Anatomy Exam 1 Mastery 1: Neurons, Outer Ear & Middle Ear

[L2] What is the neuron's pre-stimulus electrical state?

The inside of the neuron is net negative at about -65 mV relative to the outside, which is treated as about 0 mV.

[L2] Before stimulation, where is Na+ relatively concentrated and what do its gradients favor?

Na+ is higher outside and lower inside. Both its concentration gradient and the negative electrical environment inside favor Na+ moving into the cell.

[L2] Before stimulation, where is K+ relatively concentrated and what do its gradients favor?

K+ is higher inside and lower outside. Its concentration gradient favors K+ moving out, while the negative electrical environment inside favors K+ staying in.

[L2] Before stimulation, where is Cl- relatively concentrated and what do its gradients favor?

Cl- is higher outside and lower inside. Its concentration gradient favors movement into the cell, while the electrical gradient favors movement out; in the lecture, the concentration force is stronger, so net drive is inward.

[L2] Before stimulation, where is Ca2+ relatively concentrated?

Ca2+ is much higher outside than inside; intracellular Ca2+ is kept extremely low.

[L2] How does the neuron maintain its Na+ and K+ concentration gradients?

The Na-K-ATPase uses ATP to pump 3 Na+ out of the cell for every 2 K+ moved into the cell, maintaining the resting ionic gradients and negative resting potential.

[L2] How is intracellular Ca2+ kept very low before stimulation?

Ca2+ pumps remove Ca2+ from the cytoplasm, and intracellular mechanisms bind and sequester Ca2+, including storage in smooth endoplasmic reticulum.

[L2] What is an EPSP?

An excitatory postsynaptic potential is a graded depolarization of the postsynaptic cell produced when excitatory neurotransmitter causes chemically gated channels to open and Na+ enters.

[L2] What is the basic EPSP sequence?

Excitatory neurotransmitter binds postsynaptic receptors -> chemically gated Na+ channels open -> Na+ enters -> the membrane becomes less negative -> an EPSP is produced.

[L2] What is spatial summation?

Multiple presynaptic cells release excitatory neurotransmitter across different synapses at about the same time, producing a larger combined EPSP.

[L2] What is temporal summation?

The same synapse is repeatedly excited within a short time window, so the EPSPs overlap and produce greater depolarization.

[L2] How does an EPSP become an action potential?

EPSPs depolarize the neuron. If depolarization at the axon hillock is sufficient, the neuron reaches the point at which an action potential is initiated and propagated down the axon.

[L2] Why can an EPSP occur without an action potential?

An EPSP is graded and can fade without sufficiently depolarizing the axon hillock. An action potential begins only when hillock depolarization is sufficient.

[L2] How is an action potential propagated down a myelinated axon?

It is propagated by saltatory conduction: sequential depolarization and hyperpolarization occur at Nodes of Ranvier along the axon rather than continuously through the myelin.

[L2] What is the role of the Nodes of Ranvier in action-potential propagation?

They are interruptions in the myelin sheath where the action potential is regenerated; the relevant ion channels are concentrated at the nodes.

[L2] What ion movement drives the depolarizing phase of the action potential?

Na+ influx drives depolarization, making the inside of the neuron much more positive.

[L2] What happens during the hyperpolarization phase?

After depolarization, K+ moves out and Cl- moves in according to the lecture, driving the membrane potential negative and producing a hyperpolarized state.

[L2] What is the absolute refractory period?

The period after an action potential when another action potential cannot be triggered, regardless of the amount of excitatory input.

[L2] What is the relative refractory period?

The later refractory phase when another action potential can occur, but it requires greater-than-normal excitatory stimulation.

[L2] What restores the neuron toward its resting ionic conditions after an action potential?

The Na-K-ATPase pumps Na+ back out and K+ back in, rebuilding the resting concentration gradients.

[L2] What happens when the action potential reaches the presynaptic terminal?

