CHAPTER 3 Structure and Function of Muscles


Force (n)


Eccentric Concentric


67 Isometric


Lengthening velocity


0 cm/sec


Shortening velocity


Figure 3.19 The force-velocity curve. As a concentric muscle contraction decreases in speed, the force it is capable of producing increases. An isometric contraction occurs when the velocity is zero. As a muscle actively lengthens during eccentric activation, the force generated increases before it plateaus.


decreases, and the tension in the muscle increases. As the speed of contraction continues to decrease to zero, an isometric contraction occurs. During an isometric con- traction, the maximum number of cross-bridges forms. An isometric contraction produces more tension than a concentric contraction. When the load imposed exceeds the isometric force, the muscle begins to eccentrically lengthen. As the speed of lengthening increases, the muscle increases its tension and then reaches a plateau. An eccentric muscle contraction is capable of producing greater forces than the forces produced by an isomet- ric or concentric contraction. The force-velocity curve depicts the relationship between the speed of shortening or lengthening and the force produced by the muscle (Fig. 3.19).


NEURAL STRUCTURES AND MOTOR OUTPUT


Human movement and motor control are the result of elaborate interplay between the input sensory informa- tion received from joint, tendon, and muscle receptors and other sensory organs and the motor output from the CNS.


SENSORY PATHWAYS


The sensory pathways, called afferent (sensory) neurons, conduct the sensory information to the spinal cord. Some sensory information is transmitted directly to interneu- rons and efferent (motor) neurons. The impulses travel via the efferent nerves to the muscle fi bers. These simple


systems play a role in the stretch refl ex and reciprocal and autogenic inhibition (discussed in Sensory Recep- tors). Other sensory information ascends the spinal cord via pathways to the cortex where it is processed. Output impulses that infl uence the speed, direction, magnitude, and coordination of movement then descend from the cortex via nerve fi bers forming the corticospinal tract to the efferent (motor) neuron. Figure 3.20 shows the affer- ent and efferent neurons and the CNS pathways associ- ated with skeletal muscle function. Table 3.5 summarizes the neural structures involved with motor control. (The elaborate interaction among central brain centers that regulate the initiation and coordination of movement and balance is beyond the scope of this text.)


SENSORY RECEPTORS


During movement, continuous input from various sensory systems stimulates output from the CNS to make needed motor adjustments to achieve tasks. Two unique recep- tors in muscles contribute to this feedback: the Golgi tendon organ (GTO) and the muscle spindle.


Golgi Tendon Organs


The GTOs are sensory receptors located in the myoten- dinous junctions at each attachment of the muscle to the tendon. Each GTO is oriented in line with the tendon fi bers and with the muscle fi bers themselves. The GTOs are activated by tension produced in the muscle either by an active contraction or by an excessive passive stretch of the muscle. As the muscle tension increases, the fi ring of the GTO receptors escalates. When the GTO receptors are activated, they transmit nerve impulses via afferent neurons to the spinal cord and brain. The CNS responds by sending impulses through the efferent motor neurons to the contracting muscle to inhibit the muscle and reduce the force of its contraction. At the same time, the antago- nist of the contracting muscle is facilitated to contract. The agonist muscle that has excessive tension receives inhibitory input, while the antagonist receives excitatory input. This process is called autogenic inhibition.


Muscle Spindles


Muscle fi bers that make up the muscle belly and contract when stimulated by alpha motor neurons are referred to as extrafusal skeletal muscle fi bers. Within these extra- fusal muscle fi bers lie intrafusal muscle fi bers, which act as proprioceptors and form the muscle spindles. Muscle spindles are sensory receptors that function as stretch receptors. They are composed of intrafusal fi bers sur- rounded by a connective tissue sheath (Fig. 3.21). These receptors are activated in response to changes in muscle length and to the velocity of lengthening. As the muscle is stretched, the intrafusal muscle spindle fi bers are acti- vated, sending impulses through afferent neurons to the


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