Motor Systems: LMNs

Question 1

are lower motor neurons part of CNS or PNS

Answer 1

both
spinal cord –> periphery

Question 2

function of LMNs

Answer 2

directly innervates skeletal muscle to allow for voluntary movement

Question 3

LMN pools

Answer 3

1. brachial plexus
2. cervical intumescence (C6-T2)
3. lumbar intumescence (L4-S1/3)

Question 4

general somatic efferent LMNs

Answer 4

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

Question 5

motor unit

Answer 5

a single LMN and all of the myofibers it innervates

1 LMN innervates many myofibers
1 myofiber is innervated by 1 LMN

Question 6

small motor units

Answer 6

low # of myofibers innervated
allows for precise movement

Question 7

large motor units

Answer 7

high # of myofibers innervated
allows for large force generation

Question 8

ways to increase muscle activity

Answer 8

1. increase # of motor units recruited
2. increase rate of activation

Question 9

alpha motor neurons

Answer 9

innervate extrafusal muscle fibers

large, fast, heavily myelinated

Question 10

function of alpha motor neurons

Answer 10

facilitates movement and posture by causing contraction of skeletal muscle

Question 11

gamma motor neurons

Answer 11

innervate intrafusal muscle fibers

medium, slower, heavily myelinated

Question 12

function of gamma motor neurons

Answer 12

establishes readiness for action by maintaining appropriate amount of stretch in the muscle

Question 13

muscle spindle

Answer 13

intrafusal muscle innervated by gamma motor neurons + annulospiral receptor

Question 14

annulospiral receptors

Answer 14

detects stretch in intrafusal muscle fibers

sends signal to primary afferent –> spinal cord –> synapses on alpha motor neurons –> contraction of extrafusal fibers

Question 15

function of muscle spindle reflex

Answer 15

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

Question 16

gamma bias

Answer 16

gamma motor neuron activity is set at different levels for different types of behavior

Question 17

static gamma bias

Answer 17

firing of gamma neurons in a steady state; maintains constant stretch

Question 18

dynamic gamma bias

Answer 18

firing of gamma neurons in response to changes in muscle length in order to maintain appropriate stretch

Question 19

interneurons

Answer 19

provide feedback to the LMNs
usually inhibitory to LMN activity

Question 20

renshaw cells

Answer 20

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

Question 21

signs of LMN lesions

Answer 21

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