Central Processing of Somatosensory Input

Question 1

What do the dorsal columns consist of?

Answer 1

Medial gracile tract (fasciculus gracilis) and the more lateral cuneate tract (fasciculus cuneatus)

Question 2

How does sensory input travel in the dorsal column?

Answer 2

Sensory input from T6 and below travels in gracile tract, whereas input from above T6 flows in the cuneate tract

Question 3

What exists across the dorsal columns?

Answer 3

A somatosensory map of sensory inputs from lateral to medial = cervical, thoracic, lumbar, sacral

Question 4

What tracts convey unconscious proprioceptive info to the cerebellum?

Answer 4

Dorsal and ventral spinocerebellar tracts

Question 5

What is the major route by which touch and conscious proprioceptive info ascends to the cerebral cortex?

Answer 5

The dorsal column medial lemniscus pathway

Question 6

What does the first order neuron of the DCML pathway form once it enters the dorsal horn and branches?

Answer 6

Synapses deep in the dorsal horn upon the second order neuron
Long ascending axon synapsing in either dorsal column gracile nucleus or cuneate nucleus

Question 7

What happens to axons of the second order neurons of the DCML pathway?

Answer 7

Cross from the dorsal column nuclei in the great sensory decussation and ascend in the medial lemnicus to the ventral posterior lateral nucleus of the thalamus

Question 8

Where do the third order thalamic neurons of the DCML pathway project to?

Answer 8

The primary somatosensory cortex via the posterior internal capsule

Question 9

What are the capabilities of the DCML pathway?

Answer 9

Stereognosis, vibration detection, weight discrimination, fine touch, conscious proprioception

Question 10

What is stereognosis?

Answer 10

Ability to recognise an object by feeling it

Question 11

What is conscious proprioception?

Answer 11

Awareness of body position and body movements

Question 12

What results in contrast enhancement?

Answer 12

The amplification of differences in the activity of adjacent neurons as info is conveyed from one neuron to the next in a sensory pathway

Question 13

What is lateral inhibition?

Answer 13

When one neuron is active it inhibits the activity of its neighbours via inhibitory interneurons

Question 14

What is the purpose of lateral inhibition?

Answer 14

Sharpens stimulus perception = one mechanism by which info is transformed at the synapses of the DCML pathway

Question 15

What mediates general somatic info from the anterior head?

Answer 15

Two trigeminothalamic pathways

Question 16

Where are the soma of sensory neurons of the trigeminal system located?

Answer 16

The trigeminal sensory ganglion

Question 17

Where do the central terminae of the trigeminal nerve synapse?

Answer 17

Upon second order neurons in the chief sensory nucleus = these in turn decussate and project via trigeminal lemniscus

Question 18

Where does the trigeminal lemniscus project to?

Answer 18

The ventroposteriomedial neurons of the thalamus

Question 19

Where do the third order neurons of the trigeminal system relay info to?

Answer 19

The cortex via the thalamocortical neurons

Question 20

Where is the somatosensory cortex located?

Answer 20

Postcentral gyrus of the parietal cortex immediately posterior to the central sulcus and adjacent to the posterior parietal cortex

Question 21

What does the central sulcus consist of?

Answer 21

Brodmann areas (BA) 1, 2, 3a and 3b

Question 22

Where does the primary somatosensory area receive input from?

Answer 22

The VP thalamus = 70% to BA 3a and 3b, 30% to BA 1 and 2

Question 23

What are some features of BA 3a?

Answer 23

Input from proprioceptors

Sensitive to body position

Question 24

What are some features of BA 3b?

Answer 24

Input from cutaneous and FA receptors, and BA 3a
Sensitive to touch
Small lesion will cause deficits in texture and shape discrimination

Question 25

What are some features of BA 1?

Answer 25

Input from cutaneous receptors and BA 3b
Sensitive to texture discrimination
Small lesion will cause loss of texture discrimination

Question 26

What are some features of BA 2?

Answer 26

Input from joint afferents, Golgi tendon organs, deep tissues and BA 3a and 3b
Sensitive to pressure/joint position and object perception
Small lesion will cause impaired grasping and size/shape discrimination

Question 27

What do the receptive fields of S1 neurons form on the primary somatosensory cortex?

Answer 27

An orderly somatotropic map of the body surface

Question 28

What does the sensory homunculus show?

Answer 28

The relative area of cortex devoted to each body part = correlated with the density of sensory input from that body part

Question 29

How many layers does the somatosensory cortex have?

Answer 29

Six cell layers

Question 30

What layer of the somatosensory cortex do thalamic inputs to S1 terminate?

Answer 30

Mainly on neurons within layer 4 = this in turns projects cells towards the surface of the cortex and also deeper layers

Question 31

What extends across the six layers of the somatosensory cortex?

Answer 31

Vertical columns = each column consists of neurons with similar inputs and responses

Question 32

What maps are present in S1?

Answer 32

Multiple maps of the somatic sensory system

Question 33

What are some features of the somatic sensory system maps present in S1?

Answer 33

Adjacent strips of cortex map the same parts of the body in parallel to each other
Maps aren’t identical but are mirror images of each other

Question 34

What is the function of the posterior parietal cortex (SII)?

Answer 34

Receives and integrates info from S1 and other cortical and subcortical areas
Deciphers the deeper meaning of the info in S1

Question 35

What can damage to the posterior parietal cortex cause?

Answer 35

Bizarre neurological disorders with simple sensory skills remaining intact (e.g neglect syndrome)

Question 36

What is neglect syndrome?

Answer 36

Usually a result of damage to the right parietal cortex = patients believe the left side of the world doesn’t exist