In human males, the cremaster muscle is a thin layer of striated muscle found in the inguinal canal and scrotum between the external and internal layers of spermatic fascia, surrounding the testis and spermatic cord. The cremaster muscle is a paired structure, there being one on each side of the body. Anatomically, the lateral cremaster muscle originates from the internal oblique muscle, just superior to the inguinal canal, and the middle of the inguinal ligament. The medial cremaster muscle, which sometimes is absent, originates from the pubic tubercle and sometimes the lateral pubic crest. Both insert into the tunica vaginalis underneath the testis.
The cremaster develops to its full extent only in males; in females it is represented by only a few muscle loops. In female humans, the cremaster muscle is smaller and is found on the round ligament. In rats, it has been shown that cremaster muscles developed from the gubernacular bulb.
Function
Retraction
The cremaster muscle's function is to raise and lower the testes in order to regulate scrotal temperature for optimal spermatogenesis and survival of the resultant spermatozoa. It does this by increasing or decreasing the exposedsurface area of the surrounding tissue, allowing faster or slower dissipation of body heat. The cremaster muscle is an involuntary muscle and contraction can occur during arousal which can prevent injury to the testicles during sex. Contraction also occurs during ejaculation. Retraction can also occur during moments of extreme fear, possibly to help avoid injuring the testes while dealing with a fight or flight situation. Clinically, a reflex arc termed the cremasteric reflex can be demonstrated by lightly stroking the skin of the inner thigh downwards from the hip towards the knee. This stimulates the sensory fibers of the ilioinguinal nerve, which enters the spinal cord at L1. The sensory fibers stimulate the motor fibers of the genital branch of the genitofemoral nerve, which provides innervation to the cremaster muscles causing the contraction of the muscle and elevation of the testes. This causes the cremaster muscle on the same side to rapidly contract, raising the testicle. The cremaster can also be contracted voluntarily, by performing Kegels, or by flexing and tightening the abdominal muscles.
Clinical significance
The cremaster muscle occasionally experiences painful spasms or cramps in adult males which can be chronic and debilitating. Treatment for these spasms ranges from minor surgery to injection with botulinum-a toxin to the regular application of heat to relax the muscle. Surgery, including the excision of the cremaster muscle, has apparently been able to provide complete relief from this condition without significant side effects.
