Sensory Receptors & Sensory Transduction

Basic pathway of sensation

Stimulus is detected by PNS sensory receptors → transmitted by PNS sensory neurons to the CNS → integrated and interpreted by CNS neurons.

Sensory transduction

Conversion of a stimulus into an electrical signal; begins at a sensory receptor.

Sensory receptor

Region of a nerve ending where sensory transduction begins.

Encapsulated nerve ending

Sensory receptor surrounded by specialized supporting cells.

Free nerve ending

Sensory receptor that lacks specialized supporting cells and is essentially “naked.”

Basic mechanism of sensory transduction

A stimulus causes gated ion channels in the axolemma of a sensory nerve ending to open or close, changing ion flow across the membrane.

Sensory transduction and threshold

If changes in ion flow cause the neuron to reach threshold, an action potential is triggered and propagated along the axon toward the spinal cord.

Sensory receptor response

Sensory receptors may respond to stimuli with different speeds, intensities, and durations.

Rapidly adapting receptor

Receptor that responds rapidly and with high intensity but stops sending stimuli after a certain period.

Adaptation

Phenomenon in which a sensory receptor stops or reduces signaling despite continued stimulation.

Slowly adapting receptor

Receptor that responds to a stimulus with constant action potentials that do not diminish with time.

Two ways sensory receptors are classified

Sensory receptors may be classified by their location or by the type of stimulus they detect.

Exteroceptor

Sensory receptor located close to the body's surface that detects stimuli originating outside the body.

Interoceptor

Sensory receptor located within the body's interior that detects stimuli originating from within the body.

Five receptor types based on stimulus

Mechanoreceptors, thermoreceptors, chemoreceptors, photoreceptors, and nociceptors.

Mechanoreceptor

Encapsulated receptor in the skin, musculoskeletal system, and many organs that responds to mechanical deformation such as touch or internal stretch.

Mechanoreceptor ion channels

Mechanoreceptors use mechanically gated ion channels.

Thermoreceptor

Sensory receptor that responds to changes in temperature.

Cold receptors

Thermoreceptors that respond to temperatures from approximately 10°C to 40°C and are located in the superficial dermis.

Hot receptors

Thermoreceptors that respond to temperatures from approximately 32°C to 48°C and are located in the deep dermis.

Chemoreceptor

Sensory receptor that depolarizes in response to chemicals in body fluids or air.

Chemoreceptor mechanism

Chemicals bind to specific receptors and trigger ion channels to open.

Photoreceptor

Sensory receptor found only in the eye that responds to light.

Nociceptor

Sensory receptor that detects noxious stimuli perceived as pain.

Capsaicin

Chemical in hot peppers that opens specific ligand-gated ion channels in nociceptors and triggers action potentials perceived by the CNS as pain.

Capsaicin desensitization

Repeated application causes nociceptors to become desensitized and generate fewer action potentials.

Tactile (Merkel) nerve endings

Mechanoreceptors associated with Merkel cells in the floors of epidermal ridges; detect fine discriminative touch such as form and texture.

Location of Merkel nerve endings

Located in the floors of epidermal ridges and are especially numerous in the fingertips.

Merkel nerve ending sensitivity

Have the finest spatial resolution of all skin mechanoreceptors.

Tactile (Meissner) corpuscles

Mechanoreceptors located in dermal papillae that detect discriminative touch, although not as finely as Merkel nerve endings.

Bulbous corpuscles (Ruffini endings)

Mechanoreceptors in the dermis, hypodermis, and ligaments that respond to stretch and movement.

Lamellated corpuscles (Pacinian corpuscles)

Deep dermal mechanoreceptors with a layered, onion-like appearance that respond to high-frequency vibration and deep pressure.

Pacinian corpuscle stimuli

Activated by high-frequency vibratory stimuli and deep pressure.

Hair follicle receptors

Free nerve endings wrapped around the base of hair follicles in the dermis or hypodermis that respond when a hair bends.

Proprioceptors

Mechanoreceptors in the musculoskeletal system that detect movement and the position of a joint or body part.

Proprioception

Sense of the movement and position of joints and body parts detected by proprioceptors.

Mechanoreceptors for fine touch

Merkel nerve endings and Meissner corpuscles detect discriminative touch.

Mechanoreceptors for stretch and movement

Ruffini endings respond to stretch and movement.

Mechanoreceptors for vibration and deep pressure

Pacinian corpuscles respond to high-frequency vibration and deep pressure.

Mechanoreceptors associated with hair

Hair follicle receptors respond when hairs bend.

Mechanoreceptors for body position

Proprioceptors detect movement and position of joints and body parts.