Lecture 4 - Special Senses (Vision, Hearing) PDF

Lecture 4: Special Senses Sherwood Chapter 5 Objectives The Visual System Chemical Senses (Taste and Smell) The Auditory System The Vestibular System 2 The Visual System Visual System: The Eye 70% of all sensory receptors are in the eye Most of the eye is protect...

Lecture 4: Special Senses Sherwood Chapter 5 Objectives The Visual System Chemical Senses (Taste and Smell) The Auditory System The Vestibular System 2 The Visual System Visual System: The Eye 70% of all sensory receptors are in the eye Most of the eye is protected by a cushion of fat and the bony orbit Accessory structures include: Eyebrows (shade the eye, prevent perspiration from reaching the eye) Eyelids (protect the eye, contains glands that secrete a whitish, oily secretion) Eye lashes (initiate reflex blinking) Eye muscles Visual System: Extraocular Muscles Lateral Rectus Muscle: Abduction of eye Medial Rectus Muscle Adduction of the eye Superior Rectus Muscle Upward movement of the eye Inferior Rectus Muscle Downward movement of the eye Superior Oblique Muscle Downward movement of eye Inferior Oblique Muscle Upward movement of the eye Visual System: Structures of 4 the Eye 3 Cornea (1) protective, transparent covering of iris and pupil Sclera (2) Connective tissue layer 1 covering eye (whites) Conjunctiva (3) Epithelial tissue layer connects sclera to eyelid Lacrimal glands (4) 2 Secretes aqueous tear film that lubricates eye  Visual System: Structures of the Eye Lacrimal apparatus Lacrimal gland Lacrimal sac Iris - The colored part of the eyeball Pupil – central opening of the iris Regulates amount of light entering the eye during: Close vision and bright light – pupils constrict Distant vision and dim light – pupils dilate Changes in emotional state – pupils dilate when subject matter is appealing or requires Visual System: Internal Structures of the Eye Fibrous layer Sclera cornea Vascular layer Choroid (dark pigmented) Ciliary body (-lens) Iris Sensory layer retina Lens Epithelial cells filled with crystalline proteins Transparent and capable of changing shape (focusing) Retina Light sensing tissue at back of eyeball A delicate two-layered membrane  Visual System: Retinal Layers Pigmented layer – the Fibers of the Optic Nerve outer layer that absorbs stray light rays; phagocytic function Neural layer – processes visual data before sending nerve impulses into axons that form the optic nerve  Visual System: Cells of the Retina 3 types of neuronal cells within the retina 1. Photoreceptors Rods and Cones Rods (numerous ~ 120 million): Allow us to see in dim light Because rods do not provide color vision, in dim light we can see only shades of gray A person who loses rod vision mainly has difficulty seeing in dim light (shouldn’t drive at night) Are used for peripheral vision Cones (less numerous ~6 million): Respond to bright light Produce color vision  Visual System: Cells of the Retina 2. Ganglion Cell Receives visual information from the photoreceptors of the eye Sends information to the optic nerve fibers  then to brain 3. Bipolar Cell ON and OFF Dark = OFF Light = ON  Visual System: Cells of the Retina 2 types of glial cells within the retina Both have lateral functioning to integrate response of multiple cells within retina 2. Horizontal Cell Modulate signaling between photoreceptors and bipolar cells Inhibitory and release GABA upon depolarization Allow for eyes to adjust to both bright and dim light 3. Amacrine Cell Modulate signaling between ganglion and bipolar cells Inhibitory feedback via release of GABA Visual System: Cells of the Retina A more detailed look at photoreceptors Outer segment Detects the light stimulus Inner segment Metabolic area of the receptor Synaptic Terminal transmits signal to bipolar cells Visual System: Light Sensitive Photopigment Present in the outer segment of rods and cones Contained in transmembrane proteins (opsin) within discs of the outer region Contains retinal, which is a derivative of vitamin A Chemically changes when activated by light Light sensitive photopigment in rods = Rhodopsin Absorbs all wavelengths of light Brain cannot distinguish between colors + vision looks grey Light sensitive photopigment in cones = Photopsin Three types of cones that help distinguish colors Red Green Blue  Visual System: Dark Vision (Rods) When visual stimulus (photon) hits rod, absorption causes retinal to change shape This is what starts the conversion of energy (photon) to an action potential 11-cis-retinal In the dark all-trans-retinal In the light  Visual System: Photopsin IIIPhotopsin Photopsin II I Colour Vision (Cones) 3 types of photoreceptors present in the retina: Photopsin I: Sensitive to red light Photopsin II: Sensitive to green light Photopsin III: Sensitive to blue light Colour vision results from the stimulation of combinations of theses 3 types of cones Colour blindness is due to a lack of one or more of the cone types When specific wavelength hits cone, absorption causes retinal to change shape Just like rod vision  Visual In the DARK In the LIGHT System: ** Phototransdu ction cascade 1. Light hits photoreceptor and causes hyperpolarization 2. Hyperpolarization prevents Ca channels from opening 3. Less neurotransmitter (glutamate) is released Glutamate normally excitatory, but bipolar cells have special ion channels that open in the presence of less glutamate 4. Special ion channels open and bipolar cell (1st order neuron) depolarizes 5. AP reaches terminal end and neurotransmitter (glutamate) is released 6. Glutamate causes excitatory response (the “normal” response of glutamate) in ganglion cell (2nd order neuron) 7. If enough EPSP generated in ganglion 11-cis-retinal all-trans-retinal  Visual System: Optic Tracts Action potentials are sent along optic nerve fibers (ganglion cells – 2nd order neurons) The optic nerves meet at the optic chiasm Visual information is sent along optic tracts Information from contralateral side of eye will cross over Ie, right visual field of left eye will cross over to right side of brain Reaches the occipital lobe in the brain Visual Cortex  Lecture 4 Pop Quiz 1) Which cells are responsible for night vision? 2) How many types of cone cells do we have? 3) Does light depolarize or hyperpolarize the photoreceptor? 4) What are the major steps in the phototransduction cascade? Chemical Senses (Taste and Smell) The Chemical Senses Chemical senses 1) gustation (taste) and 2) olfaction (smell) Their chemoreceptors respond to chemicals in aqueous solution Taste – to substances dissolved in saliva Smell – to substances dissolved in fluids of the nasal membranes Taste Receptors (Gustation) Located along edges of small, depressed areas called fissures Stimulated when substance is dissolved in solution The stimulus converted into a nerve impulse by receptors Taste receptors are often G- protein coupled receptors  X Taste Receptors (Gustation) 5 basic tastes Sweet Sour Salty Bitter Umami Influence of other factors on taste: Taste is 80% smell Temperature and texture enhance or detract from taste Sent to the brain via CN VII (facial nerve) and CN IX (glossopharyngeal nerve) THE TONGUE MAP IS A MYT Smell Receptors (Olfaction) Receptors for smell located in olfactory epithelium (upper part of nasal cavity) Chemicals detected must be in solution in fluids that line nose Because these receptors are high in the nasal cavity, one must “sniff” to bring odours upward in the nose The Auditory System  Hearing The three parts of the ear are the inner, outer, and middle ear The outer and middle ear are involved with hearing only The inner ear functions in both hearing and equilibrium (balance) Receptors for hearing and balance: Respond to separate stimuli Are activated independently The Ear Houses two senses: Hearing Equilibrium (balance) Receptors for hearing and balance: Are mechanoreceptors Respond to separate stimuli Are activated independently 27  Anatomy of the Ear The ear is divided into three major areas: External ear (outer) – Middle ear – Internal ear – 28 External Ear Pinna (auricle) External acoustic meatus Walls are lined with ceruminous glands, which secrete cerumen (earwax) Tympanic membrane (eardrum) Sound waves in the auditory canal cause the eardrum to vibrate Separates the external and middle ear 29  Middle Ear Borders Lateral: eardrum Medial: bony wall with two openings Oval window and round window Pharyngotympanic tube Connects middle ear with the throat Allows pressure in middle ear to be equalized with external pressure Otitis Media 30  Middle Ear Ossicles: Eardrum Malleus Incus Stapes Function to transmit the vibratory motion from the