THE SPECIAL SENSES BIO 137 Anatomy Physiology I

THE SPECIAL SENSES BIO 137 Anatomy & Physiology I

Special Senses • Receptors for the special senses of smell, taste, vision, hearing, and equilibrium are anatomically distinct from one another and are concentrated in specific locations in the head • In addition to the stimuli and the receptors, there are specific afferent pathways and translation sites in the brain for information assembled from these special senses

Special Senses v Comparing the general senses and the special senses General Senses – Include somatic sensations Special Senses –Include smell, taste, vision, (tactile, thermal, pain, and hearing and equilibrium proprioceptive) and visceral –Are concentrated in specific sensations locations in the head – Are scattered throughout the body – Are relatively simple structures –Are anatomically distinct structures –Form complex neural pathways

Olfaction and Taste • Olfaction is the process of perceiving smells. Smell and taste are brought about through the interpretation of chemicals present in the environment • Olfactory and gustatory (taste) impulses travel not only to the cerebral cortex, but also to the limbic system • We can have emotional responses and strong memories to certain smells and tastes • gustation and olfaction work together. Olfaction is much stronger/more sensitive (when someone has a cold it is difficult to taste food)

Olfaction • The olfactory epithelium is located in the superior part of the nasal cavity covering the surface of the cribriform plate and extending along the superior nasal concha

Olfaction • The olfactory apparatus can detect about 10, 000 different odors, often in concentrations as low as 1/25 billionth of a milligram per milliliter of air • When an odorant binds to the receptor of an olfactory hair it initiates a cascade of intracellular events

Olfaction • Once generated, nerve impulses travel through the two olfactory nerves olfactory bulbs olfactory tract primary olfactory area in the temporal lobe of the cortex • Olfaction is the only sensory system that doesn’t go through the relay stations in the thalamus

Olfaction • Olfactory sensory pathways (centrally) are rapidly adapting, decreasing activity by 50% in the first second, and completely accommodating in 1– 2 minutes

Olfactory Nerve Pathways 1. Odorant molecules enter as gases and 2. 3. 4. 5. dissolve in liquid Bind olfactory receptor cells Nerve Impulse travels on fibers through cribriform plate of ethmoid bone Fibers Synapse with neurons of olfactory bulbs (CNI) Travel along olfactory tracts to limbic system (amygdala) and cerebral cortex

Gustation • Gustation, or taste, detects five primary tastes : sour, sweet, bitter, salty, and umami (“meaty” or “savory”)

Sense of Taste • Taste Buds are organs of taste • Humans have 10, 000 of these located on papillae of tongue, roof of mouth, cheek linings, walls of pharynx • Taste Receptors are chemoreceptors, called taste cells that are located on taste buds • Taste cells have receptive microvilli (taste hair) that project through a taste pore • Taste hair is sensitive portion that detects chemicals dissolved in saliva • Undergo rapid sensory adaptation

Gustation • In addition, the entire surface of the tongue has filiform papillae that contain tactile receptors but no taste buds • They increase friction between the tongue and food, making it easier to move food in the oral cavity

Gustation • Three cranial nerves contain axons gustatory neurons that innervate the taste buds • The facial (VII) nerve serves taste buds in the anterior 2/3 of the tongue • The glossopharyngeal (IX) nerve serves taste buds in the posterior 1/3 of the tongue • The vagus (X) nerve serves taste buds in the throat and epiglottis

Gustation • Nerve impulses propagate along these cranial nerves to the gustatory nucleus in the medulla oblongata. From there, axons carrying taste signals project to the hypothalamus, limbic system, and thalamus • Taste is perceived consciously as signals from the thalamus arrive at the primary gustatory area at the base of the somatosensory cortex in the parietal lobe

Taste Pathways • Taste signals enter the brain at the medulla from 3 cranial nerves • Facial Nerve, VII, carries impulses from anterior 2/3 of tongue • Glossopharyngeal Nerve, IX, carries impulses from posterior 1/3 of tongue • Vagus Nerve, X, carries impulses from base of tongue & pharynx • Impulses then travel to the Thalamus and to the cerebral cortex • Taste impulse then travels to the limbic system for an emotional tag

Vision • Our visual perception is dependent on the eye, its accessory structures, the optic tracts, and the 1 o visual cortex and it’s association areas • Vision is possible because of photoreceptors that detect light

