Jacewicz - CNS/PNS Motor Pathways

Question 1

What is an upper motor neuron?

Answer 1

• Motor neuron contained entirely within the CNS
• Axon terminates in brain or spinal cord

Question 2

What is a lower motor neuron?

Answer 2

• Neuron with cell body in the CNS, and axon projecting into the PNS to innervate something like a skeletal muscle fiber

Question 3

Describe the corticospinal (pyramidal) tracts.

Answer 3

• Originates in 1^o motor cortex (precentral gyrus); fibers descend from corona radiata, and converge to pass through posterior limb of internal capsule
• Descend through middle 3/5ths of crus cerebri (basis pedunculi) in anterior part of midbrain
• Fibers break up into many bundles in pons
• Descend as pyramids in anterior part of medulla
• At medullo-spinal junction, most (85%) of fibers cross midline at decussation of pyramids, forming lateral corticospinal tract
  1. Uncrossed fibers (10-15%) descend as anterior corticospinal tract
• Lateral tract terminates on LMN’s in anterior horn of spinal cord; anterior tract fibers cross midline at level where they terminate on LMN’s
• NOTE: these are the major motor fibers carrying motor instructions from higher brain centers to local circuit and LMN’s

Question 4

How are the lateral and anterior corticospinal tracts different?

Answer 4

• UMN’s from LATERAL tract synapse only ipsilaterally to innervate limb musculature
• UMN’s from ANTERIOR tract synapse bilaterally to innervate axial musculature
• NOTE: fibers from the LMN’s leave spinal cord bt juncture of lateral and ventral columns, passing into the peripheral (spinal) nerve via the ventral roots

Question 5

What are Rexed’s Laminae?

Answer 5

• Spinal cord grey matter layers
• Layers I-VI = intermediate sensory neurons
• Layers VII-VIII: local circuit, autonomic, and commissural neurons
• Layer IX: LMN’s

Question 6

Describe the somatotopic organization of LMN’s.

Answer 6

• Lateral tract in red, and ventral in blue
• LATERAL: LMN’s serving extremities (red) located laterally in ventral horn
  1. Synapse only with ipsilateral motor neurons serving distal extremities
• ANTERIOR (ventral): neurons serving more proximal trunk mm (blue) located medially
  1. Synapse bilaterally on medial motor neurons serving trunk muscles

Question 7

What is a motor unit? How does it work?

Answer 7

• Single lower alpha motor neuron innervating a group of muscle fibers within a single muscle
• Fibers are distributed evenly and widely within a single muscle to ensure smooth contraction of mm
• Action potential (AP) initiated in LMN normally brings to threshold all muscle fibers it contacts, so:
  1. Activation of a single motor unit represents smallest unit of force that can be generated by a single muscle

Question 8

What are the 2 important anatomical/physio relationships b/t LMN’s and the mm fibers they innervate?

Answer 8

• 1) Size of LMN has proportional relationship to # of mm fibers it innervates; the larger the cell body of the motor neuron, the greater the # of fibers it synapses with
  a. Contraction strength depends on motor unit size
• 2) LMN, through a trophic factor relationship with its mm fibers, determines which of 3 types of mm fibers it innervates

Question 9

How does innervation of fine mm differ from that of powerful mm?

Answer 9

• FINE MOTOR CONTROL (ex: eye, finger): single motor neuron innervates only a few mm fibers, achieving more precise control
  1. Eye mm: LMN’s contact 5-10 mm fibers each
• POWER (ex: gastroc): motor units with several thousand fibers per each LMN

Question 10

What are the 3 mm fiber types?

