Figure 24-1. The locations of vestibulospinal, reticulospinal, and rubrospinal tracts at a representative cervical level of the spinal cord. Medial vestibulospinal fibers are located in the medial longitudinal fasciculus. The general organization and somatotopy of motor neuron pools are shown on the left.
In addition to sensory feedback, the activity of lower motor neurons in the spinal cord is greatly influenced by descending projections from cells in the brainstem and cerebral cortex. These brainstem and cortical neurons are referred to as upper motor neurons, and unlike lower motor neurons, they have no direct synaptic link with muscles. Because of their origin, these descending projections are also called supraspinal systems.
Anterior horn motor neurons represent the only direct link (the final common path) between the nervous system and skeletal muscle. As such, these neurons play a central role in the production of movement. The regulation of motor neuron activity by peripheral sensory input and descending brainstem influences is crucial to the performance of normal, synergistic, and productive movements.
There are two varieties of anterior horn motor neurons, alpha and gamma, which are intermingled within the anterior horn. Alpha motor neurons innervate the ordinary, working fibers of skeletal muscles called extrafusal fibers, and gamma motor neurons innervate a special type of skeletal muscle fiber, the intrafusal fibers, which are found only within muscle spindles. Recall that the anterior horn also contains small interneurons whose axons distribute locally within the spinal gray. Interneurons are numerous in the intermediate zone and anterior horn and are functionally essential in the regulation of alpha and gamma motor neurons. Their action on motor neurons may be either excitatory or inhibitory.
The axons of both types of anterior horn motor neurons exit the spinal cord via the anterior roots and course distally in peripheral nerves. These fibers represent the final common path that links the nervous system and skeletal muscles. As the axon of an alpha motor neuron reaches the muscle it innervates, it loses its myelin sheath and forms a series of flattened boutons that indent the surface of a group of muscle fibers. This specialized type of synapse is called a neuromuscular junction or motor end plate (Fig. 24-2). Damage to or loss of the cell bodies of alpha motor neurons (also called lower motor neurons) or lesions of their axons result in a profound weakness of the skeletal muscles innervated and loss of reflexes (areflexia).
Figure 24-2. The structural elements related to the axon terminal at the neuromuscular junction (A) and the details of the synaptic cleft, its receptors, and related elements that enhance transmission and then hydrolyze the transmitter into acetate and choline (B).
Like synapses in the central nervous system, the junction between a motor axon and skeletal muscle fibers consists of presynaptic and postsynaptic components (Fig. 24-2). The presynaptic element, the axon terminal, contains round, clear synaptic vesicles (filled with the neurotransmitter acetylcholine), mitochondria, and small patches of dense material around which the vesicles aggregate at the active site. The presynaptic element is separated from the postsynaptic element by an extracellular space called the synaptic cleft. The postsynaptic membrane, the specialized portion of the muscle cell plasma membrane subjacent to the axon terminal, exhibits a large number of folds that effectively increase the surface area of the muscle cell in contact with the axon terminal (Fig. 24-2). These irregularities, called subjunctional folds, contain nicotinic acetylcholine receptors on their summit facing into the synaptic cleft (Fig. 24-2). These nicotinic acetylcholine receptors are integral membrane proteins with an extracellular domain that actually binds the acetylcholine molecule and a membrane-spanning domain that forms an ion channel (Fig. 24-2B). Such receptors are called ionotropic receptors because binding of the neurotransmitter molecule to the extracellular domain typically opens the ion channel and allows the passage of sodium and potassium ions. Thus the receptor and its associated ion channel mediate the ion flux that underlies the transmission of electrical signals from nerve to muscle. Surrounding the exterior surface of the muscle is a basal lamina that extends into the synaptic cleft, where it becomes continuous with a basal lamina formed by the Schwann cell process that encloses the axon terminal (Fig. 24-2).
When an action potential depolarizes the presynaptic element, there is an influx of calcium through voltage-gated membrane channels. Synaptic vesicles fuse with the presynaptic membrane at the active sites (which are marked by structures called dense bars) and release acetylcholine into the synaptic cleft. The transmitter binds to receptors on the postsynaptic membrane and opens ion channels. Ion flux then occurs, and a depolarizing potential called an end plate potential spreads over the surface of the muscle fiber. This potential triggers the release of calcium (from the sarcoplasmic reticulum), which elicits the movement of actin and myosin filaments, resulting in muscle contraction. Synaptic transmission is terminated by an enzyme called acetylcholinesterase, which is located in the matrix of the basal lamina in the depths of the postjunctional folds. This enzyme inactivates acetylcholine by detaching it from its receptor and hydrolyzing it to acetate and choline.
