Muscle, antagonists

Dystonia is characterized by sustained muscle contractions. In patients with dystonia, normal movements are disrupted by cocontraction of agonist and antagonist muscles, and by excessive activation of inappropriate musculature (overflow), leading to abnormal postures and slow involuntary twisting movements, which are often associated with movement execution. 

Stretching of the muscle is sensed in the muscle spindle and leads to firing in muscle spindle afferent. These nerves travel via the dorsal root and synapse in the anterior horn of the spinal cord directly with the motor neurone to that muscle. They stimulate firing of the motor neurones, which causes contraction of the muscle that has just been stretched. The muscle spindle afferent also synapses with inhibitory interneurons, which inhibit the antagonistic muscles. This is called reciprocal innervation. 

Skeletal muscle Is composed of bundles of tissue, with a tendon at either end. Usually the fixed end is referred to as the tendon of origin, the end that moves, the tendon of insertion. Muscles produce force through the process of contraction. Stretching of a muscle is passive and performed by contraction of an antagonist. For this reason, at least two muscles are necessary for free movement of joints. The principle of flexion and extension of the elbow joint by two antagonistic muscles is shown in Fig. 1. 

Fig. 1. The principle of articulating the joints by a pair of antagonistic muscles is shown for the elbow. On the upper end the tendons of the muscles are fixed.

Figure 13.2 Antagonistic muscles their role in movement. The action of antagonist muscles is exemplified by the actions involved in delivering a punch. The arm is first flexed by contraction of the biceps muscle and simultaneous relaxation of the triceps muscle. The arm is then rapidly extended by contraction of the triceps muscle and relaxation of the biceps muscle. Such movements involve considerable nervous coordination.

Figure 13.9 k soldier wounded at the Battle of Corunna (a battle in the Peninsular War) suffering simultaneous contraction of all muscles after infection with the bacterium, Clostridium tetani. Both agonist and antagonist muscles are active in this condition. The bacterium is found in the earth, especially in places where animal faeces have been present. Bacteria invade the body through a wound, especially in soldiers in battle. The bacterium secretes a toxin that is absorbed into the motor nerves which then become acutely responsive to mild stimuli. It can lead to death unless treated (from Bell 1824). The toxin is now used in cosmetic manipulation to stimulate contraction of muscles in the face to tighten the skin which removes or conceals wrinkles (Botox). 

M2 mACh-R allosteric antagonist [muscle relaxant, NM blocker]... 

On the other hand, if the muscles of the limbs of the body were to have a dead zone, then there would be times when the limbs would hang loose and flop around. A dead zone in this case would be undesirable, and so antagonistic muscle tone is usually maintained at low levels. 

FIGURE 15.3 Athree factor multiplicative model of a muscle (active torque). The model includes antagonist muscle (passive torque). 

Resistive joint torque Soft tissue, passive stretching of antagonistic muscles, and ligaments introduce nonKnearities, which can be modeled as ... 

Figure 16.4 illustrates the mechanical components of the oculomotor plant for horizontal eye movements, the lateral and medial rectus muscle, and the eyeball. The agonist muscle is modeled as a parallel combination of an active state tension generator Fag> viscosity element Bag> and elastic element TlT) connected to a series elastic element Rse- The antagonist muscle is similarly modeled as a parallel combination of an active state tension generator Tant> viscosity element Rant> and elastic element TlT) connected to a series elastic element Rse- The eyeball is modeled as a sphere with moment of inertia /p, connected to viscosity element Bp and elastic element Kp. The passive elasticity of each muscle is included in spring Kp for ease in analysis. Each of the elements defined in the oculomotor plant is ideal and linear. 

The whole hand may be operated by electromyographic signals from two antagonistic muscles in the supporting forearm stump, picked up at the skin surface. In response to tension in one muscle, the hand opens progressively and then closes to grip with an automatic reflex. The second muscle controls the mode of operation as the hand moves between the states of touch, hold, squeeze, and release. 

The end of the muscle that is attached to a nonmoving bone is called the ori n of the muscle the end attached to a moving bone is the insertion. As a muscle contracts it becomes shorter and fatter, moving one bone closer to the other. Since a muscle cannot expand, another muscle (the extensor) is required to move the bone in the opposite direction and stretch the first muscle (known as the flexor). The flexor and extensor are described as antagonistic muscles. See illustration. 

Both agonist and antagonist muscles contribute (unequally) to the net torque developed about a joint In fact, for any given joint in the body, there are many more muscles crossing the joint than there are dof prescribing joint movement llie knee, for example, has at most 6 dof, yet there are at least 14 muscles that actuate this joint One consequence of this arrangement is that the force developed by each muscle carmot be determined uniquely. Specifically, there are more unknown musculotendinous actuator forces than net actuator torques exerted about the knee that is, m > n in Eq. (6.9), which means that the matrix of muscle moment arms is not square and therefore not invertible. This is the so-called indeterminate problem in biomechanics, and virtually all attempts to solve it are based on the aj Ucation of optimization theory (see also the Qiap. 5 by Manal and Buchanan). 

The human makes flexible motions using dexterous control of muscles that are antagonistically arranged around joints. The robot hand is an antagonistic muscle-driven system (Fig. 10.2) that simplifies the musculoskeletal system, which moves joints using several muscles. Motion around a joint is made using the difference in pressure supplied to each actuator in the system. In addition, the robot hand realizes the more similar structure inspired by a musculoskeletal system because it has polyarticular muscles that make several joints move at the same time. 

Figure 10.2 Antagonistic muscle-driven system. Pi and P2 denote pressure.

Honda, Y., Miyazaki, F., Nishikawa, A., 2010. Control of pneumatic five-fingered robot hand using antagonistic muscle ratio and antagonistic muscle activity. In Proceedings of the 3rd lEEE/RAS-EMBS International Conference on Biomedical Robotics and Biomechatronics, pp. 337-342. 

Antagonist Muscle that has an opposite action from its paired muscle and limits its action. 

Figure 4 An artificial flagellum from bipolar polyelectrolyte gels working as an antagonistic muscle pair. (Adapted with permission from Ref. 12. American Chemical Society, 2007.)...

---