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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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Chapters 11 Motor System – Spinal Cord

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chapters 11 motor system spinal cord
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

six neuraxial levels
Six Neuraxial* Levels
  • Spinal Cord
  • Brainstem
  • Cerebellum
  • Diencephalon
  • Basal Ganglia
  • Cerebral Cortex

*Neuraxial = Brain and Spinal Cord Axis

functional levels
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
spinal cord
Spinal Cord
  • 43.5cm long, 1cm diameter
  • Five Spinal Segments and Spinal Nerve Groups
    • Cervical (8)
    • Thoracic (12)
    • Lumbar (5)
    • Sacral (5 fused vertebrae), "Holy Bone"
    • Coccygeal (3-5 fused vertebrae) ‘tailbone’, (coccyx = cuckoo's beak)
spine and pelvis
Spine and Pelvis
  • Spine can rotate with respect to pelvis
spinal nerves
Spinal Nerves
  • There are a total of 31 bilaterally-paired spinal nerves
    • 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.
spinal cord and vertebrae
Spinal Cord and Vertebrae


  • Vertebral Column Longer Than Spinal Cord
    • 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.
lumbar nerves
Lumbar Nerves


Conus medullaris


Cauda equina

(horses tail)

saddle area,







Filum terminale

sacral nerves
Sacral Nerves







  • Filum terminale
    • coccygeal ligament.
    • connective tissue (pia mater)
    • From medullary cone to the termination of the vertebral canal.
  • "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.
spinal cord1
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
spinal nerves1
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.
cross section
Cross Section
  • Gray matter of the Spinal Cord
    • Dorsal Horn
    • Ventral Horn
cross sections
Cross sections



White Matter (tracts)


Gray Matter (interneurons)

motor units
Motor Units
  • Lower Motor Neuron
    • 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
      • Motor Cell body
      • Efferent Fiber
      • Motor End Plate - Myoneural-Neuromuscular Junction
      • Muscle Fibers Innervated by Axon
tracts of spinal cord
Tracts of Spinal Cord
  • Neural impulses are carried through white matter
  • Three Major Bundles
    • Dorsal Column: Primarily Ascending Fibers
    • Lateral: Ascending and Descending Fibers
    • Anterior (aka Ventral): Ascending and Descending Fibers
descending tracts
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)
upper motor neurons
Upper Motor Neurons
  • Upper motor neurons: motor neurons that are NOT directly responsible for stimulating the target muscle
upper motor neurons1
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 nuclei- responsible for adjusting posture to maintain balance.
    • Reticulospinal: reticular formation - balance
corticospinal fibers
Corticospinal fibers


  • Lateral Corticospinal Tract
    • Control of Skeletal Muscle (Fingers, Toes, Forearm)
    • Skilled Manipulations
    • 90% Decussate and Form Alpha Fibers in Ventral Horn
  • 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


descending tracts1
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
descending autonomic tracts
Descending Autonomic Tracts
  • Hypothalamus:
    • Projects to Brainstem and Spinal Visceral Nuclei
    • Regulate Autonomic Function of Sympathetic and Parasympathetic Systems
ascending sensory tracts
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
types of motor nerve cells
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
motor neurons
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
  • 30 Times More Than Motor Neurons
  • Filter of Sensory and Motor Function
  • Function As Inhibitory Cells and Association Cells
motor functions of the spinal cord
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
Muscle Receptors
  • Two Types of Receptors
    • Muscle Spindle
      • Sensor inside muscle
      • Detects and Maintains Muscle Tension
    • Golgi Tendon Organs
      • Sensor on tendon
      • Monitors Degree of Muscle Tension During Contraction
      • Prevents Too Much Tension
spinal reflexes
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
spinal reflexes1
Spinal Reflexes
  • Crossed (Intrasegmental) Extensor Reflex
    • 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.
  • Spinal Cord (excitatory)
    • Epinephrine
    • Norepinephrine
    • Serotonin
  • PNS
    • Acetylcholine
clinical considerations
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
common spinal syndromes
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