Anatomy Ch. 15 Smell, Taste, Hearing, and Balance

Olfaction (smell)

Occurs in response to odors stimulating sensory receptors in the olfactory region of the nasal cavity

Olfactory epithelium

specialized epithelium that lines the superior portion of the nasal cavity. Contains cell bodies and dendrites of ~10 million olfactory neurons. Dendrites extend to the epithelial surface

Olfactory vesicles

bulbous enlargements at the ends of dendrites

Olfactory hairs

cilia on olfactory vesicles that are covered in thin mucous film

Basal cells of olfaction

replace olfactory cells every 2 months

Odorants

Airborne molecules that enter the nasal cavity and dissolve in fluid covering the olfactory epithelium. Bind to odorant receptors (chemoreceptors), 1000 different receptor molecules. Regulate many intracellular pathways involving G proteins, adenylate cyclase, and ion channels allowing for detection of ~4000 smells

7 primary classes of odorants

Camphoraceous (mothballs), Musky, Floral, Pepperminty, Ethereal (fresh pears), Pungent, and putrid

Neuronal pathways for olfaction

Complex, involving multiple cerebral areas. Stimuli causes perception of odors and emotional/ autonomic responses. Majority of neurons project to central olfactory cortex areas in the temporal and frontal lobes. Some neurons project to secondary olfactory areas involved in emotional and autonomic responses include: hypothalamus, hippocampus, and limbic system

Piriform cortex

located at the junction between the temporal and frontal lobs, amydala of temporal lobe, and orbitofrontal cortex

Sense of taste

gustation

Taste buds

sensory structures of taste. Small, oval structures located along the edge of papillae on the tongue, palate, lips and throat

Taste (gustatory) cells abundance

about 50 sensory cells per taste bud. Replaced about every 10 days throughout life

Taste hairs

microvilli that extend through the taste pore of the taste bud

Basal cells of gustation

develop into new taste cells

Supporting cells of gustation

support taste cells

4 main types of lingual papillae

filiform, vallate, foliate, and fungiform

Filiform papillae

Filament shaped. Most numerous. No taste buds. Provide rough surface on tongue

Vallate papillae

Largest and least numerous (8-12). Form a V-shaped row along the border and anterior and posterior parts of the tongue. Contain taste buds

Foliate papillae

Leaf shaped. Folds on the sides of the tongue. Contain most sensitive taste buds. Numerous in children and decrease with age

Fungiform papillae

Mushroom shaped. Scattered on the superior surface of the tongue. Contain taste buds

Tastants

Substances that dissolve in saliva and enter taste pores and stimulate taste cells. Have short connections that release neurotransmitters to secondary sensory neurons

5 taste classes

sweet, salty, sour, bitter, umami

Salty mechanism

Na+ diffuses through Na+ channels on the surface of the taste cells causing depolarization. Low sensitivity

Sweet mechanism

sugar binds to G protein-couple receptor molecules on taste hairs of taste cells. Leads to depolarization. Low sensitivity

Sour mechanism

H+ of acids cause depolarization by three mechanisms. 1. Enter the cell directly through H+ channels. 2. H+ bind to ligand-gated K+ channels and block K+ from exiting the cell. 3. H+ can open ligand-gated channels for other positive ions allowing them to enter the cell

Bitter mechanism

Alkaloid tastants stimulate via G protein mechanism. Highly sensitive. Detects toxins

Umani mechanism

Results from amino acids (glutamate). Depolarization via G protein mechanism

Infleunces on taste

Texture of food. Temperature of food. Adaptation of taste can occur within 1-2 seconds after perception complete adaptation within 5 minutes: Occurs at the level of the taste bud and in the CNS. Olfactory sensations

Neuronal pathways for taste

Axons of CNs carry information to the tractus solitarius of the medulla oblongata. Fibers from the nucleus of the tractus solitarius extend to the thalamus and decussate at the level of the midbrain. Neurons from the thalamus project bilaterally to the taste areas in the insula of the cerebrum

