![]() ![]() Touch receptors are not sensitive to light or sound they are sensitive only to touch or pressure. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve. (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair-like tethers. (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case. ![]() If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. ![]() Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 8.14) possesses specialized membranes that respond to pressure. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. There are two broad types of cellular systems that perform sensory transduction. This process is called sensory transduction. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system. Vestibular sensation, which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Senses provide information about the body and its environment. While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals. The shark, unlike most fish predators, is electrosensitive-that is, sensitive to electrical fields produced by other animals in its environment. All bilaterally symmetric animals have a sensory system, and the development of any species’ sensory system has been driven by natural selection thus, sensory systems differ among species according to the demands of their environments. In more advanced animals, the senses are constantly at work, making the animal aware of stimuli-such as light, or sound, or the presence of a chemical substance in the external environment-and monitoring information about the organism’s internal environment. Sensory information processing in animals ![]()
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