Somatosensory Pathways

Question 1

What are exteroreceptors?

Answer 1

Receptors in the skin that respond to stimuli impinging on the body

Question 2

What are proprioceptors?

Answer 2

Receptors in joints and muscles that respond to the body’s own movement

Question 3

What are nociceptors?

Answer 3

Receptors that respond specifically to tissue damage

Question 4

In which spinal laminae do proprioceptors synapse?

Answer 4

V and VI

Question 5

In which spinal laminae do exterorecptors synapse?

Answer 5

III and IV

Question 6

In which spinal laminae do nociceptors synapse?

Answer 6

I and II

Question 7

What are sensory receptors?

Answer 7

Sensory receptors are present on the ends of sensory nerve fibres, usually surrounded by a capsule which determines the stimulus which the fibre will respond to (due to differing capsule shape and structure)

Question 8

What is the sensory capsule?

Answer 8

The capsule is made of connective tissue which grows around the nerve terminal and it alters the effect of movement on the nerve ending, making a receptor either slowly or rapidly adapting

Question 9

What are ‘rapidly adapting’ sensory receptors?

Answer 9

Receptor responds at the beginning of a stimulus and fatigues after a second or so, even if the stimuli is sustained

Question 10

What are ‘slowly adapting’ sensory receptors?

Answer 10

Receptor continues firing to a sustained stimulus but at a gradually reducing rate

Question 11

Where are sensory receptors in the skin most often found?

Answer 11

In the epidermal-dermal junction

Question 12

What is the role of Merkel receptors?

Answer 12

Sustained touch and pressure

Question 13

What is the role of Meissner’s corpuscles?

Answer 13

Sensitivity to light touch

Question 14

What is the role of Pacinian corpuscles?

Answer 14

Selectively sensitive to vibration (onion-like capsule)

Question 15

What is the role of Ruffini corpuscles?

Answer 15

Sensitive to pressure/stretch

Question 16

What is the role of Peritricheal receptors?

Answer 16

Detect hair displacement

Question 17

Define ‘receptive field’

Answer 17

The area of skin innervated by a single nerve fibre

Question 18

How does the brain determine the location of a stimulus?

Answer 18

The brain uses information from many receptors simultaneously to determine the location of the stimulus

Question 19

How do chemoreceptors differ from touch receptors?

Answer 19

They do not have capsules around their endings but form ‘free’ nerve endings  very fine nerve plexus in the dermis and other tissues. These free nerve endings respond to chemical stimuli. The parent fibre of such afferents is either type Aδ or type C.

Question 20

Describe the DCML tract (information it conveys, deficits, decussation and passage)

Answer 20

•Ascending (sensory) pathway
•Contralateral deficits above medulla
•Carries information about proprioception (joint sense) and discriminative touch
•Decussation: secondary afferents decussate at the medulla after the primary afferents synapse in the gracile/cuneate nuclei
Passage - medulla –> thalamus –> somatosensory cortex

Question 21

What makes up the medial lemniscus?

Answer 21

Trigeminal nucleus lies adjacent to gracile and cuneate nuclei in lower medulla; axons from cells in the trigeminal nucleus join axons from the gracile and cuneate nuclei –> cross over as internal arcuate fibres to form the medial lemniscus

Question 22

What is the role of the VPM nucleus in the thalamus?

Answer 22

Relays information from the face to the cortex

Question 23

What is the role of the VPL nucleus in the thalamus?

Answer 23

Relays information from the limbs to the cortex (in an ordered map - homunculus)

Question 24

Where is the somatosensory cortex?

Answer 24

A strip of cortex immediately posterior to the central sulcus and is parallel to the motor cortex (post-central gyrus)

Question 25

Describe the organisation of the somatosensory cortex into a homunculus

Answer 25

Mouth is represented laterally and the most caudal dermatomes medially

Question 26

Describe the location of the first order, second order and third order neurones of the DCML tract

Answer 26

First-order neurones: peripheral nerve fibres up the dorsal columns to dorsal column nuclei in the lower medulla
Second-order neurones: conduct impulses from dorsal column nuclei up the medial lemniscus into the thalamus
Third-order neurones: conduct impulses from the VPM and VPL thalamic nuclei to primary somatosensory area of the cerebral cortex

Question 27

What information does the lateral spinothalamic tract carry?

Answer 27

Pain and temperature

Question 28

Where are the cell bodies of the lateral spinothalamic tract located?

Answer 28

Lamina I and II of dorsal horn

Question 29

How is the unpleasant/painful character of pain mediated?

Answer 29

Mediated via projections of the lateral spinothalamic tract to the limbic system of the forebrain, and this pain also stimulates general arousal and focussing of attention on the painful region.

Question 30

Where does the lateral spinothalamic tract terminate?

Answer 30

Several sites in the brainstem but NOT VPL

Question 31

What information does the anterior spinothalamic tract carry?

Answer 31

Passive touch and pressure sensation

Question 32

Where is there convergence between the anterior spinothalamic and DCML pathways?

Answer 32

In thalamic VPL nucleus