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Explain the steps involved in neurotransmitter of ANS for 6 marks

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Neurotransmitters of the Autonomic Nervous System (ANS)

Summary of neurotransmitters, receptors, and neuron types in the autonomic and somatic nervous systems

Overview

The ANS is traditionally classified based on the primary transmitter released from nerve terminals — acetylcholine (ACh) or norepinephrine (NE). Five key features govern neurotransmitter function: synthesis → storage → release → receptor action → termination.

1. Cholinergic Fibers (Release Acetylcholine)

Fibers that release ACh include:
  • All preganglionic fibers (both sympathetic and parasympathetic)
  • All parasympathetic postganglionic fibers
  • Sympathetic postganglionic fibers to sweat glands
  • Somatic motor fibers to skeletal muscle

Steps in Cholinergic Transmission:

StepDetails
SynthesisCholine + Acetyl-CoA → ACh, catalysed by choline acetyltransferase (ChAT) in the nerve terminal
StorageACh stored in small clear synaptic vesicles near the presynaptic membrane; peptide cotransmitters (e.g., VIP, substance P) in large dense-cored vesicles
ReleaseAction potential → voltage-gated Ca²⁺ channels open → intracellular Ca²⁺ rises → vesicles fuse with membrane (via VAMPs + SNAPs, the "fusion proteins") → exocytosis of ACh
Receptor BindingACh binds nicotinic receptors at ganglia and neuromuscular junction; muscarinic receptors at parasympathetic postganglionic effector organs
TerminationAcetylcholinesterase (AChE) rapidly hydrolyses ACh into choline + acetate in the synaptic cleft; choline is recycled back into the terminal

2. Adrenergic Fibers (Release Norepinephrine)

Fibers that release NE include:
  • Most sympathetic postganglionic fibers
  • Adrenal medullary chromaffin cells (release epinephrine 80% + NE 20% into the blood)

Steps in Adrenergic Transmission:

StepDetails
SynthesisTyrosine → DOPA (rate-limiting step by tyrosine hydroxylase) → Dopamine → transported into vesicle → Norepinephrine (by dopamine β-hydroxylase inside vesicle). In adrenal medulla, NE → Epinephrine (by PNMT, only in adrenal cells)
StorageNE stored in dense-cored vesicles via VMAT (vesicular monoamine transporter). Blocked by reserpine
ReleaseAction potential → Ca²⁺ influx → vesicle fusion via VAMPs/SNAPs → NE + cotransmitters released into the synapse
Receptor BindingNE/Epi bind α₁, α₂ and β₁, β₂ adrenoceptors on postsynaptic effector cells; α₂ on presynaptic terminal acts as autoreceptor (negative feedback)
Termination(a) Reuptake into terminal via NET (norepinephrine transporter) — primary mechanism; blocked by cocaine, tricyclic antidepressants. (b) Metabolism by MAO (mitochondrial) and COMT (extracellular)

3. Key Neurotransmitter Summary Table

LocationNeurotransmitter ReleasedReceptor Type
All preganglionic → ganglionAChNicotinic (N_N)
Parasympathetic postganglionic → effectorAChMuscarinic (M)
Sympathetic postganglionic → effectorNorepinephrineAdrenergic (α, β)
Sympathetic → sweat glandsACh (exception)Muscarinic
Preganglionic → adrenal medullaAChNicotinic (N_N)
Adrenal medulla → bloodEpi (80%) + NE (20%)Adrenergic (α, β)

4. Cotransmitters (Non-classic)

Many ANS neurons co-release additional substances alongside the primary transmitter:
  • ATP — cotransmitter at many ANS–effector synapses
  • Neuropeptide Y (NPY) — co-released with NE; causes prolonged vasoconstriction
  • Nitric oxide (NO) — cotransmitter at inhibitory junctions, mediates smooth muscle relaxation
  • VIP — cotransmitter in some parasympathetic neurons; vasodilatory
In summary, cholinergic transmission dominates at ganglia and parasympathetic effectors, while adrenergic transmission governs most sympathetic effector organ responses. Both pathways follow the same fundamental steps: synthesis → vesicular storage → Ca²⁺-triggered release → receptor binding → enzymatic/transporter termination.
— Katzung's Basic and Clinical Pharmacology, 16th Edition, p. 148–153; Lippincott Illustrated Reviews: Pharmacology, p. 133–134
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Yes, you can write this — the content is largely correct and relevant for a 6-mark question on neurotransmitters of the ANS. Here's a detailed review:

