Coordination and Control
Animals coordinate their internal environment and respond to external stimuli through two major systems — nervous and endocrine. This focused chapter looks at the foundation pieces of the nervous side: neurons (the cells that carry signals) and the spinal cord (the major conduit and reflex centre). Expect 2-3 MCQs from this material.
Neurons
A neuron (nerve cell) is the structural and functional unit of the nervous system. The human brain alone contains ~86 billion neurons. Neurons are highly specialised for the rapid generation and propagation of electrical signals (action potentials).
Parts of a typical neuron
- Cell body (soma / perikaryon)
- Contains the nucleus, ribosomes, mitochondria, and Nissl granules (rough ER). Carries out protein synthesis.
- Dendrites
- Short, highly branched processes that receive impulses from other neurons or from sensory receptors.
- Axon
- A single long process that conducts the impulse away from the cell body to the axon terminals. Can be over a metre long (sciatic nerve).
- Axon terminals (synaptic knobs)
- End of the axon containing vesicles of neurotransmitter released into the synapse.
- Myelin sheath
- Lipid-rich insulating layer formed by Schwann cells (in PNS) or oligodendrocytes (in CNS). Increases conduction velocity by allowing saltatory conduction.
- Nodes of Ranvier
- Gaps between adjacent Schwann cells where the axon is exposed; sites where action potentials regenerate.
- Sensory (afferent) — carry impulses from receptors to the CNS. Usually unipolar.
- Motor (efferent) — carry impulses from the CNS to effectors (muscles, glands). Usually multipolar.
- Interneurons (associative) — located within the CNS; connect sensory and motor neurons. The bulk of CNS neurons.
An unstimulated neuron maintains a membrane potential of about −70 mV. This is created by the Na+/K+ ATPase pump (3 Na+ out, 2 K+ in for every ATP) and selective leak channels. The inside of the cell is negative relative to the outside.
A wave of depolarisation propagated along the axon when threshold is reached (~−55 mV).
- Depolarisation — voltage-gated Na+ channels open; Na+ rushes in; potential rises to ~+30 mV.
- Repolarisation — Na+ channels close, K+ channels open; K+ flows out; potential drops back.
- Hyperpolarisation — brief overshoot below −70 mV.
- Refractory period — ensures one-way conduction.
Action potentials follow the all-or-nothing principle: either threshold is reached and a full impulse fires, or nothing happens.
Where two neurons meet at a synapse, the impulse crosses chemically. Action potential → Ca2+ influx into axon terminal → vesicles fuse with the presynaptic membrane → neurotransmitter (e.g., acetylcholine, dopamine, serotonin, GABA) is released → binds postsynaptic receptors → new action potential generated. Synapses ensure one-way transmission and allow integration.
Non-conducting support cells. Astrocytes (blood-brain barrier, support), oligodendrocytes (myelin in CNS), Schwann cells (myelin in PNS), microglia (immune defence), and ependymal cells (line ventricles, produce CSF). They outnumber neurons in some brain regions.
Common neuron-related disorders
- Multiple sclerosis (MS) — autoimmune destruction of CNS myelin.
- Guillain-Barré syndrome — autoimmune demyelination of peripheral nerves.
- Alzheimer's disease — loss of cholinergic neurons in cerebral cortex.
- Parkinson's disease — loss of dopaminergic neurons in substantia nigra.
- Polio — viral destruction of motor neurons in the spinal cord.
Spinal Cord
The spinal cord is a long, cylindrical part of the central nervous system that runs from the medulla oblongata down through the vertebral canal. It is about 45 cm long and ends as the conus medullaris at the level of the L1-L2 vertebrae, beyond which the lumbar and sacral nerves form the cauda equina ("horse's tail").
Protective coverings
The spinal cord is protected by the vertebral column (33 vertebrae) and three connective-tissue layers called the meninges: dura mater (outer, tough), arachnoid mater (middle, web-like), and pia mater (inner, delicate). Cerebrospinal fluid (CSF) circulates in the subarachnoid space and central canal, cushioning the cord and supplying nutrients.
