17.7 Transport of Sperm
From the seminiferous tubules, the sperm pass through the rete testis and efferent ducts into the epididymis and from there to the vas deferens.
Storage reservoir for sperm until ejaculation
The vas deferens and the portion of the epididymis closest to it serve as a storage reservoir for sperm until ejaculation, the discharge of semen from the penis.
the discharge of semen from the penis
Movement of the sperm as far as the epididymis
- Movement of the sperm as far as the epididymis results from the pressure that the Sertoli cells create by continuously secreting fluid into the seminiferous tubules.
- The sperm themselves are normally non-motile at this time.
During passage through the epididymis
- During passage through the epididymis, the concentration of the sperm increases dramatically due to fluid absorption from the lumen of the epididymis.
- Therefore, as the sperm pass from the end of the epididymis into the vas deferens, they are a densely packed mass whose transport is no longer facilitated by fluid movement.
- Instead, peristaltic contractions of the smooth muscle in the epididymis and vas deferens cause the sperm to move.
- The absence of a large quantity of fluid accounts for the fact that vasectomy, the surgical tying off and removal of a segment of each vas deferens as a method of male contraception, does not cause the accumulation of much fluid behind the tie-off point.
- The sperm, which are still produced after vasectomy, do build up, however, and eventually break down, with their chemical components absorbed into the bloodstream.
- Vasectomy does not affect testosterone secretion because it does not alter the function of the Leydig cells.
- The penis consists almost entirely of three cylindrical, vascular compartments running its entire length.
- Normally, the small arteries supplying the vascular compartments are constricted so that the compartments contain little blood and the penis is flaccid.
- During sexual excitation, the small arteries dilate, blood flow increases, the three vascular compartments become engorged with blood at high pressure, and the penis becomes rigid (erection).
- The vascular dilation is initiated by neural input to the small arteries of the penis.
- As the vascular compartments expand, the adjacent veins emptying them are passively compressed, further increasing the local pressure, thus contributing to the engorgement while blood flow remains elevated.
- This entire process occurs rapidly with complete erection sometimes taking only 5 to 10 seconds.
What are the neural inputs to the small arteries of the penis?
- At rest, the dominant input is from sympathetic neurons that release norepinephrine, which causes the arterial smooth muscle to contract.
- During erection, this sympathetic input is inhibited.
- Much more important is the activation of nonadrenergic, noncholinergic autonomic neurons to the arteries.
- These neurons and associated endothelial cells release nitric oxide, which relaxes the arterial smooth muscle.
Primary stimulus for erection
- The primary stimulus for erection comes from mechanoreceptors in the genital region, particularly in the head of the penis.
- The afferent fibers carrying the impulses synapse in the lower spinal cord on interneurons that control the efferent outflow.
- It must be stressed, however, that higher brain centers, via descending pathways, may also exert profound stimulatory or inhibitory effects upon the autonomic neurons to the small arteries of the penis.
- Thus, mechanical stimuli from areas other than the penis, as well as thoughts, emotions, sights, and odors, can induce erection in the absence of penile stimulation (or prevent erection even though stimulation is present).
- (also called impotence) is the consistent inability to achieve or sustain an erection of sufficient rigidity for sexual intercourse and is a common problem.
- Although it can be mild to moderate in degree, complete erectile dysfunction is present in as many as 10% of adult American males between the ages of 40 and 70.
- During this period of life, its rate almost doubles.
- The organic causes are multiple and include damage to or malfunction of the efferent nerves or descending pathways, endocrine disorders, various therapeutic and “recreational” drugs (e.g., alcohol), and certain diseases, particularly diabetes mellitus.
- Erectile dysfunction can also be due to psychological factors (such as depression), which are mediated by the brain and the descending pathways.
orally active cGMP phosphodiesterase type 5 (PDE5) inhibitors
- There are now a group of orally active cGMP-phosphodiesterase type 5 (PDE5) inhibitors including sildenafil (Viagra), vardenafil (Levitra), and tadalafil (Cialis) that can improve the ability to achieve and maintain an erection.
- The most important event leading to erection is the dilation of penile arteries by nitric oxide, released from autonomic neurons.
- Nitric oxide stimulates the enzyme guanylyl cyclase, which catalyzes the formation of cyclic GMP (cGMP).
- This second messenger then continues the signal transduction pathway leading to the relaxation of the arterial smooth muscle.
- The sequence of events is terminated by an enzyme-dependent breakdown of cGMP.
- PDE5 inhibitors block the action of this enzyme and thereby permit a higher concentration of cGMP to exist.
- As stated earlier, ejaculation is the discharge of semen from the penis.
- Ejaculation is primarily a spinal reflex mediated by afferent pathways from penile mechanoreceptors.
- When the level of stimulation is high enough, a patterned sequence of discharge of the efferent neurons ensues.
- This sequence can be divided into two phases:
- (1) The smooth muscles of the epididymis, vas deferens, ejaculatory ducts, prostate, and seminal vesicles contract as a result of sympathetic nerve stimulation, emptying the sperm and glandular secretions into the urethra (emission); and
- (2) the semen, with an average volume of 3 mL and containing 300 million sperm, is then expelled from the urethra by a series of rapid contractions of the urethral smooth muscle as well as the skeletal muscle at the base of the penis.
- During ejaculation, the sphincter at the base of the urinary bladder is closed so that sperm cannot enter the bladder, nor can urine be expelled from it.
Erection vs ejaculation
Note that erection involves inhibition of sympathetic nerves (to the small arteries of the penis), whereas ejaculation involves stimulation of sympathetic nerves (to the smooth muscles of the duct system).
- The rhythmic muscular contractions that occur during ejaculation are associated with intense pleasure and many systemic physiological changes, collectively termed an orgasm.
- Marked skeletal muscle contractions occur throughout the body, and there is a transient increase in heart rate and blood pressure.
- Once ejaculation has occurred, there is a latent period during which a second erection is not possible.
- The latent period is quite variable but may last from minutes to hours.