Physiology of General Somatic Sensation

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List the steps involved in transforming pressure into action potentials by the Pacinian corpuscle

  • pressure deforms the onion-like membrane
  • Channels open and Na+ and K+ flow through the membrane
  • receptor depolarizes
  • If the graded potential is above threshold, it is an action potential occurs and is propagated down the axon

How is the magnitude of the pressure stimulus encoded?

  • The magnitude of the pressure stimulus is encoded with the frequency of the action potentials

Explain why it is easier to distinguish between 1 and 2 gm weights than between 100 and 101gm weights.

  • Essentially, saturation
  • relationship between the number of action potentials and pressure is non-linear
  • this means, then, that it there is a greater difference in the # of aps at the lower end of the weight spectrum (e.g., between 1 and 2gm) than at the higher end of the weight spectrum (e.g., between 101 and 102gm)

List the receptors responsible for touch and pressure; describe the response properties of each. For what type of stimulus does each respond best?

name depth adapting speed receptive field best stimulus
hair superficial fast small localized flutter
meissner superficial fast small very fine vibration
merkel superficial slow small fine details of a surface or edge
ruffini deep slow large skin stretch (position of fingers)
pacinian deep fast large diffuse vibration (powersaw)


A Pacinian receptor is rapidly adapting like a Meissner receptor. Why is a Pacinian receptor not good at detecting texture?

  • This is true because pacinian receptors are deep, and as such have a larger receptive field

Pick up a coin with your eyes closed. What information is required to tell you this is a coin?

  • The edges, size, texture, etc.

Explain the importance of the "labeled line" in touch

  • Many of the signals coming from afferents are similar
  • To solve this problem, each sensor type has its own "line" into the brain
  • These lines are called labeled lines
  • Because of these lines, there is no need to compress information
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A schematic diagram of the dorsal column medial lemniscal system

Draw the pathway for transmission of pressure information from the finger to the primary sensory cortex

  • The pathway is called the dorsal column lemniscal system
  • It follows the path in the schematic to the right
  • If the afferent comes from the lower part of the body, it is more medial in the spinal cord
  • If the afferent comes from the upper part of the body, it is more lateral

Describe the three functions of the dorsal column nuclei

Convergence

  • Allows a few DCNs to represent a large area
  • Such as with the back, where there are few afferents
  • Consequence is large receptive fields and low tactile acuity

Inhibitory surround

  • Enhancement of center, and inhibition of surround
  • Accentuates edges of an object
  • Also enhances 2-point discrimination

Cortical gating

  • Turns off some afferents to allow better attention to others

Suppose you touched a small bump on a smooth surface. Show that the activity of DCN neurons would be relatively independent of how hard you pressed down on the surface. What does this remind you of?

Describe the arrangement of the body surface on the primary somatosensory cortex (S1)

Consists of three features:

Somatosensory map

  • Arranged in what is called a homunculus
  • distorted representation of the body

Multiple maps

  • four strips: 1, 2, 3a, 3b
  • homunculus is repeated 4 times
  • different afferents predominate in each
Area Dominant Receptor Type
1 RA Afferents from the skin (for texture)
2 SA Afferents from deep in the skin (for shape and size of objects via joint location)
3a muscle afferents
3b skin afferents

Columns

  • found in area 3b
  • each column receives information from a different type of afferent
  • this maintains the labelled line

List the similar organizing features found both in the somatosensory and visual systems

Two of the similar organizing features are multiple maps and columns

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