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