Lecture 2. Cutaneous Sensation: Touch and Pain Flashcards
Define the Somatosensory System
A network of neurons that help humans recognize objects, discriminate textures, generate sensory-motor feedback, and exchange social cues
How are cutaneous sensory fibers similar to other sensory fibers?
- Some are encapsulated with specialized endings
- Some are free nerve ending
List the (3) types of receptors of the Somatosensory System
- Mechanoreceptors
- Chemoreceptors
- Nociceptors
Most receptors of the somatosensory system are what kind of receptors?
mechanoreceptors
Chemoreceptors respond to what?
acidic substances
Nociceptors respond to what?
potential damaging stimuli
List the different mechanoreceptors (5) in the skin
- Merkel cell (disks)
- Meissner corpuscle
- Ruffini corpuscle
- Pacinian corpuscle
- Free nerve ending
Mechanoreceptor
Meissner corpuscle
- Skin Layer
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Skin Layer: Superficial layer
- Fiber Type: Aβ
- Role in Perception: Flutter, motion, more general touch
- Submodality: SA1
- Conduction Velocity: 42-72 ms-1
Mechanoreceptor
Merkels Disks (Cell)
- Skin Layer
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Skin Layer: Superficial
- Fiber Type: Aβ
- Role in Perception: Pressure, form, texture, identification of “edges and stripes”
- Submodality: SA1
- Conduction Velocity: 42-72 ms-1
Mechanoreceptor
Ruffini Corpuscle
- Skin Layer
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Skin Layer: Deeper Layer
- Fiber Type: Aβ
- Role in Perception: Stretch
- Submodality: SAII
- Conduction Velocity: 42-72 ms-1
Mechanoreceptor
Pacinian Corpuscle
- Skin Layer
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Skin Layer: Deeper Layer
- Fiber Type: Aβ
- Role in Perception: Vibration and deep pressure
- Submodality: PC
- Conduction Velocity: 42-72 ms-1
Mechanoreceptor
Free nerve ending
- Skin Layer
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Skin Layer: Superficial Layer
- Fiber Type: C
- Role in Perception: Pressure
- Submodality: RA
- Conduction Velocity: 0.5-1.2 ms-1
Mechanoreceptors are mostly what type of fiber?
Aβ
RA (nerve ending) is the only one that is not β
Thermoreceptors (Warm)
Free nerve ending
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Fiber Type: C
- Role in Perception: Warmth
- Submodality: Warm
- Conduction Velocity: 0.5-1.2 ms-1
Thermorecptors (Cold)
Free nerve ending
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Fiber Type: A𝜎
- Role in Perception: Cold
- Submodality: Cold
- Conduction Velocity: 12-36 ms-1
Nociceptors (Acute pain)
Free nerve ending
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Fiber Type: A𝜎 (acute pain)
- Role in Perception: sharp,localized pain
- Submodality: Small, myelinated
- Conduction Velocity: 12-36 ms-1
Nociceptors (Dull pain)
Free nerve ending
- Fiber Type
- Role in Perception
- Submodality
- Conduction Velocity
- Fiber Type: C (dull pain)
- Role in Perception: Burning
- Submodality: unmyelinated
- Conduction Velocity: 0.5-1.2 ms-1
What mechanoreceptors do burn patients often have issues with and why?
Meissner corpuscles and Merkel’s disk b/c the receptor’s proximity to the surface of the skin, burn pts are more likely to lose light touch and identification of objects by touch
What are Receptive Fields?
An area of skin that when stimulated with an adequate stimulus, will cause a specific neuron to alter its activity
Explain important characteristics (3) of receptive fields
- May differ in size
- Response characteristics differ with the type of receptor
- Change with location on the skin and the site in the neuro-axis of the neuron being examined
Define an Adequate Stimulus
A stimulus to which a receptor is most sensitive
Define Two-Point Discrimination
- The ability to discern that two nearby objects touching the skin are truly two distinct points, not one
Explain the relationship between the size of the receptive field and two-point discrimination
The smaller the receptive field the more we are able to distinguish between sensations
Greater two-point discrimination means what type of index?
smaller discrimination index
Lesser two-point discrimination means what type of index?
higher discrimination index
Where on the body would have a higher discrimination index/lesser two-point discrimination?
arms, legs, torso, back
Where on the body would have a smaller discrimination index/greater two-point discrimination?
face, lips, nose, fingers
How do mechanoreceptors produce a generator (action) potential?
