Chapters 11 Motor System – Spinal Cord
 Chris Rorden
University of South Carolina
Norman J. Arnold School of Public Health
Department of Communication Sciences and
Disorders
University of South Carolina
Previously: Ascending sensory fibers
Today: Descending motor fibers
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Six Neuraxial* Levels
Spinal Cord
Brainstem
Cerebellum
Diencephalon
Basal Ganglia
Cerebral Cortex
*Neuraxial = Brain and Spinal Cord Axis
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Functional Levels
Spinal Level
– Simple Reflexes
– Regulation of Higher Skilled or
Patterned Movements
Upper Levels
– Initiation, Inhibition or Facilitation of
Motor Functions
– Voluntary Motor Movements
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Spinal Cord
 43.5cm long, 1cm diameter
 Five Spinal Segments and Spinal Nerve
Groups
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Cervical (8)
Thoracic (12)
Lumbar (5)
Sacral (5 fused vertebrae), "Holy Bone"
Coccygeal (3-5 fused vertebrae) ‘tailbone’,
(coccyx = cuckoo's beak)
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Spine and Pelvis
Spine can rotate with respect to pelvis
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Spinal Nerves
 There are a total of 31 bilaterally-paired
spinal nerves
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8 cervical nerves (C1-C8)
12 thoracic nerves (T1-T12)
5 lumbar nerves (L1-L5)
5 sacral nerves (S1-S5)
1 coccygeal nerve (Co, skin of lower back)
 C1 to C7 exit vertebral canal above the
respective cervical vertebra (e.g. C1 exits
above the first cervical vertebra).
 All the other spinal nerves (C8, T*, L*,
S*,Co) leave below their corresponding
vertebra.
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Spinal Cord and Vertebrae
 Vertebral Column Longer Than Spinal Cord
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Yellow=CSF
Conus Medullaris
End of Spinal Cord at L2
Cauda Equina (Horse’s Tail – nerve roots)
Stretched nerve root fibers from L3 to S5
 Filum Terminale - fibrous tissue
– Stretched Spinal Cord Remnant Attached to
Coccyx
 Cauda equina
– Contains Lumbosacral Cistern
 Fluid Filled Space for Spinal Puncture
– Spinal cord stops growth during infancy, spine
grows through adolescence.
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Lumbar Nerves
Conus medullaris
L1
L2
Cauda equina
(horses tail)
saddle area,
sphincters,
parasympathetic
Bladder/bowel
Filum terminale
L3
L4
L5
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Sacral Nerves
S1
S2
S3
S4
S5
Co
Filum terminale
coccygeal ligament.
connective tissue (pia mater)
From medullary cone to the termination of the vertebral canal.
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Meninges
"The meninges PAD the brain
and spinal column." -- Pia;
Arachnoid; Dura.
Dentate Ligaments
– The pia mater has 21 pairs of
denticulate ligaments which attach it
to the arachnoid and dura maters.
– provide stability for the spinal cord
against motion within the vertebral
column.
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Spinal Cord
Internal Structure
– White Matter – outer parts of the cord
– Gray Matter Horns and Commissures – the internal
sections
– Varies in Shape With Level of Spinal Segment
– Dorsal Root and Root Ganglia
– Ventral Root
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Spinal Nerves
 Exit Vertebra Through Intervertebral
Foramina
 Dorsal and Ventral Rami Form Spinal
Nerve
 Dorsal Roots - Sensory Information
 Ventral Roots - Motor Information
 Except between T-2 and T-11, Ventral
Roots Form Plexi to Serve Groups of
Muscles
– A nerve plexus is a network of intersecting
nerves. They combine sets of spinal nerves
that serve the same area of the body into
one large grouped nerve.
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Cross Section
Gray matter of the
Spinal Cord
– Dorsal Horn
– Ventral Horn
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Cross sections
L4
T2
White Matter (tracts)
Gray Matter (interneurons)
S3
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Motor Units
 Lower Motor Neuron
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Lower motor neurons (LMNs) are the motor neurons bring
the nerve impulses from the upper motor neurons out to
the muscles.
Path for Efferent Impulses
Final Common Pathway (to Muscles)
Four Components
1.
2.
3.
4.
Motor Cell body
Efferent Fiber
Motor End Plate - Myoneural-Neuromuscular Junction
Muscle Fibers Innervated by Axon
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Tracts of Spinal Cord
 Neural impulses are carried through white
matter
 Three Major Bundles
1. Dorsal Column: Primarily Ascending Fibers
2. Lateral: Ascending and Descending Fibers
3. Anterior (aka Ventral): Ascending and
Descending Fibers
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Descending Tracts
 Pyramidal (aka Corticospinal) Tracts
– From cortex – Betz Cells (large pyramidal cells) in precentral
gyrus.
– Through Internal Capsule, Pes Pedunculi, Pontine Nuclei,
Pyramidal Decussation (medulla): 90% decussate, Spinal Cord
 Extrapyramidal Tract
– not directly from motor or premotor cortex
 Autonomic Pathways
– pathways from thalamus to spinal cord and brainstem –
regulates motor functions of the sympathetic and
parasympathetic systems (inspiration, vomiting, and coughing
reflexes)
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Upper Motor Neurons
Upper motor neurons: motor
neurons that are NOT directly
responsible for stimulating the
target muscle
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Upper Motor Neurons
 Upper motor neurons Tracts
– Cortico-spinal: motor cortex to spinal
nerve roots – fine voluntary movements
– Corticobulbar: Cortex to pons and medulla
– involuntary maintenance of posture
– Tectospinal – Superior Colliculus to lower
motor neurons. Involuntary correction of
head to visual stimuli
– Rubrospinal: red nucleus to LMN
– Vestibulospinal: vestibular nucleiresponsible for adjusting posture to
maintain balance.
