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Quadrupedal Mechanics - Anatomy & Physiology (view source)
Revision as of 09:58, 30 April 2013
, 09:58, 30 April 2013no edit summary
In various ways, not only joints, but also all other components of the passive musculoskeletal system of these animals can fail. While natural repair is often possible, with rest, diverse orthopedic techniques have been developed to save valuable animals. The application of each of these techniques depends on an understanding of the mechanical principles of the structures needing repair, and the materials used to assist. An orthopedic surgeon constantly depends on good anatomical knowledge.
In various ways, not only joints, but also all other components of the passive musculoskeletal system of these animals can fail. While natural repair is often possible, with rest, diverse orthopedic techniques have been developed to save valuable animals. The application of each of these techniques depends on an understanding of the mechanical principles of the structures needing repair, and the materials used to assist. An orthopedic surgeon constantly depends on good anatomical knowledge.
====4 Force production in animals====
==='''A muscle either makes movement, or prevents it: Action or heat?'''===
Forces produced within muscles achieve animal posture and motion. A muscle (Fig 4.1 a, b) consists of a large number of fibres (Fig 4.1 c) arranged in such a way that a force is developed between the two ends of the muscle when the fibres are stimulated by electrical impulses coming from nerves. This force may cause movement, but if the force is no greater than opposing forces either within the animal's body (i.e. from antagonist muscles) or acting externally on the animal's body (such as the force of gravity), no movement will result.
Mechanical and heat energy are generated from the chemical energy supplied as nutrients to the muscle. If no movement results from the generation of a muscular force, no mechanical work is done, and all the energy must be released as heat.
:::::'''Fig. 4.1 Muscle structure 1'''
:::::The semitendinosus muscle of a dog, as seen by the unaided eye (a & b), and in transverse section with the light microscope (c).
'''The contractile proteins of muscle'''
To understand the nature of a muscular force, we must appreciate the molecular and filamentous structure of muscle. Muscle is a machine largely constructed from two proteins, actin and myosin. Actin can exist in a globular form of molecular weight 45,000 and a diameter of 5.5 nm, which readily aggregates to form a long filament (Fig. 4.2 d), each filament contains two strands of spherical actin molecules, twisted on each other.
In muscle, the form of these chains is regulated by the presence of two other proteins, troponin and tropomyosin. Myosin is a larger protein, with a molecular weight of 500,000 and a more complex shape. A molecule resembles a thin rod with two small globular “heads” at one end (Fig. 4.2 d). Under the right conditions these molecules aggregate into a sheaf, a cigar shaped structure studded with projections of myosin heads along its length. Because the heads are directed towards the ends of the structure, there is a bare zone in the middle (Fig. 4.2 c).
To electron microscopists, the accumulated actin molecules and their associated proteins are known as thin filaments, and the myosin structures as thick filaments. Thin filaments are organised around the thick filaments in a regular hexagonal array. In electron micrographs of muscle, the projections of the thick filaments appear as minute crossbridges that seem to link the thin and the thick filaments.
:::::'''Fig.4.2 Muscle structure 2'''
::::::Skeletal muscle fibre in longitudinal section;
::::::a, as visualised by the light microscope;
::::::b & c, by the electron microscope; and
::::::d, as reconstructed from crystallographic X-ray
'''The appearance of muscle using electron microscopy'''
Thick filaments are 1.6 µm long, and are joined by a meshwork, anchoring the ends, called a Z disc (Fig. 4.2 c). The serially repeating filamentous array was visible to the early light microscopists, especially using polarised light. They called the region occupied by the thick filaments an anisotrophic or A band, and the region occupied by the thin filaments an isotrophic or I band. The Z disc appeared as a narrow dark band dividing the lighter I bands (Fig. 4.2 b). The whole filamentous structure between adjacent Z discs is called a sarcomere.
'''What makes the striations in skeletal muscle?'''
Sarcomeres are added in series along the length of the muscle fibre; a relaxed fibre 3 cm long would contain about 15,000 sarcomeres in series. Within the fibre, the myofilamentous pattern is broken up by other components of the fibre (mainly sarcoplasmic reticulum and mitochondria) to form bundles of myofilaments called myofibrils (Fig. 4.2 b), each about 0.5 µm in diameter. However, the greatest proportion of the transverse sectional area of a muscle fibre, and indeed of an entire muscle, is occupied by myofilaments.
'''Sliding filaments and the site of the contractile force'''
Muscle contraction is caused by a sliding of the two sets of filaments past each other. The action sites are the crossbridges between the heads of the myosin molecules and the thin filaments. Crossbridges on opposite ends of the thick filaments are directed in opposite directions. Stimulation of activity at the crossbridges therefore creates a mechanical force tending to bring the thick and thin filaments into greater overlap, decreasing the distance between the Z discs, or sarcomere length, and shortening the muscle. The sarcomere is therefore the fundamental contractile unit of muscle.
'''Linear motors'''
A muscle is a linear motor. Most man-made motors are not linear, since they take advantage of the wheel, a device not possible in animals. A man-made machine using a linear motor is the magnetic railway, where a magnetic field both lifts the train off the track, and provides a linear thrust. If we accept the fossil record, the muscle machine preceded electric motors and internal combustion engines, and indeed the also very modern invention of the wheel, by 70 million years.
:::::'''Fig. 4.3 Sliding filaments in muscle'''
:::::The sarcomere can normally contract to 44% of its fully stretched length. At its optimal length of 2.2 µm, the thin filaments are maximally in apposition with the crossbridges of the thick filaments.
'''The force of a muscle depends on sarcomere length'''
The dimensions of the sarcomere determine the extent to which muscle cells can be stretched or contracted. When fully stretched the sarcomere length cannot exceed 3.6 µm without the fibre losing the ability to contract again (Fig. 4.3). As the muscle contracts, more and more crossbridges are brought into use. The force produced increases (Fig. 4.4) until all the crossbridges between thick and thin filaments can be used. At this point, simply because of the dimensions of the filaments, the sarcomere length is 2.2 µm, i.e. 59% of the fully stretched length. This is the optimal length. With further contraction, no more crossbridges can be used, in fact there is interference by overlapping of thin filaments and the force produced declines. When the sarcomere length is 1.6 µm, the muscle can contract no further without penetration of the Z discs by thick filaments, and resulting damage. The sarcomere, and hence the entire muscle cell in which the sarcomeres are in series, is now 44% of the fully stretched length. These measurements made by electron microscopists can be verified by using a device as simple as a ruler, since they agree with measurements of the range of contraction of muscle fibres (and the macroscopically visible fibre bundles) in the limb muscles of animals.
:::::'''Fig. 4.4 Tension in muscle'''
:::::Tension is maximal at a sarcomere length of 2.2 µm. A muscle, therefore, will have only one optimal length for developing tension.
'''Getting the most out of restricted muscle performance'''
The stage of contraction at which the force produced by a muscle is optimal is therefore quite limited. The implications of this in the design of the musculoskeletal system will be discussed later in section 7.
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[[Category:Musculoskeletal System - Anatomy & Physiology]]
[[Category:Musculoskeletal System - Anatomy & Physiology]]