Diseases of the spinal cord may abolish psychogenic erections but leave reflexive ones intact. In fact, the latter may become overactive, giving rise to sustained painful erections (priapism). This indicates that the segmental mechanism for penile erection is relatively intact. There are many other nonneurologic causes for priapism, among them are sickle cell anemia and other thrombotic states and perineal trauma.
Other sexual difficulties include the premature ejaculation of semen. After lumbar sympathectomy, the semen may be ejected back into the bladder because of paralysis of the periurethral muscle within the prostate, at the verumontanum (colliculus seminalis). Polyneuropathies, such as those caused by diabetes, may be responsible; acute or chronic prostatitis may have a similar effect.
Cerebral disorders of sexual function are discussed further in Chap. 24 (see section on “Altered Sexuality”) and the development of sexual function, in Chap. 27.
Considering that the act of breathing is directed entirely by the nervous system, it is surprising how little attention it has received other than from physiologists. Every component of breathing—the lifelong automatic cycling of inspiration, the transmission of coordinated nerve impulses to and from the respiratory muscles, the translation of systemic influences such as acidosis to the neuromuscular apparatus of the diaphragm—is under neural control. Moreover, respiratory failure is one of the most serious disturbances of neurologic function in comatose states and in neuromuscular diseases such as myasthenia gravis, Guillain-Barré syndrome, amyotrophic lateral sclerosis, muscular dystrophy, and poliomyelitis. Finally, death—or brain death—is now virtually defined in terms of the ability of the nervous system to sustain respiration, a reversion to ancient methods of determining the cessation of all vital forces. Neurologists should be familiar with the alterations of respiration caused by diseases in different parts of the nervous system, the effects of respiratory failure on the brain, and the rationale that underlies modern methods of treatment. A full understanding of respiration requires knowledge of the mechanical and physiologic workings of the lungs as organs of gas exchange; but here we limit our remarks to the nervous system control of breathing.
The Central Respiratory Motor Mechanisms
It has been known for more than a century that breathing is controlled mainly by the lower brainstem, and that each half of the brainstem is capable of producing an independent respiratory rhythm. In patients with poliomyelitis, for example, the occurrence of respiratory failure was associated with lesions in the ventrolateral tegmentum of the medulla (Feldman; Cohen). Until fairly recently, thinking on this subject was dominated by Lumsden’s scheme of the breathing patterns that resulted from sectioning the brainstem of cats at various levels. He postulated the existence of several centers in the pontine tegmentum, each corresponding to an abnormal breathing pattern—a pneumotaxic center, an apneustic center, and a medullary gasping center. This scheme proves to be oversimplified when viewed in the light of modern physiologic findings. Instead, neurons in several discrete regions discharge with each breath and, together, generate the respiratory rhythm. In other words, these sites do not function in isolation, as individual oscillators, but interact with one another to generate the perpetual respiratory cycle and they each contain both inspiratory and expiratory components.