Biology Unit 11 Sensory System Video Review Sheet Answers
Chapter 47
Sensory Perception
By the end of this section, you volition be able to:
- Describe dissimilar types of sensory receptors
- Describe the structures responsible for the special senses of gustation, odor, hearing, balance, and vision
- Distinguish how different tastes are transduced
- Describe the means of mechanoreception for hearing and residue
- List the supporting structures effectually the eye and describe the construction of the eyeball
- Describe the processes of phototransduction
A major role of sensory receptors is to aid u.s. learn most the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of dissimilar types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the prison cell membrane potential of a sensory neuron. The stimulus causes the sensory jail cell to produce an action potential that is relayed into the central nervous organisation (CNS), where information technology is integrated with other sensory information—or sometimes higher cognitive functions—to go a conscious perception of that stimulus. The fundamental integration may then pb to a motor response.
Describing sensory part with the term awareness or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that find sensations. A receptor cell is changed directly past a stimulus. A transmembrane poly peptide receptor is a protein in the cell membrane that mediates a physiological change in a neuron, well-nigh often through the opening of ion channels or changes in the prison cell signaling processes. Transmembrane receptors are activated past chemicals chosen ligands. For example, a molecule in food can serve as a ligand for gustation receptors. Other transmembrane proteins, which are non accurately chosen receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion catamenia across the membrane, and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environs activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells tin can be classified into types on the basis of three different criteria: jail cell type, position, and function. Receptors tin be classified structurally on the ground of cell blazon and their position in relation to stimuli they sense. They can likewise be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, low-cal, or chemic changed the jail cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment tin exist either (1) a neuron that has a gratis nerve catastrophe, with dendrites embedded in tissue that would receive a sensation; (ii) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (three) a specialized receptor cell, which has singled-out structural components that interpret a specific blazon of stimulus (Figure 1). The hurting and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Besides located in the dermis of the peel are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch on. The cells in the retina that respond to low-cal stimuli are an example of a specialized receptor, a photoreceptor.
Receptor Classification by Cell Type
Figure 1: Receptor cell types tin exist classified on the basis of their construction. Sensory neurons can have either (a) costless nervus endings or (b) encapsulated endings. Photoreceptors in the eyes, such every bit rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar prison cell, which and so synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the pare. An interoceptor is one that interprets stimuli from internal organs and tissues, such equally the receptors that sense the increment in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located almost a moving part of the body, such as a muscle, that interprets the positions of the tissues every bit they move.
Functional Receptor Types
A tertiary classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of 3 general types. Some stimuli are ions and macromolecules that touch on transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible low-cal. For humans, the only electromagnetic energy that is perceived by our optics is visible light. Another organisms have receptors that humans lack, such equally the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the footing of the type of stimuli they transduce. Chemic stimuli tin can be interpreted by a chemoreceptor that interprets chemical stimuli, such equally an object's taste or olfactory property. Osmoreceptors answer to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such every bit pressure and vibration, every bit well as the awareness of audio and body position (rest), are interpreted through a mechanoreceptor. Another physical stimulus that has its ain type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (common cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are probable to list the five major senses—taste, odour, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this listing is balance. As well, what is referred to simply as affect can exist farther subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these bear on sensations. Other disregarded senses include temperature perception by thermoreceptors and pain perception past nociceptors.
Within the realm of physiology, senses tin can be classified as either general or specific. A general sense is ane that is distributed throughout the torso and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of bear on, equally described above, or to proprioception (body movement) and kinesthesia (trunk motility), or to a visceral sense, which is near important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the center, inner ear, tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is like to the idea of transduction. The principal sensory modalities can be described on the ground of how each is transduced. The chemical senses are taste and smell. The general sense that is commonly referred to equally bear on includes chemical sensation in the class of nociception, or pain. Pressure level, vibration, muscle stretch, and the movement of pilus by an external stimulus, are all sensed past mechanoreceptors. Hearing and balance are also sensed past mechanoreceptors. Finally, vision involves the activation of photoreceptors.
