Special Senses Lecture

Unit 3: Lecture Study Guide Special Senses ● ● There are five senses that we know of: touch, taste, smell, sight and hearing. Touch is a large group of general senses. The other four traditional senses—smell, taste, sight and hearing—are called special senses. Receptors for a fifth special sense—equilibrium, or the sense of balance—are located in the ear. ○ In contrast to the widely distributed receptors for the general senses, the special sensory receptors are localized and confined to the head region. ○ The receptors for special senses are not free nerve endings of sensory neurons but distinct receptor cells. ■ These are neuron-like epithelial cells or small peripheral neurons that transfer sensory information to other neurons in afferent (towards) pathways to the brain. The Chemical Senses - Taste and Smell ● The receptors for taste (gustation) and smell (olfaction) are classified as chemoreceptors because they respond to chemical substances—to food chemicals dissolved in saliva and to airborne chemicals that dissolve in fluids on the nasal membranes. Taste Buds ● ● ● The taste receptors occur in the taste buds in the mucosa of the mouth and pharynx (throat). ○ The majority of the 10,000 or so taste buds are on the surface of the tongue; a few others are on the posterior region of the palate (roof of the mouth), on the inner surface of the cheeks, on the posterior wall of the pharynx, and on the epiglottis (a leaf-shaped flap behind the tongue). There are three types of peg-like projections of tongue mucosa called papillae (filiform, fungiform and circumvallate. Most of the taste buds happen in two types: ○ Small fungiform papillae scattered over the surface of the tongue ○ Large circumvallate papillae arranged in an inverted V near the back of the tongue. Taste buds occur within the epithelium that covers the papillae. In the fungiform papillae the taste buds are on the apical surface, while in the circumvallate papillae they’re on the side walls. ○ Each taste bud is a globular collection of 50-100 epithelial cells that resemble a bud on a tree or a closed tulip. Each contains two major cell types: gustatory cells, or taste cells, and basal cells. ■ Long microvilli called gustatory hairs project from the gustatory cells and extend through a taste pore to the surface of the epithelium. There, these microvilli are drenched in saliva containing the dissolving molecules that stimulate taste. These molecules bind to the plasma membrane of the microvilli, including the taste cells to generate impulses in the sensory nerve fibers that innervate them. ■ Cells in the taste buds are replenished every 7-10 days by cell division of the basal cells, replacing gustatory cells that are scraped off or burned during eating. ■ If an entire taste bud is destroyed, a new one will form after its nerve ending grows back into the epithelium. Taste Sensations and the Gustatory Pathway There are five basic taste sensations: ● ● ● ● ● Sweet - sugars, saccharin, alcohol, and some amino acids Salt - metal ions Sour - hydrogen ions Bitter - alkaloids such as quinine and nicotine Umami - elicited by the amino acid called glutamate Influences of Other Sensations in Taste ● ● ● Taste is 80% smell Thermoreceptors (temperature), mechanoreceptors (touch, pressure, vibrations) and nociceptors (pain) also influence taste Temperature and texture enhance or detract from taste The Eye and Vision ● ● ● Vision is the dominant sense in humans: Approximately 70% of all the sensory receptors in the body are in the eyes and 40% of the cerebral cortex is involved in processing visual information. Only the anterior one-sixth of the eye’s surface is visible; the rest of the eye lies in the coneshaped bony orbit, where it’s surrounded by a protective cushion of fat. ○ Behind the eye, the posterior half of the orbit contains the optic nerve, the arteries and veins to the eye, and the extrinsic eye muscles. Accessory Structures of the Eye include: ○ Eyebrows ■ Consist of coarse hairs in the skin on the superciliary arches. ■ They shade the eyes from sunlight and prevent sweat from running into the eyes ○ Eyelids (or Palpebrae) ■ Thin, skin-covered folds supported internally by connective tissue structures called tarsal plates. These stiff plates give the eyelids their curved shaped and serve as attachment sites for the orbicularis oculi. ■ Protect the eyes anteriorly ■ The upper and lower lids are separated by the palpebral fissure (eye slit) and meet each other at the medial and lateral angles (canthi), or