Spinal Cord Motor Functions; the Cord Reflexes

Anterior Motor Neurons

Located in each segment of the anterior horns of the cord gray matter are several thousand neurons that are 50 to 100% larger than most of the others and are called anterior motor neurons ( Figure 55-2 ). They give rise to the nerve fibers that leave the cord by way of the anterior roots and directly innervate the skeletal muscle fibers. The neurons are of two types, alpha motor neurons and gamma motor neurons.

Alpha Motor Neurons

The alpha motor neurons give rise to large type A alpha (Aα) motor nerve fibers, averaging 14 micrometers in diameter; these fibers branch many times after they enter the muscle and innervate the large skeletal muscle fibers. Stimulation of a single alpha nerve fiber excites from three to several hundred skeletal muscle fibers, which are collectively called the motor unit. Transmission of nerve impulses into skeletal muscles and their stimulation of the muscle motor units are discussed in Chapter 6, Chapter 7 .

Gamma Motor Neurons

Along with the alpha motor neurons, which excite contraction of the skeletal muscle fibers, about one-half as many much smaller gamma motor neurons are located in the spinal cord anterior horns. These gamma motor neurons transmit impulses through much smaller type A gamma (Aγ) motor nerve fibers, averaging 5 micrometers in diameter, which go to small, special skeletal muscle fibers called intrafusal fibers, shown in Figures 55-2 and 55-3 . These fibers constitute the middle of the muscle spindle, which helps control basic muscle “tone,” as discussed later in this chapter.

Interneurons

Interneurons are present in all areas of the cord gray matter—in the dorsal horns, the anterior horns, and the intermediate areas between them, as shown in Figure 55-1 . These cells are about 30 times as numerous as the anterior motor neurons. They are small and highly excitable, often exhibiting spontaneous activity and capable of firing as rapidly as 1500 times per second. They have many interconnections with one another, and many of them also synapse directly with the anterior motor neurons, as shown in Figure 55-1 . The interconnections among the interneurons and anterior motor neurons are responsible for most of the integrative functions of the spinal cord that are discussed in the remainder of this chapter.

Essentially all the different types of neuronal circuits described in Chapter 47 are found in the interneuron pool of cells of the spinal cord, including diverging, converging, repetitive-discharge, and other types of circuits. In this chapter, we examine many applications of these different circuits in the performance of specific reflex actions by the spinal cord.

Only a few incoming sensory signals from the spinal nerves or signals from the brain terminate directly on the anterior motor neurons. Instead, almost all these signals are transmitted first through interneurons, where they are appropriately processed. Thus, in Figure 55-1 , the corticospinal tract from the brain is shown to terminate almost entirely on spinal interneurons, where the signals from this tract are combined with signals from other spinal tracts or spinal nerves before finally converging on the anterior motor neurons to control muscle function.

Renshaw Cells Transmit Inhibitory Signals to Surrounding Motor Neurons

Also located in the anterior horns of the spinal cord, in close association with the motor neurons, are a large number of small neurons called Renshaw cells. Almost immediately after the anterior motor neuron axon leaves the body of the neuron, collateral branches from the axon pass to adjacent Renshaw cells. Renshaw cells are inhibitory cells that transmit inhibitory signals to the surrounding motor neurons. Thus, stimulation of each motor neuron tends to inhibit adjacent motor neurons, an effect called lateral inhibition. The motor system uses this lateral inhibition to focus, or sharpen, its signals in the same way that the sensory system uses the same principle to allow unabated transmission of the primary signal in the desired direction while suppressing the tendency for signals to spread laterally.

Multisegmental Connections From One Spinal Cord Level to Other Levels—Propriospinal Fibers

More than half of all the nerve fibers that ascend and descend in the spinal cord are propriospinal fibers. These fibers run from one segment of the cord to another. In addition, as the sensory fibers enter the cord from the posterior cord roots, they bifurcate and branch both up and down the spinal cord; some of the branches transmit signals to only a segment or two, whereas others transmit signals to many segments. These ascending and descending propriospinal fibers of the cord provide pathways for the multisegmental reflexes described later in this chapter, including reflexes that coordinate simultaneous movements in the forelimbs and hindlimbs.

Structure and Motor Innervation of the Muscle Spindle

The organization of the muscle spindle is shown in Figure 55-3 . Each spindle is 3 to 10 millimeters long. It is built around 3 to 12 tiny intrafusal muscle fibers that are pointed at their ends and attached to the glycocalyx of the surrounding large extrafusal skeletal muscle fibers ( Video 55-1 ).

Each intrafusal muscle fiber is a tiny skeletal muscle fiber. However, the central region of each of these fibers—that is, the area midway between its two ends—has few or no actin and myosin filaments. Therefore, this central portion does not contract when the ends do. Instead, it functions as a sensory receptor, as described later. The end portions that do contract are excited by small gamma motor nerve fibers that originate from small type A gamma motor neurons in the anterior horns of the spinal cord, as described earlier. These gamma motor nerve fibers are also called gamma efferent fibers, in contradistinction to the large alpha efferent fibers (type Aα nerve fibers) that innervate the extrafusal skeletal muscle.

