Physiology of Micturation

Micturation is the discharge of urine from the bladder via the urethra.

This page continues from the general description of the human bladder and the separate labeled diagrams of the male and female bladder and urethra. It summaries the processes that lead to normal micturation (which is also known as urination).

As explained on previous pages, the bladder is a loose sack that can accommodate a range of volumes of urine - from 0 ml (immediately after the bladder has been emptied), to a maximum of around 300-400ml in normal adults, less in cases of children and adults of below average size. When the quantity of urine contained in the bladder exceeds that necessary to cause tension in the walls of the bladder this is communicated to the brain (i.e. the Central Nervous System, CNS) and is perceived consciously as a sensation recognised as due to a "full" bladder.

Urine is released from the bladder into the urethra, and then out of the body, as a result of the actions of muscles. These muscles are innervated by nerve cells called motor neurons acting at neuromusclar junctions (NMJs). The rest of this page summarises which parts of the nervous system act on which individual muscles in the processes leading to micturation (also known as urination).

Nervous System control of Micturation (recap structure of nervous system)

Recall from knowledge of the organisation of the nervous system that the Peripheral Nervous System (PNS) which includes all nervous system tissue outside of the brain and spinal cord, may be subdivided into the:

• Somatic Nervous System (SNS), the
• Autonomic Nervous System (ANS), and the
• Enteric Nervous System (ENS), which relates to the nerves of the gut.

Of these, the ENS is an involuntary part of the nervous system that relates only to the gut and therefore plays no part in micturation. However, both the SNS and the ANS are involved in this process because the SNS controls skeletal (also known as striated or voluntary muscles) and the ANS controls smooth (involuntary) muscles. More specifically, the part of the ANS that controls motor neurons (and therefore ultimately muscles) consists of the sympathetic division and the parasympathetic division. Many tissues are innervated by both of these divisions (sometimes referred to as "systems" rather than "divisions" - although they are also sections of the Autonomic Nervous System). In the cases of tissues innervated by both sympathetic and parasympathetic divisions of the ANS, the nerves of these two divisions generally have opposing effects.

Control of Muscles involved in Micturation

The detrusor muscle is the (smooth) muscle of the bladder wall and, together with the urethral (internal) sphincter muscle located at the neck of the bladder, is innervated by the sympathetic nerve fibres from the lumbar sections of spinal cord, and also by the parasympathetic nerve fibres from sacral segments 2 - 4 of the spinal cord.
Recall that these muscles are NOT under voluntary control.

However, the external urethral sphincter muscle is under voluntary control, and as such is innervated by the SNS.

Micturation Reflex

The micturation reflex is an autonomic spinal cord reflex that initiates urination.

Involuntary Action:
As mentioned above, the bladder wall can accomodate increasing volumes of urine with little change in the tension of the bladder wall until the volume of urine reaches certain a threshold (which is typically in the range 300-400 ml for normal adults). If/when this "threshold volume" (of urine) for a particular individual is reached then he/she experiences the significant discomfort associated with the increased tension in the bladder wall - and the micturation reflex is triggered. In this situation, micturation would occur involuntarily.
That is, it would have been triggered by the Peripheral Nervous System (PNS).

Voluntary Action:
The micturation reflex can also be triggered consciously - that is, by the Cental Nervous System (CNS).
This is the more usual (and convenient !) situation. Proof that the micturation (emptying) reflex can also be triggered by the brain/CNS is demonstrated by the fact that the bladder can be emptied at any volume.

In most normal cases micturation is voluntary (consciously triggered at the person's convenience). If not triggered voluntarily then the spinal cord reflexes that would give rise to immediate micturation may be temporarily inhibited by the brain until either it is appropriate to urinate, or the autonomic reflex can no-longer be inhibited.

If the autonomic reflex has been temporarily inhibited by the brain, or the micturation reflex was triggered consciously (e.g. before the volume of urine was sufficient to increase the tension in the bladder wall), then when the micturation reflex is finally triggered, it is assisted by the CNS relaxing the external urethral sphincter muscle to facilitate flow of urine from the body.

Regardless of how it is activated, once in progress, the flow of urine is aided by additional reflexes from the urethra that are stimulated by the flow of urine and which also reinforce contraction of the bladder muscle and relaxation of the sphincter muscle.

Completion of the emptying of the bladder is also facilitated by contration of the abdominal wall and pelvic floor muscles.

Next: Now review the composition of urine.