Transcript Slide 1 - Flat 18

THE AUDITORY (ACOUSTIC) SYSTEM
The auditory system is the highly specializes body
system responsible for the reception and
processing of sound sensation (stimuli). The
component parts of the system include:
1. The peripheral sound receptor organ and
associated structures (The ear).
2. The auditory pathway for conduction of nerve
impulse generated by transduction of
mechanical sound waves by the sound receptor
organ
3. Auditory (Sound) processing centers of the
brainstem and cerebral cortex
THE EAR
• The ear consists of:
The auditory apparatus and
The organs concerned with maintenance of balance
(Equilibrium).
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It is subdivided into three parts viz.
The external ear,
The middle ear and
The inner (Internal) ear (Fig 1).
THE EXTERNAL EAR
The external ear consists of
• The pinna or auricle, which is responsible for
the collection of sound waves and
• The external acoustic meatus, which transmits
the sound waves to the tympanic membrane at
the lateral wall of the middle ear.
• The pinna consists mainly of a thin plate of
yellow elastic fibrocartilagineous framework to
which is firmly attached a thin skin.
• The external features of the pinna are
illustrated in Fig 2
• The external acoustic meatus measures about
2.5cm long and runs a peculiar S-shaped sinuous
course towards the tympanic membrane at the
lateral wall of middle ear.
• The canal is covered by skin which is adherent
to the wall and contains
Hair,
Sebaceous glands and
Numerous ceruminuous glands (Wax).
• These appendages prevent the entry of
particulate body into the canal.
The tympanic membrane (Eardrum or Tympanic
diaphragm)
1. This lies between the meatus and the middle ear
(Tympanic cavity).
2. It is a thin, disc-shaped, fibrous sheet, lined
internally by mucous membrane and externally by
skin
The Tympanic Cavity (Middle Ear)
• This is the slit-like cavity in the pitrous part of the
temporal bone.
• It accommodates the acoustic ossicles, which are
responsible for the conduction of sound waves
through the cavity.
• The cavity is disc-shaped in transverse section and
comprises six walls viz. posterior, anterior, lateral,
medial, inferior and superior.
• The ossicles are connected in the order (Lateral to
medial), Malleus, Incus and Stapes. The malleus is
attached to the tympanic membrane of the lateral
wall, while the stapes is attached to the oval
window of the medial wall.
• The middle ear cavity also contains two muscles,
tensor tympany and stapedeus which control the
vibrations of the ossicles.
• It also communicates anteriorly with the
nasopharynx (Eustachian tube) and posteriorly with
the mastoid air cells
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• Four nerves are also located in the cavity. These are:
– Chorda tympani nerve
– Tympanic nerve
– The tympanic plexus, and
– The lesser petrosal nerve.
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THE INTERNAL EAR
This part is also located in the pitrous part of the
temporal bone and consists essentially of a
complicated bony labyrinth, which is made up of
a central part called the vestibule.
The vestibule communicates posteriorly with
three semicircular canals, and anteriorly with the
coiled, shell-shaped cochlear canal.
Within the entire bony labyrinth is the
membranous
labyrinth
surrounded
by
perilymph.
The membranous labyrinth adopts the shape of
the bony labyrinth, contains endolymph and
consists of three parts viz.
• Two small communicating sacs, the utricle and
the saccule, both located in the vestibule.
• Three semicircular ducts located in the
semicircular canals.
• The cochlear duct, which is located in the
cochlear canal.
• The diagram below illustrates the external
features of the bony and membranous
labyrinths.
• There are specialized sensory receptor areas in
each component of the membranous labyrinth.
These are:
1. The maculae of the utricle and saccule, which
record the direction of gravitational field
relative to the head (Vestibulation/Balancing)
2. The crista ampullaris of the semicircular canals,
which records movements in the endolymph
resulting from rotational movements of the
head (Vestibulation/Balancing).
3. The organ of corti (Spiral organ) of the cochlear
duct, which respond to vibrations induced in
the endolymph by sound waves transmitted by
the stapes at the oval window (Hearing).
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THE AUDITORY PATHWAY
The first order neurons (1st order) of the auditory
pathway are:
The spiral ganglion cells of the organ of Corti. Most
of the central processes of these cells are
myelinated.
These processes form the cochlear nerve, which
traverses the internal acoustic meatus alongside
vestibular and facial nerve fibers.
Cochlear fibers accompanied by vestibular fibers
pass to the lateral aspect of the brainstem at the
junction of the pons and medulla oblongata.
Some of the cochlear fibers terminate in the ventral
cochlear nucleus (2nd order) while others terminate
in the dorsal cochlear nucleus (2nd order).
• Axons of neurons in the ventral and dorsal
cochlear nuclei cross the midline at the caudal
aspect of the pons.
• These crossing fibers constitute the Trapezoid
body.
• After crossing, trapezoid fibers turn upwards.
Some of these ascending fibers terminate in the
Superior Olivary nucleus (3rd order) while some
others continue beyond this nucleus and are
joined by fibers from the Superior olivary
nucleus to constitute the lateral lemniscus.
• Fibers of the lateral lemniscus proceed to the
inferior colliculus where they synapse with
Neurons of the inferior colliculus (4th order
Neurons).
• Collicular neurons give rise to fibers, which
proceed to the medial geniculate body of the
thalamus.
• Neurons of the medial geniculate body (5th
order) give rise to fibers, which constitute the
auditory radiations.
• These fibers project to the inferior temporal
gyrus where they terminate in
• The primary auditory area (Brodmann’s area 41
& 42).
In summary, the auditory pathway comprises:
1st Neuron
Spiral ganglion neurons
2nd Neuron
Ventra & Dorsal cochlear neurons
3rd Neuron
Superior olivary neurons
4th Neuron
Inferior colliculus neurons
5th Neuron
Medial geniculate neurons. These
will then terminate in:
The primary auditory area of the temporal lobe
(Broadmann areas 41 and 42)