Biology

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Video

Materials

• 2 friends (1 subject and 1 assistant)
• 1 chair
• 1 screwdriver (or other blunt object)
• 1 window
• A cool place (for example, outdoors or the freezer)

Explanation

A reflex is an involuntary and almost instantaneous movement. A reflex can be either learned or innate. All reflexes you examine here are innate. This means that it is hardwired into your DNA. Once upon a time they arose as a result of the human DNA, or the DNA of our ancestors, being altered (mutated). Since the reflexes are still there, they most likely serve a purpose in the evolutionary struggle for survival.

The knee reflex (patellar reflex) is the kicking of the lower leg when the patellar tendon is subjected to a blow. The patellar tendon is the end of the large quadriceps muscle group. These muscles start off as tendons attached to the hip, turns into muscles, and then ends as a tendon bundle - the patellar tendon - that attaches just below the knee. When you hit the patellar tendon, it stretches, which could lead to tearing of the quadriceps. To prevent it from tearing, the quads strikes back - it shortens, i.e. contracts. The result is a kick of the lower leg. A torn quadriceps is very disabling so it is easy to understand why this reflex has been kept through natural selection.

The ankle jerk reflex (Achilles reflex) has the same function as the knee reflex, namely to prevent a muscle from tearing. In this case, the tendon bundle you strike belongs to the two calf muscles. A tear in any of these muscles is also very disabling, so this reflex also fulfills an important function.

Blink reflex #1 and blink reflex #2 is the subject blinking when something threatens to injure the eyes. These reflexes protect the eyesight, perhaps the most developed sense in humans.

The vestibulo-ocular reflex is the eyes moving in the opposite direction as the head moves. This makes it possible to look at something, even if the head is moving around. Without this reflex everything would be blurred when you are in motion, for example riding in a car.

The pupillary light reflex is the pupil, i.e. the opening that lets light into the eye, becoming smaller in bright light. This prevents too much light from entering the eye and damaging it. Snow blindness is the result of too much light entering the eye.

The pilomotor reflex is the hairs on the body rising. This happens when small muscles in the skin around the hair follicles contract, which is known as goose bumps. When the hairs stand up, a layer of stagnant air is formed in the "forest" of hairs. This air is quickly warmed by the skin and then acts as an insulating layer. However, this reflex had a greater effect on our ancestors who had more hair on their bodies.

The shivering reflex is the body starting to shake. It may take a while before this reflex starts, as the starting signal is that the inner parts of the body have cooled down. Shivering occurs when muscles throughout the body contract, quickly and jerkily. As always when muscles work, heat is formed.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• How hard do you have to hit to set off the knee reflex?
• How hard do you have to hit to set off the ankle jerk reflex?
• How fast does your hand have to move to set off blink reflex #1?
• How fast can the head move until the vestibulo-ocular reflex is not enough to maintain clear vision?
• Are there any differences in how the pilomotor reflex acts on different parts of the body?
• Are there any differences in how the shivering reflex acts on different parts of the body?
• Are there differences in how any of the reflexes behave in different people?

Gilla:		Dela: