Page 96 - 84 human physiolofy part-1

Chapter 5

and is thought to aid in the fast release of neurotransmitter. Efferent projections from the brain to
the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells
and on afferent dendrites under inner hair cells.

Process of Hearing

Detection of sound motion is associated with the right posterior superior temporal gyrus. The
superior temporal gyrus contains several important structures of the brain, including: (1)marking the
location of the primary auditory cortex, the cortical region responsible for the sensation of sound.
Sections 41 and 42 are called the primary auditory area of the cerebrum, and processes the basic
characteristics of sound such as pitch and rhythm. The auditory association area is located within the
temporal lobe of the brain, in an area called the Wernicke's area, or area 22. This area, near the lateral
cerebral sulcus, is an important region for the processing of acoustic energy so that it can be
distinguished as speech, music, or noise. It also interprets words that are heard into an associated
thought pattern of understanding. The gnostic area of the cerebrum, (areas 5, 7, 39 and 40) helps to
integrate all incoming sense patterns so that a common thought can be formed (correlated) using all
arriving sensory information.

Hearing Under Water

Hearing threshold and the ability to localize sound sources are reduced underwater. in which the
speed of sound is faster than in air. Underwater, hearing is by bone conduction and localization of
sound appears to depend on differences in amplitude detected by bone conduction.

Localization of Sound by Humans

Humans are normally able to hear a variety of sound frequencies, from about 20Hz to 20kHz. Our
ability to estimate just where the sound is coming from, sound localization, is dependent on both
hearing ability of each of the two ears, and the exact quality of the sound. Since each ear lies on an
opposite side of the head, a sound will reach the closest ear first, and its amplitide will be loudest in
that ear. Much of the brain's ability to localize sound depends on interaural (between ears) intensity
differences and interaural temporal or phase differences.

Two mechanisms are known to be used.

Bushy neurons can resolve time differences as small as the time it takes sound to pass one ear and
reach the other (10 milliseconds). For high frequencies, frequencies with a wavelength shorter than the
listener's head, more sound reaches the nearer ear. Human echolocation is a technique involving
echolocation used by some blind humans to navigate within their environment.

Process of Equilibrium

Equilibrioception or sense of balance is one of the physiological senses. It allows humans and
animals to walk without falling. Some animals are better in this than humans, for example allowing a
cat (as a quadruped using its inner ear and tail) to walk on a thin fence. All forms of equilibrioception

96 | Human Physiology