Lecture 11: Sensory Physiology Flashcards
Sensory Aα What is its classification of afferent fibers? Is the fiber diameter large of small? What is its conduction velocity? What receptors does it supply?
Ia and Ib large 80-120 m/s (fast) Primary muscle spindles and Golgi tendon organs
Sensory Aβ What is its classification of afferent fibers? What receptors does it supply?
II Secondary muscle spindles, skin mechanoreceptors
Sensory Aδ What is its classification of afferent fibers? What receptors does it supply?
III Skin mechanoreceptors, thermal receptors, and nociceptors
Sensory C What is its classification of afferent fibers? Is the fiber diameter large of small? What is its conduction velocity? What receptors does it supply?
IV small 0.5-2 m/s (slow) Skin mechanoreceptors, thermal receptors, and nociceptors
Motor Aα What receptors does it supply?
Extrafusal skeletal muscle fibers
Motor A(gamma) What receptors does it supply?
Intrafusal skeletal muscle fibers
Motor B What receptors does it supply?
Preganglionic autonomic fibers
Motor C What receptors does it supply?
Postganglionic autonomic fibers
What are rapidly adapting receptors
Receptors that respond only at the beginning or end of a stimulus (only active when stimulus intensity increases or decreases)
What are Slowly adapting receptors
Receptors that respond to prolonged and constant stimulation
What does Meissner corpuscle sense?
Is it RA or SA?
Touch and vibration; flutter and tapping <100 Hz
RA
Where is Meissner corpuscle located?
Glaborous skin
What does Pacinian corpuscle sense?
RA or SA
High frequency vibration
Very RA
Where is Pacinian corpuscle located?
Both hairy and glaborous skin
What does Ruffini corpuscle sense?
RA or SA
Magnitude and direction of stretch Touch, pressure, and proprioception
SA
Where is Ruffini corpuscle located?
Both hairy and glaborous skin
What does a Merkel cell sense?
RA or SA
Pressure
SA
Where is a Merkel cell corpuscle located?
Glaborous skin
What does a hair follicle receptor sense?
RA or SA
Motion across skin and directionality of that motion
RA & SA
What does a tactile free-nerve ending sense?
RA or SA
Pain and temperature
SA
What is tactile acuity
Ability to sense small or large differences in stimuli
What is a receptive field
The range of which a receptor is active
What is two point discrimination?
Ability to distinguish between two stimuli applied at a close distance. It is determined by measuring the minimum distance between the two stimuli
Where is tactile acuity the highest? Are receptive fields small or large?
Fingertips and lips
Small
Where is tactile acuity the lowest? Are receptive fields small or large?
Calf, back, thigh
Large
Where is Somatosensory area 1 located?
Post central gyus
What is the function of Somatosensory area 1?
Integration of information for position sense, size, and shape discrimination
Where is Somatosensory area 2 located?
Wall of Sylvian Fissure (under lip of S1) Receives input from S1
What is the function of Somatosensory area 2?
Responsible for comparisons between objects, different tactile sensation, and deciding what becomes a memory
What is the function of parieto-temporal-occipital association area?
High level interpretation of sensory inputs. Receives input from multiple sensory areas.
What is the law of projection?
No matter where along the afferent pathway a stimulation is applied, the perceived sensation arises from the origin of the sensation.
What phenomenon can the law of projection explain?
Phantom limb
What is pain?
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
What is nociception?
The neural process of encoding noxious stimuli
What is hypersensitivity?
Increased responsiveness of nociceptive neurons to their normal input, and/or recruitment of a response to normally subthreshold inputs
(excessive sensitivity to something that shouldn’t be that sensitive)
What is hyperaesthesia?
Increased sensitivity to stimulation, excluding the special senses
(increased sensitivity to a stimulation)
What is hyperalgesia?
Increased pain from a stimulus that normally provokes pain
(more pain than there should be)
What is allodynia?
Pain due to a stimulus that does not normally provoke pain. Classic example is the lay of sheets on skin that has been sunburned.
