TRACT
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FUNCTION
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RECEPTORS / AFFERENTS
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1ST ORDER SN
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2ND ORDER SN
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DECUSSATION
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3RD ORDER SN
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FINAL DESTINATION
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Dorsal Columns/Medial Lemniscus
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Pressure, vibration, movement
Discriminative touch (highly localized)
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Specialized skin mechanoreceptors
A-Beta (highly myelinated) SA fibers
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Located: DRG
A-Beta fibers enter s/c and ascend uncrossed in dorsal columns
F. gracilis (<=T6)
F. cuneatus (>T6)
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Located: n. gracilis and n. cuneatus in lower medulla
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Cross midline at medulla and form medial lemniscus
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VPL nucleus in thalamus
Projects to somatosensory cortex
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Postcentral gyrus in parietal lobe
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Anterolateral system (ALS) I:
Neospinothalamic tract
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Pain and thermal sense
Poorly localized touch
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Free nerve endings in skin
A-delta (thinly myelinated) SA fibers
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Located: DRG
A-delta enter s/c and project to lamina II
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Lamina I and V neurons collect edited signals from lamina II
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Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in thalamus
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VPL nucleus in thalamus
Projects to somatosensory cortex
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Postcentral gyrus in parietal lobe
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ALS II:
Paleo-spinothalamic tract
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Dull and poorly localized pain
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Free nerve endings in skin
C (non-myelinated) SA fibers
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Located: DRG
C fibers enter s/c and project to lamina II
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Lamina VI-VIII neurons collect signals from lamina II
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Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in thalamus
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Intralaminar nucleus in thalamus
Projects to cingulated cortex
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Cingulated cortex above corpus collosum
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ALS III: Spino-mesencephalic tract
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Control of pain sensation
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Free nerve endings in skin
A-delta/C (thin/unmyelinated) SA fibers
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Located: DRG
A-delta/C fibers enter s/c and project to lamina II
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Lamina I and V collect edited signals from lamina II
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Cross midline at each spinal segment
Ascend in anterolateral fasciculus to terminate in midbrain
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Mesencephalic reticular formation and raphe nucleus in periaqueductal grey
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ALS IV: Spino-reticular tract
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Sensation of poorly localized pain
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Free nerve endings in skin
A-delta/C (thin/unmyelinated) SA fibers
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Located: DRG
A-delta/C fibers enter s/c and project to lamina II
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Lamina VI-VIII neurons collect signals from lamina II
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Cross midline at each segment
Ascend in anterolateral fasciculus to terminate in reticular formation in medulla and pons
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Reaches thalamus via reticular formation
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ALS V: Anterior spinothalamic tract
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Crude touch and pressure information
Poorly localized and non-discriminative
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Free nerve endings in skin
A-beta/delta (myelinated) SA fibers
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Located: DRG
SA fibers enter s/c and project to lamina I and II
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Neurons in dorsal horn generate bilateral output fibers
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Some fibers cross, others don’t
Ascend in anterolateral fasciulus to terminate in thalamus
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VPL nucleus in thalamus
Projects to somatosensory cortex
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Postcentral gyrus in parietal lobe
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Dorsal spinocerebellar / cuneocerebellar (DSC)
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Conveys muscle sense info (strength, phase rate) used by CB to coordinate movt and posture
Collects info about individual muscles
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Muscle spindles and golgi tendon organs
A-alpha; Ia/Ib (thick myelinated) SA fibers
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Located: DRG
SA fibers enter s/c and stay in same side in lateral part of posterior funiculus
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Clarke’s n. (=
External cuneatus (>C8) in medulla (cuneocerebellar tract)
Ipsilateral
Output fibers from 2nd order project to CB via inferior CB peduncle
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Ipsilateral (no cross)
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Cerebellum (vermis)
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Ventral / rostral spinocerebellar tract
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Conveys to CB integrated info about limb behiavor as a whole from lower limbs (ventral) and upper lims (rostral SCT)
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Golgi tendon organs
A-alpha; IB (thick myelinated) SA fibers
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Located: DRG
A-alpha fibers enter s/c
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Spinal border cells of lamina V-VII
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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
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Cerebellum (vermis)
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TRACT
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FUNCTION
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ORIGIN
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1ST ORDER MN
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DECUSSATION
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SPINAL PATHWAY
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FINAL DESTINATION
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Cortico-spinal tracts: lateral / anterior
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Execute and control voluntary / involuntary motor activity
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Motor and pre-motor cortex in precentral gyrus
Produce A-alpha (thick myelinated) efferent fibers
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Projects through internal capsule towards midbrain (Cerebral peduncles), pons, medulla (pyramids)
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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
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Subcortico-spinal (lateral grp): rubrospinal tract
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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
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Red nucleus in midbrain
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Cross midline at ventral tegmental decussation (VTD)
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At s/c, rubrospinal fibers are next to lateral cortico-spinal tract
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Rubrospinal fibers project on IN in lamina V-VIII (in C and L enlargements)
Act on MN through IN networks
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Subcortico-spinal (lateral grp): raphespinal tract
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Controls and modulates pain sensation at s/c level
Related with ALS
Release serotonin (5-HT) on IN
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Dorsal raphe nuclei in midbrain
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Cross midline locally and descend in s/c next to rubrospinal and lateral cortico-spinal tract
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Project on IN and SA fibers in lamina I, II, V
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Subcortico-spinal (medial grp): tectospinal tract
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Mediates postural and head motor reflex orienting responses to moving visual stimuli penetrating visual fields
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Superior colliculus in tectus (midbrain)
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Cross midline at dorsal tegmental decussation (DTD) in midbrain
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Projects bilaterally to gray matter in anterior horn to IN and MN of axial neck and trunk muscles
Only at upper cervical levels
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Subcortico-spinal (medial grp): reticulospinal tract
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Modulates spinal reflexes thus indirectly MN excitability
Regulates muscle tone of axial muscles and posture adjustment
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Reticular formation in pons and medulla:
Pontine RS tract (medial) & medullar RS tract (lateral)
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Pontine RS travel uncrossed in medial funiculus
Medullary RS bilateral in lateral funiculus
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Gray matter interneurons of lamina VII all across s/c
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Subcortico-spinal (medial grp): lateral vestibulospinal tract
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Triggers responses to adjust posture and keep balance
Regulates activity of antigravity extensors in lumbar region (i.e. quads and tibialis anterior)
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Lateral vestibular n. in medulla
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Uncrossed in medial/lateral funiculus
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Projects to gray matter MN and IN (lamina VIII and IX) at lumbar level
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Subcortico-spinal (medial grp): medial vestibulospinal tract
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Facilitates flexor MN in upper C levels that control head position according to vestibular info
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Medial vestibular n. in medulla
Fibers join middle longitudinal fasciculus (MLF) that coordinates eye/head movt according to vestibular info
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Descend uncrossed next to midline in medial part of anterior funiculus
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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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