Somatosensory

Question 1

4 Receptors w/ Encapsulated Nerve Endings

Answer 1

Meissner’s Corpuscles -small receptive fields; superficial; rapidly adapting; stroking, fluttering, low frequency vibration; good for grip

Merkel’s Discs - small receptive fields; superficial; slow adapting; form & texture

Pacinian Corpuscles - large receptive fields; deeper dermis; rapidly adapting; high frequency vibration

Ruffini Ends - large receptive fields; deeper dermis; sloly adapting; skin stretch

Question 2

5 Types of Stimulus Coding

Answer 2

• 1- quality - type of receptor (mechanical, temp, pain)
• 2- temporal - onset/offset for RA receptors; duration for SA receptors
• 3- threshold - may only fire if gets to certain point
• 4- frequency - inc intensity means inc firing rate
• 5- location - fire when w/in receptive field (highest rate when in center of field)

Question 3

Receptive Fields (distribution & how they work)

Answer 3

• Fingers = smallest receptive fields; also most innervation density (nerves per area) so least overlap and lowest threshold for 2 pt discrimination
• Excitatory center w/ lateral inhibition - enhances difference in activity level for discrimination
  • Inhibition does not take place until level of caudal medulla of dorsal columns
  • So 2 pt discrimination = 2 puts of excitation separated by inhibition

Question 4

How does facial somatosensation work?

Answer 4

• A-beta for jaw position/proprioception
  
  • Synapse on principal sensory nucleus (pons) and/or trigeminal nucleus (medulla)
• A-delta and C for pain/temp
  
  • Only synapse in spinal trigeminal nucleus (medulla)
• Implications:
  
  • Principal sensory nucleus = gracile/cuneate nucleus (PROPRIOCEPTION ONLY)
  • Spinal trigeminal nucleus = Dorsal horn (ALL - including pain/temp)
  • Lesions… to principal sensory —> proprioception probs… to trigeminal nucleus —> pain/temp probs (ALL LESIONS CAUSE IPSAILTERAL PROBS)

Question 5

How is the 3,1,2 area broken down?

Answer 5

3a - direct thalamic input; SA proprioceptions (deep pressure and joints)

3b- direct thalamic input ; RA/SA from skin

1- weak thalamic input; RA/SA from skin

2- weak thalamic input; SA proprioception

Question 6

Columnar Organization

Answer 6

• Vertical columns share… same receptive fields, same modality, short latency b/n them
• Functional unit of processing

Question 7

Somatotropic Organization of Somatosensory Cortex

Answer 7

• Somatosensory is very similar to motor cortex
• Medial = legs/genitals (ACA)
• Lateral = upper body/face (MCA)

Question 8

ACA Stroke Deficits

Answer 8

-contralateral paralysis and loss of sensation of lower limb, gait problems, urinary incontinence

Question 9

MCA Stroke Deficits

Answer 9

• contralateral paralysis and loss of sensation to upper limbs and face, Wernicke’s aphasia (understanding problem) or Broca’s aphasia (projection problem)
• Damage to non-dominant side may lead to neglect