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After studying this chapter, you should be able to:

  • Describe the basic elements of a reflex pathway.

  • Identify the components, function, and afferent nerve fibers of the muscle spindle.

  • Explain the neuronal response initiated by striking the patellar tendon (patellar tendon or knee jerk reflex) that leads to contraction of a muscle.

  • Explain how the activity of γ-motor neurons alters the response to muscle stretch.

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

  • Define physiologic tremor, clonus, and muscle tone.

  • Identify the components and function of the withdrawal reflex pathway.

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

  • Describe how skilled movements are planned and carried out.

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

  • Describe the clinical tests and findings that distinguish between upper and lower motor neuron disorders, including the Babinski sign and clonus.

  • Identify the pathophysiology and characteristics of cerebral palsy, decerebrate rigidity, and decorticate rigidity.

  • 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, Huntington disease, and other pathologies of the basal ganglia pathways.

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


Somatic motor function depends ultimately on the activity of the spinal motor neurons and homologous neurons in the motor nuclei of the cranial nerves. These neurons, the final common pathways to skeletal muscle, are bombarded by impulses from an immense array of descending pathways, other spinal neurons, and peripheral afferents. Some of these inputs end directly on α-motor neurons, but many exert their effects via interneurons or via γ-motor neurons to the muscle spindles and back through the Ia afferent fibers to the spinal cord. It is the integrated activity of these multiple inputs from spinal, brainstem, midbrain, and cortical levels that regulates the posture of the body and makes coordinated movement possible.

The inputs converging on motor neurons have three major functions: to induce voluntary activity, to adjust body posture, and to make movements smooth and precise. The patterns of voluntary activity are planned within the brain, and the commands are sent to the muscles primarily via the corticospinal and corticobulbar systems. Posture is continually adjusted not only before but also during movement by information carried in descending brainstem pathways and peripheral afferents. Movement is smoothed and coordinated by the medial and intermediate portions of the cerebellum (spinocerebellum) and its connections. The basal ganglia and the lateral portions of the cerebellum (cerebrocerebellum) are part of a feedback circuit to the premotor and motor cortex that is concerned with planning and organizing voluntary movement.

Chapter 11 introduced somatomotor control by ...

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