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After studying this chapter, you should be able to:

  • Describe the elements of the stretch reflex and how the activity of γ-motor neurons alters the response to muscle stretch.

  • Describe the role of Golgi tendon organs in control of skeletal muscle.

  • Describe the elements of the withdrawal reflex.

  • Define spinal shock and describe the initial and long-term changes in spinal reflexes that follow transection of the spinal cord.

  • Describe how skilled movements are planned and carried out.

  • Compare the organization of the central pathways involved in the control of axial (posture) and distal (skilled movement, fine motor movements) muscles.

  • Define decerebrate and decorticate rigidity, and comment on the cause and physiologic significance of each.

  • Identify the components of the basal ganglia and the pathways that interconnect them, along with the neurotransmitters in each pathway.

  • Explain the pathophysiology and symptoms of Parkinson disease and Huntington disease.

  • Discuss the functions of the cerebellum and the neurologic abnormalities produced by diseases of this part of the brain.


All neural influences affecting muscle contraction ultimately funnel through spinal or cranial motor neurons to skeletal muscles. To voluntarily move a limb, the brain must plan a movement, arrange appropriate motion at many different joints at the same time, and adjust the motion by comparing plan with performance. The motor system “learns by doing” and performance improves with repetition. Sensory fibers provide information to the central nervous system (CNS) to promote reflex-induced changes in muscle contraction. The integrated activity of inputs from spinal, medullary, midbrain, and cortical levels as well as peripheral afferents determines the posture of the body and makes coordinated movement possible. This chapter reviews the way the brain, spinal cord, and peripheral afferents work together to regulate the activity of motor neurons for voluntary and reflex-induced movements.


When a skeletal muscle is stretched, it initiates a monosynaptic reflex to contract that muscle (stretch reflex or myotatic reflex). The sense organ is the muscle spindle located within the fleshy part of the muscle. The impulses from the spindle are transmitted to the CNS by sensory fibers that project directly on α-motor neurons that supply the same muscle. Glutamate is the neurotransmitter at the central synapse. The stretch reflex is typified by the knee jerk reflex (Clinical Box 12–1).

CLINICAL BOX 12–1 Knee Jerk Reflex

Tapping the patellar tendon stretches the muscle to elicit the knee jerk, a stretch reflex of the quadriceps femoris muscle. Stretch reflexes can be elicited from most of the large muscles of the body. The knee jerk reflex is an example of a deep tendon reflex (DTR) in a neurologic examination and is graded on the following scale: 0 (absent), 1+ (hypoactive), 2+ (brisk, normal), 3+ (hyperactive without clonus), 4+ (hyperactive with mild clonus), and 5+ (hyperactive with sustained clonus). Absence of the ...

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