When movement is limited to rotation about one axis, as it is in the phalanges of the fingers and toes, a joint is termed uniaxial: that is, it has just one degree of freedom. Although a few joints do have pure translatory movements, this is usually very limited and almost all gross movement is by rotation alone. There are therefore three possible rotations and three possible translations each one of these represents a degree of freedom.įor most joints, in the human body, translations are negligible and do not need consideration. Association fibres then run to the motor cortex this means that if the ankle starts to invert signals run very rapidly to the central control systems, which send equally rapid messages to the evertor muscles of the ankle, causing them to contract and maintain the body’s balance.Īs we have seen previously, all joint motion can be described by rotation about three orthogonal axes ( X, Y and Z) and translation in three orthogonal planes ( XY, XZ, YZ). These signals are conducted to the postcentral gyrus of the central cortex via large-diameter, myelinated neurons that have high conduction velocities. Other receptors such as Pacinian corpuscles are very much more rapidly adapting and react to stimuli with a sudden burst of high-frequency impulses, which quickly die away even when the stimulus is maintained. Some receptors such as Ruffini end organs and the receptors found in capsular ligaments and muscle spindles deliver continuous information about the relative position of muscles and joints and are used to facilitate balance, coordination and joint position sense. Stretching of the ligament causes these receptors to be stimulated and send information to the central nervous system. Ligaments have a number of neurological structures embedded within them that fall under the broad heading of mechanoreceptors. Stability actually comes from the far more powerful muscles and tendons, though the ligaments do have a crucial role to play. The image of ligaments acting like a sheet of powerful elastic to limit the motion of a joint and render it stable is one that is frequently presented to the student in fact, the integral strength of individual ligaments does very little to support most joints.
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