Listing all the unlike sensory modalities, which tin can number as many equally 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the awareness of a specific type of stimulus. For case, the full general sense of touch, which is known every bit somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or pilus movement.
Gustation (Taste)
Only a few recognized submodalities exist within the sense of sense of taste, or gustation. Until recently, only four tastes were recognized: sweetness, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means "delicious gustation," and is often translated to mean savory. Very recent research has suggested that there may as well be a sixth sense of taste for fats, or lipids.
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined past a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) incorporate the structures for gustatory transduction. At that place are four types of papillae, based on their advent (Effigy ii): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are gustatory modality buds that contain specialized gustatory receptor cells for the transduction of gustation stimuli. These receptor cells are sensitive to the chemicals independent within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.
The Tongue
Figure ii: The tongue is covered with small bumps, called papillae, which incorporate taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are institute in dissimilar regions of the tongue. The taste buds contain specialized gustatory receptor cells that reply to chemic stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the common salt crystals dissociate into the component ions Na+ and Cl–, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a stiff concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+ into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.
Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For case, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, information technology is oft sweetened so that the sour sense of taste is masked.
The outset ii tastes (salty and sour) are triggered past the cations Na+ and H+. The other tastes result from food molecules binding to a G poly peptide–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory prison cell. The sweetness taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) too actuate the sweet receptors. The affinity for each of these molecules varies, and some will gustatory modality sweeter than glucose because they bind to the M poly peptide–coupled receptor differently.
Bitter taste is like to sweetness in that food molecules demark to G poly peptide–coupled receptors. Withal, in that location are a number of different means in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some biting molecules increment Yard poly peptide activation inside the gustatory cells, whereas other bitter molecules subtract G protein activation. The specific response depends on which molecule is binding to the receptor.
I major group of biting-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are normally found in biting-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less bonny to herbivores.
Therefore, the part of biting taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are unremarkably ingested are frequently combined with a sugariness component to make them more palatable (foam and sugar in coffee, for instance). The highest concentration of bitter receptors appear to exist in the posterior tongue, where a gag reflex could nonetheless spit out poisonous food.
The gustation known as umami is oft referred to as the savory taste. Like sweet and biting, it is based on the activation of G poly peptide–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid Fifty-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that incorporate meat are often described as savory.
Once the gustatory cells are activated by the sense of taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are role of the facial and glossopharyngeal cranial fretfulness, as well as a component inside the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior 3rd of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to gustatory modality buds in the extreme posterior of the natural language, verging on the pharynx, which are more sensitive to noxious stimuli such every bit bitterness.
Watch the video linked to below to learn about Dr. Danielle Reed of the Monell Chemic Senses Centre in Philadelphia, Pennsylvania, who became interested in scientific discipline at an early age considering of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences betwixt people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as "tasters" and "non-tasters" based on the density of papillae on their tongue, which also indicates the number of taste buds. Not-tasters can gustation food, simply they are non as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are y'all very sensitive to tastes? Can you meet whatsoever similarities among the members of your family?
Olfaction (Smell)
Like taste, the sense of aroma, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region inside the superior nasal crenel (Effigy 3). This region is referred to equally the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the crenel. Every bit airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the fungus. These odorant molecules demark to proteins that keep them dissolved in the mucus and assist transport them to the olfactory dendrites. The odorant–poly peptide complex binds to a receptor poly peptide within the cell membrane of an olfactory dendrite. These receptors are M protein–coupled, and will produce a graded membrane potential in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory seedling on the ventral surface of the frontal lobe. From there, the axons carve up to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the junior and medial areas of the temporal lobe. Others projection to structures within the limbic system and hypothalamus, where smells become associated with long-term retentiveness and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one'southward birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cognitive cortex is one reason why smell can be a strong trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed past airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in identify in the cranial nervus.
The Olfactory System
Figure 3: (a) The olfactory organization begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid os and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical Schoolhouse © 2012)
Disorders of the...
