Sensory System

hearing an balance is relayed by

CNVIII, vestibulocochlear nerve

function of the vestibular system

senses and analyzes orientation with respect to gravity, linear acceleration and angular acceleration (associated with a head turn)

auditory system function

senses and analyzes sound

the space between the bone in the bony compartment of the inner ear and membrane contains

perilymph-high [Na+]

within the membrane is what fluid?

endolymph-high [K+]

perilymph has a high concentration of what ion?

Na+

endolymph has a high concentration of what ion?

K+

hair cells are made up of

short to tall sterocilia with 1 long kinocilia

hair cells are located

in the ampulla of each semicircular duct
-macular hair cells are located in the utricle and saccule

hair cells are within what fluid?

endolymph--high [K+]

Hair cells in utricle and saccule are in an area called

the macula

on top of the hair cells' sterocilia is

otolithic membrane which is a gelatinous substance with
calcium carbonate crystals (otolithic stones)

The utricle and saccule detect

static balance and linear acceleration

utricle hair cells are arranged in what plane?

horizontal plane

the saccule hair cells are arranged in what plane and detects what?

saccule hairs are arranged in a vertical plante to detect head tilt and some linear acceleration

hair cells in the semicircular canal are in a swelling at the base of each semicircular canal called

the ampulla

Ampullar hair cells are covered in

gel (detects head movement and rotation in 3 different planes)

when motion deflects the hair cells in the direction towards the kinocilia, what occurs

depolarzation occurs--> signal transduction

deflection towards kinocilia opens mechanically gated ion channels near the cilia tips of the hair cells__> K+ enters and depolarizes cell--> NT release too vestibular neuron

when motion deflects the hair cells in the direction away form the kinocilia, what occurs

hyperpolarization occurs

closes the ion channels so that no K+ can enter, thus hyperpolarizing the hair cell.

What allows membrane potential changes when hair cells move direction either away or towards kinocilia?

the presence of mechanically gated ion channels inear the cilia tips at the hair cells

vestibular nerve synapses on 4 vestibular nuclei which are

-inferior
-medial
-lateral
-superior

Vestibulospinal pathways includes

both lateral and medial vestibulospinal tracts

lateral vestibulospinal reflex mediates

vestibulospinal reflex

vestibulospinal reflex

head and body tilt upon linear acceleration which activates antigravity muscles (axial and extensor muscles) to counteract the tilt or acceleration

-info from the untricle, saccule and vestibulocerebellim synapse upon the inferior and lateral vestibular nuclei and these cell bodies project their axons as the lateral vestibulospinal tracts to activate antigravity muscles in response to head tilt and linear acceleration

Medial vestibulospinal tract mediates

vestibulo-colic reflex

vestibulo-colic reflex mediated by

Medial vestibulospinal tract

vestibulospinal reflex mediated by

lateral vestibulospinal tract

vestibulo-colic reflex

-when the body ROTATES, this activates neck muscles to turn head in opposite direction of the body rotation. This reflex helps maintain a consistent position of the head in space during body movement

info from the semicircular canals synapse upon the medial vestibular nuclei whose cell bodies project their axons via the medial longitudinal fasciculus (mlf) or as the medial vestibulospinal tracts to synapse upon neck muscles

Medial vestibular nuclei also project axons via mlf to cranial nerves what? and for what?

CNIII, IV, VI for head head and neck movements

Thalamic relay: All 4 vestibular nuclei send a few axons to

the VP of the thalamus

the cortical area for balance is

in the parietal cortex near somatosensory of face

Vestibulo-ocular reflex (VOR)

head movement results in slow eye movements in the opposite direction followed by a fast eye movement in the same direction

This reflex helps to stabilize gaze during movement of the head and body to maintain a stable visual field

Pathway for Vestibulo-ocular reflex (VOR)

-if head turns left: fluid in left horizontal semicircular canal moves and causes the hair cells to depolarize
-vestibular nerve on the left sends APs to synapse on the medial and superior vestibular nuclei
-cell bodies of the medial and superior vestibular nuclei project their axons via the mlf (median longitudinal fasciculus) to the right abducens nucleus to innervate the right lateral rectus muscle & the left occulomotor nucleus to innervate the left medial rectus muscle--> This allows eyes to move in opposite direction of the head movement

Vestibular Nystagmus as a physiological response

The direction of the nystagmus is reported as the direction of the fast eye movement that follows the slow eye movement that occurs during the vestibulo-occular reflex
-it is the same direction as the head movement
-the function is to return the eyes to the center of the orbit

Vestibular Nystagmus as a pathological condition

Spontaneous nystagmus or inappropriate nystagmus indicates a lesion in the vestibular system, cerebellum, or can be caused by some anti-epileptic medicatiosn

Couple of tests to determine vestibular disorders

-Rotatory tests
-Caloric tests

Rotatory test tests what?

