sensitive test.
• Management: pyridostigmine (symptomatic, first-line), immunosuppression (steroids, azathioprine), thymectomy, and for crisis IVIG / plasmapheresis.

Lambert–Eaton Myasthenic Syndrome (LEMS)

• Presynaptic disorder: antibodies against P/Q-type voltage-gated Ca²⁺ channels → reduced ACh release.
• Paraneoplastic, classically with small cell lung carcinoma (SCLC).
• Features: proximal weakness that improves with repeated effort/exercise (facilitation), autonomic features (dry mouth), hyporeflexia that returns after exercise.
• EMG: incremental response at high-frequency (>10–50 Hz) stimulation.
• Treatment: treat tumour; 3,4-diaminopyridine (amifampridine) blocks K⁺ channels → prolongs terminal depolarisation → more Ca²⁺ entry.

High-yield mnemonic — "MG goes Down, LEMS goes Up": On repetitive nerve stimulation, MG shows a decremental response; LEMS shows an incremental response.

Organophosphate / Carbamate Poisoning

• Irreversible (OP) or reversible (carbamate) AChE inhibition → ACh accumulation → muscarinic (DUMBELS / SLUDGE) + nicotinic (fasciculations, weakness) + CNS effects.
• Treatment: atropine (muscarinic) + pralidoxime (2-PAM) to reactivate AChE before "ageing" occurs.

Botulism

• Presynaptic: toxin cleaves SNARE proteins → no ACh release → descending flaccid paralysis, dilated pupils, dry mouth, no sensory loss. EMG mimics LEMS (incremental).

Disorder	Site	Mechanism	RNS finding
Myasthenia gravis	Postsynaptic	Anti-AChR antibodies	Decremental (low freq)
LEMS	Presynaptic	Anti–P/Q Ca²⁺ channel	Incremental (high freq)
Botulism	Presynaptic	SNARE cleavage, ↓ACh release	Incremental
OP poisoning	Synaptic cleft	AChE inhibition, ACh excess	—

Key Differentials at a Glance

• Fatigable ptosis + ocular diplopia → think MG.
• Proximal weakness improving with exercise + dry mouth + smoker → think LEMS/SCLC.
• Descending paralysis + fixed dilated pupils after canned food/infant honey → think botulism.
• Miosis, salivation, bronchorrhoea, fasciculations, farmer/suicidal ingestion → think OP poisoning.

Recently Asked / Exam Angle

• Type of Ca²⁺ channel at motor nerve terminal → P/Q type (Cav2.1); antibodies → LEMS.
• Subunit composition of the adult nicotinic NMJ receptor → α₂βδε (foetal has γ).
• SNARE proteins and their toxins — SNAP-25 (botulinum A/E), synaptobrevin (tetanus, botulinum B/D/F).
• Rate-limiting step in ACh synthesis → choline reuptake (high-affinity transporter).
• Difference between EPP and action potential (graded vs all-or-none, no refractory period).
• Decremental vs incremental RNS for MG vs LEMS — repeated frequently.
• NMDA receptor properties — Mg²⁺ block at rest, needs glycine co-agonist, Ca²⁺ permeable, role in LTP.
• Drug of choice / first-line in MG → pyridostigmine; edrophonium for the diagnostic test.
• Strychnine blocks glycine receptors; tetanus toxin blocks glycine/GABA release.
• Single-fibre EMG = most sensitive test in MG; anti-AChR = most specific.

Rapid Revision

1. Chemical synapse: unidirectional, fatigable, ~0.5 ms delay (due to Ca²⁺ entry & release).
2. NMJ transmitter = ACh; receptor = nicotinic N_M (α₂βδε); 2 ACh molecules needed to open channel.
3. P/Q-type (Cav2.1) voltage-gated Ca²⁺ channel triggers ACh release at the terminal.
4. SNARE proteins: synaptobrevin (v), syntaxin + SNAP-25 (t); synaptotagmin = Ca²⁺ sensor.
5. Botulinum = flaccid (blocks ACh release); tetanus = spastic (blocks glycine/GABA centrally).
6. MEPP = single quantum (~0.5 mV); EPP = graded, non-propagated, no refractory period.
7. EPSP (glutamate, Na⁺/Ca²⁺ in) excites; IPSP (GABA/glycine, Cl⁻ in or K⁺ out) inhibits.
8. Summation (temporal + spatial) occurs at the axon hillock, the trigger zone.
9. Presynaptic inhibition = ↓Ca²⁺ entry → ↓transmitter release (axoaxonic, GABA-mediated).
10. ACh is terminated by AChE; choline reuptake is the rate-limiting synthesis step; most others use reuptake transporters.
11. MG = postsynaptic, anti-AChR, decremental RNS, fatigue worsens with use, treat with pyridostigmine + thymectomy.
12. LEMS = presynaptic, anti–P/Q Ca²⁺ channel, incremental RNS, improves with use, linked to SCLC, treat with 3,4-diaminopyridine.