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Physical definition of sound

-sound is pressure changes in the air or other medium
-Receptors in the auditory system transduce vibratory energy (vibration of molecules in a medium)
-Sound is percieved


Perceptual definition of sound

sound is the experience we have when we hear



Size of waveform pressure variations
Determines perceived loudness



-Number of waveform repetitions per second
-Determines perceived pitch, measured in cycles/sec
-Cycles per second. 1 cycle/sec = 1 Hertz (Hz)
-Audible spectrum: 20 to 20,000 Hz.
-Spectrum varies for species (e.g., extends higher for dogs, bats), age
-Frequency of stimulus is associated with pitch perception (in vision, frequency of stimulus is associated with color perception)
-Pure tones don't exist



Combination of component frequencies that make up all natural sounds



-amount of energy in stimulus: loud or faint sounds.
-We can hear a broad range of intensities:
-from very faint sounds (rustling of leaves) to very loud ones: jet engine.



-depends on the medium in which waves travel.
-In air, sound travels at 750 miles/h (1250 Km/h) (sound barrier),
-Velocity is greater in denser media (wood, metal)


Outer Ear

Auditory canal and tympanic membrane (eardrum)


Middle Ear

-3 ossicles: hammer (malleus), anvil (incus), stirrup (stapes). ---function: interface between air in outer ear and liquid in inner ear.


Inner ear

Oval and round windows.
- Cochlea, contains the Organ of Corti (Fig. 7.5)
-Organ of Corti: Tectorial membrane, Hair cells (receptors) Basilar membrane
-Stirrup pushes oval window in, forcing round window out, creating liquid wave inside cochlea.
-Wave in cochlea induces vibrations in basilar membrane
-Hair cells on basilar membrane collide with tectorial membrane.


Function of Auditory system

-Perception of objects and events through the sounds that they make.
-What? (Recognition, identification)
-Where? (location)
-Auditory structures are arrayed according to frequency


Fourier Analysis

Mathematical procedure for breaking down complex waves into their component sine waves.


Decibel (dB)-

-Logarithmic unit used to express the ratio between two values of a physical quantity: dB=20(logP1/P0)
-Lowest threshold of hearing (reference unit)
-Simplifies calculations & representation of very large and small numbers



-Vibrations of cochlear fluid are ultimately dissipated by the round window, an elastic member in the cochlea wall.
-Cochlea is extremely sensitive
-Humans can hear differences in pure tones that differ in frequency by only .2%
-Has a internal membrane running up to its tip called organ of coti
-Major principle of cochlear coding is that different frequencies produce maximal stimulation of hair cells at different points along the basilar membrane. w/ higher membranes producing greater activation at the tip of the basilar membrane.


How sound waves travel

Waves go from outer ear-> auditory canal & cause the tympanic membrane (ear drum) to vibrate--> vibration to 3 ossicles-->triggers vibrations in oval window--> transfers vibration to fluid in cochlea


Measuring Perceived Pitch with Pure Tone Frequencies

Traditionally used in studies of frequency spectrum of auditory system
Frequency of the sound will determine the pitch you perceive
Human spectrum:
20 - 20,000 Hz
But pure tones do not exist in natural world!



Hairs in hair cells are bent, which opens ion channels on hair cells.
Receptor potential in hair cell is created, and transmitter is released.
Receptor potential produces action potentials in ganglion cells (spiral ganglion, in the cochlea) whose axons form the auditory nerve (8th pair).


Recognition of sound (what?)

-Anterior Auditory Pathway involves identifying sounds
-Basilar membrane analyses the component frequencies of sound.
-The basilar membrane is tonotopically organized:
-Different regions of membrane vibrate with different frequencies (as a keyboard).
-Base (near windows) is thick and stiff and vibrates with HIGH frequencies.
-Apex (end of membrane) is thin and flexible and vibrates with LOW frequencies.


Auditory pathway

-Spiral ganglion cells project to the cochlear nucleus, in the medulla (myelencephalon), from here to other nuclei, but eventually to the thalamus (medial geniculate nucleus),
and from here to the auditory cortex temporal lobe, FIG.7.7).
-Neurons in primary auditory cortex respond to pure tones, but neurons in seconary auditory cortex require more complex sounds. For instance, in monkeys, neurons in secondary auditory cortex respond better to monkey calls.
-Primary auditory cortex are organized in functional columns: all the neurons encountered during a vertical micro electrode penetration of primary auditory cortex tend to respond optimally to sounds in the same frequency range.
- This pathway is tonotopically organized, like cochlea, areas of primary and secondary auditory cortex organize on basis of frequency.


Sound Localization (where?):

-Posterior Auditory pathway is involved in locating sounds
-requires two ears.

-Auditory system can analyze differences in the intensity and in the time of sound arrival between the the two ears.

-Comparison of time of arrival occurs in the Medial Superior Olives, in the medulla

-Comparison of intensity of sound between the two ears is done in the Lateral Superior Olives.

-The Medial and Lateral Superior Olives are in the medulla (myelencephalon/hindbrain)


Bilateral Lesion

Following this there complete loss of hearing b/c their is shock delivered to the lesion, but sound comes back in a few weeks.


Unilateral Auditory Cortex Lesion

Disrupts ability to localize sounds in space contralateral but not ipsilateral to the lesion.



Complete deafness is rare. Two kinds
1. Nerve deafness, or inner ear deafness: damage to cochlea or hair cells, or nerve
Bilateral lesions of the primary auditory cortex in laboratory mammals produce no permament deficit in their ability to detect sounds. However, cortical lesions can disrupt the ability to localize brief sounds, and to recognize complex sounds.
2. Conductive deafness, or middle ear deafness. Patients hear their voices.
Titnitus: ringing of ears



Single cell formed by amalgamation of an ovum and a sperm. It divides into 2 daughter cells, which further divide until a mature organism is produced.


Embryos three layers

ectoderm, mesoderm and endoderm


Induction of neural plate

-A small patch of entodermal tissue on the dorsal surface of the developing embryo.
-3 weeks after conception, tissue that is destined to develop into the human nervous system becomes neural plate.
-Cells of the neural plate are referred to as embryonic stem cells (neural and glial stem cells): Stem cells must have-
-They have unlimited capacity for self-renewal
-They have the ability to develop into different types of mature cells.
-The neural plate leads to the formation of the neural tube, from where forebrain, midbrain and hindbrain develop (by 40 days after conception).
-The inside of the neural tube will eventually become the cerebral ventricles and spinal canal.



-Ability to develop into any type of cell in the body if transplanted to appropriate site
-earliest embryo cells are totipotent



Can develop into most mature nervous system cells, but not other kinds
-cells of neuron plate = embryonic stem cells



-cells lining the ventricles divide and form neurons and glial cells.


Neural Proliferation

One lips of neural groove fuse to make neural tube, cells of the tube begin to proliferate
-doesn't occur simultaneously or equally in tube
-Most cell divisions occur in ventricular zone
-Cells in different parts of neural tube proliferate in particular sequence that is responsible for the pattern of swelling & folding that gives brain the characteristic shape.
-Controlled by chem signals from 2 organizers in neural tube
1. floor plate: runs along midline of anterior surface of tube
2. Roof plate: runs along midline of dorsal surface of tube