Lecture 4

Auditory and vestibular physiology

Chemoreception – taste and olfaction

Auditory System Physiology

Ear components

3 parts:

1. external ear

2. middle ear

3. inner ear

External ear The external ear funnels sound waves to the external auditory

meatus. In some animals, the ears can be moved like radar antennas to seek out sound. From the meatus, the external auditory canal passes inward to the tympanic membrane(eardrum).

Middle ear

The middle ear is an air-filled cavity in the temporal bone that opens via the auditory (eustachian) tube into the nasopharynx and through the nasopharynx to the exterior.

The tube is usually closed, but during swallowing, chewing, and yawning it opens, keeping the air pressure on the two sides of the eardrum equalized

The three auditory ossicles,the malleus, incus, and stapes, are located in the middle ear.

The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes.

Its foot plate is attached by an annular ligament to the walls of the oval window.

Middle ear

Inner ear

The bony labyrinth is a series of channels in the petrous portion of the temporal bone. Inside these channels, surrounded by a fluid called perilymph, is the membranous labyrinth.

This membranous structure more or less duplicates the shape of the bony channels

The inner ear (labyrinth)-made up of two parts:

Path of sound• external canal

• vibrates eardrum

• vibration moves through ossicles

• Malleus

• Incus

• Stapes

• stapes vibrates oval window of cochlea

• creates pressure wave in the fluid inside

Membranous labyrinth 3 semicircular channels oriented perpendicular to each other and

following the 3D planes of space Utricle and saccule= vestibule Cochlea The semicircular channels, utricle and saccule- balance- they

contain the vestibular receptors Cochlea has a hearing function- auditory receptor (Corti organ)

Inner ear The membranous labyrinth is filled with a fluid called

endolymph K+ RICH

The bony labyrinth is filled with a fluid called perilymph

There is no communication between the spaces filled with endolymph and those filled with perilymph.

Cochlea

The cochlear portion of the labyrinth is a coiled tube which in humans is 35 mm long and makes 2.5 turns.

The basilar membrane and Reissner's membrane divide it into three chambers (scalae)

The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema

At the base of the cochlea, the scalavestibuli ends at the oval window, which is closed by the footplate of the stapes.

The scala tympani ends at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane.

The scala media is continuous with the membranous labyrinth and does not communicate with the other two scalae. It contains endolymph.

Cochlea

The cochlea is made up of three canals wrapped around a bony axis, the modiolus. These canals are: the scala tympani (3), the scala vestibuli (2) and the scalamedia (or cochlear duct) (1).

The scalae tympani and vestibule are filled with perilymph (in blue) and are linked by a small opening at the apex of the cochlea called the helicotrema. The triangular scalamedia, situated between the scalae vestibuli and tympani is filled with endolymph (in green)

Corti organ

Located on the basilar membrane

It contains the hair cells which are the auditory receptors.

The hair cells are arranged in four rows: three rows of outer hair cells and one row of inner hair cells medial to the tunnel of Corti

There are 20,000 outer hair cells and 3500 inner hair cells in each human cochlea

The tips of the hair cells go through the reticular membrane

Then they imbed in the thin tectorial membrane

At the basis of the receptor cell- dendrites of the first order neuron located in the modiolus = spiral ganglionof Corti 90-95% of the afferent fibres leave from the inner hair cells (they travel through the Corti tunnel)

only 5-10% innervate the more numerous outer hair cells

Corti organ

Inner/outer hair cells

Inner hair cells - sound perception

Outer hair cells - sound amplifiers-mechanic response to stimulation (vibration)→ otoacoustic emissions (can be registered from the external meatus with microphones)→ hearing loss screening in babies

Efferent system

The efferent auditory fibers are originated from many different sites in the central nervous system. From the superior olivary complex, they are projected to the cochlea through two different tracts:

the medial olivocochlear tract, which comprises large myelinated neurons that innervate predominantly the outer hair cells

lateral olivocochlear tract, with unmyelinated neurons, that synapses with the inner hair cells.

Function- change sensitivity of the receptor cells

Sound transmission

The ear converts sound waves in the external environment into action potentials in the auditory nerves.

The waves are transformed by the eardrum and auditory ossicles into movements of the footplate of the stapes.

