The neural circuit of the flexion reflex is depicted in Fig. 9.9, specifically for neurons that only affect the knee joint. However, it is important to note that there is significant divergence in the primary afferent and interneuronal pathways involved in the flexion reflex. This means that all major joints of a limb, such as the hip, knee, and ankle, can be engaged in a strong flexor withdrawal reflex. The details of the flexor withdrawal reflex vary depending on the nature and location of the stimulus.

Interestingly, the interneurons responsible for the flexion reflexes are also believed to be part of the central pattern generator (CPG) that generates locomotion. This demonstrates how reflex circuits can serve multiple purposes. A CPG consists of neurons and circuits that can generate rhythmic activity for motor acts, even in the absence of sensory input. When the FRA interneurons are activated, a pattern of flexor excitation and extensor inhibition occurs on one side, while the opposite pattern emerges on the other side. If the FRA interneurons on each side of the spinal cord alternate in activity, a stepping pattern, resembling walking motion, can be observed. It's worth noting that this rhythmic activity pattern in the FRA circuits can be activated by descending pathways from the brain and does not necessarily depend on activity from the FRA fibers themselves.

To confirm the involvement of these circuits in generating locomotion rhythm, spinal cord preparations were conducted to observe spontaneous locomotion. In these preparations, if the brainstem is transected and weight is supported, the spinal cord circuits can generate activity that causes the limbs to perform a normal locomotion sequence. One particular preparation involved recording electromyographic signals from the flexors and extensors of a limb, and then stimulating the FRA fibers to observe the effect on locomotion rhythm (Fig. 9.10). Prior to any stimulus, a spontaneous alternating pattern of flexor and extensor electromyographic (EMG) activity is present. If the FRA fibers were not part of the locomotion circuit, or not a crucial component responsible for generating the rhythm, the stimulus would only elicit a temporary response, such as a single burst in the flexor EMG record and brief inhibition of extensor activity. However, in this experiment, the stimulus also induced a permanent, approximately 180-degree phase shift in the locomotor rhythm. This can be deduced by comparing the times of contractions before and after the stimulus. The dashed vertical lines indicate the expected times of flexor EMG responses if the stimulus had not caused any phase shift. Prior to the stimulus, each vertical line aligns with the onset of a flexor EMG burst, while after the stimulus, each vertical line corresponds to the end of the flexor burst. Therefore, the stimulus influenced the locomotor CPG itself, highlighting the critical role of FRA interneurons in this CPG.

This experiment also highlights another important aspect – the locomotion CPG, as well as CPGs in general, can be influenced by strong afferent fiber activity. The influence of afferent fibers ensures that the pattern generator adapts to changes in the terrain during locomotion. This adaptation is crucial for maintaining proper coordination, especially when rapid changes occur during running.

Reflexes can be used to study and characterize convergence and divergence in reflex pathways and neuronal circuits in the spinal cord. Spatial facilitation, which is demonstrated in Fig. 9.11, serves as an example of convergent input in reflexes.
Fig. 9.9 , the neural circuit of the flexion reflex is shown for neurons that affect only the knee joint. Actually, however, considerable divergence of the primary afferent and interneuronal pathways occurs in the flexion reflex. In fact, all the major joints of a limb (e.g., hip, knee, and ankle) may be involved in a strong flexor withdrawal reflex. Details of the flexor withdrawal reflex vary, depending on the nature and location of the stimulus.
The interneurons subserving flexion reflexes also appear to be part of the central pattern generator (CPG) for generating locomotion and thus are an example of how reflex circuits are used for multiple purposes. A CPG is a set of neurons and circuits capable of generating the rhythmic activity that underlies motor acts, even in the absence of sensory input. For example, activation of the FRA inter-neurons leads to a pattern of flexor excitation and extensor inhibition on one side and the converse pattern on the opposite side, and if the FRA interneurons on each side of the spinal cord alternated in being active, a stepping pattern would emerge. That is, walking motion could result from alternately activating the FRA interneurons on each side. Note that such a rhythmic activity pattern in the FRA circuits need not be dependent on activity from the FRA fibers themselves (e.g., they could be activated by descending pathways from the brain).
To show that these circuits are actually involved in generating the locomotion rhythm, spinal cord preparations were made that showed spontaneous locomotion (i.e., if the brainstem is transected and weight is supported, the spinal cord circuits can generate activity that causes the limbs to perform a normal locomotion sequence). In one such preparation, the electromyographic signals from the flexors and extensors of a limb were recorded, and the FRA fibers then stimulated to demonstrate the effect on locomotion rhythm (
Fig. 9.10 ). Before any stimulus, a spontaneous alternating pattern of flexor and extensor electromyographic (EMG) activity exists. If the FRA fibers were not involved in the locomotion circuit, or at least were not a critical part of the circuits responsible for generating the rhythm (see
Fig.
9.10B ), the stimulus would be expected to produce only a transient response (i.e., a single burst in the flexor EMG record and brief inhibition of activity in the extensor EMG record) but to have no long-term effect on ongoing EMG pattern. Such a transient response is observed (see
Fig. 9.10A ; EMG records just after the stimulus). However, the stimulus also causes a permanent, approximately 180degree phase shift in locomotor rhythm, as can shown from a comparison of the times of contractions before and after the stimulus. The dashed vertical lines indicate the times at which a flexor EMG response would be expected if the stimulus had produced no phase shift from the EMG activity pattern. Before the stimulus, each vertical line is aligned with the onset of a flexor EMG burst, whereas after the stimulus, each vertical line occurs at the end of the flexor burst. Therefore, the stimulus affected the locomotor CPG itself and the FRA interneurons are a critical part of this CPG (see
Fig. 9.10C
A second important point illustrated by this experiment is that the locomotion CPG (and CPGs in general) can be influenced by strong afferent fiber activity. The afferent fiber’s influence ensures that the pattern generator adapts to changes in the terrain as locomotion proceeds. Such changes may occur rapidly during running, and locomotion must then be adjusted to ensure proper coordination.
Determining Spinal Cord Organization Through the Use of Reflexes
Convergence and divergence are important aspects of reflex pathways and of neuronal circuits in general. Several examples of these phenomena have been described in the previous discussion of the reflexes. Reflexes can be used to identify and characterize these phenomena in the spinal cord. For example, convergent input can be demonstrated through the phenomenon of spatial facilitation, which is illustrated in
Fig. 9.11