The nervous system

MDCAT Biology · Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The central nervous system is the brain and spinal cord. The peripheral nervous system is the 12 pairs of cranial nerves and 31 pairs of spinal nerves that link it to the rest of the body.

The brain has three regions. The forebrain contains the cerebrum, thalamus and limbic system (including the hypothalamus). The midbrain relays impulses and deals with sight and hearing reflexes. The hindbrain contains the cerebellum, pons and medulla oblongata.

A reflex is a fast, automatic response that does not wait for a decision by the brain. Its pathway, the reflex arc, usually passes through the spinal cord.

Rules to remember

Cerebrum (cerebral cortex): thinking, memory, intelligence, voluntary movement, senses. Each hemisphere controls the opposite side of the body; the corpus callosum joins the two
Thalamus: relays sensory impulses to the cortex. Hypothalamus: temperature, hunger, thirst, controls the pituitary. Amygdala and hippocampus: emotion and memory
Cerebellum: balance, posture, coordination of movement. Medulla oblongata: breathing, heartbeat, blood pressure. Pons: links cerebellum, medulla and cerebrum
Spinal cord: grey matter (cell bodies) inside in an H shape, white matter (myelinated axons) outside; the cerebrum has grey matter outside
Reflex arc: receptor → sensory neuron (dorsal root) → interneuron in the spinal cord → motor neuron (ventral root) → effector
Sympathetic system: prepares for emergency (faster heart, dilated pupils, slower digestion). Parasympathetic: rest (slower heart, constricted pupils, active digestion)
Receptors: chemoreceptors (taste, smell), mechanoreceptors (touch, pressure, hearing), photoreceptors (light), thermoreceptors (temperature), nociceptors (pain). Disorders: Parkinson's (low dopamine, tremor), epilepsy (seizures), Alzheimer's (loss of memory)

Common mistake

Swapping the cerebellum and the cerebrum, or the dorsal and ventral roots. Cerebellum is balance and coordination; dorsal root is sensory (in), ventral root is motor (out).

Worked example

A stroke damages the area of the left cerebral cortex that starts voluntary movements. Which part of the body is most likely to be paralysed, and can the knee-jerk reflex still occur?

Voluntary movement is started by the cerebral cortex.
Each cerebral hemisphere controls the opposite side of the body, so the left cortex controls the right side.
The knee jerk is a spinal reflex; its arc does not pass through the cerebrum.

Answer: Voluntary movement is lost on the right side of the body, but the knee-jerk reflex can still occur.

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