Neural Control & Coordination

The nervous system is the body's rapid communication network, enabling detection, processing, and response to stimuli within milliseconds. It operates through specialized cells called neurons and is organized into the central nervous system (CNS: brain and spinal cord) and the peripheral nervous system (PNS: cranial and spinal nerves).

A neuron consists of a cell body (cyton/soma) containing the nucleus and Nissl granules (rough ER for protein synthesis), short branching dendrites that receive signals, and a single long axon that transmits impulses away from the cell body. The axon may be wrapped in a myelin sheath formed by Schwann cells (in PNS) or oligodendrocytes (in CNS), with gaps called nodes of Ranvier between successive Schwann cells. Neurons are classified as sensory (afferent — carry impulses from receptors to CNS), motor (efferent — carry impulses from CNS to effectors), and interneurons (association neurons — connect sensory and motor neurons within the CNS).

The nerve impulse operates on electrochemical principles. At rest, the neuron maintains a resting membrane potential of approximately -70 mV (inside negative relative to outside), established by the sodium-potassium pump (3 Na+ out, 2 K+ in) and selective membrane permeability. When a stimulus of threshold intensity arrives, voltage-gated Na+ channels open rapidly, Na+ rushes in, and the membrane depolarizes (becomes positive inside, reaching about +30 mV) — this is the action potential. Immediately after, Na+ channels inactivate and K+ channels open, K+ flows out, and the membrane repolarizes back to negative values. A brief hyperpolarization follows before the resting potential is restored. The action potential obeys the all-or-none principle: it either fires fully at threshold or does not fire at all. In myelinated neurons, the impulse jumps from one node of Ranvier to the next — saltatory conduction — which is significantly faster than continuous conduction in unmyelinated fibres.

At the synapse, the electrical signal is converted to a chemical one. When the action potential reaches the axon terminal (synaptic knob), voltage-gated Ca₂+ channels open, Ca₂+ influx triggers exocytosis of synaptic vesicles containing neurotransmitters (commonly acetylcholine or norepinephrine). These neurotransmitters diffuse across the synaptic cleft (20 nm gap), bind to specific receptors on the postsynaptic membrane, and generate a new impulse. Enzymatic degradation (e.g., acetylcholinesterase breaks down ACh) or reuptake terminates the signal.

The brain is protected by the cranium and three meninges (dura mater, arachnoid, pia mater) and is divided into three major regions. The forebrain includes the cerebrum (largest part, divided into frontal, parietal, temporal, and occipital lobes — responsible for thinking, memory, speech, vision, hearing, voluntary movement), the thalamus (major sensory relay centre), and the hypothalamus (thermoregulation, hunger, thirst, circadian rhythm, controls the pituitary gland). The midbrain contains relay centres for visual and auditory reflexes. The hindbrain comprises the cerebellum (coordination of voluntary movements, posture, and balance — critically, it does NOT initiate movement), the pons (relay between cerebellum and cerebrum, contains the pneumotaxic centre), and the medulla oblongata (controls vital involuntary functions: cardiovascular centre, respiratory rhythmicity centre, vomiting and swallowing reflexes).

The spinal cord runs through the vertebral column and gives rise to 31 pairs of spinal nerves. It serves as a conduit for impulses between the brain and body and as the centre for spinal reflex arcs (receptor → sensory neuron → interneuron in spinal cord → motor neuron → effector).

The PNS is divided functionally into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of visceral organs), which is further subdivided into the sympathetic ("fight or flight" — increases heart rate, dilates pupils, inhibits digestion) and parasympathetic ("rest and digest" — decreases heart rate, constricts pupils, stimulates digestion) divisions.

The human eye functions as a camera-like organ. Light enters through the cornea (main refractive surface), passes through the pupil (regulated by iris muscles), is focused by the lens (accommodation — lens becomes more convex for near objects via ciliary muscles), and forms an inverted image on the retina. The retina contains photoreceptors: rods (120 million, detect dim light/scotopic vision, contain rhodopsin) and cones (6-7 million, detect colour/photopic vision, contain iodopsin, concentrated at the fovea). Common defects include myopia (short-sightedness, corrected by concave lens), hypermetropia (far-sightedness, corrected by convex lens), and presbyopia (age-related loss of accommodation).

The human ear serves both hearing and balance. Sound waves enter the external ear, vibrate the tympanic membrane, which transmits vibrations through three ossicles — malleus (hammer), incus (anvil), and stapes (stirrup, the smallest bone in the human body) — amplifying the signal 20-fold. The stapes transmits vibrations to the oval window of the cochlea in the inner ear, where fluid vibrations stimulate hair cells in the organ of Corti, generating nerve impulses carried by the auditory nerve to the brain. The vestibular apparatus (semicircular canals, utricle, and saccule) in the inner ear detects head position and rotational movement, maintaining balance and equilibrium.

The key testable concept is the resting potential value (-70 mV), the ionic basis of the action potential (Na+ influx for depolarization, K+ efflux for repolarization), and the functional distinction between cerebrum (initiates voluntary movement) and cerebellum (coordinates movement but does not initiate it).