LIFESCI 7C Lecture Notes - Lecture 4: Cochlear Duct, Semicircular Canals, Basilar Membrane

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10 Jun 2018
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Week 4:
ANIMATION: Sensing Stimulus strength and location
Strength of a signal received by sensory neuron is indicated by rate (frequency) that it fires action potentials
E.g. stronger touch → higher firing rates in sensory receptor neuron
Accomodation: receptor’s firing rate declines as it becomes “familiar” with the signal
Location of a signal’s source often determined by lateral inhibition;
pressure at one spot not only stimulates local sensory receptor neurons but also inhibits adjacent interneurons
Location of stimulus changes → location w/ strongest receptor signaling also shifts
Lateral inhibition: enhances contrast between region of receptors being stimulated and surrounding regions that are
not → increases accuracy of locating stimulus
36.3: Gravity, Movement, and Sound
hair cell A specialized mechanoreceptor that senses movement and vibration.
Allow one to orient w/ gravity, detect motion, and hear
Senses mechanical vibrations, which move stereocilia
stereocilia Nonmotile cell-surface projections on hair cells whose movement causes a
depolarization of the cell’s membrane (by opening/closing channels).
More similar to microvilli than cilia (since they cannot move on thier own)
Do not fire action potentials; when depolarized, release NT’s that later firing rate of adjacent neurons
Similar in structure, not specific for particular pitch or body orientation but instead interact w/ other
structues within their sensory organ
Hair cells sense gravity and motion
lateral line system In fish and sharks, a sensory organ along both sides of the body that uses hair cells to
detect movement of the surrounding water.
statocyst A type of gravity-sensing organ found in most invertebrates; internal chamber lined w/ hair cells w/
stereocilia that project into the chamber
statolith In plants, a large starch-filled organelle in the root cap that senses gravity; in animals, a dense
particle that moves freely within a statocyst, enabling it to sense gravity. (presses on hair cells @
“bottom” of chamber)
vestibular system A system in the mammalian inner ear made up of two statocyst chambers and three
semicircular canals.
Statocysts provide sense of gravity/body orientation w/ respect to gravity; similar to that of invertebrates
semicircular canal One of three connected fluid-filled tubes in the mammalian inner ear that contains
hair cells that sense angular motions of the head in three perpendicular planes.; provide sense of
BALANCE!
Hair cells detect the physical vibrations of sound
Sound pitch determined by frequency of sound waves; volume determined by amplitude
Sound waves cause stereocilia to bend, exciting hair cells which cause them to depolarize and release
NTs
Insects: different hairs lengths=different stiffness levels=detect different frequencies (Hz)
Also have specialized ears that sense sound by means of tympanic membrane, a thin sheet of tissue
at the surface of the ear that vibrates in response to sound waves, amplifying airborne vibrations; in
mammals, also known as the eardrum.
Hearing most widespread/developed in terrestrial vertebrates, which have external tympanic membranes that
evolved independently from insects (convergent evolution)
Vertebrate ears can also make fine distinctions between close frequencies and can detect softer sounds
because external structures amplify the sounds before they reach the hair cells
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Pinna external structure of mammalian
ears that enhance the reception of sound
waves contacting the ear.
Provides heightened sensitivity
for hearing
Structure of the human ear
outer ear The part of the human ear that
includes the pinna, the ear canal, and
tympanic membrane (eardrum)
Transmits airborne sounds into
the ear
middle ear The part of the mammalian
ear containing three small bones, the
malleus, incus, and stapes, which
amplify the waves that strike the
tympanic membrane.
Stapes connects to oval window
of the inner ear’s cochlea
inner ear The part of the vertebrate ear
that includes the cochlea and semicircular canals.
Mammalian eardrum evolved from tympanic membrane of reptiles+early amphibians
Three middle earbones also evolutionarily related to 3 bones in reptiles
Stapes functions in hearing in reptiles, while malleus and incus helps support jawbone during
feeding
Process of hearing:
1. Amplification
a. Sound vibrations received by outer ear transmitted; eardrum → middle ear, 3 bones (stapes)→
oval window → inner ear
b. Vibrations from eardrum→ middle ear→ oval window are amplified 30+ times bc larger size of
eardrum compared to oval window and lever-like action of 3 bones
2. Transfer of sound vibration to fluid pressure waves
a. Cochlea contains fluid and upper+lower canal separated by basilar membrane and cochlear
duct
i. cochlear duct A fluid-filled cavity in the cochlea, next to the upper canal, that houses
the organ of Corti.
b. Vibrations of oval window cause fluid pressure waves in both canals @ nearly the same time
3. Mechanoreception by hair cells w/in cochlea
organ of Corti structure in cochlear duct, supported by the basilar membrane, with specialized
