Demyelination is the loss or damage of the myelin sheath around axons. Consequences: Slowed nerve conduction velocity (saltatory conduction lost - impulse can no longer jump between nodes), Conduction block (complete failure of impulse transmission in severe cases), and Neurological deficits (weakness, sensory loss, visual problems, coordination difficulties). Primary demyelinating diseases: CNS = Multiple Sclerosis (MS) - autoimmune; Acute Disseminated Encephalomyelitis (ADEM) - post-viral. PNS = Guillain-Barre Syndrome (GBS) - acute; Charcot-Marie-Tooth disease - hereditary. Remyelination: Can occur (especially in PNS via Schwann cells) causing partial recovery of function. In the CNS, remyelination is limited. Oligodendrocyte precursor cells attempt remyelination in MS lesions but often incompletely.

The Frontal Lobe is the largest lobe of the brain (anterior to the central sulcus). Functions: Primary Motor Cortex (precentral gyrus) = Controls voluntary movement of contralateral body. Premotor cortex = Motor planning. Prefrontal cortex = Executive functions (planning, decision-making, impulse control, working memory, personality, social behaviour). Broca's area (dominant hemisphere) = Motor speech production. Frontal eye fields = Voluntary eye movement. Frontal lobe damage: Personality changes (Phineas Gage case), executive dysfunction, Broca's aphasia, urinary incontinence (loss of cortical inhibition of bladder), grasp reflex, and utilisation behaviour. Frontal lobe dementia (FTD - Frontotemporal Dementia): Personality change, disinhibition, early in disease.

Dementia vs Delirium: Dementia = Gradual onset (months/years), Progressive course, Normal consciousness (early), Relatively preserved attention, Usually NOT reversible. Causes: Alzheimer's (most common), Vascular, Lewy body, Frontotemporal. Delirium (acute confusional state) = Acute onset (hours/days), Fluctuating course, Impaired consciousness, Severely impaired attention, and Usually REVERSIBLE. Causes: PINCHME mnemonic = Pain, Infection, Nutrition, Constipation, Hydration, Medication, Environment. Elderly with dementia are particularly prone to delirium when unwell. Delirium is associated with increased mortality and longer hospital stays - early identification and treatment of underlying cause is essential.

The Vestibulocochlear Nerve (CN VIII) - also called the Auditory/Acoustic nerve - has two functional divisions: Cochlear division = Carries hearing signals from the organ of Corti (in the cochlea of the inner ear) > cochlear nuclei in the brainstem > auditory cortex (temporal lobe). Vestibular division = Carries balance and equilibrium signals from the semicircular canals and otolith organs (utricle, saccule) in the inner ear > vestibular nuclei/cerebellum. Disorders: Sensorineural deafness (cochlear nerve damage - noise-induced, drugs/ototoxicity, acoustic neuroma), Vestibular neuritis (inflammation causing severe vertigo, nausea, no hearing loss), and Acoustic neuroma (Vestibular schwannoma) = benign tumour of CN VIII Schwann cells causing unilateral deafness, tinnitus, vertigo; associated with Neurofibromatosis type 2 (NF2).

Wernicke's Aphasia (Receptive/Sensory aphasia) results from damage to Wernicke's area (posterior superior temporal gyrus, Brodmann area 22) in the dominant (usually left) hemisphere. Features: Fluent speech (speaks easily but output is meaningless - word salad/jargon aphasia), Paraphasias (substituting wrong words - semantic or phonemic errors), Poor comprehension (cannot understand spoken or written language), and Patient is often unaware of their deficit. Compare with Broca's aphasia (expressive): Non-fluent, effortful speech but relatively preserved comprehension. Named after Carl Wernicke (German neurologist, 1874). A stroke in the middle cerebral artery territory commonly causes Wernicke's or Broca's aphasia depending on which branch is occluded.