Accessory Eye Structures • The upper and lower palpebrae are the eyelids • CN III (Oculomotor) supplies 4 of the 6 extraocular muscles, plus the muscles that raise the upper eyelid • The conjunctiva is a clear mucous membrane that covers the white (avascular) part of the eye

Accessory Eye Structures • The lacrimal glands are each about the size an almond, situated superolateral to the eyeball. Leading from the lacrimal glands are 6 to 12 excretory lacrimal ducts • Tears (lacrimal fluid) run from the lacrimal glands, into the excretory lacrimal ducts, onto the surface of the conjunctiva, over the surface of the eyeball

Accessory Eye Structures • Tears drain into the lacrimal puncta, which are two openings on the nasal side of the extreme edge of the eyeball. Superior and inferior lacrimal canals empty the tears into the nasolacrimal sac and nasolacrimal duct • The right and left sided nasolacrimal ducts empty into each side of the nose

Accessory Eye Structures • Watery eyes occur when lacrimal fluid builds up, as when something obstructs the nasolacrimal ducts • Blocked nasolacrimal ducts can be caused by an inflammation of the nasal mucosa, such as a cold • Over production of lacrimal fluid occurs in response to parasympathetic stimulation, caused by an emotional response (crying), and tears spill over the edges of the eyelids and drain into the nasal cavity (causing nasal stuffiness)

Anatomy of the Eye The wall of the eyeball consists of three layers or tunics: 1. The fibrous tunic is the outer layer and is composed of the sclera (“white” of the eye) and the cornea (the transparent epithelium the protects the front of the eye) 2. The vascular tunic is the middle layer and is composed of the choroid, the ciliary body and the iris 3. The nervous tunic is the inner retinal layer

Middle Tunic • The choroid forms the major vascular portion that lines the internal surface of the sclera • The ciliary body consists of two parts: • The ciliary processes that secrete aqueous humor • The ciliary muscle that changes the shape of the lens to adapt to near and far vision • The iris is the colored portion of the eyeball consisting of circular and radial smooth muscle fibers

Middle Tunic • Suspensory ligaments extend inward from the ciliary processes and hold the lens of the eye in place • Lens is a clear, membrane like structure that divides the eyeball into an anterior and posterior chamber • Lens is very elastic and can change shape due to changes in ciliary muscles and suspensory ligaments = Accommodation

Aqueous Humor • fluid in anterior cavity of eye • secreted by epithelium on inner surface of the ciliary body • provides nutrients for cornea/lens • maintains shape of anterior portion of eye • Normally drains back to bloodstream • What is glaucoma?

• Vitreous humor – thick gel of posterior cavity that holds retina flat against choroid coat & maintains round shape of eyeball

Structure of Retina • Retina contains visual receptors called photoreceptors (rods and cones) • 3 layers • Outer • Photoreceptors (rods & cones) • Middle • Bipolar neurons • Inner • Ganglion cells (axons join & form optic nerve) • Light passes through ganglion and bipolar neurons before reaching photoreceptors

Retinal Layers

Anatomy of the Eye • The retina consists of two types of photoreceptor cells, rods and cones • Rods are abundant in the periphery of the retina whereas cones are found more frequently in the central areas

Anatomy of the Eye • Each eye contains ≈ 120 million rod- shaped photoreceptors that are adapted for a low light threshold (high sensitivity) - they produce low resolution, black and white images • a loss of rods with age makes it difficult to drive at night

Anatomy of the Eye • Cone-shaped photoreceptors function in bright light to produce high resolution color images • They exists in three varieties, corresponding to the type of pigment they contain: red, green or blue • The photopigments are concentrated in the outer segment of the receptor, while the inner segment contains the nucleus and organelles

Retina • The exact center of the retina is called the macula lutea, and in its center is a small depression called the central fovea (or fovea centralis) • There are no rods or nerve cells in the fovea, only a high concentration of cones - this gives us the sharp central vision • Optic disc - back of retina, site where nerve fibers leave retina • Central retinal artery enters and central retinal vein exits • BLIND SPOT • CONTAINS NO PHOTORECEPTORS

The Pupillary Response • The pupil is an opening in the center of the iris. It is composed of a radial muscle that “radiates” away from the center, and a circular muscle that is in the center • Contraction of the inner circular muscle fibers (bright light) cause the pupil to constrict • Contraction of the radial Fibers (dim light) causes it to dilate