Through voltage-or ligand ion channels
What is the mechanism that causes voltage-or ligand-gated ion channels to open in mechanoreceptors?
Physical stimuli/skin deformation (stretch) open the receptors in order to transduce signals
List the (4) features receptors code for
- Spatial location
- Modality
- Stimulus duration
- Intensity
Explain Rapid adapting receptors
Respond to changes in stimuli on the skin
Example: picking something up and putting something down
Explain slow-adapting receptors
Respond over the course of the stimulus
Example: Getting dressed in the morning and slowly losing sensation of each item as the day progresses
Which adapting receptor keeps firing as long as you’re holding something?
slowly adapting
Which adapting receptor only responds to a change in the stimulus?
rapidly adapting
Which mechanoreceptors are rapidly adapting? And what are their receptor fields?
- Pacinian corpuscles (receptive field: large, vague borders)
- Meissner’s corpuscle (receptive field: small, sharp borders)
Which mechanoreceptors are slowly adapting? And what are their receptive fields?
- Merkel’s discs (receptive field: small, sharp borders)
- Ruffini’s corpuscle (receptive field: large, vague borders)
What is the function of both rapidly and slowly adapting receptors?
Perception of texture and pattern
What perception of texture and pattern is signaled by:
- Slowly adapting receptors
- Rapidly adapting receptors
- Slowly Adapting: Place and Duration
- Rapidly Adapting: Changes in form
What does the inactivation of any of the four mechanoreceptors cause?
Inability to interpret shape
What type of fibers are contained in the dorsal root?
afferent fibers
Where does somatosensory information enter the CNS?
The dorsal root ganglia into the dorsal root
Which type of fibers bifurcate (divide) at the dorsal root entry zone?
Large myelinated fibers
After sensory information enters the spinal cord where do the branches separate too?
- One branch ascends in the posterior (dorsal) columns
- The other branch synapses in deeper laminae of the spinal cord
Which type of sensory information travels up the posterior/dorsal column?
Touch, pressure, and vibration (mechanosensory) information
Which type of sensory information travels up the lateral column?
Pain and temperature information
What location does lower body sensation enter in the posterior/dorsal column?
Most medial portion
What location does upper body sensation enter in the posterior/dorsal column?
Most lateral portion
Injury to where can lead to loss of mechanosensation in specific regions of the body?
HIGH yield
Either the medial or lateral portion of the posterior/dorsal column
Which type of fibers synapses in the dorsal horn of the spinal cord?
Small, unmyelinated fibers
Explain the Medial Lemniscal Sytem. Locations? Beginning destination and final destination?
- Located in the Dorsal/posterior column
- Begins from mechanosensory receptors from the upper and lower body through the spinal cord
- End at the medulla specifically the rostral medulla
What is the name of the tract that receives information from the lower body?
Gracile tract
What is the name of the tract that receives information from the upper body?
Cuneate tract
Explain the mechanosensory pathway from the lower body to the caudal medulla (base of the brainstem)
Mechanosensory information from the lower body enters the CNS at the lumbar spinal cord and travels up the gracile tract to the gracile nucleus in the caudal medulla
Explain the mechanosensory pathway from the upper body to the caudal medulla (base of the brainstem)
Mechanosensory information from the upper body enters the CNS at the cervical spinal cord and travels up the cuneate tract to the cuneate nucleus in the caudal medulla
After entering the caudal medulla (base of the brainstem) what is the next location for the information fibers and what occurs there?
- Location: Internal arcuate fibers
- Where BOTH will cross and continue traveling on the opposite side
What is the term to describe the crossing at the internal arcuate fibers?
