lecture 8

Question 1

How is EM radiation sensed by the body?

Answer 1

Receptor: rod and cone photoreceptors

Range: 400 to 600nm wavelength

Sensitivity and dynamic range: single photo to bright sunlight (10^10 fold)

Receptive field: single photoreceptor, concentric ganglion cell

Question 2

How is distortion of the skin sensed by the body?

Answer 2

Receptor: various encapsulated nerve endings

Range: 10nm to sub-damaging distortion

Sensitivity and dynamic range: mg, 0-1000 Hx

Receptive field: ovaloid from 10mm^2 to entire hand

Question 3

What does the somatosensory system do?

Answer 3

• mediates sensations from the whole body surface, including skin and deeper tissues

Question 4

What is the structure of skin?

Answer 4

• most of the body is covered by hairy skin
• the palmar surface of the hands and the soles of the feet are covered by glabrous skin, with skin ridges a prominent feature

Question 5

Give an overview of the somatosensory system.

Answer 5

• There are four types of mechanoreceptors in glabrous skin
• Meissner corpuscles and Merkel complexes are close to the surface
• Ruffini organs and Pacinian corpuscles are deeper in the skin
• These receptors are innervated by large myelinated axons with cell bodies in the dorsal root ganglia
• Transmission of this information to the brain generates our conscious experience of touch

Question 6

What are the layers of the skin?

Answer 6

• top dead layer
• epidermis
• dermis
• subcutaneous layer

Question 7

Where are free nerve endings located?

Answer 7

epidermis

Question 8

Where are Meissner corpuscles located?

Answer 8

right below the epidermis in the upper part of the dermis

Question 9

Where are Merkel cell-neutrite complexes located?

Answer 9

In the deep grooves of the epidermis

Question 10

Where are Ruffini corpuscles located?

Answer 10

dermis

Question 11

Where are Pacinian corpuscles located?

Answer 11

Dermis/subcutaneous layer

Question 12

What do Ruffini corpuscles respond to?

Answer 12

Skin being moved or stretched

Question 13

What do Pacinian corpuscles respond to?

Answer 13

Vibration

Question 14

How do these mechanoreceptors open sodium channels?

Answer 14

Thought to be literally by force - movement/pressure/whatever that receptor responds to opens the gate and allows the movement of sodium ions across the membrane leading to depolarisation
- however there is greater molecular complexity to it - resistance from ECM and inside of cell

Question 15

What is the key event for generating an action potential?

Answer 15

depolarisation

Question 16

What is a crucial difference between the mechanoreceptors?

Answer 16

• how they respond to an ongoing stimulus

Question 17

How can we talk about stimuli in general?

Answer 17

We can talk about stimuli as causing a transient change or a sustained change. Mechanoreceptors in turn can adapt slowly to a stimulus, or they can adapt rapidly.

Question 18

Do cells tend adapt rapidly or slowly?

Answer 18

Rapidly - prefer to detect changes. When things stay the same it doesn’t really tell us anything worth knowing e.g. clothes throughout the day

Question 19

What are the slowly adapting mechanoreceptors?

Answer 19

Merkel:
- complexes are found at the troughs of the epidermal ridges where they respond to indentation

Ruffini:
- endings are found in the upper dermis, have a sustained response to skin movement

Question 20

What are the rapidly adapting mechanoreceptors?

Answer 20

Meissner:
- receptors are found near the skin surface and have a transient response to skin movement.

Pacinian:
- receptors are located deep in the dermis and hypodermis and have a transient response to vibration

Question 21

What is the distribution/, receptor field of the different mechanoreceptors in the palm of the hand?

Answer 21

• Merkel: 25% of total, smallest RF
• Meissner: 40% of total, still a small receptor field
• Ruffini: 20%, “proprioceptive”, larger receptor fields
• Pacinian: 15% , most sensitive, vibrations, huge receptor fields

Question 22

Where else are mechanoreceptors located?

