MODULE 8- Lower Motor Neurons Circuits + Motor Control

Question 1

4 neural centers responsible for movement

Answer 1

-lower motor neurons
-upper motor neurons
-cerebellum
-basal ganglia

Question 2

what are considered the “final common path” for initiating movement

Answer 2

lower motor neurons

Question 3

basal ganglia

Answer 3

initiation of intended movement + suppression of unwanted movement

Question 4

cerebellum

Answer 4

coordination of ongoing movement

Question 5

motor cortex

Answer 5

planning, initiating, + directing voluntary movements

Question 6

brainstem centers

Answer 6

stereotyped movements, postural control, + gain adjustments

Question 7

local circuit neurons

Answer 7

sensorimotor integration + central pattern generation

Question 8

motor neuron pools

Answer 8

all the motor neurons innervating a single muscle

Question 9

lower motor neurons in the spinal cord + brainstem map what

Answer 9

the body’s musculature

Question 10

each lower motor neuron innervates muscle fibers within a single/multiple muscles

Answer 10

a single muscle

Question 11

somatotopic organization of motor neuron pools in ventral horn of spinal cord

Answer 11

-proximal muscles towards center of ventral horn
-distal muscles towards periphery of ventral horn

Question 12

describe axons of medical ventral horn

Answer 12

extend several spinal cord segments + terminate bilaterally

Question 13

axons of medial ventral horn are concerned with what

Answer 13

control of posture + locomotion

Question 14

describe axons of lateral ventral horn

Answer 14

extend over fewer spinal cord segments + terminate only ipsilaterally

Question 15

axons of lateral ventral horn are concerned with what

Answer 15

fine control of distal extremities

Question 16

2 types of lower motor neurons found in the ventral horn

Answer 16

-alpha (α) motor neurons
-gamma (γ) motor neurons

Question 17

alpha motor neurons

Answer 17

-innervate striated muscles
-make up part of the motor unit

Question 18

gamma motor neurons

Answer 18

sensory receptors arranged in parallel with muscle spindles

Question 19

the motor unit

Answer 19

-1 α-motor neuron +all the muscle fibers it innervates.
-1 α-motor neuron contacts a relatively wide area of muscle
-smallest unit of force that can be activated by the muscle

Question 20

individual motor axons branch within muscles to do what

Answer 20

to contact fibers distributed over a wide area
-allows spread of activation for smoother force
-protects the muscle from becoming dysfunctional when 1 motor neuron dies out

Question 21

where are motor neurons controlling muscles in the head, eyes + neck located

Answer 21

in the brainstem

Question 22

innervation number

Answer 22

the number of muscle fibers innervated by 1 alpha motor neuron
-aka the number of fibers innervated by a motor unit

Question 23

innervation differs across ____

Answer 23

muscle groups

Question 24

gastroc force innervation number

Answer 24

1:2000

Question 25

extraocular eye muscles innervation number

Answer 25

1:5

Question 26

poliovirus

Answer 26

invades the motor neurons, leading to muscle weakness + impaired motor control due to fewer nerve signals reaching muscle fibers

Question 27

polio- what happens to motor neurons that survive

Answer 27

develop new terminal axon sprouts that re-innervate orphaned muscle fibers

Question 28

polio- total number of motor units increase/decrease

Answer 28

decrease

Question 29

polio- muscle fibers that belong to a single motor unit increase/decrease

Answer 29

increase

Question 30

what does polio result in

Answer 30

enlarged, less efficient motor units, which are easier to fatigue, further affecting fine motor control

Question 31

how do alpha motor neurons vary

Answer 31

in size

Question 32

alpha motor neuron size is proportional to what

Answer 32

the amount of muscle force that can be generated

Question 33

slow (S) motor units

Answer 33

sustained muscular contraction -such as for maintaining posture

Question 34

fast fatigable (FF) motor units

Answer 34

generating large forces for short periods of time -such as for jumping -few mitochondria

Question 35

fast fatigue resistant (FR) motor units

Answer 35

generate 2X force as slow ones -resistant to fatigue

Question 36

what motor units have few mitochondria

Answer 36

fast fatigable (FF) motor units

Question 37

twitch contraction

Answer 37

muscle tension in response to a single AP

Question 38

see slides 13-15

Question 39

motor neuron innervation to striated muscle changes in response to ____

Answer 39

exercise

Question 40

chronic electrical nerve stimulation transforms the metabolic + contractile properties of muscle from FF to ____ type

Answer 40

S type

Question 41

size principle

Answer 41

the orderly systematic recruitment of motor neurons from S to FR to FF muscle fibers as the physical demands of the task increases

Question 42

motor units are recruited from ____ to ____

Answer 42

weakest to strongest

Question 43

as the rate of stimulation on muscle tension increases...

Answer 43

AP firing frequency increases -> temporal summation -> unfused tetanus -> fused tetanus

Question 44

increase in force occurs with increased ____

Answer 44

firing rate

Question 45

as the amount of voluntary force increases, what increases

Answer 45

the number + firing rate of active motor units

Question 46

the lowest firing rates of motor units during a voluntary movement are on the order of ____/second

Answer 46

8/second

Question 47

firing rate rises to about ____ per second

Answer 47

20-25 per second

Question 48

normal firing rates are much less than those required for ____

Answer 48

fused tetanus

Question 49

local circuitry mediates reflexes that rapidly adjust muscle tension in response to sensory input slide 23?