(Pain when there shouldn’t be pain)
What are C-fibers
Very thin
slowly conducting
unmyelinated neurons
Large receptive field
produce a throbbing dull pain sensation
Imprecise location of pain
What are A delta fibers
Fast conducting
myelinated
small receptive field
precise localization of pain
Sharp pain
What are mechanical nociceptors?
Response to mechanical forces ranging from moderate pressure with a blunt object to overtly tissue-damaging stimuli
What are chemical nociceptors?
Response to endogenous or exogenous chemical compounds, such as pro-inflammatory mediators, acids, or capsaicin
What are thermal nociceptors?
Response to noxious heat and cold will directly activate thermal receptors expressed by nociceptors
What is the function of the transient receptor potential (TRP) family of receptor ion channels?
Sense a broad range of changing environments in the body tissue: pH, inflammatory mediators, heat, cold, and exogenous chemicals.
When active are permeable to Ca2+, Na+ & K+
What activates TRPV1
Capsaicin (hot pepper)
bradykinin
intense heat
What pain conditions is TRPV1 involved with?
Migraine
dental pain
cancer pain
inflammatory pain
visceral pain
osteoarthritis
What activates TRPA1
Allyl isothiocyanate in mustard oil, wasabi and horseradish
non chilly pepper heat
What pain conditions is TRPA1 involved with?
Dermatitis
Chronic itch
Painful bladder syndrome
Migraine
Irritable bowel syndrome
Panreatitis
What activates TRMP8
Cold temperatures and agents (menthols)
What happens where there are no pain inputs to the C fibers?
Inhibitory interneurons suppress pain pathway
What happens when there are pain inputs to the C fiber?
C fibers stop the inhibitory interneurons and send a strong signal to brain
How can pain be modulated as proposed by the gate control theory?
- Aβ fiber is activated and travels to dorsal horn and synapses on an inhibitory neuron, releasing an EAA.
- Released glycine inhibits secondary sensory neuron on nociceptive pathway.
- Rubbing an area of affected skin actiaves the Aβ fiber and reduces the sensation of pain
How are pain signals reduced via descending inhibition?
- Activation of Periaqueductal grey matter by opiates, EAA & cannabinoids leads to fibers going to Locus Coeruleus (NE) and Raphe nucleus (serotonin)
- NE and serotonin released into dorsal horn and activate inhibitory interneurons
- Local inhibitory interneurons release opiates and activate mu receptors on pre-synaptic terminals of C-fibers, reducing nociception
What does the theory of central sensitization say?
- Level of spinal cord, higher brain areas, or both
- Reduces threshold of involved neurons to noxious stimuli
- Synaptic Plasticity: Persistent stimulation of NMDA receptors and intracellular signaling cascades
- Central Inflammation
What does the theory of peripheral sensitization say?
- Inflammation can make nociceptors in injured tissues more sensitive (e.g. Prostaglandins and bradykinin)
- Body more sensitive to thermal changes, activating TRPV1
What role does the primary sensory cortex play when it comes to the sensory system?
Identifies size and shape of an object
What role does the secondary sensory cortex play when it comes to the sensory system?
Cognitive interpretation of crude senses & compares objects by touch alone.
What role do the primary and secondary sensory cortices play when it comes to pain?
Receive input from nociceptors and play a role in localization of pain
What role does the insular cortex play when it comes to pain?
Interpretation of pain
Integrates all signals related to pain
What role does the amygdala play when it comes to pain?
Emotional component of pain
What role does the hypothalamus/medulla play when it comes to pain?
Integrate physiologicak change associated with visceral pain
Free nerve endings
Peripheral terminals of nociceptor axons that lack specialzed receptor cells or encapsulations. Can either be peptidergic or non-peptidergic.
Peptidergic Nociceptors
Express neuropeptides: Substance P and CGRP
Sense chronic inflammation & visceral pain
What up-regulates neuropeptides?
Chronic inflammation
Non-peptidergic Nociceptors
Does not express CGRP or Substance P
Involved in somatic chronic pain states such as diabetic neuropathy
