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Response, receptors and control of heart rate GapFill
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Heart rate is controlled by stimulation of the cardiac muscle. Waves of excitation originating in the pass through Purkyne tissue in the , causing contraction of the and then the ventricles. Information received from chemoreceptors and pressure receptors can be used to modify the heart rate via the nervous system.
The nervous system consists of a series of interconnecting pathways which respond to changes in the environment, called . Pathways generally consist of a receptor, a and an effector. For example, a Pacinian corpuscle changes its permeability to ions when it is stretched. This establishes a generator potential, which travels to either the brain or the in the central nervous system, where the information is processed to generate an appropriate response.
The human eye responds to changes in light levels detected by rods and cones on the . Rod cells can respond to low-intensity light by breaking down the pigment , but only cone cells are able to resolve dots close together and detect differences in .
Animals often have simple responses which allow them to spend more time in a favourable environment and less time in an unfavourable one. For example, a woodlouse may display a which causes it to travel and turn more quickly upon entering a dry environment, or bacteria may tend to grow towards a source of glucose in a positive chemotaxis.
Plants can respond to directional stimuli by regulating their growth, e.g. changing the distribution of in different areas to determine the relative elongation of the roots and shoots. Shoots display negative and positive phototropism as they generally grow upwards towards the sun.