The Cerebral Cortex is the thin outermost layer of grey matter (2-4 mm thick) covering the cerebral hemispheres. It is highly folded (convoluted) forming gyri (ridges) and sulci (grooves) - dramatically increasing surface area (~2500 cm2). Functional areas: Motor cortex (precentral gyrus - frontal lobe), Somatosensory cortex (postcentral gyrus - parietal lobe), Visual cortex (occipital lobe), Auditory cortex (temporal lobe), and Association areas for higher functions (reasoning, planning, language).

The Blood-Brain Barrier (BBB) is formed by tight junctions between brain capillary endothelial cells, supported by astrocyte end-feet and pericytes. Allows: O2, CO2, glucose (GLUT1), water, lipid-soluble molecules (alcohol, anaesthetics, psychoactive drugs). Blocks: most large molecules, proteins, most bacteria, and many drugs (major challenge for CNS drug delivery). Absent in circumventricular organs (area postrema/vomiting centre, hypothalamus, pituitary). Clinical relevance: CNS drug design must account for BBB penetration. Meningitis disrupts BBB causing brain oedema.

GABA (Gamma-Aminobutyric Acid) is the principal inhibitory neurotransmitter in the CNS. It acts by opening Cl- channels causing hyperpolarisation and reduced neuronal excitability. Drugs that enhance GABA activity: Benzodiazepines (diazepam/Valium - anti-anxiety, sedative, anticonvulsant), Barbiturates (phenobarbitone - sedative, anticonvulsant), and Alcohol (enhances GABA causing CNS depression). Reduced GABA activity causes seizures (epilepsy). Glutamate is the main excitatory neurotransmitter in the CNS. The balance between GABA (inhibitory) and Glutamate (excitatory) is critical for normal brain function.

The Corpus Callosum is the largest white matter commissure in the brain - a thick band of approximately 200-250 million myelinated nerve fibres that connects the left and right cerebral hemispheres, allowing them to communicate and share information. It is located in the longitudinal fissure between the two hemispheres. Functions: Allows interhemispheric communication and transfers sensory, motor, and cognitive information between hemispheres. Split-brain syndrome: Surgical cutting of corpus callosum (corpus callosotomy) done for severe epilepsy results in the two hemispheres acting independently.

The Spinal Cord extends from the foramen magnum (base of skull) to approximately L1-L2 level in adults (conus medullaris). It is approximately 45 cm long. Functions: Conducts sensory signals from body to brain (ascending tracts), Conducts motor signals from brain to body (descending tracts), and Mediates spinal reflexes. Cross-sectional anatomy: Grey matter (H-shaped) = centre (contains neuron cell bodies). White matter = surrounding (contains myelinated ascending/descending tracts). Dorsal horn (sensory), Ventral horn (motor), Lateral horn (autonomic - in thoracic/lumbar cord).

The Cerebrum is the largest part of the brain (~85% of brain weight). Its outer layer, the Cerebral Cortex (grey matter, 2-4 mm thick), is responsible for: Voluntary movement (Motor cortex - frontal lobe), Sensory perception (Somatosensory cortex - parietal lobe), Language (Broca's area - speech production; Wernicke's area - speech comprehension), Memory, learning, reasoning, consciousness, Vision (occipital lobe), and Hearing (temporal lobe). The cerebrum is divided into two hemispheres by the longitudinal fissure, connected by the corpus callosum.

Broca's Area (Brodmann areas 44 and 45) is located in the posterior inferior frontal gyrus of the dominant (usually left) hemisphere. Named after French surgeon Paul Broca (1861). Function: Motor speech production - coordinates movements of lips, tongue, and larynx needed for speaking. Broca's Aphasia (Expressive/Motor aphasia): Damage causes difficulty producing speech (non-fluent, telegraphic speech) but relatively preserved comprehension. Compare: Wernicke's area (temporal lobe) = speech comprehension - damage causes fluent but meaningless speech.

Spinal Cord Injury (SCI) damages the spinal cord, causing loss of function below the level of injury: Tetraplegia (Quadriplegia) = Injury at cervical level causing deficits in ALL four limbs + trunk. Paraplegia = Injury at thoracic/lumbar level affecting lower limbs only. Complete vs Incomplete: Complete = total loss of function below injury; Incomplete = some preservation. ASIA scale (A-E) grades severity. Complications: Neurogenic shock, Autonomic dysreflexia, Pressure sores, DVT, Respiratory failure (if C3-C5 involved - diaphragm paralysis). Spinal cord cannot regenerate. Treatment: Stabilisation, rehabilitation.

The Cerebellum (little brain) is located at the posterior cranial fossa. It represents only ~10% of brain volume but contains ~50% of all brain neurons. Functions: Coordination of voluntary movements (ensures smooth, precise movements), Balance and posture (works with the vestibular system), Motor learning (learning new motor skills - riding a bike, playing piano), and Fine-tuning of movement. Cerebellar damage causes: Ataxia (unsteady gait, incoordination), Dysdiadochokinesia (inability to perform rapid alternating movements), Intention tremor, Nystagmus, and Dysarthria (slurred speech).

The Hippocampus (Greek: sea horse) is a curved structure in the medial temporal lobe, part of the limbic system. Functions: Memory consolidation (converts short-term memories to long-term memories), Spatial navigation (contains place cells - mental map of the environment), and Learning new information. Alzheimer's disease: Hippocampus is one of the first areas damaged causing early memory loss (anterograde amnesia). HM (Henry Molaison): Famous case - bilateral hippocampectomy for epilepsy caused severe anterograde amnesia. Chronic stress/cortisol can damage the hippocampus (reduced neurogenesis).