L27 The Somatic Sensory System X

Question 1

What is the somatic sensory system?

Answer 1

The somatic sensory system is a vast and intricate network within the nervous system responsible for our conscious perception of the body and its interaction with the environment.

Question 2

What are the two major input components for the somatic sensory system?

Answer 2

1. Mechanical stimuli
2. Painful stimuli and temperature.

Question 3

Give me examples of some sensory receptors

Answer 3

1. Meissner Corpuscles
2. Pacinian corpucles
3. Buffini corpuscles
4. Merkel discs
5. free nerve endings
6. Hair follicles

Question 4

What does the Meissner (or Tactile) corpuscles detect (modality)?

Answer 4

They detect:
1. Light touch
2. Texture(Movement).

Sensitive to 30-50Hz.
Rapidly adapting

Question 6

How does the structure of the pacinian (or Lamellated) Corpuscles work? And what is its aim?

Answer 6

Gets rid of unwanted sound waves (check)

Question 7

Which sensory receptors are encapsulated/unencapsulated nerve endings?

Answer 7

• Encapsulated nerve ending - Meissner corpuscles and pacinian corpuscles
• Unencapsulated nerve endings - Merkel discs and free nerve endings.

Question 8

What are the modalities (what they detect) for free nerve endings?

Answer 8

Pain, heat and cold

Question 9

Which sensory receptors are rapidly/slowly adapting?

Answer 9

Rapidly adapting: Meissner corpuscles, Pacinian corpuscles.
Slowly adpating: Buffini corpuscles and merkel discs.

Question 10

What are the two different classes of mechanoreceptor responses?

Answer 10

Rapidly adapting or phasic receptors and slowly adapting or tonic receptors.

Question 11

When a stimulus occurs, what does the phasic receptors do?

Answer 11

Receptors give information about changes in the stimulus.

Question 12

When a stimulus occurs, what does the tonic receptors do?

Answer 12

Slowly adapting or tonic receptors, continue to respond as long as stimulus is present - Ruffini Corpuscles

Question 13

Does a larger diameter increase the conduction velocity?

Answer 13

Yes,in the somatosensory system, a larger diameter generally increases the conduction velocity of sensory axons.

Question 14

How are axons coming from the skin designed?

Answer 14

Axons coming from the skin are designated by letters (A, B, C; A = fastest/largest; C = slowest/smallest).

Question 15

How are axons coming from the muscles designed?

Answer 15

Axons coming from the muscles designated by Roman numerals:
I, II, III & IV; I = largest; IV = smallest)
‘I’ group also further broken down (Ia, Ib etc ‘a’ faster, ‘b’ slower)

Question 16

What are the features of the sensory inputs?

Answer 16

1. They are organised starting with layers in the dorsal horn of the spinal cord dorsal horn.
2. Information fro different classes of hair follicles is represented by different layers.
3. Sensory information remains spatially organised as it is carried into the brain by different pathways

Question 17

What are the two main routes that all these sensory subtypes get to the brain?

Answer 17

1. The Medial Lemniscal tracts.
2. The Spinothalamic tract

Question 18

What signals does the Medial Lemniscal route carry?

Answer 18

They carry mechanoreceptive and proprioceptive signals to the thalamus.

Question 19

What signals does the Spinothalamic tract carry?

Answer 19

They carry pain and temperature signals to the thalamus.

Question 20

What 3 neurons do sensory information travel through to reach higher centres?

Answer 20

1. First order neurons
2. Second order neurons
3. Third order neurons.

Question 22

What is different about 2nd order neurons?

Answer 22

2nd order axons cross the midline; ie are commissural

ommissural neurons - axons cross the midline of the CNS connecting acros

Question 23

What does topological organisation mean?

Answer 23

the spatial arrangement of the objects relative to one another.

Question 24

Where does the 1st order axons in the Medial Lemniscal system send its signals?

Answer 24

1st order axons from the upper body synapse on 2nd order neurons in the cuneate nucleus
1st order axons from the lower body synapse on neurons in the gracile nucleus

Question 25

Explain where 2nd order axons cross and send its signals in the Medial Lemniscal system

Answer 25

2nd order axons cross the midline and ascend in the medial lemniscus.

Question 26

Explain where 3rd order axons cross and send its signals to

Answer 26

3rd order axons reverse the topology so that lower body axons synapse on more medial cortical neurons, whereas upper body axons ‘map’ to the lateral cortex

Question 27

What does each DRG innervate ?

Answer 27

Each DRG innervates a specific domain of the body called a dermatome.

Question 28

What are Dermatomes?

Answer 28

Each sensory ganglion innervates a specific region of skin called a dermatome

Question 29

How does each dermatome arise?

Answer 29

These regions arise because the dermis of each region is derived from a specific embryonic structure called the somite

Question 30

What does topological and topographical mean?

Answer 30

Topological: the way in which constituent parts are interrelated or arranged. topographical: relating to or representing the physical distribution of parts or features on the surface of or within an organ or organism

Question 31

Each sensory neuron has a receptive field. What does we mean by that?

Answer 31

The area in which the axons spread out is called their receptive fields.

Question 32

What does the size of the receptive field depend on?

Answer 32

The size of the receptive field for any particular neuron will vary depending on where it is in the body, some regions having denser innervation than others

Question 33

How is the size of a receptive field can be measured?

Answer 33

can be measured by assessing the ability to discriminate two sharp points set apart at different distances

Question 34

What does it mean if receptive fields are large/small?

Answer 34

Where receptive fields are large, discrimination is low (legs and arms), whereas where receptive fields are small, discrimination is high (fingers).

Question 36

What does it mean if you have a larger number of nerve endings per unit area?

Answer 36

If the number of endings from different neurons is higher, therefore the amount of derived information is also higher.

Question 37

Where is the sensory modality represented?

Answer 37

In the cortex

Question 38

Where is the somatotopic map preserved?

Answer 38

The somatotopic map is preserved in the coronal plane in the postcentral gyrus

Question 39

What are brodmann areas?

Answer 39

Brodmann areas are regions of the cerebral cortex in the human (and primate) brain, defined by their cytoarchitecture – the organization of cells within the layers of the cortex.

Question 40

Is the cortex highly plastic and adaptable?

Answer 40

Yes it is highly plastic and adaptable.