Neuro563 Quiz 2 Sensory Systems and Cranial Nerves

Comprehensive flashcards covering the sensory systems (vision, auditory, vestibular, and somatosensory) and the 12 cranial nerves, their brainstem nuclei, pathways, modalities, and associated clinical reflexes.

What is the general physiological sequence through which the nervous system processes sensory information from the environment?

Stimulus $\rightarrow$ Sensory receptor $\rightarrow$ Transduction $\rightarrow$ Receptor potential $\rightarrow$ Action potentials $\rightarrow$ Sensory pathway $\rightarrow$ CNS integration $\rightarrow$ Perception.

What is meant by sensory transduction?

The process by which specialized sensory receptors convert physical or chemical stimulus energy into an electrical signal.

What are the four key properties of a stimulus encoded by the sensory system?

Modality, intensity, location, and duration.

How is the property of stimulus modality defined?

The specific type or kind of stimulus that is detected (for example, light, sound, mechanical pressure, or chemical).

How does the nervous system primarily encode the intensity of a sensory stimulus?

Through frequency coding (the rate of action potentials generated) and the recruitment of sensory receptors.

What mechanism allows the sensory system to encode the duration of a stimulus?

The sustained duration of receptor firing and the process of sensory adaptation.

What are the four primary functional classes of sensory receptors based on the type of energy they transduce?

Mechanoreceptors, chemoreceptors, thermoreceptors, and photoreceptors.

What type of energy do mechanoreceptors detect, and what senses do they mediate?

They detect mechanical displacement

touch, pressure, vibration, stretch,

sound waves, and equilibrium.

What accessory structures protect the eye from sunlight and damaging particles?

Eyebrows, eyelids, eyelashes, and tear glands.

What are the three structural coats or tunics that make up the eyeball?

The fibrous tunic, vascular tunic, and nervous tunic.

Which anatomical structures comprise the fibrous tunic of the eye?

The sclera and the cornea.

Which anatomical structures comprise the vascular tunic (uvea) of the eye?

The choroid, ciliary body, and iris.

Which anatomical structure comprises the nervous tunic of the eye?

The retina.

What is the optical function of the iris?

It controls the amount of light entering the eye by adjusting pupil diameter.

Which ocular structures focus incoming light onto the retina?

The cornea, lens, and humors.

What is the anatomical optic disc, and why is it known as the physiological blind spot?

It is the site where retinal ganglion cell axons converge and exit as the optic nerve; it contains no photoreceptors.

What is the macula lutea of the retina?

The retinal area specialized for high visual acuity, containing the fovea centralis at its center.

Identify the circled structures visible on this retinal fundus photograph.

The optic disk (papilla), macula lutea, fovea centralis, and branches of the retinal/ophthalmic vessels.

What are the functional specializations of cone photoreceptors?

They operate under bright-light conditions and mediate high visual acuity and color perception.

What are the functional specializations of rod photoreceptors?

They operate under dim-light conditions and mediate low-light, monochromatic vision.

What is the direct vertical cellular sequence of light signaling across the retina?

Photoreceptors (rods and cones) $\rightarrow$ Bipolar cells $\rightarrow$ Ganglion cells $\rightarrow$ Optic nerve fibers.

What happens to intracellular cGMP levels in photoreceptors in the dark versus in the light?

cGMP levels are elevated ($\uparrow$) in the dark and reduced ($\downarrow$) in the light.

What is the functional status of cGMP-gated cation channels in photoreceptors in the dark?

They remain open due to high cGMP concentrations.

What is the electrical state of a photoreceptor membrane in the dark?

The photoreceptor is relatively depolarized.

How does dark exposure affect glutamate release from photoreceptor terminals?

It increases ($\uparrow$) continuous tonic glutamate release.

What happens to cGMP-gated channels when light hits a photoreceptor?

cGMP levels decrease, causing cGMP-gated channels to close.

What is the membrane potential change in a photoreceptor following light exposure?

The photoreceptor becomes hyperpolarized.

How does light absorption affect glutamate release from photoreceptors?

It decreases ($\downarrow$) glutamate release onto postsynaptic bipolar cells.

In the three-neuron chain of the visual pathway, which cells constitute the first-order neurons?

Retinal bipolar cells.

In the three-neuron chain of the visual pathway, which cells constitute the second-order neurons?

Retinal ganglion cells.

In the three-neuron chain of the visual pathway, which cells constitute the third-order neurons?

Neurons of the lateral geniculate nucleus (LGN) of the thalamus.

What tract is formed by the axons of the third-order neurons in the lateral geniculate nucleus?

The optic radiations (geniculocalcarine tract), which project to the primary visual cortex.

Where is the primary visual cortex (V1 or striate cortex) anatomically located?

