Suggestion: change the view to 100%. The Senses Title Page Jim Swan These slides are from class presentations, reformatted for static viewing. The content contained in these pages is also in the Class Notes pages in a narrative format. Best screen resolution for viewing is 1024 x 768. To change resolution click on start, then control panel, then display, then settings. If you are viewing this in Adobe Reader version 7 and are connected to the internet you will also be able to access the enriched links to notes and comments, as well as web pages including animations and videos. You will also be able to make your own notes and comments on the pages. Download the free reader from [Adobe.com] 1
Receptor Classification by Type of Stimulus Mechanoreceptors - respond to a mechanical stimulus: examples are touch, pressure, stretch, hearing, balance, position and movement, vibration, muscle contraction, as well as pressoreceptors (stretch receptors in the heart) and baroreceptors (pressure receptors in blood vessels). This gives you an example of the great variety of different receptors which respond to some kind of mechanical action. 2
Thermoreceptors - respond to temperature change: example heat and cold. Photoreceptors - respond to light: example vision Chemoreceptors - respond to various chemicals such as glucose, oxygen, carbon dioxide, hormones and many, many more. Nociceptors - pain receptors from any noxious stimulus These are specialized (but not special!) receptors for non-mechanical stimuli. 3
Receptor Classification According to Origin of Stimulus Exteroceptors respond to stimuli from outside the body - vision, sound, touch, smell, temperature, pain etc. Interoceptors or visceroceptors respond to stimuli arising within the body such as chemical stimuli, deep pressure, and many others. Proprioceptors respond to muscle or tendon stretch and help the body monitor body position (body sense). Presumably the exteroceptors are the basis for the so-called five senses. As you can see from this presentation, there are a whole lot more than five senses, if you mean senses that involve unique receptors. 4
Other Classifications General vs Special Senses: Although all sensory receptors basically do the same thing, they can be classified as to their complexity. Special sense receptors are the most complex. Unencapsulated receptors - these have no special structure and are basically free nerve endings. Examples are pain receptors, temperature receptors, Merkel disks (light touch), hair root plexus. Encapsulated receptors have a special capsule which encloses a nerve ending. All sensory receptors do basically the same thing: they change a stimulus (chemical, mechanical, etc.) into a nerve impulse that travels to the CNS to be perceived, or produce a voluntary or autonomic response. 5
Free Nerve Endings The simplest receptors are free (unencapsulated) dendrites of unipolar neurons which respond to physical or chemical disturbance, perceived as pain. The basis of pain sensation is disturbance of free dendrites of unipolar neurons. 6
Pacinian Corpuscle D = 1mm Myelinated nerve fiber lamellae deep pressure receptors for mechanical and vibratory pressure. found in deep dermis and hypodermis as well as in connective tissue in joints and internal organs. As you learned in unit 1, Pacinian corpuscles allow us to perceive deep pressure and also low frequency vibrations. 7
Meissner s Corpuscle D = 150 mm responsive to low frequency touch located in dermal papillae of hairless skin, i.e. lips and palmer surfaces. Basal layer of epidermis Meissner s corpuscles consist of a nerve ending surrounded by numerous membranes. The nerve ending is depolarized as a result and sends the nerve impulse back to the CNS, which interprets it as touch in a specific location. 8
The Special Senses Special senses are more complex and have more complex structures in their receptors: e.g. Taste, smell, vision, hearing. Although all receptors do basically the same thing, act as transducers of a stimulus to a nerve impulse, certain receptors are designated as special senses because of their complexity. 9
Taste microvilli Taste bud } Neuroepithelial cell Circumvallate papillae Taste cells Basal cells Taste cells classified as neuroepithelial cells (a hybrid of a neuron and an epithelial cell). They have microvilli which respond to dissolved chemicals. They then secrete a neurotransmitter which depolarizes nerve fibers. Taste buds groups of taste cells located on papillae of tongue as well as hard palate and epiglottis. Basal cells undergo mitosis to constantly replace taste cells, about every 10 days. There are four basic tastes, and a fifth responds to glutamate which makes the others more intense. There are three types of papillae on the tongue: filiform papillae no taste buds, near tip, touch receptors. fungiform papillae most abundant, over most of tongue, possess most taste buds. circumvallate papillae large, round, back of tongue, also have taste buds. 10