In cross-section, the spinal cord shows an inner butterfly- or H-shaped grey matter (cell bodies and unmyelinated fibres) surrounded by white matter (myelinated fibre tracts) — the opposite arrangement to the brain.
- Dorsal (posterior) horn — contains sensory interneurons.
- Ventral (anterior) horn — contains cell bodies of motor neurons.
- Lateral horn (in thoracic region) — autonomic motor neurons.
- Central canal — runs the length of the cord, filled with CSF.
31 pairs of spinal nerves emerge from the cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each has two roots:
- Dorsal (posterior) root — carries sensory (afferent) fibres into the cord; bears the dorsal root ganglion housing sensory cell bodies.
- Ventral (anterior) root — carries motor (efferent) fibres out of the cord.
The two roots merge to form a mixed spinal nerve — a key principle known as the Bell-Magendie law.
- Conduction — ascending tracts (e.g., spinothalamic, dorsal columns) carry sensory information to the brain; descending tracts (e.g., corticospinal) carry motor commands down.
- Reflex centre — mediates spinal reflexes such as the knee-jerk and withdrawal reflex without the brain's conscious involvement.
The simplest functional pathway of the nervous system. The monosynaptic stretch reflex (e.g., knee-jerk) involves only two neurons; most reflexes are polysynaptic.
- Receptor — detects the stimulus (e.g., muscle spindle, pain receptor).
- Sensory neuron — carries impulse via the dorsal root into the spinal cord.
- Interneuron — processes the signal in grey matter (in polysynaptic reflexes).
- Motor neuron — carries response via ventral root.
- Effector — muscle or gland that produces the response.
Reflexes are fast, involuntary, and stereotyped — they protect the body before the brain even registers the stimulus.
Spinal cord disorders
- Poliomyelitis — viral destruction of ventral horn motor neurons → flaccid paralysis.
- Spinal cord injury — transection → paraplegia (lower limbs) or quadriplegia (all four limbs).
- Meningitis — inflammation of the meninges.
- Spina bifida — congenital failure of vertebral arch closure (folate deficiency in pregnancy).
- Multiple sclerosis — demyelination of CNS tracts.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter.
Q1. The functional unit of the nervous system is the:
The neuron is the structural and functional unit of the nervous system, specialised for generating and conducting impulses. Glial cells (astrocytes, Schwann cells) provide support but do not transmit signals.
Q2. The myelin sheath of peripheral nerves is produced by:
Schwann cells produce myelin around peripheral nerve axons. Oligodendrocytes do the same job in the central nervous system. The gaps between Schwann cells are nodes of Ranvier, where saltatory conduction occurs.
Q3. Resting membrane potential of a typical neuron is approximately:
A resting neuron is polarised at about −70 mV (inside negative). Threshold is around −55 mV; depolarisation peaks near +30 mV during an action potential.
Q4. Sensory fibres enter the spinal cord through the:
Sensory (afferent) fibres enter via the dorsal (posterior) root, whose ganglion contains sensory cell bodies. Motor (efferent) fibres exit through the ventral root — the Bell-Magendie law.
Q5. The simplest reflex arc, the knee-jerk, involves only:
The knee-jerk is a monosynaptic stretch reflex: the sensory neuron synapses directly onto a motor neuron in the spinal cord — only two neurons and one synapse, allowing the fastest possible response.
Quick Recap
- Neuron parts: dendrites (receive), cell body, axon (conduct), terminals (release neurotransmitter).
- Three functional types: sensory, motor, interneuron.
- Resting potential −70 mV; action potential is all-or-nothing; saltatory conduction in myelinated axons.
- Synaptic transmission: Ca2+ influx → vesicle fusion → neurotransmitter release.
- Spinal cord: 45 cm, 31 pairs of spinal nerves; grey matter inside (H-shape), white matter outside.
- Dorsal root = sensory; ventral root = motor (Bell-Magendie law).
- Reflex arc: receptor → sensory neuron → (interneuron) → motor neuron → effector.