Neural Control and Coordination — NEET Notes

Neural system — overview

Coordination is the process by which two or more organs interact and complement each other's functions. When a sprinter pushes off the blocks, muscles demand more oxygen, lungs ventilate faster, the heart beats harder, blood vessels redirect flow — and when the sprint ends, every one of those systems gently unwinds back to baseline. The body has two integration systems for this: the neural system for fast, point-to-point electrical signalling, and the endocrine system for slower, broader chemical signalling through hormones. This chapter is about the first one.

The neural system is built from a single type of specialised cell — the neuron — which can detect, receive and transmit different kinds of stimuli. In Hydra, neurons form a simple network with no central command. Insects add a brain and a chain of ganglia. Vertebrates take the design furthest: a centralised brain and spinal cord, fed by a vast peripheral network of nerves that reach every tissue. The neural system is what makes a stimulus on your fingertip a thought in your cortex within a tenth of a second.

Human neural system — CNS, PNS and the autonomic split

The human neural system is anatomically divided into the central neural system (CNS) — brain and spinal cord — and the peripheral neural system (PNS) — every nerve outside the CNS. The CNS does the processing; the PNS does the wiring. PNS fibres come in two functional flavours: afferent fibres carry impulses from receptors and tissues towards the CNS, and efferent fibres carry regulatory impulses away from the CNS to muscles and glands.

The PNS itself splits again. The somatic neural system relays impulses from the CNS to skeletal muscles — it is the voluntary, conscious half. The autonomic neural system handles involuntary organs and smooth muscles, and it is itself divided into the sympathetic system (fight-or-flight, accelerating) and the parasympathetic system (rest-and-digest, restoring). The visceral nervous system is the part of the PNS made up of all the nerves, fibres, ganglia and plexuses that carry impulses between the CNS and the viscera in both directions.

Neuron — the structural and functional unit

A neuron is a microscopic cell built around three structural compartments: a cell body (cyton), dendrites, and an axon. The cell body holds the typical cell organelles, the nucleus, and a population of granular bodies called Nissl's granules — densely packed rough endoplasmic reticulum used for protein synthesis. Short branching fibres projecting from the cell body are the dendrites; they carry impulses towards the cell body. A single long fibre, sometimes a metre long in human spinal motor neurons, is the axon; it carries impulses away from the cell body towards the next neuron or an effector. The distal end of the axon splits into branches; each branch terminates in a bulb-like synaptic knob that holds synaptic vesicles full of neurotransmitters.

By morphology, neurons come in three types: multipolar (one axon, two or more dendrites — found in the cerebral cortex), bipolar (one axon, one dendrite — found in the retina), and unipolar (cell body with only one axon — usually only seen in the embryonic stage). Axons themselves come in two grades: myelinated, where Schwann cells wrap around the axon to form a fatty myelin sheath broken at regular intervals by nodes of Ranvier; and non-myelinated, where the Schwann cell encloses the axon without forming a sheath. Myelinated fibres run in spinal and cranial nerves; non-myelinated fibres are commonest in the autonomic and somatic systems.

Resting potential — why a neuron is polarised

Neurons are excitable cells because their plasma membrane sits in a polarised state — there is an electrical potential difference across it even when nothing is happening. The reason is asymmetric ion distribution. When the neuron is at rest, the axonal membrane is comparatively highly permeable to K⁺ (because of always-open leaky K⁺ channels) and almost impermeable to Na⁺. The membrane is also impermeable to the large negatively charged proteins floating in the axoplasm. As a result, the axoplasm has high K⁺, low Na⁺ and a load of negative protein anions, while the extracellular fluid has high Na⁺ and low K⁺.

This asymmetry is built and maintained by the sodium-potassium ATPase, an active-transport pump embedded in the membrane. It moves 3 Na⁺ outwards for every 2 K⁺ it brings inwards, burning one ATP per cycle. Because more positive ions leave the cell than enter on each pump cycle, the inside becomes increasingly negative relative to the outside. The outer surface is positively charged; the inner surface is negative — the membrane is polarised. That standing voltage is the resting potential, and in a typical neuron it measures about −70 mV (inside negative).

Generation of action potential

An action potential is the brief, all-or-nothing reversal of membrane polarity that we call a nerve impulse. When a stimulus reaches a point A on the polarised membrane, voltage-gated Na⁺ channels open and the membrane there becomes freely permeable to Na⁺. Na⁺ floods inwards down its steep electrochemical gradient. The inside of the membrane swings from −70 mV to roughly +30 mV; the outer surface becomes negative and the inner surface positive. This reversal of polarity at site A is depolarisation, and the potential difference across the membrane during the spike is the action potential.

The rise in Na⁺ permeability is extremely short-lived. Within a fraction of a millisecond, Na⁺ channels close and voltage-gated K⁺ channels open. K⁺ rushes out of the cell, carrying positive charge with it, and the inside swings back towards negative — repolarisation. K⁺ efflux briefly overshoots the resting value (the inside becomes more negative than −70 mV) — this dip is hyperpolarisation. During a short refractory period while the channels reset and the Na⁺/K⁺ pump trims the gradients, the membrane cannot fire again. Only after this does it return to the resting state and become re-excitable.

Conduction of the nerve impulse

Once an action potential fires at site A, it propagates. The reversed polarity at A creates local currents that flow inwards from A to the still-polarised site B just ahead, and outwards from B back to A — completing a tiny circuit. This local current depolarises site B above threshold; voltage-gated Na⁺ channels open at B; an action potential is generated at B. The cycle repeats site by site along the axon. The same K⁺ efflux that repolarised site A meanwhile restores it, so the impulse never doubles back. The net effect is a wave of depolarisation–repolarisation moving in one direction down the fibre.

