Human Anatomy: Hearing and Vision Systems

Chapter 15 Outline

• Chapter 15 focuses on the lecture outline for Anatomy and Physiology, Eleventh Edition
• Separate PowerPoint slides provide figures and tables without notes and animations.
• The copyright for the content is held by McGraw Hill Education.

Special Senses

• Olfaction
• Taste
• Visual system
• Hearing and balance

15.1 Olfaction

• Seven primary odors are now recognized, yet people can distinguish around 4000 different odors.
• Olfactory hairs (cilia) of olfactory neurons are embedded in mucus.
• Odorants dissolve in mucus, binding to receptors. The mechanism is unknown, but cilia depolarize triggering action potentials in olfactory neurons.
• A single receptor can respond to multiple odors.
• Olfactory epithelium regenerates; olfactory neurons are replaced by basal cells every two months.

15.2 Taste

• Papillae are bumps on the tongue.
• Taste buds house supporting cells surrounding taste (gustatory) cells.
• Taste cells have microvilli (gustatory hairs) extending into taste pores.
• Taste cells are replaced about every 10 days.
• Taste is closely associated with smell.

Taste Types

• Sour: Most sensitive receptors are on the lateral aspects of the tongue.
• Salty: Most sensitivity with sweet, triggered by Na+ and other metal ions.
• Bitter: Highest sensitivity receptors on posterior tongue, triggered by alkaloids (often toxins).
• Sweet: Sensitivity triggered by sugars, some carbohydrates, and some proteins
• Umami (Glutamate): Scattered sensitivity caused by amino acids.

15.3 Visual System

• Eyebrows: Shade and inhibit sweat.
• Eyelids (palpebrae): have conjunctiva, palpebral fissure, and canthi (medial/lateral).
• Eyelashes: double/triple row with ciliary glands (modified sweat glands) and Meibomian glands (produce sebum).
• Conjunctiva: thin transparent mucous membrane, including palpebral and bulbar conjunctiva.
• Layers of eyelid tissues include a tarsal plate to maintain shape.

Anatomy of the Eye

• Three Tunics: fibrous, vascular, and nervous.
• Fibrous tunic: sclera and cornea
• Vascular tunic: choroid, ciliary body, and iris
• Nervous tunic: retina
• Optic nerve carries information, connecting the eye to the brain.

Lacrimal Apparatus

• Tears are produced in the lacrimal gland.
• Tears exit through several lacrimal ducts
• Tears pass over the surface of the eye.
• Tears enter lacrimal canaliculi.
• Tears are carried through the lacrimal sac to the nasolacrimal duct.
• Tears enter the nasal cavity.

Extrinsic Eye Muscles

• Various extrinsic muscles control eye movement. These include the Superior/Inferior Rectus, Superior/Inferior Oblique, and the Lateral/Medial Rectus muscles.

Chambers of the Eye

• Anterior compartment: anterior to the lens, occupied by the aqueous humor.
  • Anterior chamber: between the cornea and iris.
  • Posterior chamber: between the iris and the lens.
• Aqueous humor: helps maintain intraocular pressure; delivers nutrients. Refracts light. Ciliary process produces aqueous humor.
• Vitreous humor: posterior to lens. Jelly-like vitreous humor helps maintain intraocular pressure and holds the lens and retina in place; it refracts light.
• Glaucomatous: increased intraocular pressure.

15.4 Hearing and Balance

• External ear: Auricle, external acoustic meatus, tympanic membrane.
• Middle ear: air-filled space, auditory ossicles (malleus, incus, stapes), eustachian tube.
• Inner ear: vestibular complex (vestibule, semicircular canals; responsible for equilibrium), and cochlea (responsible for hearing).

Inner Ear

• Labyrinths (bony and membranous)
• Lymph filled: endolymph (within membranous labyrinth) and perilymph (space between bony and membranous labyrinth)
• Oval/round windows
• Cochlear chambers (scala vestibuli, scala media, scala tympani)
• Spiral organ/Organ of Corti: contains hair cells and associated nerves.

Auditory Function

• Vibrations produce sound waves
• Volume/loudness relates to wave amplitude.
• Pitch relates to wave frequency.
• Timbre relates to resonance quality or overtones.

Muscles of the Middle Ear

• Tensor tympani: inserts on the malleus and innervated by cranial nerve V.
• Stapedius: inserts on the stapes and innervated by cranial nerve VII. (Attenuation reflex muscles that contract during loud noises preventing damage during vibrations.)

Effect of Sound Waves on Cochlear Structures/Points Along the Basilar Membrane

• Sound waves cause the tympanic membrane, malleus, incus, and stapes to vibrate.
• Vibrations of the stapes and oval window cause vibrations in the perilymph of Scala Vestibuli.
• Wave vibrations in perilymph propagate through scala tympani to the round window and dampened.

Sensitivity of Hearing

• The basilar membrane is important in pitch discrimination because it responds to different frequencies at different points of the cochlea.
• Inner hair cells receive efferent inputs, causing them to change in length to enhance or diminish certain auditory inputs.
• Outer hair cell responses to frequency range differences is important for tuning.

15.5 Effects of Aging on the Special Senses

• Slight loss in ability to detect odors
• Decreased sense of taste
• Lens's of eyes lose flexibility/accommodation.
• Development of cataracts, macular degeneration, glaucoma, and diabetic retinopathy
• Decline in visual acuity and color perception.

Hearing and Balance Conditions

• Conductive hearing loss (pathology in external/middle ear)
• Sensorineural hearing loss (pathology of inner ear/cochlear nerve)
• Otosclerosis (bones of middle ear fuse)
• Tinnitus (phantom sound)
• Otitis media (middle ear infection)
• Inner ear infections
• Motion sickness (conflict of inner ear/visual/balance cues)
• Meniere disease (conflicts in inner ear/balance, fluid abnormality)

Balance

• Static labyrinth: utricle and saccule
  • Evaluates position of head relative to gravity
  • Detects linear acceleration/deceleration (as in a car)
• Kinetic labyrinth: semicircular canals
  • Evaluates head movement in three-dimensional space

Static Labyrinth

• Utricle has macula oriented parallel to skull base.
• Saccula has macula oriented perpendicular to skull base.
• Macula: Specialized epithelium with stereocilia (microvilli) and kinocilium (embedded in gelatinous).
• Otoliths (weighted structures) influence action potential rate based on gravity.
• Patterns of stimulation provide information on head position and acceleration.

Function of Vestibule in Maintaining Balance

• Endolymph in utricle, gelatinous mass, and otoliths determine displacement in response of head/body movement.
• Vestibular nerve fibers transmit action potentials from the macula depending on stimulation.

Kinetic Labyrinth

• Three semicircular canals filled with Endolymph: transverse, coronal, and sagittal planes
• Movement initiates an action potential (most intense when the rate of head movement changes)
• Movement changes translate into changes in action potential rate.

Neuronal Pathways for Balance

• Axons from vestibular ganglion pass vestibular to the vestibular nucleus.
• Vestibular nuclei influence postural muscles.
• Vestibular nuclei to other motor nuclei (oculomotor, trochlear, abducens) influence extrinsic eye muscles.
• Thalamic neurons to vestibular area of the cortex.