Week 2 - Somesthesis

Question 1

how can tactile receptors be divided? what are examples of cells?

Answer 1

1. slowly adapting mechanoreceptors, which respond to enduring stimulus (Merkel and Ruffini, types I and II)
2. rapidly adapting mechanoreceptors respond only at onset and sometimes termination of long-lasting stimulus (Meissner and Pacinian)

Question 2

how do Pacinian corpuscles respond to stimuli?

Answer 2

rapidly adapting mechanoreceptor, so its ability to respond is based on unique lamellar accessory structure
-responds w/ single AP for each phase of stimulus

Question 3

what happens on hairy skin?

Answer 3

slight movement of any hair on the body stimulates hair mechanoreceptors made of nerve fibers entwining base of hair
-although each receptor has unique properties, sensory experiences of touch, pressure, and vibration are different from tactile sensation from normal stimuli

Question 4

how is “wet” skin felt?

Answer 4

no receptor for “wet” skin, but the combination of all receptors conveys it

Question 5

what does the two-point discrimination threshhold mean?

Answer 5

ability to differentiate between stimuli of same quality that differ in place, magnitude, and spatial pattern

1. due to relationship between receptor density and receptive field size
2. receptive field is part of skin directly innervated by receptor terminals and area of adjacent tissue through which a stimulus can be conducted to receptor
3. areas of skin that are most sensitive and have greatest capacity for fine spatial discrimination have high density of receptors
   - receptors have smallest receptive fields, and largest number of receptive fields per unit area

Question 6

how do 2-point discriminations differ for fingers, arms, and back?

Answer 6

finger: 2-3 mm (highest innervation and lowest pressure thresholds)
forearm: 20 mm
back: 40 mm (lowest innervation and highest pressure threshold)

Question 7

how do Merkel’s receptors, Meissner’s corpuscles, and Pacinian corpuscles differ in stimulus resolution?

Answer 7

Merkel’s - slowly adapting, provide spatial recognition

Messner and Pacinian - rapidly adapting, provide timing info

Question 8

what are the 2 major ascending systems that carry somatosensory info to cerebral cortex?

Answer 8

1. dorsal column-lemniscal system (mediates high tactile sensations like vibration and proprioception)
2. anterolateral system (pain, temperature)
   - tactile info only crude touch and pressure sensations, poor localizing ability on body, and little capability for fine intensity discrimination

Question 9

how does somatosensory info enter spinal cord?

Answer 9

peripheral mechanoreceptors are innervated by afferent axons whose cell bodies lie in dorsal root ganglia
-on entering spinal cord, the large myelinated fibers pass medially into lateral margin of dorsal columns

Question 10

what is the laminar organization of the spinal cord for somatosense?

Answer 10

1. medial branch turns upward in dorsal columns, and continues via dorsal column pathway to brain
2. lateral branch divides many times to give off multiple terminations in cord
   - participate in mediation of local spinal activity

Question 11

what is the somatopic organization of the dorsal columns?

Answer 11

at the level of the spinal cord, the first segregation of sensory information begins

1. afferent fibers of different receptors take specific routes on way to central projections
2. within dorsal columns, these modality segregated axons are arranged in highly ordered manner
   - sacral axons enter spinal cord and are packed near midline
   - axons added at higher levels are packed at successively more lateral positions

Question 12

what is the entering central ascending pathway in the brainstem?

Answer 12

nerve fibers entering dorsal columns pass up columns on ipsilateral side to caudal medulla, where they synapse in the dorsal column (cuneate and gracile) nuclei

Question 13

how are 2nd order neuron receptive fields developed?

Answer 13

at level of dorsal column, nuclei receptor neurons converge onto single second order neurons

• receptive fields become larger b/c they get convergent input from many primary neurons, each w/ a slightly different but overlapping receptive field
• become more complex, b/c unlike simple excitatory receptive field of sensory receptor, the receptive field of the second order neurons has both excitatory and inhibitory regions

Question 14

lateral inhibition with a punctate stimulus

Answer 14

idealized point source of stimulation
-activates several touch receptors to varying degree, since primary unit influences its own secondary unit plus nearest neighbors

Question 15

two point discrimination with punctate stimuli

Answer 15

two receptive fields are stimulated by points separated by a third receptive field

Question 16

what is the second order neurons central ascending pathway in brainstem?

Answer 16

at level of dorsal column nuclei, second order neurons decussate immediately to opposite side in internal arcuate fibers, and continue ascent in medial lemniscus fiber bundle
-thus right side of brain gets input from sensory periphery on left side of brain and vice versa

Question 17

how is the medial lemniscus organized?

Answer 17

somatotopically

Question 18

where does the medial lemniscus synapse?

Answer 18

fibers synapse on ventral posterior nucleus of thalamus

Question 19

how is the VPN organized?

Answer 19

medial and lateral divisions, both receive tactile inputs

• medial: input from face
• lateral: input from trunk and limbs

Question 20

how is the representation of body surface in thalamic nuclei?

• how is sensory info processed?
• how what is represented mediolaterally and anteroposteriorly

Answer 20

sensory info about a particular modality from one part of body is processed by collections of neurons that form discrete functional units in thalamus

• axons in medial lemniscus that subserve a particular modality from a restricted body part form a bundle as they enter the thalamus
• within thalamic nuclei, place/topography is represented mediolaterally, and modality is represented anteroposteriorly

Question 21

what is the primary somatosensory cortex?

Answer 21

cortical region that gets somatosensory input from ventral posterior nucleus
-in postcentral gyrus of parietal lobe

Question 22

where do the medial and lateral divisions of VPN direct?

Answer 22

lateral: projects to medial and superior parts of postcentral gyrus
medial: projects to lateral part of postcentral gyrus

Question 23

what are the cortical sites of somatosensory processing?

Answer 23

SI can be subdivided into 4 distinct cytoarchitectonic regions designated as Brodmann’s areas 3a, 3b, 1, and 2
-most of thalamic fibers end in 3a and 3b, which then project to 1 and 2

Question 24

what did Wilder Penfield discover?

Answer 24

stimulation of postcentral gyrus produces tactile sensations in particular parts of opposite side of body

• explained how disturbance in somatosensory system could be localized clinically
• related to sensory homunculus

Question 25

what does the progression of a Jacksonian (sensory) seizure show you?

Answer 25

the direction of propagation of a seizure corresponds with the sensory cortex
-from hand –> forearm –> arm –> head –> trunk –> hip –> leg –> foot, laterally to medially

Question 26

how is segregation of peripheral inputs to SI based on modality? how is neocortex organized?

Answer 26

within any one of the architectonic areas, input from one or another submodality tends to dominate

• columnar organization of neocortex means that both architectonic and columnar regions are modality specific
• within any of the four architectonic areas of SI, there are several interrelated maps of place and modality

Question 27

what do cells of 3a/b, 1, and 2 primarily respond to?

Answer 27

3a - deep tissue input
3b - activation of rapidly or slowly adapting cutaneous receptors
2 - deep pressure
1 - activation of rapidly adapting cutaneous receptors

Question 28

what are direction-sensitive neurons?

Answer 28

process info at more complex level

• most posterior part of area SI (1 and 2, not 3s), there are vertical columns that respond only when stimulus moves across skin in certain direction
• other simpler cells are motion sensitive, when in any direction
• other complex cells that require particular orientation
• others respond only when large numbers of receptors are turned on simultaneously (grasping in hand), requiring edges in a particular direction

Question 29

what are somatic association areas?

Answer 29

posterior parietal cortex (Brodman’s 5/7)

-processing of sensory info is even more complex

Question 30

what does it mean when topographic representations are use-modifiable throughout life?

Answer 30

increase in cortical territory and receptive field size based on practice
-if encouraged to use middle 3 fingers at expense of others, the area in the cortex devoted to middle three fingers greatly expands, while the others aren’t as large