Sensory Pathways: Touch and Proprioception

Question 1

What receptors are responsible for the sensory modalities of touch and proprioception?

Answer 1

Mechanoreceptors

Question 2

Describe the structure of mechanoreceptors involved in touchand proprioception.

Answer 2

The receptor is NOT a separate entity but is actually the peripheral terminal of the peripheral axon of the primary sensory neuron.

Question 3

Describe the structure of a Pacinian corpuscle and explain how this structure relates to its function.

Answer 3

There is an axonal ending in the middle and it is wrapped around several concentric circles of epithelial cells – this allows the receptor to be very sensitive to vibration.

Question 4

What is the difference between slow adapting and fast adapting receptors?

Answer 4

Slow adapting receptors continue firing impulses for as long as the stimulus is present

Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle

Question 5

What type of receptors are mechanoreceptors?

Answer 5

Mixture of slow and fast adapting receptors

Question 6

Describe how sensory neurons vary in their properties.

Answer 6

They vary in SIZE and CONDUCTION VELOCITY

Question 7

What are the two classifications of axons?

Answer 7

Anatomical = based on axon diameter (labelled using LETTERS) eg A-beta, c fibres, A-delta

Physiological = based on conduction velocity (labelled using ROMAN NUMERALS)eg I, II, III

As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications

Question 8

Describe the general structure of sensory neurons that convey touch and proprioceptive information.

Answer 8

They are LARGE and have a FAST conduction velocity

Question 9

What is a receptive field?

Answer 9

An area of skin that is innervated by ONE sensory axon and its branches

Question 10

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

Answer 10

Lips and Mouth – high-density innervation with very small receptive fields
Upper arm – larger receptive fields and thinner innervation

Question 11

Describe how neurons can code for the intensity of a stimulus.

Answer 11

It is coded by the FREQUENCY of the action potentials going down the sensory fibres

Question 12

Which part of the spinal cord carries sensory axons for touch and proprioception?

Answer 12

Dorsal columns

Question 13

What are the bundles of axons within the spinal cord that havecome from above and below the waist called? Describe their spatial arrangement within the spinal cord.

Answer 13

Above the waist – Cuneate Fasciculus 
Below the waist – Gracile Fasciculus  
Axons from below the waist are packed more medially in the dorsal column and above the waist are more lateral
Lower = Medial 
Higher = Lateral

Question 14

Where do these neurons synapse?

Answer 14

They synapse in the Cuneate and Gracile Nuclei in the medulla

Question 15

Describe what happens after these neurons synapse and the tract that they run in.

Answer 15

The second order neurons then cross the midline (decussation) and continue up the brainstem in the MEDIAL LEMNISCUS

Question 16

Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?

Answer 16

Ventral Postero-lateral

Question 17

Describe the passage of the third order sensory neuron.

Answer 17

The third order neurone travels from the ventral postero-lateral nucleus in the thalamus to the primary somatosensory cortex

Question 18

Where does the trigeminal nerve enter the brainstem and where does it synapse with a second order neuron?

Answer 18

Pons

It synapses at the trigeminal cranial nucleus

Question 19

Describe the passage of this second order neuron.

Answer 19

The second order neuron crosses the midline (decussation) and joins the medial part of the median lemniscus

Question 20

Which thalamic nucleus is responsible for relaying sensory information from the face?

Answer 20

Ventral Postero-medial

Question 21

What is the purpose of lateral inhibition?

Answer 21

Improves the resolution of localising the stimulus

Question 22

Name the three parts of the somatosensory cortex.

Answer 22

Primary Somatosensory Cortex (SI)
Secondary Somatosensory Cortex (SII)
Posterior Parietal Cortex

Question 23

What is the posterior parietal cortex mainly involved in?

Answer 23

Spatial relationships

Question 24

Damage to the touch and proprioception pathway will halt sensory information going up to the primary somatosensory cortex. What effects will this have?

Answer 24

Anaesthesia (complete cessation of sensation)

Parasthesia (sensation is there but it isn’t normal)

Question 25

What is the most common cause of peripheral neuropathy?

Answer 25

Diabetes mellitus

Question 26

what are the 4 main sensory modalities

Answer 26

touch
proprioception
temperature
pain

Question 27

what are the 3 main receptors for these modalities

Answer 27

mechanoreceptor-touch/proprioception
thermoreceptor- temperature
nociceptor- pain

Question 28

what modalities come under “touch”

Answer 28

light touch
pressure
vibration

Question 29

what modalities come under proprioception

Answer 29

joint position
muscle length
muscle tension

Question 30

what is lateral inhibition

Answer 30

activation of one neural unit inhibits the neural units surrounding it

This facilitates pinpoint accuracy in localisation of the stimulus

So each axon will stimulate a second order neuron and inhibit neighboring first order neurons

Question 31

what is 2 point discrimination

Answer 31

minimum distance between 2 stimuli in order to perceive them as 2 separate stimuli

more densely packed receptive fields- smaller 2 point discrimination

Question 32

what is neural adaptation

Answer 32

a form of desensitisation-
if a stimulus of constant strength is maintained for a period of time, the frequency of Ads diminishes,
this length of time is different for different receptors:
phasic-Rapidly adapting
tonic-Slowly adapting

Question 33

what are the three main types of fibres and give their functions

Answer 33

A-beta fibres-innocuous mechanical stimulation

A-delta fibres-noxious mechanical and thermal stimulation

C-fibres-noxious mechanical, thermal and chemical stimulation

Question 34

give so e characteristics of nociceptor fibres

Answer 34

high threshold for activation

triggered by direct activation of ion channel proteins

Question 35

which part of the spinal cord does crude touch ascend in

Answer 35

anterior spino-thalamic (A-delta fibres)

Question 36

which part of the spinal cord does pain/temp ascend in

Answer 36

lateral spino-thalamic tract

Question 37

where do the anterior and lateral spinothalamic tracts decussate

Answer 37

at the level they enter the spinal cords (first order neurones synapse in dorsal horn and second order decussate immediately)

Question 38

where do both dorsal and spinothalamic second order neurones synapse with their third order neurones

Answer 38

VPL in thalamus - There is a topographic representation of the body in the VPL (lower extremities are lateral)