Chapter 16- Lower Motor Neuron Circuits and Motor Control

Question 1

what is the main purpose of the motor cortex ?

Answer 1

planning, initiating, and directing voluntary movements

Question 2

what is the main purpose of the brainstem ?

Answer 2

basic movements and postural control

Question 3

what is the main purpose of the basal ganglia ?

Answer 3

movement program and getting proper initiation of movement

Question 4

what is the main purpose of the cerebellum ?

Answer 4

sensorimotor coordination of ongoing movement

Question 5

what are lower motor neurons ?

Answer 5

the final common pathway to all descending systems

Question 6

what is skeletal muscle contraction initiated by ?

Answer 6

lower motor neurons in brainstem and spinal cord

Question 7

how is it possible to identify motor neurons for each muscle ?

Answer 7

the lower motor neurons that innervate each of the body’s skeletal muscles can then be seen in histological sections of the ventral horns of the spinal cord.

Question 8

how are neurons that innervate different parts of the body distributed on the spinal cord ? (somatotopic organization)

Answer 8

The map shows neurons that innervate the axial musculature (i.e., the postural muscles of the trunk) are located medially in the spinal cord. Lateral to these cell groups are motor neuron pools innervating muscles located progressively more laterally in the body.

Question 9

what are the cell bodies of neurons in the ventral horn ?

Answer 9

alpha motor neurons and gamma motor neurons

Question 10

what do gamma motor neurons do ?

Answer 10

they innervate muscle spindles
-regulate this sensory input by setting the intrafusal
muscle fibers to an appropriate length

Question 11

what are muscle spindles, what are they innervated by ?

Answer 11

The muscle spindles are embedded within connective tissue capsules in the muscle and are thus referred to as intrafusal muscle fibers. The intrafusal muscle fibers
are also innervated by sensory axons that send information to the spinal cord and brainstem about
the length of the muscle.

Question 12

what is the function of alpha motor neurons ?

Answer 12

innervate the extrafusal muscle fibers, which are the striated muscle fibers that actually generate the forces needed for posture and movement.

Question 13

each muscle fiber is innervated by how many motor neurons ?

Answer 13

usually 1, except more when the body is developing

Question 14

each motor neuron innervates how many muscle fibers ? how ?

Answer 14

there are more fibers than neurons, so the neuron branches off within muscles to synapse with different extrafusal fibers.

Question 15

when a motor neuron branches out, how does it spread out spatially ? why ? (2 reasons)

Answer 15

it spreads out evenly

1) to maximally activate the muscle
2) in case of the lesion, the movement can be better preserved

Question 16

will all motor neurons lead to the generation of the same force ?

Answer 16

no, the motor units and motor neurons vary in size and therefore in power output

Question 17

smaller motor neurons usually innervate what kind of muscle fibers ?

Answer 17

slow oxidative, fatigue resistant

Question 18

bigger motor neurons usually innervate what kind of muscle fibers ?

Answer 18

fast-twitch, fast fatiguable

have less mitochondria

Question 19

what are fast-fatigue resistant muscle fibers ?

Answer 19

an intermediate species, can generate twice the force of a slow motor unit but aren’t as big as the fast-fatiguable motor unit

Question 20

how does firing look in slow muscle fibers ?

Answer 20

easily depolarized to firing threshold, but typically maintain only slower and steady rates of firing

Question 21

medial gastroc and soleus ? S or F

Answer 21

MG is more fast

S is more slow

Question 22

how do fast and slow fibers differ in their time to peak ?

Answer 22

for slow it takes more time

Question 23

what is the size principle ? why ?

Answer 23

that the slow, smaller motor units are recruited first, then the fast fatigue resistant, then the fast fatiguable

provides a simple solution to the problem
of grading muscle force.

Question 24

which motor units have a lower threshold ?

Answer 24

the slow ones, so they’re recruited first

Question 25

what happens to the firing when you have to apply tremendous voluntary force ?

Answer 25

the number and rate of firing of active motor units increases

Question 26

how are group Ia afferents different from group II afferents ?

Answer 26

Ia:
large diameter sensory axons
respond phasically to small stretches
dynamic, looks at velocity of fiber stretch

II:
smaller diameter
static nuclear bag fibers and the nuclear chain fibers, signal the level of sustained fiber stretch by firing tonically at a frequency in proportion to the degree of stretch, with little dynamic sensitivity.

Question 27

stretching a muscle spindle leads to increased activity in which fibers ?

Answer 27

Ia afferents and alpha motor neurons which will innervate synergistic muscles
indirect inhibition of motor neurons innervating antagonist muscles

Question 28

with what kind of feedback does the stretch reflex circuit function ?

Answer 28

negative feedback loop

Question 29

how are gamma motor neurons involved with muscle spindles ?

Answer 29

they “gage” and motor control of the spindle

basically regulate muscle spindle response

Question 30

what co-activation is crucial for the spindle stretch reflex to work ?

Answer 30

alpha and gamma motor neuron co-activation

Question 31

without gamma motor neuron, what would alpha motor unit activation for muscle spindles look like ? what does this say about the role of gamma motor neurons ?

Answer 31

response of Ia fiber decreases as muscle contracts (firing rate stops in the middle)

therefore, gamma neurons can regulate the gain of muscle spindles so they can operate efficiently at any length of the parent muscle.

Question 32

how do Golgi tendon organs and spindles differ when muscle is actively contracted ? what does this say about Golgi tendon organs ?

Answer 32

When the extrafusal muscle fibers are made to contract by stimulation of their alpha motor neurons, however, the spindle is unloaded and its activity decreases, whereas the rate of Golgi tendon organ firing increases.

Therefore, the Golgi tendon organs are activated in response to contraction, when the collagen fibrils become tighter around the GTO in the extrafusal fibers, and not in response to passive stretch.

Question 33

what is the main purpose of Golgi tendon organs ?

Answer 33

protect from excessive contraction

Question 34

what axons are at work for the Golgi tendon organs ?

Answer 34

Ib fibers
inhibitory interneurons
alpha motor neurons

Question 35

what is the Golgi tendon organ pathway ?

Answer 35

Ib afferents contact inhibitory interneurons in spinal cord, which then synapse with alpha motor neurons that innervate that same muscle. It is therefore a negative feedback system.

Question 36

do reflexes go to the brain ?

Answer 36

nope

spinal cord

Question 37

what is the H-reflex ? what are its implications ?

Answer 37

a supposed stretch reflex in walking
where the stimulated leg flexes to withdraw and the opposite leg extends to support
this suggests that maybe motor output is based on reflexes

Question 38

what happens if there is a disconnection between brain and spinal cord when it comes to walking ?

Answer 38

animal can relearn to walk
meaning the walking comes from a different input mostly
is autonomous and doesn’t require sensory input or brain control

Question 39

what are the three things that lower motor neurons integrate ?

Answer 39

sensory input, reflex actions, and descending commands