Somatosensory Flashcards
Divisions of the somatosensory system
- Exteroception
- Enteroception
- Proprioception
Exteroception
- Nociception (different from pain)
- Thermoception
- Mechanoreception
Enteroception
Internal organs/tissues (e.g. heart bladder)
Proprioception
Body position
Epicritic vs. protopathic pathways
- Epicritic: judgment
- Protopathic: pain
What are the two types of skin?
Hairy vs. glabrous (not hairy)
What are the four types of mechanoreceptors?
- Merkel’s disk
- Meissner’s corpuscle
- Pacinian corpuscle
- Ruffini’s ending
Merkel’s disk
- Consists of a nerve ending and a special epithelial cell
- More superficial
- Present in hairy and glabrous skin
Meissner’s corpuscle
- Found in the ridges of glabrous skin (like the ridges of your fingerprints)
- More superficial
Pacinian corpuscle
- Found deep in dermis
- Hairy and glabrous skin
Ruffini’s ending
- Slightly smaller than Pacinian corpuscle
- Present in both hairy and glabrous skin
Do mechanoreceptors have free nerve endings?
- Yes
- They don’t have specil receptors: sometimes wrap around follicle
- Thin or unmyelinated
What is a receptive field?
The area of skin that causes change in that neuron’s membrane potential when stimulated
In what ways do mechanoreceptor receptive fields differ?
- They differ based on size (small vs. large receptive field)
- They differ based on how quickly they adapt(slow vs. rapid adaptation)
Diagram of receptor field on skin
Table of receptive fields (to memorize)
Differences in size of receptor fields
- Small RF closer to surface of skin
- Large RF found deeper in skin
- Meissner and Merkel small, Pacinian and Ruffini large (Germans small and shallow, Italians large and deep)
Differences in speed of adaptation of receptor fields
Adaptation: after a certain time, receptors will stop firing even when the stimulus is still occurring
- Rapid adaptation: APs fired only when stimulus is first placed and when stimulus is first removed, responding to changes in pressure, not absolute pressure
- Rapid adaptation: Meissner’s corpuscle and Pacinian corpuscle
- Slow adaptation: Merkel’s disk, Ruffini’s ending
RF size, adaptation, and location of Pacinian corpuscle
- RF size: large
- Adaptation: rapid
- Location: deep
RF size, adaptation, and location of Ruffini’s ending
- RF size: large
- Adaptation: slow
- Location: deep
RF size, adaptation, and location of Merkel’s disk
- RF size: small
- Adaptation: slow
- Location: superficial
RF size, adaptation, and location of Meissner’s corpuscle
- RF size: small
- Adaptation: rapid
- Location: superficial
Summary table of mechanoreceptor properties
Corpuscle mechanisms
● Both of the corpuscles are rapidly-adapting
● Rapid adaptation allows for detection of rapidly-changing / high frequency stimuli (ex. vibrations and texture detection)
● Corpuscles mediate different ranges of
frequencies
○ Pacinian responds best to 200-300 Hz
○ Meissner’s responds best to 50 Hz
● Removing capsule takes away the receptor’s rapidly-adapting capabilities and instead makes it slow-adapting: Capsules are fluid-filled and continuous stimulus eventually stops deformation of receptor
Mechanosensitive Ion Channels
The mechanoreceptors of the skin all have unmyelinated axon terminals, and the membranes of these axons have mechanosensitive ion channels that convert mechanical force into a change of ionic current.
● Force applied to channels either makes them open more or less
● Force may be applied directly to the channel or indirectly through other components of the cell like intracellular cytoskeletal components
● Internal Modulation: mechanical stimuli can trigger release of second messengers (DAG, IP3)
Are the axon terminals of mechanoreceptors of the skin myelinated or unmyelinated?
Unmyelinated
Two Point Discrimination
- Receptive Field size is determined by the degree of arborization- Two point discrimination is a simple measure of the spatial resolution that varies across the body
○ How far apart do two points have to be before your can identify them as two separate things
○ Can be said to be measure of spatial acuity - Affected by 2 factors
○ The spatial resolution is dependent on the density of touch receptors → Coverage Factor
○ Type of touch receptors (ex. fingertips have lots of Merkel’s Disks which have small receptive fields → increase acuity) - Places like hands and lips have higher density of touch receptors and, therefore,
better two point discrimination than limbs or torso → this leads cortical magnification (hands and lips are represented by large
areas of the cortex)
What are the two factors that affect two point discrimination?
