Motor system

Question 1

Define a motor unit

Answer 1

alpha motor neuron and the muscle fibers it innervates

Question 2

What is the size principle for recruitment of muscles?

Answer 2

small neurons have higher input resistances due to having fewer membrane channels –> brought to threshold with LESS synaptic input than larger motor neurons

Question 3

what effect does exercise have on the motor unit?

Answer 3

shifts motor unit phenotype from fast to slow (slowed fatigability and increased endurance)

Question 4

What provides muscles with resistance to fatigability?

Answer 4

increased numbers of mitochondria

Question 5

compare sensory info encoded by muscle spindles and golgi tendon organs

Answer 5

Muscle spindles: stretch via group Ia and II sensory afferents

Golgi tendon organs: Tension via Ib afferents

Question 6

Where are muscle spindles located?

Answer 6

PARALLEL to force-generating (extrafusal) muscle fibers

Question 7

Where are golgi tendon organs located?

Answer 7

IN SERIES at junction of muscle and tendon

Question 8

Describe muscle spindle motor neuron innervation

Answer 8

Gamma motor neurons (stretch detectors of contracted or relaxed muscle)

Question 9

Describe golgi tendon organ motor neuron innervation

Answer 9

Type Ib sensory afferents are around/in collagen strands. Tension –> pinched afferents –> firing. Analogous to chinese finger trap

Question 10

Describe muscle spindle interneuron innervation

Answer 10

Inhibitory interneuron –> relaxed antagonist muscle

Question 11

Describe golgi tendon organ interneuron innervation

Answer 11

Inhibitory and excitatory interneuron synapses –> extensor-flexor coupling

Question 12

Describe the basic stretch reflex

Answer 12

hammer tap –> muscle stretch –> muscle spindle firing –> alpha motor neuron activation (contracts stretched muscle), and inhibitory interneuron activation (relaxes antagonist muscle)

Question 13

What is the basal firing rate of Ia afferents?

Answer 13

Non-zero. Can increase (in response to stretch) or decrease (in response to shortening) firing rates –> maintained muscle tone

Question 14

What is the significance of coactivation of alpha and gamma motor neurons?

Answer 14

leads to co-contraction of extrafusal and intrafusal muscle fiber –> maintained spindle stretch sensitivity in shortened muscle

Question 15

What is the physiological process at play when you lift an empty box labeled “books”?

Answer 15

Gamma motor neurons rapidly detect mismatches between expected and actual muscle stretch –> error correction (reduced muscle contraction)

Question 16

Are gamma neurons active during reflexive contractions?

Answer 16

No!

Question 17

What happens when you step on a tack?

Answer 17

cutaneous nociceptors –> spinal interneurons –> coordinated extensor relaxation and flexor contraction on side of stimulus; extensor contraction and flexor relaxation on opposite side

Question 18

What is meant by “hierarchical organization” of the motor system?

Answer 18

Motor system is increasingly flexible and more voluntary as neurons go from periphery to forebrain

Question 19

What are central pattern generators?

Answer 19

Spinal cord circuits that lead to the ability to generate complex coordinated movements like locomotion and swimming

Question 20

Describe the motor neuron somatotopy

Answer 20

Disproportionate M1 surface area to muscles involved in fine motor tasks. Mapping reflects MOVEMENTS, not muscles (not a true homunculus)

Question 21

Which alpha motor neurons control lateral musculature?

Answer 21

laterally situated motor neurons

Question 22

Which alpha motor neurons control medial musculature?

Answer 22

medially situated motor neurons

Question 23

Describe the descending pathway controlling finger movement

Answer 23

Precentral gyrus (Brodmann’s area 4) –> layer 5 pyramidal neurons –> internal capsule –> cerebral peduncle –> pons (collateralization) –> medulla (decussate) –> lateral corticospinal tract –> alpha motor neurons –> hand and finger movement

Question 24

Describe the descending pathway controlling movement of the axial musculature

Answer 24

Precentral gyrus (Brodmann’s area 4) –> layer 5 pyramidal neurons –> internal capsule –> cerebral peduncle –> pons (collateralization) –> medulla (no decussation) –> ventral corticospinal tract –> motor neuron pools –> axial and proximal limb muscles

Question 25

How is plasticity involved in recovery from stroke?

Answer 25

areas adjacent to damage form new connections that subserve motor control over the affected part of the body. Can lead to full or partial functional recovery

Question 26

Why do people say “practice makes perfect”?

Answer 26

Practice leads to expansion of involved cortical region (plasticity)

Question 27

Describe the basic mechanism of brain-machine interfaces

Answer 27

Neural activity is recorded from the brain and decoded in the machine. Machine decipher’s motor plan of action –> control of robotic limb