Sensory System (1) Exam 2

Sensory System overview for Exam 2 PCB3703C Ahangari

Sensory System Function

Specialized epithelial cells or neurons that transduce environmental signals (mechanical force, light, sound, chemicals, temperature) into neural signals.

Mechanoreceptors

Respond to mechanical stimuli such as pressure, stretch, vibration; include Pacinian corpuscles, joint receptors, muscle stretch receptors, hair cells, and baroreceptors.

Photoreceptors

Rods detect brightness; cones detect color (wavelength).

Chemoreceptors

Detect chemical stimuli including taste, smell, osmoreceptors, and carotid body O₂ receptors.

Nociceptors

Detect pain and extreme temperatures; neurotransmitter is substance P.

A-alpha Fibers

Large alpha-motoneurons; fastest conduction velocity.

A-beta Fibers

Touch and pressure; medium conduction velocity.

A-gamma Fibers

Motor to muscle spindles; medium conduction velocity.

A-delta Fibers

Fast pain, temperature, touch; medium conduction velocity.

B Fibers

Preganglionic autonomic fibers; medium conduction velocity.

C Fibers

Slow pain and postganglionic autonomic fibers; slowest conduction velocity.

Receptive Field

Area of the body that changes a sensory neuron's firing rate when stimulated; excitatory increases firing, inhibitory decreases firing.

Sensory Transduction

Stimulus opens ion channels → inward current → depolarization → receptor (generator) potential.

Slowly Adapting Receptors

Tonic receptors that respond throughout a prolonged stimulus (muscle spindle, pressure, slow pain).

Rapidly Adapting Receptors

Phasic receptors that respond at onset and decline with constant stimulus (Pacinian corpuscles, light touch).

First-Order Neurons

Primary afferent neurons; cell bodies in dorsal root ganglia; bring sensory info to CNS.

Second-Order Neurons

Located in spinal cord/brainstem; receive input, cross midline, ascend to thalamus.

Third-Order Neurons

Located in thalamus; project to sensory cortex.

Fourth-Order Neurons

Located in sensory cortex; produce conscious perception.

Dorsal Column System

Processes fine touch, pressure, vibration, two-point discrimination; ascends ipsilaterally, crosses in medulla.

Anterolateral System

Processes pain, temperature, light touch; crosses immediately in spinal cord.

Thalamus

Organizes sensory info somatotopically; receives contralateral input; lesions cause contralateral sensory loss.

Fast Pain

Sharp, localized pain carried by A-delta fibers.

Slow Pain

Dull, aching, throbbing pain carried by C fibers.

Referred Pain

Visceral pain perceived on skin areas sharing the same spinal segment.

Two-Point Touch Threshold

Two points felt separately only if they activate different receptive fields.

Taste Pathway

Taste buds → CN VII, IX, X → medulla → thalamus → cortex.

Salt Taste

Na⁺ enters receptor → depolarization → Ca²⁺ influx → neurotransmitter release.

Sour Taste

H⁺ ions enter receptor → depolarization.

Sweet Taste

GPCR-mediated signaling.

Bitter Taste

GPCR-mediated signaling involving gustducin.

Ageusia

Loss of taste, often due to facial nerve damage.

Hypogeusia

Decreased taste sensitivity.

Hypergeusia

Increased taste sensitivity.

Glossodynia

Burning sensation of the tongue.

Geographic Tongue

Inflamed patches with white borders; cause unknown.

Olfactory Receptors

True neurons in olfactory epithelium; axons are unmyelinated C fibers.

Olfactory Transduction

Odorant → receptor → Golf → ↑ cAMP → Na⁺ channels open → depolarization → AP.

Cribriform Plate Damage

Causes hyposmia or anosmia; ammonia response intact via CN V.

Anosmia

Loss of smell.

Phantosmia

Smelling odors not present.

Dysosmia

Distorted smell perception, common in pregnancy.