somatosensory and pain

Question 1

3 subsystems of somatosensory info

Answer 1

1. cutaneous mechanoreceptors - fine touch, vibration, pressure
2. proprioception - position of joints, tension in muscle fibers, position of body in space
3. receptors for pain, temp and course touch

Question 2

cutaneous receptors

Answer 2

perception of tactile stimulation is mediated by them
located in the skin
the axons of these neurons have specialized endings which enable them to generate an action potential when exposed to various stimuli, such as indentation of the skin

Question 3

how deformation of the skin can lead to action potentials in a sensory neuron

Answer 3

deformation of capsule (i.e. pacinian corpuscle) leads to stretching of the membrane of the sensory nerve fiver ending which increases the probability of opening of stretch-sensitive cation channels. a net influx of Na+ leads to depolarization of the nerve fiber ending. if the deformation is intense enough, an action potential will be generated

Question 4

rapid adapting receptor

Answer 4

fire strongly at the offset and then quickly shut down
effective in conveying info about changes in ongoing stimulation
generally used to perceive vibration and rapidly changing patterns

Question 5

slowly adapting receptor

Answer 5

maintain firing for the duration of the stimulus
better for providing spatial attributes of the stimulus, such as size and shape

Question 6

receptive field

Answer 6

the range of input (spatial, tonal, thermal) that evokes a response

Question 7

free nerve endings

Answer 7

nociception (tissue damage, chemical) and temperature

Question 8

meissner corpuscles

Answer 8

• rapidly adapting, small receptive fields
• located on the tip of epidermal sweat ridges
• shape and texture perception

Question 9

merkel’s disks

Answer 9

slow adapting, small receptive field
- located on dermal papillae (close to skin surface)
- motion detection, grip control

Question 10

ruffini capsules

Answer 10

slowly adapting, large receptive fields
- located in the dermis
- tangential force, hand shape, motion direction

Question 11

pacinian corpuscles

Answer 11

rapidly adapting, large receptive fields, vibration sensitive
- located in dermis and deeper tissue
- perception of distant events through transmitted vibrations, tools use

Question 12

what provides the highest info requiring fine tactile discrimination

Answer 12

merkel’s disks

Question 13

proprioceptors

Answer 13

receptors for self

Question 14

3 types of receptors

Answer 14

• muscle spindles
• golgi tendon organ
• joint receptor

Question 15

muscle spindles

Answer 15

• found in all but a few striated muscles
• consist of four to eight specialized intrafusal muscle spindles surrounded by a capsule of connective tissue
• muscle stretch opens channels and activates spindles
• group Ia: rapidly adapting and signal dynamics changes
• group II afferents: produce sustained responses that signal static position
• intrafusal muscle fibers: innervated by gamma motor neurons, do not generate force, but pull on spindles and change their response to stretching
• gives info on stretching of muscles

Question 16

golgi tendons

Answer 16

• give info on muscle force
• formed by branches of Ib afferents distributed among the collagen fibers that form the tendon
• force generated by muscle causes mechanical strain and activation of receptors

Question 17

neurapraxia

Answer 17

mild and temporary nerve impairment, such as when you arm/leg falls asleep
pinching of nerve alters transmission of afferent (sensory) and efferent (motor neuron) fibers and limb function is severely impaired

Question 18

joint receptors

Answer 18

• primary source of proprioceptive info about limb position and movement
• include Ruffini ending and Pacinian corpuscles

Question 19

transmission of tactile info to the spinal cord

Answer 19

• the cell bodies of somatosensory neurons are located in the dorsal-root ganglion, located immediately adjacent to every segment of the spine
• the axons of dorsal root ganglion cells involved in mechanosensory discrimination project from the periphery, through the dorsal root ganglion and then up the dorsal part of the spinal column

Question 20

central pathways subserving touch

Answer 20

sensory fibers carrying tactile info ascend to the level of the medulla. there they synapse on neurons within one of two dorsal column nuclei. these nuclei project to the tactile part of the thalamus (ventroposterior lateral nucleus) and from there to the primary somatosensory cortex, area S1
- fibers cross to the contralateral part of the brain after the medulla

Question 21

primary somatosensory cortex

Answer 21

located in the post-central gyrus just behind the central sulcus
consists of four distinct areas (1, 2, 3a, 3b)

Question 22

secondary somatosensory cortex

Answer 22

located posterior to S1

Question 23

area 3a

Answer 23

specialized for processing proprioceptive info
gets inputs from deep cutaneous receptors and muscle stretch receptors

Question 24

area 3b

Answer 24

appears to be specialized for fine tactile discrimination
gets inputs from cutaneous receptors, especially those with small receptive fields

Question 25

info flow in S1

Answer 25

from areas 3a and 3b to area 1 then onto area 2 and finally onto SII and somatosensory association cortex (area 5)
many alternative pathways do exist

Question 26

center-surround antagonism

Answer 26

some neurons in somatosensory cortex exhibit
they have an excitatory center region and a surrounding inhibitory region
adds edge detection and possibly other useful computational properties

Question 27

lateral inhibition improves

Answer 27

discrimination of two-point stimuli
enhances the detection of edges

Question 28

differential stimulation can increase

Answer 28

2-point discrimination and increase cortical S1 area devoted to processing somatosensory input

Question 29

first pain

Answer 29

• perceptually ‘sharp’, low latency
• mediated by fast-conducting A fibers

Question 30

second pain

Answer 30

• delayed onset, spatially diffuse, longer lasting
• mediated by slow-conducting C fibers

Question 31

the inhibitory interneuron can inhibit both

Answer 31

1. the presynaptic primary sensory axon carrying pain info
2. the postsynaptic cell transmitting that pain info to the brain
   - touch inputs can gate the transmission of pain signals to the brain (squeezing a hurt finger)

Question 32

central modulation of pain

Answer 32

the brain also sends signals down the spinal column which modulate the transmission of pain. these descending fibers synpase on spinal interneurons which release enkephalin, one of the brain’s natural opioids
enkephalin acts both pre- and post-synaptically to reduce the transmission of pain signals to the brain