1. Spinal Cord
Organization and Pathways
Thomas Ahn, MD
Department of Neurology
Loyola University Medical Center
June 2012
2. Why is it important?
• Our failure to quickly diagnose, for example, a
spinal cord compression, can lead to
permanent paralysis.
• To do so, it is essential to be familiar w/ its
structures and neural tracks.
42 - 45 cm long in adults. Segments: 8 Cervical, 12 Thoracic/dorsal, 5 Lumbar, 5 Sacral and few small Coccygeal. Total 31 pairs of spinal nerves. Vertebral column =7 cervical, 12 thoracic, and 5 lumbar and the fused 5 sacral and coccyx vertebral bodies.intervertebral foramina:Cord is shorter than vertebral Column -> in Cervical region: nerve roots exit higher (C8 exit btw C7-T1). The rest from T1 exit below the equal numbered vertebral body.ConusMedullarisis the end, L1 or L2 of Vertebral column. Reason why LP done L4-5, L5-S1.filumterminaleattaches the conusmedullaris to the dural sac, supporting it (20cm). consists of pia and glial fibers and often containsa vein.2 Enlargement: more LMNs for UE (brachial plexus) and LE (lumbosacral plexus).
Dorsal = Posterior. Ventral = Anterior.The floor of the Ventral median Fissure is VentralWhite commissureA δ fibers and C fibers carrying pain sensation in the spinothalamic tractVentral corticospinal tract, which carry motor signals from the primary motor cortex (more later).The Dorsal nerve roots attached to a shallow vertical groove, the PosteroLateralsulcus. The ventral nerve roots exit in the anterolateralsulcus.
Each segment has 4roots: a pair of ventral (motor) and a dorsal (sensory) roots. Except the 1stCervical segment usually lacks dorsal roots. Each 31 pairs of spinal nerves has a ventral root and a dorsal root; each root is made up of 1 to 8 rootlets.dorsal root (spinal) ganglion: a swelling w/ cell bodies at the junction w/ ventral root. Send Afferent axons to Dorsal root.Spinal nerve outside of thevertebral column is called Peripheral nerve.
Dorsal roots contain fibers from cutaneous and deep structures.The largest fibers (Ia) from muscle spindles (spinal cord reflexes); the medium-sized fibers (A-beta) from mechanoreceptors in skin and joints. Most of the axons in the dorsal nerve roots are small (C, nonmyelinated; A-delta, myelinated) and carry information of noxious (eg, pain) and thermal stimuli.
Columns (= Horns): ventralgray column (=ventralhorn) contains cell bodies whose axons form the ventral roots, including alpha and gamma motor neurons ("lower" motor neurons).interMedioLateral gray Horn:preganglionic cells for the autonomic nervous system.T1 to L2: sympatheticneurons’ fiber leaving ventral roots to the sympathetic ganglia via the white ramicommunicantes.S2 – S4: parasympatheticneurons to the pelvic viscera.Dorsal Gray Column (=dorsal horn)
Lamina I: neurons for pain; sends axons to the contralateralspinothalamic tract.Lamina II: (=substantiagelatinosa) some small neurons for pain. Substance P (neuropeptide for pain) is naturally high in Laminas I and II.Laminas III and IV: (= nucleus proprius):forposition and light touch sense.Lamina V: both pain and visceral afferent stimuli.-Lamina VI: mechanical signals from joints and skin.-Lamina VII:large zone -dorsal nucleus (Clarke's columngiving rise toposterior spinoCerebellar tract) medially - large portion of the ventral gray horn (to ventral root) - interMedioLateral nucleus (or cell column) in thoracic (sympathetic)and upper lumbar (parasympa).-Laminas VIII and IX: medial and lateral portions of the ventral gray horn. - medial motor neuron column has LMNs that innervate axial musculature (ie, muscles of the trunk and proximallimbs). - lateral motor neuron column contains LMNs for the distal muscles of the arm and leg. - flexor muscles motor neurons located closer to the central canal medially. - extensor muscles motor neurons located more peripherally.-Lamina X: small neurons around the central canal or its remnants.