These movements set up waves in the fluid of the inner ear.

The action of the waves on the organ of Corti generates action potentials in the nerve fibers.

Sound transmission Sound wave....→ stapes→ oval window → perilymph in the

scala vestibuli →helicotrema→ perilymph in scala timpani

Wave in the perilymph transmits to endolymph in the scalamedia

Basilar membrane vibrates - resonant structure - deflected in response to waves- deformation is a traveling wave from basis to apex

APICAL CILIA OF HAIR CELLS - DEFLECTION→DEPOLARISATION

http://www.youtube.com/watch?v=1JE8WduJKV4

Basilar membrane Unique structure

Differs according to area - stiff fibres that are attached firmly to the modeolus, but are “free” at the outer end

Fibres become longer as we approach the helicotrema

Fibres at the basis are more rigid, while the ones at the apex are more flexible

This makes the basilar membrane resonate to different sound pitches!!

Sound travels along the basilar membrane BUT this resonates only in a specific RESONANT point for each of them

Basilar membrane

Frequency coding

Sound frequency- Hz/ Cicles per second

Intensity coding (loudness)

Basilar membrane vibrates directly proportional with sound intensity→more rapid rates of excitation

Spatial summation → no of stimulated cells gets higher

Outer cell stimulation → only when vibration is very high

Hair cells

The hair cells in the inner ear have a common structure

the kinocilium, is a true but nonmotile cilium,it is one of the largest processes and has a clubbed end.

The other processes are called stereocillia-about 50-70

The membrane potential of the hair cells is about -70 mV.

Hair cells

When the stereocilia are pushed toward the kinocilium, the membrane potential is decreased to about -50 mV.

When the bundle of processes is pushed in the opposite direction, the cell is hyperpolarized.

Signal transduction Very fine processes called tip links tie the tip

of each stereocilium to the side of its higher neighbor, and at the junction there appear to be mechanically sensitive cation channels in the higher process.

When the shorter stereocilia are pushed toward the higher, the open time of these channels increases. K+ is the most abundant cation in endolymph→ receptor potential→triggers Ca2+ enterance via specific channels → neurotransmitter release and produce depolarization.

Neurotransmitter- probably glutamate, which initiates depolarization of neighboring afferent neurons.

Auditory pathway

Dendrites of the neurons in the Spiral ganglion are located at the basis of the sensorial hair cells

Spiral ganglion in the modiolus. Ganglion is formed by proper bipolar nerve cells. Their myelinated axons run together to form the acoustic nerve, which unites with the vestibular nerve to form the VIIIth cranial nerve. Myelinated dendrites lose their sheaths as they perforate the bone and pass to the organ of Corti, terminating hair cells.

1st order neuron- Corti ganglion

2nd order neuron- dorsal and ventral cochlear nuclei in the pons

3rd order neuron- inferior colliculus in the midbrain

4th order neuron-geniculate medial nuclei in the methathalamus

BALANCE

Vestibular system Vestibular apparatus:

utricle, saccule and semicircular ducts.

Sensory cells of vestibular apparatus lie in the ampular cristae of canals and maculae in the utricle and saccule.

Both types of organs- hair cells- special stereocilia over the apical surface and one kinocilium:

In the utricle and saccule, the receptor organ is called macula- stereocillia and kinocilium are embedded in cuticular plate with otoliths (CaCO3 crystals)

In the semicircular ducts, the receptor organs are called ampular cristae- hairs are covered by cupulae, composed of a gelatinous material similar to otolithic membrane but lacking otoliths.

Macula (utricular and saccular)

Cristae ampullaris (semicircular canals)

Utricle and saccule:

– Linear acceleration detection

– Head position (gravity)

Semicircular canals:

-Angular motion

Function of the maculae

detect head position and linear acceleration

otoliths (small calcium carbonate particles) drag on the stereocilia when the head changes position

when the body is in anatomical position: the patch of hair cells in the UTRICLE is nearly horizontal, with the stereocilia oriented vertically

the sensory epithelium is vertical in the SACCULE, with the stereocilia oriented horizontally

when the body is in anatomical position: the patch of hair cells in the UTRICLE is nearly horizontal, with the stereocilia oriented vertically

the sensory epithelium is vertical in the SACCULE, with the stereocilia oriented horizontally

