Download HEARING

HEARING
BARORECEPTORS – THE AUDITORY SENSE AND EQUILIBRIUM
The receptors for these senses are located in the EAR and are, in a sense, modified forms of touch.
The ear has three parts:
1. EXTERNAL EAR – consists of PINNA (AURICLE) made of a framework of elastic cartilage and covered
with skin and the EXTERNAL AUDITORY MEATUS of the temporal bone. Lining the external
auditory meatus are hairs and CERUMEN (ear wax) – yellowish, bitter-tasting secretion to keep foreign
materials/insects from the passageway. External ear acts as a”funnel” to gather sound waves and channel
them to
2. MIDDLE EAR- an air-filled cavity separated from the external ear by the TYMPANIC MEMBRANE
(eardrum). Bridging the gap across the middle ear chamber are the three smallest bones of the skeleton,
the AUDITORY OSSICLES – the malleus, the incus, and the stapes (the hammer, anvil, stirrup). To
equalize pressures within the middle ear chamber with surrounding atmospheric pressures (to allow
eardrum to vibrate freely) are the EUSTACHIAN TUBES leading to the nasopharynx (ears ‘pop’ ;
middle ear infections (media otitis)).
3. INNER EAR – membrane-bound fluid-filled chamber where actual receptors are located. Consists of two
portions – the COCHLEA – the receptor for hearing and the VESTIBULE - the receptor for
static/dynamic equilibrium.
THE COCHLEA AND THE PHYSIOLOGY OF HEARING
The COCHLEA is a snail-shell shaped, membrane-bound, fluid-filled receptor for sound of the inner ear. When viewed
in cross-section , the cochlea is divided into upper and lower halves by the BASILAR MEMBRANE. Angling
into the upper half is another membrane called the TECTORIAL MEMBRANE. Resting on the basilar
membrane are the actual receptor cells of hearing. This set of structures within the cochlea is the “hearing
apparatus” is termed the ORGAN OF CORTI.
All sounds are produced by vibrations – when an object vibrates it creates alternating waves of high and low pressure that
travel thorough the surrounding medium (air or water). The rapidity of the vibrations is measured in the
NUMBER OF CYCLES PER SECOND (HERTZ Hz). When sound waves approach the ear, they are gathered
and funneled by the pinna into the external auditory meatus which channels the vibrations to the tympanum. The
tympanum is pushed and pulled by the pressure waves and set to vibrating. In the middle ear, the malleus is set to
vibrating by the tympanum to which it is attached, and in turn strikes the incus, which in turn strikes the stapes.
As the stapes is struck, it pushes against the wall of the cochlea, creating a fluid wave within the cochlea. What
began as a pressure wave in the air has been converted into a fluid wave within the cochlea.
As the fluid wave travels through the cochlea, it causes the basilar membrane to vibrate up and down also. As the
basilar membrane vibrates, the receptor cells of the organ of Corti are pushed against the tectorial membrane;
hair-like projections of the receptor cells are bent as they are pushed against the tectorial membrane. This
bending of the hairs generates impulses on branches of the AUDITORY (VIII) NERVE which transmit those
impulses to the brain. Thus hearing can be viewed as a modified form of touch – impulses generated by the
bending of hairs.
THREE BASIC QUALITIES OF SOUND
PITCH – the pitch of a sound is determined by its vibrational frequency (that is, its Hz). The more rapidly vibrating the
sound source, the higher the pitch of the sound we perceive. The basilar membrane of the organ of Corti is made
of fibers of different diameters. When the cochlear fluids are vibrating, those fibers with the diameter most
“tuned” to that particular vibrational frequency vibrate in resonance, and thus generate the greatest number of
impulses to the brain. Humans have a range of pitches we can detect of 16 - 20,000 Hz, although the range
decreases with age particularly in the higher frequencies. Dogs/bats can hear much higher frequencies.
VOLUME – the volume of a sound depends upon the force of the vibrations – the amount of difference between the
“high pressure” and “low pressure” areas of the wave. This translates into the force of the waves in the cochlear
fluids and the force with which the receptor cells of the organ of Corti are pushed against the tectorial membrane.
The greater the force involved, the louder the sound we perceived. Prolonged exposure to loud sounds can
permanently damage the receptor cells, leading to a permanent hearing loss.
TONAL QUALITY – the ‘uniqueness’ of the sound from different sources (e.g. middle C played on a piano, guitar,
clarinet, sung by a soprano). There are no ‘pure sounds’ in nature (consist of a single pitch) – rather, the sounds
we hear consist of a main pitch and accompanying secondary vibrational frequencies (harmonics). Each different
sound source produces its own unique pattern of harmonics and thus creates a unique pattern of vibrational areas
on the basilar membrane of the organ of Corti. We interpret/identify the unique vibrational patterns as sounds
from different sources.
EQUILIBRIUM SENSES
VESTIBULE – that portion of the inner ear that provides our balance and equilibrium senses, both static and dynamic
equilibrium:
static equilibrium – position of the non-moving head relative to gravity (“tilt meter”)
“Which way is up?”
dynamic equilibrium – monitors turning motions of the head
The sensory information reaching the brain from the vestibule is integrated with proprioceptive information from
throughout the body, so the brain can build a picture of not only the parts locations relative to each other, but also
in relation to the outside world.
There are two parts of the vestibule that monitor these aspects of equilibrium:
static equilibrium – within the vestibule are two chambers, the sacculus and the utriculus, which are lined with
sensory hairs and filled with a jelly-like fluid containing crystals of calcium carbonate (otoliths).
Regardless of the position of the head, the otoliths are always at the ‘bottom’ of the chambers and bend
the sensory hairs there; the brain interprets the pattern of sensory input from these hairs as “head position
relative to gravity” – which way is “up”.
dynamic equilibrium – monitoring motions of the head are three roughly circular fluid-filled passageways, the
semicircular canals, one oriented in each of the three principal axes. Within each semicircular canal is a
tuft of sensory hairs which serve as the receptor. When the head is turned, the fluid in the canals lags
behind by inertia, causing the sensory hairs to bend like strands of algae in a stream. This bending of
these hairs generates sensory impulses that travel to the brain and are there interpreted as head turning
movement. When the head stops turning, the fluid continues to flow briefly, which causes the hairs to
bend in the opposite direction and send impulses to the brain. Thus the turning and stopping of the head
provides information to the brain about turning motions of the head, which is integrated by the brain with
proprioceptive input from throughout the body into overall awareness/monitoring of bodily movements.
(DIZZINESS, SEA SICKNESS)