SPECIAL SENSES

SPECIAL SENSES Taste, smell, sight, hearing, and

balance Special sensory receptors

Large complex organs (eyes, ears)Localized clusters of receptors (taste

buds)confined to the head region

THE EYE AND VISION

EYES Visual organs 70% of all sensory receptors are in the

eyes 40% of the cerebral cortex is involved in

processing visual information

ACCESSORY STRUCTURES OF THE EYESLacrimal apparatus – keeps the surface of the eye moist

Lacrimal gland – produces lacrimal fluid

Lacrimal sac – fluid empties into nasal cavity

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ACCESSORY STRUCTURES OF THE EYES, CONT. Eyelids- anterior protection Eyelashes Meibomian glands

Modified sebaceous glands at eyelid edges Secrete oily lubricant for the eye

Ciliary glands Between eyelashes Modified sweat glands

Conjuctiva Delicate membrane that lines eyelids and covers

part of eye. Fuses with corneal epithelium Secretes mucus to keep eyes moist

EYE MOVEMENT Controlled by 6

external muscles

INTERNAL STRUCTURE OF THE EYES

Hollow sphere. Fluid filled interior- helps maintain shape Walls composed of 3 tunics

Fibrous tunic- outermost (white of the eye) Thick connective tissue Composed of two regions of connective tissue Sclera – posterior five-sixths of the tunic

White, opaque regionProvides shape and an anchor for eye muscles

Cornea – anterior transparent window Limbus – junction between sclera and cornea Scleral venous sinus – allows aqueous humor to

drain

INTERNAL EYE STRUCTURE CONT.Vascular tunic- middle coat

Composed of choroid, ciliary body, and iris Choroid – vascular, darkly pigmented

membraneBrown color – from melanocytesPrevents scattering of light rays within the eyeChoroid corresponds to the arachnoid and pia

maters Ciliary body- attachment to lens and iris

Iris- smooth muscle fibers that act like the diaphragm of a camera.

Pupil- opens to let light in

DILATION OF THE PUPIL

INTERNAL EYE STRUCTURE CONT.Sensory tunic- innermost layer (retina)

Composed of two layersPigmented layer – single layer of melanocytes Neural layer – sheet of nervous tissue

Contains three main types of neuronsPhotoreceptor cells

Rod cells – more sensitive to light • Allow vision in dim light

Cone cells – operate best in bright light• Enable high-acuity, color vision

Bipolar cellsGanglion cells

REGIONAL SPECIALIZATIONS OF THE RETINA Macula lutea –

contains mostly cones

Fovea centralis – contains only conesRegion of highest

visual acuity Optic disc – blind

spot

MEDIAL VIEW OF THE EYE

INTERNAL CHAMBERS AND FLUIDS The lens and ciliary zonules divide the

eye Posterior cavity

Filled with vitreous humor Clear, jelly-like substance Transmits light Supports the posterior surface of the lens Helps maintain intraocular pressure

INTERNAL CHAMBERS AND FLUIDS Anterior cavity

Divided into anterior and posterior chambers Anterior chamber – between the cornea and iris Posterior chamber – between the iris and lens Filled with aqueous humor

Renewed continuouslyFormed as a blood filtrateSupplies nutrients to the lens and

cornea

THE LENSA thick, transparent, biconvex disc held in place by its ciliary zonule.

HOMEOSTATIC IMBALANCE Color Blindness

Lacking one type of cone Cataracts

Lens becomes hard and opaque due to age Glaucoma

Increased pressure in eyes due to lack of drainage for aqueos humor

LIGHT AND FOCUS Structures in the eye bend light rays Light rays converge on the retina at a

single focal point Light bending structures (refractory

media)The lens, cornea, and humors

Accommodation – curvature of the lens is adjustable Allows for focusing on nearby objects

VISUAL PATHWAYS TO THE BRAINEach side of the brain receives images from both eyes.Each eye sees a slightly different view, but visual fields overlap.This gives us binocular vision.