Olfactory System: Anosmia
Blunt force trauma to the face up, such every bit that common in many car accidents, can pb to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the encephalon moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters frequently feel anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such every bit antibiotics, tin can crusade anosmia past killing all the olfactory neurons at one time. If no axons are in place within the olfactory nerve, and so the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, every bit well, such as those acquired by inflammatory responses related to respiratory infections or allergies.
Loss of the sense of odour tin result in food tasting banal. A person with an impaired sense of aroma may require additional spice and seasoning levels for food to exist tasted. Anosmia may as well be related to some presentations of balmy depression, because the loss of enjoyment of nutrient may lead to a full general sense of despair.
The ability of olfactory neurons to replace themselves decreases with age, leading to historic period-related anosmia. This explains why some elderly people common salt their food more than younger people practice. However, this increased sodium intake tin increment blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly.
Audition (Hearing)
Hearing, or audition, is the transduction of audio waves into a neural bespeak that is made possible by the structures of the ear (Effigy 4). The big, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will likewise refer to this construction as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle directly sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates later on it is struck by sound waves. The auricle, ear culvert, and tympanic membrane are often referred to equally the external ear. The heart ear consists of a infinite spanned by three small bones chosen the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in plough, articulates with the stapes. The stapes is so attached to the inner ear, where the sound waves will be transduced into a neural signal. The center ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure beyond the tympanic membrane. The tube is normally closed but volition pop open when the muscles of the pharynx contract during swallowing or yawning.
Structures of the Ear
Figure 4: The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is continued to the pharynx by the Eustachian tube. The inner ear contains the cochlea and foyer, which are responsible for audition and equilibrium, respectively.
The inner ear is often described every bit a bony labyrinth, as information technology is composed of a series of canals embedded within the temporal bone. It has 2 separate regions, the cochlea and the lobby, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through divide cobweb bundles. Nonetheless, these ii distinct bundles travel together from the inner ear to the brain stalk as the vestibulocochlear nerve. Audio is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the screw ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.
The oval window is located at the offset of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the pinnacle of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this fourth dimension travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid inside the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves friction match the frequencies of the audio waves (Figure 5). The membrane covering the round window volition bulge out or pucker in with the motion of the fluid within the scala tympani.
Manual of Sound Waves to Cochlea
Figure 5: A sound wave causes the tympanic membrane to vibrate. This vibration is amplified every bit it moves across the malleus, incus, and stapes. The amplified vibration is picked upwardly by the oval window causing pressure level waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined past the changes in amplitude and frequency of the sound waves entering the ear.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 6). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the 2 scala into neural signals. The organs of Corti lie on acme of the basilar membrane, which is the side of the cochlear duct located betwixt the organs of Corti and the scala tympani. As the fluid waves motility through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves movement the region of the basilar membrane that is shut to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.
Cross Section of the Cochlea
Figure 6: The three major spaces inside the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor pilus cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.
The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell'due south upmost surfaces (Figure seven). The stereocilia are an array of microvilli-similar structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the pilus cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the poly peptide tethers opens ion channels in the pilus cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the pilus cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels shut. When no sound is nowadays, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the pilus cell slightly depolarized.
Pilus Cell
Figure 7: The pilus cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest fellow member of their array.
Cochlea and Organ of Corti
Figure viii: LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Every bit stated in a higher place, a given region of the basilar membrane will only motility if the incoming sound is at a specific frequency. Considering the tectorial membrane but moves where the basilar membrane moves, the pilus cells in this region will also only answer to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between xx and xx,000 Hz, which is the range of sound that human ears tin observe. The unit of Hertz measures the frequency of audio waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected past hair cells at the noon, or tip, of the cochlea. Frequencies in the higher ranges of twenty KHz are encoded past hair cells at the base of the cochlea, close to the round and oval windows (Figure 9). Almost auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound moving ridge). The hair cells along the length of the cochlear duct, which are each sensitive to a detail frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible low-cal into its component colors.
Frequency Coding in the Cochlea
Effigy 9: The continuing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by loftier frequencies, whereas those at the apex of the cochlea are activated merely by depression frequencies.
