Somatic Sensory pathways Flashcards
first order neurons
RECEIVE
- Psuedo-unipolar neurons located in dorsal root ganglia (DRG) or cranial nerve ganglia
- Peripheral processes receive sensory impulse from receptor and convey it to DRG
- Central processes enter the spinal cord in the dorsal root and continue on to synapse on a second order neuron
second order neurons
CROSS
- Cell bodies located either in spinal cord or brainstem
- Most of these axons cross to opposite side of CNS
- Branches may synapse along the way in certain brainstem nuclei but most synapse on 3rd order neuron
third order neurons
TELL
- Cell bodies are usually located in the thalamus (VPL and VPM; intralaminar nuclei)
- Axons project through posterior limb of internal capsule to primary and association somatosensory cortex in parietal cortex
2 other parts of sensory pathways
internal capsule and cortex (terminates in parietal cortex)
medial bundle branches
Group I and Group II nerve fibers from muscle spindle and GTO
A beta from skin
Touch and proprioception
medial bundle branch pathways
- Direct to dorsal column pathway (FG and FC)
- SG for pain modulation
- Alpha motor neurons for motor reflexes
- Clarke’s Column and spinocerebellar pathway—unconscious proprioception; Friedrich’s
lateral bundle branches
Group III and Group IV nerve fibers from muscle
A delta and C from skin
Pain and temperature
lateral bundle branch pathways
form lateral spinothalamic and spinoreticular:
- enter lissauers tract and synapse in NP
- cross midline midline in VWC and ascend to form anterolateral pathways
form ventral spinothalamic:
- enter lissauers tract and synapse in NP
- DO NOT cross midline but ascend in ventral white matter column
others: to alpha motor neurons for motor reflexes responding to pain
types of sensory info carried in the dorsal column medial lemniscal pathway
This pathway transmits the following sensory information from the body, limbs, and back of the scalp to the primary sensory cortex:
- Light touch/pressure
- Tactile discrimination (Two point discrimination, Stereognosis)
- Conscious proprioception
(Static position sense, Kinesthetic sense, Vibration)
where are the cell bodies of the first, second, and third order neurons in the dorsal column medial lemniscal pathway?
first order cell bodies: dorsal root ganglia
second order cell bodies: nucleus gracilis and nucleus cuneatus
third order cell bodies: ventral posterior lateral nucleus of the thalamus
afferent/efferent fiber connections of first order cell bodies
Afferent fiber connection: receives sensory impulse from the receptor
Efferent: conveys impulse to the dorsal root ganglion-medial bundle
afferent/efferent fiber connections of second order cell bodies
Afferent: nucleus gracilis and nucleus cuneatus
Efferent: Medial lemniscus
afferent/efferent fiber connections of third order cell bodies
Afferent: Ventral posterolateral nucleus
Efferent: somatosensory cortex vis posterior limb of internal capsule
info carried in the fasciculus gracilis and where they are located on a cross section of the SC
crosses at medial dorsal horn
Carries proprioceptive information from lower trunk, lower extremity, and perineum
info carried in the fasciculus cuneatus and where they are located on a cross section of the SC
crosses at the lateral dorsal horn about T6
Carries proprioceptive information from upper trunk, upper extremities
what sensory info is carried in the spinocerebellar pathways???
Unconscious proprioception
Coordination of movement and balance
Information for planning movement, responding to something
what structures does friedreich’s ataxia affect and how does it present clinically?
CLARKES NUCLEUS
Lesions of spinocerebellar tracts
Bilateral decreased coordination, decreased balance, and abnormal gait
what types of sensory info are carried in the anterolateral system (ALS)?
