Session 4: sensory pathways

Question 1

Define modality

Answer 1

a type of stimulus

they have specialised receptors, which transmit information through specific anatomical pathways to the brain

Question 2

List the major somatosensory modalities

Answer 2

MECHANORECEPTORS
for touch, pressure, vibration, propiception
THERMORECEPTOR
temperature
NOCICEPTOR
nociception (pain/response to harm)

Question 3

Which sensory nerve ending are free and which are enclosed?

Answer 3

• free nerve endings - thermoreceptors and nociceptors

- enclosed nerve endings - mechanoreceptors

Question 4

What are the 3 main classifications of sensory fibres/neurones?

Answer 4

Aβ fibres - mechanoreceptors of the skin
—–> innocuous, very fast mechanical stimulation
Aδ fibres - pain and temperature
—–> noxious mechanical and thermal stimulation
—–> fast (myelinated) transmission
C-fibres - noxious mechanical, thermal and chemical stimulation
—–> slower (non-myelinated) transmission of achy pain

Question 5

What classification of sensory fibres are used for propiception?

Answer 5

Aα fibres

Question 6

Define sensory receptors

Answer 6

transducers that convert energy from the environment into neuronal action potentials

Question 7

Define the absolute threshold

Answer 7

the point of intensity at which the person can just detect the presence of a stimulus 50% of the time

Question 8

How can an increased intensity of a stimulus be transmitted/detected?

Answer 8

Increased stimulus strength and duration = increased frequency of APs = increased neurotransmitter release = greater intensity

Question 9

Describe thermoreceptors and their nerve endings

Answer 9

free nerve endings with high thermal sensitivity

Question 10

What are TRP (transient receptor potentials) channels?

What is the role of TRP channels in detection of thermal change?

Answer 10

change in temperature activates the TRP ion channels

Transient receptor potential channels (TRP channels) are a group of ion channels located on membrane. They share the common features of six transmembrane segments and permeability to cations.

Question 11

Describe the transmembrane portions of TRP (transient receptor potentials) channels

Answer 11

• 4 heat activated TRP channels -> TRPV1-4

- 2 cold activated TRP channels -> TRPM8 and TRPA1

Question 12

What re the 4 types of mechanoreceptors? What types of touch are they specific to?

Answer 12

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

Question 13

What is meant by tonic and phasic receptors?

Answer 13

Tonic sensory input adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus.

In contrast, phasic receptors adapt rapidly to a stimulus. The response of the cell diminishes very quickly and then stops. It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate.

Question 14

Describe the differences in the stimuli detected and the speed of adaptation by tonic and phasic receptors

Answer 14

tonic receptors detect continuous stimulus strength (adapt slowly or not at all)

phasic receptors detect a change in stimulus strength (adapt quickly)

Question 15

Give an example of a tonic and phasic receptor

Answer 15

tonic - Merkel cell or Ruffini corpuscle

phasic - pacinian corpuscle

Question 16

What are receptive fields?

Answer 16

the region on the skin that causes activation of a single sensory neurone when activated

Question 17

Describe the significance of different sized receptive field and relate it to areas of the body

Answer 17

small receptive field allow for the detection of fine detail over a small area = precise perception
e.g. fingers have densely packed mechanoreceptors

large receptive field have less precise perception
e.g. on the back

Question 18

Define two point discrimination

Answer 18

the minimum distance at which two points are perceived as separate
(related to the size of receptive fields)

Question 19

What are the 2 types of nociceptors? What kind of pain do they mediate?

Answer 19

Aδ fibres - mediate sharp, intense or first pain

C-fibres - mediate dull, persistent or second pain

Question 20

Describe the properties of the different types of nociceptors and their corresponding sensory neurones

(Aδ fibres - mediate sharp, intense or first pain
C-fibres - mediate dull, persistent or second pain)

Answer 20

Aδ fibres mediate sharp pain

• Myelinated – quite fast
• Type 1: Aδ-mechano-heat receptors (noxious mechanical and thermal stimuli)
• Type 2: Aδ-mechanoreceptors (purely noxious mechanical stimuli)

C-fibres mediate dull pain

• Unmyelinated – SLOW
• Respond to thermal, mechanical and chemical stimuli (polymodal)
• Chemical stimuli include inflammatory mediators
• Polymodal: respond to all the modalities – only one type of C fibre

Question 21

Where are the cell bodies of afferent neurones? (2)

Answer 21

in the dorsal root ganglia (body) and trigeminal ganglia (face)

Question 22

Describe where the different types of afferent spinal nerves terminate in the spinal chord

Answer 22

• Innocuous mechanical stimuli – Aβ-fibres terminate in deep dorsal horn
• Pain and temperature – Aδ and C-fibres terminate in superficial dorsal horn

Question 23

What is the main excitatory NT released from the pre-synaptic neurone within the dorsal horn?

Answer 23

glutamate

Question 24

What is the purpose of the interneurons?

Answer 24

Interneurons connect between different laminae and between adjacent peripheral inputs (lateral inhibition)