The Spinal Cord: The First Hierarchical Level

The spinal cord is the first level of the motor hierarchy.  It is the site where motor neurons are located.  It is also the site of many interneurons and complex neural circuits that perform the “nuts and bolts” processing of motor control.  These circuits execute the low-level commands that generate the proper forces on individual muscles and muscle groups to enable adaptive movements.  The spinal cord also contains complex circuitry for such rhythmic behaviors as walking.  Because this low level of the hierarchy takes care of these basic functions, higher levels (such as the motor cortex) can process information related to the planning of movements, the construction of adaptive sequences of movements, and the coordination of whole-body movements, without having to encode the precise details of each muscle contraction.

Motor Neurons

Alpha motor neurons (also called lower motor neurons) innervate skeletal muscle and cause the muscle contractions that generate movement.  Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction.  When the acetylcholine binds to acetylcholine receptors on the muscle fiber, an action potential is propagated along the muscle fiber in both directions (see Chapter 4 of Section I for review).  The action potential triggers the contraction of the muscle.  If the ends of the muscle are fixed, keeping the muscle at the same length, then the contraction results on an increased force on the supports (isometric contraction).  If the muscle shortens against no resistance, the contraction results in constant force (isotonic contraction).  The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, and the motor neurons that control head and facial movements are located in the motor nuclei of the brainstem.  Even though the motor system is composed of many different types of neurons scattered throughout the CNS, the motor neuron is the only way in which the motor system can communicate with the muscles.  Thus, all movements ultimately depend on the activity of lower motor neurons.  The famous physiologist Sir Charles Sherrington referred to these motor neurons as the “final common pathway” in motor processing.

Spinal cord with motor neuron in anterior horn.

Figure 1.3

Motor neurons are not merely the conduits of motor commands generated from higher levels of the hierarchy.  They are themselves components of complex circuits that perform sophisticated information processing.  As shown in Figure 1.3, motor neurons have highly branched, elaborate dendritic trees, enabling them to integrate the inputs from large numbers of other neurons and to calculate proper outputs. 

Two terms are used to describe the anatomical relationship between motor neurons and muscles: the motor neuron pool and the motor unit. 

1. Motor neurons are clustered in columnar, spinal nuclei called motor neuron pools (or motor nuclei).  All of the motor neurons in a motor neuron pool innervate a single muscle (Figure 1.4), and all motor neurons that innervate a particular muscle are contained in the same motor neuron pool.  Thus, there is a one-to-one relationship between a muscle and a motor neuron pool.

2. Each individual muscle fiber in a muscle is innervated by one, and only one, motor neuron (make sure you understand the difference between a muscle and a muscle fiber).  A single motor neuron, however, can innervate many muscle fibers.  The combination of an individual motor neuron and all of the muscle fibers that it innervates is called a motor unit.  The number of fibers innervated by a motor unit is called its innervation ratio.

If a muscle is required for fine control or for delicate movements (e.g., movement of the fingers or hands), its motor units will tend to have small innervation ratios.  That is, each motor neuron will innervate a small number of muscle fibers (10-100), enabling many nuances of movement of the entire muscle.  If a muscle is required only for coarse movements (e.g., a thigh muscle), its motor units will tend to have a high innervation ratio (i.e., each motor neuron innervating 1000 or more muscle fibers), as there is no necessity for individual muscle fibers to undergo highly coordinated, differential contractions to produce a fine movement.Control of Muscle Force

A motor neuron controls the amount of force that is exerted by muscle fibers.  There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle.

1. Rate Code.  Motor neurons use a rate code to signal the amount of force to be exerted by a muscle.  An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates.  This code is illustrated in Figure 1.5.  When the motor neuron fires a single action potential (Play 1), the muscle twitches slightly, and then relaxes back to its resting state.  If the motor neuron fires after the muscle has returned to baseline, then the magnitude of the next muscle twitch will be the same as the first twitch.  However, if the rate of firing of the motor neuron increases, such that a second action potential occurs before the muscle has relaxed back to baseline, then the second action potential produces a greater amount of force than the first (i.e., the strength of the muscle contraction summates) (Play 2).  With increasing firing rates, the summation grows stronger, up to a limit.  When the successive action potentials no longer produce a summation of muscle contraction (because the muscle is at its maximum state of contraction), the muscle is in a state called tetanus (Play 3).

2. Size Principle.  When a signal is sent to the motor neurons to execute a movement, motor neurons are not all recruited at the same time or at random.  The motor neuron size principle states that, with increasing strength of input onto motor neurons, smaller motor neurons are recruited and fire action potentials before larger motor neurons are recruited.  Why does this orderly recruitment occur?  Recall the relationship between voltage, current, and resistance (Ohm’s Law): V = IR.  Because smaller motor neurons have a smaller membrane surface area, they have fewer ion channels, and therefore a larger input resistance.  Larger motor neurons have more membrane surface and correspondingly more ion channels; therefore, they have a smaller input resistance.  Because of Ohm’s Law, a small amount of synaptic current will be sufficient to cause the membrane potential of a small motor neuron to reach firing threshold, while the large motor neuron stays below threshold.  As the amount of current increases, the membrane potential of the larger motor neuron also increases, until it also reaches firing threshold. 

Figure 1.6 demonstrates how the size principle governs the amount of force generated by a muscle.  Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle (Play 1).  With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle (Play 2 and Play 3).  Moreover, different types of muscle fibers are innervated by small and larger motor neurons.  Small motor neurons innervate slow-twitch fibers; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers; and large motor neurons innervate fast-twitch, fatigable muscle fibers.  The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods.  These fibers are used for maintaining posture and making other low-force movements.  Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons.  These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers.  Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated.  These fibers produce large amounts of force, but they fatigue very quickly.  They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism. Most muscles contain both fast- and slow-twitch fibers, but in different proportions.  Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers.

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