"Sensory partitioning" of cat medial gastrocnemius muscle by its muscle spindles and tendon organs

Detection of epimuscular myofascial forces by Golgi tendon organs

Skeletal muscles embed multiple tendon organs, both at the proximal and distal ends of muscle fibers. One of the functions of such spatial distribution may be to provide locally unique force feedback, which may become more important when stresses are distributed non-uniformly within the muscle. Forces exerted by connections between adjacent muscles (i.e. epimuscular myofascial forces) may cause such local differences in force. The aim of this exploratory study was to investigate the effects of mechanical interactions between adjacent muscles on sensory encoding by tendon organs. Action potentials from single afferents were recorded intra-axonally in response to ramp-hold release (RHR) stretches of a passive agonistic muscle at different lengths or relative positions of its passive synergist. The tendons of gastrocnemius (GAS), plantaris (PL) and soleus (SO) muscles were cut from the skeleton for attachment to servomotors. Connective tissues among these muscles were kept intact. Lengthening GAS + PL decreased the force threshold of SO tendon organs (p = 0.035). The force threshold of lateral gastrocnemius (LG) tendon organs was not affected by SO length (p = 0.371). Also displacing LG + PL, kept at a constant muscle-tendon unit length, from a proximal to a more distal position resulted in a decrease in force threshold of LG tendon organs (p = 0.007). These results indicate that tendon organ firing is affected by changes in length and/or relative position of adjacent synergistic muscles. We conclude that tendon organs can provide the central nervous system with information about local stresses caused by epimuscular myofascial forces.

Regionalization of the stretch reflex in the human vastus medialis

KEY POINTS

Regionalization of the stretch reflex, i.e. the notion that the activation of 1a afferents from a muscle region influences only the activation of motor units in the same region, has been demonstrated previously in animals but not in humans. Mechanical stretches applied to regions of vastus medialis as close as 10 mm apart resulted in recruitment of motor units localized topographically with respect to the location of the mechanical stretch. Stretch reflexes are regionalized in the human vastus medialis. The human spinal cord has the neuromuscular circuitry to preferentially activate motoneurones innervating muscle fibres located in different regions of the vastus medialis.

ABSTRACT

The localization of motor unit territories provides an anatomical basis to suggest that the CNS may have more independence in motor unit recruitment and control strategies than what was previously thought. In this study, we investigated whether the human spinal cord has the neuromuscular circuitry to independently activate motor units located in different regions of the vastus medialis. Mechanical taps were applied to multiple locations in the vastus medialis (VM) in nine healthy individuals. Regional responses within the muscle were observed using a grid of 5 × 13 surface EMG electrodes. The EMG amplitude was quantified for each channel, and a cluster of channels showing the largest activation was identified. The spatial location of the EMG response was quantified as the position of the channels in the cluster. In a subset of three participants, intramuscular recordings were performed simultaneously with the surface EMG recordings. Mechanical taps resulted in localized, discrete responses for each participant. The spatial location of the elicited responses was dependent on the location of the tap (P < 0.001). Recordings with intramuscular electrodes confirmed the regional activation of the VM for different tap locations. Selective stimulation of 1a afferents localized in a region of the VM results in reflex recruitment of motor units in the same region. These findings suggest that the human spinal cord has the neuromuscular circuitry to modulate spatially the motoneuronal output to vastus medialis regions, which is a neuroanatomical prerequisite for regional activation.