Myasthenia gravis (MG) is a disease of the neuromuscular junction. It is characterized by antibodies that bind to nicotinic acetylcholine receptors and probable lysis of postsynaptic receptors. Patients with MG may experience weakness that waxes and wanes during minutes, hours, or days; it most frequently affects ocular muscles first (about 65% of patients) and in some cases will remain confined to the extraocular muscles (about 15%, ocular myasthenia). General weakness of limb musculature occurs in about 85% of patients but rarely without concurrent ocular or brainstem (facial, tongue) involvement. If weakness of the respiratory muscles (intercostal, diaphragm) occurs, it is called a myasthenic crisis and may require rapid intervention.
Each muscle fiber receives only one motor end plate, but the number of muscle fibers innervated by a single alpha motor neuron axon varies from a few to many. The aggregate of a motor neuron axon and all the muscle fibers it innervates is called a motor unit (Fig. 24-3). In general, as the need for fine control of a muscle increases, the size or innervation ratio of its motor unit decreases. That is, the number of muscle fibers innervated by a single axon decreases. The size of a motor unit is also related to the mass of the muscle and its speed of contraction. Small muscles that generate low levels of force typically have small motor units (10 to 100 muscle fibers per motor neuron axon), whereas large, powerful muscles that generate high levels of force are usually innervated by large motor units (600 to 1000 muscle fibers per motor neuron axon). An example of small motor units are the extraocular muscles and their innervation by oculomotor, trochlear, and abducens motor neurons. On the other hand, the quadriceps muscles (rectus femoris and vastus medialis, lateralis, and intermedius) and their motor neurons in the lumbosacral spinal cord are examples of large motor units. Large muscles also contain some small motor units; the small units are initially recruited for precision, and the larger units are recruited later in the movement for increased strength.
Motor units can be divided into two categories (slow twitch and fast twitch) on the basis of the metabolic and physiologic properties of the muscle fibers and their innervation. Type I units are composed of “red” (dark) muscle fibers referred to as slow-twitch (S) fibers. These muscles are rich in mitochondria and contain a (red) heme protein that helps bind and store oxygen. Because of their ability to use glucose and oxygen from the bloodstream, these fibers can generate abundant adenosine triphosphate (aerobic metabolism) and fuel the contractile apparatus for long periods of contraction time, making these motor units resistant to fatigue. The trade-off, however, is that these muscle fibers can generate only relatively small levels of force or tension. The postural muscles (deep back muscles) are composed predominantly of this fiber type; these muscles may contract at a low level of tension but for exceedingly long times.
In contrast, the type II or fast-twitch units (white or pale muscles) generate much higher levels of force but for comparatively brief periods. Muscles used during strenuous exercise are examples of type II fibers: they contract with greater force than postural muscles but for shorter periods. The fast-twitch fibers actually come in two varieties. The fast-fatigable type (type IIb or FF) contains large stores of glycogen that provide the energy necessary to phosphorylate adenosine diphosphate (glycogen converted to lactic acid) and produce relatively greater amounts of force compared with slow-twitch fibers. However, the rapid depletion of glycogen coupled with the accumulation of lactic acid (anaerobic catabolism) contributes to the relatively brief contraction time. A second fast-twitch unit (type IIa) is actually intermediate between the type I slow-twitch and type II fast-twitch units because it exhibits sufficient aerobic capacity to resist fatigue yet is able to generate nearly as much force as the type IIb units. These units are referred to as fast fatigue-resistant fibers (FFR).
Muscles generally contain a mixture of motor units, and the proportions vary according to the demands placed on the muscle. For example, the soleus muscle is a slow-twitch postural muscle containing mainly S-type units. The relatively slow conduction time of the small-diameter alpha axons serving these motor units is adequate for the demands of this muscle. By contrast, the gastrocnemius muscle is a dynamic, powerful muscle used in running and jumping. It is considered to be a fast-twitch muscle and contains mainly FF motor units innervated by large-diameter, rapidly conducting axons.
