Unit 3 - Audition, Reflexes & Adaptation

Sound Waves are

Longitudinal waves with compressions and rarefactions

Universal Equation

v = xf and v = x/T

Speed of Sound

344m/s

Smaller Length Materials have

A higher natural frequency because they’re stiffer and oscillates faster

Spiral Cochlea

Cell bodies of cochlea nerve axons

8th Cranial Nerve

Sensory nerve that transmits sound and balance from inner ear to brain

Auditory Pathway

1. Auditory nerve heads to rostral medulla

2. Synapses in cochlear nuclei in the dorsolateral part of the rostral medulla

3. Axons sent to part of the pons with superior olivary complex

4. Inputs enters thalamus to medial geniculate nucleus

5. Travels to primary auditory cortex, layer 4

Superior Olivary Complex

Has lateral and medial superior olive in mid pons from both ears to compare sound

Tonotopic Mapping of Primary Auditory Cortex

Low frequencies (rostral) to High frequencies (c)audal

Humans Hearing Range

20 - 20000 Hz

Lateral Superior Olive

Monitors interaural level intensity different for high frequency sounds when head is greater than wavelength, ~ 2Khz or more to create sound shadow 

Medial Nucleus of the Trapezoid Body

Interneurons connected to other side’s LSO and inhibits LSO from decussating axons from other ear sending signals

Interaural Level Intensity Pathway

1. Noise from each ear excites cochlear nucleus which excites ipsilateral LSO and contralateral MNTB

2. Difference between the signals determines which ears gets more sound (If L ear > R ear, left LSO fires more)

Medial Superior Olive

Monitors interaural time differences for decreased frequency when wavelength is greater than the head (2000Hz)

Jeffress Model

MSO contains many coincidence detectors that each connect to the left and right, when sound enters an ear earlier, it’ll get a head start and hit a coincidence detector farther down than the other ear at the same time

Coincidence Detection

Only 1 neuron get input from both and the brain calculates delay

External Ear

Pinna > concha > external auditory meatus

Middle Ear

Tympanic membrane > malleus > incus > stapes 

Inner Ear

Oval window > cochlear > round window

Eustachian Tube

Connects middle ear and throat to maintain pressure or else it’ll bulge out

Tensor Tympani Muscle

Connects maleus with bone in middle ear

Stapedius Muscle

Connects stapes and bone in middle ear

Auditory Attenuation Reflex

Middle ear muscle hamper ossicles to decrease volume when it’s loud for awhile

Ossicles use mechanical advantage by

Turning large movements to result in smaller but larger force

Mechanical Advantage of Ossicles Pathway

1. Force is funneled from a large eardrum to small oval window, increasing pressure

2. Stapes displaces about 1/10 of eardrum but with increased force

Scala Vestibuli

Upper area of cochlea, near oval window

Scala Tympani

Lower area of cochlea, near round window

Basilar Membrane Lengths

Narrows at base (150um), wide at apex (500um)

Perilymph

Fluid in cochlea

Scala Media

3rd chamber with endolymph fluid and higher K ions

Tectorial Membrane

Simulates hair cells

Organ of Corti

Structure in inner ear making nerve impulses from sound

Stereocilia

Hair protruding from hair cells

Inner Hair Cells

Main receptor for sound, close to inner axis spiral (around 3500)

Outer Hair Cells

More prevalent (15000), but increases amplitude of sound by contracting itself by prestin when depolarized

Audition to Nerve Impulses Pathway

Basilar membrane moves, hair cells move against tectorial membrane that rotates on a hinge

When Hair cell is pushed to longer side

Springs stretch, opening channels to allow K and Ca in to transmit signal to afferent nerve

When Hair cell is pushed to shorter side

Springs compress, closing channel and stopping K and Ca, hyperpolarizing

Prestin

Motor protein in OHC membrane

Conductive Hearing Loss

Vibrations can’t reach inner ear

Causes for Conductive Hearing Loss

• Wax

• Ottis media

• Otosclerosis

Ottis Media

Ear infection of middle ear, common in kids because their tube is shorter and more horizontal making it harder to drain, causing pus to build and push on the eardrum

Otosclerosis

Stapes gets fused with bone onto cochlea

Sensorineural Hearing Loss

Neural processing damaged

Causes of Sensorineural Hearing Loss

• Occupational deafness

• Presbycusis

• Antibiotic Ototoxicity

• Vestibular Schwannoma

Presbycusis

Death of base hair cells from old age

Antibiotic Ototoxicity

Certain antibiotics can damage hair cells

Vestibular Schwannoma

Benign tumor compresses auditory nerves, preventing APs and weird facial sensation

Immunostaining

Using antibiotics to stain neurons

Immunohistochemistry

Using antibodies to do tissue chemistry

Antigens

Proteins recognized by immune system by antibodies where it binds to antibodies areas

