Topic 5- Central Somatosensory /Deafferentation / Plasticity

Question 1

What is the role of “first-order” sensory afferents?

Answer 1

“First-order” sensory afferents carry information from peripheral receptors to the spinal cord, moving from the periphery into the central nervous system (CNS).

Question 2

What is the function of dorsal root ganglion cells? Location?

Answer 2

Dorsal root ganglion cells are clusters of cell bodies located in the peripheral nervous system. They serve as the origin of the initial axons that transmit sensory information to the central nervous system.

Question 3

What types of receptors are found in the dorsal root ganglion?

Answer 3

The dorsal root ganglion contains mechanoreceptors, as well as pain and temperature receptors. The latter have higher thresholds and consist of smaller cells with smaller efferent fibers.

Question 4

What is the name of the pathway that carries mechanosensory information to the brain?

Answer 4

The pathway is called the dorsal column - medial lemniscus (DCML) pathway.

Question 5

What type of information does the DCML pathway carry?

Answer 5

The DCML pathway carries both cutaneous (skin-related) and proprioceptive (body position and movement) information to the brain.

Question 6

What is the role of the Dorsal Column-Medial Lemniscal System (DCMLS) in sensory transmission?

Answer 6

The DCMLS transmits discriminative touch, proprioception, and vibratory sensations via two main tracts: fasciculus gracilis (lower body) and fasciculus cuneatus (upper body), with minimal processing.

Question 7

DCLM Pathway Steps

Answer 7

1) First-order neurons carrying sensory input from mechanoreceptors synapse with second-order neurons in dorsal nuclei, namely the nucleus gracilis and nucleus cuneatus,

2)These 2nd order neurons travel through specific tracts: the internal arcuate tract (for lower body) and the medial lemniscus tract (for upper body).
Within the medial lemniscus tract, some sensory information crosses from one side of the body to the other. This is known as the “decussation of the medial lemniscus.”
3) 2nd order neurons synapse on 3rd order in VPL nucleus of thalamus and then 3rd order neurons’ sensory information to the somatosensory cortex for conscious perception. (S1)

Question 8

Where do first-order afferents synapse with second-order neurons in the DCML pathway?

Answer 8

First-order afferents synapse with second-order neurons in the dorsal column nuclei of the medulla.

Question 9

What does the term “funiculi” refer to in this context?

Answer 9

In this context, “funiculi” refers to bundles of nerve fibers that make up the dorsal column, carrying sensory information towards the brain.

Question 10

How does the DCML pathway transport information from the lower body, and where does it end up?

Answer 10

Lower body information travels within the dorsal column’s middle part, through the gracile tract, and ultimately reaches the gracile nucleus.

Question 11

What happens to upper limb information in the DCML pathway, and where does it go?

Answer 11

Upper limb information takes a more lateral route through the cuneate tract and is directed to the cuneate nucleus.

Question 12

What are the consequences of lesions to the dorsal column in the DCML pathway?

Answer 12

Lesions in the dorsal column can lead to problems with proprioception, movement imitation, tactile exploration, and understanding the direction and speed of tactile information.

Question 13

How do second-order neurons travel in the DCML pathway to reach the thalamus?

Answer 13

Second-order neurons travel through internal arcuate and medial lemniscus tracts to reach the thalamus.

Question 14

The trigeminal somatic sensory system:

Answer 14

Responsible for processing tactile (touch) and proprioceptive (body position) information from the face, follows a distinct route compared to other sensory systems.

Question 15

The trigeminal somatic sensory system Steps:

Answer 15

1) Sensory information from the face enters the central nervous system (CNS) at the level of the pons.
It then synapses onto the principal nucleus (as opposed to the gracile or cuneate nucleus).
2) The information crosses to the opposite side (midline) and ascends to the VPM thalamic nucleus.
3) This crossing, or decussation, takes place via the trigeminothalamic tract, which is a different pathway compared to other sensory systems.

Question 16

What are the sub-areas of S1?

Answer 16

S1 is comprised of the following sub-areas:

3a, which receives proprioceptive inputs.
3b, which receives cutaneous inputs.
1, which also receives cutaneous inputs.
2, which receives proprioceptive inputs.

Question 17

Which sub-area of S1 processes proprioceptive inputs?

Answer 17

Sub-area 3a of S1 processes proprioceptive inputs.

Question 18

Which sub-area of S1 is responsible for processing cutaneous inputs?

Answer 18

Sub-areas 3b and 1 in S1 process cutaneous inputs.

Question 19

What specific sensory information is each of the projection areas (1 and 2) concerned with?

Answer 19

Area 1 is concerned with texture, while area 2 is concerned with size and shape.

Question 20

What are receptive fields in S1 neurons?

Answer 20

Receptive fields are areas in S1 neurons where sensory inputs are received and processed.

Question 21

How do the sizes of receptive fields in area 3 cells compare to those in area 1 and 2 cells?

Answer 21

Receptive fields in area 3 cells are smaller than those in area 1 and 2 cells.

Question 22

How do S1 receptive fields compare to those of sensory afferents?

Answer 22

S1 receptive fields are larger than those of sensory afferents.