– Reticulospinal: reticular formation balance
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Corticospinal fibers
 Lateral Corticospinal Tract
– Control of Skeletal Muscle (Fingers, Toes,
Forearm)
– Skilled Manipulations
– 90% Decussate and Form Alpha Fibers in
Ventral Horn
LCT
ACT
 Anterior Corticospinal Tract (AKA ventral
corticospinal tract)
– 8-10% Fibers That Did Not Cross Midline
– Cross at Spinal Horn
– Control Axial and Girdle Muscles –
responsible for moving head axial movement
of head and trunk
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Descending Tracts
 Tectospinal Tract
– Response to Visual Stimulation
– Superior Colliculus to Cervical Spinal Cord
 Rubrospinal Tract
– Regulation of Muscle Tone Against Gravity
– Red N. To Motor Nerve Cells in Ventral Horn
 Vestibulospinal Tract
– Reflexive Adjustment of Body and Limbs
– Vestibular N. To Spinal Cord
 Reticular Descending Tract
– Alteration of Muscle Tone
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Descending Autonomic Tracts
Hypothalamus:
– Projects to Brainstem and Spinal Visceral Nuclei
– Regulate Autonomic Function of Sympathetic and
Parasympathetic Systems
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Ascending (Sensory) Tracts
 Fasciculus Gracilis
 Fasciculus Cuneatus
 Anterior Spinothalamic Tract
 Lateral Spinothalamic Tract
 Ventral Spinocerebellar Tract
 Dorsal Spinocerebellar Tract
 Cuneocerebellar Tract
 Spinotectal Tract
 Spinoreticular Tract
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Types of Motor Nerve Cells
Anterior Motor Neurons – exit at the ventral
horns
– Alpha and Y (Gamma) Motor Nerve Cells
– Lower Motor Neurons (Below 2nd Level in
Neuronal Pathway)
Interneurons
– Association Cells Connecting Sensory and Motor
Neuron Pools
– Often Part of Reflexive Action
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Motor Neurons
Alpha Neurons
– Major Motor Neurons
– Small
– Responsible for Voluntary and Reflexive
Movements of Head, Trunk and Extremities
– One Fiber Can Innervate 200 Muscles fibers
Y-Motor Neurons
– Smaller and Fewer
– Controlled by Reticular and Vestibular Systems
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Interneurons
30 Times More Than Motor Neurons
Filter of Sensory and Motor Function
Function As Inhibitory Cells and Association
Cells
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Motor functions of the spinal cord
Reflexive Motor Response
– Stereo-Typical (Rote) Response to Stimulus
– Involves Muscle Spindles, Afferent Fibers, Alpha
Motor Neurons, Efferent Fibers and Muscles
– Independent of Voluntary Control
– Upper Centers Become Involved to Smooth
Reaction and Return body to homeostasis
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 Muscle Receptors
 Two Types of Receptors
1. Muscle Spindle
 Sensor inside muscle
 Detects and Maintains Muscle Tension
2. Golgi Tendon Organs
 Sensor on tendon
 Monitors Degree of Muscle Tension During
Contraction
 Prevents Too Much Tension
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Spinal Reflexes
 Stretch Reflex i.e. knee reflex
– Tap Patella causing tendon change (y motor neuron)
– Muscle spindles stimulate alpha motor neuron response,
and muscle contracts
– Occurs at the L3 level
 Withdrawal (Flexor) Reflex i.e. Touching Hot Stimulus
– Protective Response to pain
– Flexion of leg or arm
– Stimulus, receptor, substantia gelatinosa, interneurons and
alpha neuron response
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Spinal Reflexes
 Crossed (Intrasegmental) Extensor Reflex
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Protective response
Involves both sides of the body
As one arm is withdrawn, the other arm is extended
Multisynaptic because it involves opposite body parts
 An example of this is when a person steps on a nail,
the leg that is stepping on the nail pulls away, while
the other leg takes the weight of the whole body.
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Neurotransmitters
Spinal Cord (excitatory)
– Epinephrine
– Norepinephrine
– Serotonin
PNS
– Acetylcholine
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Clinical Considerations
 Many Sources of Lesions
– Trauma
– Tumors or Infections
– Degenerative Conditions
 Compare the Function of One Side to the Other
 Hyper Quality of Movement (Spastic) (Upper Motor
Neuron Problems)
 Hypo Quality of Movement (Spinal or Spinal Nerve
Level - Lower Motor Neuron) Causing Flaccid
Paralysis
 Absent Reflexes and Atrophy or Muscle Wasting
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Common Spinal Syndromes
 Complete Spinal Transection
– Dislocations, tumor, myelitis
– Function lost below the lesion
– After a period of time, reflexes may become spastic in nature
 Brown-Sequard Syndrome (cord tumor, trauma,
ischemia)
– Lesion on ipsilateral half of body, ipsilateral sensory loss,
contralateral pain and temperature sensation loss
 Syringomyelia
– Developmental condition: cyst formation within spinal cord
with loss of sensation and muscle control – usually starts
between ages 25-40
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