Accommodation • Ability to adjust strength (shape) of lens in order to focus both near and far sources on retina • Controlled by ciliary muscles and suspensory ligaments • Normal image formation depends on refraction of light waves, accommodation of the lens, constriction of the pupil, and convergence of the two eyes

Refraction : Focusing Light • Refraction is the bending of light rays in order for them to be focused on the retina • Refraction occurs when light reaches the cornea and again when it reaches the lens • This is the basis for corrective measures for vision

Refraction • If the shape of the eye is normal, light waves are focused sharply on the retina • Image detected on retina is upside down & backward because of bending of light rays • Image is interpreted correctly in visual cortex

Refraction Disorders • Myopia • Nearsightedness • Light focused in front of retina • Corrected with concave lens • Hyperopia • Farsightedness • Light focused behind retina • Corrected with convex lens

HOW DOES THE BRAIN INTERPRET WHAT WE SEE? Light must reach photoreceptors in retina and be transformed into electrical (neural) signals in the CNS

Visual Transduction • The first step in visual transduction is absorption of light by a photopigment, a colored protein that undergoes structural changes when it absorbs light in the outer segment of a photoreceptor • Light absorption initiates a series of events that lead to the production of a receptor potential (number 4 in the diagram)

Phototransduction • Conversion of light stimuli into neural signals • Mechanism same for all photoreceptors • Occurs in bright light 1. Photopigment absorbs light & breaks down to 2 components 2. Hyperpolarizes rods/cones 3. Decrease in release of inhibitory neurotransmitter • What is the effect in dim light?

HOW DOES THE RETINA SIGNAL THE BRAIN IF PHOTOTRANSDUCTION INVOLVES AN INHIBITORY NEUROTRANSMITTER? • Prevent release of inhibitory neurotransmitter = Excitation • Light: remove inhibitory NT = Excite • Dark: presence of inhibitory NT = Inhibition

Visual Pathway • Fibers partially cross over in optic chiasm • Right half of brain interprets image from left eye and vice versa • How does this affect damage to a particular area of optic pathway?

Structure of Eye: Anterior to Posterior • Cornea • Aqueous Humor • Iris • Pupil • Lens • Vitreous Humor in Posterior Cavity • Retina • Choroid (continues anteriorly to form ciliary body) • Sclera

PATHWAY OF LIGHT • OPTIC NERVE • CORNEA • OPTIC CHIASM • AQUEOUS HUMOR • OPTIC TRACT • LENS • THALAMUS • VITREOUS HUMOR • PRIMARY VISUAL • PHOTORECEPTORS OF RETINA CORTEX OF OCCIPITAL CORTEX

Sense of Hearing and Equilibrium • The ear is the organ of hearing • The ear also contains receptors for equilibrium

The Ear • The ear has 3 principle regions • The external ear, which uses air to collect and channel sound waves • The middle ear, which uses a bony system to amplify sound vibrations • The internal ear, which generates action potentials to transmit sound and balance information to the brain

Auditory & Vestibular Sense External Ear: Middle Ear: Inner Ear: auricle (pinna) external auditory meatus tympanic membrane ossicles (malleus, incus, stapes) tympanic cavity Eustachian tube oval window cochlea vestibule semicircular canals

Fig 12. 9

External Ear • auricle (pinna) • collects sounds waves • external auditory meatus • directs sound waves toward tympanic membrane • lined with ceruminous glands & hairs • terminates with tympanic membrane 12 -25

Middle Ear or Tympanic Cavity • air-filled space in temporal bone • tympanic membrane (eardrum) • vibrates in response to sound waves • auditory ossicles • vibrate in response to tympanic membrane • vibrations are concentrated and amplified by ossicles • malleus, incus, and stapes • Vibration of ossicles causes movement of fluid in inner ear

Eustachian (Auditory) Tube • connects tympanic cavity (middle ear cavity) to nasopharynx • helps maintain equal pressure on both sides of tympanic membrane • usually closed by valve-like flaps in throat 12 -27

Eustachian Tubes: Air Pressure Changes • Traveling from high altitude to low altitude causes an ↑ in air pressure outside tympanic membrane • Pushes tympanic membrane inward • Swallowing, yawning or chewing allow valves in throat to open, air enters tube and equalizes pressure on inside of eardrum • “popping sound”

