Exteroception

Question 1

Somatosensory system definition

Answer 1

• collection of receptors whose ending transduce information about the body surface, the skin, some deeper tissues

Question 2

Somatotopy definition

Answer 2

• precise and orderly mapping of body surface onto the cortex

Question 3

Types of mechanoreceptors

Answer 3

• rapidly-adapting
• slowly-adapting

Question 4

Rapidly adapting receptors

Answer 4

• stimulated with steady touch ==> fire a few AP ==> stop firing even though skin is still being pressed

Question 5

Slowly-adapting receptors characteristics

Answer 5

• stimulated w/steady touch
• keep firing throughout the time skin is pressed

Question 6

Receptive field definition

Answer 6

• area of skin in which a mechanical stimulus elicits a response from the cell
• small field receptors w/sharp borders
• large field receptors w/poorly-defined borders
  
  • tend to lie in deeper tissues (dermis, subcutaneous)

Question 7

Meissner’s corpuscles (adaptation, receptive field)

Answer 7

1. rapidly adapting
2. small receptive field

Question 8

Merkel’s discs (adaptation, receptive field)

Answer 8

• slow-adapting
• small receptive field

Question 9

Pacinian corpuscles (adaptation, receptive field)

Answer 9

• rapidly-adapting
• large receptive fields

Question 10

Ruffini’s ending (adaptation, receptive field)

Answer 10

• slowly-adapting
• large receptive field

Question 11

Small vs. Large Receptive fields jobs

Answer 11

• small receptive fields = fine tactile sens of fingertips
  
  • high density gradient ==> high spatial acuity of touch @ fingertips
• large receptive fields = skin deformation over a wide area

Question 12

Mechanoreceptors axon types

Answer 12

• A(beta)
• =large, myelinated axons
• conduct @ ~35-75 m/sec

Question 13

Skeletal muscle proprioceptors axon types

Answer 13

• A(alpha)
• condution @ 80-120 m/sec

Question 14

Location of cell bodies & central processes of mechanoreceptors

Answer 14

• cell bodies @ dorsal root ganglia
• central processes ==> dorsal root ==> spinal cord
• central processes bifurcate ==>
  
  • local branches to dorsal horn in spinal reflexes
  • ascending brances entering the dorsal (posterior) columns
    
    • lower limbs = gracile fasciculus
    • upper limbs = cuneate fasciculus

Question 15

Course of sensory info from mechanoreceptors ==> cerebral cortex

Answer 15

• mechanoreceptors ==> DRG ==> spinal cord
  
  • bifurcate ==> dorsal horn (spinal reflex) & ==> dorsal column
• axons @ dorsal column synapse @ nuclei @ medulla ==> cross midline = “medial lemniscus”
• medial lemniscus fibers ==> ventral-posterior-lateral (VPL) nucleus @ thalamus
  
  • head = @ VPM nucleus
• VPM + VPL = ventro-basal complex ==> areas 3, 1, 2 @ primary somatosensory area
  
  • projections ==> primary motor, secondary somatosensory area, and association somatosensory areas

Question 16

“nearest neighbor” concept + characteristics of somatotopy

Answer 16

• neighboring cells @ cuneate nucleus or thalamus have receptive fields near to one another within the skin
• ==> somatotopic map/homunculus on cortical surface
  
  • topographic arragement/homunculus is distorted due to different density of afferents @ different regions
    
    • e.g. large lips and hands + small proximal limbs/trunk

Question 17

Columnar organization @ somatosensory system characteristics

Answer 17

• = vertical arrangement of fxnly related cells
• @ somatosensory columnar organization = vertical segregation of cells by response modality
• cortical columns = computational modules that transform info from thalamus and redistribute it

Question 18

Sensory modalities @ Broadmann’s area 3a, 3b, 1, 2

Answer 18

• 3a = muscle stretch receptors
• 3b = cutaneous receptors
• 1 &2 = respond to complex stimuli
  
  • direction and orientation-sensitive
  • some @ 2 = shape-sensitive
  • 3a/3b project onto 1 & 2 to allow for complex stimulus processing and clues for fine motor

Question 19

Lemniscal pathway/trigeminal nerve characteristics

Answer 19

• lemniscal system for head involves trigeminal nerve = ~DRG for the head
• trigeminal carries motor + touch + proprioception + pain + temp for face and head
• mechanoreceptor soma = @ trigeminal ganglion
  
  • central processes terminate in different nuclei
  • main sensory nucleus = principal nucleus ==> second order cells send axons across midline to join medial lemniscus ==> VPM