Bio 50B - Lec 3-4

What are the primary functions of the outer ear (pinna and external auditory canal)?

To collect sound waves, direct them to the tympanic membrane, and selectively amplify certain sound frequencies (especially 2,000–5,000 Hz).

How does the middle ear overcome the impedance mismatch between air and fluid in the cochlea?

• Area ratio difference: The tympanic membrane has a much larger area than the oval window, concentrating force.

• Ossicular lever action: The arrangement of the malleus, incus, and stapes creates mechanical leverage to amplify pressure.

What is the function of the attenuation reflex (acoustic reflex)?

Contraction of the tensor tympani and stapedius muscles stiffness the ossicular chain to protect the inner ear from damage due to loud, low-frequency sounds.

What are the three main fluid-filled compartments (scalae) of the cochlea, and what fluids do they contain?

Scala vestibuli – Perilymph (low K+, high Na+)Scala media (cochlear duct) – Endolymph (high K+, low Na+)Scala tympani – Perilymph (low K+, high Na+)

What generates the endocochlear potential, and what is its magnitude?

The stria vascularis actively pumps potassium (K+) into the scala media, producing an endolymphatic potential of approximately +80 mV relative to perilymph.

Explain the concept of tonotopic organization in the basilar membrane.

• Base (near oval window): Narrow, stiff, responds best to high-frequency sounds.

• Apex (near helicotrema): Wide, flexible, responds best to low-frequency sounds.

How do hair cell stereocilia movement trigger electrical depolarization?

Bending towards the tallest stereocilium pulls tip links open, allowing K+ ions from the endolymph to enter down their electrochemical gradient, depolarizing the hair cell.

How does repolarization occur in auditory hair cells?

Potassium (K+) exits the basal region of the cell into the perilymph (which has low K+) through voltage-gated potassium channels down its concentration gradient.

What is the distinct functional role of Outer Hair Cells (OHCs) versus Inner Hair Cells (IHCs)?

• IHCs: Primary sensory receptors that transmit auditory signals to the brain via type I afferent fibers.

• OHCs: Act as "cochlear amplifiers" through electromotility (motor protein prestin) to sharpen frequency tuning and boost sensitivity.

What protein powers the fast length changes (electromotility) of outer hair cells?

Prestin.

What is phase-locking in auditory nerve firing?

The tendency of auditory nerve fibers to fire action potentials at a precise phase of a sound wave's cyclical waveform (primarily active for sound frequencies below 3–4 kHz).

Which brainstem structures process binaural sound localization cues, and what specific cues do they use?

• Medial Superior Olive (MSO): Interaural Time Differences (ITD) for low-frequency sounds.

• Lateral Superior Olive (LSO) & Medial Nucleus of the Trapezoid Body (MNTB): Interaural Level Differences (ILD) for high-frequency sounds.

Trace the central auditory pathway from the cochlea to the primary auditory cortex.

Cochlear nerve → Cochlear Nuclei → Superior Olivary Complex → Lateral Lemniscus → Inferior Colliculus → Medial Geniculate Nucleus (MGN) of thalamus → Primary Auditory Cortex (A1 / Brodmann area 41).

What is conductive hearing loss vs. sensorineural hearing loss?

• Conductive: Impedance of sound transmission in outer or middle ear (e.g., earwax, otitis media, otosclerosis). Weber test lateralizes to affected ear.

• Sensorineural: Damage to inner ear hair cells or auditory nerve (e.g., noise exposure, presbycusis, acoustic neuroma). Weber test lateralizes to healthy ear.

What are the expected results of the Rinne test in a normal ear vs. conductive hearing loss?

• Normal / Sensorineural: Air Conduction > Bone Conduction (Positive test).

• Conductive loss: Bone Conduction > Air Conduction (Negative test).

What are the main refraction components of the eye, and which one provides the most refractive power?

• Cornea: Provides ~70% (2/3) of the eye's total refractive power (fixed power).

• Lens: Provides ~30% (1/3) of the power, but can change shape for accommodation.

Define Myopia and how it is corrected.

• Definition: Nearsightedness (eye is too long or refractive power is too strong). Light focuses in front of the retina.

• Correction: Concave lens (diverging lens).

Define Hyperopia and how it is corrected.

