somatosensory system Flashcards
what is included in general sensation?
tactile, pain, temp
what are tactile sensations?
crude (light) touch fine (discriminative) touch pressure vibration cutaneous tension (stretch) hair movement
where are receptors for the somatosensory system found?
skin
skeletal muscle
tendons
ligaments
where is the first order neuron in the somatosensory pathway?
out in periphery - body in dorsal root ganglion
where is the second order neuron in the somatosensory pathway?
somewhere in spinal cord or medulla
where is the third order neuron in the somatosensory pathway?
in the thalamus
what are the two tactile/proprioceptive pathways?
1: dorsal column - medial lemniscus system (DL - ML)
2: trigeminal mechanosensory system
what are the two pathways for pain, temp, and crude touch information?
1: anterolateral system (ALS) aka spinothalamic system
2: spinal trigeminal system
what are the two pathways for unconscious proprioceptive information?
1: dorsal spinocerebellar tract
2: cuneocerebellar tract
what type of information does the dorsal column - medial lemniscus system convey?
tactile (except for crude touch) and proprioceptive
what type of information does the trigeminal mechanosensory system convey?
tactile (except for crude touch) and proprioceptive
from the face via CN V and trigeminal ganglion
what type of information does the anterolateral system convey?
pain, temp, crude touch
what type of information does the spinal trigeminal system convey?
pain, temp, crude touch from the face
what kind of information does the dorsal spinocerebellar tract convey?
unconscious proprioceptive
what kind of information does the cuneocerebellar tract convey?
unconcious proprioceptive
what type of receptors are in the DL-ML pathway?
only mechanoreceptors tactile = meissner's corpuscles ruffini endings pacinian corpuscles merkel cells hair follicle receptors proprioception = muscle spindle joint receptors golgi-tendon organ
what do meissner’s corpuscles detect? where are they located?
in epidermis
detect fine touch, motion
what do ruffini endings detect? where are they located?
in dermis
detect stretch
where are pacinian corpuscles located? what do they detect?
in subcutaneous layer of the skin
vibration
what does the merkel cell neurite complex sense? where is it found?
at junction of dermis and epidermis
sense very fine touch, pressure
what does the hair follicle receptor sense?
hair movement
where are muscle spindles located? what do they sense?
in muscle fibers
sense stretch
where are joint receptors located? what do they sense?
in joints
sense joint position
where are golgi tendon organs located? what do they sense?
in muscle tendons
sense tendon movement
what is the tract by which somatosensory information from below T6 travels (first order neurons)?
in the TG-ML pathway
cell body is in dorsal root ganglia
travels up gracile tract
synapses on neuron in gracile nucleus in caudal medulla
more medial tract
what is the tract by which somatosensory information from above T6 travels (first order neurons) in the TG-ML pathway?
cell body in dorsal root ganglion
fibers travel in cuneate tract
synapse on axon in cuneate nucleus in caudal medulla
more lateral tract
where do somatosensory fibers decussate?
internal arcuate fibers in caudal medulla
second order neurons
what is the tract of the second order somatosensory fibers in the TG-ML pathway?
decussate as internal arcuate fibers in caudal medulla
travel up medial lemniscus - triangular shaped in caudal medulla - becomes more rectangular shaped in rostral medulla - at midline
tract twists - information from lower body moves lateral
synapse in thalamus in lateral nuclei in ventroposterior lateral nucleus
what is the tract of third order somatosensory fibers in the TG-ML pathway?
from VPL in thalamus
through internal capsule
terminate in cortex in primary somatic sensory areas (postcentral gyrus and paracentral lobule)
what is the pathway of first order neurons in the trigeminal mechanosensory system?
cell bodies in the trigeminal ganglion
synapse at principle nucleus of trigeminal complex in the mid-pons
what is the pathway of second order neurons in the trigeminal mechanosensory system?
from principle nucleus of trigeminal complex in mid-pons
decussate in pons
along trigeminothalamic tract (part of medial lemniscus) to thalamus (through midbrain)
synapse at ventral posterior medial nucleus of thalamus
what are the parts of the trigeminal nucleus?
spinal nucleus
primary nucleus that is wider in the mid-pons
mesencephalic nucleus above the pons
what is the pathway of third order neurons in the trigeminal mechanosensory system?
from cell bodies in ventral posterior medial nucleus of thalamus
go to sensory cortex - face area
what is the pathway of first order neurons from in the ALS?
cell bodies in dorsal root ganglia
synapse in dorsal horn in lumbar spinal cord
some can ascend or descend a few levels on the dorsolateral tract of lissauer - but still synapse in dorsal horn
what types of receptors are involved in the ALS?
mechanoreceptors for touch - all receptors are uncapsulated - bare nerve endings
thermoreceptors for temperature - TRP family
nociceptors for pain
what do nociceptors detect? what are the two different types?
