Biological Bases of Behaviour: Domain Guide

Treat the neuron as a device that has already made its decision by the time it fires. All the interesting computation happens in the summation of excitatory and inhibitory input at the cell body; the action potential is only the announcement of the verdict. This is why the all-or-none law is not a technicality but the organising fact of the whole domain: it forces the nervous system to code intensity in rate and population, which is exactly what exam items about 'a more intense stimulus' are testing.

A neurotransmitter has no fixed effect. Acetylcholine excites skeletal muscle at the neuromuscular junction and slows the heart through the vagus nerve — same molecule, opposite outcomes, because the receptors differ. Whenever an item invites you to say 'dopamine does X', the safer sentence is 'dopamine at this receptor, in this pathway, does X'. Most distractors in this domain are built from transmitter names detached from their locations.

Structures are learned through their lesions. The reason the hippocampus is described as the site where new declarative memories are consolidated is that people with bilateral medial temporal damage can no longer form them while their older memories and their capacity to learn new motor skills remain. Reading every structure as 'the thing whose loss produces this specific deficit' converts a long anatomy list into a short set of case descriptions, which is the form the test actually uses.

Start at the membrane. A neuron at rest holds an electrical charge of about −70 mV inside relative to outside, maintained by unequal distributions of sodium and potassium. Excitatory input pushes the inside less negative and inhibitory input pushes it more negative; the cell body sums these continuously over space and time. If the sum reaches threshold, near −55 mV, voltage-gated sodium channels open and sodium rushes in, driving the interior sharply positive; potassium then flows out and restores the negative interior. During the absolute refractory period that follows, no stimulus of any size can trigger another spike, which sets a ceiling on firing rate. Because the spike is all-or-none, the only variables left for coding intensity are how fast one neuron fires and how many neurons fire — a fact worth stating out loud before every item that mentions a 'stronger' stimulus.

At the axon terminal the signal becomes chemical. Vesicles release transmitter into the synaptic cleft, it binds receptors on the postsynaptic membrane, and it is then cleared by reuptake into the sending neuron or by enzymatic breakdown. This clearing step is where most drug questions live. A reuptake inhibitor removes the clearing mechanism, so transmitter stays in the cleft longer and its effect is amplified: functionally an agonist, even though the word 'inhibitor' is in its name. A receptor blocker reduces the effect: an antagonist. Build the classification on the net effect at the receptor and the naming trap disappears. The transmitters worth holding are acetylcholine (movement at the neuromuscular junction, and a system implicated in memory), dopamine (movement and reward; degeneration of nigrostriatal cells is associated with Parkinson's disease, and excess activity in other pathways features in accounts of schizophrenia), serotonin (mood, sleep, appetite), norepinephrine (arousal and alertness), GABA (the principal inhibitory transmitter, the target of sedatives), glutamate (the principal excitatory transmitter), and the endorphins (pain modulation).

Move up to organisation. The nervous system splits into central and peripheral branches; the peripheral splits into somatic (voluntary muscle and sensory afferents) and autonomic (glands, smooth muscle, viscera); and the autonomic splits into sympathetic and parasympathetic divisions that push the same organs in opposite directions. Within the brain, the hindbrain carries the life-support machinery — the medulla for breathing and heart rate, the pons, and the cerebellum for balance, coordination, and well-practised motor sequences. The midbrain's reticular formation governs arousal and the sleep–wake transition. The forebrain contains the thalamus, which relays every sense except smell to cortex; the hypothalamus, which regulates hunger, thirst, temperature, and the endocrine system; the hippocampus, where new declarative memories are consolidated; the amygdala, which tags events as threatening or emotionally significant; and the basal ganglia, involved in movement initiation and habit learning.

Cortex is organised by lobe and by side. The occipital lobe processes vision; the temporal lobe handles audition and, in the left hemisphere for most people, language comprehension; the parietal lobe carries the somatosensory strip and spatial processing; the frontal lobe carries the motor strip, speech production, and executive control. Two language regions generate a disproportionate number of items: damage to Broca's area in the left frontal lobe leaves comprehension largely intact but production halting and effortful, while damage to Wernicke's area in the left temporal lobe produces fluent speech that carries little meaning together with impaired comprehension. Each hemisphere controls and receives from the opposite side of the body, and the corpus callosum carries traffic between them — which is why severing it produces the classic split-brain findings in which information presented to one hemisphere cannot be reported by the verbal system in the other.

The endocrine system is the second control channel. It signals with hormones released into the bloodstream, so it is slower to start, broader in reach, and longer in duration than neural signalling — that speed-and-duration contrast is itself an examinable point. The hypothalamus governs the pituitary, which governs the rest: in the stress cascade the hypothalamus releases CRH, the anterior pituitary releases ACTH, and the adrenal cortex releases cortisol, which feeds back to shut the cascade down. The adrenal medulla, driven directly by sympathetic input, releases adrenaline for the fast phase of the same response. The thyroid sets metabolic rate, the pancreas regulates blood glucose through insulin and glucagon, and the gonads produce the sex hormones.

Sleep and consciousness are the same machinery observed over hours. A night proceeds through non-REM stage 1 (light, with drifting theta activity), stage 2 (sleep spindles and K-complexes), stage 3 (slow-wave sleep, with large delta waves and the greatest difficulty of arousal), and then into REM, in cycles of roughly ninety minutes. Slow-wave sleep dominates the first cycles of the night and REM periods lengthen towards morning, so someone woken early in the night and someone woken near dawn report very different experiences. Timing across days is set by a circadian clock in the suprachiasmatic nucleus of the hypothalamus, which drives melatonin release from the pineal gland; jet lag and shift work are misalignments between that clock and the external cycle.

Sensation closes the domain and is the most quantitative part of it. Each sense begins with transduction, and each has a threshold structure: the absolute threshold is the stimulus intensity detected on half of trials, and the difference threshold is the smallest detectable change. Weber's law states that the difference threshold is a constant proportion of the starting intensity, not a constant amount — adding one candle to a room lit by one candle is obvious, adding one to a room lit by a hundred is not. Signal detection theory replaces the single threshold with two independent quantities: sensitivity, meaning how well the observer separates signal from noise, and criterion, meaning how much evidence the observer demands before saying 'yes'. Shifting the criterion trades false alarms against misses without changing sensitivity at all. In vision, rods dominate the periphery and support low-light, colourless vision while cones cluster in the fovea and carry colour and detail; colour vision is trichromatic at the receptors and opponent-process further along the pathway, which is why afterimages appear in the complementary colour. In audition, place along the basilar membrane codes high frequencies while firing rate codes low ones.