Somatosensation

Question 1

what type of channel transduce touch signal?

Answer 1

Mechanically gated cation channels transduce touch signals (non specific cation channel)

Question 2

what is the different type of Mechanically gated cation channels ?

Answer 2

cation channel that activate directly through lipid tension
cation channel that directly activate via a structural protein Linking proteins may be:
intracellular (cytoskeleton)
extracellular (surrounding tissue)
Cation channel that open via indirect action through membrane structural protein
The effects of physically gated G-protein coupled receptors will be slow to activate and long-lasting => may be involved in pain sensation

Question 3

Each axon is associated with how many mechanoreceptor?

Answer 3

Each axon is associated with only a single type of mechanoreceptor.
hence it can only transmit the sensory information detected by one type of mechanoreceptor in the form of electrical signal

Question 4

Glabrous skin vs hairy skin

Answer 4

• Glabrous skin = no hair follicles. This includes lips, palms of hands, soles of feet. More sensitivity
• Hairy skin = everywhere else (even if you shave).

Question 5

where are the cell body of the sensory afferent located at?

Answer 5

The cell bodies of sensory afferents are in the Dorsal Root Ganglion

Question 6

sensory afferen

Answer 6

Sensory afferents:
Unipolar neurons
Spike initiation zone is near the specialised accessory structure
Cell body is outside the spinal cord in the DRG
Action potential is generated as a result of the receptor potential – travels along entire axon

Question 7

Action potentials can be recorded from sensory axons in awake humans

Answer 7

For each receptor type, we can characterise:
Receptive field (where on skin we can touch to change membrane potential)
Optimal stimulus properties
Frequency sensitivity
Adaptation rate: how the firing rate of a neuron changes over time in response to a constant stimulus.
Axon type and conduction velocity

Question 8

Receptive field

Answer 8

field (where on skin we can touch to change membrane potential)

The receptive field encompasses the sensory receptors that feed into sensory neurons and thus includes specific receptors on a neuron as well as collectives of receptors that are capable of activating a neuron via synaptic connection

Question 9

Adaptation rate

Answer 9

how the firing rate of a neuron changes over time in response to a constant stimulus.

Question 10

what do the mechanoreceptors differ in?

Answer 10

Mechanoreceptors differ in receptive field size and adaptation rate

Question 11

what is the size of superficial mechanorecepotors and why?

Answer 11

Superficial receptors have smaller receptive fields => they contribute more to fine position discrimination
Meissner and markel cell

Question 12

which two mechanoreceptor adapt rapidly?

Answer 12

Meissner and Pacinian corpuscles rapidly adapt

- firing rate drops in response to sustained stimuli, they respond to few initially touches

Question 13

how many axons and neurons are the deep receptors (Pacinian corpuscles and Ruffini endings) associated with?

Answer 13

The deep receptors are associated with a single axon and neuron

Question 14

how many superficial receptors are associated with a single axon and a single neuron ?

Answer 14

Multiple superficial receptors of the same type may be associated with a single axon and neuron

Question 15

Describe the structure of Pacinian corpuscles and how it relates to it’s function?

Answer 15

Concentric, fluid-filled layers of connective tissue around the axon terminal
Most sensitive mechanoreceptor, but rapidly adapt to sustained pressure
Respond to high vibration frequencies (30-500 Hz)

Like an onion – different layers can shift relative to one another. Thus, only initial part of sustained touch is signalled by Pacinian corpuscles.
However, each cycle of an oscillating stimulus can lead to shifting of the layers, causing action potentials.

Question 16

what is the advantage of corpuscle in Pacinian corpuscles for its function?

Answer 16

The physical structure of the corpuscle provides an additional, mechanical form of rapid adaptation.
-The corpuscle is like an onion – different layers can shift relative to one another. Thus, only the initial part of sustained touch is transferred by Pacinian corpuscles to a compression of the axon.

• With the capsule it undergoes deformation, absorbed the deformation and it undergoes different type of deformation when that stimuli is removed so another AP is generated. Hence it signals the initial touch and the removal touch
• However, each cycle of an oscillating stimulus can lead to shifting of the layers, causing action potentials.

Question 17

Receptor structure accounts for function – Merkel discs

Answer 17

Merkel discs (SA1) encode the shape and size of objects touching the hand.

Smaller stimuli activate fewer receptors, but activate them more strongly.
As stimulus get larger, it is actually moving in the surround of the receptive field and it is inhibiting the response of the neurons some how
Center-surround structure – required for precise localisation.

Question 18

SA1 (Merkel cell)

Answer 18

location: Superficial - tip of epidermal sweat ridges
skin type: Hairy + glabrous
relative receptive field size: Small (~2 mm)
Adaptation rate: slowly adapting
Response to sustained indentation: sustained
Best stimulus:Edges & points
Threshold for indentation or vibration:8 µm

Density is inversely proportional to RF size – highest density are Meissner’s corpuscles and Merkels disks

Question 19

RA1 (Meissner corpuscle)

Answer 19

location: Superficial - dermal papillae
skin type: glabrous
relative receptive field size: Small (~ 5 mm)
Adaptation rate: Rapidly adapting 
Response to sustained indentation: none
Best stimulus:lateral motion 
Threshold for indentation or vibration:2 µm
Superficial receptors have small RF.

Question 20

SA2 (Ruffini ending)

Answer 20

location:Deep tissue - dermis
skin type: Hairy + glabrous
relative receptive field size: Large (~ 8 mm)
Adaptation rate: slowly adapting
Response to sustained indentation: sustained
Best stimulus:skin stretch
Threshold for indentation or vibration:40 µm

Question 21

RA2 (Pacinian corpuscle)

Answer 21

location:Deep tissue - dermis
skin type: Hairy + glabrous
relative receptive field size: Large (~ 8 mm)
Adaptation rate: rapidly adapting
Response to sustained indentation: none
Best stimulus: Vibration
Threshold for indentation or vibration:0.01 µm

Question 22

Sensory pathway to the brain:the Dorsal Column–Medial Lemniscal Pathway

Answer 22

1. Axons from sensory receptors enter the spinal cord via the dorsal root
2. The axons branch, synapsing onto neurons within the dorsal horns for mediating local circuitry
3. The other axon branch ascends the spinal cord in the dorsal column, synapsing in the dorsal column nuclei
4. The axons decussate (cross to the other side of the body
5. Dorsal column neurons target neurons in the VP nucleus of the thalamus, which projects to primary somatosensory cortex (S1)

Question 23

What is “primary” about a primary sensory area?

Answer 23

Area 3b (S1):
First cortical area to receive somatosensory information - dense inputs from thalamus (VP nucleus)
neurons are responsive to somatosensory stimuli (but not other senses)
lesions impair touch sensation
Electrical stimulation evokes touch sensation

Question 24

How else might we quantitatively test touch perception

Answer 24

Performance on complex tasks can be related to mechanoreceptor properties, and tuning properties of cortical neurons
Detection of a gap (i.e. is there a gap or not)
Discrimination of grating orientation (horizontal vs vertical)
Identification of one item from a large collection of possibilities
Most common psychophysical testing