Answer 10

• SLOW FATIGUE RESISTANT (S): smaller forces that are sustained for longer periods w/o fatigue
  1. Small motor neurons, so ratio of mm fibers per neuron small; produce slow, sustained (tonic) contractions
  2. MM fibers rich in myoglobin and capillary beds, contain many mito, and func primarily via aerobic metabolism -> red, dark meat, i.e., breast of migrating birds
  3. Aka, Type 1 muscle
• FAST FATIGUE RESISTANT: b/t the 2 extremes
• FAST FATIGABLE (FF): generate lg contraction forces, but fatigue quickly
  1. Innervated by comparatively few motor neurons (mm fiber to neuron ratio very high); produce quick, forceful contractions
  2. Lesser amts of myoglobin, fewer mito, less dense capillary supply, and mainly glycolytic (non-aerobic) metabolism -> paler, white meat, i.e., breast of ground-dwelling birds
• NOTE: left image shows repetitive stimulation, and right shows sustainability in continuous stimulation

Question 11

What is the difference b/t the mm fiber types of these 2 muscles?

Answer 11

• Slow, fatigue resistant (S) mm = SOLEUS -> required to maintain variable contractions to maintain balance as we stand for long periods of time
• Fast, fatigable (FF) mm = GASTROCNEMIUS -> used for rapid, forceful mvmts in jumping and pushing off as we walk

Question 12

What is the muscle stretch reflex?

Answer 12

• Monosynaptic reflex linking muscle spindle to LMN and back to muscle (aka, deep tendon reflex); involved in maintaining muscle tone
• Reflex hammer hits patellar tendon, muscle stretched, sensed by muscle spindles (sensory organs in all mm; quads here) -> send signals via 1a sensory NN (blue) to synapse directly on alpha LMN’s
  1. Alpha LMN activated by synaptic volley, and fires AP to stretched muscle fibers, causing contraction
• At the same time, 1a sensory fibers activate INH interneurons (green) that INH LMN’s of antagonistic muscle (hamstring, in this case)

Question 13

What are the muscle sensory organs and their associated NN types?

Answer 13

• MUSCLE SPINDLE: may be assoc w/a particular motor unit, or several, depending on the muscle
  1. Give rise to Type 1a and Type II afferent NN
  2. Innervated by a gamma-neuron
• GOLGI TENDON ORGANS: located at the semi-tendonous junction where muscle fibers terminate to form a tendon
  1. Give rise to the Type Ib afferent nerves

Question 14

What types of connections are possible on alpha-motor neurons?

Answer 14

• Excitatory monosynaptic feedback: from sensory organs in bed of mm; afferent NN have cell bodies in dorsal root ganglion (common excitatory neurotransmitters are acidic AA’s)
• Inhibitory polysynaptic feedback: afferent NN from sensory organ in mm bed provides INH feedback via INH interneuron -> allows provision of excitatory info to some motor neurons and INH info to others
  1. Common INH neurotransmitters are gamma-aminobutyric acid (GABA) and glycine
• Intersegmental/cerebellar connections: info from dorsal root ganglion neurons from spinal cord segments above/below and higher motor centers
  1. EXCITATORY: via excitatory interneurons
  2. INH: via inhibitory interneurons

Question 15

What is a muscle spindle? How does it work?

Answer 15

• Designed 2 monitor/maintain mm tone; generates sensory info that triggers mm reflex arc
• Small, intrafusal (IF) mm fibers in a CT sheath; in parallel w/extrafusal (EF) mm fibers that make up most of mm -> when muscle stretched, IF mm fibers of spindle also stretched (bc in parallel w/EF)
• Specialized sensory NN endings (annulospiral; blue) encircle IF fibers, and have mem channels sensitive to mech stretch, firing off AP
• AP travels via 1a sensory N (blue) to dorsal horn of spinal cord, then ventral -> alpha motor neuron fires AP to contract EF m (red)
• Since mm spindle is in parallel w/(EF) mm, as EF contracts/shortens, so does mm spindle; w/o add’l mech, mm spindle would only func when EF mm fibers were fully extended (stretched)
• MM must maintain graded tone and strength throughout full ROM, so mm spindle served by gamma motor neuron (red) -> fires in concert w/alpha, causing simultaneous contraction of IF fibers, and keeping mech stretch sensors from becoming relaxed and nonfunctional
• MM spindle monitors and regulates muscle tone throughout full extension and flexion of a muscle; smooth process with relatively constant muscle tone throughout the movement

Question 16

Describe the anatomy of muscle spindles.

