3. Somatosensory System Flashcards
Subdivisions of somatosensory system
Proprioceptive
Enteroceptive
Exteroceptive
Exteroceptive division
Division is responsible for providing information about contact of the skin with objects in the external world, and a variety of cutaneous mechanoceptive, nociceptive (pain), and thermal receptors
Enteroceptive
Division has receptors for monitoring the internal state of the body and includes mechanoreceptors that detect distention of the gut or fullness of the bladder.
Proprioceptive
Division provides information about body and limb position and movement and relies primarily on receptors found in the joints, muscles, and tendons.
Mechanoreceptors
Touch, pressure, vibration and stretch
Location: skin, muscle, tendons, blood vessels, inner ear
Thermoreceptors
Cold, warm
Skin
Nociceptors
Stimuli causing tissue damage
Skin
Why are sensations of temperature and pain often grouped together
The sensations of pain and temperature are related and often grouped together because they are mediated by overlapping sets of receptors, share some common molecular mechanisms, and are conveyed by the same types of fibers in the PNS and the same pathways in the CNS.
Types of nerve fibers in somatosensory system
Group I/II. A-alpha and A-beta fibers
Group III. A-delta fiber
Group IV. C fiber
Group I/II. A-alpha fibers and A-beta fibres
Myelinated
Large diameter
Proprioception, light touch
Group III. A-delta fibers
Lightly myelinated
Medium diameter
Nociception
(mechanical, thermal, chemical)
Group IV. C fiber
Unmyelinated Small diamater Innocuous temeprature, itch Nociception (mechanical, thermal, chemical)
Types of mechanoreceptors
Pacinian corpuscle Meissner's corpuscle Hair follicles Ruffini's corpuscle Merkel's receptors Tactile discs
Pacinian corpuscle: location, adaptation, sensation encoded
Hairy/nonhairy skin; intramuscular
Very rapidly
Vibration, tapping
Meissner’s corpuscle: location, adaptation, sensation encoded
Nonhairy skin
Rapidly
Point discrimination, tapping, flutter
Hair follicles: location, adaptation, sensation encoded
Hairy skin
Rapidly
Velocity, direction of movement
Ruffini’s corpuscle: location, adaptation, sensation encoded
Hairy/nonhairy skin
Slowly
Strech, joint rotation
Merkel’s receptors
Nonhairy skin
Slowly
Vertical indentation of skin, light touch
Tactile discs
Hairy skin
Slowly
Vertical indentation of skin
Thermoreceptors
Respond slowly to temperature change
Slowly adapting receptors
Two types: warm receptors and cold receptors
What receptors are activated by extreme heat or cold?
Nociceptors not thermoreceptors
What channels are associated with thermosensation
TRP channels, which change to allow ions to flow through. They are associated with thermosensations. Humans have 6.
Why does spicy food make you feel hot? Why does mint make you feel cool?
Capsaicin in spicy food activated TRPV1; methol in mint activates TRPM8.
Do cold and warm receptors overlap
Yes, they have large windows
Describe structure of TRP channels
6 TM domains
A hole channel between 5th and 6th subunit, which triggers AP
6 subfamilies of thermoreceptors
TRPV4 TRPV3 TRPV2 TRPV1 TRPM8 TRP A1 Each activated at different temperatures. React to light and mechanical stimuli, chemicals and natural compounds.
Nociceptors
Respond to noxious stimuli (mechanical, thermal, and chemical) that cause tissue damage.
Two classes of nociceptors
- Thermal or mechanical
2. Polymodal
Thermal or mechanical nociceptors
Fast conducted A-delta nerve fibers; respond to mechanical or thermal stimuli such as sharp, pricking pain
Polymodeal nociceptors
Slow-conducting C fibers; respond to high-intensity mechanical, chemical, and thermal stimuli
Two phases of pain
- Fast/first pain
2. Slow/second pain
Hyperalgesia
An amplified response to a noxious stimulus (sensitization of pain)
Ex: hot water sensitizes pain caused by a paper cut
Allodynia
A painful response to a normally innocuous stimulus (elicitation of painful sensations by stimuli that do not normally provoke pain)
Ex: wearing a shirt (innocuous) can be painful when skin is sensitized by sunburn
Inflammatory soup
Caused by tissue damage: Peptides (bradykinin), lipids (prostaglandins), neurotransmiters (serotonin 5-HT, substance P, and ATP), H+, cacitonin, gener-related protein (CGRP) and neurotrophins (NGF).