K+ TRPA channels + tip - links

Depolarization - lean towards the kinocilium- K+ channel open

Hyperpolarization - lean towards the stereocilia- K+ channel close

orientation of the stereocilia within the sensory epithelium is determined by the STRIOLA, a curved dividing ridge that runs through the middle of the MACULA – in the UTRICLE, the kinocilia are oriented TOWARD the striola, and in the SACCULE they are oriented AWAY from it

in any position, some hair cells will be depolarized and others hyperpolarized in BOTH otolith organs

Signal transduction

Semicircular canals- coding of rotation

Three semicircular canals in each ear

Each canal is oriented in a different plane

Each canal is maximally sensitive to rotations perpendicular to the canal plane

Anterior (superior)- anterior and 45 degrees with the AP plane

Posterior- post and 45 degreees with the AP plane

Horizontal

Function of the cristae detect the rate of head rotation

when the head is initially moved, the endolymph and ampulla (and therefore the hair cells) turn with it in a direction opposing rotation

Depolarization in the kinocilium direction

Hyperpolarization in the stereocilium direction

HORIZONTAL CANALS (“no”)

Depolarization occurs in the SAMEdirection as the head movement (LEFT head turn produces depolarization in the LEFT horizontal canal)

ANTERIOR (SUPERIOR) (“YES”) AND POSTERIOR CANALS

anterior canals are located at ~90o to

each other

posterior canals are also located at ~90o

to each other

the directionality of the stereocilia is

different in the anterior and posterior

canals the anterior canals have their kinocilium

anterior to the stereocilia

the posterior canals have their kinocilium

posterior to the stereocilia

the natural pairing of A/P canals is:LEFT ANTERIOR with RIGHT POSTERIOR

RIGHT ANTERIOR with LEFT POSTERIOR

Signal transduction

K+ channels in the cilia

When stereocilia are bent towards the kinocilium→ K+ channel open

Depolarization of the receptor cell = receptor potential

Ca2+ channels opening → mediator release in the synaptic cleft

Action potential on the vestibular pathway

Vestibular pathway

1st order neuron = Scarpa ganglion- axons form the vestibular branch of the VIIIth cranial nerve

2nd order neuron- vestibular nuclei in the medulla oblongata

3rd order neuron- thalamus

Cortical projection= superior temporal gyrus

Chemoreception: taste & olfaction

Every cell is bathed in chemicals - food or poison, they serve as signals of

communication between cells, organs, or individuals. The ability to recognize and respond to environmental chemicals can allow cells tofind nutrients, avoid harm, attract a mate, navigate, or regulate a physiological process.Every cell in the human body is chemosensitive, and chemical signaling between cells is the basis for internal communication through endocrine systems and neurotransmission.

Chemoreception as a sensory system - the interface between the nervous system and the external and internal chemical milieu.Chemicals reach the human body by oral or nasal ingestion, contact with the skin, or inhalation, and once there, they diffuse or are carried to the surface membranes ofreceptor cells through the various aqueous fluids of the body (e.g., mucus, saliva, tears, cerebrospinal fluid [CSF], blood plasma). The nervous system constantly monitors these chemical comings and goings with a diverse array of chemosensory receptor - taste (gustation) and smell (olfaction).- chemoreceptors in the skin, mucous membranes, respiratory tract, and gut that

warn against irritating substances- chemoreceptors in the carotid bodies measure blood levels of O2, CO2, and [H+].

Taste Receptor Cells - located mainly on the dorsal surface of the tongue, concentratedwithin small projections = papillae (few millimeters in diameter). Each papilla has numerous taste buds. One taste bud contains 50 to 150 taste receptor cells, numerous basal and supporting cells that surround the taste cells, plus a set of sensory afferent axons. Most people have 2000 to 5000 taste buds, exceptional cases range from 500 to 20,000.Cells of the taste bud undergo a constant cycle of growth, death, and regeneration. Each taste cell lives about 2 weeks.

Taste cells form synapses with the primarysensory axons near the bottom of the taste bud. Receptor cells also make both electrical andchemical synapses onto some of the basal cells.