Medial Lemniscal System
Descussation
What occurs when there is a spinal cord injury on the LEFT side and why?
Loss of mechanosensation in the LEFT side b/c at the spinal cord level the fibers have not crossed so the effects are on the same side
What occurs where there is a brain stem injury on the LEFT side and why?
Loss of mechanosensation in the RIGHT side b/c at the brain stem level the fibers have crossed so the effects are on the opposite side
Explain Somatotopy
Are the different regions of the posterior (dorsal) column of the spinal cord (lateral or medial) that corresponds to different parts of our body (upper and lower)
What part of our body corresponds to the lateral region of the posterior column of the spinal cord?
Upper body
What part of our body corresponds to the medial region of the posterior column of the spinal cord?
Lower body
After the internal arcuate fibers decussate (cross over), what do they form?
Medial lemniscus tract
Where do the fibers travel after the medial lemniscus tract?
To the Ventral posterior LATERAL nucleus of the thalamus (VPLN)
After the VPLN where do the fibers ascend to?
VPLN- Ventral posterior lateral nucleus
The primary (SI) and secondary (SII) somatosensory cortex
What occurs at the primary and secondary somatosensory cortex?
Where integrating somatosensory information begins
Where is the somatosensory cortex located?
In the Parietal lobe
What is the pathway for the trigemino-thalmic-cortical (face recognition)?
- Cranial nerve carries face info. to trigeminal neuron
- trigeminal neuron info. comes through ganglion
- info. synapses and crosses immediately
- Goes to trigeminal lemniscus and VPMN
- Ends in the somatosensory cortex
Mechanosensory information from the FACE enters the brainstem via where?
Trigeminal ganglion
Where do the fibers from the trigeminal ganglion synapse?
The principal trigeminal nucleus of the caudal medulla
Where do fibers cross and travel in the trigeminal-thalamic-cortical pathway (face sensation)?
- Cross at the Trigeminal leminscus
- Travel to the Ventral posterior MEDIAL nucleus of the thalamus (VPMN)
After the VPMN where do the fibers ascend to?
trigemino-thalamic-cortical pathway
VPMN-Ventral posterior medial nucleus
The primary (SI) and secondary (SII) somatosensory cortex
For the Body, fill in the blank:
Ventral Posterior _________ Nucleus
Lateral
(L)ateral= (L)otion for Body
For the Face, fill in the blank:
Ventral Posterior _________ Nucleus
Medial
(M)edial= (M)akeup for Face
What is a dermatome?
slice of skin that one spinal/brainstem cranial nerve root carries info from
When do dermatomes develop?
At the embryological level
What are the characteristics of the somatosensory cortex?
- has plasticity
- has discrimination (touch, pain, proprioception)
What determines how big nuclei are in the ventral posterior complex?
Density of information
What part of the body uses up the largest area of the ventral posterior complex and the somatosensory cortex? and why?
- Face (VPM; medial)
- D/t a large density of information coming from a large number of neurons with small receptive fields
What is homunculus?
representation of how much brain processing area there is for each body part
What thermoreceptors are free nerve ending?
both warm and cold
What are the fiber types of thermoreceptors?
- Warm: C
- Cold: A𝜎 (acute pain)
Which nociceptors are free nerve ending?
- Small, unmyelinated
- Unmyelinated
What are the fiber types for the nociceptors?
- Small, myelinated: A𝜎 (acute pain)
- unmyleinated: C (warm/dull pain)
What are the roles of perception of the nociceptors?
- Small, myelinated: sharp, localized pain
- Unmyelinated: burning
What is an example of acute pain?
an initial sting from a bee
What is an example of dull/throbbing pain?
long-lasting pain after a bee sting
Are all A𝜎 and C fibers nociceptors?
NO
What molecules are associated with A𝜎 fibers?
A𝜎 fibers → Acute pain
Low yield
Excitatory amino acids (glutamate, aspartate, ATP)
What molecules are associated with C fibers?
