Sensory and Motor Systems Practice Flashcards

A comprehensive set of question-and-answer practice flashcards covering sensory receptors, somatosensory and somatomotor pathways, muscle reflexes, optics, retinal phototransduction, and central visual pathways.

How many pairs of cranial and spinal nerves compose the peripheral nervous system?

The peripheral nervous system is composed of $12$ pairs of cranial nerves and $30$ pairs of spinal nerves.

Where do sensory neuron cell bodies reside before entering the dorsal horn of the spinal cord?

Sensory neuron cell bodies reside within the dorsal root ganglia.

What is the main difference between dendritic and axonal signal transmission regarding stimulus intensity?

Dendrites generate an analog signal where stimulus intensity dictates the size of the excitatory postsynaptic potential (EPSP), whereas axons use a digital format where stimulus intensity dictates the frequency of all-or-none action potentials.

What are the four main broad categories of sensory receptors based on stimulus type?

The four major types are mechanoreceptors, thermoreceptors, photoreceptors, and chemoreceptors.

What are the primary structural and functional characteristics of Type A alpha sensory nerve fibers?

Type A alpha fibers are the largest myelinated and fastest transmitting nerve fibers ($80 \text{ to } 120\,m/s$), primarily responsible for conveying touch and proprioception information.

What functional distinction separates rapidly adapting mechanoreceptors from slowly adapting mechanoreceptors?

Rapidly adapting receptors fire quickly at the onset or change of a stimulus and then stop, making them ideal for detecting changes or movement; slowly adapting receptors fire continuously throughout a stimulus, making them ideal for reporting ongoing pressure or stretch.

What are the specific functional roles and locations of Meissner corpuscles, Merkel discs, Pacinian corpuscles, and Ruffini endings?

Meissner corpuscles (shallow, small field, fast-adapting) detect surface motion; Merkel discs (shallow, small field, slow-adapting) detect continuous surface pressure; Pacinian corpuscles (deep, large field, fast-adapting) detect deep vibrations; and Ruffini endings (deep, large field, slow-adapting) detect tissue stretch.

How does the mechanical structure of a Pacinian corpuscle produce rapid adaptation?

The corpuscle has a fluid-filled sac surrounding the dendritic ending; when pressure is applied, fluid rapidly redistributes within the sac to relieve mechanical force on the dendrite, causing it to stop firing despite sustained external pressure.

What types of sensory information are carried by the dorsal column-medial lemniscal pathway, and where does it decussate?

The dorsal column-medial lemniscal pathway carries proprioception and touch via fast Type A alpha and beta fibers; it travels ipsilaterally up the spinal cord and decussates at the medulla in the brainstem.

What sensory modalities travel through the anterolateral pathway, and where does decussation occur?

The anterolateral pathway carries pain, itch, and thermal sensations via smaller myelinated and unmyelinated fibers; it decussates immediately at the level of entry in the spinal cord before ascending to the thalamus.

How are somatic sensory body regions topographically mapped across the primary somatosensory cortex?

Lower body regions map medially within the postcentral gyrus, while higher body regions map progressively laterally, with an over-representation of the hands, lips, and tongue due to high receptor density.

What are the functions of the ventral corticospinal tract and the lateral corticospinal tract?

The ventral corticospinal tract works with the medial brainstem pathway to regulate posture and muscle tone, while the lateral corticospinal tract works with the rubrospinal tract (from the red nucleus) to control fine, voluntary movements of distal limbs.

What molecular sequence leads to muscle contraction at the neuromuscular junction?

Acetylcholine released from the presynaptic motor terminal binds to ligand-gated sodium channels on the sarcolemma, generating an endplate potential that triggers an action potential; this travels down T-tubules to activate voltage-gated calcium channels in terminal cisternae, dumping $Ca^{2+}$ to allow myosin-actin binding.

What is the primary role of the muscle spindle versus the Golgi tendon organ?

Muscle spindles sit in the muscle belly to detect muscle length, tone, and rate of length change; Golgi tendon organs sit in the tendon to sense tension/force placed on the tendon and bone as a protective mechanism against excessive force.

How do gamma motor neurons maintain muscle spindle sensitivity during skeletal muscle contraction?

Gamma motor neurons innervate the striated ends of intrafusal fibers, causing them to contract alongside extrafusal skeletal muscle fibers; this retains tension on the central sensory region so length changes can still be detected in a contracted muscle.

What distinguishes a monosynaptic reflex from a polysynaptic reflex?

A monosynaptic reflex contains a single synapse between a sensory and motor neuron (e.g., stretch reflex), whereas a polysynaptic reflex contains one or more interneurons (e.g., reciprocal inhibition or propriospinal withdrawal reflexes) introducing greater complexity and longer response delays.

What are the total refractive power of the human eye and the relative contributions of the cornea and lens?

The total refractive power is approximately $59\text{ diopters}$; the cornea contributes roughly two-thirds ($\sim 40\text{ D}$) due to a large refractive index difference from air, and the lens contributes one-third ($\sim 20\text{ D}$).

How does the eye adjust its lens power during the process of accommodation for near objects?

Parasympathetic stimulation causes the ciliary muscle to contract like a sphincter, releasing tension on the suspensory ligaments (zonules); the elastic lens capsule relaxes into a more spherical shape, increasing its curvature and bending power.

What structural and spatial features make the fovea centralis the site of highest visual acuity?

Interneurons and ganglion cells are pushed laterally to form the foveal pit, minimizing light scatter; it contains the highest density of cone photoreceptors and maintains a $1:1:1$ ratio of cone to bipolar cell to ganglion cell.

What physiological process occurs in a photoreceptor cell in response to light detection?

Absorption of light converts $11\text{-cis}$ retinal to all-trans retinal, breaking down rhodopsin; this activates transducin and cGMP phosphodiesterase, which degrades cGMP, closing cGMP-gated sodium channels, hyperpolarizing the cell to $-80\,mV$, and reducing glutamate release.

Why does a vitamin A deficiency cause night blindness before affecting daylight vision?

Vitamin A is the precursor for retinal; rods are far more numerous, contain larger quantities of photopigment, and are extremely sensitive to low light, making rod-mediated night vision the first to suffer from diminished retinal synthesis.

What are the functional characteristics and target layers of midget ganglion cells versus parasol ganglion cells in the visual pathway?

Midget ganglion cells ($70\%$ of population) have a $1:1:1$ ratio, convey fine spatial detail and color, and project to Parvocellular layers 3--6 of the LGN; Parasol ganglion cells ($10\%$ of population) have high convergence, are color-blind, detect movement, and project to Magnocellular layers 1--2 of the LGN.

How are retinal visual inputs organized at the optic chiasm?

Nerve fibers originating from the temporal retina remain ipsilateral (same side), while nerve fibers originating from the nasal retina decussate (cross over) to the contralateral side.

What visual field defect results from a complete lesion of the optic chiasm?

A complete lesion of the optic chiasm disrupts crossing fibers from both nasal retinas, resulting in bitemporal heteronymous hemianopia (loss of the temporal visual fields in both eyes).