Motor Systems: LMNs Flashcards
are lower motor neurons part of CNS or PNS
both
spinal cord –> periphery
function of LMNs
directly innervates skeletal muscle to allow for voluntary movement
LMN pools
- brachial plexus
- cervical intumescence (C6-T2)
- lumbar intumescence (L4-S1/3)
general somatic efferent LMNs
efferent lower motor neurons with the cell body located in the ventral horn of the spinal cord
- receives info from sensory afferents in dorsal horn
alpha and gamma LMNs
motor unit
a single LMN and all of the myofibers it innervates
1 LMN innervates many myofibers
1 myofiber is innervated by 1 LMN
small motor units
low # of myofibers innervated
allows for precise movement
large motor units
high # of myofibers innervated
allows for large force generation
ways to increase muscle activity
- increase # of motor units recruited
- increase rate of activation
alpha motor neurons
innervate extrafusal muscle fibers
large, fast, heavily myelinated
function of alpha motor neurons
facilitates movement and posture by causing contraction of skeletal muscle
gamma motor neurons
innervate intrafusal muscle fibers
medium, slower, heavily myelinated
function of gamma motor neurons
establishes readiness for action by maintaining appropriate amount of stretch in the muscle
muscle spindle
intrafusal muscle innervated by gamma motor neurons + annulospiral receptor
annulospiral receptors
detects stretch in intrafusal muscle fibers
sends signal to primary afferent –> spinal cord –> synapses on alpha motor neurons –> contraction of extrafusal fibers
function of muscle spindle reflex
detects muscle stretch required for localizing head and body position
joint flexion –> intrafusal stretch; allows for sensing change in posture to sustain posture against gravity
gamma bias
gamma motor neuron activity is set at different levels for different types of behavior
static gamma bias
firing of gamma neurons in a steady state; maintains constant stretch
dynamic gamma bias
firing of gamma neurons in response to changes in muscle length in order to maintain appropriate stretch
interneurons
provide feedback to the LMNs
usually inhibitory to LMN activity
renshaw cells
inhibitory interneurons in the spinal cord that receive input from LMN collaterals and provide negative feedback to stop LMN activity
allows for fine motor control
signs of LMN lesions
gait: weakness, short/choppy gait
- single limb: inability to support weight (nerve root signature)
- multiple limbs: collapsing, inability to rise, bunny hopping
reflexes: decreased/absent withdrawal and cutaneous trunci reflex
atrophy: rapid denervation atrophy; severe muscle loss
tone: flaccid muscle tone