Sentinel the video linked to beneath to learn more about how the structures of the ear convert audio waves into a neural point by moving the "hairs," or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The encephalon interprets the pregnant of the sounds we hear as music, speech, dissonance, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?
Watch the video linked to below to acquire more about the inner ear and to meet the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at unlike frequencies. Based on the animation, where do frequencies—from loftier to depression pitches—cause activity in the hair cells within the cochlear duct?
Equilibrium (Remainder)
Forth with audition, the inner ear is responsible for encoding information well-nigh equilibrium, the sense of balance. A similar mechanoreceptor—a pilus cell with stereocilia—senses head position, head motion, and whether our bodies are in move. These cells are located inside the vestibule of the inner ear. Head position is sensed past the utricle and saccule, whereas caput movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.
The utricle and saccule are both largely equanimous of macula tissue (plural = maculae). The macula is composed of pilus cells surrounded by support cells. The stereocilia of the pilus cells extend into a viscous gel called the otolithic membrane (Figure 10). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths substantially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the caput causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted past the brain based on the blueprint of hair-cell depolarization.
Linear Dispatch Coding by Maculae
Effigy x: The maculae are specialized for sensing linear dispatch, such as when gravity acts on the tilting head, or if the head starts moving in a direct line. The divergence in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing forcefulness that causes the stereocilia to bend in the direction of that linear acceleration.
The semicircular canals are iii band-similar extensions of the entrance hall. I is oriented in the horizontal aeroplane, whereas the other ii are oriented in the vertical airplane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 11). The base of each semicircular culvert, where information technology meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational motility, such as turning the caput while saying "no." The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a airplane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction contrary to the head movement. The semicircular canals incorporate several ampullae, with some oriented horizontally and others oriented vertically. Past comparing the relative movements of both the horizontal and vertical ampullae, the vestibular arrangement can detect the management of nearly head movements within three-dimensional (3-D) infinite.
Rotational Coding by Semicircular Canals
Effigy 11: Rotational movement of the head is encoded past the hair cells in the base of the semicircular canals. Every bit ane of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in data most the management in which the head is moving, and activation of all six canals tin give a very precise indication of caput move in iii dimensions.
Somatosensation (Touch)
Somatosensation is considered a general sense, every bit opposed to the special senses discussed in this section. Somatosensation is the grouping of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, low-cal bear upon, tickle, crawling, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the pare, but receptors are too plant in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.
2 types of somatosensory signals that are transduced by gratis nerve endings are hurting and temperature. These two modalities utilize thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the awareness of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli across a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of rut associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures to a higher place 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the power of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin tin can exist used equally a topical analgesic, such as in products such as Icy Hot™.
If yous drag your finger across a textured surface, the skin of your finger volition vibrate. Such depression frequency vibrations are sensed by mechanoreceptors called Merkel cells, as well known every bit type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure level and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings plant deep in the dermis, or subcutaneous tissue. Low-cal touch is transduced by the encapsulated endings known every bit tactile (Meissner) corpuscles. Follicles are besides wrapped in a plexus of nervus endings known every bit the hair follicle plexus. These nervus endings discover the movement of hair at the surface of the skin, such as when an insect may exist walking along the skin. Stretching of the pare is transduced by stretch receptors known every bit bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type Ii cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For case, have you lot ever stretched your muscles before or after exercise and noticed that you tin only stretch so far before your muscles spasm back to a less stretched country? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-wrinkle of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal organisation within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in Table 1.
Table 1: Mechanoreceptors of Somatosensation
| Name | Historical (eponymous) name | Location(s) | Stimuli |
|---|---|---|---|
| Costless nerve endings | * | Dermis, cornea, natural language, joint capsules, visceral organs | Hurting, temperature, mechanical deformation |
| Mechanoreceptors | Merkel'south discs | Epidermal–dermal junction, mucosal membranes | Depression frequency vibration (5–15 Hz) |
| Bulbous corpuscle | Ruffini'southward corpuscle | Dermis, joint capsules | Stretch |
| Tactile corpuscle | Meissner's corpuscle | Papillary dermis, especially in the fingertips and lips | Low-cal affect, vibrations below 50 Hz |
| Lamellated corpuscle | Pacinian corpuscle | Deep dermis, subcutaneous tissue | Deep force per unit area, high-frequency vibration (around 250 Hz) |
| Hair follicle plexus | * | Wrapped around hair follicles in the dermis | Move of hair |
| Muscle spindle | * | In line with skeletal muscle fibers | Musculus contraction and stretch |
| Tendon stretch organ | Golgi tendon organ | In line with tendons |
*No corresponding eponymous proper noun.
