Figure 2-18. Abnormal spontaneous activity. A. Positive sharp waves and fibrillations recorded from a paralyzed, denervated muscle. A typical positive sharp wave is seen above the star. The fibrillations (arrow) are 1 to 2 ms in duration, 100 to 300 mV in amplitude, and largely negative (upward) in polarity following an initial positive deflection. B. Fasciculation. This spontaneous motor unit potential was recorded from a patient with amyotrophic lateral sclerosis. It has a serrated configuration and it fired once every second or two. Calibrations: 5 ms (horizontal) and 200 μV in A; 1 mV in B (vertical).
Figure 2-19. A. Myotonia congenita (Thomsen disease). The five lines are a continuous record of activity in the biceps brachii following a tap on the tendon. The initial response is within normal limits, but it is followed by a prolonged burst of rapid activity, gradually subsiding over a period of many seconds or minutes. B. Same electrode placement as in A. Response to the fifth of a series of tendon taps. “Warmup” has occurred, and the characteristic prolonged myotonic activity is no longer evident. (See Chap. 48 for a description of the disease.)
Figure 2-20. Normal, myopathic, and reinnervated motor units. The colored muscle fibers are functional members of one motor unit, whose axon enters from the upper left and branches terminally to innervate the appropriate muscle fibers. The action potential produced by each motor unit is seen to the right: its duration is measured between the two vertical lines. The normal-appearing but uncolored fibers belong to other motor units. A. There are five muscle fibers illustrated in the active unit. B. In this myopathic unit, only two fibers remain active; the other three (shrunken) were affected by one of the primary muscle diseases. C. Four fibers that originally belonged to other motor units and had been denervated are now reinnervated by terminal sprouting from an undamaged axon. Both the motor unit and its action potential are now larger than normal. Note that only under these abnormal circumstances do fibers in the same unit lie next to one another.
In this major second part of the book, the cardinal manifestations of neurologic disease are described. In order to understand the signs and symptoms created by disordered function of the nervous system, it is necessary to first describe the normal anatomy and physiology, as they pertain to disease. Each chapter, therefore, begins with a consideration of these basic facts, followed by the manner in which they are affected by disease states and give rise to observable changes such as weakness, incoordination, abnormal movements, sensory loss, and pain.
Disorders of Motility
CHAPTER 4  Disorders of Movement and Posture
CHAPTER 5  Ataxia and Disorders of Cerebellar Function
CHAPTER 6  Disorders of Stance and Gait
The control of motor function, to which much of the human nervous system is committed, is accomplished through the integrated action of a vast array of segmental and suprasegmental motor neurons. As originally conceived by Hughlings Jackson in 1858, purely on the basis of clinical observations, the motor system is organized hierarchically in three levels, each higher level controlling the one below. It was Jackson’s concept that the spinal and brainstem neurons represent the lowest, simplest, and most highly organized motor centers; that the motor neurons of the posterior frontal region represent a more complex and less closely organized second motor center; and that the prefrontal parts of the cerebrum are the third and highest motor center. This scheme is still regarded as being essentially correct, although since Jackson’s time the importance of the parietal lobe and basal ganglia in motor control has been recognized.
More recently, functional imaging has analyzed motor organization and found it to be remarkably complex. Motor and sensory systems, although separated for practical clinical purposes, are not independent entities but are closely integrated. Without sensory feedback, motor control is ineffective. Furthermore, at the higher cortical levels of motor control, motivation, planning, and other frontal lobe activities that subserve volitional movement are preceded and modulated by activity in the parietal sensory cortex.