#17 - Motor Pathways I: Spinal Systems

Question 1

Internal Capsule Parts (3) and what they house

Answer 1

1. Anterior Limb
   
   • Frontopontine fibers
2. Genu
   
   • Corticobulbar fibers
3. Posterior limb
   
   • Corticospinal fibers
   • All corticopontine fibers other than frontopontine

Question 2

Corticobulbar Fibers leaving the pons go to CN nuclei…

Answer 2

V, VII

Question 3

Corticobulbar Fibers leaving the medulla go to CN nuclei…

Answer 3

IX, X, XII

Question 4

Corticobulbar Fibers leaving the foramen magnum to the SC go to CN nuclei…

Answer 4

XI

Question 5

5 descending motor systems that reach the spinal cord

Answer 5

1. Corticospinal system: neurons from primary motor cortex to spinal cord
2. Reticulospinal system: neurons from brainstem reticular formation to spinal cord
3. Vestibulospinal system: neurons from brainstem vestibular nuclei to spinal cord
4. Tectospinal System: neurons from midbrain tectum (superior colliculus) to spinal cord
5. Rubrospinal system: neurons from red nucleus to spinal cord

Question 6

Pyramidal vs Extrapyramidal

Answer 6

• Pyramidal: only the corticospinal (and corticobulbar) tracts
• Extrapyramidal: all other motor tracts

Question 7

Reticulospinal Tracts

Answer 7

• Brainstem Reticular Formation to SC
• Modulation of muscle tone, switch on SPG
  1. Pontine (medial) reticulospinal tract (MRST)
• Origin: pontine tegmentum
• Spinal course: anterior funiculus
• Targets: mostly interneurons
• Functions: facilitates anti-gravity, extensor muscles and increases muscle tone (gamma motor system)

2. Medullary (lateral) reticulospinal tract (LRST)

• Origin: medullary reticular formation
• Spinal course: lateral funiculus
• Targets: mostly interneurons
• Functions: suppresses extensor muscle activity and reduces muscle tone (gamma motor system)

Question 8

Vestibulospinal Tracts

Answer 8

Brainstem Vestibular Nuclei to SC

1. Lateral vestibulospinal tract
   
   • Origin: Lateral vestibular nucleus (ipsilateral)
   • Target: Ipsilateral intermediate zone and medial motor neuron groups (black dots)
   • Function: excites gamma motor neurons, which increase tone in trunk and proximal limb extensors, maintaining upright posture (vestibulo-spinal reflex: spreading arms out)
2. Medial vestibulospinal tract
   
   • Origin: Medial vestibular nucleus (bilateral)
   • Target: bilateral intermediate zone and medial motor neuron groups (white dots) for neck and shoulder muscles, ends at T1
   • Function: maintenance of head in upright position (vestibulo-collic reflex)

Question 9

Tectospinal Tract

Answer 9

• Origin: superior colliculus (midbrain tectum) to SC
• Crosses immediately in the midbrain!
• Targets: axial alpha motor neurons controlling neck muscles
• Function: reflexive head turn toward/away from visual or auditory stimulus
  
  • Is velocity-dependent: at most velocities, the reflex will turn the head toward a novel stimulus, but above threshold velocities, the tectospinal activation is protective, turning the head away form the fast-approaching stimuli

Question 10

Rubrospinal Tract

Answer 10

• Red Nucleus to SC
• Tract is most like the corticospinal tract in function
• Origin: red nucleus (midbrain), which gets inputs from numerous cortical areas iand cerebellum
• Suggests that M1, red nucleus, and cerebellum form a recurrent network involved in the feedback control of voluntary actions (since M1 also receives info from cortex and cerebellum)
• Decussates immediately in midbrain!
• Targets: intermediate zone interneurons that synapse on distal limb alpha motoneurons in neck and upper limb only
• Function: facilitates flexor activity (like corticospinal tract) in cervical spinal cord, but only in a coarse manner; we care about it in terms of abnormal posturing.

Question 11

Decorticate Posture

Answer 11

• Lesion is in the rostral midbrain or higher, above the red nucleus
  
  • RN is spared, so damage is rostral to midbrain
• Rubrospinal biased flexion overrides vestibulospinal and reticulospinal biased extension, but only for the upper limb = we only see upper limb flexion

Question 12

Decerebrate Posture

Answer 12

• Lesion of the upper pons, so damage to red nucleus itself
  
  • RN is bilaterally damaged, so damage involves the midbrain or down to the mid-pons
  • If damage extends further caudal, the LVST could also be damaged and we’d see other probs
• Activity in the intact vestibulospinal system (arising lower down in pontomedullary region) is biased for extension, causing rigidity of all four limbs

