Posture and body Equllibrium

Subconscious adjustment of tone in different muscles in relation to every movement is known as the posture. Significance of posture is to make the movement smooth and accurate and to maintain the line of gravity constant or to keep the body in equilibrium with line of gravity. Posture is not an active movement. It is the passive movement associated with redistribution of tone in different groups of related muscles.

BASIC PHENOMENA OF POSTURE

Basic phenomena for maintenance of posture are muscle tone and stretch reflex.

MUSCLE TONE

Definition

Muscle tone is defined as the state of continuous and passive partial contraction of muscle with certain vigor and tension. It is also called tonus. It is also defined as resistance offered by the muscle to stretch.

Significance of Muscle Tone

Muscle tone plays an important role in maintenance of posture. Change in muscle tone enables movement of different parts of the body. Muscle tone is present in all the skeletal muscles. However, tone is more in antigravity muscles such as extensors of lower limb, trunk muscles and neck muscles.

Development of Muscle Tone

Gamma motor neurons and muscle spindle are responsible for the development and maintenance of

muscle tone. Muscle tone is purely a reflex process. This reflex is a spinal segmental reflex. It is developed by continual synchronous discharge of motor impulses from the gamma motor neurons present in the anterior gray horn of the spinal cord.

Sequence of events

1. Impulses from the gamma motor neurons cause contraction of end portions of intrafusal fibers

(stimulus)

2. This stretches and activates the central portion of the intrafusal fibers, which initiates the reflex action

for development of muscle tone by discharging the impulses

3. Impulses from the central portion of intrafusal fibers pass through primary sensory nerve fibers

(afferent fibers) and reach the anterior gray horn of spinal cord

4. These impulses stimulate the alpha motor neurons in anterior gray horn (center)

5. Alpha motor neurons in turn, send impulses to extrafusal fibers of the muscle through spinal nerve

fibers (efferent fibers)

6. These impulses produce partial contraction of the muscle fibers resulting in development of muscle

tone (response). When the frequency of discharge from gamma motor neurons increases, the activity of muscle spindle is increased and muscle tone also increases. Stimulation of gamma motor neurons increases the muscle tone. Lesion in gamma motor neurons leads to loss of tone in muscles.

Regulation of Muscle Tone

Though the muscle tone is developed by discharges from gamma motor neurons, it is maintained continuously and regulated by some supraspinal centers situated in different parts of brain. Some of these centers increase the muscle tone by sending facilitatory impulses while other centers decrease the muscle tone by inhibitory impulses.

Supraspinal facilitatory centers

Supraspinal centers, which increase the muscle tone:

1. Motor area 4 in cerebral cortex

2. Cerebellum

3. Descending facilitatory reticular system

4. Red nucleus

5. Vestibular nucleus.

Supraspinal inhibitory centers

Supraspinal centers, which decrease the muscle tone:

1. Suppressor areas of cerebral cortex

2. Basal ganglia

3. Descending inhibitory reticular system.

 

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