Endorphins (endogenous morphine) are neuropeptides produced naturally in the CNS and pituitary gland. They bind to opioid receptors (mu, kappa, delta) in the brain and spinal cord - the same receptors targeted by morphine and other opioid drugs. Functions: Pain relief (analgesia) - reduce pain perception (activate descending pain inhibitory pathways), Euphoria (runner's high after intense exercise), Stress response (released during physical stress), and Reward and pleasure. Types: Beta-endorphin (most potent), enkephalins, dynorphins. Opioid drugs (morphine, heroin, codeine) mimic endorphins. Naloxone (opioid antagonist) blocks endorphin/opioid receptors to reverse opioid overdose. Acupuncture may work partially through endorphin release.

Adult Neurogenesis - the generation of new neurons in the adult brain - was once thought impossible but is now established in specific neurogenic niches: 1) Subgranular Zone (SGZ) of the hippocampal dentate gyrus = New neurons integrate into hippocampal circuits - important for memory and learning. 2) Subventricular Zone (SVZ) > neurons migrate to the olfactory bulb - important for smell discrimination (this pathway is less prominent in humans than in rodents). Factors that promote neurogenesis: Exercise, learning, enriched environment, antidepressants (SSRIs). Factors that inhibit neurogenesis: Chronic stress, corticosteroids, alcohol, and ageing. Importance: Adult hippocampal neurogenesis may be important for depression, anxiety, and memory - target for antidepressant action. Impaired in Alzheimer's disease.

Glial cells (Neuroglia) are non-neuronal supporting cells that outnumber neurons approximately 10:1 in the brain. Types: Astrocytes (largest, most numerous glia - structural support, form BBB, regulate synaptic environment, provide nutrients, scar formation after injury), Oligodendrocytes (CNS) = Form myelin sheath in CNS (each can myelinate up to 50 axons), Schwann cells (PNS) = Form myelin in PNS (each covers ONE segment of ONE axon), Microglia = CNS resident immune cells (phagocytose debris and pathogens), Ependymal cells = Line ventricles and central canal (produce and circulate CSF), and Radial glia = Guide neuronal migration during development. Gliomas: Tumours arising from glial cells (most common primary brain tumours). Glioblastoma multiforme (GBM): Most aggressive brain tumour.

A Nerve Plexus is a network of intersecting nerves formed by the anterior (ventral) rami of spinal nerves merging and redistributing into named peripheral nerves. Major nerve plexuses: Cervical plexus (C1-C4) = Supplies neck and diaphragm (phrenic nerve - C3,4,5). Brachial plexus (C5-T1) = Supplies entire upper limb (musculocutaneous, median, ulnar, radial, axillary nerves). Lumbar plexus (L1-L4) = Supplies anterior thigh (femoral, obturator nerves). Sacral plexus (L4-S3) = Supplies posterior thigh and entire leg (Sciatic nerve - largest nerve in the body). Erb's palsy: Injury to upper brachial plexus (C5-C6) causing waiter's tip arm position. Klumpke's palsy: Lower brachial plexus (C8-T1) causing claw hand.

The Oculomotor Nerve (CN III) has two components: Somatic motor = Controls 4 of the 6 extraocular muscles: Superior rectus (up), Inferior rectus (down), Medial rectus (inward/adduction), Inferior oblique (up and out); and Levator palpebrae superioris (elevates upper eyelid). Parasympathetic = Controls sphincter pupillae (pupil constriction - miosis) and ciliary muscle (accommodation/near vision). CN III palsy signs: Down and out eye (only lateral rectus/CN VI and superior oblique/CN IV working), ptosis (drooping eyelid), and dilated unresponsive pupil (mydriasis - parasympathetic fibres run on the outside of CN III and are compressed first). Most dangerous cause: Posterior communicating artery (PCom) aneurysm - surgical emergency. Also: Uncal herniation (raised ICP) and diabetic CN III palsy (pupil-sparing - ischaemia of inner nerve fibres).