Voltage-gated Ca2+ channels open -> Ca2+ enters the terminal -> Ca2+ triggers neurotransmitter release into the synapse.

[L2 SYNTHESIS] How does the neuron create and maintain its concentration and electrical gradients? [Professor-style]

Na+, Cl-, and Ca2+ are kept relatively higher outside, while K+ is higher inside; the inside is net negative. The Na-K-ATPase uses ATP to move 3 Na+ out for 2 K+ in, and Ca2+ pumps/sequestration keep intracellular Ca2+ extremely low. These unequal ion distributions plus the negative intracellular voltage create concentration and electrical gradients.

[L2 SYNTHESIS] How does an EPSP translate into an action potential? [Professor-style]

Excitatory neurotransmitter opens chemically gated Na+ channels, Na+ influx creates a graded EPSP, and sufficient depolarization reaching the axon hillock initiates an action potential that is then propagated down the axon.

[L3] Superior vs inferior

Superior means vertically above another structure; inferior means vertically below it.

[L3] Anterior vs posterior

Anterior means toward the front; posterior means toward the back.

[L3] Dorsal vs ventral

Dorsal refers toward the back side; ventral refers toward the belly/front side.

[L3] Rostral vs caudal

Rostral means toward the front/nose end; caudal means toward the back/tail end.

[L3] Deep vs superficial

Deep means farther from the body surface; superficial means closer to the surface.

[L3] Proximal vs distal

Proximal means closer to a point of origin or attachment; distal means farther from it.

[L3] Medial vs lateral

Medial means toward the midline; lateral means away from the midline.

[L3] What does a sagittal plane divide?

A sagittal plane divides the body or brain into left and right portions. Sagittal does not necessarily mean equal halves.

[L3] What is a midsagittal plane?

A sagittal cut through the midline that divides the structure into left and right halves.

[L3] What does a coronal plane divide?

It divides the body into anterior/posterior or ventral/dorsal portions; in the brain it separates rostral and caudal portions.

[L3] What does a horizontal plane divide?

It divides the body or brain into superior and inferior portions.

[L3] What is a transverse plane in this lecture?

It makes the same general superior/inferior division as a horizontal cut but is angled to accommodate curved structures such as the brainstem and spinal cord.

[L3] Which cranial nerves were identified as innervating or lying around the outer ear/EAC?

CN V Trigeminal, CN VII Facial, CN IX Glossopharyngeal, and CN X Vagus.

[L3] What outer-ear sensory information is associated with CN IX and CN X?

CN IX carries touch sensation from the pinna and tympanic membrane; CN X carries touch sensation from the EAC, tympanic membrane, and pharyngeal region.

[L3] What did the outer-ear lecture emphasize about CN V and CN VII?

CN V provides sensory and motor innervation to the face, eyes, and jaw; CN VII provides sensory and motor innervation to the face, lips, and parts of the mouth.

[L3] What are the four major functions of the outer ear listed in lecture?

Sound localization, collection of sound, amplification of sound, and protection from foreign bodies and moisture.

[L3] What does the pinna contribute to hearing?

It collects/directs sound, contributes high-frequency amplification, and helps localize complex sounds in the midsagittal/vertical plane.

[L3] What is the overall outer-ear amplification emphasized in lecture?

About 10-15 dB of amplification across roughly 1.5-7 kHz.

[L3] What frequency ranges are emphasized for concha, EAC, and TM resonance?

Concha: about 3-7 kHz; EAC: about 2-4 kHz; TM: about 1.5-3 kHz.

[L3] Why does the EAC have a resonance?

The canal behaves approximately like a tube that is open at one end and closed at the tympanic membrane, producing a quarter-wave resonance.

[L3] How does EAC length affect its resonant frequency?

A shorter EAC shifts resonance higher in frequency; a longer EAC shifts resonance lower.

[L3] Why is real outer-ear resonance broader than the resonance of a perfect tube?

