Penile erection physiology

Identify the structures of the male reproductive system and describe the physiology of a penile erection. 

Figure 19.17 The biochemistiy and physiology responsible for penile erection. Sexual activity itself begins with a state of arousal that leads to erection. Arousal results in part from stimulation of the sense organs. The hypothalamus coordinates the sensations and activates the autonomic nervous system. Sensory nerves from the skin of the penis and other erogenous zones stimulate the parasympathetic system. This activates nitric oxide synthase and the resultant nitric oxide, via cyclic GMP, causes vasodilation of the arterioles. This increases blood flow through the corpora cavernosa which then expands producing an erection. Pheromones secreted by the female can stimulate the odour detecting system in the nasal cavity of the male (Chapter 12 and see above). Stress, however, activates the sympathetic system releases cyclic AMP which can result in vasoconstriction of the arterioles. Other factors that can interfere with an erection are physical fatigue and alcohol.

The physiology of penile erection involves an interplay of anatomical, hemodynamic, neurophysiological, and sex hormone interaction. Penile erection is the result of a complex interaction between the central nervous system and other local factors. This physical event also can be influenced by psychological factors. 

The physiological importance of nitric oxide should also be mentioned. It plays an important role in smooth muscle relaxation, platelet inhibition, neurotransmission, immune regulation, and penile erection (Nobel Prize in 1998 for the discovery of its role in the cardiovascular system). The importance of NO in biological systems stimulated the development of electrochemical sensors and the investigation of the electrochemical behavior of that compound. 

Kim, N., Vardi, Y, Padma-Nathan, H, Daley, J, Goldstein, 1, and Saenz de Tejada, I (1993). Oxygen tension regulates the nitric oxide pathway. Physiological role in penile erection. J Clin Invest 91(2) 437-442. 

A normal penile erection requires the full functioning of several physiologic systems vascular, nervous, and hormonal. The patient must also be psychologically receptive to sexual stimuli. 

As additional information about the physiological role of NO is accumulated, new ideas and strategies for the use of NO donors are emerging. For example, several studies have demonstrated the importance of NO in penile erection (Rajfer et al., 1992 Burnett et al., 1992), and it is possible that selective delivery of NO, through the use of NO donors, may improve erectile function in impotent men. Stief etal. (1992) have shown that intracavernosal injections of 0.1-1 mg of linsidomine in patients with erectile dysfunction produced dose-dependent erectile responses with no systemic or local side effects. Porst (1993) has also shown linsidomine injection to be useful in producing penile erection, but not as effective as injected prostaglandin Ei. Further studies to optimize NO delivery for erectile responses appear to be warranted. 

Physiology of Penile Erection and Pathophysiology of Erectile Dysfunction... 

Smooth muscle relaxation is thus the key element in attaining an erection. Administration of prostaglandins can also relax penile smooth muscle and elicit erections in a manner that mimics typical physiological mechanisms. Thus, intrapenile... 

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