Columns (funiculi):dorsal, lateral, and ventral —around the spinal gray columns. - dorsal column lies between the posterior median sulcusand the posterolateralsulcus. - In the cervical and upper thoracic regions, the dorsal column is divided into a medial portion (the fasciculus gracilis) and a lateral portion (the fasciculus cuneatus). - lateral column lies between the posterolateralsulcusand the anterolateralsulcus. - ventral column lies between the anterolateralsulcusand the anterior median fissure.
Tracts: Fiber bundles with a common function are called tractsThe white matter of the cord is composed of myelinated and unmyelinated fibers. The fast-conducting myelinated fibers form bundles (fasciculi) that ascend or descend for varying distances. Glial cells (oligodendrocytes, which form myelin, and astrocytes) lie between the fibers. The lateral and ventral white columns contain tracts that are not well delimited and may overlap in their cross sectional areas; the dorsal column tracts are sharply defined by glial septa.From the precentral MOTORcortex (area 4) and the premotor area (6), large mostly myelinatedaxons (>1 million) go down through the brain stem via medullary pyramid tract and then mostly crosses over (decussates) in lowerMedulla downward into the Lateral CorticoSpinalcolumns which feeds to Ventral Horns (LMNs) either directly or via InterNeurons.About 10% descend uncrossed in the Ventral corticospinal tract and many of these fibers decussate, via the Ventral white commissure, and then to interneurons (which project to LMNs) but also connect directly to LMNs of the contralateral side.
Medial lemniscal system: carries sensations of fine touch, vibration, two-point discrimination, and proprioception from the skin and joints; they go up, without crossing, in the dorsal white column of the spinal cord to the lower Medulla. Fasciculus Gracilis carries input from LE (see picture).Fasciculus Cuneatus carries input from the UE.These tracts terminate in the Gracile and Cuneate nuclei (dorsal column nuclei) in the lowerMedulla. These 2nd order neurons send their axons across the midline via the lemniscaldecussation (also called the internal arcuate tract) and the medial lemniscus to the Thalamus. From the ventral posteroLateral (VPL)thalamic nuclei, to the somatosensory cortex.
Small-diameter sensory axons for sharp (noxious) pain, temp, and crude touch enter the spinal cord thru the dorsal root then go upone or two segments in the dorsal horn. These short tracts are thedorsoLateral fasciculus, or Lissauer's tract, then synapse with dorsal column neurons, especially in laminas I, II, and V. Then the fibers cross to the opposite side and ascend within the Spinothalamic tracts (akaventro-lateralsystem) which actually consist of 2tracts: - The ventralspinothalamic tract for light touch - the lateral spinothalamic tract for pain and temp. Project to the thalamus (ventral posterolateral (VPL), intralaminar thalamic nuclei).
Less Important: 2 ascending tracts:Dorsal and Ventral spinoCerebellar tracts.DorsalspinoCerebellar tracts: unconsciousProprioception. Afferent fibers from muscle and skin (from muscle spindles, Golgi tendon organs, and touch and pressure receptors) comes in via DRG and synapse w/2nd-order neurons in the nucleus dorsalis (Clarke's column) at the thoracic and lumbar and sacral levels.Above C8 for the UE, there is the accessory cuneate nucleus. The 2nd-order neurons from the dorsal nucleus of Clarke form the dorsal spinocerebellar tract; 2nd-order neurons from the lateral cuneate nucleus form the cuneoCerebellar tract. Both tracts remain on the ipsilateral side of the spinal cord, ascending via the inferiorCerebellar Peduncle to terminate in the Paleocerebellar cortex.Ventral Spinocerebellar Tract: proprioceptive/fine touch/vibration for coordination-movement control. 2nd-order neurons, located in Rexed'slaminae V, VI, and VII in lumbar and sacral segments of the spinal cord, go up through the SuperiorCerebellar peduncle to the Paleocerebellar cortex. The axons of the second-order neurons are largely but not entirely crossed, and this tract is of little value in localizing lesions in the spinal cord.
