 In this video, I will define receptive field and sensory transduction, describe the structural and functional classification of sensory receptors, and contrast sensory reception and perception. Sensory receptors are cells that monitor the internal and external environment. A sensory receptor has a receptive field, which is the region of the environment that is monitored by that sensory receptor. A specific example shown in this illustration is a rod, a type of photoreceptor found in the eye. A region of your visual field corresponds to the receptive field for an individual rod photoreceptor. Sensory transduction is the process where a stimulus alters the membrane permeability of a sensory receptor, producing a type of graded potential known as a receptor potential. The sensory transduction mechanism for a photoreceptor in the eye, like a rod, involves a light-sensitive G-protein-coupled receptor protein known as radopsin. Within the radopsin protein is a small molecule cofactor known as retinal. When light is absorbed by the retinal molecule within radopsin, light will cause a change in the shape of retinal from the cis to the trans isomer. This change in shape causes activation of the radopsin protein to stimulate an intracellular signaling mechanism that will ultimately lead to closing of an ion channel, causing hyperpolarization of the membrane potential, which decreases the release of the neurotransmitter glutamate from the photoreceptor cell to the dendrites of the bipolar cell in the retina of the eye. The sensory transduction mechanism in photoreceptors involves light activating the radopsin G-protein-coupled receptor protein. When radopsin is activated, it has a G-protein subunit known as transducin that will activate an enzyme known as phosphodiesterase. Phosphodiesterase catalyzes a chemical reaction that breaks down cyclic guanosine monophosphate to form guanosine monophosphate. Cyclic GMP or cyclic guanosine monophosphate is a second messenger molecule inside photoreceptors that has the function of stimulating the opening of a cyclic nucleotide-gated sodium ion channel. As the cyclic GMP concentration of the cytosol decreases in response to light, the sodium ion channel will close, leading to hyperpolarization of the photoreceptor cell and decreased release of the neurotransmitter glutamate. Another example of sensory transduction involves the mechanically gated ion channels in auditory hair cells of the inner ear. These auditory hair cells detect vibration as the basilar membrane vibrates. The auditory hair cells move relative to a more rigid tectorial membrane. The stereocilia connects from the basilar membrane to the tectorial membrane. And on the stereocilia are mechanically gated ion channels. The tether on the mechanically gated ion channel connects to the tectorial membrane, so that as the stereocilia bend, it causes mechanically gated ion channels to be pulled open. Then cations will enter the auditory hair cell, causing depolarization that will stimulate the release of neurotransmitters from the auditory hair cell. Sensor receptors can be classified structurally as either free nerve endings, encapsulated nerve endings, or specialized receptor cells. A free nerve ending is an un-encapsulated nerve ending, where the dendrites of an afferent neuron are not surrounded by a capsule of connective tissue and directly detect a stimulus, then relay that information in through the afferent fiber, the axon of that sensory neuron. An encapsulated nerve ending is the dendrites of an afferent neuron that are surrounded with a capsule of connective tissue. These encapsulated nerve endings are commonly found in the skin where they're important for detecting the mechanical stimulus for the somatosensory modality or touch. Then a specialized receptor cell is a cell distinct from a sensory neuron that will detect a stimulus in the environment and then release neurotransmitter to activate the dendrites of a neuron. We can also functionally classify sensory receptors. One functional classification of sensory receptors is chemoreceptors that detect chemicals dissolved in the extracellular fluid. We'll see that the sense of smell, also known as olfaction, is a sense that requires chemoreceptors. Similarly, the sense of taste, known as gustation, is a sense that requires chemoreceptors. But chemoreceptors are also important for monitoring the concentration of chemicals in the blood. Photoreceptors are sensory receptors that respond to light, and we'll see the rods and cones of the retina are the photoreceptors that enable vision. Mechanoreceptors are sensory receptors that respond to a physical stimulus such as pressure, vibration, or stretching. The somatosensory receptors that enable the sense commonly referred to as touch are mechanoreceptors. We'll also see other examples of mechanoreceptors such as the auditory hair cells of the inner ear that enable audition, commonly known as the sense of