✅ What's Correct

Your PointVerdict
ANS consists of preganglionic nerve, ganglion, postganglionic nerve, effector organ✅ Correct anatomical framework
Both sympathetic & parasympathetic ganglia have nicotinic receptors (Nn)✅ Correct
Nn at ganglion, Nm at neuromuscular junction✅ Correct subtypes
Nicotinic receptors blocked by d-tubocurarine (Nm) and pentamethonium/hexamethonium (Nn)✅ Correct
Muscarinic action = effect of ACh at parasympathetic postganglionic ending✅ Correct definition
Muscarinic actions: bradycardia, increased glandular secretion, smooth muscle contraction (intestine, bronchi)✅ All correct
Muscarinic receptors selectively blocked by atropine (and hyoscine butyl bromide/buscopan)✅ Correct
Adrenergic receptors at postganglionic sympathetic nerve endings✅ Correct
Adrenergic receptors classified into Alpha and Beta✅ Correct
Alpha receptor → excitatory action (except intestine)✅ Correct
Beta receptor → inhibitory action (except heart)✅ Correct

⚠️ Things to Add or Fix

  1. The question asks about "steps involved in neurotransmission" — your notes cover receptors and actions well, but are missing the key steps of neurotransmitter function:
    • Synthesis → Storage → Release → Receptor binding → Termination
    • Examiners awarding 6 marks will expect at least a brief mention of these steps for both cholinergic and adrenergic transmission.
  2. Adrenergic receptors — you wrote "α-receptor" as a third type separately, but α is already part of Alpha. Clarify: α₁, α₂, β₁, β₂ are the subtypes.
  3. Termination of action is missing:
    • ACh → broken down by acetylcholinesterase
    • NE → reuptake into nerve terminal (primary), or metabolism by MAO/COMT
  4. You mention "nicotinic action" is a stimulant effect on ganglia — add that this is also blocked by hexamethonium (ganglionic blocker), which you did write — good.

📝 Verdict

This answer will fetch ~4–4.5/6 marks. To get full marks, add 3–4 lines on the steps of neurotransmission (synthesis, storage, Ca²⁺-triggered release, termination by AChE/reuptake). The receptor classification and pharmacology part is well done.

What should I write other than these what should I add in these

Here's exactly what to add to make your answer complete for 6 marks:

🔴 Add This Section: Steps of Neurotransmission

Write this before your receptor section, right after the anatomy part.

Steps in Cholinergic Transmission (ACh)

1. Synthesis
  • Choline + Acetyl-CoA → Acetylcholine
  • Enzyme: Choline acetyltransferase (ChAT)
2. Storage
  • ACh stored in synaptic vesicles in nerve terminal
3. Release
  • Action potential arrives → Ca²⁺ influx → vesicles fuse with membrane → exocytosis of ACh
4. Receptor Binding
  • ACh binds nicotinic or muscarinic receptors → produces effect
5. Termination
  • Acetylcholinesterase (AChE) breaks ACh → choline + acetate
  • Choline is recycled back into terminal

Steps in Adrenergic Transmission (NE)

1. Synthesis
  • Tyrosine → DOPA → Dopamine → Norepinephrine
  • Rate-limiting enzyme: Tyrosine hydroxylase
2. Storage
  • NE stored in dense-cored vesicles via VMAT
3. Release
  • Action potential → Ca²⁺ influx → exocytosis of NE
4. Receptor Binding
  • NE binds α or β adrenoceptors → produces effect
5. Termination
  • Reuptake back into nerve terminal (main method)
  • Metabolism by MAO and COMT

🟡 Small Fix in Your Existing Notes

In the adrenergic receptor part, write it clearly as:
Alpha receptor subtypes → α₁ and α₂ Beta receptor subtypes → β₁ and β₂
Also add one line:
α₂ is a presynaptic autoreceptor → inhibits further NE release (negative feedback)

📋 What Your Answer Structure Should Look Like

  1. Anatomy of ANS ✅ (you have this)
  2. Steps of Cholinergic Transmission ← ADD THIS
  3. Steps of Adrenergic Transmission ← ADD THIS
  4. Cholinergic receptors (Nicotinic & Muscarinic) ✅ (you have this)
  5. Adrenergic receptors (Alpha & Beta + actions) ✅ (you have this)
Adding steps 1–5 for both ACh and NE is the main thing missing. That alone will push your answer to full 6/6 marks.
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