The EAC is not a perfect tube and the TM is not a perfect cap; the concha, canal, and tympanic membrane also contribute over overlapping frequency ranges.

[L3] How does the pinna help vertical/midsagittal localization?

The pinna changes the incoming sound spectrum in an elevation-dependent way. The auditory system interprets these spectral/HRTF cues to estimate vertical position.

[L3] What frequency region is especially important for the vertical-localization cues emphasized in lecture?

High frequencies, roughly 5-10 kHz.

[L3] What additional cue can the angle of sound entry into the EAM create?

Tiny reverberations that are added to the incoming sound and alter its spectrum.

[L3 SYNTHESIS] What roles does the outer ear fulfill for auditory-system function? [Professor-style]

It collects/directs sound into the canal, amplifies selected frequencies, contributes spectral cues for localization, and protects the auditory pathway from foreign bodies and moisture.

[L4] What does the tympanic membrane do?

It transduces acoustic sound energy into mechanical energy that can be transmitted through the ossicular chain.

[L4] What is the key functional role of the Eustachian tube?

It acts as a pressure valve for the middle-ear space so middle-ear pressure can remain near atmospheric pressure.

[L4] What spaces does the Eustachian tube connect?

The anterior middle-ear space to the posterior nasopharynx.

[L4] When is the Eustachian tube normally closed and how does it open?

It is closed at rest and opens with contraction of the levator veli palatini and tensor veli palatini muscles.

[L4] What happens to sound transmission when middle-ear stiffness impedance increases?

Low-frequency sound transmission decreases.

[L4] What happens to sound transmission when middle-ear mass impedance increases?

High-frequency sound transmission decreases.

[L4] What is an impedance mismatch in the middle-ear context?

It is the mismatch between the low impedance of air and the much higher impedance of cochlear fluid; without a matching mechanism, much of the incident sound energy would be reflected rather than transmitted.

[L4] Why is a middle-ear matching mechanism necessary?

Air is low impedance and cochlear fluid is high impedance, so direct air-to-fluid transmission would lose a large amount of sound energy.

[L4] What are the three lecture mechanisms that help overcome the air-fluid impedance mismatch?

TM-to-stapes area advantage, ossicular lever action, and buckling of the cone-shaped tympanic membrane.

[L4] How does the TM-to-stapes area relationship increase pressure?

Force collected over a relatively large effective TM area is concentrated onto the much smaller stapes footplate, increasing pressure at the oval window.

[L4] How does ossicular lever action contribute to hearing?

The malleus is larger/longer than the stapes portion of the chain, creating a lever advantage that increases effective pressure transmission toward the stapes.

[L4] How does TM buckling contribute to impedance matching?

The cone-shaped TM buckles as it vibrates, adding another mechanical pressure advantage to the middle-ear system.

[L4] How do the ossicles transmit sound mechanically?

TM motion drives the malleus, then incus, then stapes; the stapes footplate acts at the oval window to apply force to cochlear fluid.

[L4] What detail about stapes-footplate movement was emphasized?

Although the ossicles are often described as moving like a complex piston, the footplate actually pivots around a posterior connection point in the oval window.

[L4] Why does changing middle-ear impedance change hearing?

Middle-ear transmission depends on how efficiently mechanical energy is transferred; added stiffness selectively reduces low-frequency transmission and added mass selectively reduces high-frequency transmission.

[L4 SYNTHESIS] How does the middle ear overcome the impedance mismatch? [Professor-style]

It increases effective pressure at the oval window through three mechanical advantages: concentrating TM force onto the smaller stapes footplate, ossicular lever action, and TM buckling. Together these reduce the large loss that would otherwise occur when transferring sound from air to cochlear fluid.

[L4 SYNTHESIS] What does the architecture of the ossicle bones contribute to middle-ear function? [Professor-style]

Their linked orientation transmits TM motion to the stapes, and their lever geometry increases mechanical advantage so greater pressure is delivered at the oval window; the stapes footplate then pivots to drive cochlear fluid.