somatosensory and pain Flashcards
3 subsystems of somatosensory info
- cutaneous mechanoreceptors - fine touch, vibration, pressure
- proprioception - position of joints, tension in muscle fibers, position of body in space
- receptors for pain, temp and course touch
cutaneous receptors
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
how deformation of the skin can lead to action potentials in a sensory neuron
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
rapid adapting receptor
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
slowly adapting receptor
maintain firing for the duration of the stimulus
better for providing spatial attributes of the stimulus, such as size and shape
receptive field
the range of input (spatial, tonal, thermal) that evokes a response
free nerve endings
nociception (tissue damage, chemical) and temperature
meissner corpuscles
- rapidly adapting, small receptive fields
- located on the tip of epidermal sweat ridges
- shape and texture perception
merkel’s disks
slow adapting, small receptive field
- located on dermal papillae (close to skin surface)
- motion detection, grip control
ruffini capsules
slowly adapting, large receptive fields
- located in the dermis
- tangential force, hand shape, motion direction
pacinian corpuscles
rapidly adapting, large receptive fields, vibration sensitive
- located in dermis and deeper tissue
- perception of distant events through transmitted vibrations, tools use
what provides the highest info requiring fine tactile discrimination
merkel’s disks
proprioceptors
receptors for self
3 types of receptors
- muscle spindles
- golgi tendon organ
- joint receptor
muscle spindles
- 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
golgi tendons
- 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
neurapraxia
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
joint receptors
- primary source of proprioceptive info about limb position and movement
- include Ruffini ending and Pacinian corpuscles
transmission of tactile info to the spinal cord
- 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
central pathways subserving touch
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
primary somatosensory cortex
located in the post-central gyrus just behind the central sulcus
consists of four distinct areas (1, 2, 3a, 3b)
secondary somatosensory cortex
located posterior to S1
area 3a
specialized for processing proprioceptive info
gets inputs from deep cutaneous receptors and muscle stretch receptors
area 3b
appears to be specialized for fine tactile discrimination
gets inputs from cutaneous receptors, especially those with small receptive fields
info flow in S1
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
center-surround antagonism
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
lateral inhibition improves
discrimination of two-point stimuli
enhances the detection of edges
differential stimulation can increase
2-point discrimination and increase cortical S1 area devoted to processing somatosensory input
first pain
- perceptually ‘sharp’, low latency
- mediated by fast-conducting A fibers
second pain
- delayed onset, spatially diffuse, longer lasting
- mediated by slow-conducting C fibers
the inhibitory interneuron can inhibit both
- the presynaptic primary sensory axon carrying pain info
- 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)
central modulation of pain
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
