sensory pathways

Question 1

give examples of Somatosensory modalities and what receptors they are sensed by

Answer 1

Touch inc. light touch, pressure, vibration – sensed by mechanoreceptors
Proprioception inc. joint position, muscle length, muscle tension – sensed by mechanoreceptors
Temperature – sensed by thermoreceptors
Pain – sensed by nociceptors

Question 2

how are neurones specialised for these modalities

Answer 2

Individual axons of sensory nerves have modified terminals

Question 3

what are 3 classes of sensory neurones

Answer 3

Aβ-fibres: innocuous mechanical stimulation
C-fibers: noxious mechanical, thermal and chemical stimulation
Aδ-fibres: noxious mechanical and thermal stimulation

Question 4

order sensory neurones from fastest to slowest

Answer 4

Aβ-fibres, Aδ-fibres, C-fibers

Question 5

define sensory receptors

Answer 5

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

Question 6

how do thermoreceptors work

Answer 6

have free nerve endings and Transient receptor potential (TRP) ion channels
4 heat activated TRPs and and 2 cold activated

Question 7

name 4 mechanoreceptors and what they detect

Answer 7

Meissner’s Corpuscle - 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 8

what is tonic receptor (give example)

Answer 8

Tonic receptors
Detect continuous stimulus strength
Continue to transmit impulses to the brain as long the stimulus is present
Keeps the brain constantly informed of the status of the body

example (Merkel)

Question 9

What is phasic receptor (give example)

Answer 9

Detect a change in stimulus strength
Transmit an impulse at the start and the end of the stimulus
e.g. when a change is taking place

example (pacinian)

Question 10

what is the stimulation threshold

Answer 10

the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold) - The weakest detectable stimulus

Question 11

what is Stimulus intensity discrimination

Answer 11

a more intense stimulus increases NT released and hence increases freq of AP’s AND increases the number of separate receptors activated – amplitude REMAINs the same

Question 12

define receptive field

Answer 12

’The receptive field is the region on the skin which causes activation of a single sensory neuron when activated’ - can overlap

Question 13

define Lateral inhibition

Answer 13

activation of 1 neural unit – inhibits activation of other units
Occurs at interneurons in the dorsal horn of spinal cord – this increases the precision in localization of the stimulus
(prevents overlap of receptive fields)

Question 14

define 2 point discrimination

Answer 14

ability to detect that 2 stimuli are distinct from each other – the minimum distance required between 2 stimuli in order to perceive them as separate

Question 15

what is gate theory

Answer 15

non painful stimulation can inhibit primary afferent inputs of pain before they are transmitted to brain – neuronal dysfunction can cause depolarization in response to non-painful stimuli

Question 16

what is the difference between small and large receptive fields

Answer 16

Small receptive fields allow for the detection of fine detail over a small area. Precise perception

Large receptive fields allow the cell to detect changes over a wider area (less precise perception)

The fingers have many densely packed mechanoreceptors with small receptive fields

Question 17

what does dorsal column pathway detect

Answer 17

Innocuous mechanical stimuli
Fine discriminative touch
Vibration

Question 18

what is the difference between upper and lower limb innocuous sensation

Answer 18

Information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract

Information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract

Question 19

Path of 1° neurones – DC-ML pathway

Answer 19

Cell bodies of 1° neurones are in the dorsal root ganglia
1° neurones transmit information from the sensory receptors, ascend ipsilaterally up the spinal cord to the medulla.
Neurones from the upper limb synapse ascend in the cuneate fasciculus
Neurones from the lower limb ascend in the gracile fasciculus

Question 20

Path of 2°/3° neurones – DC-ML pathway

Answer 20

1° neurones from the cuneate and gracile fasciculi synapse with 2° neurones in the cuneate and gracile nuclei.
The 2° will decussate across the medulla and synapse with 3° neurones in the contralateral thalamus (VPL)
3° neurones will ascend to the somatosensory cortex

Question 21

what specific nucleus of the thalamus do secondary neurones terminate in and describe it

Answer 21

The axons of the second order neurons terminate in the ventral posterior lateral nucleus of the thalamus (VPL).

There is a topographic representation of the body in the VPL

Question 22

where are lower extremities represented in the VPL

Answer 22

Laterally

Question 23

what tracts can the spinothalamic pathway be divided into

Answer 23

anterior and lateral

Question 24

what do different tracts of the spinothalamic pathway detect

Answer 24

Pain and temperature sensations ascend within the lateral spinothalamic tract

Crude touch ascends within the anterior spinothalamic tract

Question 25

what fibre types mediate crude and fine touch

Answer 25

Crude touch is mediated by Adelta fibres (Free nerve ending)

Fine touch is mediated by Abeta fibres (Meissner’s corpuscles)

Question 26

path of 1° neurones – spinothalamic pathway

Answer 26

Cell bodies of 1° neurones are in the dorsal root ganglia
1° neurones enter the dorsal horn via the dorsal root and immediately terminate in the spinal cord (synapse with secondary neurones)

Question 27

path of 2°/3° neurones – Spinothalamic

Answer 27

2° neurones from the dorsal horn will immediately decussate across the spinal cord
They will ascend contralaterally in the spinothalamic tracts synapse with 3° neurones in the contralateral thalamus (VPL)
3° neurones will ascend to the somatosensory cortex

Question 28

key difference between dorsal column and spinothalamic tract

Answer 28

Dorsal column:
Light touch
Vibration
2-point discrimination

Spinothalamic tract:
Pain
Temperature
Coarse touch

Question 29

cause and consequence of Anterior spinal cord lesion

Answer 29

Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord

Spinothalamic tract damage causes bilateral pain and temperature loss below the level of the lesion

Retained light touch, vibration and 2-point discrimination due to intact dorsal columns

Question 30

define pain

Answer 30

An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.’

Question 31

how are different types of pain detetced

Answer 31

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 (polymodal)

Question 32

what pathway mediates emotional aspect of pain

Answer 32

Spinoreticular tract

Question 33

what is the pain matrix

Answer 33

Functional MRI (fMRI) has shown there is a ‘cerebral signature for pain’ 
Cortex: 
SI 
SII 
Insula cortex
Anterior cingulate cortex 
Prefrontal cortex 

Amygdala
Cerebellum
Brainstem

Question 34

difference between peripheral and central sensitisation

Answer 34

Peripheral: ↓ thresholds to peripheral stimuli at the site of injury

Central: ↓ thresholds to peripheral stimuli at an adjacent site to the injury
expansion of receptive field
spontaneous pain

Question 35

classify chronic pain

Answer 35

Nociceptive:
noxious stimulation of a nociceptor (somatic or viscera)
e.g. arthritis, fracture

Neuropathic: lesion or disease of the somatosensory system
e.g. diabetic, sciatica, post surgery

Question 36

which centres control descending pain modulation

Answer 36

Periaqueductal Grey (PAG), Locus coreleus (LC)

Question 37

what does the PAG do

Answer 37

Facilitation and inhibition of nociceptive processing in the dorsal horn via secretion of Monoamines:

Serotonin
Noradrenaline

Question 38

define allodynia

Answer 38

pain due to a stimulus that does not normally provoke pain

Question 39

define Hyperalgesia

Answer 39

increased pain from a stimulus that normally provokes pain

classified into primary and secondary

Question 40

what drugs target descending control systems

Answer 40

Antidepressants:
TCAs, SNRI and SSRI
(SSRI not really helpful vs SNRI or TCAs)