Ch-14 Motor Neurons and Motor Units

Flashcards reviewing the anatomy, physiology, and function of motor neurons and motor units.

What is the role of motor neurons in the CNS?

The last-order neurons that exit the CNS to target muscle fibers and provoke muscle contraction.

Where do motor neurons reside?

Spinal cord (limb and body muscles) and brain stem (face, mouth, eye, and tongue muscles).

What is a motor nucleus?

The collection of motor neurons that supply a given muscle.

Where do motor nuclei form?

Longitudinal columns that extend over several millimeters in the spinal cord.

What are gamma motor neurons?

Motor neurons that innervate the small intrafusal muscle fibers of the muscle spindle.

What are alpha motor neurons?

Motor neurons that supply the main muscle fibers of a muscle.

Where are motor neurons found in a cross-section of the spinal cord?

Ventral horn.

How are motor nuclei organized in the ventral horn?

Topographically, with distal muscles located dorsal-laterally and proximal muscles located ventrally and medially.

What are the three main types of synaptic inputs to motor neurons?

Descending, interneuronal (spinal), and peripheral.

What is the role of descending inputs to motor neurons?

Voluntary movements originating from the brain, including the cerebral cortex.

What is the role of interneuronal or spinal inputs to motor neurons?

Spinal circuits that regulate the activities of large numbers of muscles involved in locomotion.

What is the role of peripheral inputs to motor neurons?

Various forms of reflexes associated with somatosensory receptors.

What comprises a motor unit?

The motor neuron, its axon, and all the muscle fibers innervated by branches of the axon.

What is the nature of the neuromuscular junction synapse?

Unusually potent; each action potential triggers an action potential in the postsynaptic muscle fiber.

What do intramuscular EMG recordings detect?

Electrical signals that emanate into the extracellular space from the action potentials propagated along the muscle fibers.

What is a motor unit action potential?

The EMG response to a single axonal action potential.

What is a twitch in the context of muscle contraction?

The force response to a single action potential.

What are the key measures used to characterize a twitch?

Peak twitch force and contraction time (CT).

What does peak twitch force represent?

The strength of the motor unit.

What does contraction time (CT) indicate?

The speed of contraction.

What accounts for the large variation in motor unit force?

The number of muscle fibers innervated by branches of the motor axon.

What is the glycogen depletion technique used for?

To identify the muscle fibers in a whole muscle that belong to just one motor unit.

What are the three main contractile properties used to categorize motor units?

Strength, speed of contraction, and extent of fatigue during sustained activity.

What are the three types of motor units?

Type S, Type FR, and Type FF.

Describe a Type S motor unit.

Slow, weak, and highly resistant to fatigue with a prolonged contraction time (CT).

Describe a Type FR motor unit.

Fast contracting, relatively resistant to fatigue, and intermediate strength.

Describe a Type FF motor unit.

Fast, strong, and fatigable.

What is the fatigue index?

The ratio of the force produced at 2 minutes to that at the beginning of stimulation.

What is the main mechanism underlying differences in the strength of motor units?

Variation in the number of muscle fibers innervated.

What factors account for differences in contractile speed and fatigue resistance across motor unit types?

Distinct biochemical profiles of the muscle fibers innervated.

What role does myosin ATPase play in muscle fiber contraction?

The rapidity with which myosin ATPase catalyzes the breakdown of ATP determines how quickly force develops.

What are the two main types of myosin ATPases?

Fast and slow.

What is the function of oxidative metabolism in muscle fibers?

Enables muscle fibers to use glucose and free fatty acids from the blood as fuels for long periods of time, if oxygen is available.

What is the function of glycolytic metabolism in muscle fibers?

Uses glycogen stored in the muscle fibers as a fuel source, does not require oxygen.

What are the histochemical profiles of Type I muscle fibers?

Slow myosin ATPase, high concentrations of oxidative enzymes, and modest levels of glycolytic enzymes.

What are the histochemical profiles of Type IIa muscle fibers?

Fast myosin ATPase, intermediate levels of both oxidative and glycolytic enzymes.

What are the histochemical profiles of Type IIb muscle fibers?

Fast myosin ATPase, high levels of glycolytic enzymes, and meager amounts of oxidative enzymes.

What is the function of the soleus muscle?

It is a postural muscle involving long-lasting, steady, and relatively weak contractions.

What is Henneman's size principle?

Orderly recruitment of motor units based on the physical dimensions of motor neurons.

According to Ohm's Law, what is the relationship between voltage, current, and resistance?

Voltage = Current x Resistance

What does the input resistance of a neuron represent?

The overall resistance of a neuron to the passage of current.

What are the factors that influence input resistance?

Axial resistance (inversely related to cross-sectional area) and membrane resistance (inversely related to the number of leak channels).

What is rate coding?

The increase in the rate of discharge of a motor neuron in response to additional current once it has been recruited.

How does rate coding impact motor unit force?

The contraction force of the muscle fibers increases markedly with increases in the rate of action potentials delivered by the motor neuron.

List the types of Motor Units

Type S (slow), Type FR (fast, resistant), Type FF (fast, fatigable), and Type Fint (fast, intermediate).

Define motor unit recruitment

The progressive activation of motor units to generate increasing levels of force.

What is the result of increased synaptic excitation

Motor neurons emit action potentials at higher rates.

What is recruitment order based upon?

Progresses from weakest, slowest, and most fatigue resistant motor units to strongest, fastest, and most fatigable.

Name the neurophysiologist and trailblazer in the study of motor unit physiology

Elwood Henneman

How did Derek Denny-Brown record motor unit action potentials?

Using a needle electrode while human subjects performed graded muscle contractions.