Immunofluorescence Direct Method

Place foreign receptor into an organism where it makes antibodies, then fluorescently tagging the made antibodies

Immunofluorescence Indirect Method

Using the 1st antibody sample, to get another organism to create an antibody for the first one, and fluorescently tagging it so different primary antibodies have the same tail and can be used to amplify

Otolith Organs

Utricle and saccule detects head tilt and linear acceleration

Semicircular Canals

Superior, posterior, horizontal detect head rotation

Vestibular nerve joins auditory nerve into the

8th cranial nerve that enters the brainstem

Striola

Midline of otolith organs

Saccular Hair Cells point

Away from striola

Utricle Hair Cells point

To striola

Otolithic Membrane

Gelatinous layer on top of hair cells

Otoconia

CaCO₃ crystal on top help deflection by adding mass and increasing shear force

The baseline for hair cells

-40mV

As calcium concentration in the hair cell increase

Motor protein controlling tension in the gated springs slips down actin filament

Horizontal canal is tilted

30 degrees

Ampulla

Bulge in bony canal

Crista

Supporting, epithelial cell which the hair cell bodies are embedded

Cupula

Gelatinous substance hair cell bundles are embedded

Adaptation in from Angular Acceleration

Hair cell and endolymph initially deflected by inertia of fluid catches up with rotation

Scarpa’s Ganglion

Cluster of cell bodies in internal auditory canal transmitting balance and motion information

Lateral Rectus

Muscles on the outside of the eye helping movement

Medial Rectus

Muscles on the inner eye helping with movement

Vestibulo-ocular Reflex

Eyes stabilized from body movement

Vestibulo-ocular Reflex Pathway

1. Horizontal circular canal sends through scarpa ganglion into the medulla medial vestibular nucleus

2. Synapse in the abducens nucleus

3. Lateral rectus axons exit the pons and insert ipsilaterally

4. Medial rectus axons decussate again at the medial longitudinal fasciculus then synapse at the oculomotor nucleus before going to medial rectus

When looking to the left

Left horizontal canal hair cells depolarizes, the right side axons receive excitatory signals and the left side axons get inhibitory signals to relax

Oculomotor Nerve

Eye movement, reflexes

Abducens Nerve

Abduction/lateral eye movements

Vestibulocochlear Nerve

Sense of balance and hearing

Oscillopsia

Apparently motion of object and blurring of vision from bilateral loss of vestibulo-ocular reflex sometimes from hair cell damage by ototoxic medication

Positional Alcohol Nystagmus

Alcohol enters cupula causing it to become buoyant and rises to the top of the endolymph whenever it makes perceiving spinning and eyes stabilize where it slowly moves to the side then snaps back in place

Benign Paroxysmal Positional Vertigo

Otoconia breaks off from utricle and enters the posterior semicircular canal which can hit the cupula to cause dizziness in certain positions

Epley Maneuver

Series of positions to move otoconia back for BPPV

Pre Motor Cortex

Plans fine coordinated movements and six times larger in humans, but motor cortex is wrt body

Apraxia

Loss of ability to plan and execute complex voluntary motor tasks form lesion in the premotor cortex

Layer 5 of Primary Motor Cortex

Project to brain stem/spinal cord which connect to lower motor neurons

Corticobulbar Tract

Neurons in layer 5 of the proximal region from the cerebral cortex to brain stem

Internal Capsula

Stretch of white matter axons descend

Corticospinal Tract

Layer 5 neurons go to the spinal cord from the caudal medulla, either decussating or continue ipsilaterally

Pyramidal Decussation

Motor nerve fibers crossing to descend in the corticospinal tract in the medulla

Lateral Corticospinal

Motor nerves decussate in medulla synapse in the lateral ventral horn for limb movement

Ventral Corticospinal Tract

Descends ipsilaterally to the spinal cord medial ventral horn branching both sides from proximal muscle movement

Lateral Corticospinanl Tract Topography goes as

Lateral, like the STT (C,T,L,S)

Ventral Horn Neurons near

Midline controls proximal muscles

Lateral Ventral Neurons controls

Distal muscles

Motor Neurons are

Direction tuned, firing more when movement a certain angle

Georgopoulos

Figured out turning curve and M1 population vector

Motor Neuron Pools

Many neurons project to same muscles but pools don’t come from a spinal segment to muscles

Motor Unit

Single alpha motor neuron branch to many muscle fibres

Bigger Motor Units gets more

Branching from units, smaller ones get less synapses to help make smooth contractions

Excitation Contraction Coupling

Physiological process that links AP (electrical) in muscles to the release of Ca ions

Acetylcholinerase

Enzyme in junction breaking down Ach to stop continual muscle contraction