Motor system: lower motor neuron circuits

Question 1

What is a synonym for motor neuron pools?

Answer 1

Final Common path

Question 2

What is a motor neuron pool?

Answer 2

All the motor neurons innervating the same muscle.

Question 3

Where are located lower motor neuron pools in the spinal cord?

Answer 3

Ventral horn.

Question 4

How is the specific organization of lower motor neuron pools called?

Answer 4

Somatotopic organization.

Question 5

List three differences between long distance local neuron circuits and short distance local neuron circuits.

Answer 5

Long distance local neuron circuits:

• Medial ventral horn
• Axons cross the midline = bilateral innervation
• Control posture and locomotion (SYNCHRONOUS movements)

Short distance local neuron circuits:

• Lateral ventral horn
• Axons do not cross the midline = unilateral
• Control of distant limb muscles (DEXTEROUS movements)

Question 6

What is a motor unit?

Answer 6

Smallest unit capable of controlled movement → all the muscle fibers innervated by the projections of a single alpha motor neuron.

Question 7

Does size matter………… for alpha motor neurons?

Answer 7

Yes.

Question 8

Low innervation ratio is an indication of what?

Answer 8

Muscle controlling fine movement.

Question 9

What are the motor unit types?

Answer 9

Slow, fast fatigue-resistant, fast fatigable.

Question 10

What motor unit type is particularly important for forceful movements?

Answer 10

Fast fatigable.

Question 11

Slower motor units have ____ activation thresholds.

Answer 11

Lower.

Question 12

List three differences between gamma motor neurons and alpha motor neurons.

Answer 12

Gamma motor neurons:

• Small diameter
• Innervate muscle spindles
• Control excitability of sensory afferents in muscle spindles

Alpha motor neurons:

• Large diameter
• Innervate extrafusal muscle fibres
• Generate force for posture and movement

Question 13

Does motor unit plasticity exist?

Answer 13

Yes, motor units can change accordingly to the demands of the body.

Question 14

True or false: when muscle tension is increasing, the largest to smallest motor neurons are recruited.

Answer 14

False: smallest to largest.

Question 15

What is a simple solution to graded muscle contraction?

Answer 15

The muscle size principle.

Question 16

Associate the Hz to the stimulation: “twitch”, “unfused tetanus” and “fused tetanus”.

Answer 16

Respectively 5 Hz, 50 Hz and 100 Hz.

Question 17

What are the three factors controlling muscle force?

Answer 17

• Type of motor units activated
• Number of motor units activated
• Rate of firing of each motor unit

Question 18

What are the two forms that an intrafusal fibre can have in a muscle spindle?

Answer 18

Chain or bag.

Question 19

** SEEN IN SOMATOSENSORY SYSTEM **

What are the two types of sensory afferents in muscle spindles? List three differences between the two.

Answer 19

Group Ia sensory afferents:

• Dynamic sensibility
• Respond phasically
• Spiral up on bags and chain fibres

Group II sensory afferents:

• Static sensibility
• Respond tonically
• Spiral up mainly on chain fibres

Question 20

What motor neurons innervate the muscle spindles? What are its types?

Answer 20

Gamma motor neurons:

• Static
• Dynamic

Question 21

Muscle spindles monitor and maintain ____ ____ and are sensible to ____ force.

Answer 21

muscle length… passive

Question 22

Are group Ia and II afferents synapsing on alpha motor neurons? If yes, for what purpose?

Answer 22

Yes, for the stretch reflex (only carried in spinal cord).

Question 23

Explain the reciprocal innervation mechanism and its consequences.

Answer 23

Mechanism: group Ia sensory afferents EXCITE alpha motor neurons that innervate synergistic muscles AND indirectly INHIBIT alpha motor neurons innnervating antagonist muscles via interneurons (intervening reciprocal-sensory Ia-inhibitory interneurons).

Consequences: contraction of stretched muscle and relaxation of antagonist muscle.

Question 24

What is muscle tone? How is it important?

Answer 24

Muscle tone is the steady level of muscle tension. Important for adapting to demands of environment and self.

Question 25

What is responsible for regulating the muscle gain? How?

Answer 25

Gamma motor neurons by adjusting level of tension in intrafusal muscle fibers.

Question 26

True or false: often gamma and alpha motor neurons are co-activated.

Answer 26

True.

Question 27

How do gamma motor neurons prevent Ia sensory afferents to become loose when extrafusal muscles contract?

Answer 27

By innervating the contractile regions of intrafusal fibers and being often co-activated with alpha motor neurons (the gamma motor neurons).

Question 28

What is the sensory afferent in golgi tendon organs?

Answer 28

Group Ib sensory axons.

Question 29

What is the negative feedback loop mechanism of GTO?

Answer 29

- Ib sensory afferents of GTOs synapse to **inhibitory** interneurons in spinal cord that synapse alpha motor neurons of the *same muscle* = *decreasing* muscle tension - Ib sensory afferents of GTOs also synapse on **excitatory** interneurons that synapse alpha motor neurons of *antagonist* muscles

Question 30

What is the role of Golgi tendon organs?

Answer 30

Help to maintain a steady level of force to counteract fatigue or muscle damage.

Question 31

Golgi tendon organs monitor and maintain ____ ____ and are sensible to ____ force.

Answer 31

muscle force... active.

Question 32

Where is controlled locomotion?

Answer 32

In central pattern generators in spinal cord.

Question 33

What are the two states composing locomotion?

Answer 33

Swing and stance.

Question 34

How is the pace of locomotion increased?

Answer 34

Stance time reduced and swings kept constant.