Neural Control and Coordination

The human neural system has three divisions:

• Central Nervous System (CNS) — brain and spinal cord; integration and processing centre.
• Peripheral Nervous System (PNS) — cranial nerves (12 pairs) and spinal nerves (31 pairs); carry signals to and from CNS.
• Autonomic Nervous System (ANS) — controls involuntary functions; sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

A neuron is the structural and functional unit of the nervous system. It consists of:

• Cell body (soma) — contains nucleus and cytoplasm (neuroplasm).
• Dendrites — short, branched processes that receive impulses.
• Axon — long process that transmits impulses away from the cell body; covered by myelin sheath (formed by Schwann cells in PNS). Nodes of Ranvier are gaps in the myelin sheath — saltatory conduction occurs here.

Resting membrane potential (~−70 mV): Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in; inside is negative relative to outside.

Action potential: Depolarisation (Na⁺ rushes in → inside becomes positive) → Repolarisation (K⁺ rushes out) → Hyperpolarisation → Return to resting potential. Refractory period prevents reverse conduction.

Synapse: Electrical signals → chemical neurotransmitters (acetylcholine, dopamine, serotonin, norepinephrine) released into synaptic cleft → bind postsynaptic receptors → new potential generated.

The brain is protected by the cranium and three meninges (dura mater, arachnoid, pia mater). Cerebrospinal fluid (CSF) fills the ventricles and subarachnoid space.

Forebrain:

• Cerebrum — largest part; two hemispheres with gyri and sulci; conscious thought, memory, speech, voluntary movement. Corpus callosum connects the hemispheres.
• Thalamus — relay station for sensory signals.
• Hypothalamus — regulates temperature, hunger, thirst, sleep, and controls pituitary gland.

Midbrain: Visual and auditory reflexes; connects fore and hindbrain.

Hindbrain:

• Cerebellum — coordinates voluntary movement, balance, and posture.
• Pons — relays signals; involved in sleep regulation.
• Medulla oblongata — controls involuntary functions: breathing, heart rate, swallowing, vomiting.

Limbic system — amygdala, hippocampus; emotions, memory, olfaction.

Eye: Light enters through cornea → aqueous humour → pupil (size regulated by iris) → lens (focus) → vitreous humour → retina.

• Rods — scotopic vision (dim light); contain rhodopsin (visual purple); more numerous, concentrated in periphery.
• Cones — photopic vision (bright/colour); contain iodopsin; concentrated at fovea centralis (point of sharpest vision).
• Blind spot = optic disc (no photoreceptors); where optic nerve leaves the eye.

Ear:

• Sound waves → tympanic membrane → malleus → incus → stapes → oval window → cochlea.
• Organ of Corti (in cochlea) — actual auditory receptor; hair cells on basilar membrane.
• Semicircular canals — three, perpendicular to each other; sense rotational movement and help maintain dynamic equilibrium.
• Utricle and Saccule — sense linear acceleration and static equilibrium (gravity).

A reflex arc is the shortest neural pathway: Receptor → Afferent neuron → Nerve centre (spinal cord) → Efferent neuron → Effector.

Types of reflexes:

• Spinal reflexes — mediated by spinal cord alone (e.g., knee-jerk reflex — monosynaptic).
• Cranial reflexes — mediated by the brain (e.g., pupillary light reflex).
• Conditioned reflexes — acquired through learning (Pavlov's experiment).

Spinal cord:

• Extends from medulla to lumbar region.
• Grey matter (inside, H-shaped) — contains cell bodies and unmyelinated fibres.
• White matter (outside) — myelinated axons carrying ascending (sensory) and descending (motor) tracts.
• Dorsal roots carry sensory fibres; ventral roots carry motor fibres (Bell-Magendie law).