L27 The Somatic Sensory System

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 (light touch, vibrations , pressure and tension)
2. Painful stimuli and temperature.

Question 3

Give me examples of some sensory receptors

Answer 3

1. Meissner Corpuscles
2. Pacinian Corpuscles
3. Ruffini 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
Encapsulated nerve endings

Question 5

What are the modalities for the Pacinian Corpuscles?

Answer 5

Deep pressure, stretch, tickle, vibration
- Sensitive to 250-350Hz
- Rapidly adapting
- Encapsulated nerve endings

Question 6

Which sensory receptors are encapsulated/unencapsulated nerve endings?

Answer 6

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

Question 7

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

Answer 7

Pain, heat and cold

Question 8

Which sensory receptors are rapidly/slowly adapting?

Answer 8

Rapidly adapting: Meissner corpuscles, Pacinian corpuscles.
Slowly adpating: Ruffini corpuscles and merkel discs and free nerve endings

Question 9

What are the two different classes of mechanoreceptor responses?

Answer 9

Phasic (rapidly adapting) - responds to stimulus at start and end of the stimulus

Tonic (slowly adapting) - responds to stimulus as long as stimulus is applied

Question 10

Does a larger diameter increase the conduction velocity?

Answer 10

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

Question 11

How are axons coming from the skin designed?

Answer 11

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

Question 12

How are axons coming from the muscles designed?

Answer 12

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 13

How are sensory inputs organised?

Answer 13

Sensory inputs are organised by grouping different classes of sensory neuron in DRG ( spatially) which are projected by different pathways to different layers of dorsal horn

Question 14

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

Answer 14

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

Question 15

What signals does the Medial Lemniscal route carry?

Answer 15

They carry mechanoreceptive and proprioceptive signals to the thalamus.

Question 16

What signals does the Spinothalamic tract carry?

Answer 16

They carry pain and temperature signals to the thalamus.

Question 17

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

Answer 17

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

Question 18

What does the first-order neurons do?

Answer 18

They detect the stimulus and transmit to spinal cord

Question 19

What is different about 2nd order neurons?

Answer 19

Second order neurons relay the signal to the thalamus

2nd order axons cross the midline; ie are commissural

Question 20

What does topological organisation mean?

Answer 20

the spatial arrangement of the objects relative to one another.

Question 21

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

Answer 21

1st order axons from the upper body follow the lateral pathway and synapse on 2nd order neurons in the cuneate nucleus

1st order axons from the lower body follows the medial pathway and synapse on neurons in the gracile nucleus

Together they form the dorsal column nuclei

Question 22

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

Answer 22

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

Since it crosses the midline, the topology is reversed, so that the lower body axons are more lateral on reaching the thalamus

Question 23

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

Answer 23

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

(How it was originally with the 1st order axons)

Question 24

What are Dermatomes?

Answer 24

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

Question 25

How does each dermatome arise?

Answer 25

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

Question 26

What does topological and topographical mean?

Answer 26

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 27

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

Answer 27

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

Question 28

What does the size of the receptive field depend on?

Answer 28

It depends on where the particular neuron is in the body

Question 29

How is the size of a receptive field measured?

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

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

Question 32

Is the cortex highly plastic and adaptable?

Answer 32

Yes it is highly plastic and adaptable.

Question 33

What does the third order neurons generally do?

Answer 33

They carry the signal from the thalamus to the cortex