Lec 14 Sensory (2)✓mechanorecep (hearing + balance)

ANS B mechanically-gated ion channels => methods of signal transduction that is FASTEST Signal transduction for touch involves mechanical deformations of the plasma membrane

CQ

Organ of Corti?

-is the hearing sensory organ [within the cochlea] -it contains hair cells & sits on the basilar membrane

proprioception is involved in?

1)Change or maintain body position -conscious ex: reaching out to grab an object, touching ur nose w/ ur eyes closed 2)Reflex- & pattern-generating circuits -sub-conscious/automatic ex: posture, walking on diff terrains, changes in balance, carrying objects -[u adjust body position, but u are not consciously thinking about it, ure just doing it naturally, ex: if ure carrying a stack of books & someone place another book ontop, ure not gonna drop it but compensate for that weight & carry it w/o really thinking about]

4 key components within the Organ of Corti? & list its function

1)INNER hair cells -detect & transduce sound waves 2)OUTER hair cells -amplify sound waves 3)TECTORIAL membrane -hair cell stereocilia brush up against the tectorial membrane 4)BASILAR membrane -movement of the basilar membrane in response to sound waves causes the hair cells to move >hair cell stereocilia brush up against the tectorial membrane -specific movement of the stereocilia stimulates (depolarizes) hair cells [which allow hair cells to transduce the signal that could be understood by the brain]

[vertebrate proprioceptors] 3 things that mediate vertebrate proprioception? (& describe their location & what they do)

1)MUSCLE SPINDLES -on the surface of skeletal muscle -monitor length of muscles (∴think of them as: stretch receptors) -fire more than baseline when a muscle is stretched & less than baseline when a muscle is relaxed. 2)GOLGI TENDON ORGAN -in the junction b/w skeletal muscle & tendon -stimulated by changes in tendon tension (force receptors) -fires more than baseline when force is applied to the tendon - ie: when muscle is contracted -∴AP frequency indicates force strength 3)JOINT CAPSULE RECEPTORS -detect pressure, tension, & movement in the joints -function like somatosensory mechanoreceptors

gross anatomy of mammalian ear: -name the 3 components & its function

1)OUTER EAR -gathers sound waves & funnels them into the middle ear 2)MIDDLE EAR -transmits sound from air to fluid-filled inner ear 3)INNER EAR -hearing (cochlea) -balance (vestibule & semicircular canals)

Sound is sensed & encoded in the cochlea (inner ear) The cochlea has multiple compartments filled w/ 2 diff fluids:

1)Perilymph (in blue) -fills vestibular & tympanic ducts -similar to extracellular fluids 2)Endolymph (in green) -fills the cochlear duct -high in K+, low in Na+

what affects how we perceive stimuli ?

1)Receptor density -more receptors in an an area will help ↑ spatial discrimination [ie two-point sensory discrimination: finger have higher receptor density & perceive paper clip as two-points, while forearm has lower receptor density & only perceive it as one point] 2)Receptive field size -smaller receptive fields will provide greater accuracy 3)Receptive field sensitivity -surface receptors are more sensitive than deeper receptors (somatosensation)

two diff classes of mechanoreceptors? (& list some types of each class)

1)TOUCH & PRESSURE RECEPTORS a)baroreceptors: -interoceptors that detect blood pressure changes b)tactile receptors: -exteroceptors that detect touch,pressure, & vibration on the body surface c)proprioceptors: -monitor the position of the body in 3D space 2)HEARING & EQUILIBRIUM (BALANCE) a)hair cells -detect, amplify & transduce sound waves b)vestibular apparatus -monitor equilibrium

ENCODING VOLUME? [how is volume encoded?]

1)The louder the sound is, it increases the AMPLITUDE of movement of basilar membrane 2)This increases bending of stereocilia of inner hair cells towards the kinocilium -which increases depolarization 3)In result, that increases AP frequency in the primary afferent neuron -increasing AP freq in the primary afferent neuron => is what encodes volume. ---------------------------------------------------- note: There is graded neuronal signaling b/c: -movement of the stereocilia back&forth for each cycle of pressure in sound wave=> causes the firing freq of the primary afferent neuron to oscillate from low to high [get cycles of release of neurotransmitters onto primary neurotransmitters

what are hair cells?

=are sensory receptors for hearing & balance

Hair cells AT REST?