Thermal nociceptors
Coldness: TRPM8, TRPA1
Extreme heat: TRPV1 (TRPV2(
Chemical nociceptors
TRPV1 (capsaicin, H+, toxins)
TRPA1 (Acrolein)
ASICs (H+)
Mechanical nociceptors
DEG/ENaC (degenerin/epithelial Na+ channel) family, TRPA1, TRPV4
Location of somatosensory innervation of the body
Primary sensory neurons located in the dorsal root ganglion
Location of somatosensory innervation of the face
Primary sensory neurons located in the trigerminal ganglion
A common disease that illustrates the dermatomal organization of the dorsal roots
Shingles (herpes zoster virus): the rash is confined to one side of the body in a dermatomal distribution
No. of spinal nerves
31
How many of the spinal nerves are dermatomes
Only 30, C1 nerve is not dermatome (sends directly to the brain not the spinal cord)
Dermatome
An area of the skin supplied by nerves from a single spinal root
Two somatosensory pathways
- Dorsal column-medial lemniscus system
2. Anterolateral (spinothalamic) system
Dorsal column-medial lemniscus system
fine touch, pressure, vibration, proprioception, and two-point discrimination
Anterolateral (spinothalamic) system
pain, temperature, and crude touch
Why is decussation level clinically important
Because damage to the pathway below this level will produce loss of sensation on the same, or ipsilateral, side of the lesion, whereas lesions about this level will produce contralateral deficits
Order of neurons in somatosensory pathways
1st order neuron: primary afferent neuron
2nd order neuron: in the spinal cord (anterolateral system) or in the brain stem (dorsal column system); decussation.
3rd order neuron: in the somatosensory nuclei of the thalamus
4th order neuron: in the somatosensory cortex
Dorsal Column-Medial Lemniscus Pathway - order
1st order neuron: large myelinated axons (Group I/II)
2nd order neuron: in the nucleus gracilis and the nucleus cuneatus (Medulla of the brain stem); decussation
3rd order neuron: in the ventral posterior lateral (VPL) nucleus of the thalamus
4th order neuron: in the somatosensory cortex
Dorsal Column-Medial Lemniscus Pathway - sensations
fine-touch, pressure, two-point discrimination, and vibratory sensations
Graphesthesia
The ability to recognize letters or numbers traced on the skin
Anterolateral/Spinothalamic Pathways - order
1st order neuron: small lightly myelinated and unmyelinated axons (Group III/IV)
2nd order neuron: in the spinal cord; decussation
3rd order neuron: in the ventral posteroinferior (VPI) nucleus of the thalamus
4th order neuron: in the somatosensory cortex
Anterolateral/Spinothalamic Pathways - sensations
pain, temperature and crude touch
What pathway is damaged if you lose the ability to identify objects by their shape and texture but not pain and temperature sensation?
The dorsal column pathway
Structural and functional difference between the two pathways?
Type of nerve fibers; site of decussation; sensations processed.
Somatosensory homunculus
Homunculus represents each part of the body in proportion to its number of sensory neural connections but not its actual size.
Pain
an unpleasant feeling caused by noxious stimuli; a protective mechanism
Type of nerve fibers that supply nociceptors
A-delta fibres
C fibres
Fast/first pain
Transmitted via faster Adelta fibers
Rapid onset and offset
Precisely localized (e.g. pin prick)
Serves as a warning signal
Slow/second pain
Transmitted via slower C fibers
More prolonged and less intense
Poorly localized (e.g. burn)
Involves a more complex emotional process
Site of Nociceptive pain
PNS only
Site of inflammatory pain
PNS and CNS
Site of neuropathic pain
PNS and CNS
Classes of pain that involve TRP channels
Nociceptive and inflammatory, maybe neuropathic
Classes of pain that involve nociceptors AND non-nociceptors for sensing pain
Inflammatory and neuropathic
Clinical settings of classes of pain
Nociceptive: acute trauma
Inflammatory: post-operative, arthritis, etc
Neuropathic: PNS and CNS lesions, diabetic neuropathy, spinal cord injury
Function of classes of pain
Nociceptive = protective Inflammatory = healing/repair Neuropathic = pathological
What type of pain for trigeminal neuralgia that is caused by mechanical damage to the trigeminal ganglion by an artery? How to treat it?
Neuropathic pain; surgical displacement of the artery.
What is the gate control theory of pain?
Innocuous stimuli are able to suppress pain.
What is the mechanism of the gate control theory of pain?
Mechanism: activation of inhibitory interneurons by innocuous stimuli applied to Aβ fibres results in an inhibition of pain signals transmitted via C fibres to the CNS.
Referred pain
Referred pain: used for the sensation of pain that is experienced at a site other than the injured tissue
Visceral origin
Example: ischemic heart pain is referred to the left arm
Dermatomal rule
Pain is referred to a structure, which is developed from the same dermatome from which the pain producing structure is developed.
Example:
The ischemic heart pain is referred to left arm because the heart and inner aspect of left arm originate from the same dermatome.
Why morphine can relieve pain? How to treat morphine overdose?
Morphine activates the endogenous analgesia system. Naloxone can reverses morphine’s effect.
Endogenous analgesia system
The centrifugal control of somatosensation.
Opioids (enkephalin, endorphins, dynorphin)
Drugs: naloxone (antagonist), morphine (agonist)
Serotonin/catecholamines
Role of serotonin/catecholamine in centrifugal control of somatosensation
Can inhibit the interneurons or interact with the opiods system
