As we exist in the world, our bodies are tasked with receiving, integrating, and interpreting environmental inputs that provide information about our internal and external environments. Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems.
Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems. Sensory receptors code four aspects of a stimulus:.
Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor. Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function.
For example, sensory receptors in the retina are almost entirely photoreceptors. Our skin includes touch and temperature receptors, and our inner ears contain sensory mechanoreceptors designed for detecting vibrations caused by sound or used to maintain balance. Force -sensitive mechanoreceptors provide an example of how the placement of a sensory receptor plays a role in how our brains process sensory inputs. While the cutaneous touch receptors found in the dermis and epidermis of our skin and the muscle spindles that detect stretch in skeletal muscle are both mechanoreceptors, they serve discrete functions.
In both cases, the mechanoreceptors detect physical forces that result from the movement of the local tissue, cutaneous touch receptors provide information to our brain about the external environment, while muscle spindle receptors provide information about our internal environment.
Muscle spindle : Mammalian muscle spindle showing typical position in a muscle left , neuronal connections in spinal cord middle , and an expanded schematic right.
The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment Figure. The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place.
There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin.
The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane.
There are three cone photopigments, called opsins , which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.
At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon , which is described in physics as a packet of energy with properties of both a particle and a wave.
The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color.
Visible light is electromagnetic radiation with a wavelength between and nm. Wavelengths of electromagnetic radiation longer than nm fall into the infrared range, whereas wavelengths shorter than nm fall into the ultraviolet range.
Light with a wavelength of nm is blue whereas light with a wavelength of nm is dark red. All other colors fall between red and blue at various points along the wavelength scale. Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal.
Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. This molecule is referred to as cis -retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans — conformation, forming all- trans -retinal, which has a straight hydrocarbon chain Figure.
The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein.
The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the cis -retinal shape, the opsin cannot respond to light energy, which is called bleaching.
When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy. The opsins are sensitive to limited wavelengths of light.
Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of nm. The three color opsins have peak sensitivities of nm, nm, and nm corresponding roughly to the primary colors of red, green, and blue Figure.
The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision.
By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light.
Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.
Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side.
This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. Explain your answer. Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve.
For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.
Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior dorsal horn, or even the anterior ventral horn, at the level of the spinal cord where they enter.
Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord.
A branch may also turn into the posterior dorsal column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral , in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.
Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.
Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral , meaning that a cranial nerve on the right side of the head is connected to the right side of the brain.
Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves however, the vagus nerve is not associated with the somatic nervous system.
The general senses of somatosensation for the face travel through the trigeminal system. The senses are olfaction smell , gustation taste , somatosensation sensations associated with the skin and body , audition hearing , equilibrium balance , and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye.
Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs.
The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function.
The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception.
The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception. The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus free nerve endings, encapsulated endings, or specialized receptor cell , or where the cell is located relative to the stimulus interoceptor, exteroceptor, proprioceptor.
Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear.
Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy. The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion.
Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex.
The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, PA, who became interested in science at an early age because of her sensory experiences.
Answers will vary, but a typical answer might be: I can eat most anything except mushrooms! My whole family likes eating a variety of foods, so it seems that we all have the same level of sensitivity.
Figure The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. Figure The hair cells are located in the organ of Corti, which is located on the basilar membrane. The stereocilia of those cells would normally be attached to the tectorial membrane though they are detached in the micrograph because of processing of the tissue. The small bones in the middle ear, the ossicles, amplify and transfer sound between the tympanic membrane of the external ear and the oval window of the inner ear.
High frequencies activate hair cells toward the base of the cochlea, and low frequencies activate hair cells toward the apex of the cochlea. Photoreceptors convert light energy, or photons, into an electrochemical signal. The retina contains bipolar cells and the RGCs that finally convert it into action potentials that are sent from the retina to the CNS. It is important to recognize when popular media and online sources oversimplify complex physiological processes so that misunderstandings are not generated.
This video was created by a medical device manufacturer who might be trying to highlight other aspects of the visual system than retinal processing. The statement they make is not incorrect, it just bundles together several steps, which makes it sound like RGCs are the transducers, rather than photoreceptors. The sweetener known as stevia can replace glucose in food. What does the molecular similarity of stevia to glucose mean for the gustatory sense? The stevia molecule is similar to glucose such that it will bind to the glucose receptor in sweet-sensitive taste buds.
However, it is not a substrate for the ATP-generating metabolism within cells. Why does the blind spot from the optic disc in either eye not result in a blind spot in the visual field? The visual field for each eye is projected onto the retina as light is focused by the lens.
The visual information from the right visual field falls on the left side of the retina and vice versa. The optic disc in the right eye is on the medial side of the fovea, which would be the left side of the retina. However, the optic disc in the left eye would be on the right side of that fovea, so the right visual field falls on the side of the retina in the left field where there is no blind spot.
Skip to content The Somatic Nervous System. Learning Objectives By the end of this section, you will be able to: Describe different types of sensory receptors Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision Distinguish how different tastes are transduced Describe the means of mechanoreception for hearing and balance List the supporting structures around the eye and describe the structure of the eyeball Describe the processes of phototransduction.
Sensory Receptors Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Structural Receptor Types The cells that interpret information about the environment can be either 1 a neuron that has a free nerve ending , with dendrites embedded in tissue that would receive a sensation; 2 a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or 3 a specialized receptor cell , which has distinct structural components that interpret a specific type of stimulus Figure.
Receptor Classification by Cell Type. Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either a free nerve endings or b encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of c specialized receptor cells.
These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons. Functional Receptor Types A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Sensory Modalities Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight.
Gustation Taste Only a few recognized submodalities exist within the sense of taste, or gustation. The Tongue. The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink.
Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. Olfaction Smell Like taste, the sense of smell, or olfaction , is also responsive to chemical stimuli. The Olfactory System. Disorders of the….
The vibrations are transferred from SV to ST via helicotrema. As the basilar membrane vibrates, sensory hair cells of organ of Corti get stimulated. Nerve impulse generated at the base of organ of Corti will reach brain via auditory nerve. What part of the ear contains the sensory receptors for hearing? Mandira P.
Apr 19, Explanation: Sensory receptors of hearing are hair cells, present on basilar membrane of cochlea. Sensory receptors can be classified by the type of stimulus that generates a response in the receptor. Broadly, sensory receptors respond to one of four primary stimuli:.
A schematic of the classes of sensory receptors : Sensory receptor cells differ in terms of morphology, location, and stimulus. All sensory receptors rely on one of these four capacities to detect changes in the environment, but may be tuned to detect specific characteristics of each to perform a specific sensory function. In some cases, the mechanism of action for a receptor is not clear.
For example, hygroreceptors that respond to changes in humidity and osmoreceptors that respond to the osmolarity of fluids may do so via a mechanosensory mechanism or may detect a chemical characteristic of the environment.
Sensory receptors perform countless functions in our bodies. During vision, rod and cone photoreceptors respond to light intensity and color.
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