Coordination and Control — Nervous and Chemical Coordination
Animals integrate information through two parallel systems: the nervous system (fast, electrical, short-lived) and the endocrine system (slower, chemical, long-lasting). The PMDC MDCAT 2026 syllabus expects you to know neurons and synapses, the major parts of the human brain, the spinal cord, the endocrine glands and their hormones, and feedback regulation. Expect 5-7 MCQs — one of the highest-yield Biology chapters.
Neurons
The neuron is the structural and functional unit of the nervous system. Each consists of a cell body (containing nucleus and Nissl granules), short branching dendrites that receive impulses, and a single long axon that transmits impulses to the next cell.
Functional types
- Sensory (afferent) — receptor → CNS.
- Motor (efferent) — CNS → effector.
- Interneuron — entirely within CNS; integrates signals.
Myelin sheath — a lipid insulation produced by Schwann cells (PNS) or oligodendrocytes (CNS). Gaps called nodes of Ranvier allow saltatory conduction, dramatically speeding up impulse propagation.
Nerve Impulse
A nerve impulse is a wave of electrical change — an action potential — travelling along the axon membrane.
The neuron at rest is polarised: inside negative relative to outside. Maintained by:
- Na+/K+ ATPase pumps 3 Na+ out and 2 K+ in for every ATP.
- Potassium leak channels make the membrane more permeable to K+ than to Na+.
- Large negatively charged proteins inside cannot leave.
- Threshold reached at ~−55 mV.
- Depolarisation — voltage-gated Na+ channels open; Na+ rushes in; potential rises to ~+30 mV.
- Repolarisation — Na+ channels close, voltage-gated K+ channels open; K+ flows out.
- Hyperpolarisation (afterhyperpolarisation) — brief overshoot below −70 mV.
- Refractory period — absolute then relative; ensures one-way travel.
Action potentials obey the all-or-nothing law: either threshold is reached and a full impulse fires, or nothing happens. Stimulus strength is coded by impulse frequency, not amplitude.
Conduction velocity increases with axon diameter and myelination. Myelinated mammalian axons can conduct at ~120 m/s; unmyelinated invertebrate axons only ~1 m/s.
Synapse
A synapse is the junction between two neurons (or between a neuron and an effector). Most are chemical synapses using a neurotransmitter; a few are electrical (gap junctions).
- Action potential reaches the presynaptic axon terminal.
- Voltage-gated Ca2+ channels open; Ca2+ enters.
- Synaptic vesicles fuse with the membrane and release neurotransmitter into the synaptic cleft (~20 nm wide) by exocytosis.
- Neurotransmitter binds to receptors on the postsynaptic membrane.
- Ligand-gated ion channels open → postsynaptic potential (EPSP or IPSP).
- Neurotransmitter is degraded (e.g., acetylcholinesterase) or reuptaken to terminate the signal.
Major neurotransmitters
- Acetylcholine — neuromuscular junction, parasympathetic.
- Noradrenaline — sympathetic.
- Dopamine — reward, motor control (deficient in Parkinson's).
- Serotonin — mood, sleep.
- GABA — main inhibitory transmitter in brain.
- Glutamate — main excitatory transmitter in brain.
Spinal Cord
The spinal cord runs from the medulla to L1-L2 inside the vertebral column, giving rise to 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). It is protected by the vertebrae, three meninges (dura, arachnoid, pia), and cerebrospinal fluid.
In cross-section it has a butterfly-shaped grey matter (cell bodies) surrounded by white matter (myelinated tracts) — the opposite of the brain. Each spinal nerve has a dorsal root (sensory, with ganglion) and a ventral root (motor) — the Bell-Magendie law.
Functions: conduction (ascending and descending tracts) and reflex action.
Reflex arc
Receptor → sensory neuron → (interneuron in CNS) → motor neuron → effector. The knee-jerk is monosynaptic; withdrawal reflex is polysynaptic. Reflexes are fast, involuntary, and stereotyped.
Parts of Brain
The human brain has three primary divisions: forebrain (cerebrum, thalamus, hypothalamus), midbrain, and hindbrain (pons, medulla, cerebellum). The whole organ weighs ~1.4 kg.
Largest part — ~85% of brain mass. Two hemispheres connected by the corpus callosum. Outer grey matter is folded into gyri and sulci. Four lobes:
- Frontal — voluntary motor control, planning, speech (Broca's area).
- Parietal — somatic sensation, taste.
- Temporal — hearing, smell, language (Wernicke's area), memory.