intense mechanical, thermal and chemical stimuli
Adelta to conduct fast pain
C fibers to conduct slow pain
what is allodynia?
decrease in pain threshold in areas of inflammation
what is the pathway of second order neurons from in the ALS?
cell bodies in dorsal root (layer 5)
decussate through white commissure
ascend through anterolateral system - rotates laterally a little as it ascends
up through pons and midbrain
fibers synapse in VPL nucleus of thalamus
where does decussation of the neurons in ALS occur?
at the level of the spinal cord through the anterior white commissure
what is the pathway of third order neurons from in the ALS?
from ventral posterior lateral nucleus of thalamus
ascends in internal capsule to primary somatosensory cortex
what is the spinothalamic tract?
STT - part of the ALS but STT is talked about more often clinically
what is the pathway of first order neurons in the spinal trigeminal system?
cell bodies in trigeminal ganglion - part of trigeminal nerve
enter at pons
descend to medualla to synapse in spinal trigeminal tract
synapse in spinal nucleus at various levels in medulla (part of trigeminal complex)
what is the pathway of second order neurons in the spinal trigeminal system?
cell bodies in spinal nucleus of trigeminal complex (in medulla)
decussate in medulla
travel up through trigeminothalamic tract (part of medial lamniscus) through midbrain to ventral posterior medial nucleus of thalamus
where does the spinal trigeminal system decussate?
at the medulla - at any level
what is the pathway of third order neurons in the spinal trigeminal system?
from ventral posterior medial nucleus of the thalamus to somatosensory cortex
what type of information does the periaqueductal gray pathway transmit? where does it project to?
descending pathway for pain modulation
projects to targets in diencephalon and brainstem
what is the path of the periaqueductal gray pathway?
from somatosensory cortex to ventral posterior nuclear complex of thalamus
synapses there
travels to periaqueductal gray matter in caudal midbrain
synapses there
some third order fibers travel to hypothalamus
some travel to reticular formation in rostral medulla
these send fibers to the interneurons in the dorsal horn
how does the periaquaductal gray pathway regulate pain?
descending inputs synapse on interneurons in the dorsal horn
these can inhibit the c-fibers (nociceptors) that are sending pain signals to the brain
what is the gate theory of pain modulation?
explains why, if you hurt your toe, it helps to rub it
the mechanosensory fibers have a stimulatory effect on interneurons, whereas the C fibers have an inhibitory effect
if you rub your toe, that activates the mechanoreceptors and this activates the interneuron and overrides the C fiber’s inactivation of the interneuron
this inhibits pain in the same way that the periaqueductal gray pathway inhibits pain
what is the pathway of first order neurons in the thoracic, lumbar and sacral spinal cord in the spinocerebellar pathway?
muscle spindle receptors
cell body in dorsal root ganglion
synapses in spinal cord and also ascend up dorsal column to clark’s nucleus
where does the spinocerebellar pathway decussate?
it doesn’t
ipsilateral pathway
what is the pathway of second order neurons in the thoracic, lumbar and sacral spinal cord in the spinocerebellar pathway?
from clark’s nucleus
up dorsal spinocerebellar tract - along edge of cord
goes all the way up to cerebellum through inferior cerebellar peduncle
what is the pathway of third order neurons in the thoracic, lumbar and sacral spinal cord in the spinocerebellar pathway?
there are no third order neurons - second order go directly to the cerebellum
what is the pathway of first order neurons in the cervical spinal cord in the spinocerebellar pathway?
from muscle spindle afferent
cell bodies in dorsal root ganglia
ascend and synapse at external cuneate nucleus in medulla
what is the pathway of second order neurons in the cervical spinal cord in the spinocerebellar pathway?
from external cuneate nucleus in medulla
through cuneocerebellar tract into cerebellum through inferior peduncle
what is a lesion?
damage to neural tissue that causes loss of function
what is a neuropathy?
disorder of peripheral nerves (ie pathology that affects axons, myelin, etc.)
what is peripheral neuropathy? what are some common causes?
axon degeneration so lose sensation in hands and feet (stocking glove pattern)
affects the most metabolically active (longest) neurons first
can be due to toxins (alcohol, heavy metals, chemotherapy agents) metabolic disturbances (diabetes), nutritional deficiencies (B12)
what would be the result of a lesion in the DC-ML pathway?
deficits in fine tactile ability and proprioception
what would be the result of lesions int eh ALS pathway?
deficits in pain, temp, and crude touch
what is sterognosis?
ability to recognize objects by touch
what tests would you use for DC-ML?
two-point discrimination
stereognosis
graphesthesia
all test for fine touch
what is graphesthesia?
draw numbers or letters on someone and have them recognize what it is
how would you test for ALS?
look at crude touch - piece of cotton or pointy stick and see if they feel it and can differentiate
