Keywords
size principle, spasticity, Babinski sign, fasciculate, atrophy, amyotrophic lateral sclerosis
Chapter Outline
Each Lower Motor Neuron Innervates a Group of Muscle Fibers, Forming a Motor Unit, 118
Lower Motor Neurons Are Arranged Systematically, 118
There Are Three Kinds of Muscle Fibers and Three Kinds of Motor Units, 118
Motor Units Are Recruited in Order of Size, 119
Motor Control Systems Involve Both Hierarchical and Parallel Connections, 119
The Corticospinal Tract Has Multiple Origins and Terminations, 120
Corticospinal Axons Arise in Multiple Cortical Areas, 120
There Are Upper Motor Neurons for Cranial Nerve Motor Nuclei, 121
The firing rates of our motor neurons, and therefore the states of contraction of our muscles, are determined by multiple influences. Simple reflex arcs like the stretch reflex and more complex motor programs like the basic pattern generator for walking are built into the spinal cord and brainstem. Various descending pathways influence these reflex arcs and motor programs, as well as the motor neurons themselves. Finally, activity in the descending pathways is modulated by other cortical areas including the basal ganglia (see Chapter 19 ) and the cerebellum (see Chapter 20 ).
Each Lower Motor Neuron Innervates a Group of Muscle Fibers, Forming a Motor Unit
Each lower motor neuron (LMN) innervates a fraction of the muscle fibers in one muscle. The combination of a lower motor neuron and all the muscle fibers it innervates is a motor unit ( Fig. 18.1 ). Although there is a range of motor unit sizes in every muscle, their average size varies in a predictable way—those involving finely controlled muscles (e.g., extraocular muscles) contain very few muscle fibers, and those involving less finely controlled muscles may have hundreds of muscle fibers (e.g., gluteus maximus). All LMNs utilize acetylcholine as their primary neurotransmitter. When acetylcholine is released from the LMN, it interacts at nicotinic channels on the skeletal muscle resulting in a depolarization of the skeletal muscle, a chain of rapid events, and muscle contraction. Acetylcholine activity is rapidly terminated by acetylcholinesterase found in the neuromuscular synapse.
Lower Motor Neurons Are Arranged Systematically
The cell bodies of LMNs are arranged systematically in the anterior horn (just as things like body parts and retinal areas are represented systematically in pathways and cortical areas). At any given spinal level, motor neurons for more proximal muscles are located medial to those for more distal muscles, and motor neurons for flexors are located dorsal to those for extensors ( Fig. 18.2 ).
There Are Three Kinds of Muscle Fibers and Three Kinds of Motor Units
We use most muscles for multiple purposes that require different forces of contraction, from relatively weak contractions used in holding positions for long periods of time (e.g., standing) to powerful contractions that cannot be sustained for very long (e.g., sprinting, jumping). Corresponding to this, there are three different kinds of muscle fibers: slow fibers ( S ) that produce little force but do not fatigue much, fast fibers ( FF ) that produce a lot of force but fatigue quickly, and intermediate fibers ( FR ) with intermediate properties. All the muscle fibers innervated by a given motor neuron are of the same type, so there are also three kinds of motor units ( Table 18.1 ).
| Type | Force | Fatigability | Recruitment Order |
|---|---|---|---|
| S | Small | Low | Early |
| FR | Intermediate | Intermediate | Middle |
| FF | Large | High | Late |
The difference in fatigability of the motor units is related to the muscle fiber types contained in the motor unit. Different muscle fiber types have different metabolic properties. There are two types of muscle fibers: type I muscle fibers are mostly oxidative and are useful to maintain contraction for long periods of time; type II muscle fibers are primarily glycolytic and are further subdivided in type IIa (contains some oxidative too) and type IIb are mainly glycolytic. Hence, the S motor units primarily contain type I (oxidative) fibers, the FR motor units contain primarily type IIa fibers, whereas FF motor units contain primarily type IIb (glycolytic) fibers. Most muscles contain a variety of all fiber types, but the proportions of each may vary greatly.
Motor Units Are Recruited in Order of Size
If you were designing a motor control system, you would probably set it up so that increments of force produced by muscles were proportional in some way to the force already present—for example, making each increase in force 1% of the force already being produced by that muscle. That would make it easier to have fine control of movement. (Consider one alternative, that of having the increments be a fixed amount of force. The result would be adding a relatively big twitch to weak contractions and a trivial increase to strong contractions.) In fact, this proportional addition of force happens automatically because the size of the cell body of an LMN is proportional to the kind and number of muscle fibers in its motor unit. It takes less synaptic input to bring a small neuron to threshold, so as upper motor neurons increase their firing frequency, motor units are recruited in order of the amount of force they produce. This size principle ensures the smooth gradation of muscle contraction.
Motor Control Systems Involve Both Hierarchical and Parallel Connections
Reflex and motor program connections provide some of the inputs to lower motor neurons.
Upper motor neurons control lower motor neurons both directly and indirectly.
Association cortex, the cerebellum, and the basal ganglia modulate motor cortex.
