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ASCENDING WHITE MATTER TRACTS


TRACT

FUNCTION

RECEPTORS / AFFERENTS

1ST ORDER SN

2ND ORDER SN

DECUSSATION

3RD ORDER SN

FINAL DESTINATION

Dorsal Columns/Medial Lemniscus

Pressure, vibration, movement
Discriminative touch (highly localized)

Specialized skin mechanoreceptors
A-Beta (highly myelinated) SA fibers

Located: DRG
A-Beta fibers enter s/c and ascend uncrossed in dorsal columns
F. gracilis (<=T6)

F. cuneatus (>T6)

Located: n. gracilis and n. cuneatus in lower medulla

Cross midline at medulla and form medial lemniscus

VPL nucleus in thalamus
Projects to somatosensory cortex

Postcentral gyrus in parietal lobe

Anterolateral system (ALS) I:

Neospinothalamic tract

Pain and thermal sense
Poorly localized touch

Free nerve endings in skin
A-delta (thinly myelinated) SA fibers

Located: DRG
A-delta enter s/c and project to lamina II

Lamina I and V neurons collect edited signals from lamina II

Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in thalamus

VPL nucleus in thalamus
Projects to somatosensory cortex

Postcentral gyrus in parietal lobe

ALS II:

Paleo-spinothalamic tract

Dull and poorly localized pain

Free nerve endings in skin
C (non-myelinated) SA fibers

Located: DRG
C fibers enter s/c and project to lamina II

Lamina VI-VIII neurons collect signals from lamina II

Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in thalamus

Intralaminar nucleus in thalamus
Projects to cingulated cortex

Cingulated cortex above corpus collosum

ALS III: Spino-mesencephalic tract

Control of pain sensation

Free nerve endings in skin
A-delta/C (thin/unmyelinated) SA fibers

Located: DRG
A-delta/C fibers enter s/c and project to lamina II

Lamina I and V collect edited signals from lamina II

Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in midbrain




Mesencephalic reticular formation and raphe nucleus in periaqueductal grey

ALS IV: Spino-reticular tract

Sensation of poorly localized pain

Free nerve endings in skin
A-delta/C (thin/unmyelinated) SA fibers

Located: DRG
A-delta/C fibers enter s/c and project to lamina II

Lamina VI-VIII neurons collect signals from lamina II

Cross midline at each segment
Ascend in anterolateral fasciculus to terminate in reticular formation in medulla and pons




Reaches thalamus via reticular formation

ALS V: Anterior spinothalamic tract

Crude touch and pressure information
Poorly localized and non-discriminative

Free nerve endings in skin
A-beta/delta (myelinated) SA fibers

Located: DRG
SA fibers enter s/c and project to lamina I and II

Neurons in dorsal horn generate bilateral output fibers

Some fibers cross, others don’t
Ascend in anterolateral fasciulus to terminate in thalamus

VPL nucleus in thalamus
Projects to somatosensory cortex

Postcentral gyrus in parietal lobe

Dorsal spinocerebellar / cuneocerebellar (DSC)

Conveys muscle sense info (strength, phase rate) used by CB to coordinate movt and posture
Collects info about individual muscles

Muscle spindles and golgi tendon organs
A-alpha; Ia/Ib (thick myelinated) SA fibers

Located: DRG
SA fibers enter s/c and stay in same side in lateral part of posterior funiculus

Clarke’s n. (=
External cuneatus (>C8) in medulla (cuneocerebellar tract)
Ipsilateral
Output fibers from 2nd order project to CB via inferior CB peduncle

Ipsilateral (no cross)




Cerebellum (vermis)

Ventral / rostral spinocerebellar tract

Conveys to CB integrated info about limb behiavor as a whole from lower limbs (ventral) and upper lims (rostral SCT)

Golgi tendon organs
A-alpha; IB (thick myelinated) SA fibers

Located: DRG
A-alpha fibers enter s/c

Spinal border cells of lamina V-VII

Spinal border cells send axons that corss at each segment and ascend to upper pons where they cross again to reach CB via superior CB peduncle
Ends up ipsilateral




Cerebellum (vermis)



DESCENDING WHITE MATTER TRACTS


TRACT

FUNCTION

ORIGIN

1ST ORDER MN

DECUSSATION

SPINAL PATHWAY

FINAL DESTINATION

Cortico-spinal tracts: lateral / anterior

Execute and control voluntary / involuntary motor activity

Motor and pre-motor cortex in precentral gyrus
Produce A-alpha (thick myelinated) efferent fibers

Projects through internal capsule towards midbrain (Cerebral peduncles), pons, medulla (pyramids)

75-90% cross at pyramidal decussation
2 tracts:

Lateral CS: posterior lateral funiculus; 75-90% decussated and 90% project to IN of lamina VII and VIII; 10% project to MN of lamina IX of distal muscles

Anterior CS: anterior funiculus; only in cervical and upper T levels; innervates axial neck/body muscles; 25-10% cross at midline of each segment







Subcortico-spinal (lateral grp): rubrospinal tract

Muscle tone control and regulation of voluntary movement
CB uses it to make corrects on ongoing movt by modifying MN excitability
Acts on distal flexors

Red nucleus in midbrain




Cross midline at ventral tegmental decussation (VTD)

At s/c, rubrospinal fibers are next to lateral cortico-spinal tract

Rubrospinal fibers project on IN in lamina V-VIII (in C and L enlargements)
Act on MN through IN networks

Subcortico-spinal (lateral grp): raphespinal tract

Controls and modulates pain sensation at s/c level
Related with ALS
Release serotonin (5-HT) on IN

Dorsal raphe nuclei in midbrain




Cross midline locally and descend in s/c next to rubrospinal and lateral cortico-spinal tract




Project on IN and SA fibers in lamina I, II, V

Subcortico-spinal (medial grp): tectospinal tract

Mediates postural and head motor reflex orienting responses to moving visual stimuli penetrating visual fields

Superior colliculus in tectus (midbrain)




Cross midline at dorsal tegmental decussation (DTD) in midbrain




Projects bilaterally to gray matter in anterior horn to IN and MN of axial neck and trunk muscles
Only at upper cervical levels

Subcortico-spinal (medial grp): reticulospinal tract

Modulates spinal reflexes thus indirectly MN excitability
Regulates muscle tone of axial muscles and posture adjustment

Reticular formation in pons and medulla:

Pontine RS tract (medial) & medullar RS tract (lateral)




Pontine RS travel uncrossed in medial funiculus
Medullary RS bilateral in lateral funiculus




Gray matter interneurons of lamina VII all across s/c

Subcortico-spinal (medial grp): lateral vestibulospinal tract

Triggers responses to adjust posture and keep balance
Regulates activity of antigravity extensors in lumbar region (i.e. quads and tibialis anterior)

Lateral vestibular n. in medulla




Uncrossed in medial/lateral funiculus




Projects to gray matter MN and IN (lamina VIII and IX) at lumbar level

Subcortico-spinal (medial grp): medial vestibulospinal tract

Facilitates flexor MN in upper C levels that control head position according to vestibular info

Medial vestibular n. in medulla
Fibers join middle longitudinal fasciculus (MLF) that coordinates eye/head movt according to vestibular info




Descend uncrossed next to midline in medial part of anterior funiculus




Gray matter (lamina VIII and IX) at cervical levels on IN and MN of axial muscles
Contiuation of MLF in cervical medulla



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