The nervous system uses the size and functional properties of motor units as a means of grading the force of muscle contraction. When an excitatory input reaches a group of motor neurons in the anterior horn, that input will produce a larger change in the membrane potential of the smaller motor neurons than in the larger motor neurons. This is because the firing threshold of a neuron is determined by its total electrical resistance, which is inversely proportional to its surface area. Therefore, a given synaptic input to a pool of motor neurons will first recruit the smaller neurons (linked to small motor units) followed in sequence by progressively larger cells (and larger motor units). This is known as the size principle of motor neuron recruitment. Thus in a fine movement that requires sustained output with little force, the smaller cells and the small motor units (slow twitch) are activated first. As the need for a more forceful rapid movement increases, progressively larger cells and larger motor units (fast twitch) are activated, and the movement transitions smoothly from low force to high force with strong bursts of contraction. The force of contraction is also influenced by the firing rate of the participating motor neurons. As the requirement for greater force and speed of contraction increases, the synaptic input increases and recruits more of the larger neurons. The firing rate of the activated larger motor neurons also increases and enhances the speed and force of the movement.
Signals that transmit information from skeletal muscles into the nervous system enter the spinal cord via the posterior roots. For the most part, these signals are generated in specialized structures in muscles called neuromuscular spindles (commonly called muscle spindles; also known as Kühne spindles). The output of the muscle spindle signals a change in muscle length and the rate of change in muscle length.
A muscle spindle (Fig. 24-4) is a long, thin, encapsulated structure that typically contains about seven intrafusal muscle fibers, which are striated. Spindles range in length from 4 to 10 mm. The capsule of the spindle (along with its intrafusal muscle fibers) is attached to and oriented in parallel with the extrafusal fibers that constitute the bulk of the muscle.
Figure 24-4. Structure of a muscle spindle, the relation of afferent and efferent nerve fibers to the intrafusal muscle fibers of the spindle and the surrounding extrafusal muscle fibers (A), and the functional features of the spindle under certain conditions (B, C, D). Passive stretch of the muscle (B), as in a muscle stretch reflex, results in a stretch of the spindle and activation of the Ia fiber, which activates the alpha motor neuron, and the extrafusal fibers contract. The gamma motor neuron is not active, and the alpha motor neuron becomes active after receiving Ia input. Stimulation of just the alpha motor neuron (C) activates the extrafusal fibers, the extrafusal fibers contract, but the spindle is slack; except for background activity, the gamma motor neuron and the Ia fiber are silent. Under conditions of normal voluntary muscle contraction (D), the alpha and gamma motor neurons are activated by supraspinal projections, and the functional integrity of the spindle is maintained; all three fibers are active.
There are two basic types of intrafusal fibers: nuclear bag fibers and nuclear chain fibers (Fig. 24-4; Table 24-1). Like other skeletal muscle cells, intrafusal fibers are multinucleated, and the arrangement of the nuclei is the most obvious structural feature distinguishing the two types. In both types, the nuclei occupy the central (equatorial) region of the cell. In nuclear bag fibers, the nuclei are clustered centrally and give the equatorial region a swollen appearance. In nuclear chain fibers, the nuclei are arranged in a linear row, and the equatorial region is not obviously expanded. The contractile elements of both types of cells are located entirely in the two distal (polar) regions of the cell.
Table 24-1 The Muscle Spindle
The two ends of the spindle are anchored to extrafusal muscle fibers (Fig. 24-4A–D). This arrangement allows two conditions under which the equatorial region of the spindle may be stretched and the afferent fibers of the spindle activated. First, passive stretch of extrafusal muscle fibers will elongate the spindle, stretch its equatorial region, and activate its afferent fibers. Second, the intrafusal muscle fibers, when activated by a gamma motor neuron, will contract, stretch the equatorial region of the spindle, and also result in activation of its afferent fiber (Fig. 24-4A-D).
The two types of intrafusal fibers perform different sensory functions. The nuclear bag fibers are actually subdivided into two different categories that have different elastic properties and, correspondingly, different functions (Table 24-1). One type, the dynamic nuclear bag fiber, is sensitive mainly to the rate of change in muscle length. The other, the static nuclear bag fiber, signals only a change in muscle length but not the rate of that change. Nuclear chain fibers, like static bag fibers, are mainly sensitive to changes in muscle length (Table 24-1).
Intrafusal muscle fibers are associated with two types of sensory fibers, the terminals or receptive ends of which are concentrated at the equatorial (noncontractile) region of intrafusal fibers. The type Ia fiber is heavily myelinated, has a conduction velocity of 80 to 120 m/s, and is typically associated with dynamic and static nuclear bag fibers (Fig. 24-4A; Table 24-1). The distal end of this sensory fiber is wrapped around the central (noncontractile) region of the intrafusal muscle fibers. Because of this relationship, the type Ia afferent terminations are called annulospiral endings