- The spatial resolution is dependent on the density of touch receptors → Coverage Factor
- Type of touch receptors (ex. fingertips have lots of Merkel’s Disks which have small receptive fields → increase acuity)
Cortical magnification
- Places like hands and lips have higher density of touch receptors and, therefore,
better two-point discrimination than limbs or torso → this leads cortical magnification (hands and lips are represented by large areas of the cortex) - Cortical magnification is used to emphasize parts of the body that are important to our somatosensory experience. It is directly proportional to…
1. Small receptive fields
2. High density of receptors
3. High spatial acuity
Somatotopy
Areas of the body are mapped to the brain in
a systematic fashion i.e. topographic map
● Organization isn’t perfect (ex. the head
area is separated from the face area)
● Cortical magnification in action
○ Some areas have more cortical space dedicated
to them than you would expect based on their
physical size, (ex. lips and hands)
○ The relative size of the cortex devoted to an area
correlates to the density of receptors in that area
→ size of cortex for certain part of body
corresponds to its spatial acuity
Homunculus
Cortical plasticity
- Plasticity: cortical maps are not fixed
- Remove input (nerve block or amputate digit) → lose cortical area dedicated to this input and surrounding areas (e.g. neighboring fingers) grow into this region
- Overstimulate input → cortical area dedicated to this input
grows into surrounding areas - Might be able to explain phantom limbs
- High activity of body part increases area of cortex dedicated to
that body part - These can all be thought of as reallocation of cortical space
Explain how cortical plasticity might be able to explain phantom limbs
- Can still feel sensations in limb that is no longer present
- Can be all modalities and diminishes over time
- Stimulate areas of body corresponding to neighboring cortical area → cortical area of missing limb activated by lateral connections
Primary Afferent Axons
- Primary afferent axons: bring information from sensory receptors into the CNS (spinal cord)
- Enter spinal cord through dorsal root
- Cell bodies of these sensory axons are in the dorsal root ganglia
- These axons have different diameters and amounts of myelin
Diagram of pathway of primary afferent axons
Afferent =
Arriving
Diagram of axon types
A-alpha axons
- Speed of transduction: 80-120 m/sec
- Sensory receptors: proprioceptors of skeletal muscle
A-beta axons
- Speed of transduction: 35-75 m/s
- Sensory receptors: mechanoreceptors of skin
A-delta axons
- Speed of transduction: 5-30 m/s
- Sensory receptors for pain & temperature
C fibers
- Speed of transduction: 0.5-2m/sec
- Sensory receptors for temperature, pain, itch
Separations of the spinal cord
30 spinal nerves are divided into four groups:
● Cervical: C1-C8 (arms)
● Thoracic: T1-T12 (torso)
● Lumbar: L1-L5 (legs)
● Sacral: S1-S5
What is the dermatome?
- The area of skin innervated by the right and left dorsal roots of a single spinal segment
- Neighboring dermatomes overlap
What is the DCML pathway?
- Dorsal column-medial lemniscus
- This is an epicritic pathway:
○This is the path taken by touch and vibration information NOT pain or temperature
○ Information about limb position (proprioception) and tactile sensation carried in dorsal column
Spine component of DCML pathway
First, the ascending branch of large sensory neurons (bodies found in the dorsal root ganglion) enters ipsilateral dorsal column of the spinal cord
Brain component of DCML pathway
- These axon have their first synapse in the dorsal column nuclei at the junction of spinal cord and the medulla
- After this synapse, the axons decussate (left goes to right or right goes to left)
- The axons of the dorsal column nucleus ascend within the contralateral medial lemniscus (a white matter tract) through medulla, pons, and midbrain)
- These axons synapse on the ventral posterior lateral nucleus (VPL Nucleus) of the thalamus
- The thalamus projects to primary somatosensory cortex (S1)
DCML Pathway Summary
Supplemental: Trigeminal Touch Pathway
- Somatic sensation for the face and scalp
- Also decussates (touching right cheek is
processed by left cortex) - Also terminates in the VP nucleus of the thalamus (VPM instead of VPL)
Primary Somatosensory Cortex (S1)
- Primary somatosensory cortex is area 3b
- The thalamic inputs to S1 terminate mainly in layer IV
- The neurons of layer IV project, in turn, to cells in the other layers
- Cortical column (columnar organization) → S1 neurons with similar inputs and responses (ex. rapidly-adapting vs. slowly-adapting) are stacked vertically into columns
- Projections outside S1
○ Areas 1 and 2 receive inputs from area 3b.
