Central and peripheral systems — The central nervous system comprises the brain and spinal cord. The peripheral nervous system connects it with receptors, muscles and glands. These divisions describe organisation and information flow, not two systems operating independently of each other.
Somatic and autonomic divisions — The somatic division supports sensory information and control of skeletal muscles. The autonomic division regulates internal organs and glands, with sympathetic and parasympathetic branches. Voluntary versus involuntary is a useful starting distinction, but reflexes and interaction make it incomplete.
Sympathetic and parasympathetic activity — Sympathetic activity can prepare the body for action, while parasympathetic activity supports functions such as digestion and recovery. They have differing effects in particular organs rather than one being universally on and the other universally off. Coordinated activity depends on the situation.
Sensory, relay and motor neurons — Sensory neurons carry signals from receptors towards the central system, relay neurons connect processing within it, and motor neurons carry signals to effectors, the muscles or glands that produce a response. Direction is defined relative to the system and connection, not by whether a drawing's arrow points left or right.
Neuron structure — Dendrites and the cell body receive and integrate input, while an axon conducts signals towards terminals. Myelin insulates sections of many axons and supports rapid transmission between nodes. Not every neuron has the same shape, length or myelination.
Electrical signalling — An action potential is a brief change in voltage across a neuron's membrane that travels along the axon once the threshold for firing is reached. Its basic amplitude (size) is not a simple measure of stimulus intensity; firing pattern and rate can carry information. This differs from the graded postsynaptic effects that contribute to whether a neuron fires.
The synaptic junction — A synapse includes the presynaptic terminal (the sending neuron's end), a gap and a postsynaptic membrane (on the receiving cell) with relevant receptors. An electrical signal arriving at a terminal can trigger neurotransmitter release. The message crosses the chemical synapse through molecules rather than the original electrical impulse simply jumping unchanged across the gap.
Release and receptors — Neurotransmitter molecules diffuse across the cleft and bind to compatible receptors, influencing the postsynaptic cell. Receptor compatibility and intracellular processes matter. A transmitter's effect depends on receptor and circuit context, so avoid treating every transmitter as having one universal behavioural function.Two separate neurons communicate across a cleft. Excitation or inhibition depends on receptor effects; the signal does not persist indefinitely.
Excitation and inhibition — An excitatory postsynaptic effect increases the probability of firing under the relevant conditions; an inhibitory effect reduces it. Many inputs are integrated over space and time. Inhibition is not the same as switching the entire brain off, and an excitatory input need not cause an action potential on its own.
Clearing the signal — Reuptake draws a transmitter back into the releasing cell; enzymes can break it down; diffusion can carry it away from the synapse. These processes help end transmitter action, allowing signalling to change rather than accumulate indefinitely. A drug affecting reuptake alters one part of a system and does not automatically establish the original cause of a psychological difficulty.
Endocrine signalling — Endocrine glands release hormones into the bloodstream, allowing effects at distant target cells with appropriate receptors. Hormonal and neural signalling interact. Hormones often have a longer time course than individual neural impulses, but their effects are specific rather than a single general chemical influence.
Glands and control — The hypothalamus links neural information with endocrine regulation, including control of the pituitary. Pituitary hormones influence several other glands, while feedback helps regulate secretion. Calling the pituitary a master gland is shorthand that should not obscure hypothalamic control and wider feedback.
Fight or flight — Perceived threat can activate sympathetic pathways and the adrenal medulla, releasing adrenaline. Changes such as increased cardiac activity and redistribution of resources can support rapid action. The response is a coordinated physiological account, not a prediction that every threatened person literally fights or runs.
Adrenaline and recovery — Adrenaline acts through target receptors and can support physiological readiness. When the threat subsides, autonomic regulation and feedback can return activity towards an appropriate baseline. A rapid stress response and its recovery are processes, not permanently fixed personal traits.
Localisation and lateralisation
Localisation as a claim — Localisation proposes that particular brain areas make especially important contributions to particular functions. It does not mean every complex behaviour occupies one isolated spot. Functions can be distributed across networks while some components are more specialised.
Motor and somatosensory cortex — The motor cortex contributes to voluntary movement, while somatosensory cortex processes bodily sensation. Their representations are organised and often concern the opposite side of the body. The amount of cortical representation is related to processing or control demands rather than simply the physical size of a body part.