Fungiform papilla with taste buds. Epidermis Taste buds This is a fungiform papilla like those found on most of the tongue. The taste buds are found along the sides of the papilla. Composed of individual taste cells and other cells (see previous slide), they respond to the chemicals dissolved in what you take into your mouth and nose. Much of taste is also what stimulates the olfactory cells, and much of smell is what stimulates the taste cells. This is especially true for volatile substances (those that vaporize), and is the basis for the phenomenon of describing a smell as tasting like something which you have never tasted. Both taste and olfactory cells replace themselves during life, although the process seems to slow down, like everything else, in old age. A system of classification has divided taste into four basic types: sweet, salt, sour, and bitter, with a 5 th type: yummy. This is based on a Japanese word umami which means good flavor, good taste. These receptors respond to glutamate, and are the basis for the addition of sodium glutamate to foods to enhance their flavor. Too much sodium glutamate sometimes causes undesirable side effects in people who are sensitive to it. Although some texts have diagrams of where tastes are located on the tongue, this concept has been mostly disproved. Tastes appear to greatly overlap on the tongue, and we can perceive thousands of different taste combinations. 11
Olfactory bulb Olfaction Olfactory mucosa Nasal mucosa The process of olfaction (smell) begins with entrance of chemical-containing air into the nasal cavity (and also via ingested materials into the oral cavity, see previous slide). The olfactory cells are located at the top of the nasal cavity, so in order to focus on a particular odor we tend to draw air into the top of this region. 12
Olfactory Mucosa Integrating cell Cribriform plate Olfactory nerve fibers Olfactory cells modified neurons replace themselves every 60 days Olfactory cells are a type of neuron, the only neurons known to replace themselves. They do so about every 60 days. They have long processes which respond to various odors to cause the cells to depolarize and send a signal to the mitral cells, a type of cell which integrates the signals that are sent to the olfactory center in the cerebral cortex. Although attempts have been made to classify smells in the same way tastes are classified, these haven t met with much acceptance. 13
Sensory Adaptation Certain senses stop responding to a continuous stimulus, responding only when stimulus changes. Examples: touch, smell, taste. Once you have put on your clothes in the morning, you don t notice them unless they create an unusual feel. Sometime after you enter a room which has a bad smell, you stop noticing it until you leave the room again. These are examples of adaptation, the phenomenon with some senses that you only notice the stimulus when it changes. 14
Anatomy of the Eye: The Fibrous Tunic Ciliary body Sclera Cornea Anterior segment Canal of Schlemm (venous sinus) The eye is composed of three layers, or tunics: The outer fibrous tunic, composed of the: sclera the tough fibrous covering. Much of the sclera is white, called the white of the eye. cornea the transparent anterior portion of the sclera allows light to enter the eye. Behind the cornea is the anterior segment containing a watery fluid called the aqueous humor. This fluid is produced by cells in the ciliary process (part of the vascular tunic) and constantly absorbed by the venous sinus. Insufficient absorption produces an increase in pressure in the aqueous humor known as glaucoma. 15
Anatomy of the Eye: The Vascular Tunic Ciliary body Choroid coat Iris Pupil Lens Suspensory ligament The Vascular tunic is the pigmented and vascular layer. It is made up of several parts: 1) The choroid coat, in the middle between the outer fibrous and inner nervous tunics. The choroid pigment absorbs excess light and the blood vessels supply the nutrients and oxygen to the adjacent retina. 2) Iris controls the amount of light entering the eye. The iris has two sets of muscles, one which constricts the pupil and one which dilates the pupil. The anterior surface of the iris is pigmented producing the color of our eyes. 3) The ciliary body attaches to a ligament which attaches to the lens. Muscles in the ciliary body control the shape of the lens to focus the image onto the retina. The ciliary body also has the cells which produce the aqueous humor 16
The Tapetum Lucidum Tapetum lucidum a reflective portion of of choroid in in eyes of of many animals. When you see light reflected from a cat s or dog s eyes you see a bright greenish-blue color, especially in cats. That reflection is produced by the tapetum lucidum, a brightly pigmented area of the choroid which helps these animals see better in dim light. Humans do not have a tapetum lucidum. The reflection which produces red eye in photographs is due to a pigment called visual purple found in the light receptor cells. 17
