Forbes – Spinal Cord – NS Exam 2
Spinal cord basics:
 Lumbar cistern – expansion of subarachnoid space; vertebral L2 – S2
 Cauda equina – collection of lumbar, sacral & coccygeal nerve roots w/in lumbar cistern, travelling from
intervertebral foramina  exit the spinal canal
 Filum terminale – continuation of pia beyond the caudal end of SC. Incorporates into the coccygeal
ligament – attaches to the periosteum of the coccyx
 Conus medullaris is the end of the SC, not end of vertebral column! Remember these are different!
 Spinal nerves 1-7 exit ABOVE cervical vertebrae.
 Spinal nerves C8 and below exit BELOW thoracic, lumbar & sacral vertebrae
o NOTE: there are 8 cervical nerves & 7 cervical vertebrae
 Epidural space is significant – has fat and venous plexus. (v. the ‘virtual’ epidural space in skull)
 Sympathetic ganglia have POST-ganglionic nerve cell bodies.
 Lateral horn has PRE-ganglionic nerve cell bodies.
Internal organization of the SC:
 Gray matter – has nerve cell bodies and synapses
o GM larger at cervical (C4-T1) & lumbosacral (L2-S3) b/c more nerve fibers related to innervation
of limbs
 White matter – has axons that form tracts
o less WM in lower lvls b/c there are less nerve fibers – descending tracts are done, ascending are
still building
 Cord gets smaller caudally
 Shape of cord:
o Cervical segments are large and oval
o Lumbar & sacral segments are smaller and rounder
 Lateral horn is present at lvls of T1-L2 (intermediolateral cell column)
 Posterior funiculus divided in 2 – f. cuneatus & f. gracilis (above T6)
Functional organization of the SC:
 Functional components of a spinal nerve: GSA, GVA, GSE, GVE.
o All sensory and motor activity associated with the body is related to the SC.
 Sensory innervation
o Spinal nerves carry sensory (primary afferent) input  DRG that are GSA & GVA
 Primary afferent (1st order) cell bodies are in the DRG
 Each spinal nerve is associated w/ a dermatome
 Motor innervation
o Ventral root & spinal nerves carries
motor output from SC  skeletal
muscles (GSE) & preganglionic
autonomic neurons (GVE) that will
terminate on postganglionic
autonomics

o KNOW SC segments that innervate specific muscles!
Reflex activity
o Monosynaptic (ex: Knee jerk) & Polysynaptic (ex: flexor)
o Can be confined to SC (1+ levels) or include brain stem connections
o Influenced by descending tracts (helps modulate) & local inputs
o Damage to any afferents, efferents or des. tracts alters reflexes!
Gray matter of the dorsal horn:
 Alar plate derivative = SENSORY
 Has projection neurons (send axons up SC for asc. tracts) & interneurons (processes are close to cell body
& make synaptic connections w/ nearby neurons)
 Input from 1⁰ afferents / 1st order neurons of dorsal root (GSA & GVA) & from desc. tracts
 Areas:
o Substantia gelatinosa (of Rolando) – near apex of dorsal horn – extends length of cord – ass. w/
Lissauer’s tract/dorsolateral fasciculus & marginal zone of dorsal horn  pain & temp info via
spinothalamic tract
o Body of dorsal horn (principal sensory nucleus) – remainder of dorsal horn – extends length of
cord – has projection neurons & interneurons
o Nucleus dorsalis (Clark’s nucleus) – ventro-medial location in dorsal horn – from C8/T1 to L2/L3 in
cord – has projection neurons that send info proprioceptive information via dorsal spinocerebellar
tract (DSCT)
Gray matter of the lateral horn:
 aka: intermediolateral cell column
 from T1 – L1/L2 (cord)
 has cell bodies of preganglionic sympathetic neurons – preganglionic axons exit SC via ventral roots
 Parasympathetic preganglionic cell bodies also in lateral region of spinal GM at S2-4
Gray matter of the ventral horn:
 Basal plate derivative = MOTOR
 Has alpha motor neurons that innervate extrafusal skeletal muscle fibers (aka lower motor neurons
(LMNs) and anterior horn cells), gamma motor neurons that innervate intrafusal muscle fibers,
interneurons & projection neurons that give rise to asc. tracts
 Input from primary afferents of dorsal root, descending tracts like lateral corticospinal tract (LCST) &
interneurons that get info from primary afferents & desc. tracts
 Functional localization:
o Medial motor cell column present at all lvls – innervates axial muscles
o Lateral motor cell colum present at cervical and lumbar lvls – innervates the limbs
 Other motor cell groups in ventral horn of certain cervical segments:
o Phrenic nucleus – medial part of ventral horn at C3-C5 cord segments.
o Spinal accessory nucleus – lateral part of ventral horn at C1-C5/6 cord segments.
