An important aspect of interneuronal systems is that the laterally placed interneurons project ipsilaterally to motor neurons that supply the distal or the proximal limb muscles, whereas the medial interneurons project bilaterally. This arrangement of the lateral interneurons allows the limbs to be controlled independently. In contrast, the bilateral arrangement of the medial interneurons allows bilateral control of motor neurons to the axial muscles to provide postural support to the trunk and neck.
Although motor neurons are the final common pathway from the CNS to muscles and thus shape how neuronal activity is transformed into muscular contraction, each motor neuron directly acts on only a single muscle. Normal movements (or postures), however, are rarely, if ever, caused by the isolated contraction of an individual muscle. Rather, they reflect the coordinated activity of large groups of muscles. For example, elbow flexion involves an initial burst of activity in flexor muscles, such as the biceps, and in relaxation of extensor muscles, such as the triceps. This activity is then succeeded by a burst of activity of the triceps and then a second burst of activity in the biceps to stop the flexion movement at the desired position. Furthermore, other muscles are also activated during the elbow flexion to maintain overall balance and posture.
As the elbow flexion example shows, different roles are played by each muscle during a movement. The muscle that initiates, and is the prime cause of, the movement is called the agonist. Muscles that act similarly to the agonist are called synergists, whereas muscles whose activity opposes the action of the agonist are antagonists. In addition, muscles can act as fixators to immobilize a joint and in postural roles. Moreover, the relationship two muscles have to each other may depend on the specific movement being performed. For example, during elbow flexion, the triceps acts an antagonist to the biceps. In contrast, during supination of the forearm without rotation occurring about the elbow, the biceps (which also acts to supinate the forearm) is again an agonist, but the role of the triceps is that of an elbow fixator.
Thus motor control requires flexibly linking (and unlinking) the activity of groups of motor neurons that connect to different muscles. The circuits of the spinal cord are a major mechanism used by the CNS for this aspect of motor control. Indeed, descending pathways from the brain target primarily the interneurons of the spinal cord, although there are some descending axons that synapse directly onto motor neurons.
Spinal cord circuitry has several levels of organization. The most basic is the segmental level: that is, a circuit that is largely confined to a single or several neighboring segments and that is repeated again at many levels. The basic spinal reflexes covered below (i.e., the myotatic, inverse myotatic, and flexion reflexes) are mediated by such circuits. Superimposed on this segmental organization is the propriospinal system, which is a series of neurons whose axons run up and down the spinal cord to interconnect the different levels of the cord. This system allows the coordination of activity at different spinal levels, which is important for behavior involving both the forelimbs and the hind limbs, such as •Fig. 9.2 Muscle Proprioceptors. Skeletalmusclescontainsensoryreceptorsembeddedwithinthemuscle(spindles)andwithintheirtendons(Golgitendonorgans).A, Schematicviewofamuscle,showingthearrangementofaspindleinparallelwithextrafusalmusclefibersandatendonorganinserieswithmusclefibers.B, Structureandinnervation(motorandsensory)ofamusclespindle.C, Structureandinnervationofatendonorgan.A B C Ib afferent neuron Muscle fibers Tendon 250 µm Bag fibers Chain fibersGolgi tendon organ