Ch-14 Motor Neurons and Motor Units

Anatomy of Motor Nuclei

Motor neurons are the last-order neurons in the CNS.
Their axons exit the CNS to target muscle fibers, causing muscle contraction.
Motor neurons reside in:
- Spinal cord: Control limb and body muscles.
- Brain stem: Control face, mouth, eye, and tongue muscles.
A motor nucleus is a collection of motor neurons that supply a given muscle.
Motor nuclei in the spinal cord form longitudinal columns.
Types of motor neurons:
- Gamma motor neurons: Innervate intrafusal muscle fibers of the muscle spindle.
- Alpha motor neurons: Supply the main muscle fibers (unless otherwise noted, "motor neuron" refers to alpha motor neurons).
The number of motor neurons within a motor nucleus is relatively small (a few hundred).
Bigger muscles tend to have larger numbers of motor neurons.
Motor neurons reside in the ventral horn of the spinal cord.
Motor nuclei supplying different muscles occupy different regions of the ventral horn and are topographically organized.
In the lumbar spinal cord, motor nuclei supplying distal foot muscles are located dorsal-laterally.
Motor nuclei innervating progressively more proximal muscles are located more ventrally and medially.

Inputs to Motor Neurons

Motor neurons have large dendritic arbors and receive many synaptic contacts (10,000 to 50,000).
Three main types of synaptic inputs:
- Descending:
  - Arise from neurons in the brain, including the cerebral cortex.
  - Important for voluntary movements.
- Interneuronal (spinal):
  - Arise from interneurons within the spinal cord.
  - May be the largest source of inputs.
  - Associated with spinal circuits for complex processes like locomotion.
  - The spinal cord is not simply a conduit between the brain and periphery.
- Peripheral:
  - Arise from somatosensory receptors.
  - Often associated with reflexes.

Motor Units

A motor unit consists of a motor neuron, its axon, and all the muscle fibers innervated by the axon's branches.
In adult muscle, each muscle fiber receives input from just one motor neuron.
The neuromuscular junction is a potent synapse; each action potential triggers an action potential in the postsynaptic muscle fiber.
Each action potential in a motor neuron will activate all its innervated muscle fibers, causing them to contract together.
The motor unit is the indivisible output element of the nervous system.
The activity of single motor neurons is amplified through the simultaneous activation of hundreds of muscle fibers.
Intramuscular electromyographic (EMG) recordings detect electrical signals from action potentials propagated along muscle fibers.
Motor neurons are the only neurons of the CNS that can be recorded in awake, healthy human subjects due to the accessibility of skeletal muscle and motor unit configuration.

Contractile Properties of Motor Units

Mechanical properties of motor units are studied by stimulating single motor axons and recording the force response and EMG response.
The force response to a single action potential is called a twitch.
The time course of the twitch is longer than the motor unit action potential due to the relatively slow time course of calcium release and reuptake.
Key measurements for characterizing a twitch:
- Peak twitch force: Strength of the motor unit.
- Contraction time (CT): Speed of contraction.

Twitch Force

There is a wide range of contractile properties among motor units within a muscle.
The distribution of motor units based on twitch force is highly skewed, with many small-force units and few large-force units.
The force range of motor units within a muscle is large (greater than 100-fold).
The number of muscle fibers innervated by branches of the motor axon is the variable that most readily accounts for the large variation in motor unit force.
Greater the number of muscle fibers belonging to a motor unit, the stronger it is.
Glycogen depletion technique is used to identify muscle fibers belonging to a single motor unit.

Motor Unit Types

Three important contractile properties:
- Strength.
- Speed of contraction.
- Extent of fatigue during sustained activity.
Three categories (or types) of motor units often can be identified based on these
- Type S (slow):
  - Prolonged CT.
  - Weak.
  - Highly resistant to fatigue.
- Type FR (fast, relatively resistant):
  - Fast contracting.
  - Relatively resistant to fatigue.
  - Intermediate strength.
- Type FF (fast, fatigable):
  - Fast.
  - Strong.
  - Fatigue rapidly.
Fatigue index: Ratio of the force produced at 2 minutes to that at the beginning of stimulation; quantifies the extent of fatigue.

Factors Determining CT and Fatigability

Differences in contractile speed and fatigue resistance arise from distinct biochemical profiles of muscle fibers.
Development of force depends on interactions between actin and myosin filaments, requiring energy from ATP breakdown.
The rapidity with which myosin ATPase breaks down ATP is a critical determinant of how quickly force develops.
Two main types of myosin ATPases:
- Fast: Found in fast motor unit types (FF and FR).
- Slow: Found in type S motor units.
Muscle fibers with high concentrations of enzymes for oxidative metabolism can use glucose and fatty acids for long periods.
Muscle fibers relying on glycolytic metabolism use glycogen, which can be rapidly depleted.
Histochemical profiling identifies different biochemical types of muscle fibers:
- Type I: Slow myosin ATPase, high oxidative enzymes, modest glycolytic enzymes (innervated by type S motor units).
- Type IIa (IIx in humans): Fast myosin ATPase, intermediate oxidative and glycolytic enzymes (type FR motor units).
- Type IIb: Fast myosin ATPase, high glycolytic enzymes, meager oxidative enzymes (type FF motor units).
The main factor underlying variation in strengths of different motor unit types is the number of fibers innervated, not fiber size.

