Somatosensory System

Question 1

What does the somatosensory system do?

Answer 1

Information transmitted from the skin, muscles, and joints to CNS
-> Touch, pressure, vibration, limb position/proprioception, heat/cold, itch, and pain

Subsystems
- Cutaneous Mechanoreceptors (mechanical deformation of the receptor by discriminative touch, vibration and pressure)
- Receptors of muscles, tendons, and joints (proprioception)
- Pain, temperature, and non-discriminative (sensual) touch

Question 2

What is a pseudo unipolar neuron?

Answer 2

-Single axon that bifurcates
- One process peripherally, one process centrally

Usually first order neuron

Question 3

Where are the cell bodies of somatosensory neurons?

Answer 3

Reside in ganglia adjacent to the spinal cord (ganglia sits outside of dorsal horn)

Periphery to CNS
- 3 neuron pathway

Question 4

Describe free nerve endings

Answer 4

• Unmyelinated
  -> since there is no myelin sheath it travels slower
• found in receptors that detect pain, temperature, itch, non-discriminative touch

Question 5

What receptor type is proprioception (sensory function)?

Answer 5

Muscle spindle

(Fastest and biggest diameter)
-> highly myelinated since it is so fast

Question 6

What receptor type is touch (sensory function)?

Answer 6

Merkel, Meissner, Pacinian, and Ruffini cells

(Second fastest and second biggest diameter)

Question 7

What is a receptive field?

Answer 7

• Area of skin innervated by a SINGLE afferent (bringing info towards CNS) neuron
• Smaller distally and larger proximally (more precision distally)
  (if i touch a receptive field on my forearm, but im touching different areas, the same afferent neuron is firing)
• Increased density of receptive fields distally (more receptive fields/sensory neurons in the finger tips for example)

Question 8

What is two point discrimination?

Answer 8

A clinical test that checks how sensitive skin is

-Minimum distance that one is able to perceive two simultaneously applied stimulus (feeling them as two rather than 1 touch)

Question 9

Describe Cutaneous Mechanoreceptors

Answer 9

• Detect and transmit information about touch sensory stimuli
• Often encapsulated by specialized receptor cells (with the exception of free nerve endings)

Types
- Meissner’s Corpuscles
- Merkel’s Disc
- Pacinian Corpuscles
- Ruffini Endings
- Free Nerve Endings

Question 10

What do Merkel’s Disc do?

Answer 10

Function: Shape and texture perception; slowly adapting

Stimuli: Edges, points, corners, curvature

Located in Epidermis

Question 11

What is Meissner Corpuscles?

Answer 11

Function: Motion detection; grip control; rapidly adapting

Stimuli: Skin motion

Located in Epidermis

Question 12

What is Pacinian Corpuscles?

Answer 12

Function: Transmission of vibration; rapidly adapting

Stimuli: vibration/deep touch

Located in Subcutaneous layer

Question 13

What is Ruffini Ending?

Answer 13

Function: Hand shape/motion detection; slowly adapting

Stimuli: Skin stretch

Located in Dermis

Question 14

What innervates hair follicles?

Answer 14

Innervated by a variety of mechanoreceptive afferents

Question 15

What do Free Nerve Endings detect?

Answer 15

Coarse/Crude Touch
-Pleasant touch, tickle, or itch

Nociceptors (Pain)
- Respond to stimulus that may damage or threaten tissue

Thermoreceptors
-Hot/cold
- Responsive within a range that is non-damaging to tissue

Question 16

What is Sensory Transduction?

Answer 16

• Stimulus triggers an alteration in cation channel permeability
• If sufficient stimulus of receptor potential -> progression to an AP

Question 17

Describe Muscle Spindles

Answer 17

• Embedded within skeletal muscle
• Fusiform shape (tapered at both ends)
  -> Intrafusal fibers: muscle fibers within
  -> Extrafusal fibers: muscle fibers outside (force producing)
• Responds to changes in muscle length and velocity of length change
  CENTRAL REGION OF SPINDLE MUST BE TAUT TO DETECT STRETCH

Two sensory afferents
1. Primary Endings (Type Ia)
- Rapidly adapt to change in muscle length
- Detect velocity/direction of movement
2. Secondary Endings (Type II)
- Produce sustained responses to constant muscle lengths
- Detect static position of a limb
Muscle stretch -> Tension of fibers stimulates mechanically gated ion channels triggering an AP

• Contractile at each end however provide minimal to force
• Contraction maintains sensory activity of spindle
• Motor Efferent: Gamma motor neuron

Question 18

What are Golgi Tendon Organs (GTO)?

Answer 18

• Responsive to TENSION of tendons via both active contraction or passive stretch of the muscle
  -> Ib sensory afferents

Question 19

What are Joint Receptors

Answer 19

• Responsive to mechanical deformation of the joint capsule and ligaments
• Sensory afferents
  -> Include both slowly and rapidly adapting neurons