Elimination of Urine

In lab we discussed how as urine forms in the nephrons, it enters the collecting ducts and runs into the renal pelvis and then into the ureters. From there, the urine flows through the ureters, via openings in the trigone, into the bladder. Urine is stored in the bladder for some time, and then is expelled through the urethra.

Last semester you learned that the bladder is lined by transitional epithelium. Remember, that is the only epithelium that can undergo the daily stretching and relaxing that the bladder repeatedly performs.

What I want to work with you on now is micturition. Micturition is the scientific word for urination. We all know that when a human is born, they are unable to control their timing of micturition. But, as the infant develops into a child, one day that child is suddenly able to control their timing of micturition. How is that possible? After all, we have learned that voluntary and involuntary behaviors are controlled by different portions of the nervous system (central and autonomic, respectively).

We all possess a reflex behavior called the micturition reflex. This reflex occurs when our bladder gets rather distended. There are sensory neurons (in stretch receptors) that lie within the bladder wall and report when it is distended. The idea of this reflex is that one needs to urinate before the bladder ruptures from getting too distended!

In an infant, this micturition reflex is their only control over their bladder. Let's start with an empty bladder in our infant. When the bladder is empty or only a little full, there is an internal sphincter muscle that remains contracted at the junction between the urethra and the bladder. The internal sphincter muscle is composed of smooth muscle. Then, the bladder fills, it distends, and sensory neurons report the distention back to the spinal cord. The sensory information inhibits the motor neurons to the internal sphincter muscle. These motor neurons are visceral (involuntary) motor neurons. Motor neuron inhibition leads to relaxation of the internal sphincter muscle, and the bladder empties.

What about in an adult? Well, we all have another sphincter muscle, the external sphincter muscle (or external urethral sphincter). The external sphincter muscle wraps around the urethra, near the bladder. This muscle is composed of skeletal muscle, and is under voluntary control by other, voluntary, motor neurons in the spinal cord. So, when an adult's bladder distends, even if the internal sphincter relaxes, the adult can willingly maintain a contraction of the external sphincter muscle.

Both the involuntary motor neurons to the internal sphincter muscle and the voluntary motor neurons to the external sphincter muscle lie in the sacral spinal cord.

So, why can't kids control their urination before they are 2? (or 3?). In order to have voluntary control over anything, we have to be able to decide to do it in our motor cortex, and then signal the motor neurons to do it. The signaling from the motor cortex to the spinal motor neurons requires the corticospinal tract through the spinal cord. Remember that?

The corticospinal tract has a long way to grow. It takes longer for this tract to complete its journey than any other. So, when an infant is born, the connection from the motor cortex to the spinal motor neurons is not yet complete. It takes time (somewhere around 1 1/2 to 2 years) for this tract to finish growing. After that, it takes time for the child to realize that they really do have control over micturition. And then, of course, toilet training is more than a physiological process-- kids have to want to do it.