hair cells with stereocilia, that functions to convert mechanical vibrations to electrical impulses.
Stereocilia of mammalian hair cells form a “v’ and project into rigid, immotile tectorial membrane
tectorial membrane A rigid membrane in the cochlear duct, against which the
stereocilia of hair cells in the organ of Corti bend when stimulated by vibration, setting
off an action potential.
sound/pressure waves in the air→ vibrations in solid structures in the ear → waves of fluid → (electrical) nerve
signal
Fluid motions in cochlear channels → motions in basilar membrane relative to tectorial membrane →
Stereocilia of thei hairs cells are bent back and forth in localized regions of the cochlea→
release of excitatory NT’s → action potentials in postsynaptic neurons → SOUND sensed in
auditory complex
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auditory cortex The area of the brain that processes sound.
Sounds characterized by amplitude (loudness) and frequency (pitch)
Louder sounds → larger fluid vibrations → stereocilia bend more → increasing release of ENT’s →
increased firing rate of postsynaptic sensory neuron → increased intensity of hearing=loudness!
Ability to discriminate different frequencies=result of differences in mechanical properties of BASILAR
MEMBRANE
@ apex, basilar membrane widest but thinnest and most flexible: excited by lower frequencies
@ base, basilar membrane is narrow but thick and stiff: excited by higher frequencies
aging=increasing stiffness @ base=losing ability to hear high pitched sounds
Sound amplitude and frequency, together with vestibular sensing of gravity and head motion, are transmitted by
the vestibulocochlear nerve (another cranial nerve) to the brain
Case 7: How have sensory systems evolved in predators and prey?
echolocation Using sound waves to locate an object; bats find insect prey by emitting short bursts of high-
frequency sound that bounce off surrounding objects and are reflected to the bat’s ears.
Bats can resolve (measure of image sharpness) prey and other objects @ less than 1mm; HELLA
GOOD
Using hearing to see REALLY WELL!
On the other hand, prey like moths have evolved the ability to emit sounds that interfere w/ bat’s sonar signal
Evolution of sophisticated sensory systems in both groups of animals+accompanying brain organization=result
of evolutionary arms race
36.4: Vision
Variety of animal light-sensing organs all rely on same light-senstiive protein, opsin (universal photoreceptor
protein)
opsin= photosensitive protein that converts the energy of light photons into electrical signals in the
receptor cell.
Ancient molecules that evolved early as receptor proteins senstiive to many different stimuli including
light
G protein-coupled receptors, which activate G proteins, leading to cellular response → change
in membrane potential
Suggests evolution for animal eyes about 500 million years ago in cambrian period
Animals see the world through different types of eyes
The way the visual world is perceived depends on the structure in the eye in which the receptors are embedded
eyecup An eye structure found in flatworms that contains photoreceptors that point up and to the left or
right.
Simple light-sensitive photoreceptors
Pigmented epithelium blocks light from behind, light only detected above and in front of animal
Flatworms respond to light by seeking dark regions to hide from potential predators; uses
nervous system to compare light intensity received by photoreceptors of eyecups, then moves
in that direction
compound eye An eye structure found in insects and crustaceans that consists of a number of
ommatidia, individual light-focusing elements. Involves lens
ommatidia (singular, ommatidium) Individual light-focusing elements that make up the
compound eyes of insects and crustaceans; the # of ommatidia determines the resolution
(sharpness) of the image.
Each ommatidium is sensitive to narrow angle of light;
Compound eyes provide mosaic image bc individual light regions are sensed by separate
ommatadia (image likely sharpened in brain); resolution not nearly as good as single-lens eyes
Extremely good at detecting motion and rapid flashes of light; also have good color vision and
can perceive UV light
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Document Summary

Strength of a signal received by sensory neuron is indicated by rate (frequency) that it fires action potentials. E. g. stronger touch higher firing rates in sensory receptor neuron. Accomodation: receptor"s firing rate declines as it becomes familiar with the signal. Location of a signal"s source often determined by lateral inhibition; pressure at one spot not only stimulates local sensory receptor neurons but also inhibits adjacent interneurons. Location of stimulus changes location w/ strongest receptor signaling also shifts. Lateral inhibition: enhances contrast between region of receptors being stimulated and surrounding regions that are not increases accuracy of locating stimulus. Hair cell a specialized mechanoreceptor that senses movement and vibration. Allow one to orient w/ gravity, detect motion, and hear. Stereocilia nonmotile cell-surface projections on hair cells whose movement causes a depolarization of the cell"s membrane (by opening/closing channels). More similar to microvilli than cilia (since they cannot move on thier own)

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