<P> A receptor cell converts the energy in a stimulus into an electrical signal . Receptors are broadly split into two main categories: exteroceptors, which receive external sensory stimuli, and interoceptors, which receive internal sensory stimuli . </P> <P> In the visual system, sensory cells called rod and cone cells in the retina convert the physical energy of light signals into electrical impulses that travel to the brain . The light causes a conformational change in a protein called rhodopsin . This conformational change sets in motion a series of molecular events that result in a reduction of the electrochemical gradient of the photoreceptor . The decrease in the electrochemical gradient causes a reduction in the electrical signals going to the brain . Thus, in this example, more light hitting the photoreceptor results in the transduction of a signal into fewer electrical impulses, effectively communicating that stimulus to the brain . A change in neurotransmitter release is mediated through a second messenger system . Note that the change in neurotransmitter release is by rods . Because of the change, a change in light intensity causes the response of the rods to be much slower than expected (for a process associated with the nervous system). </P> <P> In the auditory system, sound vibrations (mechanical energy) are transduced into electrical energy by hair cells in the inner ear . Sound vibrations from an object cause vibrations in air molecules, which in turn, vibrate the ear drum . The movement of the eardrum causes the bones of the middle ear (the ossicles) to vibrate . These vibrations then pass into the cochlea, the organ of hearing . Within the cochlea, the hair cells on the sensory epithelium of the organ of Corti bend and cause movement of the basilar membrane . The membrane undulates in different sized waves according to the frequency of the sound . Hair cells are then able to convert this movement (mechanical energy) into electrical signals (graded receptor potentials) which travel along auditory nerves to hearing centres in the brain . </P> <P> In the olfactory system, odorant molecules in the mucus bind to G - protein receptors on olfactory cells . The G - protein activates a downstream signalling cascade that causes increased level of cyclic - AMP (cAMP), which trigger neurotransmitter release . </P>

Where is the mechanical stimulation of sound converted into a nerve impulse
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