In the occipital lobe along the banks of the calcarine sulcus.

Which retinal axons decussate at the optic chiasm?

Axons originating from the nasal hemiretina of each eye.

Which retinal axons remain uncrossed at the optic chiasm?

Axons originating from the temporal hemiretina of each eye.

What is the functional consequence of nasal hemiretinal fiber decussation at the optic chiasm?

All information from the left visual field projects to the right visual cortex, and all information from the right visual field projects to the left visual cortex.

To which part of the retina does information from the upper visual field project?

To the lower (inferior) retina.

To which part of the retina does information from the lower visual field project?

To the upper (superior) retina.

What is Meyer's loop in the visual radiation pathway?

Fibers carrying information from the upper visual field (lower retina) that loop anteriorly around the inferior horn of the lateral ventricle before reaching the lower bank of the calcarine sulcus.

To which portion of the calcarine cortex do fibers conveying lower visual field information project?

To the upper portion (cuneus) of the calcarine sulcus.

Where do fibers representing the fovea project within the primary visual cortex?

Directly to the most posterior portion of the cortex surrounding the calcarine sulcus.

What visual field deficit results from a complete lesion of one optic nerve?

Ipsilateral monocular vision loss (complete blindness in the affected eye).

What visual field deficit results from a midline lesion of the optic chiasm?

Bitemporal hemianopia.

What visual field deficit results from a lesion of the left optic tract?

Right contralateral homonymous hemianopia.

What visual field deficit results from a lesion of the primary visual cortex?

Contralateral homonymous field loss (often with macular sparing).

Which cranial nerve mediates both hearing and equilibrium?

The vestibulocochlear nerve (CN VIII).

What type of specialized mechanoreceptor cells transduce stimuli in both the cochlea and vestibular apparatus?

Hair cells (named for their apical stereocilia).

What specific physical stimulus activates hair cells in the cochlear division of CN VIII?

Mechanical vibrations caused by sound waves.

What specific physical stimulus activates hair cells in the vestibular division of CN VIII?

Mechanical displacement resulting from head position and head movement in relation to gravity.

What anatomical component represents the external funnel of the auditory system, and what is its role?

The pinna (auricle); it captures and channels sound waves into the external auditory canal.

Which membrane separates the external auditory canal from the middle ear cavity?

The tympanic membrane (eardrum).

What is the primary mechanical role of the middle ear and auditory ossicles?

To amplify sound wave vibrations and convey them from the air-filled canal to the fluid-filled inner ear.

Name the three auditory ossicles in sequence from the tympanic membrane to the oval window.

Malleus (hammer), incus (anvil), and stapes (stirrup).

Which two membrane-covered openings separate the middle ear from the fluid-filled inner ear?

The oval window and the round window.

In which inner ear structure are sound vibrations transduced into electrical signals?

The organ of Corti within the cochlea.

Where is perilymph found within the cochlea?

In the scala vestibuli and scala tympani.

What is the ionic composition of cochlear perilymph?

High $\text{Na}^+$ and low $\text{K}^+$ concentration (similar to extracellular fluid).

Where is endolymph found within the cochlea?

In the scala media (cochlear duct).

What is the ionic composition of endolymph?

High $\text{K}^+$ and low $\text{Na}^+$ concentration.

Why is the high potassium concentration of endolymph critical for auditory hair cells?

It maintains an electrical potential and chemical gradient that drives $\text{K}^+$ influx into hair cells upon stereocilia bending, causing depolarization.

What mechanical event directly causes depolarization of auditory hair cells?

Basilar membrane movement pushing stereocilia against the tectorial membrane, causing them to bend toward the tallest stereocilium and opening mechanically-gated ion channels.

What neurotransmitter is released by cochlear hair cells onto first-order sensory neurons?

Glutamate.

Where are the cell bodies of first-order auditory neurons located?

In the spiral ganglion of the modiolus within the cochlea.

Where do first-order auditory axons project and synapse after entering the brainstem via CN VIII?

Onto the ipsilateral cochlear nuclei in the medulla oblongata.

Why are cochlear nuclei classified as monaural?

Because they receive and process sound input exclusively from one ear (the ipsilateral ear).

What is the first binaural structure in the central auditory pathway?

The superior olivary complex in the pons.

What is the primary function of the superior olivary complex?

Sound localization based on interaural time differences and interaural intensity differences.

Through which ascending brainstem tract do auditory fibers travel from the superior olive to the midbrain?

The lateral lemniscus.

What is the role of the inferior colliculus in auditory processing?

It serves as a major integrative center for auditory reflexes, sound localization, frequency processing, and orienting attention to biologically relevant sounds.

Which thalamic relay nucleus receives auditory input from the inferior colliculus?

The medial geniculate nucleus (MGN / MGB).

