Serotonin (5-Hydroxytryptamine / 5-HT) is a monoamine neurotransmitter produced mainly in the Raphe nuclei of the brainstem. Functions: Mood regulation (low serotonin linked to depression and anxiety), Sleep regulation (converted to melatonin in the pineal gland), Appetite suppression, Pain modulation (descending pain control), GI motility (90% of body's serotonin is in the GI tract - enterochromaffin cells), and Platelet aggregation. SSRIs (Selective Serotonin Reuptake Inhibitors) - fluoxetine (Prozac), sertraline - block serotonin reuptake causing increased synaptic serotonin - treat depression, anxiety, OCD, and PTSD. Serotonin Syndrome: Excess serotonin causes fever, agitation, hyperreflexia, clonus - potentially fatal.

The Sciatic Nerve is the largest (widest) and longest nerve in the human body. Origin: Sacral plexus (L4, L5, S1, S2, S3). It exits the pelvis through the greater sciatic foramen (below piriformis) and runs down the posterior thigh. At the popliteal fossa, divides into: Common peroneal (fibular) nerve (supplies anterior and lateral leg - dorsiflexion, eversion) and Tibial nerve (supplies posterior leg and foot - plantarflexion, toe flexion). Sciatica: Compression of sciatic nerve or its nerve roots (usually L4-L5 or L5-S1 disc prolapse) causes pain radiating from back down the leg (dermatomal distribution), numbness, and tingling. Treatment: physiotherapy, NSAIDs, epidural steroids, and surgery (microdiscectomy). Piriformis syndrome: Piriformis muscle compresses sciatic nerve.

Proprioception (from Latin proprius = one's own) is the sense of the body's position and movement in space - sometimes called the sixth sense. Receptors: Muscle spindles (detect muscle length and stretch - trigger stretch reflexes), Golgi tendon organs (GTO) (detect muscle tension - trigger protective inhibitory reflexes to prevent over-contraction), and Joint receptors (detect joint position and movement). Proprioceptive signals travel via the dorsal column-medial lemniscal pathway (fine touch, proprioception, vibration) to the brain. Cerebellum uses proprioceptive input to coordinate movement. Loss of proprioception causes sensory ataxia (unsteady with eyes closed - positive Romberg's test). Common in vitamin B12 deficiency (subacute combined degeneration of the cord).

The Cerebral Aqueduct (Aqueduct of Sylvius) is a narrow canal (~3 mm diameter) passing through the midbrain that connects the third ventricle (above) to the fourth ventricle (below), allowing CSF to flow between them. Ventricular system: Lateral ventricles (in cerebral hemispheres) > via foramina of Monro > 3rd ventricle (diencephalon) > via cerebral aqueduct > 4th ventricle (brainstem/cerebellum) > via foramina of Magendie and Luschka > subarachnoid space. Aqueduct stenosis (congenital or acquired) = most common cause of obstructive hydrocephalus - blockage prevents CSF flow - pressure builds in ventricles above - raised ICP.

Dopamine Pathways in the brain: 1) Nigrostriatal = Substantia nigra > Striatum - Movement control (degenerated in Parkinson's). 2) Mesolimbic = Ventral tegmental area (VTA) > Nucleus accumbens - Reward, pleasure, motivation, addiction (the pleasure centre). 3) Mesocortical = VTA > Prefrontal cortex - Cognition, working memory, executive function (low in schizophrenia - cognitive symptoms). 4) Tuberoinfundibular = Hypothalamus > Pituitary - Inhibits prolactin secretion. Addiction: All addictive substances (alcohol, cocaine, heroin, nicotine) and behaviours (gambling, gaming) activate the mesolimbic dopamine system causing dopamine surge in nucleus accumbens, reward learning, craving, and compulsive use despite consequences. Antipsychotics block dopamine receptors (mainly D2) to reduce psychosis but can cause movement side effects.

The Temporal Lobe (located below the lateral sulcus/Sylvian fissure) contains the primary auditory cortex (Heschl's gyrus - Brodmann areas 41 and 42) which processes hearing. It is also responsible for: Memory (hippocampus is in the medial temporal lobe), Language comprehension (Wernicke's area - posterior superior temporal gyrus - dominant hemisphere), Emotion (connected to amygdala), Face recognition (fusiform face area), and Smell (piriform cortex). Temporal lobe damage: Wernicke's aphasia (fluent but meaningless speech, poor comprehension), memory problems, and auditory agnosia (cannot recognise sounds). Temporal lobe epilepsy: Most common form - deja vu, automatisms, olfactory hallucinations (uncinate fits).

A TIA (Transient Ischaemic Attack) is a temporary disruption of blood supply to the brain causing brief neurological symptoms that resolve completely within 24 hours (usually within minutes). It is caused by a small clot that temporarily blocks a cerebral artery and then dissolves/moves. Symptoms: Same as stroke (weakness, speech difficulty, visual loss, dizziness) but fully reversible. Significance: Strong warning sign for impending stroke - 10% risk of stroke within 48 hours, 15-20% within 90 days. ABCD2 score assesses short-term stroke risk after TIA. Treatment: Aspirin + Clopidogrel (dual antiplatelet), statin, blood pressure control, carotid endarterectomy if stenosis present. Urgent investigation required.

A Ganglion (plural: ganglia) is a cluster of neuron cell bodies located outside the CNS (in the PNS). Types: Dorsal root (spinal) ganglia = Contain cell bodies of sensory neurons - located on dorsal roots of spinal nerves. Herpes zoster virus lies dormant here. Autonomic ganglia = Contain cell bodies of postganglionic autonomic neurons. Sympathetic ganglia: Paravertebral (sympathetic chain) and prevertebral (e.g., coeliac ganglion). Parasympathetic ganglia: Located near or within target organs (e.g., ciliary, pterygopalatine, submandibular, otic). Cranial nerve ganglia: (e.g., trigeminal ganglion/Gasserian ganglion - sensory cell bodies of CN V). In the CNS, equivalent clusters of cell bodies are called nuclei (not ganglia).

The Parietal Lobe is located between the frontal lobe (separated by the central sulcus) and the occipital lobe. Functions: Primary Somatosensory Cortex (postcentral gyrus, Brodmann areas 3, 1, 2) = Processes touch, pressure, pain, temperature, proprioception from the contralateral body. Somatosensory association cortex = Integrates sensory information, body image. Spatial processing = Visuospatial awareness and navigation. Calculation and language = Angular gyrus (reading and calculation). Parietal lobe damage (dominant - usually left): Gerstmann syndrome (finger agnosia, acalculia, agraphia, left-right disorientation) and Ideomotor apraxia. Non-dominant (right): Hemispatial neglect (ignores left side of space/body), dressing apraxia, and constructional apraxia.

The Olfactory Nerve (CN I) is the first cranial nerve and carries the special sense of smell (olfaction). It is the shortest cranial nerve and the only sensory nerve that does NOT relay through the thalamus before reaching the cortex (goes directly to the olfactory cortex/piriform cortex). Pathway: Olfactory receptor neurons in the nasal mucosa (roof of nasal cavity) > axons pass through cribriform plate of ethmoid bone > olfactory bulb > olfactory tract > olfactory cortex (medial temporal lobe). Anosmia (loss of smell): Caused by head trauma (cribriform plate fracture), COVID-19 (commonly), nasal polyps, Parkinson's disease (early sign), Alzheimer's disease, and zinc deficiency. Smell is closely linked to memory and emotion (limbic system).