eye corners. ● The medial canthus contains a reddish elevation called the lacrimal caruncle (“a bit of flesh”) ● Glands here produce the gritty “eye sand” that you find when you wake up ○ Conjunctiva ■ A transparent mucous membrane that covers the inner surfaces of the eyelids as the palpebral conjunctiva and folds back over the anterior surface of the eye as the bulbar conjunctiva. ● The bulbar conjunctiva, which covers the whites of the eyes but not the cornea, is a very thin membrane and the blood vessels are clearly visible beneath it. ■ When the eye is closed, the slit-like space that forms between the eye surface and ○ ○ the eyelids is called the conjunctival sac, which is where a contact lens would go. Lacrimal Apparatus ■ Consists of the lacrimal gland and associated ducts ■ Lacrimal glands secrete tears ● Tears contain mucus, antibodies and lysozyme ● They drain into the nasolacrimal duct Extrinsic Eye Muscles ■ There are six straplike extrinsic eye muscles, which originate from the walls of the orbit and insert onto the outer surface of the eyeball, that control eye movement, hold in the eyes in place and maintain shape of the eyeball. ● Four rectus muscles originate from the annular ring ● Two oblique muscles move the eye in the vertical plane Structure of the Eyeball ● ● Because the eyeball is generally shaped like a globe, it’s said to have two poles: The anterior pole and the posterior pole. There’s an external wall and an internal cavity, which contains a fluid called humors; ○ ○ ● The lens separates the internal cavity into two segments: The anterior segment, which contains aqueous humor ■ The anterior segment is divided into two chambers: ● Anterior chamber ○ Lies between the cornea and the iris ● Posterior chamber ○ Lies between the iris and the lens ○ The posterior segment, which contains the jelly-like vitreous humor The external wall consists of three layers: ○ ○ Fibrous layer ■ This layer is the most external layer ■ Consists of dense connective tissue arranged into two different regions: ● Cornea (anteriorly) ○ Transparent to allow light into the eye ○ Forms the anterior sixth of the fibrous layer ○ A round window bulges outward from its junction with the sclera ○ Consists of a thick layer of dense connective tissue sandwiched between a superficial corneal epithelium and a deep corneal endothelium ○ The junction between the cornea and the sclera is called the limbus ● Sclera (posteriorly) ○ Opaque white, seen as the “whites of the eye” ○ Forms the posterior five-sixth of the fibrous layer. ○ Protects the eyeball ○ Provides shape and a sturdy anchoring site for the extrinsic eye muscles Vascular layer ■ This layer has three parts: ● Choroid ○ The choroid is a very vascular, darkly pigmented membrane that forms the posterior five-sixths of the vascular layer. ○ The many blood vessels nourish the other layers of the eye ○ The brown color of the choroid is melanin, produced by melanocytes, which helps absorb light, thereby preventing light from scattering within the eye and creating visual confusion ● Ciliary body ○ ○ ○ ○ ○ ○ ● Iris The halo of fine fibrils that extends from these processes to attach around the entire circumference of the lens is called the ciliary zonule ○ The visible, colored part of the eye. ○ It lies between the cornea and the legs and its base attaches to the ciliary body The round opening in the middle is called the pupil, allowing light to enter the eye ■ The pupil regulates the amount of light entering the eye: ● Close vision and bright light - pupils constrict ● Distant vision and dim light - pupils dilate ○ ○ Anteriorly, continuous with the choroid layer is the ciliary body A thickened ring of tissue surrounding the lens Anchors the suspensory ligament that holds the lens in place Mostly consists of smooth muscle called the ciliary muscle, which focuses the lens Nearest the lens, the posterior surface of the ciliary body is thrown into radiating folds called ciliary processes Sensory layer (retina) ■ This layer contains two things: ● Retina ○ The retina itself has two layers: ■ A thin pigmented layer ● Lies against the choroid, is a single layer of flat-tocolumnar melanocytes. ● Functions like the choroid to absorb light and prevent it from scattering within the eye ■ A far thicker neural layer ○ It’s a sheet of nervous tissue that contain: ○ Photoreceptors - light-sensitive cells converting light into electrical signals that the brain uses for vision; come in two types: ■ Rod cells ■ Respond to dim light ■ Used for peripheral vision ■ Cone cells ■ Respond to bright light ■ Have high-acuity color vision ■ Found in the macula lutea ■ concentrated in the fovea centralis ○ Bipolar cells - separate visual signals from light and dark contrasts ○ Ganglion cells - run along the inner surface of the retina and leave the eye as the optic nerve ○ Interneurons - (amacrine and horizontal cells) ● Optic nerve ○ Axons from the ganglion cells run along the internal surface of the retina and converge posteriorly to form the optic nerve, which runs from the eye to the brain Regional Specialization of the Retina ● In certain regions of the eye, the retina differs from its “typical” structure: ○ In the anterior part of the eye, the neural layer ends at the posterior margin of the ciliary body. This junction is called the ora serrata (“sawtoothed mouth”). The pigmented layer extends anteriorly beyond the ora serrata to cover the ciliary body and to form the pigmented layer of the posterior iris. ○ The posterior part of the eye contains three key things: ■ Lying precisely at the eye’s posterior pole is the macula lutea (“yellow spot”) ■ At the center of the macula lutea is a tiny pit called the fovea centralis (“central pit”). The fovea centralis contains only cone cells and provides maximal visual acuity. ■ A few mm medially to the FC the optic disc is the site where the optic nerve leaves the eye ● It lacks photoreceptors and is called the blind spot The Ear and Equilibrium ● The ear, the receptor organ for hearing and equilibrium, has three main regions: ○ External ear ■ Functions involved with hearing ■ Consists of the: ● Auricle, or pinna ○ The auricle is what we see on the outside ○ It’s composed of the helix (rim) and the lobule (earlobe) ● External acoustic meatus ○ A short tube (about 2.5cm long) running medially, from the auricle to the eardrum. ● ○ ○ Tympanic membrane (eardrum) ○ Thin connective tissue membrane that vibrates in response to sound ○ It transfers sound energy to the middle ear ossicles ○ Serves as a boundary between outer and middle ear Middle ear (Tympanic Cavity) ■ A small, air-filled space inside the petrous part of the temporal bone ■ Lined by a thin mucous membrane and is shaped like a hockey puck standing on its side ■ Its lateral boundary is the tympanic membrane and its medial boundary is the wall of bone that separates it from the inner ear ■ There are two small holes that penetrate the medial wall: ● A superior oval window ● An inferior round window ■ Superiorly, the middle ear arches upwards as the epitympanic recess; it’s superior boundary is the roof of the petrous region of the temporal bone ■ The posterior wall of the middle ear opens into the mastoid antrum, a canal leading to the mastoid air cells in the mastoid process ■ The anterior wall of the middle ear lies just behind the internal carotid artery, the main artery to the brain, which contains the opening of the pharyngotympanic tube ● The pharyngotympanic tube connects the middle ear to the pharynx ● It functions in equalizing pressure in the middle ear cavity within the external air pressure ■ The inferior boundary of the middle ear is a thin, bony floor, under which lies the important internal jugular vein. ■ Auditory ossicles ● The tympanic cavity is spanned by the three smallest bones in the body, which transmit vibrations of the eardrum across the cavity to the fluid in the inner ear. ● From lateral to medial, there is: ○ Malleus (or hammer), which looks like a club with a knob ○ Incus (or anvil), which resembles a tooth with two roots ○ Stapes, which looks like the stirrup of a saddle. ○ The handle of the malleus attaches to the eardrum, and the base of the stapes vibrates against the oval window. Internal ear ■ The internal ear, also called the labyrinth, because of its mazelike, complex shape, lies within the thick, protective walls of the petrous part of the temporal bone. ■ The internal ear consists of two main parts, the bony labyrinth and the membranous labyrinth: ● The membranous labyrinth ○ A continuous series of membrane-walled sacs and ducts that fit loosely within the bony labyrinth, like the way an airtube might fit inside a tire. ○ The main parts of the membranous labyrinth are the: ■ Semicircular ducts, one inside each semicircular canal ■ Utricle and saccule, both inside the vestibule ● ● ● ● ● ● ■ Cochlear duct, inside the cochlea ○ The membranous labyrinth is filled with a clear fluid called endolymph The bony labyrinth ○ The bony labyrinth is filled with a clear fluid called perilymph ■ The perilymph is continuous