Coordination and Control
Animals coordinate their internal environment and respond to stimuli through two systems — nervous and endocrine. This chapter builds the nervous foundation at MCAT mechanism depth: the neuron, how it generates a resting and action potential, how impulses travel, the reflex arc and spinal cord, and the overall organization of the nervous system (CNS vs PNS, somatic vs autonomic). The synapse, brain regions, and endocrine system are covered in the companion Nervous and Chemical Coordination notes. This is high-yield for the Bio/Biochem section.
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 specialized 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 meter 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 the PNS) or oligodendrocytes (in the 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.
Non-conducting support cells. Astrocytes (blood-brain barrier, support), oligodendrocytes (myelin in the CNS), Schwann cells (myelin in the PNS), microglia (immune defense), 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 the cerebral cortex.
- Parkinson's disease — loss of dopaminergic neurons in the substantia nigra.
- Polio — viral destruction of motor neurons in the spinal cord.
The Nerve Impulse
Neurons carry information as brief electrical signals. At rest the membrane is polarized; a stimulus that reaches threshold triggers a self-propagating action potential.
An unstimulated neuron sits at about −70 mV, inside negative relative to outside. This is set up by:
- The Na+/K+ ATPase pump moving 3 Na+ out and 2 K+ in per ATP, building the ion gradients (high Na+ outside, high K+ inside).
- K+ leak channels that make the membrane far more permeable to K+ than to Na+, so the resting potential lies close to the K+ equilibrium potential.
- Large intracellular anions (proteins) that cannot cross the membrane.
When a stimulus depolarizes the membrane to threshold (~−55 mV), an all-or-none impulse fires:
- Depolarization — voltage-gated Na+ channels open; Na+ rushes in; the potential shoots up to ~+30 mV.
- Repolarization — Na+ channels inactivate and voltage-gated K+ channels open; K+ flows out, restoring the negative interior.
- Hyperpolarization — K+ channels close slowly, causing a brief overshoot below −70 mV.
- Refractory period — the absolute phase (Na+ channels inactivated) prevents any second impulse; the relative phase needs a stronger-than-normal stimulus. Together they force one-way conduction.
Action potentials obey the all-or-nothing principle: threshold is either reached (a full spike fires) or not (nothing happens). Stimulus strength is coded by impulse frequency, not amplitude.
In myelinated axons the insulating myelin sheath forces the impulse to regenerate only at the nodes of Ranvier, so the signal appears to "jump" from node to node — saltatory conduction. It is much faster and more energy-efficient than the continuous conduction of unmyelinated axons. Velocity also rises with axon diameter: large myelinated mammalian axons conduct at ~120 m/s, versus ~1 m/s in thin unmyelinated axons.
Organization of the Nervous System
The nervous system is divided anatomically into the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS) — all the nerves outside it. The PNS is subdivided by function.
- CNS — brain + spinal cord; integrates and processes information.
- PNS, sensory (afferent) division — carries input from receptors to the CNS.
- PNS, motor (efferent) division, split into:
- Somatic — voluntary control of skeletal muscle.
- Autonomic — involuntary control of smooth muscle, cardiac muscle, and glands; further split into sympathetic and parasympathetic.
The two autonomic divisions are antagonistic; their balance keeps the body in homeostasis.
| Effect / target | Sympathetic | Parasympathetic |
|---|---|---|
| Overall role | Fight or flight — mobilizes energy | Rest and digest — conserves & restores |
| Origin (CNS) | Thoracic + lumbar (T1–L2) | Cranial nerves (III, VII, IX, X) + sacral (S2–S4) |
| Main neurotransmitter | Noradrenaline (postganglionic) | Acetylcholine |
| Heart rate | ↑ Increases | ↓ Decreases |
| Blood pressure | ↑ Increases | ↓ Decreases |
| Pupil | Dilates (mydriasis) | Constricts (miosis) |
| Bronchi | Dilate | Constrict |
| Digestion / peristalsis | Inhibited | Stimulated |
| Bladder | Relaxes (retention) | Contracts (urination) |
| Adrenal medulla | Releases adrenaline + noradrenaline | No effect |
| Liver | Glycogenolysis → releases glucose | Promotes glycogen synthesis |
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 gray matter (cell bodies and unmyelinated fibers) surrounded by white matter (myelinated fiber 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 the 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) fibers into the cord; bears the dorsal root ganglion housing sensory cell bodies.
- Ventral (anterior) root — carries motor (efferent) fibers 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 center — 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 the impulse via the dorsal root into the spinal cord.
- Interneuron — processes the signal in gray matter (in polysynaptic reflexes).
- Motor neuron — carries the response via the 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. During the depolarization phase of an action potential, which channels open?
Depolarization is driven by voltage-gated Na+ channels opening once threshold (~−55 mV) is reached, letting Na+ rush in and drive the potential toward +30 mV. Voltage-gated K+ channels open later, during repolarization. The Na+/K+ pump maintains the ion gradients but does not itself generate the spike.
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 the nodes of Ranvier, where saltatory conduction occurs.
Q3. The resting membrane potential of a typical neuron is approximately:
A resting neuron is polarized at about −70 mV (inside negative), set up by the Na+/K+ pump and K+ leak channels. Threshold is around −55 mV; depolarization peaks near +30 mV during an action potential.
Q4. During the "fight-or-flight" response, the sympathetic nervous system:
The sympathetic division mobilizes the body for action — raising heart rate and blood pressure, dilating the pupils and bronchi, and inhibiting digestion. Its main postganglionic transmitter is noradrenaline. The parasympathetic ("rest and digest") division produces the opposite effects.
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); sensory / motor / interneurons.
- Resting potential ~−70 mV, set by the Na+/K+ ATPase and K+ leak channels.
- Action potential: depolarization (Na+ in) → repolarization (K+ out) → hyperpolarization; all-or-none; refractory period forces one-way travel.
- Saltatory conduction in myelinated axons (node to node) is fast and energy-efficient.
- Organization: CNS (brain + spinal cord) vs PNS; PNS motor = somatic (voluntary) + autonomic (sympathetic + parasympathetic).
- Spinal cord: 45 cm, 31 pairs of nerves; gray 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.