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Response, receptors and control of heart rate Typeit
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The retina of the human eye contains two types of cell; cells cannot detect colour but respond to low-intensity light, whereas cone cells come in three different types and have better visual acuity.
Animals detect stimuli using sensory receptors. A coordinator in the central nervous system (either the spinal cord or the ) then organises a response, which is brought about by an effector.
An example of a receptor is a , which responds to changes in mechanical pressure. When its plasma membrane is deformed, ions enter the neurone and cause depolarisation, which creates a generator potential.
Rapid, involuntary nervous responses are called . They involve the three types of neurone - sensory neurones, intermediate neurones and neurones.
The main way that plants respond to change is via tropisms controlled by plant factors such as auxins. The uneven distribution of an auxin called causes cells in certain parts of roots or shoots to elongate - it can cause shoots to bend towards light in positive , or away from the effect of gravity, known as negative gravitropism.
Animals can also have basic responses to changing conditions - are directional responses, and are responses that result in the animal changing its speed or rate of turning.
Animals' heart rates are controlled via two systems; the sympathetic nervous system generally causes an increase in heart rate, while the nervous system usually does the opposite. For example, an increase in carbon dioxide concentration reduces the pH of the blood, which triggers an increase in the number of impulses sent to the , the control centre in the brain. This leads to the sending electrical waves across the atria at a greater rate, which in turn causes the waves to pass through the atrioventricular node more quickly, causing contraction of the .