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