Cornea Anterior segment contains aqueous humor Ciliary body: ciliary processes ciliary muscle Lens Posterior segment contains vitreous humor The Aqueous humor is produced by ciliary processes and absorbed into the Canal of Schlemm at same rates. Glaucoma is due to insufficient absorption of the aqueous humor. It can be hereditary or caused by abnormal constriction of the canal. The excess pressure which results causes damage to ocular structures and blood vessels and can lead to blindness. The Vitreous humor is a gelatinous fluid that holds the retina in place. 18
Lens Bends light Cornea Ciliary body: ciliary processes ciliary muscle The iris regulates amount of light entering eye by constricting or dilating the pupil. The pupil also contributes to visual acuity by bending the light toward the center of lens where focusing is most accurate. 19
Lens Cornea Ciliary body: ciliary processes ciliary muscles Lens capsule Susensory ligament The lens is composed of proteins secreted by cells called lens fibers. The natural shape of the lens is convex. Ciliary muscles attach to the suspensory ligament to regulate the tension on the lens to make it more convex or less convex for fousing the image. 20
The Refractory Apparatus - Structures which affect light path. Cornea most important in bending light Aqueous humor light passes without affect Pupil bends light toward lens Lens most important in focusing for near objects. Vitreous humor light passes without bending. All of these structures play a role in bending, or not bending, the light on its ultimate journey to stimulate receptor cells in the retina. 21
Convex Lens Converges Light Rays and Inverts the Image The convex lens functions to produce convergence of the light rays onto the receptor surface, the retina. A more convex lens is needed for close images, a less convex lens for distant images. The natural shape of the lens works for distant images, but contraction of the ciliary muscles is needed for more convex images, to look at things which are close. 22
Focusing for Distant Objects Distant objects reflect light in rays that are nearly parallel to one another. When the ciliary muscles are relaxed the suspensory ligament pulls lens into flattened (less convex) shape. In this way it doesn t produce converge of the rays when looking at distant (more than a few feet) images. 23
Focusing for Near Objects Reflected light rays from close objects are diverging as the reach the eye. The ciliary muscles contract to relieve the tension on the lens, which causes the lens to achieve it more convex natural shape. In this way it produces convergence of the rays to focus them on the retina. 24
Refractive Disorders Normal eye Myopic eye Hyperopic eye Myopia nearsightedness caused by an eyeball too long for the convexity of the lens. The correction is a concave lens to reduce convergence. Hyperopia farsightedness caused by an eyeball too short for the convexity of the lens, or to weakened ciliary muscles. The correction is a convex lens to increase convergence. Presbyopia old eyes the loss of elasticity impairs focusing at various distances. The correction is different lenses for different circumstances, as in bifocals. Astigmatism irregular curvature of a misshapen lens or cornea. The correction is a lens which counters the abnormal curvature. The new procedures of various types use lasers or other devices to reshape the cornea, which changes the convergence or divergence of light entering the eye so as to compensate for the defect. Unfortunately this does not usually allow satisfactory regulation by the ciliary muscles so often a set of reading glasses is still needed. 25
Other Disorders Strabismus orientation of two eyes does not match one another. Generally occurs early in life and may be due to weak muscles or improper neural control of eye. Also called lazy eye. In this condition, the brain suppresses the visual image from the deviating eye to prevent double vision. About 50% of childhood amblyopia is strabismic. Nystagmus neurological sign characterized by rapid uncontrolled eye movement. May be due to neurological dysfunction in inner ear. Amblyopia is the general term for impaired vision that is not due to a pathology of the eye itself. 26
The Eye muscle insertion choroid ciliary sclera retina body Suspensory optic disk ligament lens Posterior segment (vitreous body) posterior chamber ora serrata Sagittal Section Anterior segment iris cornea anterior chamber Above is seen a section performed on an actual eyeball. 27
The Retina The Sensory Tunic Pigmented layer Neural layer Nerve fibers Optic nerve Optic disk Blood vessels The innermost layer is the Sensory or Nervous Tunic. It is composed of the retina, the neural structure which contains the receptor cells to light. The neural layer contains the receptor cells, the rods and cones. Nerve fibers from this layer converge at the optic disk to become the optic nerve. The optic disk is also called the blind spot because it has no receptors. Since it is located is different portions of the visual field in each eye, the image from one eye compensates for the blind spot in the other eye. Behind the neural layer is the pigmented layer which absorbs excess light which could disrupt the image. This layer also contains vitamin A for replenishment of the pigment which absorbs light in the receptor cells. 28