Taste receptors• Clustered in taste buds

• Associated with lingual papillae

• OR free in the pharynx, larynx, oesophagus

• Contain basal cells which appear to be stem cells

• Gustatory cells extend taste hairs through a narrow taste pore

Gustatory discrimination

•Primary taste sensations• Sweet, sour, salty, bitter, umami (aminoacid glutamate)

•Taste sensitivity shows significant individual differences, some of which are inherited

•The number of taste buds declines with age

Taste buds

•Receptors for taste are modified epithelial cell present in taste buds located on the tongue, roof of the mouth and pharynx

Taste receptors

Olfaction• The receptors for olfaction, which are bipolar neurons, are in the nasal epithelium

in the superior portion of the nasal cavity

• The only neurons to come into direct contact with the external enviroment

• They are first-order neurons of the olfactory pathway.

• Basal stem cells produce new olfactory receptors that regenerate!

Olfactory Epithelium

• 1 square inch of membrane holding 10-100 million receptors

• Covers superior nasal cavity and cribriform plate

Cells of the Olfactory Mucosa

• Olfactory receptors• bipolar neurons with cilia or olfactory hairs

• Supporting cells • columnar epithelium

• Basal cells = stem cells• replace receptors monthly

• Olfactory glands (Bowmann)• produce mucus

Olfaction: Sense of Smell

• Odorants bind to receptors

• Na+/ Ca2+/ Cl- channels open

• Depolarization occurs

• Nerve impulse is triggered

Olfactory receptors use a G-protein coupled

transduction mechanism

Olfactory transduction involves specific receptors, G protein–coupled signaling, and a cyclic nucleotide–gated ion channel

When an odorant binds to the olfactory receptor protein it stimulates a G-protein that activates

adenylate cyclase; cAMP binds to and opens channels permeable to Na+/Ca2+ and Cl- channels.

The resulting current flow depolarizes the receptor cell (receptor potential) causing it to spike.

Its axon terminal in the olfactory bulb then releases transmitter (glutamate).

The chain of events in the main olfactory receptor cells that leads to action potentials in the olfactory nerve (i.e., cranial nerve I [CN I]):

Step 1: The odorant binds to a specific olfactory receptor protein in the cell membrane of a cilium of an olfactory receptor cell.Step 2: Receptor activation stimulates a heterotrimeric G protein called Golf.Step 3: The α subunit of Golf in turn activates an adenylyl cyclase (specifically, ACIII), which produces cAMP.Step 4: The cAMP binds to a CNG cyclic nucleotide–gated cation channel Step 5: Opening of this channel increases permeability to Na+, K+, and Ca2+.Step 6: The net inward current leads to membrane depolarization and increased [Ca2+]i.Step 7: The increased [Ca2+]i opens Ca2+-activated Cl− channels called anoctamin2 (ANO2). Opening of these channels produces more depolarization because of the relatively high [Cl−]i of olfactory receptor neurons.Step 8: If the receptor potential exceeds the threshold, it triggers action potentials in the soma that travel down the axon and into the brain

OLFACTORY BULB

Olfactory receptor axons terminate on mitral

cell dendrites in a restricted encapsulated

structure called a glomerulus; a glomerulus

contains the dendritic bush of one mitral cell but

many olfactory receptor axons. All the olfactory

receptor axons ending in one glomerulus are

from receptors expressing same olfactory

binding protein. So each mitral cell codes for one kind of

odorant molecule. This is the primary basis

of olfactory coding.

Olfactory Pathway

• Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb• pass through 40 foramina in cribriform plate

• Second-order neurons within the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe• conscious awareness of smell begins

• Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs

80

Adaptation & Odor Thresholds

•Adaptation = decreasing sensitivity

•Olfactory adaptation is rapid• 50% in 1 second• complete in 1 minute

• Low threshold• only a few molecules need to be present• methyl mercaptan added to natural gas as warning

Bibliography

Boron Medical Physiology, 3rd edition

Vestibular and auditory transduction:

Chapter 15 - Sensory Transduction, pages 371-382,

Chapter 16 - Circuits of The Central Nervous System, pages 405-407

Chemoreception:

Chapter 15 - Sensory Transduction, pages 354-359