Auditory System I: Anatomy, Physiology and Control

Question 1

Pinna/External ear

Answer 1

• Varies in size and shape between dog breeds.
• Highly mobile
• Deep ear canal for better hearing (dogs hearing is 4x better than humans) worse for infection!

Question 2

Blood and nerve supply to the external ear

Answer 2

• Auricular arteries branch from carotid artery follow helical margins
• Venous drainage to maxillary vein
• Sensation from V and VII

Question 3

Scutiform cartilage

Answer 3

The scutiform cartilage (Cartilago scutiformis) is a thin irregular triangular plate, of variable size and shape according to the species. It is located rostromedially to the base of the auricle, to the surface of the temporal muscle. It transmits the action of other muscles, of which it receives the terminal insertions, to the cartilage of the auricle to which it is attached by several muscular beams.

Question 4

Middle Ear

Answer 4

Sound is transmitted from the tympanic membrane to the oval window, via first of all the ossicles (malleus, incus, stapes), then the middle ear wall, then the middle ear cavity:
tympanic membrane → ossicles → middle ear wall → middle ear cavity → oval window
Amplification due to the bony lever is only 1.5 times, as the stapedius muscle prevents the stapes from vibrating too much.
The tympanic membrane is 20 times larger than the oval window.

he middle ear is housed in the temporal bone and is essentially the small air-filled space known as the tympanic cavity ( Fig. 9.24/ 5 ). It is lined with a thin mucous membrane and communicates with the nasopharynx by the auditory tube ( Fig. 9.24/ 7 ). The lateral wall of the cavity incorporates the tympanic membrane ( Fig. 9.24/ 4 ). The medial wall is formed by the petrous part of the temporal bone, which houses the internal e

Question 5

Stapedius

Answer 5

The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes.

Question 6

tensor tympani

Answer 6

The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or thunder.

Question 7

The inner ear/ the cochlear

Answer 7

• The cochlear contains inner and outer hair cells
• Outer hair cells amplify sound
• Inner hair cells convert sound to an electrical signal, uses glutamate as neurotransmitter
• Vestibulocochlear nerve (CN VIII) carries information to brain
• Hair cells at different regions of the cochlear detect sounds with different pitches.
• Damages to hair cells decreases hearing sensitivity
The cochlea is a portion of the inner ear that looks like a snail shell (cochlea is Greek for snail.) The cochlea receives sound in the form of vibrations, which cause the stereocilia to move. The stereocilia then convert these vibrations into nerve impulses which are taken up to the brain to be interpreted.

Question 8

vestibular duct

Answer 8

The vestibular duct or scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the cochlear duct.

It is separated from the cochlear duct by Reissner’s membrane and extends from the vestibule of the ear to the helicotrema where it joins the tympanic duct.

Question 9

cochlear duct

Answer 9

The cochlear duct (or scala media) is an endolymph filled cavity inside the cochlea, located between the tympanic duct and the vestibular duct, separated by the basilar membrane and Reissner’s membrane (the vestibular membrane) respectively. The cochlear duct houses the organ of Corti.

Question 10

tympanic duct

Answer 10

The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of the human. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as vestibular duct.

The purpose of the perilymph-filled tympanic duct and vestibular duct is to transduce the movement of air that causes the tympanic membrane and the ossicles to vibrate, to movement of liquid and the basilar membrane. This movement is conveyed to the organ of Corti inside the cochlear duct, composed of hair cells attached to the basilar membrane and their stereocilia embedded in the tectorial membrane. The movement of the basilar membrane compared to the tectorial membrane causes the stereocilia to bend. They then depolarise and send impulses to the brain via the cochlear nerve. This produces the sensation of sound.