Vision
Vision is the special sense of sight that is based on the transduction of lite stimuli received through the eyes. The eyes are located inside either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the middle (Effigy 12). The eyelids, with lashes at their leading edges, aid to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a sparse membrane known every bit the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the olfactory organ. Tears produced by this gland menstruum through the lacrimal duct to the medial corner of the eye, where the tears menstruation over the conjunctiva, washing away foreign particles.
The Centre in the Orbit
Figure 12: The eye is located within the orbit and surrounded by soft tissues that protect and support its role. The orbit is surrounded by cranial bones of the skull.
Move of the eye within the orbit is accomplished by the contraction of half dozen extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure thirteen). Iv of the muscles are arranged at the fundamental points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting musculus. For example, when the superior rectus contracts, the centre rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. Nevertheless, the tendon of the oblique muscles threads through a pulley-like slice of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea ways that wrinkle of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the middle, in opposition to the superior oblique. Rotation of the middle by the ii oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the middle must also rotate slightly to compensate for the superior rectus pulling at approximately a xx-degree angle, rather than straight up. The aforementioned is true for the inferior rectus, which is compensated by wrinkle of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with tiptop of the eye by the superior rectus (see Figure 12).
The extraocular muscles are innervated past three cranial fretfulness. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nervus. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stalk, which coordinates center movements.
Extraocular Muscles
Figure xiii: The extraocular muscles movement the heart within the orbit.
The centre itself is a hollow sphere equanimous of iii layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is non visible, though humans are unique compared with many other species in having then much of the "white of the eye" visible (Figure fourteen). The transparent cornea covers the anterior tip of the middle and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly equanimous of the choroid, ciliary torso, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens past suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus low-cal on the dorsum of the centre. Overlaying the ciliary torso, and visible in the anterior eye, is the iris—the colored part of the heart. The iris is a polish muscle that opens or closes the pupil, which is the hole at the centre of the eye that allows calorie-free to enter. The iris constricts the pupil in response to bright light and dilates the educatee in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.
The eye is also divided into ii cavities: the inductive cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. Information technology is filled with a watery fluid called the aqueous humour. The posterior crenel is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more than viscid fluid called the vitreous sense of humor.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar jail cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an activity potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and exit the eye every bit the optic nerve (encounter Effigy 14). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nervus begins. This creates a "bullheaded spot" in the retina, and a respective blind spot in our visual field.
Structure of the Center
Figure xiv: The sphere of the eye can exist divided into anterior and posterior chambers. The wall of the eye is composed of iii layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The middle of the retina has a small indentation known as the fovea.
Notation that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A meaning corporeality of lite is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small-scale area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and just contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure fourteen). Every bit one moves in either management from this central indicate of the retina, visual acuity drops significantly. In improver, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accurateness of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to one. The difference in visual acuity between the fovea and peripheral retina is hands evidenced by looking directly at a discussion in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your optics off that give-and-take, detect that words at the kickoff or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not equally conspicuously identified. As a upshot, a large part of the neural part of the eyes is concerned with moving the optics and head and so that important visual stimuli are centered on the fovea.
Light falling on the retina causes chemic changes to paint molecules in the photoreceptors, ultimately leading to a change in the action of the RGCs. Photoreceptor cells accept ii parts, the inner segment and the outer segment (Figure 15). The inner segment contains the nucleus and other mutual 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-jump discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the jail cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a item wavelength of calorie-free. The wavelength of visible light determines its color. The pigments in human being eyes are specialized in perceiving three dissimilar main colors: cerise, green, and blue.
