The Somatosensory System

Question 1

List the major somatosensory modalities

Answer 1

Touch
Temperature
Nociception
Proprioception

Question 2

Which receptors have free nerve endings and which have enclosed nerve endings?

Answer 2

FREE: thermoreceptors and nociceptors

ENCLOSED: mechanoreceptors

Question 3

Classify the different sensory neurone axon types

Answer 3

(in order of decreasing diameter and myelination, and hence transmission speed)

Aβ-fibres: innocuous mechanical stimulation (so mechanoreceptors)

Aδ-fibres: noxious mechanical and thermal stimulation (nociceptors and thermoreceptors)

C-fibers: noxious mechanical, thermal and chemical stimulation (nociceptors and thermoreceptors)

Question 4

What are sensory receptors?

Answer 4

Sensory receptors are transducers that convert energy from the environment into neuronal action potentials

Question 5

What types of ion channels expressed by thermoreceptors mediate the ability to detect thermal changes?

Answer 5

• Transient receptor potential (TRP) ion channels
• 4 heat activated ones = TRPV1-4
• 2 cold activated = TRPM8
  and TRPA1

Temperatures above neutral = C-fibres
Temperatures below neutral = Aδ-fibres

Question 6

What are the four different types of mechanoreceptors, and what roles do they each have?

Answer 6

1. Meissner’s corpuscle:
   - Fine discriminative touch, low frequency vibration
2. Merkel cells:
   - Light touch and superficial pressure
3. Pacinian corpuscle:
   - Detects deep pressure, high frequency vibration and tickling
4. Ruffini endings:
   - Continuous pressure or touch and stretch

Question 7

State what is meant by Stimulus Threshold

Answer 7

The point of intensity at which the person can just detect the presence of a stimulus 50% of the time - known as the absolute threshold

Question 8

How do you interpret stimulus intensity?

Answer 8

Increased stimulus strength and duration = increased neurotransmitter release = greater intensity

Question 9

What are tonic receptors? Give an example.

Answer 9

• Do not adapt/very slowly adapting
• Detect continuous stimulus strength and continue to transmit impulses to the brain as long the stimulus is present
• Keeps the brain constantly informed of the status of the body

e.g. Merkel cells

Question 10

What are phasic receptors? Give an example.

Answer 10

• Fast adapting
• Detect a change in stimulus strength
• Transmit an impulse at the start and the end of the stimulus

e.g. Pacinian receptor

Question 11

What is a receptive field?

Answer 11

The receptive field is the region on the skin which causes activation of a single sensory neurone

Question 12

Describe how the receptive fields in the lips, mouth and fingers vary from the receptive fields of the upper arm and back.

Answer 12

Lips, mouth and fingers:

• Densely packed mechanoreceptors
• Small receptive fields allow for the detection of fine detail over a small area - precise perception

Upper arm and back:
- Large receptive fields allow the cell to detect changes over a wider area - less precise perception

Question 13

Define two point discrimination. What is it related to?

Answer 13

Minimum distance at which two points are perceived as separate.
Related to the size of the receptive field

Question 14

Define somatosensory dermatome

Answer 14

A dermatome is an area of skin that is supplied by sensory neurones from a single dorsal root of a spinal nerve (forms a part of a spinal nerve)

Question 15

Aβ-fibres, Aδ-fibres and C-fibres synapse with others neurones in which areas of the dorsal horn?

Answer 15

Aδ-fibres and C-fibres: superficial layers of dorsal horn (lamina I and II)

Aβ-fibres: deeper layers of dorsal horn (lamina III, IV, V)

Question 16

Define Lateral Inhibition

Answer 16

Lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors by disabling the spread of action potentials from excited neurons to neighboring neurons in the lateral direction => enhanced sensory perception
Means that receptive fields don’t overlap
(Mediated by inhibitory interneurons within dorsal horn of spinal cord)

Question 17

Recall the pathway of the 1st order sensory neurones (dorsal column pathway)

Answer 17

Aβ fibers enter via the dorsal horn and enter the ascending dorsal column pathways;
- Information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract. Neurones synapse in the Gracile Nucleus (medulla)

• Information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract. Neurones synapse in the Cuneate Nucleus (medulla)

Question 18

Where do the 2nd order sensory neurones decussate, name the tract that they form and where do they terminate?

