Nervous System Organisation and Neural Communication

There are two levels to master: organisational and cellular.

At the organisational level, learn the CNS/PNS division first, then the somatic/autonomic division of the PNS, then the sympathetic/parasympathetic division of the ANS. For each division, know what it controls, which neurotransmitters it uses, and (for ANS) the opposing effects of sympathetic and parasympathetic on key organs.

At the cellular level, prioritise the action potential sequence — resting potential, threshold, rising phase, falling phase, refractory periods. Understand *why* each step happens in terms of ion channels and ion flows, not just that it happens. Then link this to synaptic transmission: how an electrical signal becomes chemical, and how EPSP/IPSP summation determines whether an action potential fires. Finally, know the major neurotransmitters, their functions, and how drugs exploit their mechanisms — SSRIs, cocaine, donepezil.

These two levels connect: the ANS uses noradrenaline at sympathetic effectors and ACh at parasympathetic — so understanding neurotransmitters clarifies the pharmacology of ANS drugs (e.g., beta-blockers, atropine).

Organisation: CNS · PNS · afferent · efferent · somatic nervous system · autonomic nervous system (ANS) · sympathetic · parasympathetic · fight-or-flight · rest-and-digest · blood-brain barrier (BBB)

Neurons: soma · dendrites · axon · axon hillock · myelin · node of Ranvier · saltatory conduction · glial cells · astrocytes · oligodendrocytes · Schwann cells

Action potential: resting membrane potential (−70 mV) · threshold (−55 mV) · depolarisation · repolarisation · Na+/K+ ATPase · all-or-none principle · absolute refractory period · relative refractory period

Synaptic transmission: neurotransmitter · vesicle · synaptic cleft · EPSP · IPSP · temporal summation · spatial summation · ionotropic · metabotropic · reuptake

Neurotransmitters: glutamate · GABA · dopamine · serotonin · acetylcholine · noradrenaline · SERT · DAT · SSRIs · acetylcholinesterase

Apply to examples: Use drug mechanisms to apply neurotransmitter knowledge. SSRIs block SERT (serotonin reuptake transporter) → more serotonin in the cleft → mood effects. Cocaine blocks DAT (dopamine) → dopamine accumulates → euphoria and addiction. Donepezil inhibits acetylcholinesterase → more ACh → used in Alzheimer's. These are the kinds of application questions that appear in biopsychology exams.

Compare sympathetic vs parasympathetic: Do this systematically by organ — heart (sympathetic ↑ rate, parasympathetic ↓ rate), pupils (dilate vs constrict), digestion (inhibited vs stimulated), bronchioles (dilate vs constrict). Know the neurotransmitters: sympathetic postganglionic = noradrenaline; parasympathetic postganglionic = ACh.

Critique: The biggest conceptual challenge in this area is that correlation ≠ causation in neurotransmitter research — serotonin depletion correlates with depression, but that doesn't mean low serotonin *causes* depression (the monoamine hypothesis is contested). Similarly, neurotransmitter-level explanations are reductionist — they explain mechanism but not the psychological meaning of a disorder or behaviour.

Neural communication mechanisms underpin all of psychiatry and pharmacology. Every antidepressant, antipsychotic, and anxiolytic works by modifying synaptic transmission — understanding the mechanism tells you why they work and why they have side effects. Demyelinating diseases like multiple sclerosis become clinically legible once you understand saltatory conduction and why myelin damage slows or blocks signals. The ANS distinction explains the physical experience of anxiety (sympathetic activation) and why beta-blockers can treat performance anxiety (they block β-adrenergic receptors on the heart). The nervous system is the foundation that all clinical neuroscience sits on.

Confusing the two-neuron chain in the ANS: all preganglionic neurons use ACh (nicotinic receptors). The difference is the postganglionic: sympathetic = noradrenaline (adrenergic receptors); parasympathetic = ACh (muscarinic receptors). Getting this wrong is the most common ANS error.

Getting the action potential sequence wrong: the most common mistake is saying that K+ channels open *before* Na+ channels close — in fact they open roughly at the same time. Know the sequence: Na+ in → peak → Na+ channels inactivate + K+ channels open → K+ out → repolarisation → afterhyperpolarisation.

**Describing saltatory conduction without explaining *why* it's faster**: jumping from node to node is faster because myelin increases resistance between nodes, forcing current to flow along the axon rather than leaking out. Without this explanation, you've only named the phenomenon.

Saying neurons "fire" without specifying the threshold: the all-or-none principle requires that depolarisation reach threshold (~−55 mV). Below threshold, no AP fires regardless of stimulus strength. This detail is frequently tested.