Answer 16

• Sensory organs w/cross striations similar to those in skeletal mm fibers (2 forms based on location of nuclei: nuclear chain and bag; tx them as identical)
• Anatomically, much smaller in length and diameter than skeletal mm fibers, and attached to one or mm fibers by EC matrix
• Contractile, so innervated by gamma-motor neurons that can adjust tension of spindle, which has same effect as changing length of mm fibers, allowing dynamic range of this sensory organ to be adjusted for a wide range of muscle lengths
• Afferent nerves, coming from annulospiral ring or flower-spray endings, provide info about length & changes in length of the mm fibers assoc w/spindle

Question 17

Compare the two types of afferent NN from muscle spindles.

Answer 17

• TYPE 2: encodes mm fiber length info in freq of its AP’s -> INC length = INC freq of AP’s, & vice versa
• TYPE 1A: encodes both length and velocity info in freq of AP’s -> freq of AP’s INC dramatically during rapid INC in fiber length, and less dramatically in case of less rapid INC in mm fiber length (and vice versa for DEC)
  1. While static, freq of AP reflects length of mm fiber

Question 18

Describe muscle spindle feedback.

Answer 18

• Type 1a and II afferent NN in muscle spindles feedback on their associated motor neurons through a monosynaptic, excitatory connection
  1. Alpha-motor neuron efferent/sensory afferent excitatory circuit
• Motor unit is monitoring its length, allowing controlled movement, or lack of mvmt
  1. Ex: able to hold forceps in a static position without observing your hands, or close your eyes and touch your nose

Question 19

What is the golgi tendon organ (GTO)?

Answer 19

• Monitor and maintain muscle force, protecting mm & tendons against generating excessive, potentially harmful force
• Encapsulated afferent NN endings (blue) at junction of mm and tendon (connected in series)
• Specialized NN terminals convey info via 1b sensory afferent NN that synapse on INH local circuit neurons in spinal cord
  1. Interneurons INH alpha motor neurons connected to same muscle as the GTO
• Excessive muscle contraction causes GTO to fire AP’s that INH, attenuate force of contracting mm

Question 20

How does the GTO work?

Answer 20

• Stretch of GTO (blue) stimulates INH circuit neuron (black) to INH LMN (brown), causing biceps (in this case) to relax
• At same time, 1b afferent stimulates excitatory interneuron (purple) to activate LMN (red) of the antagonistic mm (triceps) to contract
• NOTE: higher cortical and brainstem centers also maintain control over muscle spindle and GTO reflex mechs via corticospinal and o/descending motor pathways that modulate local circuit neurons involved in reflex arcs (and in turn regulate LMN)

Question 21

What is the flexor reflex?

Answer 21

• Uncomfortable stimulus on skin will produce reflex withdrawal
• Agonist muscle activation + antagonist muscle inhibition -> agonist mm excitatory stimulus via excitatory interneuron

Question 22

How does the gamma motor neuron reflect positive bias?

Answer 22

• Gain on muscle tone -> higher the gain, the greater the muscle tone, and force of contraction
• NOTE: muscle spindles facilitate muscle stretch, and help maintain normal muscle tone

Question 23

What are 2 important influences on the muscle spindle (gamma motor neuron gain) and GTO?

Answer 23

• Local reflex circuitry
• Upper motor neuron input

Question 24

How can you distinguish a UMN from an LMN (table)?

Answer 24

• Weakness is a prominent sign of injury to either pathway, but significant differences in o/features
• UMN’s: spastic motor responses with INC muscle tone (b/c UMN’s usually inhibit LMN reflex arcs), hyperactive mm stretch reflexes, clonus, Babinski sign (extensor plantar response)
  1. CLONUS: repetitive plantar extension of foot when it is forcefully flexed upward, reflecting unchecked muscle stretch reflex that continues to operate via a feedback loop
• LMN’s: flaccid muscle response w/DEC mm tone and DEC muscle stretch reflexes; fasciculations may be seen, and the muscle will become atrophic in short time (bc neurotrophic relationship bt motor neuron and muscle)
  1. FASCICULATIONS: with loss of LMN input to mm, muscle fiber mem hyperexcitable, and may spontaneously discharge, creating brief, involuntary contractions of few muscle fibers, causing a flicker of movement under the skin

Question 25

Name 7 pathways/tracts that regulate LMN's, and their origins.