○ 3b → 1 = texture information
○ 3b → 2 = size and shape
Posterior Parietal Cortex (PPC)
- Areas 5 and 7 (posterior to S1)
- PPC neurons have large RFs with stimulus preferences that are a challenge to characterize because they are so elaborate
- Concerned not only with somatic sensation but also with visual stimuli (integration of senses), movement planning, and even a person’s state of attentiveness → create “seamless, complete representation of perceived object”
Agnosia
Damage to the posterior parietal cortex (PPC) can cause an inability to recognize objects even though sensory systems are intact
Neglect syndrome
Happens contralaterally (hemi-neglect) ex. a patient with right parietal cortex lesion will neglect everything on the left side of his/her world including own body
Nociceptors
Free, branching, unmyelinated nerve endings that signal body tissue is being damaged or is at risk of being damaged
Is nociception the same as pain?
No!
- Pain can happen without
nociception, and nociception can happen without pain
- Pain is the perception of discomfort, stinging, aching, throbbing, or throbbing sensations
○ Nociception is the process that triggers the sensation of pain (i.e. the actual neural signals)
Hyperalgesia
The elevation of pain sensitivity: two types i.e. primary and secondary
Analgesia
Lack of pain: Not as great as it sounds ex.
people born with inability to feel pain have much shorter lives because they don’t know when they are having tissue
damage
Referred pain
- “Referred pain” is the sensation of pain in a certain place despite there being no tissue damage
- Axons carrying pain information from visceral organs enter the spinal cord at the same location as the axons from the periphery (e.g.: skin)
● Sometimes crossing over of signals occurs → pain is “referred” to the outside of your body (where we have body maps)
● Example: During heart attack, may feel radiating pain in the left arm→ activation of nociception in the heart’s ventricles causes pain to be felt in the left arm (this radiating pain is most commonly seen in men)
What are the four types of nociceptors?
- Mechanical
- Thermal
- Chemical
- Polymodal
What are mechanical nociceptors sensitive to?
Pressure on the surface of the body that could cause tissue damage
What are thermal nociceptors sensitive to?
Dangerous temperatures
What are chemical nociceptors sensitive to?
Specific chemicals that signal tissue damage (e.g. free hydrogen ions from lactic acid when working out)
What are polymodal nociceptors sensitive to?
More than one type of stimulus
TRPv Receptors
TRPV receptors: heavily studied nociceptors, TRP = transient receptor potential
- Have TRPV channels, specialized VG NaV1.7 channels, regular VG Na+ channels, and non-selective ion channels (Na+, K+, Ca++)
- V stands for vanilloid (capsaicin found in hot peppers is an example)
- Process: capsaicin → TRPV
channels activated and open → depolarization → regular VG Na+ channels open → further depolarization → AP
- Blocking the TRPv channel, VG NaV1.7 channel, or regular VG Na+ channel will disturb nociception
SCN9A Channelopathy
- Codes for NaV1.7 channel (uniquely expressed in nociceptive cells)
- People with congenital insensitivity to pain have a mutation in this channel
- Some people can also have a mutation in this channel that causes it to be
overactive → creates lower threshold for pain called “erythromelalgia”
Slow vs. Fast Pain
- Axons carrying pain from nociceptors are either thinly myelinated Aδ fibers OR unmyelinated C fibers
- Aδ fibers mediate fast pain: Sharp, stinging, local, transient
○ Aδ fibers are myelinated which means faster conduction - C fibers mediate slow pain: Dull, aching, poorly localized, enduring
○ C fibers are unmyelinated so they have slowest conduction in somatosensory system
What types of fibers carry fast pain?
