The central nervous system, sensory, relay and motor neurones, the reflex arc as a fast automatic response, the structure and function of the eye, and how the eye focuses light and adjusts to light intensity.

What this dot point is asking

CCEA Double Award wants the central nervous system, the three types of neurone, the reflex arc as a fast automatic response, then the structure of the eye and how it focuses and adjusts to light. The reflex arc and accommodation (focusing) are the two processes you must be able to describe in order.

The nervous system

There are three types of neurone (nerve cell):

• A sensory neurone carries impulses from a receptor (such as the skin or eye) to the CNS.
• A relay neurone carries impulses within the CNS, connecting sensory to motor neurones.
• A motor neurone carries impulses from the CNS to an effector (a muscle that contracts or a gland that secretes).

Where two neurones meet there is a tiny gap called a synapse, where a chemical carries the signal across.

The reflex arc

A reflex action is a fast, automatic response that does not involve conscious thought, which protects the body from harm. The pathway is the reflex arc:

stimulus to receptor to sensory neurone to relay neurone (in the spinal cord) to motor neurone to effector to response.

Because the impulse does not have to travel up to the conscious brain, the response is very quick - for example pulling your hand off a hot pan before you feel the pain.

The structure of the eye

Light enters through the transparent cornea, passes through the pupil (the hole), and is refracted by the lens onto the retina at the back. The retina contains light receptors that send impulses along the optic nerve to the brain. The coloured iris controls the size of the pupil, and the ciliary muscles and suspensory ligaments change the shape of the lens.

Focusing and adjusting to light

Accommodation is how the eye focuses:

• For a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter and refracts light more.
• For a distant object, the ciliary muscles relax, the suspensory ligaments tighten, and the lens becomes thinner and refracts light less.

The iris reflex adjusts to light intensity. In bright light, circular muscles in the iris contract and the pupil gets smaller, protecting the retina. In dim light, radial muscles contract and the pupil gets larger, letting more light in.

Examples in context

Example 1. Why reflexes are protective. The blink reflex closes the eye when something approaches it, far faster than a conscious decision could. Because the relay neurone shortcuts the response in the CNS, the eye is protected before the brain has even registered the threat.

Example 2. Coming out of a dark cinema. Stepping into bright sunlight makes you squint while the pupil quickly shrinks. The iris reflex reduces the light reaching the retina to protect the receptors, showing the iris responding to light intensity.

Try this

Q1. Name the type of neurone that carries impulses from a receptor to the CNS. [1 mark]

• Cue. Sensory neurone.

Q2. What happens to the pupil in bright light? [1 mark]

• Cue. It gets smaller.