Where is the primary auditory cortex located, and which Brodmann areas correspond to it?

In the superior temporal gyrus of the temporal lobe; Brodmann areas 41 and 42.

Which Brodmann area corresponds to the secondary auditory cortex (auditory association area)?

Brodmann area 22.

Trace the sequential stations of the central auditory pathway from cochlea to cortex.

Cochlea $\rightarrow$ Spiral ganglion (CN VIII) $\rightarrow$ Cochlear nuclei $\rightarrow$ Superior olivary complex $\rightarrow$ Lateral lemniscus $\rightarrow$ Inferior colliculus $\rightarrow$ Medial geniculate body $\rightarrow$ Auditory cortex.

What is conductive hearing loss, and what are its primary causes?

An acoustic-mechanical disturbance of sound conduction located in the external auditory canal, across the tympanic membrane, or within the ossicular chain.

What is sensory hearing loss?

Hearing impairment caused by dysfunction of hair cells in the cochlea or their synaptic connections to the cochlear nerve.

What is neural hearing loss?

Hearing impairment resulting from cochlear nerve dysfunction, characterized by delayed impulse conduction and disturbed neural encoding.

What is central hearing loss, and how does it manifest clinically?

Dysfunction of the auditory pathway or auditory cortex; tone perception may remain intact while speech perception in noise, localization, and rapid processing are impaired.

What sensory structures comprise the vestibular labyrinth?

The otolith organs (utricle and saccule) and the three semicircular canals.

What specific type of motion is detected by the semicircular canals?

Rotational or angular acceleration of the head.

What specific type of motion is primarily detected by the utricle?

Linear acceleration in the horizontal plane and head tilt relative to gravity.

What specific type of motion is primarily detected by the saccule?

Linear acceleration in the vertical plane (such as riding in an elevator).

What are otoconia, and where are they located?

Small crystals of calcium carbonate embedded in the gelatinous cap overlying hair cells in the otolith organs (utricle and saccule).

How do otoconia stimulate hair cells during head tilt?

Gravity pulls on the dense otoconia, shifting the otolithic membrane and mechanically bending hair-cell stereocilia.

How are the three semicircular canals anatomically oriented relative to each other?

At right angles ($90^\circ$) to each other, allowing detection of rotational acceleration in all three spatial planes.

What is the ampulla of a semicircular canal?

A localized dilation or swelling at the base of each semicircular canal containing sensory receptor hair cells.

What is the cupula within the ampulla of the semicircular canal?

A gelatinous membrane spanning the ampullary lumen into which hair cell stereocilia and kinocilia project.

How does head rotation cause deflection of the cupula?

Endolymph lag due to fluid inertia exerts pressure against the cupula, deflecting it in the direction opposite to head rotation.

What is the kinocilium?

The single tallest specialized stereocilium located at one edge of the hair-cell bundle.

What mechanical and electrical change occurs when stereocilia bend toward the tallest stereocilium (kinocilium)?

Tip link tension opens mechanically-gated $\text{K}^+$ channels, leading to $\text{K}^+$ influx and hair cell depolarization.

What electrical change occurs when stereocilia bend away from the kinocilium (toward the shortest stereocilium)?

Tip links relax, closing mechanically-gated $\text{K}^+$ channels and causing hair cell hyperpolarization.

Where are the cell bodies of first-order vestibular neurons located?

In Scarpa's (vestibular) ganglion.

Where are second-order vestibular neurons located, and what are their four subnuclei?

In the pons and medulla; superior, inferior, medial, and lateral vestibular nuclei.

What is the function of central vestibular projections to extraocular motor nuclei (cranial nerves III, IV, VI)?

Mediation of the vestibulo-ocular reflex (VOR) to keep gaze stabilized during head motion.

What is the functional role of central vestibular projections descending to the spinal cord?

To maintain posture and balance reflexes via vestibulospinal pathways.

Which thalamic nucleus relays vestibular information to the cerebral cortex for conscious perception?

The ventral posterior (VP) nucleus of the thalamus.

Which sensory modalities are mediated by the Dorsal Column–Medial Lemniscal (DCML) pathway?

Fine (discriminative) touch, vibration, and conscious proprioception.

Where do first-order axons of the DCML pathway ascend, and where do they first synapse?

They ascend ipsilaterally in the dorsal funiculus (fasciculus gracilis and cuneatus) and synapse in the gracile and cuneate nuclei of the lower medulla.

At what anatomical level and via what fibers does the DCML pathway decussate?

In the medulla oblongata via the internal arcuate fibers (sensory decussation).

Which thalamic nucleus receives second-order axons from the medial lemniscus?

The ventral posterolateral (VPL) nucleus.

Which sensory modalities are mediated by the spinothalamic (anterolateral) pathway?

Pain, temperature, and crude touch.