In unmyelinated fibres this wave moves continuously, regenerating at every patch of membrane. In myelinated fibres the myelin sheath insulates the axon, leaving only the nodes of Ranvier exposed. The impulse jumps from one node to the next — saltatory conduction — which is many times faster than continuous conduction and uses far less ATP because only the nodes need re-polarising.

Synaptic transmission — electrical vs chemical

An impulse arriving at the end of an axon must somehow cross over to the next neuron. The junction where this happens is a synapse, formed by the membrane of a pre-synaptic neuron and the membrane of a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two architectures.

At an electrical synapse, the pre- and post-synaptic membranes lie in very close proximity, joined by gap junctions. Electrical current flows directly from one neuron to the next, very much like impulse conduction along a single axon. Transmission is always faster than at a chemical synapse — but electrical synapses are rare in our nervous system.

At a chemical synapse, the two membranes are separated by a fluid-filled synaptic cleft. The axon terminal of the pre-synaptic neuron contains synaptic vesicles loaded with neurotransmitters. When an action potential arrives at the terminal, voltage-gated Ca²⁺ channels open, vesicles fuse with the plasma membrane and release their neurotransmitter into the cleft. The transmitter diffuses across, binds specific receptors on the post-synaptic membrane, and opens ion channels. The resulting flow of ions generates a new potential in the post-synaptic neuron — excitatory (pushing it towards firing) or inhibitory (pushing it away). Major neurotransmitters at chemical synapses include acetylcholine (ACh), dopamine, serotonin, glutamate (the dominant excitatory transmitter in the CNS), and GABA (the dominant inhibitory transmitter).

Central neural system — the human brain

The brain is the command-and-control organ of the body — site of voluntary movement, balance, regulation of involuntary organs, thermoregulation, hunger and thirst, the circadian rhythm, endocrine control, and the entire library of cognition: vision, hearing, speech, memory, intelligence, emotion. It is protected by the skull and, beneath the bone, by three meninges: the tough outer dura mater, the thin middle arachnoid, and the inner pia mater in direct contact with brain tissue. Anatomically it divides into three regions — forebrain, midbrain, hindbrain.

The brain stem — three regions: midbrain, pons, medulla oblongata — forms the connection between the brain and the spinal cord. Within the forebrain, the cerebrum is split longitudinally into a left and right cerebral hemisphere by a deep cleft, and the two hemispheres communicate through a tract of nerve fibres called the corpus callosum. The cerebral cortex is grey matter (neuron cell bodies); the deeper layer of myelinated fibre tracts is white matter. The hypothalamus is the body's thermostat — NEET 2019 tested this directly. The limbic system together with the hypothalamus is what NEET 2023 named as the centre for sexual behaviour and the expression of excitement, pleasure, rage and fear.

Reflex action and the reflex arc

A reflex action is an automatic, quick, involuntary response to a stimulus. Pulling your hand away from a hot pan happens before you are consciously aware of the heat — and that is the point. Reflexes are spinal short-cuts that protect the body without waiting for the brain to deliberate. The pathway that makes it possible is the reflex arc, and it has five components arranged in a single linear chain.

Reflexes processed through the spinal cord are spinal reflexes (the knee-jerk, the hand-withdrawal); those routed through the brain are cerebral reflexes (pupillary light reflex, salivation at sight of food). Reflexes you are born with — sneezing, blinking, sucking — are simple or unconditioned reflexes. Reflexes built by learning, like salivating at the sound of a bell, are conditioned reflexes — Pavlov's contribution. The spinal cord can carry out every reflex below the neck without ever consulting the brain, which is the whole evolutionary point of the arc.

Sensory reception — the eye and the ear

The neural system is only as informed as the receptors that feed it. The two NEET-favourite sense organs are the eye (photoreception) and the ear (mechanoreception).

The eye — retina, rods, cones

The wall of the eyeball has three layers: the tough outer sclera (dense connective tissue of collagen), the vascular middle choroid, and the inner sensory retina. Light enters through the transparent cornea (a dense collagen matrix with a corneal epithelium — the most sensitive part of the eye), is regulated by the iris (the visible coloured portion that controls pupil diameter), and is focused by the lens (held in place by suspensory ligaments attached to the ciliary body) onto the retina.

The retina has two kinds of photoreceptor cells. Rods are highly sensitive and operate in dim light; they contain the photopigment rhodopsin, made of opsin (protein) and retinal (a derivative of vitamin A, which is itself derived from carotene — which is why a carotene-rich diet supports good vision). Cones operate in bright light and give colour vision, and they are concentrated at the fovea centralis — the point of greatest visual acuity. The blind spot is where the optic nerve leaves the eyeball and no photoreceptor cells are present. Light absorbed by retinal isomerises it, triggering a cascade that generates a nerve impulse in the optic nerve, which carries it to the visual cortex of the cerebrum.

The ear — cochlea and organ of Corti

The ear handles two jobs — hearing and balance. The mechanism of hearing is a chain of physical and ionic events. Sound waves enter the external ear and vibrate the tympanic membrane. The three ear ossicles — malleus, incus and stapes — amplify the vibration and transfer it through the oval window into the fluid of the inner ear. The fluid moves the membranes of the cochlea, a coiled part of the bony labyrinth that looks like a snail's shell. Sitting on the basilar membrane inside the cochlea is the organ of Corti, the actual auditory receptor — its hair cells convert the fluid movement into nerve impulses carried by the auditory nerve to the brain. The Eustachian tube connects the middle ear to the pharynx and equalises pressure across the tympanic membrane.