Neural Control and Coordination

Animals coordinate the activities of their organs through two systems — the neural system (fast, electrical, short-lived) and the endocrine system (slower, chemical, longer-lasting); this chapter is about neural control. The human neural system has two divisions. The central nervous system (CNS) — the brain and spinal cord — is where information is processed and commands are issued. The peripheral nervous system (PNS) is all the nerves that connect the CNS to the rest of the body. The PNS fibres are of two kinds: afferent (sensory) fibres carry impulses toward the CNS, and efferent (motor) fibres carry impulses away from it. Functionally the PNS is further split into the somatic nervous system (carries signals to the skeletal muscles, voluntary) and the autonomic nervous system (to the involuntary smooth muscle and organs).

The structural and functional unit of the neural system is the neuron (nerve cell). A neuron has three parts: the cell body (cyton), which contains the cytoplasm with characteristic granules called Nissl's granules; the dendrites, short branched processes that receive signals and carry them toward the cell body; and the single long axon, which carries the impulse away from the cell body to the next cell, ending in branched synaptic knobs.

By structure, neurons are multipolar (one axon, many dendrites — in the cerebral cortex), bipolar (one axon and one dendrite — in the retina) or unipolar (only an axon — in the embryo). Axons are also myelinated or non-myelinated. In myelinated fibres the axon is wrapped by Schwann cells forming a myelin sheath with gaps called the nodes of Ranvier; the impulse 'jumps' from node to node (saltatory conduction), making conduction much faster. Non-myelinated fibres lack these gaps and conduct more slowly.

For NEET, fix the CNS/PNS split, the afferent-vs-efferent and somatic-vs-autonomic distinctions, the three neuron parts (note Nissl granules in the cyton), the three structural types with their examples, and the role of the myelin sheath and nodes of Ranvier in speeding conduction. The next topic explains how the impulse is actually generated and passed on.

A resting neuron is polarised. The axon membrane keeps the outside positive and the inside negative, a difference called the resting potential (about −70 mV). It is maintained by the sodium–potassium pump, which pumps 3 Na⁺ out for every 2 K⁺ in, and by the resting membrane being far more permeable to K⁺ than to Na⁺; so the inside has high K⁺ and the outside high Na⁺, with a net negative charge inside.

When a stimulus is applied at a point, the membrane there suddenly becomes highly permeable to Na⁺: Na⁺ rushes in, the inside becomes positive and the polarity is reversed — this is depolarisation, and the brief reversal is the action potential (nerve impulse). Almost at once the Na⁺ channels close and K⁺ channels open, so K⁺ moves out and the resting potential is restored — repolarisation. The action potential at one point depolarises the next point, so the impulse moves along the axon as a self-propagating wave; conduction is always away from the point of stimulation.

Where one neuron meets the next is a synapse, and impulses cross it in two ways. At an electrical synapse the membranes are very close, so the current flows directly — transmission is extremely fast but uncommon. At a chemical synapse (the common type) the two neurons are separated by a fluid-filled synaptic cleft. The arriving impulse makes the synaptic knob release a neurotransmitter (such as acetylcholine) from synaptic vesicles into the cleft; the transmitter diffuses across, binds receptors on the next neuron and starts a fresh impulse.

Because the transmitter is released on one side and received on the other, the chemical synapse conducts in only one direction. For NEET, remember the resting state (−70 mV, Na⁺/K⁺ pump 3:2, more K⁺-permeable), the ionic basis of the action potential (Na⁺ in = depolarisation, K⁺ out = repolarisation), and the chemical-synapse sequence ending in a one-way transmission via a neurotransmitter.

The brain is the central command centre of the body, lodged in the cranium and protected by three membranes, the meninges (dura mater, arachnoid and pia mater), with cerebrospinal fluid (CSF) cushioning it. It is divided into three regions — forebrain, midbrain and hindbrain — and their parts and functions are heavily examined.

The forebrain is the largest region. Its main part is the cerebrum, divided into two cerebral hemispheres joined by a band of fibres, the corpus callosum. The outer cerebral cortex is grey matter (cell bodies) thrown into folds, and it controls thinking, memory, intelligence, the senses and voluntary actions. The forebrain also contains the thalamus (a major relay station for sensory and motor signals) and the hypothalamus, which controls body temperature, hunger and thirst, contains centres for many emotions, and links the nervous and endocrine systems by controlling the pituitary gland. The inner parts form the limbic system, involved in emotions and motivation.

The midbrain is a short region between the thalamus and the hindbrain; it bears four rounded lobes, the corpora quadrigemina, and controls some visual and auditory reflexes. The hindbrain has three parts: the cerebellum, which coordinates movement, posture and balance; the pons, a bridge of fibre tracts; and the medulla oblongata, which connects to the spinal cord and contains the vital centres that control respiration, heartbeat (cardiovascular reflexes) and gastric secretions.

The midbrain and the hindbrain (pons + medulla) together form the brain stem, which connects the brain to the spinal cord. For NEET, fix each part with its job: cerebrum (thinking/voluntary), corpus callosum (links hemispheres), thalamus (relay), hypothalamus (temperature/hunger/thirst + pituitary), cerebellum (balance/coordination) and medulla (respiration/heart). The next topic covers how the nervous system produces rapid involuntary responses.

Not every response needs the brain to 'decide'. A reflex action is a sudden, automatic, involuntary response to a stimulus — like jerking the hand off a hot object — and it is usually mediated by the spinal cord so that the response is very fast. The pathway it follows is the reflex arc, a fixed NEET sequence: a receptor detects the stimulus → an afferent (sensory) neuron carries the impulse to the CNS → the impulse is passed within the CNS (often through an interneuron in the spinal cord) → an efferent (motor) neuron carries the command out → the effector (a muscle or gland) produces the response. The classic example is the knee-jerk reflex.

The part of the PNS that controls involuntary functions is the autonomic nervous system (ANS), which acts on cardiac and smooth muscle and on glands without conscious control. It has two divisions that usually have opposite (antagonistic) effects, keeping the body balanced.

The sympathetic nervous system prepares the body for emergencies — the 'fight-or-flight' response. It speeds up the heart rate, raises blood pressure, dilates the pupils and the bronchi, releases glucose for energy and slows down digestion — diverting resources to face a threat. The parasympathetic nervous system does the opposite, the 'rest-and-digest' state: it slows the heart rate, constricts the pupils and stimulates digestion, conserving energy and restoring calm.

Together these maintain a steady internal state by pushing in opposite directions as conditions change. For NEET, the high-yield points are the exact order of the reflex arc (receptor → sensory neuron → CNS → motor neuron → effector), that simple reflexes are processed by the spinal cord, and the antagonistic actions of the two ANS divisions — sympathetic (fight-or-flight: heart up, pupils dilate, digestion down) versus parasympathetic (rest-and-digest: heart down, pupils constrict, digestion up).