Answer 18

• Decussate in the caudal medulla.
• Form the contralateral medial lemniscus tract
• Terminate in the ventral posterior lateral nucleus of the thalamus

Question 19

Where do the 3rd order sensory neurones project to?

Answer 19

Project to the somatosensory cortex
Somatotopy = correspondence of an area of the body to a specific point on the central nervous system; Size of somatotopic areas is proportional to density of sensory receptors in that body region

Question 20

Pain and temperature, and crude touch sensory information (2nd order neurones) ascend within which tracts?

Answer 20

Pain and temperature - lateral spinothalamic tract

Crude touch - anterior spinothalamic tract (mediated by Aδ-fibres)

Remember: 1st order terminate upon entering the spinal cord; 2nd order neurones decussate immediately and terminate in VPL nucleus of thalamus)

Question 21

What method is used to test the integrity of ascending pathways?

Answer 21

Quantitative sensory testing (for pain and temp, discriminative touch etc.)

Question 22

Which nociceptor fibres mediate which types of pain?

Answer 22

Aδ fibers mediate sharp, intense or first pain;
Type 1: noxious mechanical
Type 2: noxious heat

C-fibres mediate dull, aching or second pain
- Noxious thermal, mechanical and chemical stimuli

Question 23

State the major excitatory neurotransmitter released from sensory afferents in response to noxious stimuli (essential for pain signalling)?

Answer 23

Glutamate

Question 24

What two components make up the pain pathway? State the relevant tracts

Answer 24

Sensory component: Lateral spinothalamic tract

Emotional component: Spinoreticular tract

Question 25

Explain the significance of the pain matrix

Answer 25

Functional MRI (fMRI) has shown there is a ‘cerebral signature for pain’

Involves many areas of the cortex (SI, SIII, Insula cortex, Anterior cingulate cortex, Prefrontal cortex) and other brain areas (Amygdala, Cerebellum, Brainstem)

Question 26

Briefly describe the Gate Control Theory

Answer 26

Gate Control Theory of Pain describes how non-painful sensations can override and reduce painful sensations,
i.e. Inhibition of primary afferent inputs (stimulated by noxious stimulus) by touch/Aβ-fibres, before they are transmitted to the brain through ascending pathways (via inhibitory interneurones)

Question 27

What is the descending pain modulatory system?

Answer 27

• Descending influences on spinal nociceptive processing involves the Periaqueductal Grey (PAG) and the rostral ventromedial medulla (RVM)
• These are linked to a number of higher level brain areas including; cingulofrontal regions, the amygdalae and the hypothalamus (helps explain the role that emotions and cognition have in processing nociceptive information)
• (In dorsal horn) Inhibition of substance P from noxious mechanical stimulation is via release of noradrenalin, and thermal nociceptive stimuli via the release of serotonin.

Essentially, “strong emotions” block pain.

Placebo activates descending inhibition in humans

Question 28

Define Allodynia

Answer 28

Pain due to a stimulus that does not normally provoke pain

Question 29

Define Hyperalgesia

Answer 29

Increased pain from a stimulus that normally provokes pain

• Primary (at the site of stimulus)
• Secondary (adjacent site to stimulus)

Question 30

Distinguish between central and peripheral sensitisation

Answer 30

CENTRAL = “Increased responsiveness of nociceptive neurones in the central nervous system to their
normal or sub-threshold afferent input.”; due to neural plasticity => recruitment of additional, sub-threshold synaptic inputs to nociception

PERIPHERAL = “Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to
the stimulation of their receptive fields.”; initiated by inflammatory mediators in damaged tissue