The Neuron (nerve cell) is the structural and functional unit of the nervous system. Each neuron has: Cell body (soma) containing nucleus and organelles; Dendrites - short branching processes that receive signals; Axon - long process that carries signals away from the cell body. The human brain contains approximately 86 billion neurons. Neurons communicate via electrical impulses and chemical messengers (neurotransmitters). Unlike most body cells, mature neurons generally cannot divide (post-mitotic) - making nerve injury often permanent.

The nervous system is divided into: CNS (Brain + Spinal cord) - the processing and integration centre - Protected by skull, vertebral column, meninges, and CSF. PNS (All nervous tissue outside the CNS) - 12 pairs of cranial nerves + 31 pairs of spinal nerves and their branches. The PNS is further divided into: Somatic NS (voluntary control of skeletal muscles) and Autonomic NS (involuntary control of visceral organs - Sympathetic + Parasympathetic).

Dendrites are the short, highly branched extensions of the neuron that receive incoming signals from other neurons and carry them toward the cell body. The word dendrite comes from Greek dendron meaning tree. Each neuron may have thousands of dendrites forming a dendritic tree, greatly increasing the surface area for receiving signals. Dendrites contain receptor proteins for neurotransmitters released at synapses. The more dendrites a neuron has, the more connections (synapses) it can form with other neurons.

The Axon (nerve fibre) is the long, slender extension of the neuron that carries electrical impulses (action potentials) AWAY from the cell body. Key facts: Can be up to 1 metre long (e.g., neurons supplying leg muscles via the sciatic nerve). Usually one axon per neuron (may branch into axon collaterals). Ends in axon terminals (synaptic knobs) which release neurotransmitters. May be covered by myelin sheath (speeds up conduction). Mnemonic: Axon = Away from cell body.

The myelin sheath is a fatty insulating layer that wraps around axons of many neurons. Schwann cells form myelin in the PNS (Peripheral Nervous System). Oligodendrocytes form myelin in the CNS (Central Nervous System). Functions of myelin: Electrical insulation, speeds impulse conduction via saltatory conduction (impulse jumps between Nodes of Ranvier). Myelinated fibres conduct at 70-120 m/s vs unmyelinated at 0.5-2 m/s. Multiple Sclerosis (MS) = autoimmune destruction of myelin in the CNS causing neurological deficits.

Nodes of Ranvier are small gaps in the myelin sheath at regular intervals (~1 mm apart) along a myelinated axon. Saltatory conduction (from Latin saltare = to leap) occurs when the nerve impulse jumps from one node to the next - rather than travelling continuously along the entire axon. This dramatically increases speed of conduction (up to 100x faster) and reduces energy expenditure (Na+/K+ pumps only work at nodes). Named after Louis-Antoine Ranvier (French pathologist, 1871). In demyelinating diseases like MS, destruction of myelin eliminates nodes causing slowed/blocked conduction.

There are 12 pairs of cranial nerves that arise directly from the brain. Mnemonic: On Old Olympus Towering Tops A Finn And German Viewed Some Hops. I - Olfactory (smell), II - Optic (vision), III - Oculomotor (eye movement), IV - Trochlear (eye movement), V - Trigeminal (face sensation + chewing), VI - Abducens (eye movement), VII - Facial (facial expression + taste), VIII - Vestibulocochlear (hearing + balance), IX - Glossopharyngeal (throat + taste), X - Vagus (visceral organs), XI - Spinal Accessory (neck/shoulder muscles), XII - Hypoglossal (tongue movement).

Hydrocephalus (water on the brain) is a condition where excess CSF accumulates in the ventricles of the brain, causing increased intracranial pressure (ICP). Causes: Obstructive (Non-communicating) = Blockage within the ventricular system (e.g., aqueduct stenosis, tumour). Non-obstructive (Communicating) = Impaired CSF reabsorption at arachnoid granulations (e.g., after meningitis, subarachnoid haemorrhage). In infants: enlarging head circumference, bulging fontanelle, sunset sign. In adults: headache, vomiting, papilloedema (raised ICP). Treatment: Ventriculoperitoneal (VP) shunt - drains CSF from ventricle to peritoneum.

The Reticular Activating System (RAS) - also called Ascending Reticular Activating System (ARAS) - is a diffuse network of neurons in the brainstem reticular formation extending from the medulla to the thalamus. Functions: Maintains consciousness and wakefulness, Regulates the sleep-wake cycle (circadian rhythm), Filters sensory input (determines what reaches conscious awareness), and Controls arousal and attention. Damage to the RAS causes coma or persistent vegetative state. General anaesthetics suppress RAS causing loss of consciousness. Caffeine stimulates RAS increasing alertness. Benzodiazepines depress the RAS.

There are 31 pairs of spinal nerves arising from the spinal cord: Cervical (C1-C8) = 8 pairs (neck, shoulders, arms), Thoracic (T1-T12) = 12 pairs (chest, upper abdomen, back), Lumbar (L1-L5) = 5 pairs (lower back, anterior legs), Sacral (S1-S5) = 5 pairs (buttocks, posterior legs, bladder), and Coccygeal (Co1) = 1 pair (coccyx region). Each spinal nerve has a dorsal root (sensory - carries afferent signals) and a ventral root (motor - carries efferent signals). The dorsal root ganglion contains the cell bodies of sensory neurons.