tests the integrity of the semicircular canals

Caloric tests

-tests 1 side of the semicircular canals
-cool or warm water is placed in the external auditory meatus which will evoke nystagmus
-when cold water (30°C) for 30 seconds is placed in the meatus, the eyes will deviate towards the ear that has received the cold water, which will cause the nystagmus towards the opposite side
-after 5 min, irrigate the ear with warm water--> nystagmus to the same side

COWS
Cold opposite, warm same

Vestibular lesion signs and symptoms

-Spontaneous nystagmus
-decreased antigravity muscle reflexes
-motion sickness
-vertigo (spinning feeling)

how does hearing worK?

sound waves cause the tympanic membrane to vibrate which causes the 3 middle ear bones to oscillate thus causing fluid movement in the cochlea, thus allowing the activation of hair cells and the cochlear nerve

tensor tympani muscle

-attached to the tympanic membrane
-when contracted, causes the tympanic membrane to become stiff
-innervated by CNV3

The cochlea consists of a

bony and membranous part

The 3 parts of the membranous labyrinth

1) Scala vestibuli
2)Scala media aka cochlear duct
3) Scala tympani

Scala vestibuli

-opens to oval window from the stapes
-contains perilymph (high in Na+)

Scala media

-aka cochlear duct
-contains endolymph (high in K+)

Vestibular (Reissner's) membrane

separates scala vestibuli from scala media

Scala tympani

-contains perilymph

basilar membrane separates

scala media from tympani

what separates scala media from tympani

basilar membrane

the basilar membrane runs what length and what sits on top of it?

-runs the length of the cochlea
-the Organ of Corti sits on the basilar membrane

what fluid does the scala tympani contain

perilymph (high in Na+)

what fluid does the scala media contain

endolymph (high in K+)

scala vestibuli contains what fluid

perilymph (high in Na+)

The receptor organ for hearing

The Organ of Corti which consists of about 1 inner hair cell & 4 outer hair cells

the sterocilia of the hair cells project into the endolymph of the scala media and contact the tectorial membrane

Signal Transduction for hearing

-sound waves-->tympanic membrane-->middle ear bones-->pressure waves--> through the oval window--> cochlea-->move perilymph in the scala vestibuli and tympani-->oscillates the basilar membrane-->disturbance in the endolymph in the scala media--> bends sterocilia of hair cells of the Organ of Corti

-if stereocilia bend towards the large kinocilium a mechanically gated ion channel opens allowing K+ to enter and depolarize the cell which can then
release neurotransmitter to elicit an action potential in the auditory nerve-->changes in NT release --> change in the rate of impulse transmission--> brain interprets this as different sound intensities

what hair cells form 95% of the synapses with the primary sensory afferent, so they are important
for sound transduction

the inner hair cells, even though are outnumbered 4:1 by outer hair cells

outer hair cells acts as a cochlear amplifier by being able to increase the movement of the basilar membrane so it acts as a cochlear amplifier

function of outer hair cells

acts as a cochlear amplifier by being able to increase the movement of the basilar membrane so it acts as a cochlear amplifier

How does pitch discrimination work?

-at the base of the cochlea, the basilar membrane is SHORT and LIGHT, so it can receive high frequencies
-at the apex of the cochlea, the basilar membrane is WIDER and RELAXED, so it can receive lower frequencies

Therefore, the membrane is tonotopically organized

at the base of the cochlea, the basilar membrane is
-so it can receive what frequencies

SHORT and LIGHT

HIGH frequencies

at the apex of the cochlea, the basilar membrane is
-so it can receive what frequencies

WIDER and RELAXED

LOW frequencies

the auditory pathway
-afferent neurons
-1st CNS synapse

-afferent neuron: Auditory nerve with cells bodies located in spiral ganglion
-1st CNS synapse: auditory nerve synapses on dorsal and ventral cochlear nuclei (in rostral medulla)

the auditory pathway : dorsal and ventral cochlear nuclei projections

-ventral cochlear nuclei project contralaterally to trapezoid body--> superior olovary complex--> lateral lemniscus--> inferior colliculus--> MGN-->transverse gyri of Heschl
-dorsal cochlear nuclei projects its axons contralaterally via the lateral lemniscus to synapse on the inferior colliculus-->MGN (medial geniculate nuclei) of the thalamus--> transverse gyri of Heschl

Thalamic relay of hearing

Cell bodies from the inferior colliculus project their axons via the brachium of the inferior colliculus to the medial geniculate nucleus (MGN) of the thalamus

Cortical Area for hearing

cell bodies of MGN nucleus of thalamus project axons to the transverse gyri of Heschl

function of ventral cochlear nuclei

timing of sounds

timing of sounds what nuclei

ventral cochlear nuclei

function of dorsal cochlear nuclei

vertical sound location

vertical sound location

dorsal cochlear nuclei

Superior Olivary nuclei's auditory function

first place to compare input from both ears, analyze timing and
intensity of sound, help to locate a sound source

first place to compare input from both ears, analyze timing and intensity of sound, help to locate a sound source

