Neural and Chemical Coordination — NEET Biology

The body's two coordinating systems — fast electrical (nervous) and slower chemical (endocrine) — together give NEET 4–6 questions a year, and they are heavy on exact recall: how a nerve impulse is generated and crosses a synapse, and which gland makes which hormone with which disorder of excess or deficiency. This chapter builds the neuron and impulse first, then the whole endocrine map in the gland-by-gland form the exam quotes almost verbatim.

Part A — Neural Coordination

1. The neuron and the nervous system

The neuron is the structural and functional unit: a cell body (with the nucleus), branching dendrites (receive impulses) and a single long axon (carries impulses away). Many axons are wrapped in a myelin sheath (made by Schwann cells) with gaps called nodes of Ranvier — myelination speeds conduction (saltatory).

The nervous system has two divisions:

• Central nervous system (CNS): brain + spinal cord.
• Peripheral nervous system (PNS): somatic (voluntary, to skeletal muscle) + autonomic (involuntary), which is sympathetic ("fight or flight") and parasympathetic ("rest and digest").

2. The nerve impulse — resting and action potential

A neuron at rest is polarised: the inside is negative (~−70 mV) relative to the outside. This resting potential is maintained by the Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in) and the membrane being more permeable to K⁺; outside is Na⁺-rich, inside K⁺-rich.

An action potential (impulse) is a wave of depolarisation:

1. A stimulus opens Na⁺ channels → Na⁺ rushes in → the inside becomes positive (depolarisation, ~+30 mV).
2. K⁺ channels open → K⁺ leaves → the inside returns negative (repolarisation).
3. The Na⁺/K⁺ pump restores the original ionic distribution.

The impulse is all-or-none and self-propagates along the axon.

Worked example 2.1. What ion movement causes the rising (depolarisation) phase of the action potential? The inflow of Na⁺ ions. When a stimulus opens the voltage-gated Na⁺ channels, Na⁺ rushes into the axon down its gradient, reversing the membrane potential from −70 mV to about +30 mV — the depolarisation that constitutes the impulse.

3. The synapse and neurotransmitters

Where one neuron meets the next is a synapse. At a chemical synapse the impulse cannot jump the gap directly:

1. The impulse reaches the axon terminal → Ca²⁺ enters → synaptic vesicles release a neurotransmitter (e.g. acetylcholine) into the synaptic cleft.
2. The transmitter binds receptors on the next (post-synaptic) membrane → opens ion channels → a new impulse begins.

Transmission is one-way (transmitter is only on the pre-synaptic side).

Worked example 3.1. Why does a nerve impulse travel in only one direction across a chemical synapse? Because the neurotransmitter is stored and released only from the pre-synaptic (axon) terminal, and the receptors are only on the post-synaptic membrane. The chemical can therefore cross only from the pre- to the post-synaptic neuron, making transmission unidirectional.

4. The brain, spinal cord and reflex arc

The brain has three regions:

• Forebrain: cerebrum (thinking, memory, voluntary action), thalamus (relay), hypothalamus (temperature, hunger, thirst, links to endocrine).
• Midbrain: reflexes of eye and ear.
• Hindbrain: cerebellum (balance, coordination of movement), pons, medulla oblongata (heart rate, breathing, blood pressure — vital centres).

A reflex action is an automatic response via a reflex arc: receptor → sensory neuron → CNS (spinal cord, via an interneuron) → motor neuron → effector — bypassing conscious brain control (e.g. knee-jerk, withdrawing from a pin).

Worked example 4.1. Which brain part controls balance, and which controls heart rate and breathing? The cerebellum (hindbrain) controls balance and fine coordination of movement. The medulla oblongata houses the vital centres for heart rate, breathing and blood pressure.

Part B — Chemical Coordination

5. The endocrine system

Endocrine glands are ductless and secrete hormones directly into the blood. Hormones act on distant target cells with specific receptors, in tiny amounts. The main glands:

6. Hypothalamus and pituitary

• Hypothalamus — links the nervous and endocrine systems; makes releasing/inhibiting hormones that control the pituitary, plus ADH and oxytocin (stored in the posterior pituitary).
• Pituitary ("master gland") — sits below the hypothalamus.
  • Anterior lobe: GH (growth hormone), TSH, ACTH, FSH, LH, prolactin.
  • Posterior lobe: stores ADH (water) and oxytocin (childbirth, milk ejection).

GH disorders: excess in childhood → gigantism, in adults → acromegaly; deficiency in childhood → dwarfism.

Worked example 6.1. Why is the pituitary called the master gland, yet is itself controlled? It secretes hormones (TSH, ACTH, FSH, LH) that regulate other endocrine glands — hence "master." But its own anterior-lobe secretion is directed by releasing/inhibiting hormones from the hypothalamus, so the hypothalamus is the true overall controller.

7. Thyroid, parathyroid and adrenal

• Thyroid — secretes thyroxine (T₄/T₃), which needs iodine; it sets the basal metabolic rate. Deficiency of iodine → goitre; underactivity → hypothyroidism (cretinism in children, myxoedema in adults); overactivity → hyperthyroidism (Grave's disease). Also secretes calcitonin (lowers blood Ca²⁺).
• Parathyroid — parathormone (PTH) raises blood Ca²⁺ (opposes calcitonin).
• Adrenal gland (on each kidney):
  • Medulla: adrenaline & noradrenaline — the "emergency / fight-or-flight" hormones (raise heart rate, BP, blood glucose).
  • Cortex: glucocorticoids (cortisol — glucose metabolism, stress), mineralocorticoids (aldosterone — Na⁺/water/BP).

Worked example 7.1. A person in a hilly region develops goitre (enlarged thyroid). What is the usual cause? Dietary iodine deficiency. Thyroxine synthesis requires iodine; when iodine is scarce the thyroid cannot make enough hormone and enlarges (goitre) trying to compensate — common where soil/water iodine is low, hence iodised salt.

8. Pancreas and gonads

• Pancreas (endocrine islets of Langerhans):
  • β-cells → insulin (lowers blood glucose; its lack → diabetes mellitus).
  • α-cells → glucagon (raises blood glucose). Insulin and glucagon are antagonistic.
• Gonads:
  • Testes → testosterone (male secondary sexual characters, sperm).
  • Ovaries → oestrogen & progesterone (female characters, menstrual cycle, pregnancy).

Other sources: pineal gland → melatonin (sleep–wake cycle); the heart (ANF), kidney (renin, erythropoietin) and GI tract also secrete hormones.

Worked example 8.1. How do insulin and glucagon act antagonistically on blood glucose? Insulin (β-cells) lowers blood glucose — promoting its uptake by cells and storage as glycogen. Glucagon (α-cells) raises blood glucose — breaking glycogen down to glucose. The two together keep blood sugar within narrow limits; loss of insulin causes diabetes mellitus.

9. Common traps NEET sets here

• Resting potential ~−70 mV; depolarisation = Na⁺ in; repolarisation = K⁺ out.
• Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in.
• Synapse is one-way (transmitter pre-synaptic, receptors post-synaptic); Ca²⁺ triggers release.
• Cerebrum = thinking; cerebellum = balance; medulla = heart/breathing; hypothalamus = temperature/hunger + endocrine link.
• Pituitary is the master gland but is controlled by the hypothalamus.
• Thyroxine needs iodine; deficiency → goitre/cretinism.
• Calcitonin lowers Ca²⁺; PTH raises Ca²⁺ (don't reverse).
• Adrenaline = fight-or-flight; insulin lowers, glucagon raises glucose.
• ADH & oxytocin are made by the hypothalamus but released from the posterior pituitary (it only stores them).

10. Memory aids

• "Na⁺ in to depolarise, K⁺ out to repolarise" — the two impulse phases.
• "Cerebrum thinks, Cerebellum balances, Medulla keeps you alive" — brain regions.
• "Master pituitary, boss hypothalamus" — who controls whom.
• "Iodine for the thyroid, no iodine → goitre" — thyroxine.
• "Calci-tonin tones Ca down, PTH pushes Ca up" — calcium hormones.
• "Insulin In (stores sugar), Glucagon Gives (releases sugar)" — antagonistic pair.

11. Exam protocol

1. Neuron parts; resting potential (−70 mV, Na⁺/K⁺ pump); action potential (Na⁺ in → K⁺ out).
2. Synapse: Ca²⁺ → neurotransmitter (acetylcholine) → one-way transmission.
3. Brain: cerebrum, cerebellum, medulla, hypothalamus roles; reflex arc pathway.
4. Pituitary (master, but hypothalamus-controlled); GH disorders (gigantism/acromegaly/dwarfism).
5. Thyroid (thyroxine/iodine, goitre; calcitonin) and parathyroid (PTH) — the Ca²⁺ pair; adrenal (adrenaline, cortisol, aldosterone).
6. Pancreas (insulin ↓ / glucagon ↑ glucose; diabetes mellitus); gonads (testosterone; oestrogen/progesterone); pineal (melatonin).