Cranial nerves involved in taste

Facial (VII), Glossopharyngeal (IX), Vagus (X)

Facial nerve (CN VII) role in taste

Chorda tympani – branch of the facial nerve that transmits taste sensation from anterior 2/3 of the tongue

Glossopharyngeal nerve (CN IX) role in taste

Carries taste sensation from posterior 1/3 of the tongue, vallate papillae, and superior pharynx

Vagus nerve (CN X) role in taste

Carries taste sensation from the root of the tongue and epiglottis

3 parts of the ear

external, middle, and inner

Section(s) of the ear involved in hearing only

External and Middle ear

Section(s) of the ear involved in hearing and balance

Inner ear

Auricle (pinna)

Fleshy external portion of the ear
Primarily elastic cartilage covered in skin
Helps collects sound waves and directs them towards the external acoustic meatus

External acoustic meatus

Canal lined with hairs and ceruminous glands

Ceruminous glands

Produces cerumen (ear wax)
Helps prevent foreign objects from reaching the tympanic membrane
Overproduction of cerumen may block external acoustic meatus

Tympanic membrane

Thin, semitransparent membrane
Separates external from middle ear
Sound waves cause vibration

3 layers of tympanic membrane

Simple cuboidal epithelium (inner surface)
Connective tissue (middle layer)
Thin stratified squamous epithelium (outer surface)

Middle ear

air-filled cavity consisting of four openings

Four openings of the middle ear

Round window, oval window, one to mastoid air cells in the mastoid process of the temporal bone, and one to the auditory tube that opens into the pharynx to equalize air pressure

Round and oval windows

Two covered openings that separate the middle ear from the inner ear

3 auditory ossicles

malleus (hammer), incus (anvil), and stapes (stirrup)

Auditory ossicles function

Transmit vibrations from the tympanic membrane to the oval window

How the auditory ossicles are connected

Handle of malleus attaches to tympanic membrane
Head of malleus attaches via small synovial joint to incus
Incus attaches via synovial joint to stapes
Foot of stapes fits into the oval window and is held in place by the annular ligament

Sound attenuation reflex

Protects delicate ear structures from damage by loud noises
Most effective in response to low-frequency sounds
Reduces energy reaching the oval window by a factor of 100
Does not protect from sudden noises, protects from longer than about 10 minute of noise
Involves Tensor tympani m. and Stapedius m.

Tensor tympani m.

attached to the malleus, innervated by CN V
Only stimulated by extremely loud noises

Stapedius m.

attached to stapes, innervated by CN VII
Primarily involved in sound attenuation reflex

Bony labyrinth

tunnels and chambers in the temporal bone

Membranous labyrinth

smaller set of membranous tunnels and chambers similar in shape to the bony labyrinth

Perilymphatic cells

cover then inner surface of the endosteum and outer layer of the membranous labyrinth

Endolymph

fills the membranous labyrinth
High concentration of K+
low concentration of Na+

Perilymph

fills space between the membranous labyrinth and bony labyrinth
similar to CSF
Low concentration of K+
High concentration of Na+

3 regions of the bony labyrinth

vestibule, semicircular canals, cochlea

Region(s) of the bony labyrinth involved in balance

Vestibule and semicircular canals

Region(s) of the bony labyrinth involved in hearing

cochlea

Cochlea regions

Three regions created by the arrangement of the membranous labyrinth: Scala vestibuli, Scala tympani, and Cochlear duct (Scala media)

Scala ventibuli

extends from oval window to helicotrema at apex of cochlea, filled with perilymph

Scala tympani

extends from helicotrema to round window parallel to the scala vestibuli, filled with perilymph

Cochlear duct (Scala media)

formed by membranous labyrinth, filled with endolymph

Vestibular membrane

wall of the membranous labyrinth that borders the scala vestibuli

Vestibular membrane composition

Double layer of squamous epithelium
Simplest region of the membranous
Thin with little to no mechanical effect on transmission of sound waves