At rest: -some channels are open, some are closed -partially depolarized membrane potential -some neurontransmitter release from hair cell -some AP firing afferent neuron ------------------------------ [hair cells @ rest is not activated or deactivated -even when hair cell is @ rest, it does NOT mean its silent, it can still be firing APs -rarely do u have 100% channels closed, or 100% channels opened]

5 Types of tactile (touch) receptors ?

At the surface => has smaller receptive fields: 1)Merkel disks -non-laminated (slowly adapting) -sensitive to superficial touch & pressure (braille) 2)root hair plexus -laminated (rapidly adapting) -sensitive to hair movement 3)free nerve endings -pain, temp, light touch Below the surface - larger receptive fields 4)Ruffini endings -non-laminated (slowly adapting) -sensitive to skin & joint stretch, aids proprioception 5)Pacinian corpuscles -laminated (rapidly adapting) -sensitive to vibrations ------------------------------- [receptor that is SLOWLY ADAPTING = means it does NOT adapt to stimuli / it does NOT get used to the stimuli ∴ u never lose that sensation as long as the stimuli is there] [receptor that is RAPIDLY ADAPTING= means that the receptor adapts to the stimuli quickly ∴ it senses the stimuli & begins to ignore it; only signal at the start of the stimuli & sometimes at the end.]

ANS B The extracellular fluid around a hair cell has a much higher K+ conc than the fluid around neurons, reversing the electrochemical gradient for K+ -The fluid of the cochlear duct ( the endolymph) has a much higher [K+] (potassium conc) than the extracellular fluid that surrounds typical neurons.

CQ In neurons, when a K+channel opens, K+leaves the cell, causing the cell to hyperpolarize. Why would the opposite happen in hair cells? --------------- in neurons: -K channel opens -K LEAVES cell in hair cells: -K channel opens -K ENTERS the cell WHY?

ANS: C increased AP firing in the left primary afferent neurons near the apex of the basilar membrane -increased AP firing => encodes for VOLUME -if near apex of basilar membrane=> that is encoding a low freq sound

CQ low freq: -would be able to travel into the cochlear further. -near the apex (not proximal) left ear

ENCODING FREQUENCY [how is freq encoded?]

Mechanical differences in the basilar membrane encodes for freq. The basilar membrane is: 1)stiff & narrow @ the proximal end (close to the oval window) but 2)flexible & wide @ distal end High freq sound • is not going to go very far into the cochlea • bc it does not have enough power to penetrate further along the basilar membrane. The stiff&narrow basilar membrane at the proximal end of basilar membrane causes the high freq sound to stop & not penetrate much further (so it doesnt get that far into the cochlea). [stiffness&narrow facilitates high freq movement, but high freq cant move in more flexible/floppy membrane] • stays close to the base/proximal end of basilar membrane Low freq sound • is able to come in & penetrate alil further into the cochlea along towards the apex of the basilar membrane Neurons from the basilar membrane form connections with specific areas of the auditory cortex. This is termed "place coding" & this is a form of 'labelled line' coding. [ultimately this is a way the auditory cortex is wired to the cochlea] -[the auditory cortex knows (depending on which neurons are stimulating & which areas of the cortex is being stimulated) whether that sound closer to the APEX (low freq sound) or to the BASE (high freq sound), b/c of that it knows whether the sound is high / low freq.]

surface mechanoreceptors a) have smaller receptive fields (∴ provide greater accuracy) b)surface receptors are more sensitive (receptive field sensitivity is higher) deep tissue mechanoreceptors: a) have larger receptive fields (∴ decrease in accuracy) b)deeper receptors are less sensitive (receptive field sensitivity is lower)

PEW 2 diff features of touch stimuli: 'surface mechanoreceptors' vs. 'deep tissue mechanoreceptors'

Lateral lines?

Some animals (ie Sharks, teleosts & amphibians (X. laevis)) have additional sensory organs called lateral lines. Lateral lines: -used to detect water movement & vibrations. -[it works similarly to the semicircular canals] it also have hair cells within a cupula

ENCODING SOUND LOCALIZATION [how is sound localization encoded?]

The brain [take info from both ears & hair cells] uses info on: 1)time lags; 2)differences in sound intensity to encode sound localization. ex: -sound arrives @ right ear 1st => sound located to the right -sound louder in right ear => sound located to the right In some animals, the pinna can move to help w/ sound localization -[ie rabbits] ------------------ [key: sound localization: >NOT encoded in the cochlea > it is the brain that encodes for sound localization!]

where does sensory encoding of BALANCE & EQUILIBRIUM occur? what is it comprised of?

VESTIBULAR APPARATUS IS COMPRISE OF: A) 3 semicircular canals + ampullae: -anterior -lateral -posterior B)Vestibule [in vestibule, it has:] -macula of utricle -macula of saccule --------------------------------------------------- cochlea vs. a vestibular apparatus: -Similar: all use hair cells -Different: adequate stimuli (what they are sensitive to) [note: this is moving outside of cochlea, but it is still in the inner ear]

Describe hair cell ACTIVATION (hair cell activation is caused....)