HOMEOSTATIC IMBALANCE Age-related macular degeneration (AMD)

Involves the buildup of visual pigments in the retina

Wet

HOMEOSTATIC IMBALANCE Retinopathy in diabetes

Vessels have weak walls – causes hemorrhaging and blindness

THE EAR: HEARING AND BALANCE

ANATOMY OF THE EAR receptor organ for hearing and

equilibrium Composed of three main regions

Outer ear – functions in hearingMiddle ear – functions in hearing Inner ear – functions in both hearing and

equilibrium

THE OUTER (EXTERNAL) EAR

The part we think of as the ear.The auricle (pinna)

Helps direct soundsExternal acoustic meatus

Canal lined with skin Contains hairs, sebaceous glands, and

ceruminous glands (secrete yellow wax)Tympanic membrane

Forms the boundary between the external and middle ear

THE MIDDLE EAR The tympanic cavity

A small, air-filled space Located within the petrous portion of the temporal

bone Contains ossicles that transmit vibration from

eardrum to fluids of inner ear. Malleus (hammer) Incus (anvil) Stapes (stirrup)

Medial wall is penetrated by: Oval window Round window

Pharyngotympanic tube (auditory or eustachian tube) Links the middle ear and pharynx

THE INNER (INTERNAL) EAR Inner ear – also called the bony

labyrinth Lies within the petrous portion of the

temporal bone behind the eye socket cavity consisting of three parts

Semicircular canalsVestibule Cochlea

THE INNER EAR, CONT. Membranous labyrinth

Series of membrane-walled sacs and ducts within the bony laryrinth.

Consists of three main parts Semicircular ducts Utricle and saccule Cochlear duct

Filled with a clear fluid – endolymph Confined to the membranous labyrinth

Bony labyrinth is filled with perilymph Continuous with cerebrospinal fluid

THE MEMBRANOUS LABYRINTH

Figure 16.20

THE VESTIBULE The central part of the bony labyrinth

(actually a cavity) Lies medial to the middle ear

Utricle and saccule – suspended in perilymph Two egg-shaped parts of the membranous labyrinth

House the macula – a spot of sensory epithelium that contains receptor cells Monitor the position of the head when the

head is stillContains columnar supporting cellsReceptor cells – called hair cells

Synapse with the vestibular nerve

THE SEMICIRCULAR CANALS Lie posterior and lateral to the vestibule Anterior and posterior semicircular

canalsLie in the vertical plane at right angles

Lateral semicircular canal Lies in the horizontal plane

THE SEMICIRCULAR CANALS

THE SEMICIRCULAR CANALS

Semicircular duct – snakes through each semicircular canal

Membranous ampulla – located within bony ampullaHouses a structure called a crista ampullaris

Responsible for maintaining static equilibrium.

STRUCTURE AND FUNCTION OF THE CRISTA AMPULLARIS

Figure 16.22

THE COCHLEA A spiraling chamber in the bony

labyrinth contains receptors for hearing

EQUILIBRIUM AND AUDITORY PATHWAYS The equilibrium pathway

Transmits information on the position and movement of the head

Most information goes to lower brain centers (reflex centers)

The ascending auditory pathway Transmits information from cochlear

receptors to the cerebral cortex

DISORDERS OF EQUILIBRIUM AND HEARING: MOTION SICKNESS Motion sickness – carsickness,

seasicknessPopular theory for a cause – a mismatch of

sensory inputs

DISORDERS OF EQUILIBRIUM AND HEARING: MENIERE’S SYNDROME Meniere’s syndrome – equilibrium is

greatly disturbedExcessive amounts of endolymph in the

membranous labyrinth

Normal Meniere’s

DISORDERS OF EQUILIBRIUM AND HEARING

Deafness Conduction deafness

Sound vibrations cannot be conducted to the inner earRuptured tympanic membrane, otitis

media, otosclerosis

Normal tympanicmembrane

Otitis mediaRuptured tympanicmembrane

DISORDERS OF EQUILIBRIUM AND HEARING:

Deafness Sensorineural deafness

Results from damage to any part of the auditory pathway

mild severe

THE CHEMICAL SENSES: TASTE AND SMELL

TASTE – GUSTATION Taste receptors

Occur in taste buds Most are found on the surface of the tongue Located within tongue papillae (circumvallate

and fungiform) Collection of 50-100 epithelial cells Contain three major cell types

Supporting cellsGustatory cells-respond to chemicals in saliva

Contain long microvilli – extend through a taste pore

Basal cells

TASTE BUDS

TASTE SENSATION AND THE GUSTATORY PATHWAY Four basic qualities of taste

Sweet (responds to sugars and amino acids)Sour (respond to hydrogen ions or acidity)Salty (respond to metals) Bitter (responds to alkaloids)

No structural difference among taste buds

GUSTATORY PATHWAY Taste information reaches the cerebral

cortexPrimarily through the facial (VII) and

glossopharyngeal (IX) nervesSome taste information through the vagus

nerve (X)Sensory neurons synapse in the medulla

Located in the solitary nucleus

SMELL (OLFACTION) Receptors occupy a postage-stamp sixe

area in the roof of each nasal cavity. Olfactory receptor cells

Neurons with olfactory hairs that transmit to the olfactory nerve.