Superior Olivary nuclei

Inferior Olivary nuclei's function

integrates function from all brainstem nuclei

integrates function from all brainstem nuclei

Inferior Olivary nuclei

auditory lesions can be a results of

-aging
-loud noise exposure
-pathology
-toxins

Unilateral lesion of cochlea, auditory nerve, cochlear nuclei

impaired hearing in ipsi ear

Unilateral lesion above the Superior Olivary nucleus

inability to locate and analyze sounds
from the contralateral field

Auditory cortical areas lesion

speech comprehension problems/ language comprehension problems

Signs/Synptoms of auditory lesions

-hearing loss
-tinnitus (ringing of the ear)
-speech comprehension impairment

Conductive dysfunction/lesion

interference of sound waves--outer or middle ear problems

sensorineural deficit

nerve defect--problem somewhere between cochlear and CNS

Conductive lesions can be caused by: (2)

1) ostosclerosis- reduced movement of ossicles
2) otitis media- middle ear infection

Sensorineural lesions can be caused by (3)

1) acoustic neuroma (vestibular Schwannoma)
2) damage to hair cells- via genetics, high intesity sounds, ototoxic drugs
3) Meniere's Disease

Acoustic neuroma (vestibular Schwannoma)

-most common type of CNS hearing loss
-caused by damage to CN VIII cia Schwannoma (Schwann cell tumor)
-Bell palsy is a common secondary symptom, as CN VIII neuroma can compress CNVII in internal auditory canal

Meniere's Disease

-Triad of symptoms:
1) Tinnitus (ringing in ear)
2) Fluctuating hearing loss
3) Episodic Vertigo
-cause: idiophatic, traumatic, post-syphilis, viral
-Pathophysiology: trauma/damage to endolymphatic sac, can't absorb endolymph fluid which results in fluid overload
-Vestibular membrane ruptures from buildup of inner fluid and pressure--> hair cell toxicity occurs from the mixing of endolymph and perilymph fluids--> resulting in hair cell death

Testing for hearing tests

-Rinne's Test
-Weber's test

Rinne's Test

-Place tuning fork on mastoid process
-When patient can no longer hear the sound, place the turning fork 2.5 cm in front of external auditory meatus
-if patient cannot hear the sound, then there is a conductive problem in the external or middle ear

Weber's Test

-place tuning fork in the middle of the forehead
-if hearing is normal, it will sound the same in both ears
-if patient has sensorineural deficit, the ringing is not as loud in the affected side.
-If patient has a conductive deficit, the ringing is louder on the affected side

the cornea shape is fixed, but the lens can change its curvature due to

due to parasympathetic innervation (Edinger-Westphal nucleus of CN III) to the ciliary muscle.

If ciliary muscle contracts, the lens

has more curvature or "rounds up" which allows for near vision

if ciliary muscle relaxes, the lens

has less curvature, which is needed for far vision

the pupil contains what muscles that control the amount of light entering the retina

pupillary dilator muscles and pupilary sphincter muscles

pupillary dilator muscle is under what ANS control

S-ANS; opens pupil to let in more light

pupilary sphincter muscles is under what ANS control

P-ANS; closes pupil to let less light in

3 types of cones

-red
-green
-blue

classes of ganglion cells

-M (magnocellular)- respond to large objects and detects motion
-P (parvocellular)- small, more numerous cells that perceive form and color

M (magnocellular) ganglion cells

responds to large objects and detects motion

P (parvocellular) ganglion cells

small, more numerous than M cells that perceive form and color

Optic pathway: 1st CNS synapse

ganglion cells of the retina--> optic nerve--> some fibers cross in the optic chiasm--> continue on as optic tract-->synapse on LGN or pretectal area

Optic pathway: Synapses and decussations

-axons of the ganglionic cells from the nasal hemiretina (temporal visual fields) cross at the optic chiasm
-ganglion cells can synapse on LGN which project axons through Meyer's loop in the temporal lobe--> synapse in the lingual gyrus of the occipital cortex or LGN can project their axons via the parietal liip to the cuneate gyrus
-ganglion cells can project--> superior colliculus--> synapse on pretectal area--> projects to--> Edinger Westphal nucleus (preganglionic parasympathetic cells body)-->project-->synapse on cilary ganglion (postganglionic parasympathetic) which innervate the constrictor pupillae muscle (for pupillary light reflex)

Meyer's loop and Parietal loop together are called

the optic or visual radiations

cortical area for occipital pathway

Area 17 of the occipital cortex including the lower lingual gyrus and upper cuneus gyrus