The Special Senses PART A Eye and Associated

The Special Senses PART A Eye and Associated

The Special Senses PART A Eye and Associated Structures 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, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles Eyebrows

Functions include: Shading the eye Preventing perspiration from reaching the eye Palpebrae (Eyelids)

Protect the eye anteriorly Palpebral fissure separates eyelids Canthi medial and lateral angles (commissures) Palpebrae (Eyelids)

Lacrimal caruncle contains glands that secrete a whitish, oily secretion (Sandmans eye sand) Tarsal plates of connective tissue support the eyelids internally Eyelashes Project from the free margin of each eyelid Initiate reflex blinking

Palpebrae (Eyelids) Lubricating glands associated with the eyelids Meibomian glands (modified sebaceous glands) Ciliary glands lie between the hair follicles (sweat and sebaceous glands)

Palpebrae (Eyelids) Figure 15.1b Conjunctiva Transparent mucous membrane that: Lines the eyelids as the palpebral conjunctiva

Covers the whites of the eyes as the ocular or bulbar conjunctiva Lubricates and protects the eye Lacrimal Apparatus Consists of the lacrimal gland and

associated ducts Lacrimal glands secrete tears Located on the lateral portion of the eye Lacrimal Apparatus Tears Contain mucus, antibodies, and lysozyme

Enter the eye via superolateral excretory ducts Exit the eye medially via the lacrimal punctum Drain into lacrimal canaliculus, lacrimal sac and then into the nasolacrimal duct Lacrimal Apparatus Figure 15.2

Extrinsic Eye Muscles Six extrinsic eye muscles Enable the eye to follow moving objects Maintain the shape of the eyeball Structure of the Eyeball

A slightly irregular hollow sphere with anterior and posterior poles The wall is composed of three tunics fibrous, vascular, and sensory

The internal cavity is filled with fluids called humors The lens separates the internal cavity into anterior and posterior segments Structure of the Eyeball Figure 15.4a Fibrous Tunic

Forms the outermost coat of the eye and is composed of: Opaque sclera (posteriorly) Clear cornea (anteriorly) The sclera protects the eye and

anchors extrinsic muscles The cornea lets light enter the eye Vascular Tunic or Uvea Has three regions: choroid, ciliary body, and iris Choroid region

A dark brown membrane that forms the posterior portion of the uvea Supplies blood to all eye tunics Vascular Tunic or Uvea Ciliary body A thickened ring of tissue surrounding the lens

Composed of the ciliary muscles (smooth muscle) anchor the suspensory ligament that holds the lens in place Ciliary processes Secrets the aqueous humor Vascular Tunic: Iris

Pupil central opening of the iris Regulates the 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 the subject matter is appealing or requires problem-solving skills

Pupil Dilation and Constriction Figure 15.5 Sensory Tunic: Retina

A delicate two-layered membrane Pigmented layer the outer layer that absorbs light and prevents its scattering Neural layer, which contains: Photoreceptors that transduce light energy Bipolar cells and ganglion cells Horizontal and amacrine cells Sensory Tunic: Retina

Figure 15.6a The Retina: Ganglion Cells and the Optic Disc Ganglion cell axons: Run along the inner surface of the

retina Leave the eye as the optic nerve The optic disc: Is the site where the optic nerve leaves the eye Lacks photoreceptors (the blind spot) The Retina: Ganglion Cells and the Optic Disc

Figure 15.6b The Retina: Photoreceptors Rods: Respond to dim light Are used for peripheral vision Cones:

Respond to bright light Have high-acuity color vision Macula lutea mostly cones Fovea centralis only cones 24 Blood Supply to the Retina

The neural retina receives its blood supply from two sources The outer third receives its blood from the choroid The inner two-thirds is served by the central artery and vein Small vessels radiate out from the optic disc and can be seen with an ophthalmoscope The Special Senses

PART B Inner Chambers and Fluids The lens separates the internal eye into Anterior segment Posterior segment

Inner Chambers and Fluids The posterior segment is filled with a clear gel called vitreous humor that: Transmits light Supports the posterior surface of the lens Holds the neural retina firmly

against the pigmented layer Contributes to intraocular pressure Inner Chambers and Fluids Anterior Segment

Filled with aqueous humor A plasmalike fluid Drains via the canal of Schlemm Supports, nourishes, and removes wastes Anterior Segment Figure 15.8 Lens

A biconvex, transparent, flexible, avascular structure that: Allows precise focusing of light onto the retina Is composed of epithelium and lens fibers Lens