The Macula Lutea Macula lutea Fovea centralis Optic disk This is what an ophthalmologist or optometrist sees when he or she looks into your eye with an ophthalmoscope. The dark area is the macula lutea, the yellow spot within which is the fovea centralis, or central cone, the area of greatest visual acuity because it has the highest concentration of receptors, mostly the color sensing cones. But the examination is to look at the blood vessels. Early damage from diabetes and other disorders can often be seen in the structure of the vessels which serve the retina. A disorder of unknown cause known as macular degeneration is the most common cause of blindness in the US. 29
Retinal Structure Pigmented layer Rods Cones Bipolar cells Ganglion cells Horizontal cells Amacrine cells Here you see the rods and cones, as well as other cells found in the retina. The rods are dim light, black and white receptors only. The cones respond to different wavelengths of light to produce color vision. The other cells function to integrate the nerve impulses produced. The structure of the retina is surprising. The nerve fibers and other cells actually lie in front of the rods and cones. This disrupts the image in most of the retina, except in the fovea centralis, the central cone where receptor cells lie close to the surface so that light strikes them directly. This is the area at the center of our vision where the greatest sharpness of vision occurs. 30
dim light receptors very sensitive to light requires dark adaptation found around the periphery of the retina, outside the macula lutea monochromatic vision Rods vs. Cones function in bright light require more light to be stimulated no adaptation time required, except in extremely bright light mostly in the macula lutea, only receptor in the fovea centralis color vision Here is a comparison of the characteristics of the rods and cones. 31
Visual Pathway 1) Optic nerve 2) Optic chiasma 3) Optic tract 1 2 3 4 5 4) Midbrain 5) Thalamus 6) Visual cortex 6 Part of the image from each eye goes to each side of the cerebral cortex, so that the cortex can put together a single three-dimensional image from the two eyes. 32
The Ear Auricle Middle ear Inner ear External ear External auditory canal The external ear captures the sound vibrations, the middle ear concentrates the vibrations on the window (oval window) leading into the cochlea, where the actual sensory receptors transform the vibrations into nerve impulses which travel to the cerebral cortex. 33
Ossicles: stapes incus malleous Tympanic membrane Middle and Inner Ear Semicircular canals Vestibule Vestibular n. cochlear n. of VIII Oval window Round window Cochlea Int. jugular v. Eustachian canal Sound vibrations pass through the external auditory canal to reach the tympanic membrane or eardrum. Vibrations pass from the tympanic membrane to the ossicles, the three tiniest bones in the body. The ossicles concentrate the strong vibrations about 10 times onto the oval window, damping the weak vibrations in the process. Vibrations pass from the oval window into canals in the cochlea, which contains the sense organ for hearing. The cochlea transforms these into nerve impulses which travel to the cerebral cortex. The semicircular canals and vestibule contain the sense organs for movement and position which are important for maintaining balance. 34
The Middle Ear View from the medial side Malleus Incus Stapes Tympanic membrane Tensor tympani muscle Footplate of stapes Eustachian canal Stapedius muscle In the middle ear magnified vibrations are transferred from the tympanic membrane to the oval window. Stapedius and tensor tympani muscles damp vibrations to reduce damage to ossicles and tympanic membrane and to allow the next sound to be heard. The eustachian canal equalizes pressure in the middle ear cavity. The malleous attaches to the tympanic membrane and the footplate of the stapes inserts onto the oval window. 35
Middle Ear Cavity Malleus head Malleus handle Incus Chorda tympani Stapes Round window Tympanic membrane Here you see an actual photo of the middle ear cavity. Conduction deafness is due to inelasticity of ligaments which connect the ossicles. It is treated with a standard amplifying hearing aid in which sound passes through bone around the ossicles to reach the eardrum. Nerve deafness is due to impairment or damage to the sense organ in the cochlea. It can be congenital, or caused by exposure to intense sound. It cannot be corrected with an amplifying hearing aid. But new devices are in development which can pick up sounds and stimulate the auditory nerve for those with this type of deafness. 36
The Ossicles and Inner Ear Semicircular canals Ampulla utricle vestibule{saccule Incus Malleus Stapes Cochlea The connection between the ossicles and the inner ear. 37