Peripheral Nervous System - Afferent Division and Special Senses

Comprehensive Question and Answer flashcards covering sensory receptor types, transduction pathways, visual mechanisms, auditory processing, and vestibular physiology from Chapter 6 notes.

What is the primary function of the afferent division of the peripheral nervous system?

To transmit information from the external and internal environment to the central nervous system (CNS) by detecting environmental changes via sensory receptors.

What are the two structural types of sensory receptors?

1. Specialized endings of an afferent neuron.
2. A separate cell that signals the afferent neuron by releasing chemical messengers.

How do receptor potentials generated in specialized afferent endings differ from those in separate receptor cells?

In specialized endings, a stimulus opens stimulus-sensitive channels to produce a receptor potential that directly opens adjacent voltage-gated $\text{Na}^+$ channels to initiate an action potential. In separate receptor cells, local depolarization opens voltage-gated $\text{Ca}^{2+}$ channels, triggering neurotransmitter exocytosis to open chemically gated channels on the afferent fiber ending.

What are the six main classes of sensory receptors and their corresponding stimuli?

• Mechanoreceptors: pressure, vibration, stretch, fine touch • Thermoreceptors: temperature • Photoreceptors: sight (light) • Chemoreceptors: chemical composition and $\text{H}^+$ • Nociceptors: pain • Osmoreceptors: solute concentration

What are the key physiological features of a receptor potential?

It is a graded potential with no refractory period that displays summation; a stronger stimulus causes greater membrane permeability, producing a larger graded potential that generates an action potential if threshold is reached.

How does an increase in stimulus strength affect signal transmission along an afferent neuron?

An increased stimulus strength leads to a higher magnitude of receptor potential, an increased action potential frequency along the afferent neuron, and a greater quantity of neurotransmitter released from afferent terminals.

What is the functional difference between tonic receptors and phasic receptors?

Tonic receptors adapt slowly or not at all and continuously relay information to the CNS (e.g., muscle stretch receptors for posture). Phasic receptors adapt rapidly, stopping responses to a maintained stimulus, and exhibit an off response (depolarization) when the stimulus is removed.

Where do receptor adaptation and habituation occur in the nervous system?

Receptor adaptation occurs at the individual neuron level, whereas habituation is a behavioral change occurring at the central nervous system (CNS) level.

What are the three structural layers of the eye?

1. Sclera (tough outer layer) and Cornea (transparent anterior part of sclera).
2. Choroid (middle pigmented layer with blood vessels, including specialized ciliary body and iris).
3. Retina (innermost layer, composed of an outer pigment layer and inner nervous tissue layer with rods and cones).

What are the functions of the vitreous humor and aqueous humor?

Vitreous humor is a clear jellylike substance between the lens and retina that maintains eyeball shape. Aqueous humor is a clear watery fluid between the cornea and lens that carries nutrients; it is produced continuously by the ciliary body and drains into a canal at the edge of the eye.

What physiological change causes glaucoma?

Glaucoma is caused by increased eye pressure due to an excessive accumulation or increased volume of aqueous humor.

How do autonomic nervous system pathways control pupillary constriction and dilation?

Parasympathetic stimulation contracts the circular (constrictor) muscle to cause pupillary constriction in bright light. Sympathetic stimulation contracts the radial (dilator) muscle to cause pupillary dilation in dim light.

How do convex and concave optical lenses bend incoming light rays?

Convex lenses cause light rays to converge to a focal point, whereas concave lenses cause light rays to diverge.

How does the ciliary muscle adjust the shape of the lens during accommodation for near vision compared to far vision?

For far vision, sympathetic stimulation relaxes the ciliary muscle, pulling suspensory ligaments taut to flatten the lens (decreasing refractive strength). For near vision accommodation, parasympathetic stimulation contracts the ciliary muscle, slackening suspensory ligaments and allowing the lens to become spherical (increasing refractive strength).

What structural change in the eye causes presbyopia?

Presbyopia is caused by age-related stiffness of the lens occurring when central lens cells (lacking nuclei and organelles) die due to poor nutrient access, preventing the lens from becoming spherical during near vision.

What are the three neural cell layers of the retina from outermost to innermost?

