(2) Isotonic. In isotonic exercises, the active muscles change in length. As
the prime mover decreases in length, the antagonistic muscle increases in length.
However, both muscles are producing tension.
Section IV. NERVOUS CONTROL OF SKELETAL MUSCLES
Generally, skeletal muscle tissue contracts in response to a signal from the
nervous system. The skeletal muscles of one side of the body are controlled by the
opposite side of the brain. Thus, injury to the left side of the brain tends to result in
paralysis of the right side of the body.
5-22. NEUROVASCULAR BUNDLE AND MOTOR POINT
The nerves of the body extend from the CNS to the individual muscles. Going to
the individual skeletal muscle is the neurovascular bundle. This contains the NAVL for
that muscle within a common FCT sheath. The point where this bundle enters the
muscle is called the motor point. In a clinic or laboratory, this is the last point where a
stimulus can be applied to make the whole muscle contract.
5-23. SENSORY INPUT TO THE CNS FROM THE SKELETAL MUSCLE
The central nervous system (CNS) receives information from the individual
skeletal muscle. It also sends commands for action to the muscle.
a. General Sensations. The usual general sensations of pain, temperature,
pressure, etc., are included in the input from the skeletal muscle.
b. Stretch Receptors. Associated with the individual skeletal muscle are two
sense organs which analyze the degree of tension or stretch of the muscle as a whole.
(1) The stretch reflex. The muscle spindle is located within the substance of
the fleshy belly of the muscle. The muscle spindle is very sensitive to the length of the
muscle. It continuously sends information about the specific length of the muscle.
(2) The Golgi tendon organ reflex. Another stretch receptor is the Golgi
tendon organ. As its name implies, it is located in the tendon of the muscle. When it
informs the CNS that the stretch is excessive, the CNS commands the muscle to relax.
5-24. MOTOR COMMANDS FROM THE CNS TO THE SKELETAL MUSCLE
a. Motor Homunculus. The various parts of the body are represented in the
substance of the brain. If one plots the sequence and the amount of tissue devoted to