Answer 49

8-10 intrafusal fibers arranged in parallel -dynamic nuclear bag fibers -static nuclear bag fibers -nuclear chain fibers

Question 50

_____ coil around fibers

Answer 50

large-diameter afferents

Question 51

Ia afferent innervates what

Answer 51

all 3 types of fibers

Question 52

what does Ia afferent signal

Answer 52

velocity

Question 53

II afferent innervates what

Answer 53

nuclear chain fibers

Question 54

what do II afferent signal

Answer 54

length

Question 55

activation of ____ pulls central region of intrafusal fiber

Answer 55

gamma motor neuron

Question 56

activiation of gamma motor neuron contracts intrafusal fibers, making which afferents more sensitive

Answer 56

Ia

Question 57

stretching muscle leads to what

Answer 57

increased discharge (APs) from Ia afferents

Question 58

alpha motor neurons of agonist/antagonist increase discharge (APs) causing extrafusal muscle fibers to contract

Answer 58

agonist -monosynaptic

Question 59

reciprocal innervation

Answer 59

-excite agonist muscle that is stretched -inhibits antagonist muscle -monosynaptic

Question 60

disynaptic pathway is mediated by what

Answer 60

reciprocal Ia inhibitory interneurons

Question 61

what happens during extrafusal contraction

Answer 61

the spindle afferents are quiet

Question 62

during extrafusal contraction, why are spindle afferents quiet

Answer 62

because the muscle spindle is not stretched – it is not loaded

Question 63

during alpha + gamma coactivation, what happens

Answer 63

when extra + intrafusal fibers contract, there is no change in spindle afferent firing, as they are being stretched or loaded

Question 64

stretch reflex operates as a feedback loop to regulate what

Answer 64

muscle length

Question 65

gain of stretch reflex depends on the level of what

Answer 65

gamma motor neuron activity

Question 66

high gain of stretch reflex

Answer 66

small stretch -> large increase in alpha motor neuron activation

Question 67

low gain of stretch reflex

Answer 67

larger stretch is required to produce same amount of tension in extrafusal fibers

Question 68

gain of stretch reflex is high during what tasks

Answer 68

difficult tasks or uncertain conditions -ex: walking across a narrow beam or standing on a moving bus

Question 69

golgi tendon organ

Answer 69

encapsulated nerve endings at junction of tendon + muscle

Question 70

what axons innervate GTOs

Answer 70

Ib axons

Question 71

how are GTOs arranged with extrafusal fibers

Answer 71

in series

Question 72

GTOs sense changes in ____ of the muscle

Answer 72

tension

Question 73

GTOs are sensitive/insensitive to passive stretch

Answer 73

insensitive

Question 74

GTOs work by positive/negative feedback system

Answer 74

negative

Question 75

decreased/increased GTO leads to APs of the Ib neuron

Answer 75

increasd

Question 76

which horn do GTOs enter

Answer 76

dorsal horn

Question 77

GTOs excite/inhibit alpha motor neuron of agonist muscle

Answer 77

inhibit

Question 78

GTOs excite/inhibit alpha motor neuron of antagonist muscle

Answer 78

excite

Question 79

GTOs prevent what

Answer 79

excessive tension

Question 80

GTOs maintain what during muscle fatigue

Answer 80

steady tone

Question 81

2 functions of GTOs

Answer 81

-prevent excessive tension -maintain steady tone during muscle fatigue

Question 82

under passive stretch or tension, what happens to muscle spindle activity

Answer 82

increases + is maintained

Question 83

under passive stretch or tension, what happens to GTO activity

Answer 83

relatively unaffected

Question 84

when load is increased...

Answer 84

the increased force is transmitted to the GTO -> increase in firing rate of its Ib afferents

Question 85

flexion reflex

Answer 85

-cutaneous (nociceptor) stimulated -excites interneuron circuits in the spinal cord

Question 86

flexion reflex causes inhibition of flexor/extensor of stimulated foot

Answer 86

extensor

Question 87

flexion reflex causes excitation of flexor/extensor of stimulated foot

Answer 87

flexor

Question 88

flexion reflex is ___ in the other limb

Answer 88

opposite -crossed extension reflex

Question 89

what does flexion reflex lead to

Answer 89

withdrawal of the limb from painful stimuli

Question 90

central pattern generator

Answer 90

local circuits in the spinal cord for generating rhythmic stereotypical patterns of movement, such as in walking, running, etc.

Question 91

central pattern generator extra info

Answer 91

-alternating between stance phase and swing phase -flexors + extensors activated reciprocally -increased speed means less time between each cycle -module receives input from excitatory rhythm-generating interneurons, which are reciprocally inhibited by interneurons

Question 92

what alternates during stepping

Answer 92

contractions of flexor + extensor muscles

Question 93

after recovering from surgery (transection of spinal cord), are the cats hindlimbs able to walk on treadmill

Answer 93

yes -reciprocal bursts of electrical activity can be recorded from flexors during the swing phase and from extensors during the stance phase of walking

Question 94

the basic rhythmicity of stepping is produced by what

Answer 94

neuronal circuits contained entirely within the spinal cord -separate CPG for each leg

Question 95

are supraspinal commands necessary for producing the basic motor pattern for stepping

Answer 95

NO

Question 96

what initiates + modulates CPG activity

Answer 96

mesencephalic locomotor region

Question 97

do CPG networks require sensory input

Answer 97

NO

Question 98

what are CPG networks strongly regulated by

Answer 98

input from limb proprioceptors (e.g., speed changes)

Question 99

lower motor neuron syndrome

Answer 99

caused by damage to lower motor neurons -depending on the extent of damage, can lead to paralysis, weakness, or loss of tone -muscles may exhibit fasciculations and fibrillations – spontaneous twitches characteristic of degenerating motor units + fibers, respectively

Question 100

how is lower motor neuron syndrome clinically diagnosed

Answer 100

EMG

Question 101

amyotrophic lateral sclerosis (ALS)/Lou Gehrig's disease

Answer 101

degeneration of lower motor neurons +/or upper motor neurons