Understanding Physiology of Erection and Ejaculation

Introduction

During sexual intercourse, penis erection and ejaculation are closely associated functions. Erection is an event resulting in blood filling the corpora cavernosa, two cylindrical tubes, along with the length of the penis. Ejaculation represents the climax of sexual intercourse and comprises emission, that is, the secretion of semen. It is the propulsion of semen. The functionality of the penis depends on the balance between several factors, such as several central and peripheral neurologic factors associated with the molecular physiological, psychological, vascular, and endocrinological factors. Despite many research studies on the physiology of erection and ejaculation being published, much regarding this topic is still unknown. This article summarizes the physiology of the mechanism of erection and ejaculation.

What Is the Physiology of an Erection?

• Penis erection is the physiologic process in which the penis becomes engorged with blood in the presence of sexual arousal or sometimes spontaneously.

• Cerebral, autonomic, and molecular mechanisms orchestrate the erection response during sexual intercourse. In addition, a smooth musculature corpora cavernosa in the penis and smooth muscles of arterial blood vessels play a key role in erection.

• During the flaccid state, the smooth muscles remain moderately contracted, thus only allowing a small amount of blood flow for nutritious purposes in two long cylindrical structures, corpora cavernosa. Sexual stimulus induces the release of neurotransmitters from the brain, which results in the relaxation of the contracted smooth muscles, resulting in the following events. First, sexual stimulation causes the release of neurotransmitters from the brain that leads to dilatation of the arterial blood vessels by increased blood flow. Then the trapping of incoming arterial blood flow and reduction in venous blood outflow occurs, which increase in po2 (partial pressure of oxygen) to about 90 mm Hg and an increase in intracavernous (pressure in two cylindrical structures in the penis) around 100 mm Hg and then further result in an increase of pressure to several hundred mm Hg by contraction of ischiocavernosus muscle giving rise to a rigid erection.

• The angle of an erect penis is determined by its size and attachment to the ligaments, which are connected to the surface of the pubic bone. In men with a long and heavy penis or loose suspensory ligament, the angle may not be greater than 90 degrees, even with a fully rigid erection.

What Is the Role of the Cerebral Mechanism Behind an Erection?

Erotic visual stimuli or thoughts induce cerebrally controlled erection. The main cerebral structure involved in the erection of a penis is the hypothalamus part of the brain.

The most important brain neurotransmitter for an erection is a hormone named dopamine, which stimulates oxytocin and releases other important neurotransmitters. In addition, several other brain neurotransmitters involved in the erection process, to varying degrees, are:

• Norepinephrine.

• Nitric oxide.

• Alpha-melanocyte-stimulating hormone.

• Opioid peptides.

What Is the Role of Autonomic Control Behind an Erection?

The main mechanism through this reaction pathway is the central suppression of sympathetic nervous system stimulation. The spinal cord contains the autonomic neurons that innervate the erectile tissue of the penis, and the motoneurons innervate the perineal muscles. The peripheral and supraspinal origin information activates these spinal neurons that control erection. Both peripheral and supraspinal input is capable of either inducing erection or modulating or inhibiting already present erection.

What Is the Role of the Molecular Mechanism Behind Erection?

During sexual arousal, Nitrous oxide is released from the cavernous nervous system and activates certain neurotransmitters, eventually leading to smooth muscle relaxation and vasodilation. This eventually relaxes the muscle, and vasodilation leads to blood flow in the corpora cavernosa, resulting in increased intracorporal pressure of more than 100 mm Hg during full erection, markedly reducing the venous outflow.

What Is the Physiology of Ejaculation?

The autonomic nervous system heavily controls the physiological process of ejaculation. There are two main phases of the erection process:

• Emission

• Impulsion.

What Is the Emission Phase in the Ejaculation Process?

• The first step in the admission phase is the closure of the bladder neck, preventing retrograde spillage of seminal fluids into the bladder. This process is followed by the ejection of prosthetic secretions, 10 % of the final semen volume, into The prostatic urethra.

• Subsequently, the pelvic plexus sends dense autonomic nerve supply, both sympathetic and parasympathetic, to the organs involved in the emission process.

• The neural autonomic system transmits input to produce emission from genital stimulation. The emission phase of ejaculation is under significant control of the brain and can be induced physically or with erotic stimulation.

What Is the Expulsion Phase in the Ejaculation Process?

• Emission is followed by expulsion as the process of ejaculation climaxes. The expulsion process refers to the ejection of semen through the urethral opening in the penis. The semen is pushed through the urethral meatus (urethral opening) by the rhythmic contraction of pelvic muscles associated with muscles attached to the penis and pelvis. The bladder neck remains closed to facilitate forward semen expulsion.

• The exact triggering mechanism for this expulsion is still unknown. However, there is no evidence that men can voluntarily control expulsion. Numerous studies have suggested that rhythmic contraction during orgasm can occur in the absence of emission of the seminal fluid process, that is, in dry ejaculation, such as in prostatectomy cases.

• Moreover, studies establish that the expulsion process is a continuation of the process initiated by emission after reaching a certain threshold of final activation.

What Is the Role of Neurochemical Regulation in the Ejaculation Process?

The ejaculation process involves many neurotransmitters. Definitively, the exact role of this neurotransmitter is difficult, given the variety of sexual parameters being affected because of different sites of action in the spinal and supraspinal pathways and the presence of multiple receptor types. However, some molecules that play a major role in ejaculations are neurotransmitters such as nitric oxide, serotonin, and dopamine.

What Is the Role of Hormonal Regulation in the Ejaculation Process?

Although the hormonal system majorly regulates men's sexual functions, the clinical studies to evaluate this regulation of ejaculation are very few, and the knowledge about the effect of hormones on the ejaculation process is still lacking. Some hormones that have been proven by studies to be regulating the ejaculation process are:

• Oxytocin.

• Prolactin.

• Thyroid hormone.

• Glucocorticoid.

• Estrogens.

• Androgens.

What Happens After the Ejaculation of Seminal Fluid?

After ejaculation, the penis returns to its flaccid state by relaxation of arterial muscles and subsequent contraction. This mild to moderate contraction reduces blood flow to the usual rate and volume level. The pressure on the veins is also reduced, resulting in more outflow of venous blood, and blood flow continues at its normal pre-erection pace.

Conclusion

Erection and ejaculation is a complex process involving several anatomic organs and structures associated with extensive neurochemical and hormonal regulation. Although orgasm is closely associated with ejaculation, it is a distinct physiological process, entirely different from ejaculation. In addition, many aspects of these physiological processes are still unknown. Therefore, research is needed to comprehend the mechanism of physiology and pathological dysfunction to identify the definitive treatment and management of ejaculatory and erectile dysfunctions.