2.2 Central & Peripheral Nervous Systems & the Senses

Organizing the Nervous System

The nervous system has two main divisions. The central nervous system (CNS) is the brain and spinal cord — the processing centre. The peripheral nervous system (PNS) is every nerve outside the CNS, carrying signals in and out. Biology 30 expects you to match each brain region to its function and to distinguish the PNS subdivisions, so build a clear mental map before exam day.

Think of the CNS as the command centre that interprets information and decides on responses, while the PNS is the wiring that gathers sensory data and delivers the commands. Sensory neurons of the PNS feed information into the CNS, and motor neurons of the PNS carry instructions out to muscles and glands.

The Brain: Three Regions to Know

Three brain structures appear most often on the diploma exam:

• Cerebrum — the large, folded outer brain; controls conscious thought, reasoning, memory, sensory interpretation, and voluntary movement. Divided into left and right hemispheres.
• Cerebellum — sits below and behind the cerebrum; coordinates balance, posture, and fine muscle movement. Damage causes jerky, uncoordinated movement.
• Medulla oblongata — connects the brain to the spinal cord; controls involuntary vital functions such as heart rate, breathing rate, and blood pressure.

Also useful: the hypothalamus links the nervous and endocrine systems and regulates temperature, hunger, and thirst, while the cerebral cortex is the cerebrum's outer grey-matter layer.

The Spinal Cord & PNS

The spinal cord is a bundle of nerves running through the vertebral column. It relays signals between the body and the brain and serves as the integration centre for many reflexes, as you saw in the reflex arc.

The peripheral nervous system divides into two functional parts:

The somatic division governs movements you choose; the autonomic division runs automatically in the background.

Sympathetic vs Parasympathetic

The autonomic division has two antagonistic branches that balance each other:

• Sympathetic — the "fight-or-flight" response. It speeds the heart, dilates pupils and bronchioles, releases glucose, and diverts blood to muscles. It dominates during stress or danger.
• Parasympathetic — the "rest-and-digest" response. It slows the heart, constricts pupils, and stimulates digestion. It dominates during calm and recovery.

Because they oppose each other, the body fine-tunes organ activity by shifting the balance. For example, the sympathetic branch raises heart rate while the parasympathetic branch lowers it, keeping the cardiovascular system responsive.

A useful exam framing: picture a sudden scare. The sympathetic branch instantly readies the body for action — heart pounding, pupils wide, airways open, glucose mobilized. Once the threat passes, the parasympathetic branch takes over to slow the heart, restore digestion, and conserve energy. Most organs receive input from both branches, so the body never simply switches one off; it shifts the dominance between them moment to moment.

The Eye: Focusing Light

Light passes through several structures before reaching the photoreceptors:

1. Cornea — transparent front layer; does most of the light bending.
2. Pupil — opening whose size is set by the iris (the coloured muscle).
3. Lens — fine-tunes focus by changing shape.
4. Retina — light-sensitive inner layer holding the photoreceptors.
5. Optic nerve — carries impulses to the brain. Where it leaves the eye there are no receptors — the blind spot.

The retina holds two photoreceptor types. Rods are sensitive in dim light and give black-and-white vision. Cones need bright light and detect colour (red, green, blue). The fovea, packed with cones, gives the sharpest vision.

Accommodation

Accommodation is the eye's adjustment of the lens to focus on objects at different distances, controlled by the ciliary muscle.

• Near object: ciliary muscle contracts, suspensory ligaments slacken, and the lens becomes rounder (more convex) to bend light more.
• Distant object: ciliary muscle relaxes, ligaments tighten, and the lens flattens to bend light less.

Keep the relationship straight: a rounder lens is for close vision, a flatter lens for far vision.

The Ear: Hearing & Balance

The ear handles two senses. For hearing, sound waves enter the outer ear and vibrate the eardrum (tympanic membrane). Three tiny bones (hammer, anvil, stirrup) amplify the vibration and pass it to the fluid-filled cochlea. Hair cells inside the cochlea convert the vibrations into nerve impulses, sent along the auditory nerve to the brain.

For balance, the semicircular canals detect rotational movement and head position. Fluid shifting inside them bends hair cells, signalling the brain about orientation — which is why spinning makes you dizzy after you stop. Hearing and balance are therefore separate functions handled by different inner-ear structures.

A common exam point: the cochlea distinguishes pitch and volume of sound, while the semicircular canals and associated vestibular structures handle equilibrium. Both rely on the same trick — fluid movement bending sensitive hair cells that convert mechanical motion into nerve impulses — but they feed different parts of the brain and serve completely different senses. Do not confuse the cochlea (hearing) with the semicircular canals (balance) on a labelling question.