eardrum to the inner ear 31 Inner Ear Consists of bony chambers (called bony labyrinth) within temporal bone Cochlea Vestibule Semicircular canals 32 A closer look at the inner ear The Cochlea Spiral Organ: Fluid-filled structure Contains the Organ of Corti (sensory organ of hearing) Houses thousands of hair cells (receptors for hearing) 34 Organ of Corti Contains hair cells (hearing receptors) Sound waves that reach inner ear set cochlear fluids into motion Motion vibrates the basilar membrane Hair cells on basilar membrane are stimulated and transmit impulses via cochlear nerve to auditory cortex 35 Sound Transduction Auditor y Pathwa y 37 Deafness Sensorineural deafness: Impairment in hair cells, cochlear nerve or to neurons in the auditory cortex Prolonged drug exposure (aspirin; antibiotics) Prolonged loud noise exposure Conduction deafness: Results when something interferes with the conduction of sound vibrations to the inner ear Injury to the eardrum Buildup of earwax 38 The Vestibular System The Vestibular Apparatus SEMICIRCULAR CANALS Anterior VESTIBULE Posterior Lateral 40  Anatomy of the Vestibular Apparatus Bony labyrinth: Filled with perilymph Suspended in the perilymph is a membranous labyrinth The membranous labyrinth contains a thicker fluid called endolymph 41  Equilibriu m Responds to head movements Equilibrium receptors of the inner ear are part of the vestibular apparatus Static equilibrium - Vestibule Dynamic equilibrium – Semicircular Canals 42 Static Equilibrium Receptors in the vestibule called maculae Respond to gravity, and report on head position when the body not moving (i.e static) Help keep our head erect Macula is patch of hair cells embedded in otolithic membrane (gel) studded with otoliths (tiny stones) Otoliths roll in response to changes in head position Creates pull on gel, which slides over hair cells and bends them Bending of hair cells sends impulses along vestibular nerve to cerebellum 43 Dynamic Equilibrium Receptors found in semicircular canals called cristae ampullaris (contain hair cells) Covered with gelatinous cap called the cupula Respond to angular or rotational body movements Canals oriented in 3 planes of space (anterior, posterior, lateral) anterio When head moves in angular r direction, endolymph lags behind The cupula then drags against later al endolymph, causing it to bend posteri Bending stimulates hair cells or and impulses are transmitted via vestibular nerve to cerebellum 44  Balance/ Equilibrium Depends on inputs from internal ear, from vision, and from stretch receptors of muscles and tendons The equilibrium receptors of internal ear collectively send signals to brain to initiate reflexes needed to make simplest changes in position Receptors in semicircular canals of inner ear transmit signals via vestibulocochlear nerve  Balance and Orientation Pathways Three modes of input for balance and orientation Vestibular receptors Visual receptors Somatic receptors These receptors allow our body to respond reflexively 46 Input and output of the Vestibular Nuclei  Homeostatic Imbalance: Motion Sickness Appears to be a sensory input mismatch Example: travelling in a vehicle The vestibular apparatus detects no motion because you are sitting still The visual inputs indicate that you are moving and sends impulses that disagree with the vestibular information The brain receives conflicting information and somehow leads to motion sickness (research still unclear how this happens) Once the stimulus ends (car ride over) the symptoms start to disappear (sometimes immediately, sometimes hours) 48  Lecture 4 Pop Quiz 1) What is the function of the middle and inner ear? 2) What is responsible for transmission of impulses to auditory cortex? 3) What’s the difference between sensorineural and conduction deafness? 4) What’s the difference between vestibules and semicircular canals? 5) Give an example of static eqilibrium 6) Describe how dynamic equilibrium works 7) Application: Your friend has BPP vertigo, and small calcium carbonate crystals have formed in their anterior semicircular canal. Your friend is totally fine unless they do any extreme movement, like a cartwheel, in the frontal (anterior) plane. Describe how vertigo causes dizziness, and why your friend only has an issue when they do extreme movements in a specific plane.