Gearing effects of the patella (knee extensor muscle sesamoid) of the helmeted guineafowl during terrestrial locomotion.

Human patellae (kneecaps) are thought to act as gears, altering the mechanical advantage of knee extensor muscles during running. Similar sesamoids have evolved in the knee extensor tendon independently in birds, but it is unknown if these also affect the mechanical advantage of knee extensors. Here, we examine the mechanics of the patellofemoral joint in the helmeted guineafowl Numida meleagris using a method based on muscle and tendon moment arms taken about the patella's rotation centre around the distal femur. Moment arms were estimated from a computer model representing hindlimb anatomy, using hip, knee and patellar kinematics acquired via marker-based biplanar fluoroscopy from a subject running at 1.6 ms-1 on a treadmill. Our results support the inference that the patella of Numida does alter knee extensor leverage during running, but with a mechanical advantage generally greater than that seen in humans, implying relatively greater extension force but relatively lesser extension velocity.

Haramiyids and Triassic mammalian evolution.

Isolated teeth referred to the family Haramiyidae are among the earliest known fossil evidence of mammals. First discovered in European Late Triassic deposits a century and a half ago, haramiyids have been interpreted as related to multituberculates, a diverse and widespread lineage that occupied a rodent-like niche from the Late Jurassic to the Early Tertiary. Nonetheless, haramiyid relationships have remained enigmatic because the orientation and position of the teeth in the upper or lower jaw could not be determined with certainty; even their mammalian status has been questioned. The discovery of haramiyid dentaries, a maxilla and other skeletal remains in the Upper Triassic of East Greenland reveals haramiyids as highly specialized mammals with a novel pattern of puncture-crushing occlusion that differs dramatically from the grinding or shearing mechanisms of other Early Mesozoic mammals.

Neuromuscular diversity in archosaur deep dorsal thigh muscles.

The living members of the clade Archosauria, crocodilians and birds, differ markedly in the morphology of their deep dorsal thigh muscles. To investigate whether this diversity is accompanied by differences in motor pattern and muscle function, the hindlimbs of representative archosaurs were studied by electromyography and cineradiography during terrestrial locomotion. In a crocodilian, Alligator, the iliofemoralis and pubo-ischio-femoralis internus part 2 are both active during the swing phase of the stride cycle. This appears to be the primitive motor pattern for archosaurs. There are four avian homologues of these muscles in the helmeted guineafowl, Numida. These are primarily active in the propulsive phase (iliotrochantericus caudalis and iliotrochantericus medius), the swing phase (iliotrochantericus cranialis) and a speed-dependent combination of the propulsive and/or swing phases (iliofemoralis externus). Differences between Alligator and Numida in the number and attachment of deep dorsal muscles are associated with dissimilar motor patterns and functions. Evolutionary modifications of neuromuscular control must be recognized when evaluating avian locomotor history, but are rarely considered by paleontologists. Even within the deep dorsal thigh muscles of Numida, developmentally and anatomically similar muscles are active out-of-phase. Therefore, although the actions of two adjacent muscles appear equivalent, their functions may differ dramatically. The diversity of deep dorsal thigh muscles in modern birds may be a good model for studying the relationship between activity pattern and peripheral morphology.

Evidence for compartmental identity in the development of the rat lateral gastrocnemius muscle.

In adult rats, each neuromuscular compartment of the lateral gastrocnemius muscle (LG) is exclusively innervated by a primary branch of the LG nerve. In neonates, however, a small percentage of LG cells receives inputs from more than one primary nerve branch; these inputs are known as cross-compartmental. Cross-compartmental inputs are normally lost from the medial compartment of LG (LGm) by the 8th postnatal day. To investigate the mechanisms involved in the elimination of cross-compartmental inputs, muscle fibers in the LGm compartment were denervated by cutting the LGm nerve branch in 1-4 day old rat pups and in adult rats. We then assessed the degree of cross-compartmental innervation within the "denervated" compartment using intracellular recordings from neonatal muscle fibers or immunohistochemical staining for nerve cell adhesion molecule (N-CAM) and neurofilament protein in adult muscles. Following LGm axotomy in neonates, cross-compartmental innervation is more extensive than in controls and is present as late as 20 days after birth. Thus, in the absence of "native" LGm axons, neonatal cross-compartmental inputs proliferate by axonal sprouting and the formation of new synapses on vacant LGm fibers. In contrast, axotomized adults do not form new cross-compartmental inputs over the same time period. The differential response of neonates and adults to muscle nerve branch denervation is evidence for the existence of some form of compartment-specific recognition. We propose that compartmental identity either arises or becomes relatively more potent during ontogeny and normally acts selectively to eliminate foreign axons and deter the formation of new cross-compartmental inputs.