Lens epithelium anterior cells that differentiate into lens fibers Lens fibers cells filled with the transparent protein crystallin With age, the lens becomes more compact and dense and loses its

elasticity Light Electromagnetic radiation all energy waves from short gamma

rays to long radio waves Our eyes respond to a small portion of this spectrum called the visible spectrum Different cones in the retina respond to different wavelengths of the visible spectrum Light Figure 15.10

Refraction and Lenses When light passes from one transparent medium to another Light passing through a convex lens

(as in the eye) is bent so that the rays converge to a focal point When a convex lens forms an image, the image is upside down and reversed right to left Refraction and Lenses Figure 15.12a, b Focusing Light on the Retina

Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors Light is refracted:

At the cornea Entering the lens Leaving the lens The lens curvature and shape allow for fine focusing of an image Focusing for Distant Vision

Light from a distance needs little adjustment for proper focusing Far point of vision the distance beyond which the lens does not need to change shape to focus (20 ft.) Figure 15.13a Focusing for Distant Vision

Focusing for Close Vision Close vision requires: Accommodation changing the lens shape by ciliary muscles to increase refractory power Constriction the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye

Convergence medial rotation of the eyeballs toward the object being viewed Focusing for Close Vision Figure 15.13b Problems of Refraction

Emmetropic eye normal eye Myopic eye (nearsighted) the focal point is in front of the retina Corrected with a concave lens Hyperopic eye (farsighted) the focal point is behind the retina Corrected with a convex lens

Problems of Refraction Figure 15.14a, b Photoreception Photoreception process by which the eye detects light energy

Rods and cones contain visual pigments (photopigments) Arranged in a stack of disklike infoldings of the plasma membrane Special epithelial cells - release neurotransmitters that stimulates neurons Figure 15.15a, b Rods

Sensitive to dim light and best suited for night vision

Absorb all wavelengths of visible light Perceived input is in gray tones only Sum of visual input from many rods feeds into a single ganglion cell Results in fuzzy and indistinct images Cones

Need bright light for activation (have low sensitivity) Have pigments that furnish a vividly colored view Each cone synapses with a single

ganglion cell Vision is detailed and has high resolution Chemistry of Visual Pigments Rhodopsin

Retinal is a light-absorbing molecule Synthesized from vitamin A Two isomers: 11-cis and 11-trans Opsins proteins 4 types that will absorb different wavelengths of light Excitation of Rods

The visual pigment of rods is rhodopsin (opsin + 11-cis retinal) Light phase Rhodopsin breaks down into all-trans retinal + opsin (bleaching of the pigment) Excitation of Rods

Dark phase All-trans retinal converts to 11-cis form 11-cis retinal is also formed from vitamin A 11-cis retinal + opsin regenerate rhodopsin Photoreception

Photoreception Bleaching and Regeneration of Visual Pigments Signal Transmission in the Retina Figure 15.17a Signal Transmission

Figure 15.17b Excitation of Cones

Visual pigments in cones are similar to rods (retinal + opsins) There are three types of cones: blue, green, and red Intermediate colors are perceived by activation of more than one type of cone Method of excitation is similar to rods

Light Adaptation Going from dark to light: Fast bleaching of rods and cones Glare Rods are turned off Retinal sensitivity is lost Cones are turned on Visual acuity is gained

57 Dark Adaptation Going from light to dark: Cones stop functioning and rods pigments have been bleached out by bright light We see blackness

Rods are turned on Rhodopsin accumulates in the dark and retinal sensitivity is restored 58 Visual Pathways

Axons of retinal ganglion cells form the optic nerve Medial fibers of the optic nerve decussate at the optic chiasm Most fibers of the optic tracts continue to

the thalamus Fibers from the thalamus form the optic radiation Optic radiations travel to the visual cortex Visual Pathways Figure 15.19 Visual Pathways

Some nerve fibers send tracts to the midbrain ending in the superior colliculi A small subset of visual fibers contain melanopsin (circadian pigment) which: Mediates pupillary light reflexes

Sets daily biorhythms Depth Perception Achieved by both eyes viewing the same image from slightly different

angles Three-dimensional vision results from cortical fusion of the slightly different images If only one eye is used, depth perception is lost and the observer must rely on learned clues to determine depth Cortical Processing

Primary visual cortex (striate) Basic dark/bright and contrast information Visual association area (Prestriate) Form, color, and movement Chemical Senses

Chemical senses gustation (taste) and 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

Sense of Smell Olfactory epithelium Superior nasal concha Olfactory receptors Bipolar neurons Olfactory cilia

Supporting cells Basal cells Olfactory glands Olfactory Receptors Physiology of Smell Odorants dissolved in secretion bind to the receptor