Motor Unit Populations

Different muscles may have varying proportions of the three main types of motor units, endowing individual muscles with distinct contractile properties.
Soleus muscle: High proportion of type S motor units, highly resistant to fatigue (fitting for a postural muscle).
The proportion of motor unit types can vary for the same muscle in different individuals.
Individuals with high proportions of type FF units in the quadriceps may have an advantage in activities requiring explosive muscle contractions.
Those with a high prevalence of type S motor units would likely have an advantage in endurance activities.

The Selection Problem

The CNS faces the challenge of selecting which motor units to activate to achieve a desired level of muscle force.
Total muscle force is approximately the linear sum of individual motor unit forces.
Random selection of motor units would be computationally challenging for the CNS.

Fixed Sequence of Recruitment

Motor units are recruited in a fixed sequence, regardless of the type of contraction.
During decreasing force phase of the contraction, motor units turn off in the opposite sequence to that which they were recruited such that “first on” is “last off ” and “last on” is “first off”.

Orderly Recruitment

A motor unit starts firing once a certain level of isometric force is attained (force threshold).
Early recruited motor units have low force thresholds, and later recruited units have high thresholds.
Motor units are activated in a highly organized way: from weakest to strongest (orderly recruitment).
Fine resolution of force is an in-built control feature; subtle adjustments in force can be accomplished by drawing upon a large population of weak motor units.

Uniformity of Synaptic Input to Motor Neurons

All motor neurons in a motor nucleus receive practically the same synaptic input.
Evidence comes from anatomical and electrophysiological studies.
Axons of individual neurons ramify extensively to contact a large proportion of the motor neurons making up a motor nucleus.

Differences in Intrinsic Excitability Determine Recruitment Order

There are systematic variations in the intrinsic excitability of motor neurons. Intrinsic excitability is how readily individual neurons are depolarized in response to the same level of excitatory synaptic (or injected) current.
Rheobase current: Minimum current needed to bring a neuron to the spiking threshold.
Neurons with high resistances require less current to be brought to the threshold than neurons with lower resistances such that \Delta V{Th} = I{rh} \times R_i, where
- \Delta V_{Th} is the change in threshold voltage
- I_{rh} is the rheobase current
- R_i is the input resistance
Rearranging the above equation, I{rh} = \frac{\Delta V{Th}}{R_i}, suggesting that the greater the input resistance, the lower the reobase current, and vice versa.
Axial resistance is inversely related to cross-sectional area; large-diameter neurons have lower resistances.
Total membrane resistance is inversely related to the number of leak channels in the membrane.

Henneman’s Size Principle

Small-diameter motor neurons have high input resistance, while large motor neurons have low input resistance.
The degree of membrane depolarization in any neuron will be the product of the synaptic current received and the input resistance of the neuron.
Motor neurons are activated in order from smallest to largest.
Motor units associated with the smallest motor neurons produce the weakest forces, while those with the biggest neurons have the strongest motor units.
Motor unit recruitment progresses “automatically” from weakest to strongest.
Smallest units are Type S motor units; during weak muscle contractions, most of the motor units activated will be of that type.
Fatigue-resistant Type S motor units will be the main contributors to such activity, enabling long-lasting contractions with little fatigue.
The set of ideas that explain the orderly recruitment of motor units based on the physical dimensions of motor neurons is referred to as Henneman’s size principle.

Rate Coding

Once sufficient current has been delivered to a motor neuron to recruit it, it responds to additional current by increasing its rate of discharge.
The relationship between steady-state frequency and the intensity of injected current is often linear.

Impact of Rate Coding on Modulation of Motor Unit Force

The contraction force of the muscle fibers supplied by a motor neuron increases markedly with increases in the rate of action potentials delivered by the motor neuron.
Rate coding can provide profound modulation of force produced by single motor units.
Temporal summation of many individual twitches leads to progressively larger forces with higher frequencies.
A sigmoid-shaped relationship forms when steady-state force is plotted as a function of the frequency of action potentials delivered to muscle fibers.
The "code" used to dictate the intensity of muscle contraction involves both rate coding and recruitment.
As the degree of synaptic excitation increases, progressively more motor units are recruited, increasing the total muscle force.
At the same time, once the motor units are recruited, increasing the synaptic excitation causes their motor neurons to emit action potentials at higher rates (rate coding).

Summary

This chapter provides a detailed overview of the motor system, focusing on motor neurons and motor units. Key concepts include the anatomy of motor nuclei, inputs to motor neurons, the composition and properties of motor units, the selection problem faced by the CNS, recruitment and rate coding, and Henneman's size principle. The chapter emphasizes the importance of motor neurons as the final common pathway for behavior and highlights the mechanisms underlying the precise control of muscle force.