hearing. Thermoreceptors respond to changes in temperature. There are thermoreceptors that are widely distributed throughout the body. Lots of thermal receptors found in the skin that are free nerve endings, which will respond to changes in the temperature of the skin. The last functional class of sensory receptors are nosoceptors that detect tissue damage, producing the perception that we commonly refer to as pain. Chemicals that are released from damaged cells in a tissue activate the receptor proteins on the dendrites of free nerve endings to stimulate nosoception or pain. Olfactory receptors are an example of chemoreceptors that are structurally classified as free or unencapsulated nerve endings. The dendrites of the olfactory receptor neurons are exposed in the mucus of the olfactory epithelium in the superior nasal cavity. The odorants are molecules that dissolve in the mucus and bind to the receptor proteins on the surface of the dendrites of the olfactory receptor neurons. Merkle cells are an example of specialized receptor cells in the epidermis that are functionally classified as mechanoreceptors. The merkle cells have mechanically gated ion channels that respond to bending of the superficial layer of the skin known as the epidermis. As the epidermis bends, it will cause mechanically gated ion channels to be pulled open in merkle cells, causing depolarization that stimulates the release of neurotransmitter from the merkle cell to the dendrites of a sensory neuron, the afferent neuron that will then relay sensory information into the central nervous system. Meisner's corpuscles are an example of an encapsulated nerve ending found in the skin located in the dermis of the skin, which is the connective tissue layer forming the majority of the skin just deep to the epidermis. Meisner's corpuscles are found in the superficial region of the dermis known as the papillary layer of the dermis. These Meisner's corpuscles are functionally categorized as mechanoreceptors. They contain mechanically gated ion channels that will open in response to a mechanical force pushing on the surface of the skin. Meisner's corpuscles will detect light pressure or medium frequency vibrations on the surface of the skin as the connective tissue capsules surrounding the dendrites and attached to the mechanically gated ion channels. The pressure on the surface of the skin causes the connective tissue capsule to move relative to the mechanically gated ion channels on the dendrites. This leads to opening of the ion channels, creating depolarization that will then stimulate an action potential in the afferent fiber, the sensory axon that relays information into the central nervous system. Up to this point, this video has focused on the process of reception, where sensory receptors detect a stimulus. The sensory receptors then relay information in through a sensory pathway with an afferent neuron that carries information into the central nervous system. Within the central nervous system, perception is the interpretation of sensory information. Perception occurs in specific regions of the brain for distinct sensory modalities. A sensory modality is the sensory information detected by a specific sensory receptor and transmitted through a specific sensory pathway and then interpreted by a specific region of the brain. For example, the somatosensory cortex is located in the post-central gyrus of the parietal lobe shown in the pinkish-purple color in this illustration. This somatosensory processing region receives the general senses of pressure and vibration, itch, pain, changes in temperature, and also proprioception, the position of the body. It receives this sensory information coming from sensory receptors that are widely distributed throughout the body, and the somatosensory processing region will interpret this sensory information in order to produce the conscious perception of the feeling of touch or the feeling of the position of the body. Another example is the special sense of audition or hearing. We call audition a special sense because there are specialized sensory receptor organs that contain the auditory hair cells that detect sound. And then the sensory information is relayed in through an auditory pathway to primary auditory cortex located in the lateral temporal lobe shown in the blue color in this illustration. The primary auditory cortex is the location where the sensory information for hearing is perceived, leading to the conscious perception of sound. Similarly, there's a distinct region involved in processing visual sensory information that's found in the occipital lobe and receives sensory information coming from the photoreceptor cells in the eyes. So each of the special senses, vision, hearing, taste or gustation, smell or olfaction, each of these has specialized sensory organs where the sensory receptors are located. In contrast, the general senses come from receptors that are widely distributed in the body.