Pathway of Voluntary Movement Quiz

Pathway of Voluntary Movement

• Movement begins with the upper motor neuron (UMN) in the motor cortex firing action potentials (APs).
• These APs travel down descending pathways from the brain to the spinal cord.
• The motor neuron system (MNS) is part of the central nervous system (CNS), but their axons are long and extend to the periphery.
• It takes about 30-35 milliseconds for a signal to travel from thought to action.
• The final common pathway for motor output is the lower motor neuron (LMN) in the spinal cord sending APs to muscles.
• The LMN is the main controllable element in the motor pathway and determines the force output of the muscles.

The Motor Unit

• A motor unit consists of one motor neuron and all the muscle fibers it innervates.
• One motor neuron can innervate multiple muscle fibers.
• Muscles contain hundreds of motor units.
• The type of muscle fiber in a motor unit is determined by the motor neuron.
• The size of a motor unit (number of muscle fibers innervated) determines the level of fine control possible.

Motor Unit (MU) Size and Control

• Muscles requiring fine control have a smaller innervation ratio (fewer muscle fibers per motor unit) compared to larger muscles requiring more force.
• Slow twitch muscle fibers are more centrally located, deep within the muscle to receive an adequate blood supply.
• Fast twitch muscle fibers tend to be located more peripherally.

Variability in Muscle Fiber Types

• The proportion of slow twitch (type I) and fast twitch (type II) muscle fibers varies between individuals.
• Humans are mostly made up of slow twitch fibers optimized for endurance.
• It's possible to make fast twitch fibers act more like slow twitch fibers and vice versa, but not through actual type conversion.

Properties of Muscle Related to Movement

• Length-Tension Relationship: A muscle can produce the most force when it is at its optimal length.
  • Excessive shortening or stretching decreases force production since fewer cross-bridges can be formed.
• Whole Muscle Length-Tension Relationship: Longer muscles generate more tension due to the contribution of passive components (elastic connective tissue).
  • These passive elements contribute to the total muscle tension curve, which is the sum of active and passive tension.
• Load-Velocity Relationship: The speed of muscle shortening is dictated by the resistance it encounters.
  • A muscle can shorten at its maximum velocity (Vmax) with no resistance.
  • When resistance increases, the velocity of shortening decreases.
• Power: The power of a muscle contraction is measured in watts and is calculated by multiplying force and velocity.
  • Peak power output occurs at about half of Vmax.
  • Motor units control the speed, strength, and power of muscle contractions.

Electromyography (EMG)

• Surface EMG: Records the electrical activity of a muscle during contraction by placing electrodes on the skin.
  • Provides an overall measure of electrical activity in the muscle.
  • Does not allow for the identification of individual motor unit activity.
• Intramuscular EMG: Records electrical activity directly from within a muscle using a needle electrode.
  • Allows for the recording of individual motor unit activity.
  • Enables the observation of the firing rate of motor units.

Muscle Force Gradation

• The CNS uses two mechanisms to control muscle force:
  • Recruitment: Activating more motor units.
  • Rate Coding: Increasing the firing rate of motor units.
• Small motor units are easier to activate (lower threshold) than large motor units.
• Smaller motor units have a higher surface area to volume ratio, leading to concentrated excitatory currents and easier excitation.
• Larger motor units are recruited later and do not fire for as long as smaller motor units.
• All motor unit types are used during different phases of movement, with type IIB fibers being recruited primarily during high-intensity bursts.