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 thestimulus 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)
Physiological = based on conduction velocity (labelled using ROMAN NUMERALS)
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 conveytouch 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

What is the main sensory nerve of the face?

Answer 18

Trigeminal Nerve (CN V)

Question 19

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

Answer 19

Pons

It synapses at the trigeminal cranial nucleus

Question 20

Describe the passage of this second order neuron.

Answer 20

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

Question 21

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

Answer 21

Ventral Postero-medial

Question 22

What is lateral inhibition?

Answer 22

Lateral inhibition takes place in the cuneate and gracile nuclei
Each axon has lateral branches that are inhibitory on neighbouringaxons
So each axon will stimulate a second order neuron and inhibit neighboring first order neurons

Question 23

What is the purpose of lateral inhibition?

Answer 23

Improves the resolution of localising the stimulus

Question 24

Name the three parts of the somatosensory cortex.

Answer 24

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

Question 25

What is the posterior parietal cortex mainly involved in?

Answer 25

Spatial relationships

Question 26

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

Answer 26

Anaesthesia (complete cessation of sensation)

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

Question 27

What is the most common cause of peripheral neuropathy?

Answer 27

Diabetes mellitus