Upper Motor Neuron (UMN) lesion features: Spastic paralysis (increased tone), Hyperreflexia (exaggerated reflexes), Positive Babinski sign (big toe extends upward - abnormal in adults), No muscle wasting (initially), and Clonus. Causes: Stroke, MS, spinal cord compression above the lesion. Lower Motor Neuron (LMN) lesion features: Flaccid paralysis (reduced/absent tone), Hyporeflexia/Areflexia, Negative Babinski, Muscle wasting (atrophy), and Fasciculations (spontaneous muscle twitching). Causes: Polio, Guillain-Barre syndrome, nerve compression.

The Medulla Oblongata is the lowest part of the brainstem, continuous with the spinal cord. It contains vital autonomic control centres: Cardiac centre (regulates heart rate and force), Vasomotor centre (regulates blood vessel tone and blood pressure), Respiratory centre (controls breathing rhythm with the pons), and Reflex centres (coughing, sneezing, swallowing, vomiting, hiccupping). The medulla is known as the vital centre because damage to it is rapidly fatal. It contains the pyramidal decussation where corticospinal tracts cross - explaining why the left brain controls the right side of the body and vice versa.

The Limbic System is a set of interconnected brain structures forming the emotional brain. Key structures: Hippocampus (memory formation, spatial navigation), Amygdala (fear, aggression, emotional processing), Hypothalamus (homeostasis, drives - hunger, thirst, sex), Cingulate cortex (attention, emotion-cognition integration), Olfactory bulb (smell - directly connected to limbic system - explains why smells evoke powerful memories), and Mammillary bodies (memory - damaged in Korsakoff's syndrome from thiamine/B1 deficiency due to chronic alcoholism). The limbic system connects the primitive brainstem with the higher cognitive cerebral cortex.

In both sympathetic and parasympathetic divisions of the ANS, the preganglionic neurotransmitter is always Acetylcholine (ACh) acting on nicotinic receptors at the ganglia. The difference lies in the postganglionic neurotransmitter: Parasympathetic postganglionic = Acetylcholine (acts on muscarinic receptors) - cholinergic. Sympathetic postganglionic = Noradrenaline (Norepinephrine) (acts on adrenergic receptors) - adrenergic. Exception: Sweat glands (eccrine) are sympathetically innervated but use ACh. Adrenal medulla: Sympathetic preganglionic fibres directly innervate it causing release of Adrenaline (80%) + Noradrenaline (20%) into blood.

The Hypothalamus is a small but critically important structure in the diencephalon. Functions: Temperature regulation (thermostat of the body), Hunger and satiety (feeding centre and satiety centre), Thirst (osmoreceptors - control of ADH release), Sleep-wake cycle (circadian rhythm regulation), Emotion and behaviour (rage, fear, pleasure), Endocrine control (controls the anterior pituitary via releasing hormones and produces ADH and Oxytocin released by posterior pituitary), and Autonomic NS control (sympathetic and parasympathetic responses).

The Amygdala (Greek: almond) is an almond-shaped structure in the medial temporal lobe, part of the limbic system. Functions: Fear processing and fear conditioning (the fear centre of the brain), Aggression and threatening behaviour, Emotional memory (attaches emotional significance to memories), and Threat detection (triggers fight-or-flight response via hypothalamus). Kluver-Bucy Syndrome: Bilateral amygdala damage causes loss of fear, hypersexuality, hyperorality (putting everything in mouth). Post-Traumatic Stress Disorder (PTSD): Hyperactive amygdala causes exaggerated fear responses.

Guillain-Barre Syndrome (GBS) is an acute autoimmune demyelinating polyneuropathy of the peripheral nervous system. Often triggered by a preceding infection (most commonly Campylobacter jejuni gastroenteritis, also influenza, CMV, EBV, COVID-19). Mechanism: Molecular mimicry - antibodies against pathogen cross-react with peripheral nerve myelin (gangliosides). Features: Ascending weakness (starts in legs, progresses upward), Areflexia (absent reflexes), Can progress to respiratory failure (diaphragm paralysis - 25% need ventilation), and Sensory symptoms. CSF: Albuminocytological dissociation - raised protein with normal WBCs. Treatment: IV immunoglobulin (IVIg) or Plasmapheresis. Most patients recover fully.

The Thalamus is a large, egg-shaped structure in the diencephalon - the gateway to the cortex. It: Receives almost all sensory information (except smell/olfaction - which goes directly to cortex), Relays and filters sensory signals to appropriate cortical areas, and Processes pain, temperature, touch, vision, hearing, taste, proprioception. Also involved in: Motor control (receives cerebellar and basal ganglia output - relays to motor cortex), consciousness, and sleep-wake regulation. Thalamic strokes can cause sensory deficits on the opposite side of the body.

Cerebrospinal Fluid (CSF) is a clear, colourless fluid that fills the subarachnoid space, ventricles of the brain, and the central canal of the spinal cord. Production: ~500 mL/day by the choroid plexus in the ventricles. Total volume at any time: ~150 mL (reabsorbed at arachnoid granulations). Functions: Mechanical cushion (protects brain from injury), Buoyancy (reduces effective brain weight from 1400g to ~25g), Nutrient and waste transport, and Maintains stable chemical environment. CSF analysis (lumbar puncture) diagnoses: Meningitis (cloudy CSF, raised WBCs), Subarachnoid haemorrhage (xanthochromic CSF), and Multiple sclerosis (oligoclonal bands).

The Basal Ganglia are a group of subcortical nuclei: Caudate nucleus + Putamen = Striatum (main input), Globus pallidus (internal and external), Subthalamic nucleus, and Substantia nigra (midbrain). Functions: Voluntary movement control (modulates cortical motor output - selects and initiates movements, suppresses unwanted movements), Habit and procedural learning, and Reward processing (dopamine pathway). Parkinson's disease: Dopamine depletion in substantia nigra causes disrupted basal ganglia circuit. Huntington's disease: Degeneration of striatum (caudate mainly) causes chorea (involuntary jerky movements).