Video Transcript
A bright light is pointed into your
eye. Your pupil contracts to restrict
the light entering. In this reflex action, what is the
effector?
To answer this question, let’s
first recall what a reflex action is. A reflex action is a response to a
stimulus that occurs almost instantaneously and involuntarily, meaning that this
response is initiated without the need for conscious thought. If we look at the question, it
describes such an action occurring to change the size of the pupil and asks us to
say what the effector is in this scenario. But what is an effector? An effector is the part of a body,
usually a muscle or gland, that carries out a response to a stimulus. A stimulus is any change in the
internal or external environment that can influence the organism’s activity, for
example, a change in the surrounding temperature or a change in blood glucose
levels.
The question asks about a response
in the pupil, so let’s look at this in more detail. The size of the pupil changes in
response to changes in light intensity. In dim light, the diameter of the
pupil increases to allow more light into the eye to help us see. In bright light, the diameter gets
smaller to protect the light-sensitive cells in the retina, the rods and cones, from
being damaged. The size of the pupil is controlled
by small muscles in the iris, the colored part of your eye.
There are two types of muscles in
the iris: the circular and radial muscles. As the name suggests, the circular
muscles go around the iris, as shown in the diagram here. The radial muscles radiate out from
the pupil and are anchored on the outer edge of the iris. Muscles become shorter when they
contract. So when the circular muscles
contract, the circumference of each gets shorter and the diameter of the pupil is
reduced. When the radial muscles contract,
they pull outwards, because they are fixed around the edge of the iris and so they
widen the pupil. All pairs of muscles work
antagonistically, in other words, against each other. So when the circular muscles
contract, the radial muscles relax and vice versa.
In a reflex arc, there is an order
of events. The first is the stimulus, and the
last is the response, which is carried out by the effector. If we return to the question, here
the stimulus is the light being shone into the eye and the response is the pupil
contracting.
Having reviewed the key facts about
the response of the eye to changes in light intensity, we can now answer the
question correctly. In this reflex reaction, the
effector is the muscles around the pupil.