Photoreceptor
Figure fifteen: (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
At the molecular level, visual stimuli crusade changes in the photopigment molecule that atomic number 82 to changes in membrane potential of the photoreceptor cell. A single unit of measurement of light is called a photon, which is described in physics as a packet of energy with backdrop of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible low-cal corresponding to a particular colour. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiations longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas lite with a wavelength of 720 nm is dark red. All other colors autumn between scarlet and blue at various points along the wavelength scale.
Opsin pigments are actually transmembrane proteins that comprise 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 concatenation to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal'due south flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-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 16).
The shape alter of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins event in activation of a Thou poly peptide. 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 11-cis-retinal shape, the opsin cannot answer to lite 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. Subsequently 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 than light energy.
Retinal Isomers
Figure 16: The retinal molecule has two isomers, (a) ane before a photon interacts with information technology and (b) one that is altered through photoisomerization.
The opsins are sensitive to limited wavelengths of lite. Rhodopsin, the photopigment in rods, is almost sensitive to light at a wavelength of 498 nm. The three color opsins have superlative sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the chief colors of red, green, and bluish (Figure 17). 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 agile. In a darkened room, there is not enough light to actuate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to lite that a single photon can result in an activeness potential from a rod'south corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. Past comparison the activity of the iii different cones, the brain can extract color information from visual stimuli. For example, a bright blue lite that has a wavelength of approximately 450 nm would actuate the "ruddy" cones minimally, the "green" cones marginally, and the "blueish" cones predominantly. The relative activation of the 3 different cones is calculated by the brain, which perceives the colour as blue. All the same, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our depression-lite vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of greyness. If yous recollect that yous can encounter colors in the dark, it is most probable considering your encephalon knows what color something is and is relying on that memory.
Comparison of Color Sensitivity of Photopigments
Figure 17: Comparison the acme sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.
Sentinel the video linked to beneath to learn more most 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 nervus to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic jail cell that so projects to the visual cortex in the occipital lobe where "seeing," or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that "specialized cells in the retina called ganglion cells convert the low-cal rays into electrical signals." What aspect of retinal processing is simplified past that argument? Explain your answer.
Sensory Fretfulness
Once whatsoever sensory cell transduces a stimulus into a nervus impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors accept 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.
Spinal Nerves
Mostly, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the peel, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nervus nears the spinal string, it splits into dorsal and ventral roots. The dorsal root contains merely the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches volition synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or fifty-fifty the inductive (ventral) horn, at the level of the spinal cord where they enter. Other branches volition travel a short distance upwardly or downward the spine to collaborate with neurons at other levels of the spinal cord. A branch may too plough into the posterior (dorsal) column of the white matter to connect with the encephalon. 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 betwixt the unlike 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
Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations beneath the neck, the right side of the body is continued 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 more often than not ipsilateral, meaning that a cranial nerve on the correct side of the head is connected to the right side of the brain. Some cranial fretfulness contain just sensory axons, such every bit the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus fretfulness (nevertheless, the vagus nerve is not associated with the somatic nervous system). The full general senses of somatosensation for the confront travel through the trigeminal system.
Chapter Review
The senses are olfaction (smell), gustation (sense of taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this listing represents the special senses, or those systems of the torso that are associated with specific organs such as the natural language or middle. 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 full general senses tin exist 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 carve up from the somatic nervous system function in that they do non 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 prison cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional nomenclature is based on how the cell transduces the stimulus into a neural indicate. Chemoreceptors respond to chemical stimuli and are the ground for olfaction and gustatory modality. Related to chemoreceptors are osmoreceptors and nociceptors for fluid remainder and pain reception, respectively. Mechanoreceptors reply to mechanical stimuli and are the ground for virtually aspects of somatosensation, too as existence 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 data from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial fretfulness, connected to the encephalon. Spinal nerves accept mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal string 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 encephalon to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such every bit the olfactory, optic, and vestibulocochlear fretfulness, 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.
Source: https://guides.hostos.cuny.edu/bio140/14-47
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