Coordination and Control — Nervous and Chemical Coordination
Animals integrate information through the nervous system — fast, electrical and precisely targeted. The PMDC MDCAT 2026 syllabus lists three subtopics for this chapter: receptors as transducers, neurons (their structure, the nerve impulse, the classification of reflexes and the reflex arc), and the brain — its main parts and their functions. Expect 5-7 MCQs — one of the highest-yield Biology chapters.
Receptors
A receptor is a cell or organ specialised to detect a particular stimulus. Its job is transduction: converting the energy of that stimulus — light, sound, pressure, heat, a chemical — into the electrical language of the nervous system, a nerve impulse. A receptor responds most readily to one adequate stimulus, which is why the eye reports light and the ear reports sound.
| Type | Stimulus detected | Examples |
|---|---|---|
| Photoreceptors | Light | Rods and cones of the retina |
| Mechanoreceptors | Touch, pressure, stretch, sound, gravity | Pacinian corpuscles in skin; hair cells of the cochlea and vestibular apparatus |
| Chemoreceptors | Chemicals | Taste buds, olfactory epithelium, carotid-body CO2 sensors |
| Thermoreceptors | Temperature change | Warm and cold receptors in the skin and hypothalamus |
| Nociceptors | Tissue damage | Free nerve endings signalling pain |
Receptors are also grouped by where the stimulus comes from: exteroceptors (external environment — skin, eye, ear), interoceptors (internal organs — blood pressure, CO2) and proprioceptors (position and tension in muscles and joints).
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.
Reflexes and the reflex arc
A reflex is a rapid, involuntary, stereotyped response to a stimulus. The nerve pathway it travels is the reflex arc, and it has five components:
- Receptor — detects the stimulus and generates an impulse.
- Sensory (afferent) neuron — carries the impulse to the CNS.
- Centre (interneuron in the CNS) — relays or processes the signal.
- Motor (efferent) neuron — carries the command outwards.
- Effector — the muscle or gland that responds.
Because the response is generated in the spinal cord or brain stem rather than the cerebrum, it happens before the stimulus is consciously perceived — the hand is withdrawn first, the pain is felt afterwards.
Classification of reflexes:
- Unconditioned (inborn) — present from birth and identical in everyone: blinking, sneezing, coughing, the knee-jerk, pupillary constriction.
- Conditioned (acquired) — learned by association during life, as in Pavlov’s dogs salivating to a bell.
- Monosynaptic — one synapse only, sensory neuron straight onto motor neuron: the knee-jerk (stretch) reflex.
- Polysynaptic — one or more interneurons in between: the withdrawal reflex from a hot object.
- Somatic reflexes act on skeletal muscle; autonomic (visceral) reflexes act on smooth muscle, cardiac muscle and glands.
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).
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 essential function of a sensory receptor is to act as a:
Whatever the stimulus — light at a rod, pressure at a Pacinian corpuscle, a chemical at a taste bud — the receptor converts that form of energy into the one language the nervous system reads, an electrical impulse. That conversion is transduction. Effectors (muscles, glands) sit at the other end of the pathway.
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. The knee-jerk reflex is described as monosynaptic because its arc contains:
"Monosynaptic" counts synapses, not neurons: the sensory neuron synapses directly onto the motor neuron in the spinal cord with no interneuron between them, which is why the response is so fast. A withdrawal reflex is polysynaptic — at least one interneuron is inserted.
Q5. During the depolarisation phase of an action potential:
Once threshold (~−55 mV) is reached, voltage-gated Na+ channels open and Na+ floods inwards, driving the potential to about +30 mV. K+ efflux belongs to repolarisation, and the overshoot below −70 mV is hyperpolarisation.
Quick Recap
- Receptors are transducers: photo-, mechano-, chemo-, thermo- and nociceptors, each converting its stimulus into a nerve impulse.
- Neuron = cell body + dendrites (receive) + axon (transmit); sensory / motor / interneurons.
- Myelin from Schwann cells (PNS) or oligodendrocytes (CNS); nodes of Ranvier give saltatory conduction.
- Resting potential −70 mV (Na+/K+ ATPase, 3 Na+ out : 2 K+ in); threshold ~−55 mV; peak ~+30 mV.
- Action potential is all-or-nothing; stimulus strength is coded by frequency, not amplitude.
- Reflex arc: receptor → sensory neuron → centre (interneuron) → motor neuron → effector. Knee-jerk is monosynaptic; withdrawal is polysynaptic. Unconditioned reflexes are inborn, conditioned ones are learned.
- Brain: cerebrum (cognition, 4 lobes), cerebellum (balance, posture), medulla (heartbeat, breathing), hypothalamus (homeostasis), thalamus (sensory relay), midbrain (visual/auditory reflexes), pons (bridge).