The Neuromuscular Junction (NMJ) is the specialised synapse between the terminal of a somatic motor neuron and a skeletal muscle fibre. Mechanism: Action potential arrives causing Ca2+ influx, Acetylcholine (ACh) released from motor nerve terminal, ACh binds nicotinic receptors on motor end plate of muscle, Depolarisation causes muscle action potential and muscle contraction, and Acetylcholinesterase rapidly breaks down ACh to terminate the signal. Diseases: Myasthenia Gravis (autoimmune - anti-AChR antibodies - reduced receptors - muscle fatigue). Lambert Eaton syndrome affects presynaptic calcium channels. Botulinum toxin (Botox) blocks ACh release causing flaccid paralysis. Suxamethonium (succinylcholine) is a depolarising NMJ blocker used for intubation.

A Dermatome is an area of skin supplied by sensory fibres from a single dorsal root (spinal nerve). Dermatomal maps allow clinicians to localise neurological lesions: C3-C4 = Neck and shoulder, T4 = Nipple level, T10 = Umbilicus level, L1 = Inguinal region, L4 = Medial lower leg (knee jerk reflex), S1 = Lateral foot (ankle jerk reflex), and S3-S5 = Perineum and genitalia. Clinical uses: Radiculopathy (nerve root compression - disc prolapse) causes pain/sensory loss in a dermatomal pattern. Shingles (Herpes Zoster): Rash follows a single dermatome - typically thoracic (bandlike chest pain and rash). Testing sensation in dermatomal distribution helps localise spinal cord injury level.

Phantom Limb Pain (PLP) is the sensation of pain in a limb that has been amputated - the brain still receives and misinterprets signals as coming from the absent limb. Mechanisms: Cortical reorganisation (after amputation, the somatosensory cortex representing the amputated limb is invaded by adjacent cortical areas causing abnormal processing and pain), Aberrant peripheral nerve signals (neuromas/stump nerve endings generate spontaneous signals), and Spinal cord changes (central sensitisation). Treatment: Mirror therapy (V.S. Ramachandran) = Patient performs movements with the intact limb reflected in a mirror causing brain to see the phantom limb move normally and reducing pain. Also: Amitriptyline, Gabapentin, Pregabalin, Ketamine, and Virtual Reality. Mirror therapy exploits neuroplasticity to remap the cortex.

Myasthenia Gravis (MG) (grave muscular weakness) is an autoimmune disease caused by antibodies against nicotinic acetylcholine receptors (AChR) at the neuromuscular junction (or against MuSK - muscle-specific kinase - in seronegative MG). Consequences: Reduced functional AChRs causes less muscle response to ACh and muscle weakness. Characteristic feature: Fatigable weakness (weakness that WORSENS with repeated use and improves with rest). Common presentations: Ptosis (drooping eyelids), diplopia (double vision), bulbar weakness (dysarthria, dysphagia), and limb weakness. Myasthenic crisis: Life-threatening respiratory failure. Investigation: Tensilon (edrophonium) test, anti-AChR antibodies, CT thorax (associated thymoma in 15%). Treatment: Pyridostigmine (acetylcholinesterase inhibitor), steroids, thymectomy, immunosuppressants.

The Gate Control Theory of Pain was proposed by Melzack and Wall (1965). Key concept: A gate in the dorsal horn of the spinal cord (substantia gelatinosa) can open or close pain transmission. Gate CLOSES (pain inhibited): Large-diameter A-beta fibres (touch, vibration) activate inhibitory interneurons, inhibit pain transmission (A-delta and C fibres), and less pain reaches brain. Gate OPENS (pain transmitted): Small-diameter fibres (pain - A-delta and C) cause pain signals to pass to brain. This explains: Rubbing an injured area reduces pain (stimulates large fibres - closes gate). Clinical applications: TENS (Transcutaneous Electrical Nerve Stimulation) = stimulates large fibres - closes pain gate. Spinal cord stimulation for chronic pain. Cognitive factors (anxiety, attention) also modulate the gate.

