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
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
These muscles are innervated by nerve cells called motor
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),
- 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
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.
The micturation reflex
is an autonomic spinal cord reflex that initiates urination.
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).
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
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
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
Next: Now review the composition