Sensory Systems

• Sensory system - collection of several cell types that work together to accomplish a specific receptive process by transducing various stimuli into nerve impulses.

• Somatic Sensory Systems - pain, touch, stretch, and temperature receptors and associated neurons

• Special Sensory Systems:

¤ visual system - photoreceptors¤ Olfactory - chemoreceptors¤ Taste – chemoreceptors¤ auditory - mechanoreceptors¤ equilibrium sense – mechanoreceptors

Visual System• Dominant sense in humans

• visual receptors-photoreceptors

• Structure of the eye

¤ External structures• canthi-medial and lateral corners of the eye

• palpebrae-eyelids

• caruncle-small, fleshy structure in the medial corner-contains sebaceous &sweat glands white secretion

• tarsal glands-posterior to eyelashes-secrete lubricant; infection/inflammation Chalazion

• ciliary glands-modified sweat glands between hair follicles

infection/inflammation of ciliary or other non-tarsal glands sty

Fig 15.1a

Visual System (con’t)

¤ External Structures of the eye (con’t)• conjunctiva-transparent mucous membrane that lines the eyelid

and whites of the eyeinflammation of the conjunctiva – conjunctivitis

infection of the conjunctiva – pinkeye• lacramal apparatus-produces protective lacramal fluid

containing mucous, antibodies, lysozymes tears produced in lacramal gland lacramal canal l. sac nasolacramal duct nasal cavity

¤ Internal Structures of the eye (eyeball)• Segments- divided into anterior and posterior segments by lens

posterior segment contains vitreous humor-clear gel made of collagenous fibers; supports the lens and retina anterior segment contains aqueous humor-plasma-like fluid that supplies nutrients and O2 to the lens,cornea and retinal cells-continually drained and replaced; drainage problem leads to glaucoma increased pressure in the eye.

Internal Structures of the Eye-(eyeball) (con’t)

• Wall- comprised of 3 layers (tunics) « 1) sclera-(fibrous tunic) fibrous outermost layer of the posterior segment of

the eyeball-fuses with the cornea» cornea-clear collagenous protective layer covering the front of the eye;

covered by epithelial sheets on both sides that allows regeneration« 2) uvea-middle layer (vascular tunic); contains blood vessels, iris and ciliary

body » iris-colored part of the eye-made of smooth muscle that acts a as a

diaphragm to open and close pupil (opening for light)» ciliary body-smooth muscular structure that control lens shape and

secretes aqueous humor « 3) retina (sensory tunic)- photosensitive layer; comprised of;

» pigmented epithelial layer-absorbs scattered light, stores VitA» neural layer-contains photoreceptors that mediate phototransduction

and other neurons involved in vision photoreceptors bipolar cells ganglion cells opticnerve

» optic disc-area where optic nerve exits the eye- “blind spot” retinal detachment-separation of vascular layer and sensory layer

• Lens-biconvex structure that changes shape to focus light on retina; made of crystalline proteins; cataract-clouding of the lens

Fig 15.4a

Phototransduction

• Phototransduction-transduction of light into a neural signal

¤ photoreceptors-light-detecting cells (neurons) of the retina that contain the visual pigments and convert the incoming light into a neural signal (change in membrane potential)rod photoreceptors-work in dim light; use rhodopsin;cone photoreceptors- work in bright light; coneopsin visual pigments- proteins containing 11-cis retinal that absorbs light

¤ visual transduction mechanism: light

11-cis retinal *rhodopsin *G-protein PDE all trans retinal

cGMP membrane cGMP-gated 5’GMP potential ion channels close

¤ Visual pathway to the brain: retina thalamus (relay) visual cortex and other areas

Olfactory System

• Odorant molecules detected by chemoreceptor cells called olfactory receptor cells (neurons)

• olfactory epithelium- located in the upper part of the nasal cavity-contains the olfactory receptor cells

• olfactory cilia- non-motile cilia on the olfactory receptor cells that contain the receptor surface for odorant molecules

• fiber bundles of olfactory nerves-axons of olfactory receptor cells that carry neural signal to olfactory bulb (in frontal lobe of brain)

• Physiology/trandsuction of olfaction: Dissolved odorant molecules bind protein receptors on olfactory cilia *G-protein ion channels

membrane potential olfactory bulb thalamus olfactory cortex

limbic system

Sense of Taste

• Mediated by chemoreceptor cells (gustatory cells-epithelial cells) located on taste buds; continually regenerated.

• Gustatory hairs (microvilli)- projections of gustatory cells that

contain the protein receptors for taste molecules within their the gustatory receptor membrane

• sensory nerve fibers-connect with gustatory cells to send neural signal to the gustatory cortex in the brain

• physiology/transduction of taste- different transduction mchanisms for different chemical stimuli; some involve a G-protein mechanism all involve conversion of a chemical signal neural (electrical) signal via ion channels that change the membrane potential of the gustatory receptor membrane

Auditory System• Structure of the ear

¤ outer ear- auricles, helix-collect sound waves external auditory canaltympanic membrane (ear drum)

¤ middle ear-tympanic cavity ; mucosal-lined; connected to nasopharynx through the pharyngotympanic (auditory) tube; contains ossicles that transmit vibrations in tympanic membrane to inner ear.

¤ inner ear- comprised of vestibule, semi-circular canals, cochlea cochlea-contains organ of corti-receptor organ of

hearing that contains auditory hair cells (mechanoreceptors) that detect movement; auditory nerve connected to the cochlea

• Mechanism of hearing: “sounds set up vibrations in the air that beat against the ear drum that pushes a chain of tiny bones that press fluid in the inner ear against membranes that set up shearing forces that pull on the hair cells that stimulate nearby neurons that give rise to impulses that travel to the brain which interprets them, and you hear”

• Tinnitis-ringing or clicking sound in the ears in the absence of auditory stimuli caused by inflammation of the middle ear, cochlear nerve degeneration, side effect of medications

• Otitis media- middle ear inflammation/infection-can result from sore throat

Cochlea

Semi-circular canals

Fig 15.25a

Equilibrium Sense

• Response to head movements-sense of orientation and balance• Involves input from inner ear, vision, and stretch receptors• Vestibular apparatus-semi-circular canals and vestibule contain

equilibrium mechanoreceptors that sense position and acceleration

• Maculae-sensory receptors located in vestibule; contain hair cells that sense linear acceleration but not rotational motion; when hairs are bent membrane potential vestibular nerve fibers

• Cristae-sensory receptors located in the semi-circular canals that contain hair cells that respond to angular or rotary movements; hairs are bent when fluid moves within the semicircular canals hair cells excited via membrane potential vestibular apparatus

• Impulses from the vestibular apparatus are sent via vestibular nerve fibers to the vestibular complex of the brain stem and the cerebellum. These centers initiate responses that fix the eyes on objects and activate muscles to maintain balance

• motion sickness-equilibrium disorder due to vestibular and visual sensory input mismatch- dramamine, bonine depress vestibular inputs