Depolarization Action potential Olfactory Pathway

Olfactory receptor Olfactory nerves Synapse with mitral cells Cells that process odor signals Olfactory tract The olfactory cortex The hypothalamus, amygdala, and limbic system Taste Buds

Most of the 10,000 or so taste buds are found on the tongue Taste buds are found in papillae of

the tongue mucosa Papillae come in three types: filiform, fungiform, and circumvallate Fungiform and circumvallate papillae contain taste buds Taste Buds Structure of a Taste Bud

Taste bud consists of three major cell types Supporting cells insulate the receptor Basal cells dynamic stem cells Gustatory cells (taste cells) special epithelial cells Gustatory hair Taste pores

Taste Sensations There are five basic taste sensations Sweet sugars, saccharin, alcohol, and some amino acids Salt metal ions Sour hydrogen ions Bitter alkaloids such as quinine and nicotine

Umami elicited by the amino acid glutamate Physiology of Taste In order to be tasted, a tastant: Must be dissolved in saliva Must contact gustatory hairs

Binding of the food chemical: Depolarizes the taste cell membrane, releasing neurotransmitter Initiates a generator potential that elicits an action potential Gustatory Pathway

Facial nerve Anterior 2/3 of the tongue Glossopharyngeal Posterior 1/3 of the tongue Vagus Pharynx

To the solitary nucleus of the medulla Gustatory Pathway These impulses then travel to the thalamus, and from there fibers branch to the:

Gustatory cortex (taste) Hypothalamus and limbic system (appreciation of taste) Trigeminal nerve provide other information about the food Influence of Other Sensations on Taste

Taste is 80% smell Thermoreceptors, mechanoreceptors, nociceptors also influence tastes Temperature and texture enhance or detract from taste The Special Senses PART C

The Ear: Hearing and Balance The three parts of the ear are the

inner, outer, and middle ear The outer and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium Receptors for hearing and balance: Respond to separate stimuli Are activated independently The Ear: Hearing and Balance

Outer Ear The auricle (pinna) is composed of: The helix (rim) The lobule (earlobe) External auditory canal Short, curved tube filled with ceruminous glands

Outer Ear Tympanic membrane (eardrum) Thin connective tissue membrane that vibrates in response to sound Transfers sound energy to the middle ear ossicles Boundary between outer and middle ears

Middle Ear (Tympanic Cavity) A small, air-filled, mucosa-lined cavity Flanked laterally by the eardrum Flanked medially by the oval and round windows

Middle ear communicates with mastoid cells Middle Ear (Tympanic Cavity) Pharyngotympanic tube connects the middle ear to the nasopharynx Equalizes pressure in the middle ear cavity with the external air pressure

Middle and Internal Ear Ear Ossicles The tympanic cavity contains three small bones: the malleus, incus, and stapes Transmit vibratory motion of the eardrum to the oval window

Dampened by the tensor tympani and stapedius muscles Ear Ossicles Inner Ear Bony labyrinth

Tortuous channels worming their way through the temporal bone Contains the vestibule, the cochlea, and the semicircular canals Filled with perilymph Membranous labyrinth Series of membranous sacs within the bony labyrinth Filled with endolymph Inner Ear

Mechanisms of Equilibrium and Orientation Vestibular apparatus equilibrium receptors in the semicircular canals and vestibule. Also special type of epithelial cells Maintains our orientation and balance in space

Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium The Vestibule

The central egg-shaped cavity of the bony labyrinth Suspended in its perilymph are two sacs: the saccule and utricle The saccule extends into the cochlea The Vestibule

The utricle extends into the semicircular canals These sacs: House equilibrium receptors called maculae Respond to static equilibrium The Vestibule

Anatomy of Maculae Contain supporting cells and hair cells Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane Anatomy of Maculae

Otolithic membrane jellylike mass studded with tiny stones called otoliths Utricular hairs respond to horizontal movement

Saccular hairs respond to vertical movement Anatomy of Maculae Effect of Gravity on Utricular Receptor Cells Otolithic movement in the direction of the kinocilia:

Depolarizes vestibular nerve fibers Increases the number of action potentials generated Effect of Gravity on Utricular Receptor Cells Movement in the opposite direction:

Hyperpolarizes vestibular nerve fibers Reduces the rate of impulse propagation From this information, the brain is informed of the changing position of the head Effect of Gravity on Utricular Receptor Cells

The Semicircular Canals Three canals that lie in the three planes of space Membranous semicircular ducts line

each canal and communicate with the utricle The ampulla is the swollen end of each canal and it houses equilibrium receptors in a region called the crista ampullaris These receptors respond to dynamic equilibrium The Semicircular Canals