Middle Ear Infections • Mucous membranes lining eustachian tube are continuous with lining of middle ear • Passageway for bacteria from throat or nasal passages to middle ear • Children have shorter eustachian tubes, so infection more frequent in children • Insertion of ‘tubes’ through tympanic membrane into eustachian tube allows ears to drain

Inner Ear • Complex system of intercommunicating chambers and tubes called a labyrinth • • Osseous labyrinth • bony canal in temporal bone filled with fluid perilymph • Membranous labyrinth • tube that lies within osseous labyrinth • filled with fluid endolymph

Inner Ear: Labyrinths

Inner Ear Oval window – entrance to inner ear, covered by stapes 3 Parts of Labyrinths • cochlea • functions in hearing • semicircular canals • functions in equilibrium • vestibule • functions in hearing and equilibrium 12 -29

Cochlea • Coiled tubular system • 3 fluid filled longitudinal compartments • Scala vestibuli • Cochlear Duct • Scala tympani • Helicotrema – apex where fluid in top and bottom is continuous

Sense of Hearing: Cochlea Scala vestibuli • upper bony compartment of labyrinth • leads from oval window to apex of spiral • filled with perilymph Scala tympani • lower bony compartment of labyrinth • extends from apex of the cochlea to round window • filled with perilymph These are separated by cochlear duct filled with endolymph

Inner Ear: Cochlea

Sense of Hearing: Cochlear duct • portion of membranous labyrinth within cochlea • filled with endolymph Vestibular membrane • forms roof of cochlear duct Basilar membrane • forms floor of cochlear duct • Sense organ for hearing located here 12 -31

Cochlear Anatomy • Scala vestibuli • Vestibular membrane • Cochlear duct • Basilar membrane • Floor of cochlear duct • Organ of Corti located here • Scala tympani

Sense of Hearing: Organ of Corti • Organ of Corti is located on the upper surface of the basilar membrane • Hearing receptor cells are located on the organ of Corti • Hair cells covered by a tectorial membrane • These receptors are mechanoreceptors

Movement of Sound • Vibrations of ossicles in middle ear cause movement of fluid in inner ear • Results in vibrations of basilar membrane, and bending of hair cells on Organ of Corti • Hair cells bend against tectorial membrane • Nerve impulse initiated on cochlear branch of CN VIII

Auditory Pathway • Vibrations to basilar membrane • Hair receptor cells bend against tectorial membrane • Ca++ enters receptor cells • Neurotransmitter from hair cells released to outside • Stimulates nearby sensory fibers • NI carried along cochlear branch of Vestibulocochlear nerve (CNVIII) • Medulla Oblongata (fibers cross-over here) • Thalamus • Temporal Cortex • Impulses from each ear interpreted on both sides of brain

The Sense of Hearing: Auditory Nerve Pathways 12 -34

Sound Pathway • Auricle, External Auditory Meatus • Tympanic Membrane, Malleus, Incus, Stapes (through oval window to) • Perilymph of scala vestibuli, vestibular membrane • Endolymph of cochlear duct • Hair cells of Organ of Corti. NT released & stimulates sensory neuron • Travels along Cochlear branch of vestibulocochlear nerve (CNVIII) • Medulla to Thalamus to • Primary auditory cortex of temporal lobe

Hearing Loss • Conductive • Interference with transmission of vibrations to inner ear • Physical blockage, eardrum rupture, middle ear infections, restriction of ossicle movement • Can be treated with hearing aids • Sensorineural • Sound waves transmitted, no neural processing • Damage to Vestibulocochlear Nerve or cochlea • Can sometimes be treated with cochlear implants

Sense of Equilibrium • Feeling of equilibrium is derived from two senses • Involves semicircular canals & vestibule • Vestibule connects semicircular canals to cochlea Static Equilibrium • senses movement of head when body is still • vestibule Dynamic Equilibrium • senses rotation and movement of head when they suddenly move or rotate • Semicircular canals 12 -36

Sense of Static Equilibrium: Vestibule • Receptor cells send info along vestibular branch of CN VIII • Motor impulses are then sent out to help maintain balance

Vestibular Sense: How the body handles movement • Sensory receptors in inner ear give you information about movement, gravity and vibration • Keeps you upright when you bend or rotate the head (postural adjustments) • Quick start and stop movements give very intense vestibular stimulation

Vestibular Input • Jumping up and down • Running • Swinging • Spinning • Vestibular system is the primary organizer of sensory input