AMERICAN SOCIETY OF BIOMECHANICS Presented at the Twenty-First Annual Meeting of the American Society of Biomechanics Clemson University, South Carolina September 24-27, 1997

INTRODUCTION

Short latency excitatory Ia reflex connections were determined between pairs of human wrist muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone.

REVIEW AND THEORY

For the cat ankle, it has been demonstrated that the primary spindle afferents in the ankle flexor and extensor muscles form heteronymous connections upon the motor nuclei innervating functional synergists (Eccles et al., 1957; Hultborn, 1976). This simple pattern of synergist excitation is not, however, found in the cat forelimb (Fritz et al., 1989) or the human ankle (Meunier et al., 1993; Mao et al., 1984). There are only fragmentary comparable data in humans on monosynaptic excitatory reflex connections in the wrist. The goal of the present study was to determine the excitatory projections of large muscle afferents from the major wrist flexor and extensor muscles in humans to determine monosynaptic connections linking these muscles.

PROCEDURES

Muscle spindle afferent excitation was elicited by tendon tap for the flexor carpi ulnaris (FCU), flexor carpi radialis (FCR), extensor carpi ulnaris (ECU), extensor carpi radialis (ECR) and extensor digitorum communis (EDC) muscles, or by electrical stimulation of the median nerve for the FCR. Single motor unit (SMU) activity was recorded in 6 subjects from pairs of forearm muscles (FCU & FCR; ECR & ECU; ECR & EDC; ECU & EDC) using bipolar intramuscular electrodes (Calancie et al., 1985). Subjects was asked to contract the muscles slightly to simultaneously recruit a motor unit in each of the two muscles recorded. Input to a single motoneurone was assessed from short latency changes (20-30 ms post stimulus time) in the motoneurone response probability (Pr ), determined from the peristimulus time histogram constructed between the stimulus and the respective motor unit spike train (Bawa et al., 1993). Data for all motor units from all subjects for a single muscle are summarized by the average response probability of all units examined under that condition (i.e., tendon tapped homonymous muscle response versus simultaneous heteronymous muscle response).

RESULTS

A basic assumption in the data collection and analysis was that a homonymous motor unit should respond to the spindle input at a latency shorter than 30 msec after the stimulus. Results are summarized in Table 1.

DISCUSSION

Almost all homonymous motoneurones examined responded to Ia afferent input. Bidirectional facilitation was observed between the two primary flexor muscles and between almost all of the extensor muscle pairs examined. The effectiveness of the afferent Ia volley was usually greater on homonymous than on heteronymous motoneurones, similar to that observed in the cat wrist (Fritz et al., 1989) and the human (Cody et al., 1989). One reflex Ia excitatory connection between forearm muscles was observed routinely which is seldom present in the cat forearm. A weak ECU short latency facilitation by EDC Ia afferents was observed in humans, while in the cat it is not present (Fritz et al., 1989).

The latency of the heteronymous ECR response when the ECU tendon was tapped, was later than the homonymous ECU response, and later than the homonymous ECR response. It was included in the analysis because it was the first excitatory response observed and it falls within the period of time defined as the M1 spinal response to muscle stretch. It is possible that ECU Ia reflex connections to the ECR are only through an oligosynaptic pathway which delays the increase in firing probability. Whatever the cause, it is apparent that while there is a Ia excitatory reflex connection from the ECU to the ECR, it does not have as short of a latency as observed between other forearm muscles. This observation is particularly interesting when combined with the fact that a reflex connection was not observed from the ECR to the ECU. It is clear that in humans the two principal wrist extensors are not connected through a monosynaptic excitatory reflex pathway.

Stimulation	Muscle studied	Number of motor units#	Time of peak onset (ms)*	Peak width (ms)*	Pr *
Median nerve	FCR	13/13	17	1.7	0.55
Median nerve	FCU	17/17	18	1.6	0.27
FCR Tendon	FCR	6/6	22	4.3	0.63
FCR Tendon	FCU	6/5	24	6.5	0.30
FCU Tendon	FCU	18/18	25	5.3	0.31
FCU Tendon	FCR	17/13	28	6.2	0.15
ECR Tendon	ECR	12/12	25	4.2	0.38
ECR Tendon	ECU	14/0	NA	NA	0
ECU Tendon	ECU	8/7	23	3.4	0.17
ECU Tendon	ECR	8/8	31	6.4	0.22
ECR Tendon	ECR	13/13	22	5.9	0.48
ECR Tendon	EDC	13/7	23	6.6	0.16
EDC Tendon	EDC	13/13	23	4.0	0.30
EDC Tendon	ECR	12/11	23	5.2	0.25
ECU Tendon	ECU	8/8	24	3.6	0.23
ECU Tendon	EDC	8/4	21	4.6	0.09
EDC Tendon	EDC	10/10	23	5.3	0.26
EDC Tendon	ECU	9/7	26	3.2	0.08

Table 1: Excitatory short latency connections between wrist muscles.
# Number of units studied / number of responding units
* Mean
Pr = response probability
NA = Not applicable, no heteronymous motor units responded.

It has been demonstrated that muscles which have a very similar mechanical action on a common joint tend to have Ia connections between them which are bidirectional and balanced, i.e. of approximately equal strength in both directions (Eccles et al., 1957, Fritz et al., 1989). In the cat wrist, where there are many degrees of freedom and even adjacent muscles do not have exactly the same mechanical action, bidirectional and balanced Ia connections do not exist among all the flexors and extensors (Fritz et al., 1989).

It is apparent that in the human wrist, as in the cat wrist, the primary wrist extensors, ECU and ECR which have very different mechanical actions, are not strongly interconnected through the reflexes tested. As a result, they can be more free to operate independently to serve the multiple degrees of freedom at the wrist than if they were bilaterally connected through strong Ia reflexes. In contrast, the FCU and the FCR wrist flexors in humans, are connected through a balanced, bidirectional short latency Ia pathway despite their very different mechanical actions. It may be that the great importance of the hand grasping action, usually requiring a combined effort of both the FCU and the FCR as well as the finger flexors, has demanded that tight reflex Ia connections exist between the FCR and the FCU, despite the different actions these muscles have during other wrist movements.

REFERENCES

Bawa P et al. J Physiol (Lond) 47: 445-464 1993

Calancie B et al. J Neurophysiol 53:1179-1193 1985

Cody FWJ et al. J Physiol 411:379-392 1989

Eccles JC et al. J Physiol 137:22-50 1957

Fritz N et al. J Physiol 419:321-351 1989

Hultborn H Prog Brain Res 44:235-255 1976

Mao CC et al. Exp Brain Res 56:341-50 1984

Meunier S et al. Exp Brain Res 96:534-44 1993

ACKNOWLEDGMENTS

Supported by the Natural Research and Engineering Council of Canada and the Medical Research Council of Canada.