13: Somatic Sensation

Question 1

What are the 4 sensory receptors of skin?

Answer 1

• Temperature (thermoreceptors)
• Position of limbs (proprioceptors)
• Touch receptors (mechanoreceptors)
• Pain (nociceptors)

Question 2

Describe thermoreceptors

Answer 2

• Different types more active at warm or cold temperature ranges, especially responsive to changing temperature

Question 3

Describe proprioceptors

Answer 3

• (Muscle spindles, golgi tendon organs, joint capsule) skin mechanoreceptors

Question 4

Decribe touch/mechanoreceptors

Answer 4

• Sensitive to mechanical deformation (mechanoreceptors)

- Various types

Question 5

Describe nocicepters (pain)

Answer 5

• Respond to extreme mechanical, temperature, and/or chemical stimuli

Question 6

What are four types of skin touch receptors?

Answer 6

• Meissner’s corpuscles
• Pacinian corpuscles
• Merkel cell endings
• Ruffini’s corpuscles

Question 7

Draw out/decribe a pacinian corpuscle

Answer 7

slides

Question 8

What are four types of sensory coding?

Answer 8

• Modality
• Intensity
• Location
• Duration

Question 9

What is modality?

Answer 9

(eg; touch vs temperatre)

-Specificity of receptors. ‘labelled line’. (slides)

Question 10

What is intensity?

Answer 10

-Frequency of action potentials in each axon, number of axons activated

Question 11

What is location?

Answer 11

Mapping of receptive fields of individual primary afferents to specific cortical locations (somatotopic representation)

Question 12

Describe duration

Answer 12

• ‘rapidly adapting’ receptors respond briefly, even if stimulus is sustained. Detect movement, changing pressure.
• ‘slowly adapting’ receptors signal true duration of stimulus

Question 13

Where does the anterolateral/ventral pathway cross over, and what is it used for

Answer 13

• Crosses over low in the spinal cord

- For general or broad sensations pain, temperature

Question 14

Where does dorsal column pathways cross over, an what is it for?

Answer 14

• Crosses over high in the spinal cord, in medulla

- For fine, detailed pathways

Question 15

What is lateral inhibition?

Answer 15

A fundamental mechanism to increase accuracy of sensory information

Slides

Question 16

What is lateral inhibition mediated by?

Answer 16

Inhibitory interneurons

Question 17

When is lateral inhibition used

Answer 17

• In somatic sensation, lateral inhibition localises sensation to a restricted area of skin
• Lateral inhibition is used wherever accurate location of a stimulus is required -or where a pattern of input needs to be discriminated (eg; vision, smell..)

Lateral inhibition results in centre-surround inhibition

Question 18

Locate the somatosensory cortex

Answer 18

slides

Question 19

Describe the somatotopic map

Answer 19

-

Question 20

Describe pain (noiception) and its use

Answer 20

slides

Question 21

What are the two types of nerve fibres that carries pain/noiciception?

Answer 21

• “C”-fibres

- “Aδ” -fibres

Question 22

Describe C fibres

Answer 22

• Smallest diameter, unmyelinated axons
• Slow conduction velocity
• Signal ongoing damage (or potential damage)
• Polymodal

Question 23

Describe “Aδ” -fibres

Answer 23

• Small diamter myelinated axons
• Faster conduction velocity
• Signal acute onset of painful stimulus
• Primary mechanoreceptors

Question 24

Describe sensitation in pain pathways

Answer 24

slides

Question 25

Describe the gate control model for pain

Answer 25

slides

Question 26

What are two chemical methods of inhibiting pain

Answer 26

- Endogenous opiates | - Endocannabinoids

Question 27

Describe endogenous opiates

Answer 27

- Released at synapses on pain-pathway neurons - "natural analgesia" system, acute response to pain, stress - Site of action of 'centrally acting' painkillers (morphine, codeine)

Question 28

Describe endocannabinoids

Answer 28

- Synthesised and released by neurons (but not stored in vesicles), pain receptors, as well as centrally - Apparently mediate some presumed effects of endogenous opiates

Question 29

Describe referred pain

Answer 29

-Occurs when pain is poorly localised - Feelings from viscera (gut) is referred to the body surface - eg; heart pain to neck and arm - activated by stretch, ischemia -except in brain -Also, nerve compression can cause pain to be felt in region of nerve termination (slipped disc)

Question 30

What is a phatom limb

Answer 30

Sensation "felt" in region that no longer exists

Question 31

What are two causes of a phantom limb

Answer 31

1. Ongoing activity in nerves that used to come from that part 2. Invasion of cortical representation for that part by intact body regions - Eg; cortex representation of amputated hand may become activated by inputs from face Fibres from the face then occupy the 'territory' of the hand -allowing it to become its own sensory input.