Answer 22

• in the muscle: spindles
• in the tendons: golgi tendon organ
• they are activated by physical deformation forces
• they signal information about muscle stretch or position and tendon force
• not considered part of somatosensation

Question 23

What are receptive fields?

Answer 23

• it’s really about what the system can do
• resolution
• how sensitive/the smallest difference it can detect
• depending on how centrally in a particular receptive field something is acting, the associated receptor will respond strongly or weakly
• the closest together to points of contact can be while still detecting that it is two points of contact is a way you can define resolution.
• e.g. fingertips = a few mm, thigh = a few cm

Question 24

What do all four somatosensory mechanoreceptors connect to?

Answer 24

• afferent axon type = A-Beta fibres
• 6 - 12 µm diameter
• 33 - 75 m/s
• large and myelinated

Question 25

What has the biggest, fastest conducting fibres in the body?

Answer 25

- muscle spindles

Question 26

Are the conducting fibres connecting to pain receptors fast?

Answer 26

- not particularly: slowest and thinnest of the somatic sensory afferents that link receptors to the central nervous system - counterintuitive - however sustains well beyond the stimulus - most slow to adapt

Question 27

What are dermatomes?

Answer 27

- a dermatome is a region of skin that is supplied by the sensory nerves of a single spinal segment - this means that location of sensory loss can be indicative of which part of the CNS is damaged - especially if it is evidently a dermatomal region - relates to the form the embryo was in when the dermatomes/connections were made

Question 28

What nerve supplies the skin of the face?

Answer 28

- trigeminal nerve

Question 29

Where is the cell body for the mechanosensory afferent fibres/mechanoreceptor endings?

Answer 29

- dorsal root ganglion

Question 30

How does the axon travel?

Answer 30

From receptor endings, all the way through the body, past its cell body (offshoot), and then into the spinal cord. From here it does two things: goes to the brain, make local synaptic connections in the spinal cord too. The pain and temperature afferent fibre obligatorily makes synaptic connections in the spinal cord - second cell axon crosses over and then travels up to the brain on the other side important to note that the information for touch and the information for pain travel in completely different paths when they get to the CNS

Question 31

What happens when we get to the brain?

Answer 31

Gets the medulla - first bit of the brain stem reached - it meets a nucleus called the dorsal column nucleus. Second neuron in the chain sits here - this neuron decussates (crosses over to the other side) Project all the way up through the brainstem to the thalamus In the ventral, posterior part of the thalamus they meet the third neuron in the pathway. Information goes from here to the cortex e.g. primary somatic sensory cortex

Question 32

Where is the primary somatic sensory cortex located?

Answer 32

Behind the central sulcus, in the post central gyrus.

Question 33

How is the primary somatic sensory cortex divided?

Answer 33

- into four areas based on subtle differences in the cellular architecture - 1, 2, 3b, 3a

Question 34

How does the information get mapped out in the primary somatic sensory cortex?

Answer 34

- somatopic map - areas of the skin that are near each other activate neurons that are near each other in that strip of cortex - so what you get is a sort of map of the body from the medial to lateral part of the cortex - not a continuous map - hands have a large amount of the cerebral cortex - lips and tongue large - humunculus - reflection of scaling of somatosensory importance - not really one - 4 or 8, 16 - most of the input goes to area 3b - also interpret texture (1), size/shape (2) - not isolated bits of cortex, they exchange information

Question 35

Where does the information go?

Answer 35

- a lot to secondary somatosensory cortex and then to amygdala and hippocampus (important for salience, what things mean) - parietal areas 5, 7 (used for motor planning, how to use to touch to guide movements/actions) - interplay of motor and sensory information - separate at tease information out at the level of the primary somatic sensory cortex --> slowly adapting and fast adapting seem to have different areas within their particular regions

Question 36

Is this regional mapping fixed?

Answer 36

No - very plastic. E.g. if you amputate a finger, the cerebral space that was devoted to that finger starts respond to the neighbouring fingers or if you increase stimulation of a particular area over time more of the cerebral cortex will be devoted to these areas