A Lumbar Puncture (LP / Spinal Tap) involves inserting a needle into the subarachnoid space at the L3-L4 or L4-L5 interspace (below the level of the spinal cord - conus medullaris ends at L1-L2) to collect CSF. The safest level is below L2 to avoid spinal cord injury (only the cauda equina - nerve roots - are present at this level). CSF analysis diagnoses: Bacterial meningitis (cloudy CSF, raised WBCs/neutrophils, high protein, low glucose), Viral meningitis (clear CSF, raised WBCs/lymphocytes, mildly raised protein, normal glucose), Subarachnoid haemorrhage (xanthochromic/yellow CSF and blood), Multiple Sclerosis (oligoclonal bands/IgG), and Cryptococcal meningitis (India ink stain positive). Raised ICP: LP is contraindicated (risk of brain herniation - must do CT first to exclude mass).

Ascending Tracts (sensory pathways) carry information from the body TO the brain. Main ascending tracts: Dorsal Column-Medial Lemniscal (DCML) pathway = Carries fine touch, vibration, proprioception, two-point discrimination. Travels ipsilaterally in dorsal columns, decussates in medulla, then goes to contralateral cortex. Spinothalamic tract (Anterolateral system) = Carries pain, temperature, crude touch, pressure. Crosses at the SAME spinal cord level (within 1-2 segments), travels contralaterally, goes to thalamus then cortex. Brown-Sequard Syndrome (hemisection of spinal cord): Ipsilateral loss of proprioception/fine touch + motor loss; Contralateral loss of pain/temperature - because these tracts cross at different levels.

The Pupillary Light Reflex is a critical clinical reflex. Afferent limb: Optic nerve (CN II) carries light signal from retina to pretectal nucleus in midbrain. Efferent limb: Oculomotor nerve (CN III, parasympathetic fibres) signals constrictor pupillae causing pupil constriction. Both pupils constrict when light shines in ONE eye: direct reflex (same eye) and consensual reflex (opposite eye). Clinical significance: Absent direct reflex but intact consensual = CN II (afferent) damage on that side. Absent both reflexes in one eye = CN III (efferent) damage on that side (e.g., posterior communicating artery aneurysm). Fixed dilated pupils bilaterally = severe brainstem damage, herniation, or drugs (atropine). Used in ICU/emergency to assess level of consciousness and brainstem function.

The Nervous System is the body's most complex and vital system. Summary of functions: 1) Sensory function (detects changes/stimuli inside and outside the body), 2) Integration (processes, interprets, and stores information - memory, learning), 3) Motor function (activates muscles and glands to produce responses), 4) Homeostasis (regulates temperature, blood pressure, breathing, heart rate via ANS), 5) Higher functions (consciousness, thought, language, creativity, emotion, personality), 6) Endocrine control (hypothalamus controls the pituitary and entire endocrine system), and 7) Immune modulation (neuroimmune interactions). The nervous system works in milliseconds (action potentials) compared to the endocrine system (hormones - minutes to hours). Together they form the neuroendocrine system - the body's dual control network. Without the nervous system, no organ system could function in a coordinated manner.

Descending Tracts (motor pathways) carry commands FROM the brain TO muscles. Main descending tracts: Corticospinal Tract (Pyramidal tract) = Most important voluntary motor pathway. Arises from motor cortex, passes through internal capsule, through brainstem, decussates at pyramidal decussation (medulla), then lateral corticospinal tract controls distal limb muscles (fine motor control). Corticobulbar tract = Controls cranial nerve motor nuclei (facial expression, speech, swallowing). Extrapyramidal tracts (Rubrospinal, reticulospinal, vestibulospinal, tectospinal) = Control posture, balance, and gross movements. UMN lesion (above anterior horn): Spastic paralysis, hyperreflexia. LMN lesion (at/below anterior horn): Flaccid paralysis, wasting, fasciculations.