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SPINAL CORD
INTRO
Spinal cord importance
Major source of somatic sensory input
Major source of motor output (expression in behavior)
Groundplan simplest, helps understand brainstem
Nomenclature in human spinal cord:
Dorsal = posterior; ventral = anterior
SEGMENTAL ORGANIZATION
Embryonic somites associated with…
One vertebral segment
Single spinal nerve on each side
Each nerve defines spinal segment
Cervical (8 segments; supply neck, upper trunk, arm)
Thoracic (12 segments; thorax, abdomen)
Lumbar (5 segments; lower abdomen, legs)
Sacral, coccygeal (5 and 2-3 segments; pelvic)
Cauda equina
Differential growth of vertebral column and cord means lower column
contains only spinal roots
Relevance to relatively safe "lumbar puncture" for CSF sample
SPINAL NERVES
Relationship to cord
Dorsal and ventral rootlets
Fusion at dorsal root ganglion
Connective tissue components
Epineurium (continuous with dura)
Perineurium (septa between big bundles of axons)
Endoneurium (surrounds each axon; relevance to regeneration)
Fiber heterogeneity
Thickness
Myelination
A, B, C scheme
Are axons in spinal nerve sensory or motor? (both!)
"Afferents" and "efferents"
MOTOR FIBERS (efferents)
Ventral roots
Bell (1811): mechanical stimulation of roots produces movement
Where are cell bodies of the axons?
In spinal gray
In the segment associated with the spinal nerve
Fiber classes (3)
A-alpha
Innervate striated muscle
Arise from "Alpha motor neurons"
A-gamma
Innervate contractile fibers of muscle spindles (see below)
B fibers
Autonomic pre-ganglionics
SENSORY FIBERS (afferents)
Where are cell bodies? (dorsal root ganglia)
Classes of peripheral endings
Cutaneous
Dermatomes: (segmental spinal organization as visualized on
body surface)
Herpes zoster --> shingles ("lights up" individual dermatomes)
Tendon and joint
Viscera (walls)
Muscle spindles
Sensory structures (stretch receptors)
But "motor" too
Not force-generating
Aimed at sensory function
Role of gamma innervation
Keep receptor "on-line"
An example of central control of sensitivity (a common
principle in sensory systems)
Fiber calibers
Varied
Relate to function
Nomenclatures (2)
A, B, C scheme
I - IV
Cartoon of functional relevance
Fat fibers = tactile and proprioceptive
Thin fibers = pain and temperature
Ventral Root Afferents
INTERNAL ANATOMY OF CORD
Gross features
Core vs. Rind
Root entry
Posterior median sulcus
Anterior median fissure
Central Canal
White Matter
Funiculi (3)
Ventral white commissure
Lissauer's fasciculus
Origin of axons?
Dorsal root ganglia (primary sensory)
Brain (descending)
Spinal cord itself
"Propriospinal"
May run only one segment, or whole length of cord
Course near gray matter
Gray Matter
Dorsal Horn
Substantia gelatinosa
Marginal nucleus (big cells at top of substantia gelatinosa)
Nucleus proprius
Intermediate zone
Gray commissure
Ventral horn
Rexed's laminae
I marginal
II substantia gelatinosa
III,IV - proprius
V, VI - base of dorsal horn
VII intermediate zone
IX motor nuclei
Longitudinal variations on foregoing theme
Gray matter
Enlargements
Cervical (C4-T1)
Lumbar (L1-S2)
Why do enlargements exist? Limbs.
More motorneurons
More sensory neurons (recipients of afferents)
More interneurons
Thoracic specializations
Comparatively little gray matter
But does have special nuclei not evident elsewhere
Intermediolateral cell column
Levels: T1-L2
Preganglionic sympathetic motorneurons
Clarke's column (nucleus dorsalis); precerebellar
relay
White matter gradient
Cervical (like urban expressway; local plus remote traffic)
Sacral (suburban cul-de-sac; local traffic only)
TWO REPRESENTATIVE TRACTS
ASCENDING sensory (DORSAL COLUMN SYSTEM; main fine tactile and
proprio)
Origin: Dorsal root ganglia
Ultimate target: somatosensory cortex (postcentral gyrus)
Primary afferents ascend in DORSAL COLUMNS
SYNAPSE: DORSAL COLUMN NUCLEI of caudal medulla
Decussation of secondary fibers
Ascent of secondary fibers as MEDIAL LEMNISCUS
Synapse in VPL nucleus of thalamus
Projection of third order fibers to postcentral gyrus
Implications of crossing (deficits are CONTRALATERAL [i.e, on side
opposite] to lesion)
DESCENDING motor (CORTICOSPINAL TRACT; fine voluntary
movement)
Origin: primary motor area and surrounding cortex
Target: spinal gray
Direct connection (no synapses along extent)
Pathway within brain: internal capsule, cerebral peduncle, medullary
pyramid
At spino-medullary junction, pathway splits:
Main component: "lateral corticospinal tract"
Decussation
Lateral funiculus (hence "lateral cortico...")
Synapse in spinal gray at all levels of cord
Minor component: "anterior corticospinal tract"
No decussation at spinomedullary junction
Anterior funiculus (hence "anterior cortico...")
Decussation at last minute (if at all)
Synapse in spinal gray
Implications of predominantly crossed pathway: damage to the
pathway in brain produces deficit contralateral to lesion
ANATOMICAL DETECTIVES (demyelination in lateral funiculus)
What's stained
What's wrong?
What caused this?
Where is the lesion?
Which side?