L8. Central Somatosensory Pathways

Question 1

Does the mechanoreceptor sensory information through the afferent neuron relay ipsilaterally or contralaterally through the spinal cord?

Answer 1

Ipsilaterally

(red line)

Question 2

What is significant about the regional segregation of the dorsal columns?

Answer 2

The dorsal columns have a generally representative mapping of the body with different areas or sources of senses in the body represented by different regions of the dorsal columns.

The lower down from the body the information originates from, the closer to the midline of the dorsal columns the information relays through

(lower = midline and upper = lateral)

Question 3

What is the 3 neurone sequence pathway from periphery to somatosensory cortex of the mechanoreceptive pathway?

Answer 3

1. Peripheral nerve synapsing in the caudal medulla
2. Caudal medulla to the thalamus
3. Thalamus to the somatosensory cortex

Question 4

Describe the main tactile mechanoreceptive pathway

Answer 4

• Peripheral afferent nerve ending through to the dorsal root ganglia and into the dorsal horn of the spinal cord
• Travels ipsilaterally up the spinal cord through the white matter to the caudal medulla
• Makes the first major synapse in the medulla just on top of the dorsal columns (at the dorsal column nucleus/grassile/cuneate nucleus)
• Second order neurons then dessucate across the medulla to travel to the thalamus contralaterally
• Second order neurons (called the medial lemniscus) travel to the ventral posterolateral nucleus of the thalamus where they synapse again
• The third order neuron projects up to the somatosensory regions of the cortex

Question 5

Is the tactile mechanoreceptive pathway the only pathway available for carrying sensory information to the CNS?

Answer 5

NO

Other pathways exist such as for propriception, motor control (to the cerebellum particullary)

Question 6

The thalamus also has somatotopic order (ie representation or mapping of the body within it).

The posterior ventral thalamus has 2 divisions (lateral and medial) what does this signify?

Answer 6

Information from the body is relayed to the lateral division

Information from the face (trigeminal pathway) is relayed to the medial division

Question 7

What is the major somatosensory centre in the cortex that mechanoreceptor information is relayed to from the thalamus?

Where is it located?

Answer 7

The ventral part of the thalamus projects to the primary somatic sensory cortex (S1) which is a ridge behind the central sulcus

(also called the post-central gyrus)

However a secondary somatic sensory cortex is also present located behind the frontal lobe and above the temporal (unsure whether it is parallel or heirachial signaling to this one)

Question 8

What is the pre-central gyrus responsible for?

Answer 8

Motor control

Question 9

What is important to note about the primary somatosensory cortex in terms of somatotopy

Answer 9

The primary somatosensory cortex is further subdivided into distinct areas: 1, 2, 3a and 3b

Question 10

Describe the somatotopic order in the primary somatic sensory cortex from the medial fold to the lateral fold of the cortex

Answer 10

Representative of body parts and regions

The larger the information being processed from a body part, the larger the amount of cortex is assigned to receive and process the information (eg. hands and digits have large areas)

Question 11

Describe what is meant by parcellation of the sensory information in the cortex?

Answer 11

The different areas (1, 2, 3a and 3b) recieve and process different regions and types (categories) of information. It is unknown why this is

Question 12

Neurons in the primary somatosensory cortex form funcationally distinct columns. Appreciate this

Answer 12

Information is mixed and somewhat segregated

However the differences in functionality of the 4 regions is largely unknown,

Question 13

What is the main subregion of the somatosensory cotex that information is relayed from the thalamus to?

Answer 13

Region 3b

Which then seems to relay information to region 1 (texture?) and 2 (size and shape)

Question 14

How does the information running through the subdivisions converge together?

Answer 14

They all converge to the secondary somatosensory cortex where the information is synthesised in its totality before being further relayed

Question 15

Where is information sent once it has reached the secondary somatosensory cortex? (2 main regions) Why is this so?

Answer 15

1. Amygdala and hippocampus for memory and association of the experience and touch
2. To the motor and premotor cortical areas to respond to the experience and enviroment

Question 16

Is the somatic sensory cortex and the regionalisation fixed and permanent?

Answer 16

No, it is plastic

The cerebral cortex is rapidly reassigning and reevaluating territory in resonse to different stimuli (eg. in the case of amputation)

(Also the case if over stimulation of certain digits, stimulate one digit over the others will cause the region allocated to that digit to increase). Also the case in occular regions

Question 17

At what stage of the pathway allows for the plasticity of the cortical region?

Answer 17

It is unknown where in the pathway.

It is believed that there is some plasticity at every level of the pathway.

Question 18

Is the territory reassignment a remapping?

Answer 18

No.

The region is responsible for sensation of that region only (eg. nerve damage or repair where if the ulnar and medial nerves were somehow switched, the CNS would still feel the ulnar nerve and belive it was where it originally was). The CNS doesn’t adapt this way.

Eg. phantom limb

Question 19

What is dysaesthesia?

Answer 19

Having one place of the body stimulated and feeling the stimulus in another place in the body (mis-localisation)