COMD 4333 Exam 3

Auditory System

Explain the different forms of energy (mechanical, hydraulic, electrical, neurochemical) created in the auditory system from the external auditory meatus/tympanic membrane to the auditory nerve (cranial nerve VIII).

1. Sound waves → Mechanical energy (movement)

• Vibration/pressure from sound waves moves tympanic membrane and ossicles

• Mechanical energy → Hydraulic energy (fluid)

• Movement of tympanic membrane & ossicles causes movement of fluid in cochlea

2. Hydraulic energy → Electrical energy (nerve impulses)

• Fluid movement causes movement of hair cells in cochlea

3. Electrical energy → Neurochemical energy (Action potential)

• Hair cells release glutamate, exciting the auditory nerve (CN VIII)

4. Brain Pathway

• CN VIII → cochlear nucleus → superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate body → auditory cortex (Heschl’s gyrus)

Describe how the Organ of Corti functions. Be sure to mention the basilar membrane, fluid, inner and outer hair cells, and tectorial membrane.

• Explain what happens when the basilar membrane moves, including what happens to the stereocilia, what tip-links are, where the potassium channels are, what causes opening of calcium channels, and what happens when calcium enters the hair cells.

Produces nerve impulses in response to sound vibrations.

• Basilar Membrane: what the Organ of Corti sits on top of

• Fluid: Endolymph (high in K+) causes Basilar Membrane to move up and down and thus displaces Organ of Corti

• Inner Hair Cells: 1 row, sensory only, each synapse with multiple spiral ganglion neurons

• Outer Hair Cells: 3 rows, sensory and motor onto each spiral ganglion neuron

  • Stereocilia: extend from hair cells in proximty to Tectorial Membrane (on top of Organ of Corti); arranged by length; Kinocilium being the tallest type of sterocillia

• Tip Links: connect Sterocillia

When fluid moves → basilar membrane moves → organ of corti moves, stereocilia bend toward kiniocilium → tip links open → influx of K+ (potassium) ions from Endolymph enter hair cell

• => Depolarization

• Depolarization opens Ca (calcium) channels, causes Ca to enter hair cells

• Ca enters hair cells → triggers release of neurotransmitter Glutamate

• Glutamate binds to receptors on Spiral Ganglion Neurons; their axons create auditory/cochlear portion of CN VIII

Cilia bend away from Kinocilium → tip links close → no influx/no further response from hair cell

What are outer hair cell microtubules and how do they impact hearing?

remember, outer hair cells receive efferent signals that can activate Microtubules.

Microtubules are structures that lengthen OHC cell bodies. They enhance our hearing via Electromotility.

• Electromotility: conversion of Electrical energy → Mechanical forces

• Increases effect of Basilar Membrane movement and thus displacement of cilia

DETECTION OF LOW INTENSITY SOUNDS

How do they impact hearing?

• OHC microtubules improve detection of low intensity sounds.

• 100-fold increase in effect of actual basilar membrane movement

They decreases the need for a stronger signal to movie basilar membrane/triggering action potential; OHC are already lengthened enough to shear against tectorial membrane

• increased effect of basilar membrane, allows us to hear sounds better

Describe the auditory pathway starting from the cochlear nerve neurons all the way to Heschl’s gyrus. Identify where there are crossovers and synapses.

C-SLIMA

spiral ganglion axons —> brainstem, synapse in cochlea nuclei

1. Cochlear Nuclei synapse

   1. ipsilateral, monaural processing

   2. 80% cross over to contralateral SOC

   3. ALL processing is Binaural beyond cochlear nuclei

2. CROSS OVER Superior Olivary Complex WHERE SOUND LOCALIZATION OCCURS synapse

   1. receives information from both ears

   2. localizes based on time delay and intensity difference

3. Lateral Leminiscus no synapse

   1. Tract between SOC and Inferior Colliculus/SOC extend through Lateral Leminiscus

4. Inferior Colliculus synapse

   1. Some signals sent from Inferior Colliculus to superior colliculus for auditory-visual integration

   2. synapse in Inferior Colliculus

5. Medial Geniculate Body (in thalamus) synapse

   1. from ipsilateral Inferior Colliculus

6. A1/Heschl’s Gyrus synapse

   1. from Ipsilateral MGN

What thalamic nuclei does the information from the auditory system synapse in?

