Motor System Flashcards
Movements are controlled by multiple heirarchical systems
Motor neurons Spinal cord Brainstem Motor cortex Basal ganglia Cerebellum
The motor unit
Motor neuron and the muscle fibers it innervates
Slow (S)” motor units
Small alpha motor neurons with fine caliber axons
- Innervate a small number of ‘slow oxidative’ muscle fibers
- Generate small forces
- Fatigue slowly
- Recruited first due to high input resistance of neuron
Fast fatigue-resistant (FR)” motor units
- Intermediate sized alpha motor neurons with medium caliber axons
- Innervate an intermediate number fast oxidative glycolytic muscle fibers
- Generate large forces
- Fatigue slowly
- Recruited second
Fast fatigable (FF)” motor units
Large alpha motor neurons with thick axons
- Innervate a large number fast glycolytic muscle fibers
- Generate large forces
- Fatigue quickly
- Recruited last
Recruitment Order
Slow (S) motor units–> Fast fatigue-resistant (FR) motor units–> Fast fatigable (FF) motor units
Orderly recruitment = size principle
Oxidative fibers primarily use oxidative phosphorylation to generate ATP. As a consequence, they are richly endowed with ____________, and surrounded by capillaries that deliver oxygen.
mitochondria
Oxidative fibers contain a lot of ___________, an oxygen-binding protein that can store oxygen and speed its delivery to mitochondria within the muscle cell.
myoglobin
Myoglobin imparts a red color to the cell, hence the terms “red muscle” for oxidative fibers and “white muscle” for ___________ that have low levels of myoglobin.
glycolytic fibers
Neuromuscular synaptic transmission
NT: Ach
Ach opens Na channels on muscle fiber and generates AP
Alpha motor neurons
large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction.
What degenerates in ALS
-Amytrophic Lateral Sclerosis
Alpha motor neurons
Each muscle fiber innervated by _____ motor neuron
ONE
Exercise
Endurance exercise tends to slow the contractile properties of motor units and increase endurance and strength.
High intensity strength training increases amount of contractile protein
Effects not entirely on motor unit phenotype; also central changes that may alter recruitment
Changing use through exercise can change relative distributions
Cross innervation of small alpha motor neurons onto fast twitch muscle fibers results in:
the conversion of the fast-twitch muscle to a slow twitch fiber type and vice versa
Suggests that the motor neuron identity governs muscle phenotype