Answer 25

- Corticospinal: precentral gyrus - Vestibulospinal: vestibular nuclei - Reticulospinal: reticular formation - Rubrospinal: red nucleus - Colliculospinal: superior colliculus - Local circuit neurons - Dorsal root sensory neurons

Question 26

What are the highest levels of motor control? Be specific.

Answer 26

- **Brodmann area 4** (light green): contains _1^o motor cortex_ where lg motor neurons, _Betz cells_, reside, and give rise to axons making up corticospinal tract 1. Caudal boundary marked by central sulcus - **BA 6**: _premotor_ (blue), _supplementary motor areas_ (dark green); influence outflow of activity from 1^o motor cortex - **BA 3, 1, 2** (light orange): _1^o somatosensory cortex_; important in planning mvmt - **BA 5, 7**: _parietal association cortex_ (dark orange); indirectly assoc w/motor control

Question 27

What is the motor homunculus? Describe layout and clinical significance. What might you see in the case of occlusion of the R anterior cerebral artery?

Answer 27

- Motor on the right (purple); sensory on left (blue) - _Large areas of cortex dedicated to functions that require precise control_, like lower face and tongue for articulation and hand for dexterity - Useful when relating neuro symptoms/signs to location of a lesion in the brain - EX: occlusion of right anterior cerebral aa causes _ischemic stroke in R paramedian brain, where leg is represented_ -\> would produce contralateral leg weakness, but not be expected to compromise control and mvmt of hand or face

Question 28

Describe the somatotropic organization of the corticospinal/bulbar tracts.

Answer 28

- Right middle figure = _internal capsule_: posterior portion (caudal to angle) is posterior limb, and anterior portion is anterior limb 1. Bulbar fibers pass nearest angle of internal capsule (genu), with arm, trunk, leg fibers arranged in that order moving postero-laterally - Maintain somatotropic arrangement in brainstem - Cerebral peduncle: face/bulbar fibers most ventral-medial and legs dorsal-lateral - After pyramidal fibers cross in lower medulla and upper cervical spinal cord, maintain somatotropic arrangement such that _leg fibers are most lateral and arm fibers most medial_ 1. The few trunk fibers that travel in the lateral tract lie b/t the leg and arm fibers - NOTE: majority of trunk fibers have not crossed over, but travel on the ipsilateral side of the spinal cord as the anterior or ventral corticospinal tract

Question 29

What is the somatotropic org of the LMN's in the cervical and lumbosacral enlargements?

Answer 29

- LMN's somatotopically arranged such that _neurons serving extremities located laterally_ in ventral horn, and those serving trunk mm medial - Descending _corticospinal fibers innervating arm positioned medially_ (rather than peripherally) b/c they peel off first to synapse on their target motor neurons in the ventral horns, getting out of the way of fibers descending to innervate the legs - Ventral corticospinal tract synapse on _trunk and axial muscle motor neurons lying most medially_ and near the tract

Question 30

What is the somatotropic org of the corticobulbar tracts?

Answer 30

- Note the somatotropic organization here; will learn more about this later

Question 31

What is the vestibulospinal tract?

Answer 31

- Fibers originate from vestibular nuclei - Terminates at _cervical/thoracic spinal cord levels_ (as does reticulospinal tract) to innervate neck and trunk muscles - This innervation provides information coming from the vestibular formation to control _coordinated mvmts of neck and trunk muscles_

Question 32

What is the reticulospinal tract?

Answer 32

- Fibers originate from neurons in reticular system, located diffusely in brainstem - Terminates largely at _cervical/ thoracic spinal cord_ level (as does vestibulospinal tract) to innervate neck and trunk muscles - This innervation provides information coming from reticular formation to control _coordinated mvmts of the head and trunk muscles_

Question 33

What is the rubrospinal tract?