How are frequency, intensity, and localization coded?

• What neuroanatomical structures are involved in how these attributes of sound are coded?

1. Frequency

   1. Place Coding: hair cell location on basilar membrane (base = high; apex = low)

   2. Characteristic Frequencies: hair cells respond best (fire more action potentials) to specific sound frequencies

   3. Tonotopic Organization: specific frequencies map to specific places

2. Intensity

   1. number of Action Potentials

   2. number of Neurons sending action potentials

3. Localization

   1. Interaural Time and Intensity Differences

      1. The difference in arrival time of a sound between two ears.

      2. The difference in intensity between the two ears.

   2. sound wave travels around the head; reaches second ear later and a bit softer

neuroanatomical structures: cochlea, C-SLIMA

What benefit comes from having binaural stimuli in the superior olivary complex? Hint: interaural time delay

The benefit of having binaural input in the superior olivary complex is that it allows the brain to detect interaural time differences, or the timing differences in when a sound reaches each ear.

• Because sound reaches one ear slightly before the other, neurons in the superior olivary complex compare the timing of input from both ears. This enables the system to precisely localize sound from the left or right side.

How is the primary auditory cortex organized and where is it?

• organization: Tonotopic Organization

• location: dorsal surface of Superior Temporal Gyrus

What is cortical deafness? How is it caused? How does it compare to pure word deafness?

Cortical Deafness

• what is it? rare form of hearing loss

• cause: Bilateral damage to Heschl’s Gyrus, intact peripheral auditory system & pathways to cortex

• Impaired speech comprehension, repetition, recognition of noise/music, impaired hearing insensitivity

Pure Word Deafness

• what is it? rare form of hearing loss

• cause: damage to White Matter between primary and association areas

• Impaired Comprehension of speech sounds (they sound like noise; uninterpretable)

• Preserved reading, writing, speaking, GOOD auditory perception of NOISES

Vestibular System

What is the difference between semicircular canals and otolith organs?

3 Semicircular Canals

1. Anterior semicircular canal

2. Posterior semicircular canal

3. Lateral semicircular canal

• At right angles

• Function: rotation and angular acceleration

SEMICIRCULAR = ROTATION, ANGULAR

2 Otolith Organs

1. Utricle (horizontal)

2. Saccule (vertical)

• Function: linear acceleration

UTRICLE, SACCULE, LINEAR

-le = linear

Explain how the semicircular canals are related to maintaining balance (e.g., with head movement) and what causes action potentials in vestibular nerve axons (include the ampulla, cupula, cristae, and hair cells)

• help maintain balance by detecting rotational head movements

base of each canal is an enlarged region called the ampulla, which contains the crista ampullaris (cristae), a ridge of sensory hair cells. The stereocilia of these hair cells project into a gelatinous structure called the cupula.

• ampulla: at base of each canal, contains crista (sensory organ, contains hair cells); these hair cells, which have sterocilia on top of them, encased in cupula (gelatinous structure)

• movement of head causes → movement of endolymph in semicircular canals causes → movement of cilia toward kinocilium opens → K+ channels causes depolarization opens → calcium channels triggers → glutamate release

• GLUTAMATE BINDS TO RECEPTORS ON SCARPA GANGLION NEURONS (vestibular portion of CN VIII) → creates EPSPs

  • these EPSPs send information about head movement to the brain, helping maintain balance and spatial orientation

  • WHAT CAUSES ACTION POTENTIALS IN VESTIBULAR NERVE AXONS? Glutamate released from depolarized hair cells binds to receptors on Scarpa’s ganglion neurons, generating EPSPs that summate to trigger action potentials in vestibular nerve axons.

movement of cilia away from kinocilium → closing of K+ channels → hyperpolarization & no signal

example: As your head rotates, the fluid in the semicircular canals lags behind due to inertia. This pushes the cupula in the ampulla, bending the hair cells in the cristae. In the left ear, the stereocilia bend toward the kinocilium, increasing firing in the vestibular branch of the Vestibulocochlear nerve, while in the right ear they bend away, decreasing firing.

