Sensory Tracts Flashcards
Ascending tracts specifics
Are long projections that originate from the spinal cords and go UP to the brainstem or forebrain (opposite of descending motor tracts)
Typically represent various types of sensory input rising to higher levels of the CNS And may or may not be in the conscious level
Consists of the:
- dorsal columns
- Anteriolateral system
- posterior/anterior spinocerebellar tracts
Posterior (dorsal) column- medial lemniscus pathway (PCMLP)
Receives afferents carrying fine discriminative touch, vibratory sense and proprioception from the periphery/body
Primary nuclei are located in the dorsal root ganglion, and sends their fibers to the dorsal horns to travel in the gracile/cuneate tracts where they ascend to the caudal medulla
- at this point they decussate once they synapse on the nucleus gracilis and nucleus cuneatus
They then travel from medulla to the thalamus (called the medial lemniscus pathway after the medulla decussation)
Finally thalamus -> primary somatosensory cortex (postcentral gyri)
Somatotopic organization of the dorsal column of the (PCMLP)
Fasciculus gracillis
- contains information from lower limbs
- exists at all levels of the spinal cord
(Lateral -> medial organization all the way down)
Fasciculus cuneatus
- contains information from upper limbs
- exists in C1-T5 only (lateral -> medial organization)
How does the medial lemniscus change in somatotopically as it ascends?
At the medulla level ( where it becomes the medial lemniscus) :
- most ventral -> dorsal
- LE/Trunk/UE
At medulla-pons junction
- rotates clockwise 60 degrees
At the middle-rostral pontine levels
- rotates clockwise another 45 degrees
At mid brain levels
- rotates clockwise another 45 degrees
- at this point is almost 180 change from the medulla level
Where do tertiary neuron go?
From the VPL in the thalamus -> primary sonatosensory cortex -> secondary somatosensory cortex and superior parietal lobule
Trigemino-thalamic tract
primary sensory neurons begin in the trigeminal ganglion
- these provide sensory afferents from the face and head
Secondary neurons start from either the principal trigeminal sensory nuclei or the spinal trigeminal nuclei and go to the trigeminal lemniscus or trigeminothalamic tract respectively
- these both go to the VPM nuclei of the thalamus, synapse.
Tertiary sensory neurons then leave VPM and move to the primary somatosensory cortex
Anterolateral system
Possess 4 tracts and all are for transmission of pain, temperature sensation and crude (non-discriminative) touch
Direct pathway = (neo)spinothalamic tract
* most important*
Indirect pathways:
- spinomesencephalic
- paleospinothalamic
- spinoreticular
Spinothalamic tract specifics
Carries fast pain sensation, pain intensity and pain localization
Small primary pain afferents enter dorsal horn from periphery site and synapse here
Secondary decussate through the anterior white commissure at the same spinal cord level and forms the anterior and lateral spinothalamic tract
Spinothalamic tract then synapses on the VPL nucleus and sends it to the primary somatosensory cortex
Spinomesencephalic tract
Is an indirect pathway of the anterolateral system
Starts in dorsal horn of the spinal cord -> the PAG and superior colliculi
Not known for sure but presumably helps inhibit/modulate pain sensation.
Paleospinothalamic tract
An indirect pathway of the anterolateral system
Starts in the dorsal horn of the spinal cord and gores to the reticular formation of the brainstem where it projects to the intra-laminar thalamic nuclei
They then diffuse into the cerebral cortex limbic regions and do some sort of pain modulation (not known for sure)
Spinoreticular tract
An indirect pathway of the anterolateral system
Starts in the dorsal horn of the spinal cord and goes to the medullary reticular formation and the pontine reticular formations of the brainstem where it projects to the intra-laminar thalamic nuclei
They then diffuse into the cerebral cortex limbic regions and do some sort of pain modulation (not known for sure)
Generalized anterolateral system somatotopic organization
Medial -> lateral
- leg
- trunk
- arm
- neck/face
Posterior spinocerebellar tract
Receives efferent fibers from muscle spindles and golgi tendon organs from muscles in the dorsal horn of the spinal cord.
Secondary neurons travels from the nucleus dorsalis of Clarke in spinal cord to the inferior cerebellar peduncle
Tertiary neurons then travel from inferior cerebellar peduncle -> spinocerebellum and synapse
conveys information about individual muscles and groups of muscles in the LOWER limbs and caudal trunk (nonconscious proprioception)
DONT DECUSSATE
Cuneocerebellar tract
Receives efferent fibers from muscle spindles and golgi tendon organs from muscles in the dorsal horn of the spinal cord.
Secondary neurons start in the fasciculus cuneatus from the accessory cuneate nucleus in the midbrain and spinal cord and ascend to the inferior cerebellar peduncle
Tertiary neurons then move from inferior cerebellar peduncle -> spinocerebellum
conveys information about individual muscles and groups of muscles in the UPPER limbs and caudal trunk (nonconscious proprioception)
DONT DECUSSATE
Anterior spinocerebellar tract
- Starts in the dorsal root ganglion
With secondary neurons decussating and traveling up to the pons to enter the superior cerebellar peduncle. - tertiary neurons start from the superior cerebellar peduncle, decussate again and travels to the spinocerebellum
Conveys information about whole limb movements and postural adjustments to the cerebellum ipsilaterally (since it decussate twice)
Rostral spinocerebellar tract
Similar to anterior spinocerebellar tract except conveys upper limb movements and postural adjustments and does NOT DECUSSATE
- also enters pons from the inferior cerebellar peduncle.
Propriospinal tracts (fasciculus proprius)
Association bundles (contain both ascending and descending fibers)
- also fibers can be uncorssed or crossed
- super confusing so dont need to know much
Located in anterior/lateral/posterior funiculi adjacent to the gray matter And connect different segments of the spinal cord
Function to mediate intrinsic reflexes of the spinal cord
Medial longitudinal fasciculus (MLF)
Bundle of ascending and descending fibers that arise from the superior and medial vestibular nucleus
Ascending portion:
- travel to the CN3/4/6 nuclei and then terminate here
- Carries informational to these cranial nerves about what direct the eyes should move (act as an in between for the cortex and the nerves on the actual muscles).
Descending portion:
- travel to the ipsilateral cervical spinal cord segments and inhibts motor neurons in these segments
- works to control the position fo the head
Brown-sequard syndrome
A hemisection of the spinal cord
At the level of the lesion
- motor loss on ipsilateral side
Below the segmental level of lesion
- ipsilateral loss of proprioception, vibration and upper motor neurons
- contralateral loss of pain and temp sensation
Amyotrophic lateral sclerosis (Lou Gehrig’s disease)/ (ALS)
Progressive degenerative disease in which the corticospinal tracts and ventral horns degenerate
Begins with degeneration of lower limb motor neurons and then upper limbs later on
Unknown cause of disease
Syringomyelia
Expansion of the central canal via developmental or acquired abnormalities (unknown)
Causes sensory loss
- segmental loss of pain and thermal sensation due to interruption of crossing fibers oft he spinothalamic tracts in the same/adjoining segment levels of the lesions
- DOES NOT affect tactile,vibratory and proprioception sensations.*
