Exercise 4: Special Senses

The eye

External anatomy – use a partner to see these structures

Eyebrows - on supraorbital ridge

Eyelids – muscles for blinking

Eyelashes – many touch receptors

Conjunctiva – transparent mucous membrane that lines the eyelids and covers the white of the eye, small glands secrete lubrication so lids slide easily on the eye surface

Lacrimal glands – superior to the eye, slightly lateral to the midline, dilute saline tears with antibiotic enzymes and antibodies secreted onto conjunctiva and upper eyelids,

Lacrimal canals - 2 tiny red spots on the medial margin, collect and drain lacrimal solution into nasolacrimal ducts in lacrimal bone

Extrinsic eye muscles – attach to outer surface of eye within the orbit, 6 muscles,

Internal anatomy – use eye model and text illustrations

Fibrous tunic – 2 layered covering of the outer surface of the eyeball

Sclera – white of the eye

Cornea – transparent covering of the iris and pupil

Anterior chamber – space between the cornea and the iris and pupil

Iris – visible colored area of the eye, iris has 2 layers of smooth muscle and bundles of elastic fibers which open and close the pupil

Pupil – central opening of the eye through which light passes

Lens – clear disc of crystalline protein, light passes through

Suspensory ligaments – on inner surfaced of iris, attach lens to ciliary body

Ciliary muscles – smooth muscle of iris, change shape of lens for focus

Posterior cavity – space behind the iris

Retina – sensory tunic, inside lining of the posterior cavity

Cones – photoreceptor for color vision, 3 iodopsin photo pigments, blue, green, red

Rods – photoreceptor with rhodopsin for black and white or dim vision,

Macula lutea – contains few rods and high concentration of small cones

Fovea centralis – area in the center of the macula lutea packed with cones

Optic disc – blind spot, no photoreceptors, nerve fibers form optic nerve

Optic nerve – sends sensory projections to brain

The Ear

External anatomy

Auricle – pinna, outer portion of ear, elastic cartilage

Helix – shell shaped rim of auricle

Lobule – dangling ear lobe

External auditory canal (meatus) – passage from exterior to inner structures

Ceruminous glands – skin of external canal has modified sweat glands that produce a waxy secretion cerumen )

Tympanic membrane – eardrum, connective tissue membrane separates outer ear from middle ear

Internal Anatomy

Tympanic cavity – middle ear

Ossicles – 3 middle ear bones

Malleus – hammer, eardrum pushes on hammer as sound waves enter canal

Incus – anvil, hammer pushes on incus

Stapes – stirrup – incus moves stapes, stapes pushes on oval window

Eustachian tube – auditory tube, pharyngotympanic tube, allows air to pass between middle ear and oral cavity

Inner Ear (labyrinth)

Cochlea – organ for perception of sound, spiral, perilymph

Oval Window - vestibular window, stapes attached to oval window, sets perilymph in motion which moves the cochlear hair cells (cilia), and depolarization sends an action potential along axons to the cochlear nerve

Round Window - cochlear window, perilymph flow bulges round window outward releasing pressure in cochlea

Vestibulocochlear nerve – auditory nerve, made up of fibers of the cochlear and vestibular nerves

Vestibule – structures for sensing up and down, utricle lined with hair cells and perilymph, has otolithic jelly like membrane containing calcium deposits called otoliths, the heavy membrane in response to perilymph movement presses down on the hair cells generating action potentials through the vestibular nerve, as you

change positions of the head different hair cells are activated and signals are sent to the brain about the new position

Semicircular canals - equilibrium, balance, perception of motion, rotational motion of the head, as the head moves perilymph moves and bends the hair cells (cilia) creating action potentials from the semicircular canals and utricle pass through axons that become the vestibular nerve which merge with cochlear nerve to form the vestibulocochlear nerve (auditory nerve)