Visual and auditory areas — Visual cortex in the occipital region processes visual information, and auditory areas in the temporal region process sound. Early sensory specialisation does not imply that recognising a person or understanding a conversation is completed in one area alone.
Language networks — Broca's area is associated with aspects of speech production and language organisation; Wernicke's area is associated with aspects of comprehension. Modern interpretation places these regions in wider networks. Lesion findings support contributions but do not make the traditional labels complete accounts of language.
Lesion evidence — A deficit after damage can indicate that an area or its connections contribute to a function. Damage may be extensive, affect connected regions or cause adaptation, so a single case rarely isolates one process perfectly. Compare pre-existing differences, lesion extent and converging evidence.
Hemispheric lateralisation — Lateralisation concerns differences in the relative contributions of the hemispheres. Language is often more left-lateralised, but people vary and both hemispheres contribute to many tasks. The popular division into wholly logical left-brained and wholly creative right-brained people is not supported by that claim.
Corpus callosum — The corpus callosum is a major connection supporting communication between the hemispheres. Cutting some connections for clinical reasons creates a special research context. A split-brain participant is not simply a typical intact brain with two completely independent personalities.
Visual fields and hemispheres — Information from the left visual field initially projects predominantly to the right hemisphere, and right-field information to the left. A visual field is not the same as one eye: each eye receives information from both sides of the field. Fixation and brief presentation help control which hemisphere initially receives a stimulus.Visual-field routing concerns halves of the visual scene, not one whole eye assigned to one hemisphere. Naming and manual responses test different pathways.
Split-brain testing — A briefly presented left-field object may be difficult to name verbally if the relevant language output is left-lateralised and transfer is disrupted, while the person may select it with the left hand. The contrast concerns the tested pathway and response, not proof that the object was not perceived at all.
Evaluating split-brain evidence — Small clinical samples, differences in surgery, epilepsy histories and task demands limit generalisation. Converging results can support specialised functions while leaving substantial integration in everyday life. Consider whether the task tests naming, recognition, drawing or action before describing a hemisphere's ability.
Plasticity and functional recovery
Plasticity — Plasticity is the nervous system's capacity to change with experience, learning or injury. Change can involve the strength or organisation of connections and patterns of activity. It does not mean unlimited recovery or that every structural difference is caused by the experience with which it is associated.
Experience and evidence — Associations between expertise and brain characteristics can suggest experience-related change, but pre-existing differences and selection are alternatives. Longitudinal measurements and appropriate comparisons provide stronger evidence of change. Establishing change still does not identify every cellular mechanism.
Recovery after trauma — Functional recovery concerns regaining abilities after injury, potentially through restoration, reorganisation, compensation or rehabilitation. Improvement in a task does not necessarily mean the original tissue regenerated. Distinguish recovery of a function from a new strategy that achieves the same outcome.
Unmasking and reorganisation — Previously less-used pathways may contribute more after damage, and nearby or connected areas may alter their roles. Collateral sprouting describes growth of new branches from surviving axons. These are possible mechanisms requiring evidence, not automatic explanations of every improvement.
Factors influencing outcome — Age, injury location and extent, health, experience, support and rehabilitation can influence recovery. A group association with age or education does not fix an individual's potential. Plastic change can also be maladaptive, so greater change is not always better.
Methods for studying the brain
Choosing a method — Brain methods answer different questions about structure, activity and timing. Compare spatial resolution (how precisely a location can be identified), temporal resolution (how precisely timing can be tracked), inference, invasiveness (how much a procedure enters or disrupts the body), experimental control and relevance to the task. No one method is best for every research question.
fMRI and the BOLD signal — Functional magnetic resonance imaging uses changes related to blood oxygenation as an indirect indicator of activity. It can locate task-related patterns with useful spatial detail, but the blood-flow response (haemodynamic response) is slower than neural firing. BOLD stands for blood-oxygen-level-dependent, naming the signal used as this indirect measure. More signal is not a direct count of thoughts or a simple proof that a region alone caused a task.
fMRI design and inference — A task comparison can help identify activity associated with a process, but conditions may differ in several cognitive demands. Inferring one precise mental state from activity in a region is risky if that region participates in many processes. Motion, analysis choices and sampling also affect interpretation.
