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Spinal cord motor neurons and motor units

Terms in this set (57)

upper motoneurons (UMN)
higher motor areas in the cerebral cortex -> synapse on LMN -> motor commands
lower motoneurons (LMN)
motoneurons in the spinal cord or brainstem (cranial nerves) -> synapse w/ skeletal muscle fibers directly
what is a decrease in strength of muscle contraction?
weakness
what is muscular weakness but not complete paralysis due to loss of innervation?
paresis
what is complete paralysis due to loss of innervation?
plegia
a lesion at any level of the pyramidal pathway, from the cerebral cortex to the muscle fibers can cause ____
muscle weakness
what is weakness of a single limb?
monoparesis (plegia)
what is weakness of left/right side of the body?
hemiparesis (plegia)
what is weakness in both legs?
paraparesis (plegia)
what is weakness in all 4 limbs?
tetraparesis (plegia) or quadriparesis (plegia)
contraction is under ____ control
voluntary
what is required to initiate contraction?
alpha motoneuron
action potentials propagated on motoneurons cause release of ACh at the ______ junction
neuromuscular
what is required for skeletal muscle function?
connection to the motoneuron (intact innervation)
what is it called when innervation is lost and skeletal muscle becomes relaxed and paralyzed? what happens eventually?
flaccid paralysis
- eventually, muscle also atrophies if innervation is not restored
what enters the dorsal horn via dorsal root (central process of primary sensory neuron in the dorsal root ganglion)?
sensory information
interneurons process ___
information (reflexes)
what contains the lower motoneurons (LMNs) in the ventral (anterior) horn?
spinal cord
what executes motor commands from descending pathways?
spinal cord
what coordinates motor activity via reflexes integrated in the spinal cord?
spinal cord
what is a set of motoneurons innervating fibers within the same muscle?
motorneuron pool
what are muscle fibers innervated by the same motor neurons?
motor unit
strength of the whole muscle can be increased by ____
motor unit recruitment
number of contracting fibers (and whole muscle contraction strength) can be increased by ____
motor unit recruitment
what constitutes the majority of the skeletal muscle -> used to generate force?
extrafusal (contractile) muscle fibers
what are specialized muscle fibers in the muscle spindles?
intrafusal muscle fibers
what are motor spindles that run parallel with the extrafusal muscle fibers and sense muscle length (proprioceptors)?
muscle spindles
what are intrafusal muscle fibers which run parallel to the actual contractile muscle fibers (extrafusal fibers)?
muscle spindles
nerves on the intrafusal fibers can detect the length of the muscle and when the ____ shortens or lengthens
muscle
what senses muscle tension via afferent nerves?
golgi tendon organ
what innervates extrafusal muscle fibers?
alpha motor neurons
what innervates specialized intrafusal muscle fibers in the muscle spindles?
gamma motor neurons
alpha and gamma motor neurons are activated ____
simultaneously
what maintains muscle spindle sensitivity in the contracting muscle?
alpha gamma coactivation
neuronal circuit is also called the
reflex arc
what is the pain stimulus from the hand touching a hot stove an example of?
crossed extensor reflex
what activates the flexors and inhibits the extensors on the same side to withdraw the hand from painful stimulus?
crossed extensor reflex
what stimulates the extensors and inhibits the flexors on the other side to support the body at the same time?
crossed extensor reflex
lower motor neurons receive ____ input from the cerebral motor cortex via the pyramidal pathway
direct
what receives indirect input from the brain stem via the extrapyramidal pathways?
lower motor neurons
what is the tract from the motor cortex to the brain stem (cranial) motor nuclei?
corticobulbar (corticonuclear) tractwhat co
what is the tract from the motor cortex to the spinal cord anterior horn?
corticospinal tract
what is the system that goes from the motor cortex to the brainstem and spinal motor neurons directly -> voluntary movement?
pyramidal system
what system includes pathways involved in motor control -> adjustments of muscle tone, posture, supraspinal reflexes?
extrapyramidal system
lateral/corticospinal tracts are _____ tracts
pyramidal
rubrospinal, reticulospinal, olivospinal, and vestibulospinal tracts are ___ tracts
extrapyramidal
what is the interruption of the flow of impulses along the final common path? what is it accompanied by?
flaccid paralysis; hypotonia
why do decreased/absent superficial and deep reflexes occur?
b/c lower motor neurons form the efferent limbs of all skeletal muscle reflexes
spontaneous twitches/fasciculation and decrease in bulk (atrophy - denervated muscles after weeks to months) occurs in ___ syndrome
lower motor neuron syndrome/flaccid paralysis
what contains fibers that cross in the medulla after going through the pyramid?
lateral corticospinal tract (LCST)
what contains fibers that do not cross in the medulla, but some cross in the spinal cord segment, some don't (axial muscles)?
anterior (ventral) corticospinal tract (ACST or VCST)
what tract extends through the entire brain and spinal cord?
pyramidal tract
what tract is susceptible to vascular and traumatic damage at any CNS level?
pyramidal tract b/c it extends through the entire brain and spinal cord
pyramidal tract contains mostly _____ nerve fibers that make it susceptible to damage in demyelinating diseases such as MS and ALS
myelinated
what are the principal signs of the upper motor neuron syndrome?
paralysis, increased muscle tone, exaggerated myotatic reflexes, extensor plantar response (Babinski sign)
lower motoneuron signs
everything lowered (less muscle mass, muscle tone, reflexes, and downgoing toes)
upper motoneuron signs
everything up (tone, DTRs, toes)
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