The sacral region, or sacrum, is located at the rear of the pelvis and consists of five sacral vertebrae that are fused together in the adult human. It forms the rear wall of the pelvis, and is connected to the coccyx at its lower end. Four pairs of openings are located on either side of the sacrum to allow passage of the sacral nerves. Some branches of the sacral nerves along with some of the lumbar nerves join to form the sacral plexus and sciatic nerve.
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The sacral plexus and sciatic nerve provide both sensory and motor nerves for part of the pelvis, the back of the thighs, the lower part of the legs and the feet. The area between the last lumbar vertebra and the sacrum experiences a lot of strain due to the twisting and bending of the body, which all hinges on this area. This can cause a hernia of the disc, which puts pressure on the nerve causing extreme pain in some of the areas controlled by it, and is commonly called sciatica. People who suffer from osteoporosis or from rheumatoid arthritis are more prone to stress and fatigue fractures in the sacrum, and hence to sciatica.
The control of the rectal muscles is affected by the pudendal nerve, one of the sacral nerves. When the rectum receives waste matter from the colon, it compresses it into a solid mass called a stool. When the rectum is full and ready to be emptied it sends a signal to the brain by means of sacral receptors. When the body is ready to evacuate stools, a signal is sent to the anal sphincter, which normally acts as a plug. The sphincter muscles are relaxed and the rectal muscles force the stool out of the body. When there are problems with the sacral nerves they can cause scoliosis or haemorrhoids.
The pudendal nerve also controls the functioning of the bladder in much the same way as that of the rectal organs. The bladder receives waste products from the kidneys by way of the two ureters. When the bladder becomes filled up the distension stimulates stretch receptors, which send a signal to the brain via the sacral nerves. Urine leaves the bladder by way of the urethra, which has two sphincters that are usually closed in the same way as the anal sphincter does with the rectum. When it is ready the brain sends a signal via the sacral nerves to relax the sphincters, emptying the bladder.
The pudendal nerve also supplies both the male and female external genitalia, and at sexual climax is the route by which the feelings of orgasm in both sexes are transmitted to the brain. A difficult childbirth or even the act of riding a bicycle can compress this nerve, giving a transitory loss of function. Penile erection is also caused by somatic stimulation of the sacral nerves, and damage to or pressure on a sacral nerve is thought to be one possible cause of erectile dysfunction.
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