Responses in animals

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Responses in Animals


Control of heart rate involves the brain and autonomic nervous system.

  1. The sinoatrial node (SAN) generates electrical impulses that cause the cardiac muscles to contract.
  2. The rate at which the SAN fires (i.e. the heart rate) is unconsciously controlled by a part of the brain called the medulla oblongata.
  3. Animals need to alter their heart rate to respond to internal stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen.
  4. Stimuli are detected by pressure receptors and chemical receptors:

-         There are pressure receptors called baroreceptors in the aorta and the vena cava. They’re stimulated high and low blood pressure.

-         There are chemical receptors called chemoreceptors in the aorta, the carotid artery (a major artery in the neck) and in the medulla. They monitor the oxygen level in the blood and also carbon dioxide and pH (which are indicators of oxygen levels).

  1. Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (which are part of the autonomic nervous system).


Here are some examples –


Stimulus – High blood pressure.

Receptor – Baroreceptors detect high blood pressure.

Neurone and neurotransmitter – Impulses are sent to the medulla, which sends impulses along parasympathetic neurones. These secrete acetylcholine, which binds to receptors on the SAN.

Effector – Cardiac


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