- Occipital — vision.
The cerebrum integrates higher functions: thought, reasoning, language, conscious memory.
Located behind the medulla. Coordinates voluntary muscular activity, balance, posture, and fine motor learning. Damage → ataxia (clumsy uncoordinated movement).
Lowest part of the brainstem; continuous with the spinal cord. Houses vital centres for heartbeat, breathing, blood pressure, vomiting, swallowing, sneezing, coughing. Damage is rapidly fatal.
Below the thalamus. Master regulator of homeostasis: thermoregulation, hunger, thirst, sleep-wake cycle. Controls the autonomic nervous system and the pituitary gland (releasing/inhibiting hormones & ADH/oxytocin).
- Thalamus — sensory relay station to the cerebral cortex.
- Midbrain — visual and auditory reflexes (e.g., pupillary reflex).
- Pons — bridge between cerebellum and rest of brain; respiratory regulation.
- Limbic system — emotion, memory (hippocampus, amygdala).
Autonomic nervous system — Sympathetic vs Parasympathetic
The autonomic NS is the involuntary motor system, controlling smooth muscle, cardiac muscle, and glands. It has two antagonistic divisions whose balance keeps the body in homeostasis.
| Effect / target | Sympathetic | Parasympathetic |
|---|---|---|
| Overall role | Fight or flight — mobilises energy | Rest and digest — conserves & restores |
| Origin (CNS) | Thoracic + lumbar (T1–L2) | Cranial nerves (III, VII, IX, X) + sacral (S2–S4) |
| Main neurotransmitter | Noradrenaline (post-ganglionic) | Acetylcholine |
| Heart rate | ↑ Increases | ↓ Decreases |
| Blood pressure | ↑ Increases | ↓ Decreases |
| Pupil | Dilates (mydriasis) | Constricts (miosis) |
| Bronchi | Dilate | Constrict |
| Digestion / peristalsis | Inhibited | Stimulated |
| Saliva | Thick, scant | Watery, abundant |
| Sweat glands | Stimulated | No effect |
| Bladder | Relaxes (retention) | Contracts (urination) |
| Adrenal medulla | Releases adrenaline + noradrenaline | No effect |
| Liver | Glycogenolysis → releases glucose | Promotes glycogen synthesis |
Endocrine System
The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. Hormones act on distant target cells with specific receptors. Endocrine signalling is slower than nervous signalling but produces sustained, widespread effects.
The hypothalamus controls the pituitary ("master gland") via releasing/inhibiting hormones.
- Anterior pituitary — secretes GH (growth hormone), TSH, ACTH, FSH, LH, prolactin.
- Posterior pituitary — stores and releases ADH (vasopressin) and oxytocin produced in the hypothalamus.
Disorders: gigantism / acromegaly (excess GH), dwarfism (deficient GH), diabetes insipidus (deficient ADH).
Thyroid (anterior neck) secretes thyroxine (T4) and triiodothyronine (T3) — raise basal metabolic rate — and calcitonin (lowers blood Ca2+). Iodine deficiency → goitre; hyperactivity → Graves' disease; underactivity → cretinism (children) or myxoedema (adults).
Parathyroid glands secrete PTH, which raises blood Ca2+ — the antagonist of calcitonin.
β-cells secrete insulin — lowers blood glucose by promoting cellular uptake and glycogen synthesis. α-cells secrete glucagon — raises blood glucose by glycogenolysis. Type 1 diabetes — autoimmune destruction of β-cells; type 2 — insulin resistance.
Sit atop the kidneys. The cortex secretes mineralocorticoids (aldosterone — Na+ retention), glucocorticoids (cortisol — stress, raises glucose), and a small amount of sex steroids. The medulla secretes adrenaline (epinephrine) and noradrenaline — the "fight-or-flight" hormones.
Ovaries secrete oestrogen and progesterone (menstrual cycle, pregnancy); testes secrete testosterone (male secondary sexual characters and spermatogenesis). Both controlled by FSH and LH from the anterior pituitary.