1. Rods and cones (outermost photoreceptor layer).
2. Bipolar cells (middle layer).
3. Ganglion cells (innermost layer, whose axons form the optic nerve).

Why does the fovea provide the highest visual acuity in the retina?

Because bipolar and ganglion cells are pulled aside at the fovea, allowing light to strike photoreceptors directly, and the fovea contains exclusively cones.

How do rods and cones differ in sensitivity, acuity, and cellular convergence?

Rods have high sensitivity (specialized for night vision) but low acuity due to high convergence ($> 100$ rods onto one bipolar cell). Cones have lower sensitivity (activated only in bright light) but high acuity due to little convergence, with each cone reporting a very small receptive field.

What are the two structural components of photopigments, and how does retinal change upon absorbing light?

Photopigments consist of an opsin protein and retinal (a Vitamin A derivative). Light absorption converts retinal from its inactive $11\text{-}cis$ conformation to its active $\text{all-}trans$ conformation.

What sequence of molecular events occurs during phototransduction in response to light?

Absorption of light converts retinal to $\text{all-}trans$ -> activates transducin -> activates phosphodiesterase -> breaks down $\text{cGMP}$ -> closes $\text{Na}^+$ channels in outer segment -> hyperpolarizes photoreceptor -> closes voltage-gated $\text{Ca}^{2+}$ channels in synaptic terminal -> decreases neurotransmitter release.

Why are photoreceptors unique among sensory receptors regarding membrane potential at rest?

Photoreceptors are the only sensory receptors in the human body that are depolarized at rest in the dark and become hyperpolarized upon stimulation by light.

What are the three types of cones responsible for color vision and their peak absorption wavelengths?

• Blue cone ($S$-type): peak absorption around $400\text{--}450\,\text{nm}$ • Green cone ($M$-type): peak absorption around $530\,\text{nm}$ • Red cone ($L$-type): peak absorption around $560\,\text{nm}$

Which physical properties of sound waves determine pitch, intensity, and timbre?

• Pitch (tone) depends on sound wave frequency, measured in Hertz ($\text{Hz}$). • Intensity (loudness) depends on sound wave amplitude, measured in Decibels ($\text{dB}$). • Timbre (quality) depends on overtones.

How do auditory ossicles amplify sound energy as it travels from the outer ear to the inner ear?

The ossicles (malleus, incus, stapes) transmit vibrations from the large tympanic membrane to the much smaller oval window, concentrating force per unit area to amplify sound pressure by approximately $20\times$.

What are the three fluid-filled channels of the cochlea?

1. Scala vestibuli (filled with perilymph).
2. Scala media or cochlear duct (filled with endolymph, which has a high $\text{K}^+$ concentration).
3. Scala tympani (filled with perilymph).

How do tip links on stereocilia generate a receptor potential in hair cells of the Organ of Corti?

Bending stereocilia toward the tallest stereocilium stretches tip links, pulling open mechanically gated cation channels to let $\text{K}^+$ enter from endolymph and depolarize the hair cell. Bending away slacks tip links, closing channels and hyperpolarizing the cell.

How does the structure of the basilar membrane allow pitch discrimination?

The narrow, stiff end near the oval window vibrates maximally in response to high-frequency pitches, whereas the wide, flexible end near the helicotrema vibrates maximally in response to low-frequency pitches.

What are the components of the vestibular system and what types of head motion do they detect?

• Semicircular canals: detect angular acceleration and deceleration (rotation) along three perpendicular axes. • Utricle: detects horizontal linear acceleration and static head tilt (vertical hair orientation). • Saccule: detects vertical linear acceleration (horizontal hair orientation).

What structures enclose the sensory hair cells in the semicircular canals and the utricle?

In semicircular canals, hair cells are ensheathed by a gelatinous mass called the cupula inside the ampulla. In the utricle, hair cells are embedded in a gelatinous layer topped with otoliths (calcium carbonate crystals).

How does stereocilia displacement relative to the kinocilium regulate vestibular nerve firing rates?

Bending stereocilia toward the kinocilium depolarizes the hair cell, increasing neurotransmitter release and raising action potential frequency in synapsing vestibular nerve fibers. Bending away hyperpolarizes the hair cell, decreasing neurotransmitter release and reducing action potential frequency.