Conscious pain and temperature sensation and your emotional/autonomic responses to pain
Course touch, pain, temperature, itch, TICKLE
the 3 main pathways making up the ALS
- spinothalamic tract (lateral and ventral)
- spinoreticular tract
- spinomesencephalic tract
function of spinothalamic tract
lateral: main pathway for transmitting pain information to cerebral cortex
Lesion: Will reduce ability to feel chronic severe pain
ventral: mostly redundant/accessory pathways; isolated lesion will not cause clinically significant deficits
function of spinomesencephalic tract
carries signals from spinal cord to periaqueductal grey in the midbrain
involved in descending pain modulation
*endogenous pain
function of spinoreticular tract
carries signals from spinal cord to reticular formations in pons and medulla
lesion: automatic/reflex withdrawal from pain
arteries that might supply blood to the different portions of the sensory pathways
- Posterior cerebral artery: thalamic pain syndrome
(Non-trackable pain; low threshold, non-localized hemi-pain, typically reports pain at rest, contralateral to lesion) - Middle cerebral artery (Contralateral UE deficits)
- Anterior cerebral artery (Contralateral LE deficits)
location of the periaqueductal gray matter (PAG)
midbrain
function of the periaqueductal gray matter (PAG)
Primary source of input for reticular nuclei whose axons form the reticulospinal pathways that are involved in sensory regulation
how does the PAG regulate pain
- Descend to spinal cord to terminate in the substantia gelatinosa where they inhibit the transmission of pain arising from dorsal horn in NP
- Electrodes placed in PAG can produce a strong analgesic effect and decrease pain even in those suffering from chronic pain syndromes
- Projects from the thalamus and cortex to the PAG may explain the ability to suppress pain during periods of high stress
location of substancia gelatinosa
dorsal horn of SC
function of substantia gelatinosa
gate theory of pain modulation
gate theory and substantia gelatinosa to regulate pain
Group I touch fibers will inhibit the input other groups
- Pain fibers come in, go down to NP and excite cells there: send signal via ascending pain pathways
If at same time there is touch AND proprioceptive information coming it and it goes through SG and then to NP giving it inhibitory signal
- you can stimulate pain and a mechanoceptor will override and get to your brain first
*FAST ACTING PAIN RELIEF
- Rubbing a body segment with firm pressure can help alleviate pain
2 descending sensory regulatory pathways
Medullary Reticulospinal (Lateral Column) Pontine Reticulospinal (Ventral Column)
location and termination of medullary and pontine reticulospinal pathways
Located in brainstem
Go down to substantia gelatinosa in the SC and inhibit pain by causing a block of the nucleus Proprius from sending pain signals
shingles (herpes zoster)
This is an infection of dorsal root ganglia or cranial nerve ganglia with varicella-zoster virus.
Some unknown stimulus triggers the virus to become infectious again and the virus irritates and inflames the spinal nerve causing pain.
The virus is released into the skin from the nerve endings causing painful red eruptions on the skin. Itching and burning or tingling sensations can precede these skin lesions.
syringomyelia
lesion around the central canal of the spinal cord affects the ventral white commissure and is usually present in the cervical spine
Causes bilateral deficits in pain and temperature sensation a few segments above and below the lesion
nerve lesions
Peripheral: Impairment of all sensation in the area of skin supplies by that peripheral nerve
Dorsal root/Spinal nerve: Impairment of all sensation in the dermatome supplied by that nerve
clinical lesions types of deficits: direct effects
- astereognosis (hand only-lateral cortex and UE sensation)
- impaired ability to detect, localize, or discriminate touch/pressure
- impaired 2 point discrimination
- impaired ability to consciously detect static position/kinesthetic/vibration
unconscious proprioception pathways
- provide sensory info to the cerebellum to allow it to coordinate movement and balance
- structure includes nucleus dorsalis and spinocerebellar tracts
- tracts exit the brainstem and the level of the pons/medulla go to the cerebellum
- no synapses in thalamus, no termination in cortex
lesions of spinocerebellar tracts
disease: friedreich’s ataxia
- bilateral decreased coordination, decreased balance, abnormal gait
endogenous pain control
- a strong pain signal goes up spinomescenphalic tract, to PAG, sends signal down, along the way activates norepinephrine and serotonin
- LONG LASTING (but takes longer to activate)
ex) opiates
PAG surrounds the
cerebral aqueduct
stretch reflex
agonists activate
synergists and antagonists inhibit
(at rest, 1b afferents (GTO) are activating alpha motor neurons of the muscle they arise from; so during movement 1b excite the antagonist)