| Hormone | Gland | Main action | Hyper / hypo disorder |
|---|---|---|---|
| Growth hormone (GH) | Anterior pituitary | Stimulates growth, protein synthesis | Hyper: gigantism / acromegaly · Hypo: dwarfism |
| TSH | Anterior pituitary | Stimulates thyroid to release T3/T4 | — |
| ACTH | Anterior pituitary | Stimulates adrenal cortex (cortisol) | — |
| FSH / LH | Anterior pituitary | Gametogenesis, ovulation, sex hormones | Infertility |
| ADH (vasopressin) | Posterior pituitary | Water reabsorption in kidney | Hypo: diabetes insipidus |
| Oxytocin | Posterior pituitary | Uterine contractions, milk ejection | — |
| Thyroxine (T4) | Thyroid | Raises basal metabolic rate | Hyper: Graves' · Hypo: cretinism / myxoedema; iodine deficiency → goitre |
| Calcitonin | Thyroid (C-cells) | Lowers blood Ca2+ | — |
| PTH | Parathyroid | Raises blood Ca2+ (antagonist of calcitonin) | Hypo: tetany · Hyper: bone demineralisation |
| Insulin | Pancreas (β-cells) | Lowers blood glucose → glycogen | Hypo: Diabetes mellitus (type 1, type 2) |
| Glucagon | Pancreas (α-cells) | Raises blood glucose by glycogenolysis | — |
| Cortisol | Adrenal cortex | Stress response, raises glucose, anti-inflammatory | Hyper: Cushing's · Hypo: Addison's |
| Aldosterone | Adrenal cortex | Na+ retention, K+ excretion (kidney) | Affects BP regulation |
| Adrenaline | Adrenal medulla | "Fight-or-flight" — ↑ HR, ↑ BP, ↑ glucose | — |
| Testosterone | Testes (Leydig) | Male sex characters, spermatogenesis | Hypogonadism |
| Oestrogen / Progesterone | Ovaries | Menstrual cycle, pregnancy maintenance | Cycle disorders |
Feedback Mechanism
The endocrine and nervous systems regulate themselves through feedback loops.
Output reverses the original change, restoring the set point. Examples:
- High blood glucose → insulin release → glucose uptake → glucose falls → insulin release stops.
- Low T4 → hypothalamus releases TRH → pituitary releases TSH → thyroid releases T4 → T4 rises and inhibits TRH/TSH.
- Body temperature, blood pressure, blood Ca2+.
Output amplifies the change. Examples:
- Childbirth — oxytocin causes uterine contraction → pushes baby's head onto cervix → more oxytocin.
- Blood clotting — activated platelets recruit more platelets.
- LH surge at ovulation.
Positive feedback always ends with a definite event (delivery, vessel sealed, ovulation), or it would be runaway.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter.
Q1. Saltatory conduction occurs in:
In myelinated axons, action potentials "jump" from one node of Ranvier to the next, dramatically increasing conduction velocity — saltatory conduction. Unmyelinated axons conduct continuously and slowly.
Q2. The principal inhibitory neurotransmitter of the human brain is:
GABA (gamma-aminobutyric acid) is the main inhibitory transmitter in the CNS, producing IPSPs by opening Cl⁻ channels. Glutamate is the main excitatory transmitter.
Q3. Which part of the brain coordinates posture and balance?
The cerebellum coordinates voluntary movement, posture, balance, and fine motor learning. Damage causes ataxia. The medulla controls vital autonomic centres; the hypothalamus regulates homeostasis.
Q4. Insulin is secreted by which cells of the pancreas?
β-cells of the islets of Langerhans secrete insulin in response to high blood glucose. α-cells secrete glucagon (raises blood glucose). Acinar cells of the exocrine pancreas secrete digestive enzymes — not hormones.
Q5. Childbirth (parturition) is regulated by which kind of feedback?
Oxytocin causes uterine contractions, which push the baby's head onto the cervix; cervical stretch signals the hypothalamus to release more oxytocin. Each round amplifies the next — classic positive feedback. The loop ends when the baby is delivered.
Quick Recap
- Neurons: dendrites → cell body → axon → terminals; sensory / motor / interneurons.
- Resting potential −70 mV; action potential is all-or-nothing; saltatory conduction in myelinated axons.
- Synapse: Ca2+ influx → vesicle fusion → neurotransmitter release. Major NTs: ACh, NA, dopamine, serotonin, GABA, glutamate.
- Spinal cord: 31 pairs of nerves; dorsal root sensory, ventral root motor (Bell-Magendie); reflex arc.
- Brain: cerebrum (cognition), cerebellum (balance), medulla (vital reflexes), hypothalamus (homeostasis), thalamus (relay).
- Endocrine glands: hypothalamus, pituitary (FLAT PiG anterior; OA posterior), thyroid, parathyroid, pancreas, adrenals, gonads.
- Feedback: negative (most homeostasis); positive (childbirth, blood clotting, LH surge).