Describe the otolithic organs including their names, where they are located, the maculae, otolithic membrane, and otoconia. How are action potentials generated in the vestibular nerve axons?

2 Otolithic Organs

1. Utricle (horizontal)

2. Saccule (vertical)

IN VESTIBULE OF INNER EAR!

• macula: sensory patches inside utricle/saccule, embedded with hair cells

• otolithic membrane: gelatinous structure; where cilia is embedded

• otoconia: calcium carbonate crystals, on top of otolithic membrane

• head movement shifts → otolithic membrane (shifting enhanced by weight of otoconia)

• cilia bend toward kinocilium → open K+ channels → depolarization → open Ca2+ channels → release of neurotransmitter

Glutamate then binds to receptors on neurons in Scarpa's ganglion, which are part of the vestibular portion of the Vestibulocochlear nerve. This binding produces excitatory postsynaptic potentials (EPSPs); if these EPSPs reach threshold, they generate action potentials in vestibular nerve axons that transmit information about head position and linear movement to the brain. If the stereocilia bend away from the kinocilium, the opposite occurs (hyperpolarization and reduced firing rate).

sensory patches within the inner ear's utricle and saccule, vital for balance by detecting linear acceleration and head tilt. They consist of hair cells embedded in a gelatinous membrane weighted with calcium carbonate crystals (otoconia) that shift with gravity.

What is the function of the vestibular system? Where do different projections from the vestibular nuclei go to carry out these functions?

maintain balance, spatial orientation, and stable vision during movement

1. eye movement; head/eye position; visual fixation with head movement

2. movement and reflex coordination

3. maintain balance/equilibrium; proprioception

4. visercal-autonomic activities; cause of sea-sickness/motion sickness

vestibular nuclei projections

1. superior colliculus (midbrain)

2. cerebellum (vestibulocerebellum)

3. spinal cord

4. reticular formation

Olfaction and Gustation

What kind of receptors do olfaction and gustation rely on?

Smell (olfaction) and taste (gustation) are Chemical Senses sensed by

• Chemoreceptors.

Where are the olfactory receptors? Where are the gustatory receptors?

Olfactory Receptors

• olfactory epithelium

Gustatory Receptors

• tongue, (pharynx, palate, epiglottis)

Know that the thalamus plays a role in sensory processing for every sense except olfaction.

What is anosmia? What is ageusia?

• What are the similarities and differences between conductive anosmia and sensorineural anosmia?

Anosmia

• loss of sense of smell

2 types of Anosmia

1. Conductive anosmia: blockage of transmission

   1. Cold

   2. Allergies

   3. Tumors

2. Sensorineural anosmia: caused by damage to neurons/pathways involved in cell

   1. TBI

   2. Alzheimer’s Disease

   3. Parkinson’s Disease

   4. Healthy Aging

both are associated with a loss of sense of smell, but differ in causes.

Ageusia

• loss of sense of taste

• Impairments often lead to loss of appetite and weight loss

Visual System

Describe the function of the different types of cells found in the retina (rods, cones, bipolar cells, ganglion cells)

• Compare and contrast the functions and distribution of rods and cones

Rods

• type of photoreceptor, responds to light

• function: detect shape, size, brightness

• light doesn’t matter; fine in strong or low

• location: distributed throughout retina, not Fovea

Cones

• type of photoreceptor, responds to light

• function: detect color (r, b, g), fine detail

• best in strong light

• location: primarily in Fovea

RODS: SHAPE; SIZE; BRIGHTNESS; LIGHT DONT MATTER; DISTRIBUTED THROUGHOUT RETINA

CONES COLOR (r, b, g) AND FINE DETAL; BEST IN STRONG LIGHT; FOVEA

Bipolar Cells

• connects Photoreceptors to Ganglion Cells

Ganglion Cells

• their axons extend out of eye to form Optic Nerve

• AXONS OF GANGLION CELLS FORM OPTIC NERVE (CN II)

rods, cones, bipolar cells, ganglion cells

Be able to label all regions of the visual pathway from the retina to the occipital lobe including where the synapses are.