Aδ fibers (myelinated)
What types of fibers carry slow pain?
C fibers (unmyelinated)
What are the two key differences between the spinothalamic (anterolateral) and DCML pathways?
Unlike the DCML pathway, the spinothalamic pathway:
- First synapses onto 2nd order neuron found in dorsal horn INSTEAD of in brainstem
- Decussates immediately in spinal cord instead of in brainstem
Spinothalamic pathway
Table of comparison of DCML and spinothalamic pathways
Diagram of comparison of architecture of DCML and spinothalamic pathways
Important things to note about the 2 somatosensory pathways
- They can be separately damaged: Loss of DC/ML damages the sense of proprioception and fine tactile sensation vs. loss of spinothalamic damage sense of pain, temperature, and itch
- You should understand the order of the levels spinal cord: Damaging the spinal cord damages all pathways that start below it, lumbar damage affects legs vs. cervical damage affects arms (and everything below, including legs)
- The pathways decussate at one point: If you damage the pathway before the decussation, the impairment occurs on the same side of the body BUT if you damage the pathway after the decussation, the damage occurs on the opposite side of the body
Lesion to DCML in the dorsal column
- Lesion is in dorsal column (or anything below medulla)
- Results in loss of touch and vibration on the ipsilateral side for everything below the lesion (nothing has decussated yet)
- If they mention cutting at specific portions of the spinal cord, information coming in from nerves ABOVE the lesion point will be unaffected (e.g. if you cut the lumbar spinal cord, both arms are fine)
DCML lesion above medulla
- A lesion to anything above the medulla will result in loss of all touch and vibration on the contralateral side (all info has decussated by now)
- No matter where you cut after the medulla, no new sensory information is coming in above, so EVERYTHING is lost on the opposite side
Sometimes, instead of a lesion, they’ll instead ask about the effect of something else, like disrupting synaptic transmission in a certain part of the nervous system. This would have the same effect, just make sure there is actually a DCML synapse there, they may try to trick you!
Lesion to spinothalamic pathway
- A lesion to anything above decussation in spinal cord results in loss of sensation of pain and temp on the contralateral side (all info has decussated
- If you cut in the spinal cord, you lose all sensory info coming in below the lesion, but above the lesion will be unaffected
- If you cut above C1, no new sensory info is entering, so ALL pain and temp sensation is lost
Hyperalgesia
- Hyperalgesia: characterized by excessive response to noxious stimuli (greater pain intensity) or to stimuli that aren’t normally painful (lower pain threshold), causes a release of inflammatory “soup”
○ Chemicals such as bradykinin, prostaglandin and substance P increase sensitivity of nociceptors
○ Substance P also induces vasodilation, which leads to increased levels of histamine from mast cells - Primary is directly at site of damage
- Secondary is around site of damage
Primary hyperalgesia
- Injury occurs in periphery
- Damaged cells release inflammatory substances (prostaglandins, bradykinin, histamine, K+ )
- Inflammatory substances activate nociceptors
a. AP travels back up the nociceptor, sending information to the brain
b. Chemicals such as substance P will trigger the release of additional inflammatory
substances - Substance P sensitizes nociceptor → lower threshold for nociceptive activation
Axon reflex
- C fibers are activated by pain stimulus
- Signal travels towards CNS (remember these axons have cell bodies in dorsal root ganglion)
- Sometimes axons branch.
- If the axon is not in refractory period, the signal travels on the branches back towards skin → back-propagating pain signal depolarizes these branching axons
○ Nociceptors at the end of branched axons are activated
○ Substance P released → further increases inflammation (and can further sensitize already activated nociceptors)
Secondary Hyperalgesia
Sensitizes skin in regions surrounding (but not directly at) the primary site of injury
● Axon reflex causes release of Substance P
● This can also result in sensitization of
nociceptors in regions outside the zone of primary hyperalgesia
Descending Pain Control
- Descending pain control pathway: periaqueductal gray (PAG) around the cerebral aqueduct signals to raphe nuclei in the medulla → affects nociceptive inputs in the dorsal horn of the spinal cord
- This pathway can be activated by emotional factors or opioids (PAG and dorsal horn neurons have opioid receptors)!