• Definition: Farsightedness (eye is too short or refractive power is too weak). Light focuses behind the retina.

• Correction: Convex lens (converging lens).

What is Presbyopia and what causes it?

• Definition: Age-related loss of accommodation (difficulty focusing on near objects).

• Cause: Loss of elasticity of the crystalline lens and weakening of the ciliary muscle over time.

Explain the mechanism of Accommodation for near vision.

• Parasympathetic activation contracts the ciliary muscle.

• Zonular fibers (suspensory ligaments) relax/slacken.

• Lens becomes more spherical/rounder (increases refractive power).

Explain the eye's mechanism for distant vision.

• Sympathetic activation / relaxation of the ciliary muscle.

• Zonular fibers tighten/pull tight.

• Lens becomes flattened (decreases refractive power).

What are the key functional differences between Rods and Cones?

• Rods: High sensitivity (night vision/scotopic), low acuity, achromatic (1 photopigment), high convergence onto ganglion cells.

• Cones: Low sensitivity (day vision/photopic), high spatial acuity, color vision (3 photopigments: RGB), low convergence (1:1 in fovea).

What makes the Fovea centralis specialized for high visual acuity?

• High density of cones (no rods).

• Inner retinal layers are pushed aside to let light hit photoreceptors directly.

• Minimal convergence (1 cone → 1 bipolar cell → 1 ganglion cell).

• What is the visual photopigment in rods, and what are its two components?

• Rhodopsin

• Opsin: G-protein coupled receptor protein.

• Retinal (11-cis retinal): Light-absorbing chromophore (derived from Vitamin A).

What chemical change occurs to retinal when hit by a photon of light?

11-cis retinal photoisomerizes into all-trans retinal.

What is the state of photoreceptor membrane potential and neurotransmitter release in the DARK?

• Membrane Potential: Depolarized (~ -40 mV) due to the "dark current" ($Na^+$/$Ca^{2+}$ entry through cGMP-gated channels).

• Neurotransmitter: High continuous release of Glutamate.

What is the state of photoreceptor membrane potential and neurotransmitter release in the LIGHT?

• Membrane Potential: Hyperpolarized (~ -70 mV) because cGMP-gated $Na^+$/$Ca^{2+}$ channels close.

• Neurotransmitter: Reduced (decreased) release of Glutamate.

Step-by-step phototransduction cascade from light absorption to hyperpolarization:

• Photon absorption activates Rhodopsin ($\rightarrow$ Metarhodopsin II).

• Activates G-protein Transducin ($G_t$).

• Transducin activates Phosphodiesterase (PDE).

• PDE hydrolyzes cGMP to 5'-GMP (lowers cGMP concentration).

• cGMP-gated cation channels close $\rightarrow$ Hyperpolarization.

How do ON-center bipolar cells respond to light in the center of their receptive field?

• Light decreases glutamate release from the photorceptor.

• Reduced glutamate action on mGluR6 receptors causes the ON-center bipolar cell to depolarize (sign-inverting).

How do OFF-center bipolar cells respond to light in the center of their receptive field?

• Light decreases glutamate release from the photoreceptor.

• Reduced glutamate action on AMPA/Kainate receptors causes the OFF-center bipolar cell to hyperpolarize (sign-conserving).

Which retinal cell type is responsible for lateral inhibition and center-surround receptive field organization?

Horizontal cells (they release GABA to inhibit neighboring photoreceptors).

Which retinal cells produce true Action Potentials?

Ganglion cells (and some Amacrine cells). Photoreceptors, Bipolar, and Horizontal cells use graded potentials.

Trace the primary visual pathway from retina to cortex.

Retina → Optic Nerve →Optic Chiasm → Optic Tract → Lateral Geniculate Nucleus (LGN) of Thalamus → Optic Radiations → Primary Visual Cortex (V1 / Striate Cortex / Area 17).

What visual deficit occurs from a lesion to the Optic Chiasm?

Bitemporal Hemianopia ("tunnel vision") — loss of peripheral vision in both eyes due to disruption of crossing decussating nasal fibers.

What visual deficit occurs from a complete lesion of the Left Optic Tract?

Right Homonymous Hemianopia — loss of the right visual field in both eyes.