Sensory Innervation of the Muscle Spindle

The receptor portion of the muscle spindle is its central portion. As shown in Figure 55-3 and in more detail in Figure 55-4 , sensory fibers originate in this area and are stimulated by stretching of this midportion of the spindle. One can readily see that the muscle spindle receptor can be excited in two ways:

• 1.

  Lengthening the whole muscle stretches the midportion of the spindle and, therefore, excites the receptor.

• 2.

  Even if the length of the entire muscle does not change, contraction of the end portions of the spindle’s intrafusal fibers stretches the midportion of the spindle and therefore excites the receptor.

Two types of sensory endings, the primary afferent and secondary afferent endings, are found in this central receptor area of the muscle spindle.

Primary Ending

In the center of the receptor area, a large sensory nerve fiber encircles the central portion of each intrafusal fiber, forming the primary afferent ending or annulospiral ending. This nerve fiber is a type Ia fiber averaging 17 micrometers in diameter, and it transmits sensory signals to the spinal cord at a velocity of 70 to 120 m/sec, as rapidly as any type of nerve fiber in the entire body.

Secondary Ending

Usually one but sometimes two smaller sensory nerve fibers—type II fibers with an average diameter of 8 micrometers—innervate the receptor region on one or both sides of the primary ending, as shown in Figures 55-3 and 55-4 . This sensory ending is called the secondary afferent ending; sometimes it encircles the intrafusal fibers in the same way as the type Ia fiber, but often it spreads like branches on a bush.

Division of the Intrafusal Fibers Into Nuclear Bag and Nuclear Chain Fibers—Dynamic and Static Responses of the Muscle Spindle

There are also two types of muscle spindle intrafusal fibers: (1) nuclear bag muscle fibers (one to three in each spindle), in which several muscle fiber nuclei are congregated in expanded “bags” in the central portion of the receptor area, as shown by the top fiber in Figure 55-4 , and (2) nuclear chain fibers (three to nine), which are about half as large in diameter and half as long as the nuclear bag fibers and have nuclei aligned in a chain throughout the receptor area, as shown by the bottom fiber in the figure. The primary sensory nerve ending is excited by both the nuclear bag intrafusal fibers and the nuclear chain fibers. Conversely, the secondary ending is usually excited only by nuclear chain fibers. These relations are shown in Figure 55-4 .

The Primary and the Secondary Endings Both Respond to the Length of the Receptor—“Static” Response

When the receptor portion of the muscle spindle is stretched slowly, the number of impulses transmitted from both the primary and the secondary endings increases almost directly in proportion to the degree of stretching, and the endings continue to transmit these impulses for several minutes. This effect is called the static response of the spindle receptor, meaning that both the primary and secondary endings continue to transmit their signals for at least several minutes if the muscle spindle remains stretched.

The Primary Ending (but Not the Secondary Ending) Responds to Rate of Change of Receptor Length—“Dynamic” Response

When the length of the spindle receptor increases suddenly, the primary ending (but not the secondary ending) is stimulated powerfully. This stimulus of the primary ending is called the dynamic response, which means that the primary ending responds extremely actively to a rapid rate of change in spindle length. Even when the length of a spindle receptor increases only a fraction of a micrometer for only a fraction of a second, the primary receptor transmits tremendous numbers of excess impulses to the large 17-micrometer sensory nerve fiber, but only while the length is actually increasing. As soon as the length stops increasing, this extra rate of impulse discharge returns to the level of the much smaller static response that is still present in the signal.

Conversely, when the spindle receptor shortens, exactly opposite sensory signals occur. Thus, the primary ending sends extremely strong signals, either positive or negative, to the spinal cord to apprise it of any change in length of the spindle receptor.

Control of Intensity of the Static and Dynamic Responses by the Gamma Motor Nerves

The gamma motor nerves to the muscle spindle can be divided into two types: gamma-dynamic (gamma-d) and gamma-static (gamma-s) . The first of these gamma motor nerves excites mainly the nuclear bag intrafusal fibers, and the second excites mainly the nuclear chain intrafusal fibers. When the gamma-d fibers excite the nuclear bag fibers, the dynamic response of the muscle spindle becomes tremendously enhanced, whereas the static response is hardly affected. Conversely, stimulation of the gamma-s fibers, which excite the nuclear chain fibers, enhances the static response while having little influence on the dynamic response. Subsequent paragraphs illustrate that these two types of muscle spindle responses are important in different types of muscle control.

Continuous Discharge of the Muscle Spindles Under Normal Conditions

Normally, when there is some degree of gamma nerve excitation, the muscle spindles emit sensory nerve impulses continuously. Stretching the muscle spindles increases the rate of firing, whereas shortening the spindle decreases the rate of firing. Thus, the spindles can send to the spinal cord either positive signals (increased numbers of impulses to indicate stretch of a muscle) or negative signals (reduced numbers of impulses) to indicate that the muscle is unstretched.