Basilar membrane

wall of the membranous labyrinth bordering the scala tympani
More complex

Acellular portion of the basilar membrane composition

collagen fibers, ground substance, and sparse elastic fibers

Cellular portion of the basilar membrane

thin layer of vascular connective tissue overlaid with simple squamous epithelium

Basilar membrane dimensions

Width increases from 0.04 mm near the oval window to 0.5 mm near the helicotrema
Collagen fibers run across the membrane and decrease in width as the increase in length
Near the oval window the membrane is short and stiff and responds to high-frequency vibrations
Near the helicotrema is wide and limber and responds to low-frequency vibrations

Spiral organ (organ of Corti)

Located in the cochlear duct
Location of hair cells and supporting epithelial cells
Contains the Stereocilia and Tectorial membrane
Hair cells arranged in four rows (3500-4000 hair cells/row)

Stereocilia

hairlike microvilli on the hair cells

Tectorial membrane

where the tips of stereocilia of the outer hair cells are embedded

Hair cells primarily responsible for hearing

inner

Hair cells involved in regulating the tension of basilar membrane

Outer

Tip link

connects the tips of each stereocilium in a hair bundle to the side of the next longer stereocilium

Gating spring

pair of microtubule strands that attach to the gate of the gated K+ channel
Allows for very brief closure and is much faster than other gating mechanisms

Hair cells neurons

Have no axon
Basilar regions are covered by synaptic terminals of sensory neurons
Cell bodies of the afferent neurons are located in the cochlear modiolus and grouped into cochlear (spiral) ganglion
Axons of sensory neuron form the cochlear nerve
Cochlear nerve joins with the vestibular nerve to form the CN VIII

Auditory function

Created by vibration of matter (air, water, solid)

Two major features of sound

volume and pitch

Volume

loudness
Function of sound wave amplitude (height)

Pitch

Function of sound wave frequency (number of waves per second)

Timbre

resonance quality or overtones of sounds

"Pure" sound waves

represented by a smooth sigmoid curve

Jagged sound curves

formed by numerous superimposed curves of various amplitudes and frequencies

Effect of different frequency sound waves on the Basilar membrane

Causes vibrations in different regions of the membrane

High pitch noises cause vibration on the Basilar membrane near the:

Base

Low pitch noises cause vibration on the Basilar membrane near the:

apex

Lymph surrounding the hair cells

Apical portion is surrounded by endolymph, while the basal portion is surrounded by perilymph

Endocochlear potential

charge difference between the endolymph and perilymph

K+ ion channel opening causes

K+ ions flow into the hair cell

Cause of inward movement of K+ ions

Slight depolarization of the heair cell

Opening of voltage gated Ca2+ channels causes

Ca2+ flows into the cell causing further depolarization

Effect of neurotransmitter (glutamate) release

Increase in action potential frequency at the basal region of the hair cell

Opening of K+ voltage-gated channels in the basal region

K+ leaves the hair cell causing repolarization

Neurotransmitter release from inner hair cells induces

action potentials in the sensory neurons

Two structural and functional parts of balance

Static labyrinth and Dynamic labyrinth

Static labyrinth

utricle and saccule of the vestibule
Primarily involved in evaluating the position of head relative to gravity and responds to linear acceleration or deceleration

2-3 mm patches of specialized epithelium such as

Utricular macula and Saccular macula

Utricilar macula location

oriented parallel to the base of the skull

How the utricilar macula functions

detects horizontal acceleration by detecting the movement of fluid within the structure due to acceleration. If accelerating forward fluid moves towards the back of the head. If accelerating backwards fluid moves towards the front of the head.

How the saccular macula functions

detects vertical movements of the head by the movement of fluid due to gravity. If the head moves down gravity pulls the fluid inferiorly and the nerve fibers detect that.