C fibers → dull pain
Low yield
- Substance P
- Neurokinins A&B
- Cholycstokinin
- Calcitonin Gene Related Peptide (CGRP)
- Vasointestinal Peptide (VIP)
True or False. We have pain receptors in our body
FALSE
Rather nociceptors respond to very intense and possibly harmful stimuli
What is an example of how temp and pain can be carried by the same fibers?
capsaicin-molecule in chili pepper that gives them their spice
What does capsaicin do?
binds to VR-1 (vanilloid) receptor to allow Ca2+ and Na to start signaling
VR-1 is synonymous to TRPV1
What is the main difference between discriminative touch and pain response?
Discriminative touch signaling stays on the same side until it reaches the brain stem, while pain response information crosses right away
Explain how nociceptive information enters the CNS
Enter the spinal cord via the dorsal horn and cross to the lateral side immediately via the anterior white commissure
A patient with a LEFT spinal cord lesion will lose:
- Discriminative touch on what side?
- Temperature and pain sensation on what side?
- Loss of discriminative touch on the LEFT side
- Loss of temperature and pain sensation on the RIGHT side
What is the anterior commissure in the spinal cord?
a small bundle of fibers connecting 2 hemispheres in the brain
What occurs at the anterior white commissure in the spinal cord?
where nociceptive information is processed
What is Lissauer’s tract?
2-3 segments above and below the dorsal root entry zone (DREZ) that add redundancy
Redundancy-allows a set of neurons to perform multiple tasks at once
What is the benefit of Lissauer’s tract from an evolution standpoint?
Helps us maintain nociceptive perception in case of injury
What is the pathway of the pain signal?
Anterolateral ((Neospinothalamic) Tract
comes in at dorsal horn -> crosses at the ant. white commissure -> goes to anterolateral tract -> enters brainstem -> ends at primary somatic sensory cortex
Where does nociceptive information from BOTH the upper and lower body enter the CNS?
Dorsal root ganglion
Where does nociceptive information from the body cross and travel to in the spinal cord?
IMMEDIATELY crosses at the anterior white commissure to the anterolateral portion of the spinal cord
After entering the anterolateral portion of the spinal cord where does nociceptive info. from the body travel to?
- Travels up the spinothalamic/neospinothalamic tract to the ventral posterior LATERAL nucleus (VPLN) of the thalamus
After the VPLN, where do the nociceptive fibers from the body ascend to?
The primary (SI) and secondary (SII) somatosensory cortex to begin integrating somatosensory information
What is the pathway of nociceptive info. from the face?
Trigeminal System
comes in from the pons -> drops to the medulla -> crosses at the ant. white commissure-> travels up the spinal thalamic tract -> ends at VPMN and primary somatic sensory cortex
Where does nociceptive information from the face enter the CNS?
At the PONS
Where does nociceptive information of the face travel down and cross into the spinal cord?
- Down the medulla
- Cross at the anterior white commissure
After crossing the ant. white commissure of the spinal cord where does nociceptive info. from the face travel to?
Travel up the trigeminal-thalamic tract to the ventral posterior MEDIAL nucleus of the thalamus.
After the VPMN, where do the nociceptive fibers from the face ascend to?
The primary (SI) and secondary (SII) somatosensory cortex to begin integrating somatosensory information
Will an injury to the medulla lose sensation on one or both sides of the face?
both
The nociceptive pathway goes through what entire system?
limbic
What are the (6) different targets of the anterolateral system?
- Anterior cingulate/insular cortex
- Amygdala
- Hypothalamus
- Superior colliculus
- Reticular formation
- Periaqueductal Gray
The periaqueductal grey is responsible for what response?
pain modulation (control)
The superior colliculus is responsible for what response?
Directs our attention to the source of pain
The reticular formation is responsible for what response?
Arousal (via release of ACh) in order to respond to pain
What anterolateral system target is responsible for fight or flight?
Hypothalamus
What anterolateral system target is responsible for negative emotion?
Amygdala
What anterolateral system target is responsible for avoidance/emotional component that drives behavioral responses?
