Motor Neurons and Motor Units

Flashcards on motor neurons, motor units, and recruitment.

Motor Neurons

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

Motor Nucleus

The collection of motor neurons that supply a given muscle.

Gamma Motor Neurons

Small intrafusal muscle fibers of the muscle spindle are innervated by these motor neurons.

Alpha Motor Neurons

Main muscle fibers of a muscle are supplied by these motor neurons.

Ventral Horn

Where do motor neurons reside in the spinal cord?

Descending, Interneuronal/Spinal, Peripheral

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

Voluntary Movements

What is the role of descending inputs to motor neurons?

Interneuronal/Spinal Inputs

Inputs that arise from interneurons within the spinal cord itself.

Peripheral Inputs

Inputs from various somatosensory receptors that make synaptic contacts onto motor neurons.

Motor Unit

A motor neuron, its axon, and all the muscle fibers innervated by branches of the axon constitutes this functional entity.

Spiking activities of a few motor units

A thin needle electrode is placed into the muscle in order to record this.

Intramuscular electromyographic (EMG) recordings

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

Twitch

The force response to a single action potential.

Peak twitch force, Contraction Time (CT)

Name two key measurements made to characterize a twitch.

The strength of the motor unit

What does the peak twitch force represent?

The speed of contraction

What does the contraction time (CT) indicate?

Wide range of contractile properties among motor units making up a muscle

What is a consistent observation of experiments done in many mammalian species, including humans, regarding contractile properties?

Glycogen Depletion Technique

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

Number of muscle fibers innervated by branches of the motor axon

What is the variable that most readily accounts for the large variation in motor unit force?

Strength, Speed of Contraction, Extent of Fatigue

List three important contractile properties of motor units.

Type S

Motor unit type: slow, weak, and highly resistant to fatigue.

Type FR

Motor unit type: fast contracting, relatively resistant to fatigue, intermediate strength.

Type FF

Motor unit type: fast, strong, and fatigues rapidly.

Fatigue Index

The ratio of the force produced at 2 minutes to that at the beginning quantifies the extent of fatigue in motor units.

Variation in the number of muscle fibers innervated

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

Myosin ATPase

What biochemical element accounts for differences in contractile speed and fatigue resistance across motor unit types?

Type I fibers

Fibers with slow myosin ATPase, high oxidative enzymes, and modest glycolytic enzymes that are innervated by type S motor units.

Type IIa (or IIx in humans) fibers

Fibers with fast myosin ATPase, intermediate oxidative and glycolytic enzymes that make up the type FR motor units.

Type IIb fibers

Fibers with fast myosin ATPase, high glycolytic enzymes, and meager oxidative enzymes that are innervated primarily by type FF motor units.

Soleus

Muscle that typically has a high proportion of type S motor units, making it highly resistant to fatigue.

Which motor units to activate to achieve a desired level of muscle force

What is the major challenge that confronts the CNS in regard to motor units?

Linear Sum

The total muscle force is equivalent to this of the individual motor unit forces

Motor units recorded were activated in the same sequence regardless of the type of contraction

What did Derek Denny-Brown observe about motor units during graded muscle contractions?

"First on" is "last off," and "last on" is "first off"

What is the general rule of thumb related to recruitment and derecruitment of motor units?

low, high

Early recruited motor units will have force thresholds and later recruited units will have thresholds.

Strongest

Motor units are activated in a highly organized way: from those that are the weakest toward those that are the _.

Orderly Recruitment

Term that describes activating motor units from weakest to strongest.

Practically the same synaptic input

For a given motor nucleus, do all motor neurons receive distinct inputs or the same synaptic input?

Intrinsic Excitability

Even with similar synaptic input to motor neurons, variations exist in the motor neurons' _.

Rheobase Current

The minimum current needed to bring a neuron to the spiking threshold.

~15 to 20 mV

What is the typical range of membrane depolarization needed to reach the spiking threshold (∆VTh)?

Input Resistance (Ri)

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

∆VTh = Irh × Ri

What is the relationship between rheobase current (Irh) and input resistance (Ri)?

Axial resistance is inversely related to cross-sectional area.

Why do larger-diameter neurons have lower axial resistances?

Henneman’s Size Principle

The set of ideas that explain orderly recruitment of motor units based on the physical dimensions of motor neurons.

Small motor neurons have high input resistance and thin axons, while large motor neurons have low input resistance and thick axons

Describe the general relationship between motor neuron size, input resistance, and axon diameter.

Rate of Discharge

Once a motor neuron is recruited, it responds to additional current by increasing this.

Contraction force increases markedly with increases in the rate of action potentials

What is the impact of rate coding on the modulation of motor unit force?

Temporal Summation

The term for tetanic responses caused by the temporal summation of many individual twitches, lead to progressively larger forces with higher frequencies.

Rate Coding, Recruitment

What two processes dictate the intensity of muscle contraction?

Transduction Process

How is biological signals (i.e., action potentials delivered to muscle fibers) transformed into mechanical contractile forces