- Hippocampus
- A seahorse-shaped structure in the medial temporal lobe, critical for the formation of new explicit (declarative) memories — both episodic (personal events) and semantic (facts). Long-term potentiation (LTP) — the synaptic strengthening mechanism underlying memory formation — was first demonstrated in hippocampal tissue. The hippocampus also plays a key role in spatial navigation (cognitive maps) and is one of the few brain regions where neurogenesis continues in adults. Bilateral hippocampal damage produces anterograde amnesia: inability to form new long-term memories (famously illustrated by patient H.M., Henry Molaison).
- Amygdala
- An almond-shaped structure in the anterior medial temporal lobe, involved in processing emotionally significant stimuli — particularly fear, threat, and reward. The amygdala modulates memory consolidation for emotionally arousing events (flashbulb memories), responds rapidly to faces expressing fear (via a fast subcortical "low road" bypassing the cortex), and triggers autonomic fear responses (fight-or-flight). Hyperactivation is associated with anxiety disorders and PTSD; hypoactivation (or bilateral damage) produces Urbach-Wiethe disease, characterised by fearlessness.
- Thalamus
- A paired structure sitting at the top of the brainstem, acting as the brain's main relay station. Virtually all sensory information (except smell) travels through the thalamus before reaching the cortex. Specific thalamic nuclei relay sensory modalities: the lateral geniculate nucleus (vision), medial geniculate nucleus (audition), and ventral posterior nucleus (touch). The thalamus also participates in regulating consciousness and arousal (through connections with the reticular formation) and is involved in certain memory circuits (thalamic amnesia).
- Hypothalamus
- A small but vital structure at the base of the forebrain (below the thalamus), controlling autonomic functions and hormone release. It regulates hunger, thirst, body temperature, sleep-wake cycles, sexual behaviour, and the stress response. The hypothalamus controls the pituitary gland (the "master gland") via releasing hormones and direct neural connections, making it the link between the nervous system and the endocrine system. The HPA axis (hypothalamus–pituitary–adrenal cortex) is the central pathway of the stress response.
- Basal ganglia
- A collection of interconnected nuclei (caudate nucleus, putamen, globus pallidus, subthalamic nucleus, substantia nigra) deep within the cerebral hemispheres. The basal ganglia are critical for initiating and smoothly executing voluntary movements, for procedural (habit) learning, and for action selection. The cortico-striato-thalamo-cortical loops are disrupted in Parkinson's disease (loss of dopaminergic neurons in the substantia nigra → bradykinesia, rigidity, tremor) and Huntington's disease (striatal degeneration → chorea). The basal ganglia are also implicated in OCD and reward-based decision-making.
- Corpus callosum
- The largest white matter commissure, connecting the two cerebral hemispheres across the longitudinal fissure. Contains approximately 200 million axon fibres. Enables the hemispheres to share and integrate information. Complete or partial agenesis of the corpus callosum (present from birth) can occur without obvious symptoms, revealing remarkable plasticity; surgical callosotomy (severing for epilepsy control) produces the split-brain syndrome studied by Sperry and Gazzaniga.
- Limbic system
- A loosely defined set of structures forming a ring ("limbus") around the brainstem, proposed by Paul MacLean (1952) as the emotional brain. Typically includes the hippocampus, amygdala, cingulate cortex, fornix, mammillary bodies, and septal nuclei. Modern neuroscience regards it as an oversimplification — these regions have highly diverse functions and are interconnected with the entire brain. The concept remains useful heuristically but has been largely replaced by specific network analyses.