Introduction to Sensory Receptors and Sensory Neurones The Somatosensory System Flashcards
What are the sensory modalities?
- Fine discriminatory touch [Light touch, pressure, vibration, flutter and stretch (mechanosensation)]
- Joint and muscle position sense (proprioception)
- Temperature (thermosensation)
- Pain (nociception)
- Itch (pruriception)
What are the broad categories of sensory modalities?
- Exteroceptive division (cutaenous senses) registers information from the surface of the body by numerous receptor types
- Proptioceptive division monitors posture and movement (sensors in muscle, tendons and joints)
- Enteroceptive division reports upon the internal state of the body and is closely related to autonomic function
How is sensory stimulus converted to electrical activity?
Stimulus opens cation selective ion channels in peripheral terminal of primary sensory afferent eliciting a depolarising receptor potential
Action potentials ariving at the central terminal cause the graded release of ___________ on to ______ order neurones
Action potentials ariving at the central terminal cause the graded release of neurotransmitter on to second order neurones
What is adaptation rate?
Does the sensory unit discharge action potentials continuously during the stimulus, or does it respond preferentially to a changing stimulus
What is the sensory unit for touch, pressure, vibration?
Skin mechanoreceptors
What is the sensory unit for proprioception?
Joint and muscle mechanoreceptors
What is the sensory unit for temperature?
Cold and warm thermoceptors
What is the sensory unit for pain?
Mechanical, thermal and polymodal nociceptors
What is the sensory unit for itch?
Itch receptors
What is the role of low threshold units?
To respond to low-intensity (non-damaging) stimuli
- low threshold mechanoreceptors: mediate fine discriminatory touch
- low threshold thermoreceptors: mediate cold through to hot
What is the role of high threshold units?
Respond to high (noxious, potentially damaging), but not (normally) low, intensity stimuli
- high threshold mechanoreceptors: respond to high intensity mechanical stimuli
- thermal nociceptors: respond to extreme degrees of heat (>45 degrees celcius)
- chemical nociceptors: respond to substances in tissue [e.g. as in inflammmation: prostaglandins, bradykinin, serotonin (5-HT), histamine, K, H and ATP)
- polymodal nociceptors resopond to at least two of the above
Where are slowly adapting sensory units found?
What information do they provide?
Stretch receptors
Information about position, degree of stretch, or force
Where are fast adapting sensory units found?
What information do they convey?
Some muscle spindle afferents, hair follicle afferents
Detects changes in stimulus strength (range of movement) number of impulses proportional to rate of change of stimulus.
Where are very fast adapting sensory units found?
What information do they convey?
Pacinian corpuscle
Responds to very fast movement, such as rapid vibration
What do Group I, Aα axons conduct? How fast are they and are they myelinated?
From proprioceptors of skeletal muscle
80-120ms-1
Thick myelin
What do Group II, Ab axons conduct? How fast are they and are they myelinated?
Mechanoreceptors of skin
35-75ms-1
Moderately myelinated
What do Group III, Aδ axons conduct? How fast are they and are they myelinated?
Pain, temperature
5-30ms-1
Thinly myelinated
What do Group IV, C axons conduct? How fast are they and are they myelinated?
Temperature, pain, itch
0.5-2.0ms-1
Not myelinated
The peripheral teminal of cutaenous afferent fibres branch into many fine processes, the tips of these can be;
Free nerve endings (partially naked)
Associated with specialised structures
Receptive field is inversely related to ________ _______
Receptive field is inversely related to innervation density
How does sensory acuity correlate to receptive field size?
Inverse relationship
How is two-point discrimination tested?
By applying simultaenously two sharp point stimuli, separated by a variable distance, at different sites on the body surface
Regions with the highest discriminative capacity have the _______ receptive fields?
Regions with the highest discriminative capacity have the smallest receptive fields?
What should two-point discrimination be on the finger tip, palm and forearm?
Fingertip - 2mm
Palm- 10mm
Forearm- 40mm
What are meissner’s corpuscles?
Found in locations where two point discrimination is highest, not present in hairy skin.
Where are merkel’s discs found?
In the same places as meissner’s corpuscles but also present in hairy skin
What are ruffini endings?
Responsible for detecting pressure, found in dermis and joint capsules
What are pacinian corpuscles?
Responsible for detecting pressure, found in dermis and fascia
What are the subdivisions of skin low threshold mechanoreceptors?
FA1
FA2
SA1
SA2
Which physiological sub-class do;
- merkel cell-neurite complexes
- meissner corpuscles
- ruffini endings
- pacinian corpuscles
belong to?
SA1
FA1
SA2
FA2
Spinal cord consists of ____ (cell bodies and sensory afferent terminals) and _____ (fibre tracts) matter the relative proportions of which vary along its length
____ matter subdivided into ______ (posterior) and ______ (anterior) horns and ten distinct laminae of Rexed
Spinal cord consists of grey (cell bodies and sensory afferent terminals) and white (fibre tracts) matter the relative proportions of which vary along its length
Grey matter subdivided into dorsal (posterior) and ventral (anterior) horns and ten distinct laminae of Rexed
Where are nociceptors located in the dorsal horn?
Laminae I and II
Where are LTMs located in the dorsal horn?
Laminae III to VI
Where are proprioceptors located in the dorsal horn?
Laminae VII to IX