Examples of pain without nociception
- Phantom limb: pain happens from abnormal activation in CNS (part of S1
dedicated to the missing limb) instead of stimulus on the periphery - Neuropathy is when damage to peripheral nerves can cause numbness or pain
Example of nociception without pain
Opiates can affect perception of pain (raises threshold for pain) by activating endogenous opioid receptors found in descending pain pathway
Supplemental: Temperature
- Thermoreceptors (and thus, temperature sensitivity) is not spread
uniformly on the skin - Temperature sensitivity mediated by 6 distinct types of TRP channels
- Adapt to stimuli → sudden change of temperature is what causes the most firing → is perceived most strongly by brain
- Cold carried by Aδ and C vs. warm carried by only C
Supplemental: Itch
- Pain and itch axons seem to be distinct but can interact (ex. pain can suppress itch)
- Itch signal carried by C fibers which are sensitive to histamine → activates TRPV 1 channels
- Certain peptide NTs may be involved in itch transmission
If you damage a pathway (DC/ML or spinothalamic) before decussation, where will impairment occur?
On the same side of the body
If you damage a pathway (DC/ML or spinothalamic) after decussation, where will impairment occur?
On the opposite side of the body
Rapid adaptation will increase a sensory neurons sensitivity to
a) vibrating stimuli
b) nociceptive stimuli
c) temperature
d) itch
a) Vibrating stimuli
A lesion of the medial lemniscus on the left side of the Pons would result in loss of
a) touch in the right foot
b) nociception in the right hand
c) touch in the left hand
d) nociception in the left foot
a) Touch in the right foot
If there is a lesion in the medial lemniscus on the left side of the pons, it will affect the sensory signals that have already crossed over from the right side of the body. Therefore, a lesion here would result in a loss of fine touch, vibration, and proprioception on the right side of the body.
Compared to a 1 square centimeter patch of skin on your back, a 1 square centimeter patch of
skin on the tip of your finger has
a) a higher density of mechanoreceptors
b) larger mechanoreceptor receptive fields
c) a larger representation in primary somatosensory cortex
d) More than one of the above
e) All of the above
d) More than one of the above
a) A higher density of mechanoreceptors: The skin on the tip of the finger has a higher density of mechanoreceptors compared to the skin on the back. This allows for finer discrimination of tactile information, which is crucial for tasks requiring precision, like feeling textures or manipulating small objects.
b) Larger mechanoreceptor receptive fields: This is incorrect. The mechanoreceptors on the tip of the finger have smaller receptive fields, allowing for higher spatial resolution. In contrast, mechanoreceptors on the back have larger receptive fields, resulting in less precise spatial discrimination.
c) A larger representation in primary somatosensory cortex: The fingertip has a larger representation in the primary somatosensory cortex than the skin on the back. This reflects the brain’s need to process detailed sensory information from areas that require fine tactile perception, like the fingers.
The decreased sensitivity to pain when a person is in a very stressful situation
a) is due to activation of mu opiod receptors (MORs) in the spinal cord
b) involves increased activity in neurons of the raphe nucleus
c) can be reversed by the administration of naloxone
d) All of the above
d) All of the above
a) Activation of mu opioid receptors (MORs) in the spinal cord: Stress-induced analgesia is partly mediated by the release of endogenous opioids, which activate mu opioid receptors in the spinal cord to reduce pain signals. Activation of MORs inhibits pain transmission.
b) Increased activity in neurons of the raphe nucleus: The raphe nucleus in the brainstem is involved in descending pain modulation. Under stress, neurons in the raphe nucleus can increase activity, releasing serotonin and other neurotransmitters that help suppress pain.
c) Reversal by naloxone: Naloxone is an opioid receptor antagonist. It can reverse stress-induced analgesia by blocking opioid receptors, which confirms that endogenous opioids (like endorphins) are involved in this pain-suppression response.
