Nervous System: Structure & Function

The nervous system is the body's master communication and control network, integrating information from the environment and coordinating responses in all other organ systems. It is divided into two anatomical parts: the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), consisting of all nervous tissue outside the CNS — cranial nerves, spinal nerves, ganglia, and peripheral nerve endings.

The PNS is further divided functionally. The somatic nervous system controls voluntary movements via motor neurons to skeletal muscle and carries sensory information from the body surface and special senses. The autonomic nervous system (ANS) controls involuntary functions of smooth muscle, cardiac muscle, and glands, and is itself subdivided into sympathetic and parasympathetic divisions.

The fundamental unit of the nervous system is the neuron. A typical neuron has three main regions: the cell body (soma), which contains the nucleus and is the metabolic centre; dendrites, which are branched extensions that receive incoming signals; and a single axon, which conducts action potentials away from the soma toward target cells. Axons may be very long (up to 1 metre in lower-limb motor neurons). Many axons are surrounded by a myelin sheath — concentric layers of lipid-rich membrane produced by Schwann cells in the PNS and oligodendrocytes in the CNS. Between adjacent myelin segments are unmyelinated gaps called nodes of Ranvier, which are critical for saltatory conduction. At their terminals, axons branch into synaptic knobs that release neurotransmitters.

The resting membrane potential of a neuron is approximately −70 mV (inside negative relative to outside). This is maintained by: (1) the Na+/K+-ATPase pump, which actively transports 3 Na+ out and 2 K+ in per cycle, and (2) the selective permeability of the membrane — K+ can leak out through resting K+ channels, while Na+ cannot easily enter, leaving net negative charge inside.

An action potential is generated when a stimulus depolarises the membrane to the threshold of approximately −55 mV. This triggers opening of voltage-gated Na+ channels, causing rapid Na+ influx and depolarisation to about +40 mV. Na+ channels then inactivate while voltage-gated K+ channels open, allowing K+ efflux and rapid repolarisation back to and briefly below resting potential (hyperpolarisation). The absolute refractory period (during Na+ channel inactivation) prevents re-excitation and ensures unidirectional propagation.

Synaptic transmission converts an electrical signal into a chemical one. The action potential reaches the axon terminal, opening voltage-gated Ca2+ channels; Ca2+ influx triggers vesicle fusion and neurotransmitter release into the synaptic cleft. Neurotransmitters diffuse across and bind postsynaptic receptors, producing either an excitatory postsynaptic potential (EPSP — depolarisation) or inhibitory postsynaptic potential (IPSP — hyperpolarisation). Key neurotransmitters include: acetylcholine (ACh — neuromuscular junction, parasympathetic), dopamine (reward, motor control), serotonin (mood, sleep), noradrenaline (sympathetic, arousal), GABA (major inhibitory neurotransmitter in the CNS), and glutamate (major excitatory neurotransmitter in the CNS).

A reflex arc is the simplest neural circuit: sensory receptor → afferent neuron → (interneuron in spinal cord) → efferent motor neuron → effector. The patellar (knee-jerk) reflex is monosynaptic; the withdrawal reflex is polysynaptic. Reflex testing in clinical examination assesses the integrity of spinal cord segments.

The sympathetic division ("fight or flight") has preganglionic neurons from thoracolumbar cord (T1–L2), uses noradrenaline at target organs, and increases heart rate, dilates pupils, diverts blood to muscle, and inhibits digestion. The parasympathetic division ("rest and digest") has preganglionic neurons from craniosacral regions, uses acetylcholine throughout, and slows heart rate, increases digestion, and constricts pupils.