EEG — Electroencephalography records voltage differences at the scalp arising from coordinated neural activity. It tracks changes rapidly but has limited precision in locating deep or closely spaced sources. The signal is not a direct recording of every individual neuron.
ERPs — Event-related potentials are patterns extracted from EEG time-locked to repeated events, often by averaging. Averaging reduces unrelated activity while preserving consistent timing. An ERP component's latency and amplitude need interpretation within the task; a label alone does not reveal a unique psychological process.
Post-mortem examination — Post-mortem research examines tissue after death and can investigate detailed structure or pathology. It cannot directly observe the person's ongoing task processing. Cause of death, illness history, treatment and retrospective behavioural information can complicate interpretation.
Converging methods — Rapid electrical measures and spatially informative imaging can constrain complementary aspects of a theory. Lesion or intervention evidence can add information beyond association, with its own limitations. A strong conclusion explains how the methods converge rather than simply claiming that more methods guarantee truth.
Legacy requirement: biological rhythms and sleep
Rhythm timescales — Circadian rhythms cycle approximately every 24 hours, ultradian rhythms recur more than once within a day, and infradian rhythms have periods longer than a day. Classification depends on cycle duration rather than the size of the response. A sleep-wake cycle and the changing stages within a night's sleep therefore illustrate different timescales.
Circadian sleep-wake cycle — The sleep-wake pattern is influenced by internal timing and environmental cues alongside sleep pressure and behaviour. Isolation from normal time cues can reveal a free-running period, but artificial light, activity and prior routines may still influence it. One person's cave study is detailed evidence, not a population estimate or complete separation of all cues.
Ultradian sleep stages — During a typical night, non-REM and rapid-eye-movement (REM) stages recur in cycles often described as roughly 90 minutes, with substantial variation. Deep non-REM sleep tends to be more prominent earlier and REM periods often lengthen later. Brain activity, eye movements and muscle tone help identify stages; a person does not pass through a single stage once and remain there all night.
Sleep evidence and applications — EEG records can be related to eye movement, waking reports and laboratory awakenings, but waking can disrupt the process being measured. A correlation between REM and dream reports does not establish that all dreaming occurs only in REM. Sleep diaries and actigraphy (using a wearable movement recorder to estimate rest and activity patterns) provide different kinds of information, each with measurement limits.
Infradian rhythms — The menstrual cycle is a common infradian example, with hormonal regulation and individual variation in length. Seasonal patterns can also illustrate longer cycles. Claims that menstrual cycles reliably synchronise through cohabitation require strong evidence; chance overlap, variable cycle length and retrospective reporting can create an apparent pattern.
Endogenous pacemakers — An endogenous pacemaker is an internal timing mechanism. The suprachiasmatic nucleus in the hypothalamus coordinates circadian timing using information about light from the retina. It influences downstream systems, including the timing of melatonin release from the pineal gland through a multi-step pathway. Melatonin signals biological night rather than acting as the sole master clock.A functional route with intermediate neural steps omitted, not a direct anatomical wire from SCN to pineal. Sleep pressure also affects sleep.
Exogenous zeitgebers — Exogenous zeitgebers are external timing cues, especially light, that entrain internal rhythms to environmental time. Social schedules and activity can contribute to timing too. Entrainment is adjustment of phase and synchronisation, not evidence that the internal clock ceases to exist when a cue is removed.
Evaluating pacemaker evidence — Lesion and transplant research in animals can test timing mechanisms more directly than a human association, but disruption may affect several functions and extrapolation needs caution. Ethical costs and species differences matter. Converging human isolation and light-exposure findings can strengthen an account without making a single animal experiment proof of every human sleep problem.
Timing and individual variation — Chronotype (a person's tendency towards earlier or later daily sleep and activity timing), age, shift work and travel can influence the relation between internal timing and external demands. A controlled light study should specify intensity, duration and timing rather than assume all light has an identical effect. Observed tiredness may reflect sleep loss as well as circadian mismatch, so separate the proposed mechanisms.
Test yourself
50 questions · Random sets of 10. These quick checks support revision; practise longer explanations and justified judgements too.
Mr Kidman's Classroom