• What regions do fibers in the optic tract branch of to? For what functions?

• What information crosses over at the optic chiasm?

• What thalamic nuclei does the information from the visual system synapse in?

1. Retina

2. Optic Nerve (CN II)

3. Optic Chiasm

   1. Where fibers decussate

      1. Information that crosses over: Nasal retinal field axons (temporal retinal field axons stay ipsilateral)

4. Optic Tract

   1. Some fibers branch off from optic tract to synapse on:

      1. Suprachiasmatic nucleus of hypothalamus: function=circadian rhythm

      2. Superior colliculus: function-orientation of head to visual stimuli, allows for blindsight

1. what thalamic nuclei does the info from the visual system synapse in? Lateral Geniculate Nucleus (thalamus)

   1. Synapse of contralateral visual field axons; NEW NEURON STARTS

2. Optic Radiation

3. Occipital Lobe/V1

retina, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, optic radiation, V1

Know the difference between visual and retinal fields.

Visual Fields

What you see; External area visible to eyes without movement

• Refers to the actual environment you see (external space)

• Divided into left and right visual fields

👉 Example:

• Everything to your left of fixation = left visual field

• Everything to your right = right visual field

Retinal Fields

image refracted onto retina; focused representation of visual fields

• Refers to where light hits on the retina inside the eye

• Image is inverted and reversed

👉 Key rule:

• Left visual field → right side of each retina

• Right visual field → left side of each retina

• Left visual field hits:

  • Nasal retina of left eye

  • Temporal retina of right eye

• Right visual field hits:

  • Nasal retina of right eye

  • Temporal retina of left eye

For each eye, the visual field on the same side projects to the nasal retina, while the opposite visual field projects to the temporal retina due to image reversal by the lens.

Where is the primary visual cortex located? How is it organized?

• location: occipital lobe

• retinotopic organization: ordered mapping of the visual field from the retina onto target areas of the brain

Be able to label and explain the visual field defects caused by damage to the optic nerve, optic chiasm, optic tract, geniculocalcarine fibers, and primary visual cortex.

1. Optic Nerve lesion

   1. Monocular Blindness: loss of vision in ipsilateral eye

Damage to the optic nerve causes monocular blindness because it interrupts all visual information from one eye before any fibers cross to the opposite side.

2. Optic Chiasm lesion

   1. Bitemporal Heteronymous Hemianopia (“tunnel vision”): Loss of temporal (peripheral) visual fields in both eyes

   2. Caused by damage to crossing nasal fibers

Damage to the optic chiasm causes bitemporal hemianopia because it disrupts the crossing nasal retinal fibers, which carry information from the temporal (peripheral) visual fields of both eyes.

3. Optic Tract lesion

   1. Homonymous Hemianopia: Loss of vision in same visual half of each eye

Damage to the optic tract causes homonymous hemianopia because each optic tract carries information from the opposite visual field of both eyes.

4. Geniculocalcarine Fibers lesion

1. Homonymous quadrantanopsia: Loss of vision in same visual quadrant of each eye (L/R)

Damage to the geniculocalcarine fibers causes homonymous quadrantanopsia, which is the loss of vision in the same quadrant of both eyes, because these fibers carry information from specific portions (quadrants) of the contralateral visual field from the lateral geniculate nucleus.

5. Primary Visual Cortex lesion

   1. Homonymous Hemianopsia with central sparing

      1. Blindness in opposite field of vision in each eye

      2. Central vision can be spared

Damage to the primary visual cortex causes homonymous hemianopia with macular sparing because each hemisphere processes the opposite visual field, and central vision is often preserved due to dual blood supply.

What kind of information is processed in V3, V4, and V5?

