Urination is the process of disposing urine from the urinary bladder. The process of urination is usually under voluntary control. When control over urination is lost or absent, this is called urinary incontinence.
Urinary System: Micturition Reflex is activated when the urinary bladder wall is stretched and it results in micturition, which is the elimination of urine from the bladder. the mictiration occurs in the spinal cord, specifically in the sacral region. that is modified by the higher centersin the brain the pons and cerebrum. it is due ro the presence of urine in the bladder stimulates the stretch recptors, which preduces action potentional.
The action potentials are carried by sensory neurons to the sacral segments of the spinal cord through the pelvic nerves. the parasympathetic fibers carry the action potentials to the urinary bladder in the pelvic nerves. This causes the wall of the bladder to contract. In addition, decreased somatic motor action potentials cause the external urinary sphincter, which consists of skeletal muscle, to relax. When the external urinary sphincter is relaxed urine will flow from the urinary bladder when the pressure there is great enough to force urine to flow through the urethra. The micturition reflex normally produces a series of contractions of the urinary bladder.
Action potentials carried by sensory neurons from stretch receptors in the urinary bladder wall also ascend the spinal cord to a micturition center in the pons and to the cerebrum. Descending potentials are sent from these areas of the brain to the sacral region of the spinal cord, where they modify the activity of the micturition reflex in the spinal cord. The micturition reflex, integrated in the spinal cord, predominates in infants. The ability to voluntarily inhibit micturition develops at the age of 2-3 years, and subsequently, the influence of the pons and cerebrum on the spinal micturition reflex predominates. The micturition reflex integrated in the spinal cord is automatic, but it is either stimulated or inhibited by descending action potentials. Higher brain centers prevent micturition by sending action potentials from the cerebrum and pons through spinal pathways to inhibit the spinal micturition reflex. Consequently, parasympathetic stimulation of the urinary bladder is inhibited and somatic motor neurons that keep the external urinary sphincter contracted are stimulated.
The pressure in the urinary bladder increases rapidly once its volume exceeds approximately 400-500 ml, and there is an increase in the frequency of action potentials carried by sensory neurons. The increased frequency of action potentials conducted by the ascending spinal pathways to the pons and cerebrum results in an increased desire to urinate.
Voluntary initiation of micturition involves an increase in action potentials sent from the cerebrum to facilitate the micturition reflex and to voluntarily relax the external urinary sphincter. In addition to facilitating the micturition reflex, there is an increased voluntary contraction of abdominal muscles, which causes an increase in abdominal pressure. This enhances the micturition reflex by increasing the pressure applied to the urinary bladder wall.
The initiation of urination is caused by the stretch in the wall of the baldder. But also irritaion such as the urinary bladder or The urethera bacterial infections or other conditions can also initiate the desire to urinate, even though the urinary bladder may be nearly empty.
During urination, muscles in the wall of the bladder contract, forcing urine out of the bladder and into the urethra. At the same time, sphincter muscles surrounding the urethra relax, letting urine pass out of the body. Urinary incontinence will occur if the bladder muscles suddenly contract or muscles surrounding the urethra suddenly relax.
In wealthy countries, humans usually urinate at a toilet (also known euphemistically as a "bathroom"). Public toilets are often separate for men and women, and may be partitioned for reasons of modesty in some countries.
Those for men also often have urinals, since most men can urinate standing up, and most women cannot. Urinals may have partitions between them to avoid men being able to see the penises of other men, a social taboo in many countries.
Men's trousers usually have a fly allowing men to urinate without lowering the whole trousers. The fly has buttons or a zipper. Either just the fly is opened or also the fastening at the waist (including, if present, the belt). Additionally, the fly of the underpants is used or their frontside is lowered. All combinations are possible. Trousers without fly, like some jogging trousers, have usually an elastic waist band allowing lowering the front side like underpants.
A novelty, still rare, are public urinals for women, allowing them also to urinate standing up. This is done using a special tool [1] or with the finger-assist method [1].
A common transgression is urinating in the street. Often this is done after consumption of alcoholic beverage: the alcohol causes production of additonal urine as well as a reduction of inhibitions. This is sometimes referred to as a "New Orleans Piss".
In dogs and other animals, urination can mark territory or express submissiveness.
