L14: Somesthesis (mechanoreceptors)

Question 1

Tactile receptors can be divided into 2 major groups, which are?

Answer 1

1) slowly adapting mechanoreceptors, which respond to an enduring stimulus
2) rapidly adapting mechanoreceptors, which respond only at the onset and sometimes the termination of a long lasting stimulus

Question 2

List the 4 mechanoreceptors that are found in glabrous (hairless) skin and state whether they are slowly adapting or rapidly adapting

Answer 2

1) Pacinian Corpuscles (lamellar corpuscles) = rapidly adapting mechanoreceptor found in deep tissues or immediately beneath skin, responsible for sensitivity to vibration & pressure
2) Meissner’s Corpuscles = encapsulated nerve ending that excites a large myelinated sensory nerve fiber. They are rapidly adapting.
3) Merkel Receptors = slowly adapting mechanoreceptors that transmit an initially strong but partially adapting signal and then a continuing weaker signal.
4) Ruffini’s end organs = multi-branched, encapusulated nerve endings that adapt very little,

Question 3

Slight movement of any hair on the body stimulates what type of receptors?

Answer 3

another type of receptor that is made up of nerve fiber entwining the base of the hair

Question 4

What is an important feature of somesthesis?

Answer 4

An impt feature of somesthesis is an ability to differentiate btw stimuli of the same quality that differ in place, magnitude & spatial pattern. This could be done due to relationship btw receptor density & receptive field size

Question 5

In terms of somesthesis (epicritic sensation), what does receptive field refer to?

Answer 5

portion of skin directly innervated by the receptor terminals and the area of adjacent tissue thru which a stimulus can be conducted to the receptor

Question 6

Areas of skin that have high density of receptors are more or less sensitive for fine spatial discrimination? These areas of skin have small or large receptive fields?

Answer 6

areas of skin with high density of receptors are most sensitive and have the greatest capacity for fine spatial discrimination. These areas have the smallest receptive fields but have the largest NUMBER of receptive fields per unit area.

Question 7

receptive field size: compare fingertips to forearm to back.

Answer 7

fingertips are most sensitive to 2-point discrimination. Therefore, it will have the smallest receptive field size (2-3 mm). Forearm will have 20 mm and back will have 40 mm.

Question 8

Areas of high sensile tactility vs areas of low sensile tactility: compare and contrast their receptor fields and receptor densities

Answer 8

high sensile tactility = high receptor densities, small receptive fields

low sensile tactility = low receptor densities, high receptive fields

Question 9

How do mechanoreceptors give us spatial temporal discrimination ability?

Answer 9

Different mechanoreceptors give us different info (e.g. meissner = time; mercekl = spatial) which overall will contribute to all the somatosensory info/experience.

Question 10

Describe the laminar organization of the spinal cord

Answer 10

the gray matter of the spinal cord has different layers. from dorsal to ventral, it’s labeled I to IX.

Question 11

List and explain the 2 major ascending systems carrying somatosensory information to the cerebral cortex.

Answer 11

1) dorsal column/medial lemniscus system
- mediates tactile sensations including vibration sense & proprioception

2) anterolateral system aka spinothalamic system
* carries info chiefly abt pain & temperature
- mediates tactile info based on crude touch and pressure sesnations = poor localizing ability on body and little capability for fine intensity discrimination

Question 12

Large myelinated fibers carrying epicritic information pass into the spinal cord in which specific region?

Answer 12

medially into the lateral margin of the dorsal columns

Question 13

The dorsal columns are somatotopically organized. Explain

Answer 13

More medially = lower limb, trunk

More laterally = upper limb, neck

Question 14

As the dorsal column fasiculi (gracile & cuneate fasicului) synapse at the gracile & cuneate nuclei in the caudal medulla, receptive fields of second order neurons become larger why?

Answer 14

receptive fields become larger b/c they receive convergent input from many primary neurons, each with a slightly different but overlapping receptive field.

Question 15

In what ways are second order neurons receptive fields become more complex than the sensory receptor?

Answer 15

second order neurons’ receptive field has both excitatory & inhibitory regions unlike sensory receptor’s receptive field which only has a simple excitatory receptive field.

Question 16

when there’s a punctuate stimulus (pin to skin), does only one region of receptors get activated?

Answer 16

No, several touch receptors are activated to a varying degree. Each of the primary units influences its own secondary unit and it also influences its nearest neighbors.

Question 17

Gracile and cuneate fasiculi synapse in caudal medulla at gracile and cuneate nuclei. They decussate and then ascend as medial lemniscus. Where does medial lemniscus synapse in the thalaus?

Answer 17

medial lemniscus will synapse at the VPL nuclei of the thalamus.

Question 18

epicritic and protopathic sensations of face and head synapse at which thalamic nuclei?

Answer 18

VPM nuclei

Question 19

Within the thalamic nuclei, how are “place” and “modality” represented?

Answer 19

place - mediolaterally

modality -anteroposteriorly

Question 20

Somatosensory info eventually will reach post-central gyrus, the primary somatosensory cortex. Explain where specifically in the primary somatosensory cortex that VPL and VPM info are sent.

Answer 20

VPL (contains somatosensory info from limbs and trunks) will project to the medial and superior portions of the post-central gyrus

VPM (contains somatosensory info from face and head) will project to the lateral portion of the post-central gyrus

Question 21

The primary somatosensory cortex can be divided into 4 distinct cytoarchitectonic regions. List them. Where do most of the thalamic fibers terminate?

Answer 21

Brodmanns areas 3a, 3b, 2 and 1
3a - respond to deep tissue input
3b - respond primarily to activation of cutaneous receptors
2 - respond primarily to pressure
1- respond to activation of rapidly adapting cutaneous receptors

-Most of the thalamic fibers terminate in areas 3a and 3b. The cells in 3a and 3 b in turn project to areas 1 and 2

Question 22

Explain the somatotopic representation in primary somatosensory cortex (S1)

Answer 22

more medially = legs, trunk, arms, hand, and face most laterally

Question 23

Jacksonian seizures

Answer 23

seizures that affect areas of the body in the order that follows the somatotopic topography of the primary somatosensory cortex.
ex. spreading from finger to hand, then arm, down across shoulder and down ipsilateral leg (moving laterally to medially)

Question 24

Are there fairly complete somatotopic maps in all 4 regions of the primary somatosensory cortex or as a whole, they form one map?

Answer 24

there are 4 independent and complete maps in S1, one for each of the areas 3a, 3b, 2, and 1.

Question 25

In addition to architectonic regions (3a, 3b, 2, and 1) that are modality specific, there is also a columnar organization of the neocortex that is also modality specific. Within any one of the 4 architectonic areas of S1 there are, one or several interelated maps of place and modality?

Answer 25

several interrelated maps of place and modality

Question 26

There are cells in S1 which process info at a more complex level. For example, there are cells that are not found in 3a or 3b, but in 1 and 2 that are direction-specific. What does that mean?

Answer 26

these vertical columns of cells located in the most posterior portion of area S1 respond only when a stimulus moves across the skin in a particular direction.

Question 27

What are cells that respond to stimulation in any direction?

Answer 27

simple cells that are only motion sensitive

Question 28

The processing of sensory info becomes even more complex farther posteriorly in the posterior parietal cortex in regions known as?

Answer 28

somatic association areas

Question 29

What does it mean that topographic representations are use-modifiable thru out life?

Answer 29

practice or experience plays a role in the plasticity of somatoptic maps

For. instance, encouraging monkeys to use their middle 3 fingers at the expense of the others saw that the area of cortex devote to the middle 3 fingers was greatly expanded.