Being injected with a compound that blocks all action potentials in axons in the right side of your cervical spinal cord will cause you to lose all of the following functions EXCEPT:
a) the sensation of pain in your left foot
b) the ability to write your name with your right foot (You learned how to do this when you were
6 years old)
c) touch in your right foot
d) knee jerk reflex in your right le
d) Knee jerk reflex in your right leg
a) The sensation of pain in your left foot: Pain sensation (nociception) travels via the spinothalamic tract, which crosses over to the opposite side of the spinal cord shortly after entering. Since the compound was injected on the right side of the cervical spinal cord, it would block the transmission of pain signals coming from the left side of the body. Therefore, you would lose the sensation of pain in your left foot.
b) The ability to write your name with your right foot: Motor commands for skilled voluntary movements travel through the corticospinal tract, which runs down the spinal cord on the same side as the target muscles after crossing at the level of the medulla. Since the injection was on the right side of the cervical cord, motor control of the right foot would be lost, affecting your ability to write with it.
c) Touch in your right foot: Fine touch, proprioception, and vibration sense are carried by the dorsal column-medial lemniscus pathway. Signals from the right foot would travel up the right side of the spinal cord to the medulla, where they cross over. Therefore, blocking the right side of the cervical cord would also affect touch sensation in the right foot.
d) Knee jerk reflex in your right leg: The knee jerk reflex (a simple spinal reflex) does not require signals to travel up to the cervical spinal cord; it is mediated by a reflex arc within the lumbar spinal cord. Since this reflex is local to the lumbar region, it would remain intact despite the cervical spinal cord being blocked.
A phantom limb sensation of a missing left hand could be elicited by stimulating the
a) right hand
b) right side of the face
c) left side of the face
d) left foot
c) Left side of the face
Phantom limb sensations occur because the brain regions that previously processed sensations from the missing limb can be “re-mapped” to nearby areas in the sensory cortex. For the hand, the nearby area in the somatosensory cortex corresponds to the face.
Damage to which part of the brain is most likely to cause a person to not recognize their left
foot as being part of their own body.
a) primary visual cortex
b) primary somatosensory cortex
c) posterior parietal cortex
d) prefrontal cortex
c) Posterior parietal cortex
The posterior parietal cortex is crucial for integrating sensory information and maintaining a sense of spatial awareness and body ownership. Damage to this area, particularly in the right hemisphere, can lead to a phenomenon known as hemispatial neglect or asomatognosia. In this condition, individuals may fail to recognize parts of their body (such as their left foot) as their own or may ignore them entirely.
What is the function of the posterior parietal cortex?
Crucial for integrating sensory information and maintaing a sense of spacial awareness and body ownership
Piezo1 and 2 Mechanoreceptors
- Mechanically-gated cation channels
- Extremely sensitive to slightest pressure
(on the milliNewton scale) - Essential for machanotransduction in
somatosensory neurons - Also essential in many other tissues
where sensation of mechanical force is
important (bone growth, red blood cells,
blood vessels, bladder, and many more
Mu opioid receptors
- Mu-opioid receptors or
(MOR’s) are found in neurons of the PAG, medulla and dorsal horn and play a major role in
regulating pain. - They are also found elsewhere in the central NS, peripheral NS and body.
When comparing somatic sensation in the skin on the fingertip compared to the skin on the
back, all of the following factors vary to a large degree EXCEPT
a) mechanoreceptor coverage factor
b) mechanoreceptor receptive field size
c) cortical magnification factor
d) performance on two-point discrimination tasks
a) Mechanoreceptor coverage factor
Explanation:
b) Mechanoreceptor receptive field size: Receptive fields are much smaller on the fingertips compared to the back, allowing for higher spatial resolution and more precise tactile discrimination in the fingers.
c) Cortical magnification factor: The cortical representation (cortical magnification) of the fingertip is much larger than that of the back. This reflects the brain’s need to process more detailed sensory information from the fingertips.
d) Performance on two-point discrimination tasks: Due to smaller receptive fields and greater cortical magnification, the fingertips are much better at distinguishing between two closely spaced points than the back.
a) Mechanoreceptor coverage factor: This factor, which refers to the general distribution or density of mechanoreceptors in a given area of skin, does not vary as significantly between these two areas when compared to the other factors listed. The difference in tactile sensitivity is more strongly influenced by receptive field size, cortical magnification, and two-point discrimination ability than by overall coverage factor.
How do the mechanoreceptor receptive fields in the fingertips compare to those on the back?
