Sensory Pathways

Question 1

What are the somatosensory modalities

Answer 1

touch, thermosensation, nociception, proprioception

Question 2

What is proprioception

Answer 2

mechanical displacement of muscles, joints

Question 3

Which receptors have free nerve endings

Answer 3

thermoreceptors, nociceptors

Question 4

Which receptors has enclosed nerve endings

Answer 4

mechanoreceptors

Question 5

What are the 3 types of sensory neurons and what do they differ in

Answer 5

Abeta, Adelta, C fibres

they differ in diameter, speed (myelination), function

Question 6

Describe Abeta fibres

Answer 6

largest, thick myelin (fastest), innocuous mechanical stimulation

Question 7

Describe Adelta fibres

Answer 7

has myelin, noxious mechanical + thermal stimulation

Question 8

Describe C fibres

Answer 8

smallest, unmyelinated, noxious mechanical + thermal + chemical stimulation

Question 9

Sensory receptor definition

Answer 9

transducer that converts energy from the environment into neural APs

Question 10

Describe thermoreceptors

Answer 10

Adelta + C fibres, free nerve endings, not uniformly distributed - this affects sensitivity

Question 11

What are thermoreceptors linked to

Answer 11

transient receptor potential (TRP) ion channels

Question 12

Which TRP ion channels are C fibres associated with

Answer 12

C fibres detect heat - so, TRPV1-4

Question 13

Which TRP ion channels are Adelta fibres associated with

Answer 13

Adelta fibres detect cold - so, TRPM8, TRPA1

Question 14

State the 5 types of mechanoreceptors

Answer 14

Merkel cells (light touch, superficial pressure), Pacinian corpuscle (deep pressure, tickling, high frequency vibration), Meissner’s corpuscle (low frequency vibration, fine + discriminative touch), Ruffni endings (continuous pressure, touch, stretch), hair follicles

Question 15

Describe nociceptors

Answer 15

free nerve endings; Adelta for sharp (pin-prick) pain, cold; C fibre for dull aching pain, heat

Question 16

Stimulus threshold definition

Answer 16

point of intensity at which a person can detect presence of stimulus 50% of the time (absolute threshold)

Question 17

What is stimulus intensity

Answer 17

how fast a neutron fires

Question 18

How do you get high stimulus intensity

Answer 18

increased stimulus strength AND duration leads to increased NT release and therefore increased stimulus intensity

Question 19

Name the 2 types of adapting receptors

Answer 19

tonic, phasic

Question 20

Describe tonic receptors and give an example

Answer 20

detects continuous stimulus strength, adapts v slowly, impulses continuously transmitted to brain as long as stimulus is present; example: Merkel cells

Question 21

Describe phasic receptors and give an example

Answer 21

detect change in stimulus strength, adapt quickly, impulse transmitted at start and end of stimulus; example: Pacinian corpuscle: sudden pressure excites receptor and another impulse transmitted when pressure released

Question 22

Receptive field definition

Answer 22

region on the skin which causes activation of a single sensory neuron when stimulated

Question 23

3 examples of different receptive fields

Answer 23

fingers have densely packed mechanoreceptors with small receptive fields, arm has small receptive fields for detection of fine detail over small area - precise perception, back has large receptive fields for detection of changes over wider area

Question 24

The smaller the receptive field, the … the precision of perception

Answer 24

greater

Question 25

What is 2 point discrimination

Answer 25

minimum distance at which 2 points are perceives as separate; related to receptor field size

Question 26

Where are cell bodies of the body located

Answer 26

dorsal root ganglion

Question 27

Where are cell bodies of the face located

Answer 27

trigeminal ganglia

Question 28

Info from lower body received at what spinal level and carried in which tract

Answer 28

lumbar level, gracile tract

Question 29

info from upper body received at what spinal level and carried in which tract

Answer 29

cervical level, cuneate tract

Question 30

info from face received at what level

Answer 30

pons

Question 31

2 types of dorsal horn neurons

Answer 31

``` projections neurons (going to brain), interneurons (remain in spinal cord) *Abeta fibres are deep: laminas 3-5; Adelta + C fibres are superficial: laminas 1-2 ```

Question 32

What happens when receptive fields overlap

Answer 32

difficult to distinguish between the 2 stimuli locations

Question 33

What prevents the overlap of these receptive fields

Answer 33

lateral inhibition; mediated by inhibitory interneurons within dorsal horn of spinal cord, leads to enhanced sensory perception

Question 34

Describe the ascending pathway for touch, proprioception

Answer 34

Abeta fibres enter dorsal horn and enter dorsal column pathways. Info from lower/upper limbs travel ipsilaterally along gracile/cuneate tracts. 1st order neurons terminate in gracile/cuneate nucleus in medulla. 2nd order neurons decussate in caudal medulla, forming CONTRALATERAL medial leminscus tract. They terminate in ventral posterior lateral nucleus (VPL) of thalamus. 3rd order neutron projects from VPL to somatosensory cortex

Question 35

What is crude touch

Answer 35

sensory modality that allows the subject to sense that something has touched them, without being able to localise where they were touched (contrasting fine touch)’’

Question 36

Difference between fine and crude touch in terms of fibres

Answer 36

fine touch mediated by Abeta fibres (Meissner's corpuscle), crude touch mediated by Adelta fibres (free nerve ending)

Question 37

Pain and temperature sensations ascend within which part of the spinothalamic tract

Answer 37

lateral

Question 38

Crude touch ascends within which part of the spinothalamic tract

Answer 38

anterior

Question 39

Describe the ascending pathway for pain, temperature, crude touch

Answer 39

Spinothalamic (anterolateral) pathway. 1st order neurons terminate in the dorsal horn upon entering. 2nd order neurons decussate immediately in spinal cord, forming spinothalamic tract and ascend. The neurons terminate in VPL of thalamus. 3rd order neurons to somatosensory cortex

Question 40

Clinical importance: examples of sensory testing

Answer 40

pin pricks to detect sharp vs blunt perception of stimulus; brushing to detect whether innocuous stimuli perceived as painful; graded tuning fork to identify normal/abnormal vibration sensation;

Question 41

What does anterior spinal cord lesion result in

Answer 41

blocked anterior spinal artery causes ischaemic damage to anterior part of spinal cord - dorsal columns intact but damaged spinothalamic tract. Pain/temp loss below the level of lesion; retained light touch, vibration, 2-point discrimination

Question 42

Which pathway is associated with sensational pain

Answer 42

lateral spinothalamic tract

Question 43

Which pathway is associated with emotional pain

Answer 43

spin-reticular tract

Question 44

What is gate control theory

Answer 44

when elbow gets hit, you activate C fibres. Rubbing the elbow stimulates Abeta fibres which blocks C fibres by stimulating inhibitory neurons. There is reduced C fibre input to somatosensory cortex, resulting in decreased stimulus intensity.