Specificity of intramuscular activation during rhythms produced by spinal patterning systems in the in vitro neonatal rat with hindlimb attached preparation

Abstract

In intact adult vertebrates, muscles can be activated with a high degree of specificity, so that even within a single traditionally defined muscle, groups of motor units can be differentially activated. Such differential activation might reflect detailed control by descending systems, potentially resulting from postnatal experience such that activation of motor units is precisely tailored to their mechanical actions. Here we examine the degree to which such specific activation can be seen in the rhythmic patterns produced by isolated spinal motor systems in neonates. We examined motor output produced by the in vitro neonatal rat spinal cord with hindlimb attached. We recorded the activity of different regions within the posterior portion of biceps femoris (BFp; i.e., excluding the anterior/vertebral head). We found that in the rhythms evoked by bath application of serotonin/N-methyl-d-aspartate (5-HT/NMDA), all regions of BFp were active during extension. However, the regions of BFp were activated in a specific sequence, with the activation of rostral regions consistently preceding those of more caudal regions in both afferented and deafferented preparations. In the rhythms evoked by cauda equina (CE) stimulation, rostral and middle regions of BFp remained active in extension, but the caudal region of BFp was usually active in flexion. Stimulation of L5 and S2 dorsal roots typically evoked rhythms with all regions of BFp active during extension; although the same rostral to caudal sequence of activation observed in 5-HT/NMDA evoked rhythms could also be observed in these rhythms, we also observed cases with reversed sequences, with activity proceeding from caudal to rostral. S2 dorsal root stimulation occasionally evoked rhythms with flexor activity in caudal BFp, similar to CE-evoked rhythms. Taken together, these results suggest a high degree of individuated control of muscles by spinal pattern generating networks, even at birth.

Fig. 1.

A: schematic indicating the different regions of the biceps femoris (BF) that were recorded from. We recorded from 3 sites along the medial surface of BF: a rostral region (rostral BFp [rBFp]) with insertion near the femur, a caudal region (caudal BFp [cBFp]) with an insertion on the tibia toward the ankle, and a middle region (middle BFp [mBFp]) intermediate to the other two. Note that none of these recordings was from the anterior head of BF, which has its origin on spinal vertebrae, since it is not readily accessible from medial recordings. B: an example of the activity on iliopsoas (IP) and the different regions of BF. In this experiment, the 3 different regions of BF were partly dissected with a small pin and separated with dissecting pins and afferents were intact. As can be seen in the figure, although all regions of BF were active during extension, there was a sequential activation of different regions so that rostral regions preceded more caudal regions.

Fig. 2.

A: distribution of mean phases across all cycles for the animal illustrated in Fig. 1B. The phase of the rhythmic cycle is indicated on the x-axis, with 0 and 360° indicating the onset of iliopsoas on 2 adjacent cycles and the transition between flexion and extension occurring approximately at 180°. B: shows another example of the distributions of mean phases in a deafferented animal (not the same as in A). C: the mean phase on rostral and middle regions of BFp plotted against one another for all individual cycles for the animal shown in B. A unity line is superimposed, indicating the position in the plot which would be expected if the 2 regions had the same mean phase on individual cycles.

Fig. 3.

Summary of all animals with rhythms evoked by serotonin/N-methyl-d-aspartate (5-HT/NMDA). A: the mean phase of each muscle for individual animals with afferents intact. Each interconnected line indicates a single animal. In cases in which not all regions of BFp had ≥6 significantly modulated cycles, only individual data points are indicated. B: the same information for deafferented animals. C: summary of the differences in mean phases between regions of BFp (r/m: rostral vs. middle; r/c: rostral vs. caudal; m/c: middle vs. caudal) for afferented and deafferented animals.

Fig. 4.

An example of rhythms evoked at varying concentrations of 5-HT/NMDA. In the rhythm evoked by the low concentration of 5-HT/NMDA there was a clear sequential activation of BFp from rostral to caudal. With a slightly higher concentration of 5-HT/NMDA in the same animal (B), the rhythm became more intense and faster and the delays between different regions of BFp were reduced. Display gains in A and B are the same.

Fig. 5.

Rhythms evoked by cauda equine (CE) stimulation. A: an example of a rhythm evoked by CE stimulation. Note that the reduced activation of iliopsoas is accompanied by a reduced activation of caudal BFp on the fourth cycle in the figure. B: the distribution of mean phases for all cycles for the animal shown in A. C: summary of the responses for all individual animals for CE-evoked rhythms, showing the consistency of the flexor-related activity in caudal BFp. D: the averaged difference in mean phase between each region of BFp for CE-evoked rhythms.

Fig. 6.

Rhythms evoked by L5 dorsal root (DR) stimulation. A: an example of a rhythm evoked by contralateral L5 DR stimulation. The mean phases for all rhythms evoked by ipsilateral L5 DR stimulation are shown in B; the mean phases for rhythms evoked by contralateral L5 DR stimulation are shown in C. The difference in mean phases between BFp regions are shown in D for ipsilateral L5 DR rhythms and in E for contralateral L5 DR evoked rhythms.

Fig. 7.

Rhythms evoked by S2 DR stimulation. An example of an ipsilateral S2 DR stimulation evoked rhythm is shown in A. Conventions for the rest of the figure are the same as those in Fig. 6.