Posture is body position, which is designed to maintain support against the force of gravity. Put another way, it is the arrangement of vertical position relative to the body supports in contact with the ground. In humans, the support area when standing is covered by the feet, a relatively small area, and the center of gravity is high. Therefore relatively little tilt will destabilize the vertical position. The body is equipped with mechanisms that sense changes in the orientation of the body and of the support area, and with mechanisms that respond to alter posture so as to maintain stability.
Two sources of information monitor changes in body position and ground support: the head and the feet. The feet have pressure receptors, which monitor changes in pressure distribution points; the head has special senses, the vestibular and visual systems, which monitor motion and position of the body relative to the external environment. The head also possesses a brain which integrates information from the feet and the other two systems, and which can instruct the musculature to respond appropriately. Posture maintenance is therefore an excellent paradigm of the integration of the so-matosensory and motor systems.
The vestibular system informs the brain about the velocity of body movement relative to the semicircular canals, and about the static body position relative to the direction of gravity from the point of view of the semicircular canals. The semicircular canals respond extremely fast to body changes, and are able virtually to warn in advance of an impending upset to body balance.
An important aim is to keep the head upright regardless of body position, the head righting reflex. This in turn keeps the eyes in the optimal orientation; if the head tilts, for example, the eyeballs will
Greenstein, Color Atlas of Neuroscience © 2000 Thieme rotate reflexly in an attempt to retain their optimal position for sensing body orientation. Visual responses, like those of the vestibular system may be static or dynamic. Static responses are those to non-moving objects such as a tree or the horizon. Dynamic responses are those to moving objects such as trains or revolving doors. The visual responses are slower than those of the vestibular system.
To maintain posture, the brain also needs to know the position of the head relative to the body. This is provided by the neck reflexes, which are mediated by joint proprioceptors, mainly around the vertebrae. This does not mean that the head always has to be kept upright; posture can be maintained whatever the position of the head relative to the body.
From the scheme provided opposite, it is clear that the orientation of the body in space is sensed by three independent mechanisms. Swimmers, for example, are kept informed of their orientation under water, even with closed eyes and no pressure on their feet, purely through the responses of the vestibular system. And even if the eyes are kept open while the body revolves rapidly, the person will nevertheless still fall to the ground due to the violent disturbances in the semicircular canals.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.