Answer 33

- Fibers that originate in the RED NUCLEUS - Travels closely with the lateral corticospinal tract, and participates in the _control of arm muscles_

Question 34

What is the colliculospinal (tecto-) tract?

Answer 34

- Fibers originate from the SUPERIOR COLLICULUS - Terminates largely at _cervical spinal cord_ level to innervate neck muscles -\> this innervation provides info from the superior colliculus to _coordinate head movements with eye movements_ - Collicular vision is what turns your head/eyes to a bug crawling on wall before you think about it; reflex rxn to pay immediate attention to sudden disturbances in visual periphery -\> useful from an evolutionary standpoint if you didn’t want to get eaten or if scouring landscape for something to eat

Question 35

What might lesions at these arrows produce?

Answer 35

- BLACK ARROWS (i.e., stroke): would cause pts to devo weakness of contralateral face and limbs 1. Above or rostral to pyramidal decussation - BLUE ARROW (below decussation): pt would devo ipsilateral weakness of arm/leg when lesion was high in cervical cord or only in ipsilateral leg if below cervical cord - RED ARROWS: if lesion involved only the anterior or ventral horn cells, or the peripheral nerves, pt would devo weakness of ipsilateral limb served by that lower motor neuron pop or peripheral nerve - NOTE: muscle weakness is a symptom and sign of both upper and lower motor neuron lesions

Question 36

What is the Babinski sign? What does it mean?

Answer 36

- NORMAL: stroking lateral sole of foot with blunt object, enough to cause discomfort = plantar flexion (big toe goes down; left image) - POST-INJURY to UMN's or corticospinal tract: same maneuver elicits extensor plantar response (big toe goes up w/or w/o fanning of the other toes; right image) 1. Primitive reflex that exists in newborns and disappears w/maturation of the UMN system at 3-6 months of age

Question 37

What do clinicians use the muscle stretch reflex to assess?

Answer 37

- Used to assess the integrity of the upper and lower motor neuron systems

Question 38

Middle-aged man with R-handed weakness case on phone. Where is the lesion? Left frontal cortex. Pure motor phenomenon that also affects facial expression, so has to be at least at the level of the pons. Difficulty with language expression means this lesion must be in the cortex. Without this, you would not know where, except that it was between frontal cortex and mid-pontine area. Why are his muscle stretch reflexes increased in his right arm? What is the likely diagnosis?

Answer 38

- UMN lesions cause loss of inhibition of the alpha motor neuron and thereby increase reflexes - LIKELY DX: stroke of the left frontal cortex 1. Symptoms got better (DIA) 2. Not a tumor of the motor strip because this grows gradually and progressively —\> can eventually rupture a blood vessel and cause a stroke

Question 39

64-y/o man presents w/complaints that for past 6 mos he has difficulty opening jars w/either hand, and more recently noted difficulty walking. He also has noticed twitching of his arm and shoulder mm. On exam, mm tone in arms is DEC, stretch reflexes DEC, and atrophy of interossei of both hands. Arm strength DEC bilaterally, w/distal mm weaker than prox. Prox leg strength normal, but difficulty standing on toes. Stretch reflexes in legs INC bilaterally, and positive Babinski bilaterally. Brief contractions of small portions of biceps, pectorals, and deltoids. Anatomical diagnosis?

Answer 39

- Lesions of upper and lower motor neurons - ALS may present like this - Cervical spondylosis can also cause this by damaging the LMN’s in the upper body, and the ascending UMN’s from the lower body

Question 40

49-y/o F with slowly progressive weakness in both legs over 2 yrs. Now, she has trouble climbing stairs, and getting out of lounge chair. Strength, muscle tone, and reflexes are normal in her arms. In her lower extremities, however, it is 4/5. Tone in legs in INC as are muscle stretch reflexes at knees and ankles. Bilateral Babinski signs. Sensation appears unaffected. What is your anatomical diagnosis?

Answer 40

- Midline intra-hemispheric lesion -\> parasagittal meningioma (see attached image) - Lesion in the brainstem will most likely NOT be selective - You would want to image the cervical spine in this patient to look for cervical degeneration (another possibility)