Question 11

Sound transduction

Answer 11

• A sound wave is a vibration through a medium, (air, solids, liquids)
• Sound waves are detected by the pinna and travel through the ear
canal to the tympanic membrane.
• The tympanic membrane is displaced in turn moving the bones of the middle ear.
• The movement reaches the oval window and conveys vibration to the fluid in the cochlear in the inner ear.
• The cochlear contains 30,000 hair cells which detect movement through mechano-transduction

Question 12

Semi-circular canals and balance

Answer 12

The vestibular labyrinth, that is contained within the bony labyrinth of the inner ear is the part of the ear that is involved with the vestibular sense - balance.

The vestibular labyrinth contains the saccule, the utricle and the semicircular ducts - the semicircular ducts being housed within the semicircular canals.

There are sensory hair cells within the vestibular labyrinth, similar to those in the other regions of the inner ear, which detect movement.

However, these sensory hair cells are lodged in the ampullary cupulae or in otoliths (minute calcareous particles), rather than in the tectorial membrane as in the rest of the ear.

The ampulla is a swelling at the base of the semicircular ducts. The sensory hair cells project upwards from the ampulla into the cupula, which is a gelatinous mass.
The ampullary cupulae detect flow around the semicircular canals, which are filled with endolymph, and there is an inertia of fluid for detection of angular acceleration.
Angular acceleration is the detection of motion of the head in any direction.
Otoliths are denser than endolymph - they are calcareous and crystalline.
They are contained within the maculae, and detect gravity and linear acceleration.
Linear acceleration is the detection of motion along a line, for example when you lean to one side.
Movement of the sensory hair cells triggers impulses, which are carried by the vestibular portion of the vestibulocochlear nerve

Question 13

ampulla

Answer 13

An ampulla is a part of the inner ear that surrounds sensory receptors that are responsible for movement related sensory experiences like spatial awareness and pressure change. Ampullae (the plural of ampulla) are located throughout the semicircular canals of the inner ear.

Question 14

Crista

Answer 14

the sensory structure within the ampulla of a semicircular canal within the inner ear (see illustration). The cristae respond to changes in the rate of movement of the head, being activated by pressure from the fluid in the semicircular canals. 2. one of the infoldings of the inner membrane of a mitochondrion.

Question 15

Perilymph

Answer 15

Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid.

Question 16

Endolymph

Answer 16

Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. It is also called Scarpa’s fluid, after Antonio Scarpa.

essential to normal hearing, as well as balance and movement.

Question 17

semicircular canals

Answer 17

The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals

The semicircular duct system of vertebrates is part of the sensory organ of balance, and one of the smallest and most delicate structures of the head. Located inside the bony labyrinth of the inner ear, it monitors rotational head motion through a cascade of biomechanical events

Question 18

function of the semi- circular canals in balance

Answer 18

The membranous semicircular duct can be thought of as a fluid-filled tube with a partition (the cupula) in the middle (Fig. 22.6B). Because the utricles are located medially, compared with the horizontal canals, on each side of the head, the hair cells of the complementary left and right semicircular canals are oppositely polarized. An example is seen in rotational head movements made in the horizontal plane (Fig. 22.9). When the head is stationary (no angular acceleration), the endolymph and the cupula remain still and the afferents from the two horizontal semicircular canals fire at the same (resting) rate (Fig. 22.9A). When the head turns to the right or left, however, the horizontal semicircular ducts turn with it, but the endolymph lags behind owing to inertial forces and the viscous drag between the fluid and the duct wall. The lagging endolymph deflects the cupula, which in turn deflects the stereocilia of the hair cells.

Question 19

Problems associated with the ear

Answer 19

• Ear infection- otitis externa and media • Lack of balance and postural control
• Vertigo

Question 20

Clinical tests for the ear

Answer 20

• Observation of gait, posture and head position provides information about the function of the vestibular nerve.
• Test physiological nystagmus by moving the head side to side and up and down also test the vestibular system.
• A hand clap or whistle to elicit a response tests hearing and the cochlear system.

Question 21

Other functions of the ear

Answer 21

• Body language and communication • Temperature control