Motor system Flashcards
Define a motor unit
alpha motor neuron and the muscle fibers it innervates
What is the size principle for recruitment of muscles?
small neurons have higher input resistances due to having fewer membrane channels –> brought to threshold with LESS synaptic input than larger motor neurons
what effect does exercise have on the motor unit?
shifts motor unit phenotype from fast to slow (slowed fatigability and increased endurance)
What provides muscles with resistance to fatigability?
increased numbers of mitochondria
compare sensory info encoded by muscle spindles and golgi tendon organs
Muscle spindles: stretch via group Ia and II sensory afferents
Golgi tendon organs: Tension via Ib afferents
Where are muscle spindles located?
PARALLEL to force-generating (extrafusal) muscle fibers
Where are golgi tendon organs located?
IN SERIES at junction of muscle and tendon
Describe muscle spindle motor neuron innervation
Gamma motor neurons (stretch detectors of contracted or relaxed muscle)
Describe golgi tendon organ motor neuron innervation
Type Ib sensory afferents are around/in collagen strands. Tension –> pinched afferents –> firing. Analogous to chinese finger trap
Describe muscle spindle interneuron innervation
Inhibitory interneuron –> relaxed antagonist muscle
Describe golgi tendon organ interneuron innervation
Inhibitory and excitatory interneuron synapses –> extensor-flexor coupling
Describe the basic stretch reflex
hammer tap –> muscle stretch –> muscle spindle firing –> alpha motor neuron activation (contracts stretched muscle), and inhibitory interneuron activation (relaxes antagonist muscle)
What is the basal firing rate of Ia afferents?
Non-zero. Can increase (in response to stretch) or decrease (in response to shortening) firing rates –> maintained muscle tone
What is the significance of coactivation of alpha and gamma motor neurons?
leads to co-contraction of extrafusal and intrafusal muscle fiber –> maintained spindle stretch sensitivity in shortened muscle
What is the physiological process at play when you lift an empty box labeled “books”?
Gamma motor neurons rapidly detect mismatches between expected and actual muscle stretch –> error correction (reduced muscle contraction)
Are gamma neurons active during reflexive contractions?
No!
What happens when you step on a tack?
cutaneous nociceptors –> spinal interneurons –> coordinated extensor relaxation and flexor contraction on side of stimulus; extensor contraction and flexor relaxation on opposite side
What is meant by “hierarchical organization” of the motor system?
Motor system is increasingly flexible and more voluntary as neurons go from periphery to forebrain
What are central pattern generators?
Spinal cord circuits that lead to the ability to generate complex coordinated movements like locomotion and swimming
Describe the motor neuron somatotopy
Disproportionate M1 surface area to muscles involved in fine motor tasks. Mapping reflects MOVEMENTS, not muscles (not a true homunculus)
Which alpha motor neurons control lateral musculature?
laterally situated motor neurons
Which alpha motor neurons control medial musculature?
medially situated motor neurons
Describe the descending pathway controlling finger movement
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
Describe the descending pathway controlling movement of the axial musculature
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
