Lecture 8: Lower Motor Systems

Question 1

The four systems of motor
control

Answer 1

1. Local spinal cord and
   brainstem
   • “Final common path”
2. Descending control pathways
   • Upper motor neurons
   • Voluntary and skilled movements
3. Basal ganglia
   • Suppresses unwanted movements
4. Cerebellum
   • “Tweaking” motor commands

Question 2

Describe the three ways that motor neurons are orderly arranged at the level of the spinal cord.

Answer 2

• Spatial map of the body’s musculature
• Along the length
• Distinct clusters in the ipsilateral ventral horn
• Cylindrically shaped and distinct distribution of different motor pools

• Across the medial to lateral

• Axial musculature is in the medial cord
• Distal musculature is farthest from the midline

Question 3

Alpha motor neurons

Answer 3

Innervate extrafuscal muscle fiers that generate force and movement.

Question 4

Beta motor neurons (very basic)

Answer 4

innervate extra and intrafuscal muscle fibers

just know that they exist

Question 5

What do gamma motor neurons do?

Answer 5

innervate intrafuscal muscle fibers, which generate no actual movement, but set the gain of the muscle fibers by changing the length of the intrafuscal muscle fibers.

Question 6

Describe the local circuit neurons in the spinal cord. What do they do? (talk about long and short circuits)

Answer 6

• Interneuron processes remain in the spinal cord
• Mediate simple reflexes
• Contribute to the coordination of motor neurons involved in fine motor behaviors, balance, and locomotion
  
  • Long distance local circuit neurons are medial, and control posture/balance.
  • Short distance local circuit neurons are lateral in the ventral horn and control fine motor behaviors.

Question 7

Motor unit

Answer 7

A motor neuron, and all the muscle
fibers it innervates
• Most extrafusal muscle is innervated
by a single alpha motor neuron
• Motor neuron must branch within
muscles to synapse onto many fibers

Question 8

What are the differences between small and large alpha-motor units?

Answer 8

Motor units and alpha motor neurons can vary in size

• Small alpha-motor neurons innervate few muscle
fibers = small force
• Large alpha-motor neurons innervate many muscle
fibers = greater force

Question 9

Red muscle fibers

Answer 9

• “Red” (S) muscle fibers make up most of the skeletal
muscles used for long exertions like standing

• Smaller alpha-motor neurons
• Contract slowly, relatively low force, resistant to fatigue (hours)

Question 10

Pale muscle fibers (fast fatiguing)

Answer 10

“Pale” (FF) muscle fibers are used for brief exertions
(like jumping)

• Large alpha-motor neurons
• Contract quickly, high force, fatigue quickly (<1 to 5 min)
due to fewer mitochondria, higher activation threshold

Question 11

“also red” or FFR muscle fibers

Answer 11

“Also Red” (FFR) muscle fibers used for moderate for
exertions (like running)

Question 12

Describe motor unit plasticity

Answer 12

Training improves muscle performance primarily through changes in the
motor unit

• Increases in firing rate and decrease in firing latency

Question 13

What the size principle of the motor unit.

Answer 13

Increases in muscle tension are
brought about through increases in
the number of motor units recruited

• Order in which motor units are
recruited based on stimulation level

• S units –> FFR –> FF

• Muscle tension is also controlled by
frequency

• Individual motor unit 8-25Hz
• Multiple motor units firing asynchrously

• Allows for smooth motionIncrea

increase force production

Question 14

What three factors lead to the most force?

Answer 14

More motor units

Higher frequency

Asynchronous

Question 15

Describe the monosynaptic stretch reflex?

Answer 15

Monosynaptic Reflex

• Best known is the patellar tendon
tap (knee-jerk reflex)

• Reflex arc involves only the primary
afferent neuron, a synapse, and the
efferent neuron

• Tap causes muscle stretch detected
by muscle spindle and conveyed to
spinal cord

• Results in contraction of the muscle
that was stretched and inhibition of
the antagonist muscle

• Occurs in addition to sensory
information asecending the dorsal
column

• Almost all muscles can exhibit a
stretch reflex

Question 16

How do the Ia afferents associate with the muscle to mediate stretch reflex? (muscle spindle)

Answer 16

Large diameter (Ia afferents) coiled around
 intrafusal fibers (either nuclear bag or
 nuclear chain)

• Forms ‘annulospiral’ primary endings

Question 17

How do gropu II afferents play into the muscle spindle reflexes?

Answer 17

Group II afferents form secondary endings
• Combined

• Group Ia afferents respond to small
stretches
• Group II afferents respond to sustained
fiber stretch

Question 18

Group Ia afferents respond to _______ stretches

Answer 18

small

rapidly adapting.

Question 19

Group II afferents respond to _________ fiber stretch.

Answer 19

sustained. *slow adapting*

Question 20

Golgi tendon organs

Answer 20

Golgi Tendon Organs (GTOs)

• Encapsulated afferent nerve endings

• Located at the junction of a muscle and
tendon\

• Sensitive to muscle tension, but relatively
insensitive to muscle stretch

• Protects the muscle from damage

Question 21

How are GTOs part of a negative feedback loop?

Answer 21

• GTO afferent Ib fibers stimuluate inhibitory neurons that suppress activity in the alpha motor neurons that innervate homonymous muscle
• Can also activate antagonistic muscle
• Prevents muscle damage and maintains a steady level of muscular tone

Question 22

Flexion-withdrawl reflex pathways

Answer 22

used to withdraw a limb from a painful stimulus

involves slowly conducting afferent axons and multiple synpatic links

leads to withdrawl of the limb

Excitation of ipsilateral flexion muscles, inhibition of ipsilateral extensor muscles.

Question 23

Cross extension reflex

Answer 23

Happens with flexion withdrawl reflex

opposite reaction in the contralateral limb

Pain stimul leads to flexion on one side, extension on opposite.

Question 24

Sources of converging input in flexion reflex pathways and how they can modify a reflex

Answer 24

other spinal cord neurons

upper motor neuron pathways

functional purpose is unclear

seems to be involved in modulating the responsiveness of the local circuitry

can even inhibit/suppress activity

Question 25

Central pattern generators

Answer 25

local circuits with the spinal cord

can produce repetitive or cyclic movements without sensory feedback

ex. locomotion, rhythmic movements of the tongue, respiration, heartbeat

control the timing adn coordination of complex patterns of movement and can even adjust them to altered circumstances.

Question 26

stance phase

Answer 26

limb in contact with ground and pushing forward (extension)

Question 27

swing phase

Answer 27

limb is lifted and brought forward (flexion)

Question 28

What happens to the pattern from the CPG with different types of locomotion?

Answer 28

Order of swing/stance (flexion/extension) is important

increases in speed increase the cycle frequency and decrease the cycle length.

When transitioning from different types of locomotion, pattern changes in CPG

Question 29

How are multiple limbs controlled with CPG?

Answer 29

each limb has its own

connected to one another through local interneurons

receive input from descending sources and from sensory neurons, but are not required.

Question 30

Can decerebrate and spinal transected cats still walk?

Answer 30

Yep. no descending input needed

can still be modified by sensory input

can be pharmacologically activated (local injection causes walking)