Sensory Receptors Flashcards
General Mechanism of Sensation
Sensory receptors transmit “information” in a series of action potentials
Differential sensitivities of sensory receptors allows for the CNS to “decode” the action potentials into “sensations”
Each receptor is highly sensitive to one type of stimulus (adequate stimulus)
Labeled Line principle
Each type of sensation is projected to a specific area of the CNS
Allows electrical impulses to be decoded into perceived sensation
Somatosensory Neuron
Cell body resides in the DRG
Peripheral and central processes
Peripheral process is specialized for specific stimuli
AP propagates from the periphery to the central process, then to a synapse with a CNS neuron (spinal cord, brainstem)
The frequency of APs is dependent on the stimulus intensity
Somatosensory Receptors
Specialized endings of primary afferent neurons
Send signals from periphery to CNS
Somatosensory Receptors occur where?
throughout the body
Skin, viscera, muscles, joints, connective tissue
Somatic sensation includes
Touch, pressure, vibration, body position, tickle, temperature, and pain (and itch)
Receptors transform energy into AP by stimulating
mechanical, chemical, and/or thermal energy into electrical energy (e.g., action potential)
Mechanoreceptors
detect mechanical deformation (compression or stretch)
Touch, pressure (sustained touch), stretch or vibration
Thermoreceptors
detect heat and cold
specific receptor for each thermal modality
Nociceptors
detect tissue damage (chemical or physical)
Chemoreceptors
detect chemical environment of body systems
ex. blood oxygen levels, bodily fluid concentrations
Electromagnetic receptors
retinal (light) receptors
Discriminative Touch
Discriminate between the quality of touch (soft or coarse) and which area is being touched (2pt discrimination)
Pressure and VIbration
Pressure is sustained touch
Vibration – rapidly alternating movements/pressure
Crude Touch
(light touch, non-discriminative touch)
Indicates whether or not area is being touched and the strength of the touch
Superficial receptors
Meissner’s corpuscles–light touch, vibration
Merkel’s disks-pressure
Hair follicle receptors-hair displacement
Subcutaneous Receptors
Pacinian corpuscle-light touch, pressure, vibration
Ruffini’s endings-stretch of skin
Free nerve endings-poorly localized touch, pressure, tickle, itch
Also act as nociceptors
Meissner’s Corpuscles
Encapsulated, located in the ridges of superficial glaborous skin
Highly concentrated in the finger tips
Sensitive to:
Light touch, vibration and discriminative touch
Rapidly adapting
Merkel’s Disks
Un-encapsulated; located in the superficial skin (hairy and glaborous)
Groups of 4-6 disks innervated by one fiber
Concentrated in the finger tips
Sensitive to discriminative touch
Slowly Adapting
Pacinian Corpuscles
Encapsulated; located within:
Dermis, subcutaneous fat, intramuscular connective tissue, and capsules of synovial joints
Detects pressure, vibration, acceleration
Important function as joint motion receptors
Rapidly adapting
Rufinni’s Endings
Encapsulated, flower-spray neural endings
Dermis, joints, glabrous and hairy skin
Detects light touch Also warm (thermo) receptors
Slowly Adapting
Hair Follicle: Hair end organ
Nerve endings embedded in hair follicle, surrounding hair shaft
Deflection of hair deforms follicle, creating a stimulus for the receptor
Light (crude) touch information
Consist of rapidly and slowly adapting types
Thermoreceptors
Cold and warm receptors are most responsive to sudden changes in temperature:
Thermoreceptors are “sensitive” to the effect of temperature on their cell metabolism
Altered metabolism of the receptor serves as stimulus for it to fire action potentials
Pain receptors also are stimulated by extremes of heat/cold
Thermoreceptors for cold
Krause’s bulb
slow adapting
Thermoreceptors for heat
Ruffini’s endings
slow adapting
Nociceptors
Stimulated by mechanical, thermal or chemical stimuli (if stimulus is related to tissue damage)
Chemical substances that stimulate nociceptors
Bradykinin, serotonin, histamine, K+ and H+ ions (acids), acetylcholine, other inflammatory molecules
What increases sensitivity to pain
Prostaglandins and substance P increase sensitivity to pain (sensitize free nerve endings)
Rate and extent of tissue damage is directly correlated with the level of pain perceived
Hyperalgesia
increased sensitivity to painful stimuli
Free Nerve Endings
No specialized receptor “ending”
Responsive to mechanical, chemical, and thermal stimuli
Respond to thermal changes, light touch and chemicals related to tissue damage
Slowly Adapting
Some do not adapt at all, continue firing as long as tissue insult remains
Common cause of tissue damage
Bacterial infection, tissue ischemia, tissue contusion
Fast Pain
Elicited by mechanical and thermal stimuli
Transmitted on A-delta fibers
Sharp, easily localized pain
Slow Pain
Elicited by chemical stimuli and persistent thermal and mechanical stimuli
Transmitted by unmyelinated C-fibers; dull aching type pain
Receptor potential
Stimulus causes a change in the membrane electrical potential of a sensory receptor
They causes a change in ion permeability of the receptor membrane and results in depolarization of receptor, If membrane depolarization reaches threshold, then APs are generated
Freq of APs increase
as depolarizing current increases
Stronger stimulus results in increased AP frequency
Receptor continues to send action potentials as long as stimulus is present (continued depolarization)
Receptor Adaptation
Diminishing rate of discharge of a somatosensory neuron occurs with continued stimulation of constant intensity (adaptation)
Adaptation allows the nervous system to focus on new or altered stimuli without the distraction of the constant stimulus
Adaptation takes place over a range of durations
Somatosensory receptors are either slowly-adapting or rapidly-adapting (some do not adapt at all)
Adaptation Rate: Rapid
Rapidly adapting: (phasic receptors)
Receptor responds transiently at the onset of the stimulus and at the end of or a change in the stimulus
e.g.,Pacinian corpuscles – a type of mechanoreceptor
Adaptation Rate: Slow
Slowly Adaptating: (tonic receptors)
Continued presence of stimulus is signaled by a persistent response from the receptor
Slow adapting examples
Examples:
Nociceptors, Merkel’s receptors – a mechanoreceptor
Chemoreceptors of CV system (blood content measures)
Muscle spindles, Golgi tendon organ
Influential factors for Adaptation Rate
Properties of excitability of the membrane of the sensory neuron
The non-neuronal accessory structure that surrounds the axon
Referred Pain
Pain felt in part of the body that is remote from the tissue causing the pain
Most frequently occurs with pain (injury) in visceral organs (abdomen/thorax)
Visceral Pain fibers and skin pain fibers
terminate at the same areas of spinal cord
Thus the CNS perceives the pain as coming from the skin rather than abdominal/thoracic organs