Question 23

What happens to the firing of S1 neurons when light touch is applied, and how does it change across areas 3, 1, and 2?

Answer 23

Light touch elicits cell firing over larger areas as you move from area 3 to area 1 and then to area 2 in S1.

Question 24

What is the trend in the specialization of inputs in later stages?

Answer 24

In later stagesof processing , there are more specialized inputs in S1.

Question 25

Topographic Map

Answer 25

The arrangement of receptive fields in the somatosensory cortex forms a map of the entire body.

Question 26

What is the concept of the sensory homunculus?

Answer 26

Represnets how the brain perceives the relative sizes and locations of different body parts based on their sensitivity to touch and sensory input. in the brain where body parts with greater tactile discrimination have larger representations.

Question 27

What determines the size of a body part’s representation in the sensory homunculus?

Answer 27

The importance of a body part for tactile discrimination determines the size of its representation in the sensory homunculus. The larger the representation, the more neural circuitry is devoted to processing those parts.

Question 28

Which two motor control skills have the largest representations in the sensory homunculus?

Answer 28

Manual dexterity and speech are the two motor control skills with the largest representations in the sensory homunculus.

Question 29

What is the basic principle of organization in the central nervous system (CNS)?

Answer 29

The basic principle is called “columnar organization,” where different receptor groups are separated and represented in distinct columns.

Question 30

How is columnar organization used in the somatosensory system?

Answer 30

In the somatosensory system, columns represent different receptor groups, ensuring that all neurons in a column receive inputs from the same local area of the skin and respond to a single class of receptors

Question 31

Information in S1 is arranged by:

Answer 31

1. receptor type (columns)
2. location of origin (homunculus)

Question 32

What is muscimol, and how is it used in the context of somatosensory research?

Answer 32

Muscimol is a GABA agonist used to temporarily deactivate or “knock out” specific areas in the primary somatosensory cortex.

Question 33

What happens when you use muscimol to temporarily deactivate the primary somatosensory cortex?

Answer 33

Temporary deactivation of the primary somatosensory cortex results in deficits in discriminating the size, shape, and texture of objects.

Question 34

What is a distinctive feature of the receptive fields in S2?

Answer 34

Receptive fields in S2 are characterized by their large and bilateral nature.

Question 35

Sensory Gating

Answer 35

The ability to focus on relevant sensory information while disregarding less important information.

Question 36

How does the central nervous system (CNS) control ascending sensory pathways?

Answer 36

The CNS can control ascending pathways through a process known as sensory gating, which involves inhibition.

Question 37

Where along the DCML pathway can inhibition occur?

Answer 37

Inhibition can occur along the Dorsal Column - Medial Lemniscus (DCML) pathway.

Question 38

What does the term “plasticity” refer to in the context of sensory and motor maps?

Answer 38

Plasticity refers to the ability of sensory and motor maps to be modified through training or use.

Question 39

What is another term for the process of modifying sensory and motor maps through training or use?

Answer 39

This process is also known as “functional remapping.”

Question 40

What is deafferentation, and how can it occur?

Answer 40

A condition where peripheral lesions or amputations result in modified somatosensory maps. It occurs when afferent inputs from the periphery are interrupted, often following an injury or amputation.

Question 41

What changes occur in the brain’s representation after amputation, particularly in terms of how cells respond to stimuli?

Answer 41

After amputation, cells that would normally respond to stimuli from the amputated limb may begin responding to stimuli from other areas. For example, tissue that usually responds to hand stimulation might now be processed by the brain as if it were part of the face. This phenomenon reflects the re-mapping of sensory representation in the brain after amputation.

Question 42

Remapping of referred sensations

Answer 42

Phenomenon where the brain associates stimuli from one area with sensations in a different, unrelated area.

Question 43

What characterizes the phantom phenomena in relation to the central nervous system (CNS) and the periphery?

Answer 43

In the phantom phenomena, the CNS operates independently of the periphery, regardless of whether the limb is present.

Question 44

What can result from peripheral nerve damage or dysfunction, and what is this condition called?

Answer 44

Peripheral nerve damage or dysfunction can lead to the loss of sensation and the development of pain. This condition is known as neuropathy.

Question 45

What happens when a nerve fiber is compressed, and what sensations are affected in a specific order?

Answer 45

When a nerve fiber is compressed, sensations are affected in a specific order. It begins with the loss of sensations related to cold and acute pain, followed by sensations of heat. The last to be affected is the ability to sense slow or deeper pain.

Question 46

What occurs if the compression on a nerve fiber is relieved?

Answer 46

When the compression on a nerve fiber stops, there can be a reversal of sensations, often experienced as a “pins and needles” feeling.

Question 47

What is Tables Dorsalis caused by?

Answer 47

Untreated syphillis

Question 48

What are 2 effects of tables dorsalis?

Answer 48

1) Destroyes large diameter neurons in the dorsal root ganglion (but not small diameter ones) - so pain and temp are intact
2) Leads to dorsal coloum degeneration

Question 49

Symptoms of dorsalis tables

Answer 49

Weakness, paraesthias, loss of coordination & proprioception