Question 13

Corticopontine System

Answer 13

• Largest descending motor control system in the CNS
• Cortical input to ipsilateral pons (which then goes to contralateral cerebellum via MCP)
• Conveys motor plan and integrates sensory data to the cerebellum
• Makes up the majority of internal capsule

Question 14

Sections of Internal Capsule and their contents

Answer 14

• Anterior limb: Frontopontine fibers
• Genu: corticobulbar fibers
• Posterior limb:
  
  • Corticospinal fibers
  • Temporopontine fibers
  • Parietopontine fibers
  • Occipitopontine fibers

Question 15

Answer 15

Question 16

Hand Muscle Motoneuron Location

Answer 16

• Dorsal part of C7-T1 ventral horn
• Direct innervation from corticospinal tract
  
  • Thought to allow individual, fractionated had, forearm movements

Question 17

Lateral Corticospinal Fiber Targets, Function

Answer 17

• 80% end in intermediate zone
• 20% end on alpha motor neurons, particiularly in dorsolateral part of ventral horn
• Function:
  
  • Strongest influence on flexor muscle groups
  • Make adaptive CPG modifications
  • Excites whatever it lands on (including inhibitory interneurons)

Question 18

MRST

Answer 18

• Pontine (medial) reticulospinal tract (MRST)
• Origin: pontine tegmentum
• Spinal course: anterior funiculus
• Targets: mostly interneurons
• Functions: facilitates anti-gravity, extensor muscles and increases muscle tone (gamma motor system)

Question 19

LRST

Answer 19

• Medullary (lateral) reticulospinal tract (LRST)
• Origin: medullary reticular formation
• Spinal course: lateral funiculus
• Targets: mostly interneurons
• Functions: inhibits extensor muscle activity and reduces muscle tone (gamma motor system)

Question 20

Tectum

Answer 20

• Inferior colliculi
  
  • Maps sounds in space
  • Sends info to superior colliculus
• Superior colliculi
  
  • Maps retinotopic info
  • Integrates auditory & visual stimuli
  • Drives tectospinal tract for reflexive head movement

Question 21

UMN Lesion Symptoms

Answer 21

• Spasticity
  
  • Paresis
  • Hypertonia
  • Hyperreflexia
• Clonus
• Babinski sign

Question 22

LMN Lesion Symptoms

Answer 22

• Almost always ipsilateral
• Paralysis = loss of movement
• Paresis = diminution of movement
• Hypotonia = decrease in muscle tone
• Hyporeflexia = decreased reflexes
• Areflexia = absent reflexes
• Fasciculations = spontaneous activity of muscle fibers at rest
• Muscle atrophy

Question 23

Spasticity vs Rigidity

Answer 23

• Spasticity = pyramidal damage (corticospinal tract)
  
  • Combination of paresis, hyperreflexia, and hypertonia
  • Variable features that distinguish it from extrapyramidal rigidity:
    
    • More resistance in one direction (increased tone in flexors with corticospinal tract damage)
    • Velocity dependent (more noticeable with fast movements)
• Rigidity = extrapyramidal damage
  
  • Rigidity is a kind of hypertonia
  • Indicates damage to basal ganglia or vestibulospinal tracts
    
    • When BG only are involved, a “cogwheel” effect can be seen; circular, jerking rigidity in flexion and extension in the background tremor, that continues throughout an entire range of movement
  • Muscles are stiff, but rigidity features same resistance to all directions of movement (flexion and extension)
  • Is not velocity dependent

Question 24

Clonus

Answer 24

• A pattern of involuntary and rhythmic muscle contraction resulting from lesion in descending motor pathways
• Two widely accepted hypotheses:
  
  • Caused by hyperactive stretch reflexes via self-excitation, producing rhythmic stimulation of LMN
  • Caused by CPG activity arising as a consequence of peripheral sensory events, producing the rhythmic effect
• May be seen at ankle, knee, wrist, jaw, and elbow, or any muscle with a frequency of 5-8 Hz.

Question 25

Hyperreflexia Explanation

Answer 25

• UMN lesion symptom
• Due to loss of supraspinal inputs to neurons that inhibit alpha/gamma motor neurons
  
  • Loss of excitatory corticospinal fiber innervation to inhibitory interneurons, which inhibit alpha/gamma motor neurons via spindle type II pathways, results in exaggerated stretch reflex
  • Fast adapting type Ia fibers encode changes in muscle length and velocity without temporally precise reference to immediate length
• BASICALLY: Both supraspinal inputs and type Ia pathways work to inhibit the type II alpha/gamma motor neurons. So, if you take away the supraspinal input, there’s less inhibition, so you see hyperreflexia.