Reflexes

In situations where there is a danger to the body, reflex actions can save precious fractions of a second. This is achieved through what is termed a reflex arc. A reflex arc begins with a receptor. This passes a signal to a sensory neuron that connects to the spinal cord. In the spinal cord, a relay neuron sends the signal directly to a motor neuron. The motor neuron connects to an effector which is generally a muscle so that the body can respond. The relay neuron also sends a signal to the brain so that you are aware of what is happening.

Two common examples of reflex actions are taking your hand away from something sharp and the iris of an eye changing the size of the pupil. In the first example, receptors detect the sharp object cutting into the skin. The signal travels rapidly through a reflex arc and the part of the body touching the sharp object (e.g. your hand if you accidentally get hold of a rose bush or bramble) rapidly releases the object or moves the body away. Fractions of a second saved here could be crucial - just imagine if the sharp object is entering the body near to one of your main arteries ...

In the second example above, too much light entering the eye could seriously damage it, so it is important for any animal to be able to limit the amount of light entering. This is done using the colored muscle called the iris. Receptors in the eye are the starting point for a reflex arc that relaxes or contracts the iris, regulating the size of the pupil and therefore controlling the amount of light reaching the easily damaged light receptors of the retina.

Fast reflexes can make all the difference, but don't worry - you wont need them to play this quiz on our automatic response mechanism!