'Like the arrangement of notes in a musical chord, each muscle synergy specifices how a particular muscle should be activated together with others. Just as one note belongs to several different chords, each muscle belongs to more than one synergy'
The muscles do not act alone
Muscles are not activated independently but instead are coactivated in synergies. The muscles within a synergy receive a common command signal during postural responses. In this way the many muscles of the body are controlled by just a few signals, reducing the time needed to compute the appropriate postural response
Anticipatory postural adjustments
The nervous system has advanced knowledge of the effects of voluntary movement on postural alignment and stability and activates anticipatory postural adjustments, often in advance of the primary movement
- An example of this is when you rapidly lift your arms in front of your body while standing, this will produce a reaction where your hips will extend, the knees will flex and your ankles will dorsiflex, moving the body's center of mass forward relative to the feet
The nervous system plans foot placement several steps in advance using visual information about the terrain and sorrounding environment
At times information about motion from any one sensory system may be ambiguous, for this reason multiple sources of sensory information must be integrated in posturalcenters to determine what orientation and motion of the body in space are appropriate
Somatosensory, vestibular, and visual information is integrated to provide a coherent picture of the position and velocity of the body in space and to generate and update motor commands that maintain balance and orientation.
The vestibular system consists of the otolithic organs and the semicircular canals
The otolithic organs of the vestibular apparatus provide information about the direction of gravity
The semicircular canals measure the velocity of head rotation
It should be noted, that the vestibular system is located in the head and therefore provide information about acceleration of the head but not about the rest of the body
- In order for the brain to know whether only the head is tilted, or if the entire body is tilted, it needs additional information from the somatosensory system
Loss of somatosensory fibers
A study was conducted on a cat which had it's somatosenory fibers destroyed by vitamin B6 intoxication, where the motor neruons and muscle strength are still functioning, but afferent information about muscle length and force is diminished
The results showed that that the postural response in gluteus medius was severly delayed after the intoxication. The delay of 20 ms induced ataxia and difficulty in maintaining balance
The results also demonstrated that the loss of largediameter somatosensory fibers (group I fibers) delayed the activation of the hind limbs while mainting posture, thus giving problems with recovery of balance following displacement testing.
Loss of vestibular balance
Immediately following vestibular loss, neck muscles get abnormally activated during ordinary movements, and often the head and trunk are moved together as a unit.
Recuperation from this involves greater reliance on the remaining sensory information
Loss of vestibular balance often leads to:
- Stiffness in head-trunk control
- Instability, especially when visual and somatosensory information is unavailable for postural orientation
Vestibular information is critical for balance when visual information is reduced and the support surface is not stable, for example at night, on a sandy beach, or a boat's deck