How Balance and Posture Are Maintained

Receptors in The Vestibular Apparatus And Elswhere Keep us Upright

Sep 6, 2009 John Richard Roberts

The fact that human beings that can stand upright and move around in the environment is due to a remarkable system of information collection and integration.

Maintainance of balance and posture requires Information from millions of receptor cells to be sent to several areas of the brain for processing. The major centres for this processing are the brain stem, the cerebellum - where much movement is coordinated, and the cerebral cortex where conscious awareness of body position and movements occurs.

Major Receptor Areas for the Control of Posture and Balance.

Information which the brain uses to orient the body in space in static and dynamic situations comes from five major sources:

• The vestibular apparatus in the inner ears,
• The eyes.
• The muscles.
• The Joints.
• Pressure receptors in the skin.
• Each of these areas contains the sensory receptors which signal to the brain such things as head position, muscle tension, joint position and touch.

The Vestibular Apparatus

Situated in the labyrinth of each of the inner ears this is made up of two otolith organs and the semicircular- canals. The otolith organs contain a gel infiltrated with particles of calcite. These organs determine which way the head is in relation to gravity by the movements of the comparatively dense calcite gel against movement receptors.

The semicircular canals are fluid-filled but have no particles of calcite. They signal movement or more correctly acceleration by the fluid flowing past receptor cells within the canals as the body moves.

Problems with the vestibular apparatus are the major causes of balance disorders, for example benign positional vertigo occurs when there is a dysfunction with the otolith organs. Meniere's disease results from a dysfunction of the membranes in the vestibular apparatus.

The Eyes and Balance

The eyes provide information on orientation and movement using reference points in the visual field. Tracking movement requires complex integration of information.

If the eye detects an object moving across the visual field, which means across the retina, the brain needs to know whether this is apparent movement due to the head moving or a real object moving. If the head is moving then this will be signalled by the vestibular apparatus and the receptors in the neck vertebrae. In addition to this the brain uses information on the movement of eye muscles to know if the eyes have moved or not.

The system isn’t full-proof. It is a common experience to be sitting on a stationary train next to another which has started to move. Because the train adjacent fills up the whole visual field, there are no other clues to determine what is moving and what is not, consequently it feels as if the stationary train is moving.

The contribution made by the eyes to balance can be felt simply by standing still for several minutes with the eyes closed. Most people will start to gently sway back and forwards in an unconscious effort to maintain balance.

Muscles and Posture Control

In addition to providing movement the muscles control posture and therefore assist balance. Muscles and tendons have receptors which signal to the spinal cord and brain the degree of stretch in the muscle fibres and the tension in the muscle.

This allows the body to constantly adjust muscle length and tension to cope with whatever posture is adopted. With humans posture control is more complex because we stand on just two legs: quadrupeds are inherently more stable.

All the muscles are important in this but those of the legs, pelvis and neck particularly so.

Joints and Limb Position

Joints have receptors which tell the brain where the limb is in space. Most people can move an arm or leg into a certain position with their eyes closed and know what that position is.

The receptors in the joints of the neck vertebrae are particularly important in balance since these receptors provide information on head position.

Skin Receptors and Balance

It is probable that stretch receptors in the skin over joints help to provide information in joint position and therefore supplement the information coming from the joint receptors themselves, however it is the pressure receptors in the soles of the feet that play the crucial role in balance.

They do this by feeding the spinal cord and brain information on different pressures on various parts of the soles of the feet and signal such things as tipping forwards or backwards.

There is considerable overlap in the information provided to the brain to maintain balance. Because of this if any part does not function properly it is still possible to maintain reasonable balance.

Those with vestibular disorders, which often give rise to severe vertigo can learn through rehabilitation techniques to cope fairly well. Blind people usually have little trouble in maintaining balance.

Some diseases, diabetes and neurological disorders for example can cause damage to joint and skin receptors so that information received by the brain is faulty, however, if the other parts of the system are functioning correctly then balance can be maintained.

Resources

Human Anatomy and Physiology by Carola, Harley and Noback. Pub. McGraw Hill 1992

Neurophysiology by RHS Carpenter. 2002

The copyright of the article How Balance and Posture Are Maintained in Anatomy & Physiology is owned by John Richard Roberts. Permission to republish How Balance and Posture Are Maintained in print or online must be granted by the author in writing.

Cross Section of Semicircular Canal