[incoming sound waves cause the stereocilia to bend back & forth.] when stereocilia bends TOWARDS THE KINOCILIUM, the tip links are stretched -which opens K+ channels (mechanoreceptors) on the tips of the stereocilia 1)K+ enters the cell => membrane depolarization 2)depolarization opens VG-Ca2+ channels 3)Ca2+ enters the cell => results in release of neurotransmitter (glutamate) from the hair cell 4)Increased neurotransmitter release onto the primary afferent neuron 5)Increased AP firing in the primary afferent neuron (note: its not the hair cell that is creating the AP, it is the primary afferent neuron)

tip links ?

connect the mechanosensitive K channels on 1 stereocilium to those on the adjacent stereocilium

ANS: B bc 1)surface receptors are more sensitive than deep receptors 2)smaller receptive fields means it would provide more accuracy in how the stimuli is perceived

cq

ans: b

cq

SEMICIRCULAR CANALS encode for? & how does it do it.

encodes dynamic equilibrium (= 3 axes of rotational head movement). Rotation is detected in semicircular canals: -there are cristae (sensory organs) located in the ampullae -cristae hair cells are embedded in a cup-shaped gaelatinous mass called a cupula -semicircular canals are filled w/ endolymph -as the head turns, the endolymph shifts due to the rotational acceleration, pushing the cupula over & activating/deactivating the hair cells. (increased inertiaa is compared to endolymph alone) [refer to pic below[ =========================================== [Endolymph will move alot more quckly than the cupula (aka: endolymph alone increases the inertia). Endolymph would rush by & the cupula will slow move & follow it. This is why on spinning ride: -u feel the spinning in the beginning but as long as spinning remains constant u eventually stop feeling it. -this is bc the endolymph is moving & the cupula is moving. Movement of the cupula is what causes that sensation of movement & then it catches up. Once it catches up, the hair cells are no longer bending (hair cells at rest) & therefore its not sensing anything anymore. (EVEN THO WE SITLL MOVING, ITS NOT ENCODING, FIRING, SAYING WE'RE MOVING, ITS ADAPTED AT THAT POINT)]

UTRICLE & SACCULE encode for? & how does it do it.

encodes static equilibrium (=linear head movements) -macula of utricle: horizontal acceleration & head tilt -macula of saccule: vertical acceleration & head tilt [horizontal accel=> ie being in moving car vertical accel => being in elevator] [otoliths in the maculae moves on top of this gelatinous fluid , & the hair cells are embedded in this gelatinous fluid ] As the the head moves, it causes the otoliths in the maculae to slide. Otoliths in maculae increases inertia, & the sliding of the otoliths moves/drags the gelatinous fluid along.. which will bend the hair cells. -(depending on the orientation of the hair ells - stereocilium & kinocilium - it will become activated /deactivated. -[the hair cells can come back to rest] ---------- [Instead of using endolymph to push capula to cause movement of hair cells in the semicircular canals, -here it is the otoliths in maculae causing the shifting & movement of hair cells]

hair cells have?

extracellular structures that extend from the apical end: 1)kinocilium: -is flexible, longer than stereocilia >[can shift/bend] 2)sterocilia -is rigid -is connected to the kinocilium & to each other with tip links >[cant flex/bend but can wobble side-to-side]

mechanoreceptors

mechanoreceptors transform mechanical stimuli (pressure, vibration, movement) into electrical signals.

What mediates the sense of BODY POSITION IN 3D SPACE (PROPRIOCEPTION) in vertebrates?

proprioceptors

What mediates the sense of TOUCH (SOMATOSENSATION) in vertebrates?

tactile receptors

Problem: sound transfers poorly between air (outer ear) & the fluid-filled inner ear What is this solved?

the incoming sound waves are amplified by the the middle ear. -the middle ear contains the tympanic membrane & 3 bones (malleus, incus & stapes) that act as levers to amplify the sound waves [Sound waves enter ear -rattle the tympanic membrane & causes these bones to act as levers (they all work tgt to amplify & transmit the sound waves into the oval window] [the 3 bones => amplifies the sound]

Describe hair cell DEACTIVATION

when stereocilia are deflected AWAY FROM THE KINOCILIUM, the tip links are relax [theyre no longer pulling on K channel, ∴:] -K+ channels close 1)K+ is prevented from entering the cell -this causes polarization/hyperpolarization of the hair cell 2)hyperpolarization of the hair cell results in the VG Ca channel to be closed 3)No longer get Ca into the cell. 4)This results in a reduction in neurontransmitter (glutamate) release onto the primary afferent neuron 5)Resulting in a reduction in AP spiking in primary afferent neuron