They are much smaller on the fingertips
Which of the following is the largest in number?
a) the number of cell bodies in all dorsal root ganglia
b) the number of alpha motor neurons in the spinal cord
c) the number of gamma motor neurons in the spinal cord
a) the number of cell bodies in all dorsal root ganglia
b) The number of alpha motor neurons in the spinal cord: Alpha motor neurons are responsible for innervating skeletal muscles and are fewer in number than DRG neurons. Each alpha motor neuron typically controls multiple muscle fibers, allowing for fewer alpha motor neurons to manage larger muscle groups.
c) The number of gamma motor neurons in the spinal cord: Gamma motor neurons are even fewer in number than alpha motor neurons. They primarily innervate muscle spindles to regulate muscle tone and proprioception, rather than directly causing muscle contraction. Their numbers are smaller because they only need to manage the sensitivity of muscle spindles rather than directly controlling muscle fibers.
Which lobe of the brain is the primary somatosensory cortex located in?
Parietal lobe
Damage to the left side of your thoracic spinal cord would result in loss of all of the following
EXCEPT
a) Pain in your right foot
b) Movement of your left foot
c) Touch in your left foot
d) Movement of your right hip
d) Movement of your right hip
Explanation:
a) Pain in your right foot:
Spinothalamic Tract: Pain sensation is carried by the spinothalamic tract, which decussates (crosses over) shortly after entering the spinal cord. Damage to the left side of the spinal cord would affect pain sensation on the right side of the body, including the right foot. Therefore, this would be lost.
b) Movement of your left foot:
Alpha Motor Neurons: Motor control for the left foot is mediated by alpha motor neurons that exit the spinal cord via the ventral roots on the same side. Damage to the left side of the spinal cord would impact the motor pathways for the left foot, resulting in loss of movement. Therefore, this would also be lost.
c) Touch in your left foot:
Dorsal Column Pathway: Touch sensation is carried by the dorsal column-medial lemniscus pathway, which crosses over at the level of the medulla. Damage to the left side of the thoracic spinal cord would lead to loss of touch sensation in the left foot. Therefore, this would also be lost.
d) Movement of your right hip:
Motor Control for the Right Hip: The motor control for the right hip is mediated by descending motor pathways that decussate in the medulla (the corticospinal tract). Damage to the left side of the thoracic spinal cord would not affect the ability to move the right hip, as it is controlled by the right side of the spinal cord.
Areas of skin where two point discrimination performance is most accurate
a) have small mechanoreceptor receptive fields
b) have a high density of mechanoreceptors
c) have a high magnification factor in somatosensory cortex
d) More than one of the above
e) All of the above
e) All of the above
When drawing a clock, a person puts all of the numbers on the right side. This deficit is likely caused by damage to their ___
Right posterior parietal cortex (neglecting left side – contralateral neglect)
That painfully hot small potato that was dropped into your outstretched hand in question 29
would activate all of the following EXCEPT
a) Mechanoreceptors
b) Thermal nociceptors
c) Mechanical nociceptors
d) Polymodal nociceptors
c) Mechanical nociceptors
The sensation of pain is primarily processed in the ___ of the spinal cord
Dorsal horns
A stimulus vibrating at 100 Hz
a) Will excite rapidly adapting receptors more effectively than slowly adapting receptors
b) Will excite slowly adapting receptors more effectively than rapidly adapting receptors
c) Will excite free nerve endings more effectively than Pacinian corpuscles
a) Will excite rapidly adapting receptors more effectively than slowly adapting receptors
The fastest conducting axons in the peripheral nervous system
a) Form neuromuscular junctions with muscle cells
b) Have axons that innervate muscle spindles
c) Carry information from receptors in the skin
d) Carry information about temperature
b) Have axons that innervate muscle spindles
Primary sensory neurons in the somatic sensory system have cell bodies in the
a) ventral horn
b) dorsal horn
c) dorsal root ganglia
c) dorsal root ganglia
The dorsal horn of the spinal cord is indeed grey matter, but it does not contain the cell bodies of primary sensory neurons. Instead, it is the site where primary sensory axons terminate and synapse with second-order neurons in the somatic sensory pathway. The cell bodies of the primary sensory neurons are located in the dorsal root ganglia (DRG), outside the spinal cord.
