Lecture 8: Tactile sensation and perception

Question 1

What does the somatosensory system mediate?

Answer 1

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

Question 2

What is most of the body covered by?

Answer 2

hairy skin

Question 3

What is the palmar surface of the hands and soles of the feet covered by?

Answer 3

glabrous skin, with skin ridges a prominent feature

Question 4

What are the four types of mechanoreceptors in glabrous skin?

Answer 4

Meissner corpuscles and Merkel complexes are close to the surface, while Ruffini organs and Pacinian corpuscles are deeper in the skin

Question 5

What are mechanoreceptors innervated by?

Answer 5

large myelinated axons with cell bodies in the dorsal root ganglia

Question 6

What generates our conscious experience of touch?

Answer 6

transmission of information sent through mechanoreceptors to the brain

Question 7

Where are the Merkel cell-neurite complexes located?

Answer 7

at the tips of epidermal ridges

Question 8

Where are the Meissner and Ruffini corpuscles located?

Answer 8

Meissner: near the skin surface
Ruffini: upper dermis

Question 9

Where are the Pacinian corpuscles located?

Answer 9

deep in the dermis and hypodermis

Question 10

What is the role of corpuscles?

Answer 10

they are highly sensitive to vibrations and deformation which leads to the mechanical opening of ion channels

Question 11

What happens when a touch stimulus is too weak to be detected?

Answer 11

the receptor potential will not reach threshold and an action potential will not be fired

Question 12

Which receptors respond to the distortion of skin, what is their range, what is their sensitivity and what is their receptive field?

Answer 12

various encapsulated nerve endings respond to the distortion of skin
they have a range of 10nm to sub-damaging distortion, their sensitivity is mg and their receptive field is ovaloid from 10mm^2 to entire hand

Question 13

Which receptors respond to EM radiation, what is their range, what is their sensitivity and what is their receptive field?

Answer 13

rod and cone photoreceptors respond to EM radiation
they have a range of 400 to 600nm wavelength, their sensitivity is a single photon to bright sunlight (10^10 fold) and their receptive field is a single photoreceptor, concentric ganglion cell

Question 14

What are the slow adapting mechanoreceptors?

Answer 14

Merkel complexes which respond to indentation and Ruffini endings which have a sustained response to skin movement

Question 15

What are the rapidly adapting mechanoreceptors?

Answer 15

Meissner receptors which have a transient response to skin movement and Pacinian receptors which have a transient response to vibration

Question 16

What are the characteristics of Merkel?

Answer 16

superficial

25% smallest receptive field

Question 17

What are the characteristics of Ruffini?

Answer 17

deep

20% “proprioceptive”

Question 18

What are the characteristics of Meissner?

Answer 18

high density

40% of total more sensitive

Question 19

What are the characteristics of Pacinian?

Answer 19

low density

15% most sensitive to vibrations

Question 20

What does sensory discrimination depend on?

Answer 20

the entire system, peripheral and central mechanisms

Question 21

What are the two types of peripheral nerve axons?

Answer 21

can either be efferent (carrying signals from the CNS) or afferent (carrying signals to the CNS)

Question 22

Do efferent and afferent peripheral nerve axons have the same pathways?

Answer 22

no, although mixed in the peripheral nerves, they have separate pathways (the roots) connecting to the CNS

Question 23

Where do sensory neurons travel in the spinal cord?

Answer 23

through the dorsal root

Question 24

What are the dorsal column nuclei?

Answer 24

location of the second neuron in the touch pathway
from here the neuron projects across to the other side of the nervous system to the thalamus where the third neuron in the pathway is located

Question 25

Where does the third neuron in a touch pathway project?

Answer 25

a part of the cerebrum called the somatosensory cortex

Question 26

What is matter in front of the central sulcus involved in and what is matter behind the central sulcus involved in?

Answer 26

in front of the central sulcus = action

behind the central sulcus = sensation / perception

Question 27

What is the post-central gyrus?

Answer 27

S1 of the somatic sensory cortex

Question 28

What is somatotopic order?

Answer 28

when a specific part of the body is associated with a distinct location in the central nervous system

Question 29

How can sensory systems be defined?

Answer 29

in terms of resolution and dynamic range

Question 30

What is the receptive field?

Answer 30

defines the elements of detection

Question 31

What does detection require?

Answer 31

transduction of physical energy into nervous system activity

Question 32

What are some useful generalities about cortical representations?

Answer 32

are often topographically mapped to the (contralateral) periphery
cortical territory is unevenly magnified - usually in proportion to peripheral receptor density
cortical territory is plastic