Anterior cingulate/insular cortex
What is allodynia?
perception of normally non-nociceptive stimuli being painful
Getting a tap on the site of a new shot is an example of what type of sensation a patient can undergo.
allodynia
What is hyperalgesia?
nociceptive stimuli are perceived as being more painful than expected
Getting pushed on the arm after getting a shot is an example of what type of sensation a patient can undergo.
hyperalgesia
What is peripheral sensitization?
allodynia and/or hyperalgesia due to peripheral mechanisms
What is central sensitization?
allodynia and/or hyperalgesia due to central mechanisms
What occurs in peripheral sensitization?
- Release of neuropeptides and NTs at the free nerve ending as a result of local damage (e.g. glutamate, substance P, histamine, etc)
- Sensitization of the area around the site of injury
What occurs in central sensitization?
- Neurochemistry (sensitization of NMDA ( N-methyl-D-aspartate) receptors alters synaptic efficacy)
- Anatomic reorganization in spinal cord (loss of primary afferent results in sprouting of existing afferents in dorsal horn)
NMDA-glutamate receptor
Explain descending pain modulation
- Spinotectal fibers synapse in the periaqueductal grey (PAG) which projects to:
- Raphe nucleus: serotonergic projections
- Lateral Tegmental Nucleus: noradrenergic projections
- These projections synapse on
enkephalinergic interneurons in the dorsal horn
Explain pain control in the spinal column
- You have serotonin and norepir (input coming from raphe nuclei) coming down
- This activates and releases an enkephalin neuron which inhibits the C fiber (nociceptor) from synapsing onto the dorsal horn projection neuron to the anterolateral system
Explain Phantom Limb
- It is when you lose a peripheral body, but you feel like you still have that limb.
- Trigger points are going to become those areas of skin that were processed proximally at the somatosensory cortex
- Which of the following correctly identifies the mechanoreceptor responsible for sensing deep pressure/vibration and its respective adaptation type?
A. Meissner corpuscle; Rapidly adapting
B. Pacinian corpuscle; Slow adapting
C. Meissner corpuscle; Slow adapting
D. Pacinian corpuscle; Rapidly adapting
d.Pacinian corpuscle; Rapidly adapting
- All of the following regarding the dorsal column/medial lemniscal system are true EXCEPT:
A. Mechanosensory information from the lower body synapses at the gracile nucleus
B. After entering the cerebrum, fibers synapse at the ventral posterior lateral nucleus
C. Mechanosensory information decussates at the anterior white commissure
D. Mechanosensory information from the upper body enters the CNS via a cervical dorsal root ganglia
C. Mechanosensory information decussates at the anterior white commissure
- A 37-year-old man is rushed to the ED following a severe motorcycle accident which has altered somatosensory perception in his lower limbs. MRI indicates the patient has a lesion in the right portion of his spinal cord. What symptoms is the patient likely experiencing?
A. Loss of both mechanoreceptive and nociceptive sensation in the right leg
B. Loss of mechanoreceptive sensation on the right leg and nociceptive sensation on the left leg
C. Loss of both mechanoreceptive and nociceptive sensation in the left leg
D. Loss of mechanoreceptive sensation on the left leg and nociceptive sensation on the right leg
B. Loss of mechanoreceptive sensation on the right leg and nociceptive sensation on the left leg
- The anterolateral system sends projections to varying portions of the CNS to elicit physiological responses after pain stimulation. Which of the following structures directs our attention to the source of pain?
A. Superior colliculus
B. Insular cortex
C. Periaqueductal Gray
D. Somatosensory cortex
A. Superior colliculus
- The perception of normally non-nociceptive stimuli as being painful is often caused by a form of sensitization. Which of the following would lead to peripheral/local sensitization?
A. Loss of primary afferents in the dorsal horn
B. Sensitization of NMDA receptors on spinal interneurons
C. Release of neuropeptides from C fibers
D. Sensitization of NMDA receptors directly on the anterolateral tract
C. Release of neuropeptides from C fibers
