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Dr. SaadKlemanAbd
]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [
RECIPROCAL INHIBITION
In many cases, muscles work in antagonistic pairs (e.g., the biceps and triceps muscles of
the arm).
When a muscle produces movement by shortening, it is
referred to as an agonist,muscles that oppose the action of the
agonist are antagonists.
(e.g., flexion of the elbow), the contraction of biceps is
accompanied by relaxation of its antagonist, the triceps muscle;
this phenomenon is known as reciprocalinhibition and is
mediated by spinal interneurons.
THE REVERSE MYOTACTIC REFLEX
The reverse myotactic reflex is mediated by Golgi tendon organs
 It is the reflex inhibition of muscle contraction stimulated by active contraction of the
muscle itself.
 protects a muscle from a potentially damaging overload during extreme contraction.
 under normalloading conditions it regulates muscle tension during a sustained
contraction.
For example, when picking up a fragile object such as an egg, a steady force that is not too
powerful must be applied.
THE FLEXOR WITHDRAWAL REFLEX
Spinal interneurons are not always inhibitory,
flexor withdrawal response, which protects the affected limb by rapidly removing it from an
injurious stimulus.
Thesteps of the flexor withdrawal reflex
responsecan be illustrated, for example,
when a painful stimulus is applied to one leg:
■Pain receptors are activated at the site of
stimulation.
■Afferent pain fibers enter the dorsal root
and send collaterals to several spinal
segments.
■Excitatory interneurons that synapse with
alpha motor neurons serving flexors are
stimulated. Contraction of flexors removes
the limb from the aversive stimulus.
■Reciprocal
inhibition
suppresses
contraction of the extensors of the affected limb.
■In this example, where one leg would suddenly be lifted, the person would rarely fall
down because the postural support of the other limb would be simultaneously increased.
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