What is an upper motor neuron?
- Motor neuron contained entirely within the CNS
- Axon terminates in brain or spinal cord
What is a lower motor neuron?
- Neuron with cell body in the CNS, and axon projecting into the PNS to innervate something like a skeletal muscle fiber
Describe the corticospinal (pyramidal) tracts.
- Originates in 1o 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
How are the lateral and anterior corticospinal tracts different?
- 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
What are Rexed's Laminae?
- Spinal cord grey matter layers
- Layers I-VI = intermediate sensory neurons
- Layers VII-VIII: local circuit, autonomic, and commissural neurons
- Layer IX: LMN's
Describe the somatotopic organization of LMN's.
- 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
What is a motor unit? How does it work?
- 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
What are the 2 important anatomical/physio relationships b/t LMN's and the mm fibers they innervate?
- 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
How does innervation of fine mm differ from that of powerful mm?
- 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
What are the 3 mm fiber types?
- 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
What is the difference b/t the mm fiber types of these 2 muscles?
- 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
What is the muscle stretch reflex?
- 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)
What are the muscle sensory organs and their associated NN types?
- 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
What types of connections are possible on alpha-motor neurons?
- 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
What is a muscle spindle? How does it work?
- 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
Describe the anatomy of muscle spindles.
- 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
Compare the two types of afferent NN from muscle spindles.
- 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
Describe muscle spindle feedback.
- 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
What is the golgi tendon organ (GTO)?
- 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
How does the GTO work?
- 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)
What is the flexor reflex?
- Uncomfortable stimulus on skin will produce reflex withdrawal
- Agonist muscle activation + antagonist muscle inhibition -> agonist mm excitatory stimulus via excitatory interneuron
How does the gamma motor neuron reflect positive bias?
- 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
What are 2 important influences on the muscle spindle (gamma motor neuron gain) and GTO?
- Local reflex circuitry
- Upper motor neuron input
How can you distinguish a UMN from an LMN (table)?
- 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
Name 7 pathways/tracts that regulate LMN's, and their origins.
- Corticospinal: precentral gyrus
- Vestibulospinal: vestibular nuclei
- Reticulospinal: reticular formation
- Rubrospinal: red nucleus
- Colliculospinal: superior colliculus
- Local circuit neurons
- Dorsal root sensory neurons
What are the highest levels of motor control? Be specific.
- Brodmann area 4 (light green): contains 1o 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 1o motor cortex
- BA 3, 1, 2 (light orange): 1o somatosensory cortex; important in planning mvmt
- BA 5, 7: parietal association cortex (dark orange); indirectly assoc w/motor control
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?
- 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
Describe the somatotropic organization of the corticospinal/bulbar tracts.
- 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
What is the somatotropic org of the LMN's in the cervical and lumbosacral enlargements?
- 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
What is the somatotropic org of the corticobulbar tracts?
- Note the somatotropic organization here; will learn more about this later