The cord gray matter is the integrative area for the cord reflexes. Figure 1 shows the typical organization of the cord gray matter in a single cord segment. Sensory signals enter the cord almost entirely through the sensory roots, also know as the posterior or dorsal roots. After entering the cord, every sensory signal travels to two separate destinations: One branch of the sensory nerve terminates almost immediately in the gray matter of the cord and elicits local segmental cord reflexes and other local effects, and another branch transmits signals to higher levels of the nervous system—that is, to higher levels in the cord itself, to the brain stem, or even to the cerebral cortex.
Fig1. Connections of peripheral sensory fibers and corticospinal fibers with the interneurons and anterior motor neurons of the spinal cord.
Each segment of the spinal cord (at the level of each spinal nerve) has several million neurons in its gray matter. Aside from the sensory relay neurons discussed in Chapters 48 and 49, the other neurons are of two types: (1) anterior motor neurons and (2) interneurons.
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 percent larger than most of the others and are called anterior motor neurons (Figure 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.
Fig2. Peripheral sensory fibers and anterior motor neurons innervating skeletal muscle.
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 anywhere from three to several hundred skeletal muscle fibers, which are collectively called the motor unit.
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 2 and 3. These fibers constitute the middle of the muscle spindle, which helps control basic muscle “tone,” as discussed later in this chapter.
Fig3. Muscle spindle, showing its relation to the large extrafusal skeletal muscle fibers. Note also both motor and sensory innervation of the muscle spindle.
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 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 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 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 sur rounding motor neurons. Thus, stimulation of each motor neuron tends to inhibit adjacent motor neurons, an effect called lateral inhibition. This effect is important for the following major reason: 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.