Changes in muscle spindle firing in response to length changes of neighboring muscles

Skeletal muscle force can be transmitted to the skeleton, not only via its tendons of origin and insertion but also through connective tissues linking the muscle belly to surrounding structures. Through such epimuscular myofascial connections, length changes of a muscle may cause length changes within an adjacent muscle and hence, affect muscle spindles. The aim of the present study was to investigate the effects of epimuscular myofascial forces on feedback from muscle spindles in triceps surae muscles of the rat. We hypothesized that within an intact muscle compartment, muscle spindles not only signal length changes of the muscle in which they are located but can also sense length changes that occur as a result of changing the length of synergistic muscles. Action potentials from single afferents were measured intra-axonally in response to ramp-hold release (RHR) stretches of an agonistic muscle at different lengths of its synergist, as well as in response to synergist RHRs. A decrease in force threshold was found for both soleus (SO) and lateral gastrocnemius afferents, along with an increase in length threshold for SO afferents. In addition, muscle spindle firing could be evoked by RHRs of the synergistic muscle. We conclude that muscle spindles not only signal length changes of the muscle in which they are located but also local length changes that occur as a result of changing the length and relative position of synergistic muscles.

Effects of epimuscular myofascial force transmission on sarcomere length of passive muscles in the rat hindlimb

Results from imaging studies and finite element models suggest epimuscular myofascial effects on sarcomere lengths in series within muscle fibers. However, experimental evidence is lacking. We evaluated epimuscular myofascial effects on (1) muscle belly, fiber, and mean sarcomere length and (2) sarcomere length distribution within passive fibers of the rat tibialis anterior (TA) and soleus (SO) muscles. Hindlimbs (n = 24) were positioned in predefined knee (55°, 90°, 125°, 160°) and ankle (either 90° or 125°) angles, and fixed in a formaldehyde solution. Varying knee joint angle causes changes in muscle–tendon unit length of SO and TA’s synergists, but not of SO and TA. Whole fibers were taken from SO and TA and photographed along their length. Mean sarcomere length was assessed for the entire fiber and for the proximal, intermediate, and distal thirds (fiber segments) separately. Mean sarcomere length of the fiber was not affected by knee angle, neither for SO (mean: 2.44 ± 0.03 μm and 2.19 ± 0.05 μm for ankle angles of 90° and 125°, respectively) nor for TA (mean: 2.33 ± 0.05 μm and 2.51 ± 0.07 μm for ankle angle set to 90° and 125°, respectively). Only for TA, a significant interaction between knee angle and fiber segment was found, indicating changes in the distribution of lengths of in-series sarcomeres. Thus, while epimuscular myofascial force transmission did not cause mean sarcomere length changes within passive SO and TA, it did alter the length distribution of sarcomeres within passive TA.

Increased H-reflex response induced by intramuscular electrical stimulation of latent myofascial trigger points

BACKGROUND

Myofascial trigger points (MTrPs) present with mechanical hyperalgesia and allodynia. No electrophysiological evidence exists as to the excitability of muscle spindle afferents at myofascial trigger points MTrPs. The purpose of this current study was to explore whether an H-reflex response could be elicited from intramuscular electrical stimulation. If so, to assess the possibility of increased reflex response at MTrPs.

METHODS

The H-reflex latency and the conduction velocity were first determined from electrical stimulation of the tibial nerve in 13 healthy subjects. Then an intramuscular monopolar needle electrode was inserted randomly into a latent MTrP or a non-MTrP in the gastrocnemius muscle. Electrical stimuli at different intensities were delivered via the intramuscular recording electrode to the MTrP or non-MTrP.

RESULTS

The average conduction velocity (44.3 +/- 1.5 m/s) of the electrical stimulation of tibial nerve was similar (p>0.05) with the conduction velocity (43.9 +/- 1.4 m/s) of intramuscular electrical stimulation. The intramuscular H-reflex at MTrPs was higher in amplitude than non-MTrPs (p<0.001). The reflex threshold was lower for MTrPs than non-MTrPs (p<0.001).

CONCLUSION

The current study provides first electrophysiological evidence that intramuscular electrical stimulation can evoke H-reflex, and that higher H-reflex amplitude and lower H-reflex threshold exist at MTrPs than non-MTrPs respectively, suggesting that muscle spindle afferents may be involved in the pathophysiology of MTrPs.

Selfishness examined: Cooperation in the absence of egoistic incentives

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.

On the function of muscle and reflex partitioning

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems.