Lect 5 Pain and Temperature Flashcards
temperature and pain receptors
thermoreceptors
nociceptors
nociceptors
respond to different stimuli
- mechanical (crushing, cutting)
- thermal (noxious cold or heat)
- chemical (bradykinin, prostaglandin, histamine)
primaray afferent axons for pain and temp
A-delta and C-fibers
A-delta fibers
sharp localized pain
lightly myelinated
1st pain
C-fibers
dull and unlocalized pain, aching sensations
unmyelinated
2nd pain
Neuronal chain example 1st,2nd,3rd order
be able to draw this
1st order: periphery to dorsal horn of spinal cord; ipsilateral
2nd order: Spinal cord to thalamus; contralateral
3rd order: Thalamus to cerebral cortex
Anterolateral system
Pain and temp are carried here
Second order neurons travel within the anterior lateral aspect of the spinal cord and anterior lateral aspect of the brainstem tegmentum
Anterolateral system tracts
spino –> anterolateral system, all second order neurons originate in spinal cord
spinothalamic, spinoreticular, spinohypothalamic and spinomesencephalic
Spinothalamic tracts
axons of ALS that originate in the spinal cord and terminate in the thalamus
carries noxious & innocuous temp info, crude non-discriminative sensory info (not major touch pathway)
2 tracts
neospinothalamic tract
paleospinothalamic tract
Neospinothalamic tract
this neuronal chain carries sharo, localized pain info to primary somatosensory cortex
this cortical area allows for conscious awareness of the location and nature of the painful stimulus
cut from sharp object
Paleospinothalamic tract
neuronal chain carries dull, aching and poorly localized pain to secondary somatosensory cortex (S-II) and other cortical areas
-most fibers that carry this type of pain info take a multisynaptic course through the reticular formation before reaching the cerebral cortex
Lissauers Tract
ascends or descends 1-3 segments
substantia gelatinosa
aka lamina II
major site for pain and temperature
a delta fiber termination
1st order neuron of spinothalamic tract
generally A-delta fibers, C-fibers
lamina II
A delta fibers may synapse directly on lamina II interneurons or lamina I and V projection neurons, which have dendrites in lamina II
Laminae I and V
comprised of projection neurons. axons of these neurons form the neospinothalamic tract
can have dendrites and interneurons in lamina II
2nd order neurons of neospinothalamic tract
projection neurons in lamina I and V
have dendrites that extend into the substantia gelatinosa
communicated directly or thru interneurons with primary pain afferents
paleospinothalamic tract 2nd order neurons
laminae VI-VIII receive C fiber input via interneurons that project from lamina II
are projection neurons, their axons form the paleospinothalamic tract
White commissure of spinothalamic tract
where it crosses midline
Neospinothalamic tract 2nd order neuron synapse
synapses on Ventral posterior lateral Thalamic Nerve (VPL)
neospinothalamic 3rd order neuron synapse
synapses on primary somatosensory cortex (S-I)
Paleospinothalamic tract 2nd order neuron synapse
synapses on intralaminar thalamic nuclei (ILN)
10-40% of paleo fibers have been found to pass to the thalamus
Paleospinothalamic tract 2nd order neuron synapse
S-II primary somatosensory cortex
anterior cingulate, insula
Course of neo and paleospinothalamic tracts 3rd order
from VPL or ILN of thalamus -> 3rd order neurons project axons through posterior limb of the internal capsule to reach the cerebral cortex
S-I location & function
primary somatosensory cortex, post central gyrus
cortical area allows for conscious awareness of location and nature of painful stimulus
S-II location and function
paleo
somatosensory cortex, anterior cingulate cortex and insula
carries dull, aching and less well defined pain and sensation
majority of fibers take a multisynaptic course through the reticular formation
Anterior cingulate
thought to mediate the suffering and emotional components of pain
insula
contributes to emotional aspects of pain(c fibers)
Overview of spinothalamic tract
Primary afferent (soma DRG) –> ascend/descend in lissauers tract –> Synapse in substantia gelatinos –> 2nd order neurons cross to contralateral ALS –> medulla –> pons –> midbrain –> Thalamus (VPL or ILN) –> cortext (S I, S II, anterior cingulate, insula)
location of spinothalamic tract in medulla
ALS maintains similar position in the anterolateral tegmentum
location of spinothalamic tract in pons
ALS maintains position in anterolateral tegmentum
locaiton of spinothalamic tract in midbrain
ALS maintains position in anterolateral tegmentum
Visceral afferent fibers
respond to excessive pressure, tension and inflammation in visceral organs
afferents enter the spinal cord and may by interpreted by the cerebral cortex as pain
Why do we have pain in our left arm during a heart attack?
visceral afferents from the heart enter the upper thoracic spinal cord segments. In the spinal cord, these visceral pain afferents converge on the same spinothalamic tract cells reached by somatic pain fibers.
Brain interprets the activation of the spinothalamic tract fibers as pain in somatic region (chest and left arm)
Where does the diaphragm refer pain?
shoulder
Spinoreticular fibers
second order axons of LAS originated in spinal cord and terminate in the reticular formation
laminae VI thru VII/VIII receive c fiber input and give rise to spinoreticular fibers
**may be described as collateral branches of paleospinothalamic tract
Spinoreticular fibers project to…
Thalamus
amygdala - involved in emotional response to pain
hypothalamus
Synapse of spinoreticular fibers on thalamus
Intralaminar thalamic nuclei (ILN)
ILN projects to..
widespread cortical areas including anterior cingulate cortex and insula
and frontal lobe (mediates heightened attention to painful stimuli)
-chronic pain often disrupts concentration
spinohypothalamic tract
receives indirect input from reticular formation
receives bilateral pain information directly from the spinal cord
hypothalamicspinal fibers
involved in regulation of autonomic outflow in response to painful stimuli
hypothalamic spinal fiber projection
to the lateral horn of the spinal cord (location of preganglionic sympathetic neurons)
Spinohypothalamic fiber projections
cross caudally and then move vertical ipsilaterally
Some cross contralaterally near hypothalamus at the supraoptic decussation
carry C pain and A delta pain
spinomesencephalic tract
a delta fibers pass to lamina I and V, give rise to spinomesencephalic fibers, may be considered part of the neospinothalamic tract
pass through cerebral aqueduct
important in descending pain control pathway
spinomesencephalic tract synapse
periaqueductal gray (PAG)
periacqueductal gray
receives input from spinomesencephalic tract and hypothalamus, amygdala and several cortical areas
Descending pain control pathway
believed to be reason why we do not feel pain in serious situation
general steps:
1. PAG receives input
2. Excitation of Enk interneurons
3. Inhibition of tonically active GABA interneurons
4. Excitation of serotonergic neurons
5. Release of serotonin in spinal cord
PAG excitation
from the spinomesencephalic tract & other areas (amygdala, hypothalamus)
Enk containing inhibitory neurons excitation
inhibits release of GABA from the tonically active GABAergic interneurons
Inhibition of GABA neurons
disinhibit the PAG neurons that projecet to the raphe nucleus, this activates the descending pain control pathway
Excitation of serotonergic neurons
in the raphe nucleus leads to release of serotonin from neurons at level of spinal cord
serotonin in spinal cord
leads to inhibition of S-T tract neurons directly and indirectly by exciting inhibitory interneurons which in turn release enkephalin to inhibit S-T tract neurons
opiates
opium, morphine control pain by activating opiate receptors in the PAG, initiate descending pathway
