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.
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