A sensory receptor is a cell, or group of cells, that detects a stimulus. Receptors are transducers: they convert the energy of a stimulus into the electrical energy of a nerve impulse.Each type of receptor is specific to one type of stimulus:• photoreceptors detect light (rods and cones in the eye)
• mechanoreceptors detect pressure, touch or movement (for example, Pacinian corpuscles in the skin)
• chemoreceptors detect chemicals (for example, taste buds, and carbon dioxide receptors in the carotid bodies)
• thermoreceptors detect temperatureWhen a stimulus is detected, it changes the permeability of the receptor membrane to ions, usually by opening Na+ channels. This produces a small depolarisation called a generator potential. The bigger the stimulus, the bigger the generator potential.If the generator potential reaches threshold, an action potential is produced in the sensory neurone. If it does not, no impulse is sent. A stronger stimulus produces a larger generator potential and a higher frequency of action potentials.Example: the Pacinian corpuscle. In this pressure receptor, the ending of a sensory neurone is surrounded by many layers of connective tissue (lamellae) separated by gel. Pressure deforms the lamellae, which stretches the neurone membrane and opens stretch-mediated sodium ion channels. Na+ diffuses in, producing a generator potential.
Many receptors show adaptation: when a stimulus stays constant, the frequency of impulses falls. This is why you stop noticing your clothes, and it lets the nervous system concentrate on changes in the environment.
• mechanoreceptors detect pressure, touch or movement (for example, Pacinian corpuscles in the skin)
• chemoreceptors detect chemicals (for example, taste buds, and carbon dioxide receptors in the carotid bodies)
• thermoreceptors detect temperatureWhen a stimulus is detected, it changes the permeability of the receptor membrane to ions, usually by opening Na+ channels. This produces a small depolarisation called a generator potential. The bigger the stimulus, the bigger the generator potential.If the generator potential reaches threshold, an action potential is produced in the sensory neurone. If it does not, no impulse is sent. A stronger stimulus produces a larger generator potential and a higher frequency of action potentials.Example: the Pacinian corpuscle. In this pressure receptor, the ending of a sensory neurone is surrounded by many layers of connective tissue (lamellae) separated by gel. Pressure deforms the lamellae, which stretches the neurone membrane and opens stretch-mediated sodium ion channels. Na+ diffuses in, producing a generator potential.
Many receptors show adaptation: when a stimulus stays constant, the frequency of impulses falls. This is why you stop noticing your clothes, and it lets the nervous system concentrate on changes in the environment.