with the cerebrospinal fluid that fills the subarachnoid space. ○ A cavity in the petrous bone consisting of a system of twisting channels that has three parts; from posterolateral to anteromedial: ■ Se micircular canals The semicircular canals house the receptors for rotational acceleration of the head The three semicircular canals of the bony labyrinth lie posterior and lateral to the vestibule Each canal describes two-thirds of a circle and has an expansion at one end called an ampulla. ○ The ampullae house equilibrium receptors in a structure called crista ampullaris, or “crest of the ampulla” Each canal lies in one of the three planes of space: ○ Anterior and posterior semicircular canals lie in the vertical planes at right angles to each other ○ Lateral semicircular canal lies almost horizontally ○ Snaking through each canal is a part of the membranous labyrinth called the semicircular duct Vestibule ○ ○ The central egg-shaped cavity of the bony labyrinth Suspended in its perilymph are two sacs: ■ Utricle - extends into the cochlea ■ Saccule - extends into the semicircular canal ■ Both of these sacs contain equilibrium receptors called maculae, which respond to gravity and changes in the position of the head ■ Maculae are the sensory receptors for static equilibrium, containing supporting cells and hair cells that respond to vertical and horizontal movement Vestibular apparatus - These are equilibrium receptors in the semicircular canals and vestibule. They maintain our orientation and balance in space. Vestibular receptors monitor static equilibrium. Semicircular canal receptors monitor dynamic equilibrium. ● Cochlea ○ A spiraling chamber in the bony labyrinth that extends from the anterior vestibule ○ It's the size of a split pea ○ From its attachment to the vestibule at its base, it coils for about two and a half turns around a pillar of bone called the modiolus. ■ The modiolus is shaped like a screw whose tip lies at the apex of the cochlea, pointing anterolaterally. ○ Running through the bony core of the modiolus is the cochlear nerve, which is the cochlear division of the vestibulocochlear nerve. ○ The coiled part of the membranous labyrinth within the cochlea is called the cochlear duct (or scala media). ■ This is the location of the sensory receptors for hearing ■ This duct winds through the cochlea and ends blindly in the cochlear apex. ■ It’s filled with endolymph ○ Within the cochlea, the endolymph-filled cochlear duct (scala media) lies between two perilymph-filled chambers of the bony labyrinth, the scala vestibuli and scala tympani. ■ The scala vestibuli is continuous with the vestibule near the base of the cochlea, where it abuts the oval window. ■ The scala tympani, on the other hand, ends at the round window at the base of the cochlea. ■ The scala tympani and scala vestibuli are continuous with each other at the apex of the cochlea, in a region called the helicotrema. ■ The “roof” of the cochlear duct, separating it from the scala vestibuli, is the vestibular membrane. ■ The external wall of this duct is the stria vascularis, which is an unusual epithelium that contains capillaries and secretes the endolymph of the inner ear. ■ The floor of the cochlear duct consists of the osseous spiral lamina plus an attached sheet of fibers called the basilar membrane, which supports the spiral organ (of corti), the receptor epithelium for hearing. This tall epithelium is very similar to the receptor epithelia for equilibrium. It consists of columnar supporting cells and one row of inner and three rows of outer hair cells, which are the receptor cells. ● The cochlear branch of the vestibulocochlear nerve (VIII) runs from the organ of Corti to the brain ■ At the cell’s apex, the tips of the hairs, the stereocilia, are embedded in a gel-like tectorial membrane. Auditory and Equilibrium Pathways ● ● ● ● ● The ascending auditory pathway transmits auditory information primarily from the cochlear receptors of the inner hair cells to the cerebral cortex. First, impulses pass through the cochlear nerve to the cochlear nuclei in the medulla. From there, some neurons project to the superior olivary nuclei, which lie at the junction of the medulla and pons. Beyond this, the axons ascend in the lateral lemniscus (a fiber tract) to the inferior colliculus (the auditory reflex center in the midbrain), which projects to the medial geniculate nucleus of the thalamus. Axons of the thalamic neurons then project to the primary auditory cortex, which provides conscious awareness of sound.