 Nerve rootlets leave SC dorsal to denticulate ligament & pass rostrally along cord & enter
skull through the foramen magnum!
Blood supply of the Spinal Cord:
 Vertebral arteries:
o Anterior spinal artery (more than 2/3 of cord)
o Posterior spinal arteries
 Radicular / segmental medullary artery
o Branches of arteries along vertebral column like intercostals & lumbar a
o Anastomose w/ spinal arteries along length of cord
o Great radicular artery = artery of Adamkiewicz @ T12
 Venous drainage
o Series of 6 venous channels exist around cord  radicular veins  epidural venous plexus
 at the intervertebral spaces there are connections that allows venous blood to drain into
thoracic, abdominal & intercostals veins
Introduction to CNS tracts:
(Nolte pg 73-78)
 Tracts carry information from one part of CNS to another – travel through White Matter of SC & Brain.
 Neuraxis: brain & SC
 White matter: nerve fibers or axons + associated glial cells
 Funiculus: white column
o Fasciculus: division of a funiculus. Ex: fasciculus gracilius + fasciculus cuneatus = dorsal funiculus
 Tract: nerve fibers in CNS w/ similar origin, termination & function
 Projection neurons: cell bodies whose axons form tracts
 Modality: type of sensation – tactile, painful, thermal
 Nucleus: group of nerve cell bodies in brain or SC that have similar inputs, outputs, functions
 Decussation: something that crosses midline – often ascending or descending the neuraxis
 Commissure: also fibers that cross the midline, but this occurs at same lvl. Ex: corpus callosum
Intro to Ascending Tracts:
 Transmit sensory info from periphery to brain – usually w/ 3 major neurons but interneurons do exist
o 1⁰ neuron / primary afferent – cell body in peripheral sensory ganglion
o 2⁰ neuron – cell body in spinal cord (or brain stem) GM – axon terminals in thalamus
o 3⁰ neuron – cell body in thalamus – axon terminals in cerebral cortex
o This terminology (1⁰, 2⁰, 3⁰) only for
sensory!
 Anterolateral system: Spinothalamic tract,
Spinoreticular tract, Spinomesencephalic
(spinotectal) tract
 Dorsal Column-Medial Lemniscal system
(DCML)
o Fasciculus cuneatus & Fasciculus
gracilis
 Spinocerebellar tracts: Dorsal
spinocerebellar tract, cuneocerebellar tract,
ventral spinocerebellar tract

1st order afferents / primary afferents carry sensory input to ascending tracts from sensory receptors
o Pseudounipolar cell body in DRG – peripheral process ass. w/ sensory receptor – central process
enters spinal cord via dorsal root. [reminder: NO SYNAPSES in DRG!]
o Central processes segregate at dorsal root (size / function)
 Lateral division: small diameter nerve fibers – Pain & Temp – enter Lissauer’s tract &
synapse in dorsal horn (incl substantia gelatinosa)
 Medial division: large diameter nerve fibers – Touch, vibration & proprioception – enter
dorsal or posterior columns (f. gracilis / f. cuneatus) & sypase in SC & Brain stem
o
Primary afferents bifurcate to make ascending & descending branches
o Descending – short / reflexes
o Ascending branches feed ascending tracts
 Small fibers from lateral division are short & synapse on neurons in dorsal horn
 Large fibers of medial division vary in length & synapse on neurons in dorsal horn
& medulla
***EACH primary afferent: 1. may make 1000s of synapses 2. may participate in reflexes 3. send info
to 1+ ascending tracts. A given primary afferent is NOT dedicated to a particular tract!!!
Introduction to Descending Tracts:
 Typically carry motor information to Lower Motor Neurons (LMNs). [Can also be involved in sensory.]
 Originate in Cerebral Cortex or Brain Stem  Terminate in Brain Stem or Spinal Cord
 Upper Motor Neurons = tracts
 Final common pathway = Lower Motor Neuron
o LMN is motor neuron whose axons leave the CNS & innervate skeletal muscle (last neuron bxt
CNS and effector)
 Motor unit: one LMN, its axon & all skeletal muscle fibers innervated by the terminals of that LMN
 Pyramidal tracts: Corticospinal tract (CST) & Corticobulbar tract (CBT)
 Others: vestibulospinal tract, reticulospinal tracts, medial longitudinal fasciculus, Raphe spinal tract