• V3 – form

• V4 – color & form

• V3a & V5 – motion

form / color and form / motion

Describe the dorsal and ventral visual pathways and what kinds of deficits can occur with damage to each.

Dorsal Visual Pathways

• “Where” pathway

• Goes to parietal lobe

• Function: To determine locations of objects in space

Damage:

• Difficulty localizing objects in space

• Can name & describe object, would have difficulty picking it up

• Visual disorientation, difficulty using a map

Ventral Visual Pathways

• “What” pathway

• Goes to inferior temporal lobe

• Function: To identify objects/images

Damage:

• Agnosia- inability to recognize objects, words, faces, etc.

Define/describe apperceptive agnosia, associative agnosia, achromatopsia, akinetopsia, cortical blindness, and blindsight.

Apperceptive agnosia

• Difficulty recognizing an object due to a perceptual deficit

• WAY TO TEST: ability to draw or copy something; bad drawinf

• Difficulty recognizing something because they have a hard time perceiving it

Associative agnosia

• Difficulty recognizing or attaching meaning to an item with preserved perception

• WAY TO TEST: can copy pictures perfectly but could not name the object RECOGNIZING WHAT SOMETHING LOOKS LIKE, WITH WHAT IT IS; perfect drawing, cant name it 

• Ex: show a key, cant visually identify it, but if given to hold it, they can use other senses to identify 

Achromatopsia CHROM-TOPSIA

• difficulty recognizing colors (severe = no color vision)

Akinetopsia KINE-TOPSIA

• difficulty recognizing movement, loss of perception of movement

• V3a, V5

Cortical Blindness

• profound vision loss caused by damage to the brain's occipital cortex (VI) rather than the eyes

Blindsight

• neurological phenomenon where patients with damaged primary visual cortex (V1) report being blind but can unconsciously detect, localize, and react to visual stimuli aka without conscious visual awareness

• patients with damaged primary visual cortex (VI) can respond to visual stimuli in their blind fields without conscious perception.

  • Patients might accurately guess the orientation of a line, point to a light, or navigate around obstacles despite reporting that they see nothing.

What are the names, numbers, and functions of the cranial nerves involved in the senses above (auditory, vestibular, vision, olfaction, gustation)?

1. 👃 Olfaction (Smell)

• Olfactory nerve (CN I)

• → Function: Carries sensory information for smell from the olfactory epithelium to the brain

2. 👁 Vision

• Optic nerve (CN II)

• → Function: Carries visual information from the retina to the brain

3. 👅 Gustation (Taste)

• Facial nerve (CN VII)

• → Function: Taste from anterior 2/3 of the tongue

• Glossopharyngeal nerve (CN IX)

• → Function: Taste from posterior 1/3 of the tongue

• Vagus nerve (CN X)

• → Function: Taste from epiglottis and lower pharynx

4. 👂 Auditory (Hearing)

• Vestibulocochlear nerve (CN VIII)

• → Function: Hearing (cochlear branch)

5. ⚖ Vestibular (Balance)

• Vestibulocochlear nerve (CN VIII)

• → Function: Balance, head movement, spatial orientation (vestibular branch)

CN II Optic Nerve

UNILATERAL LOSS OF ALL VISION

• monocular blindness

Optic Chiasm

Loss of vision in temporal visual

fields = tunnel vision

• Bitemporal heteronymous hemianopsia

Optic Tract

Loss of vision in same visual half of each eye

• Homonymous hemianopsia

Example: Right optic tract lesion

• You lose:

  • Left visual field from left eye

  • Left visual field from right eye

Each optic tract carries one entire visual field from both eyes, not one eye.

• Axons from contralateral visual field

Interruption of portion of geniculocalcarine fibers

Loss of vision in same visual quadrant of each eye (L/R)

• Homonymous quadrantanopsia

these fibers carry quadrant-specific information from the contralateral visual field of both eyes, so a lesion removes the same visual quadrant bilaterally.

V1 one side damage

Blindness in opposite field of vision in each eye

○ Central vision can be spared

• Homonymous hemianopsia with central sparing