Crista Ampullaris and Dynamic Equilibrium Each crista has support cells and hair cells that extend into a gel-like mass called the cupula Dendrites of vestibular nerve fibers encircle the base of the hair cells

Crista Ampullaris and Dynamic Equilibrium Activating Crista Ampullaris Receptors

Cristae respond to changes in velocity of rotatory movements of the head Directional bending of hair cells in the cristae causes: Depolarizations, and rapid impulses reach the brain at a faster rate Hyperpolarizations, and fewer impulses reach the brain The result is that the brain is informed of rotational movements of the head

Rotary Head Movement The Cochlea A spiral, conical, bony chamber that: Extends from the anterior vestibule Coils around a bony pillar Contains the cochlear duct, which

ends at the cochlear apex Contains the organ of Corti (hearing receptor) The Cochlea The cochlea is divided into three chambers: Scala vestibuli Scala media

Scala tympani The Cochlea The scala tympani terminates at the round window

The scalas tympani and vestibuli: Are filled with perilymph Are continuous with each other via the helicotrema The scala media is filled with endolymph The Cochlea

The floor of the cochlear duct is composed of: The bony spiral lamina The basilar membrane, which supports the organ of Corti The cochlear branch of nerve VIII runs from the organ of Corti to the brain The Organ of Corti

Is composed of supporting cells and outer and inner hair cells (special type of epithelial cells) Afferent fibers of the cochlear nerve attach to the base of hair cells

The stereocilia (hairs): Protrude into the endolymph Touch the tectorial membrane The Cochlea Properties of Sound Sound is: A pressure disturbance

(alternating areas of high and low pressure) originating from a vibrating object Represented by a sine wave in wavelength, frequency, and amplitude Properties of Sound

Frequency the number of waves that pass a given point in a given time Pitch perception of different frequencies (we hear from 20 20,000 Hz) Properties of Sound

Amplitude intensity of a sound measured in decibels (dB) Loudness subjective interpretation of amplitude Properties of Sound Frequency and Amplitude

Transmission of Sound to the Inner Ear Figure 15.31 Pathways of Sound

Outer ear pinna, auditory canal tympanic membrane vibrates Middle ear malleus, incus, and stapes Amplifies the sound Conducts the vibration to the oval window Movement at the oval window applies pressure to the perilymph of the

vestibular duct 118 Pathway of Sound Pressure waves vibrate basilar membrane on the cochlear duct Hair cells of the Organ of Corti are

pushed against the tectorial membrane Opens mechanically gated ion channels Causes a graded potential and the release of a neurotransmitter (probably glutamate) Pathway of Sound The neurotransmitter causes

cochlear nerve to transmit impulses to the brain, where sound is perceived Excitation of Hair Cells in the Organ of Corti Resonance of the Basilar Membrane

Sound waves of low frequency (inaudible): Travel around the helicotrema Do not excite hair cells Audible sound waves: Penetrate through the cochlear duct Excite specific hair cells according to frequency of the sound

Resonance of the Basilar Membrane Auditory Pathway to the Brain Impulses from the cochlea pass to the cochlear nerve

From there, impulses are sent to the inferior colliculus (auditory reflex center) Auditory Pathway to the Brain From there, impulses pass to the auditory cortex

Auditory pathways decussate so that both cortices receive input from both ears Simplified Auditory Pathways Auditory Processing

Pitch is perceived by: The primary auditory cortex Cochlear nuclei Depending on the position of the hair cell stimulated Loudness is perceived by: Varying thresholds of cochlear cells The number of cells stimulated Deafness

Conduction deafness something hampers sound conduction to the fluids of the inner ear (e.g., impacted earwax, perforated eardrum, osteosclerosis of the ossicles) Sensorineural deafness results from damage to the neural structures at any

point from the cochlear hair cells to the auditory cortical cells Deafness Tinnitus ringing or clicking sound in the ears in the absence of auditory stimuli

Menieres syndrome labyrinth disorder that affects the cochlea and the semicircular canals, causing vertigo, nausea, and vomiting Developmental Aspects All special senses are functional at birth

Chemical senses few problems occur until the fourth decade, when these senses begin to decline Developmental Aspects

Vision is not fully functional at birth Babies are hyperopic, see only gray tones, and eye movements are uncoordinated Depth perception and color vision is well developed by age five and emmetropic eyes are developed by year six With age the lens loses clarity, dilator muscles are less efficient, and visual acuity is drastically decreased by age 70

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