Autonomic outflow and the bladder

Why it matters

Heart rate, digestion, bladder control and many other automatic functions are run by the autonomic nervous system, and much of its output leaves through the spinal cord ^[1]. Which levels it leaves from explains why an injury at a given level affects particular organs ^[2].

What it is

The autonomic nervous system has a sympathetic and a parasympathetic division ^[1]:

• The sympathetic division, linked to the fight-or-flight response, connects to the organs from the thoracic and upper lumbar cord, so it is called the thoracolumbar system ^[1].
• The parasympathetic division, linked to rest and digest, is called the craniosacral system because its preganglionic neurons are in brainstem nuclei and in the lateral horn of the sacral cord ^[1].

How it works

Sympathetic neurons in the lateral horn send axons to the chain ganglia beside the vertebral column or to collateral ganglia in front of it; ganglionic neurons then project to the target organs ^[1]. There are typically 23 chain ganglia on each side, and because the ganglia lie next to the column, preganglionic sympathetic fibres are relatively short and myelinated ^[1].

Parasympathetic preganglionic fibres from the sacral cord travel in spinal nerves to terminal ganglia near or within the target organ, so their preganglionic fibres are long and postganglionic fibres short ^[1].

When

Normal voiding is a spinal reflex: stretch receptors in the bladder wall send impulses to the sacral cord ^[2].

1. When bladder volume reaches about 150 mL^[2], an urge to void is sensed but is easily overridden ^[2].
2. During filling, sympathetic activity through the hypogastric nerves suppresses detrusor contraction, and the voluntary external sphincter stays contracted ^[2].
3. With further stretch, signals in the sacral pelvic nerves activate parasympathetic neurons; the detrusor contracts and the internal sphincter relaxes, while the cord inhibits the motor neurons of the external sphincter ^[2].

If voluntary restraint fails, incontinence occurs as bladder volume approaches 300 to 400 mL^[2].

The micturition reflexStretch receptors in the bladder wall signal the sacral cord. Further stretch activates parasympathetic neurons, which contract the detrusor and relax the internal sphincter, while the cord inhibits the motor neurons of the external sphincter.

Step through

• Excitatory synapse (filled arrowhead)
• Inhibitory synapse (bar)
• Modulatory (open circle)
• Signal or data flow, not a synapse (dashed)
• Midline crossing (decussation)

• During filling, sympathetic activity through the hypogastric nerves suppresses detrusor contraction; voluntary control of the external sphincter runs through the pudendal nerve.^[2]

Simplified: Brainstem and cortical control of the reflex are not drawn; the inhibition of the external sphincter is drawn as one link from the afferent signal.

Text description of this diagram

1. Stretch receptors in the bladder wall signal the sacral cord. Further stretch activates parasympathetic neurons, which contract the detrusor and relax the internal sphincter, while the cord inhibits the motor neurons of the external sphincter.
2. Stretch receptors (left side, Bladder) to Pelvic nerve (left side, Nerves); signal or data flow (not a synapse).
3. Pelvic nerve (left side, Nerves) to Parasympathetic neurons (left side, Sacral cord); excitatory synapse; activates.
4. Pelvic nerve (left side, Nerves) to Pudendal motor neurons (midline, Sacral cord); inhibitory synapse; cord inhibits.
5. Parasympathetic neurons (left side, Sacral cord) to Detrusor (midline, Bladder); excitatory synapse; acetylcholine.
6. Pudendal motor neurons (midline, Sacral cord) to External sphincter (right side, Bladder); signal or data flow (not a synapse); less drive.

Why it is built this way

Children learn to override the reflex by controlling the external sphincter ^[2]. Voluntary control of that sphincter runs through the pudendal nerve, from the sacral cord via S2 to S4; a group of sacral neurons called Onuf's nucleus contracts the external sphincter and maintains continence ^{[2, 3]}.

When it does not behave

The micturition reflex may be preserved after a spinal cord injury that causes paraplegia or quadriplegia, but the external sphincter may not relax in every case, so periodic catheterisation may be needed to empty the bladder ^[2]. Voluntary micturition requires an intact spinal cord and a working pudendal nerve ^[2].

Common misconceptions

Check yourself

Why is the sympathetic system called thoracolumbar?

Its connections to the organs emerge from the thoracic and upper lumbar spinal cord ^[1].

Which nerve carries voluntary control of the external urethral sphincter?

The pudendal nerve, from the sacral cord via S2 to S4 ^[2].

Read next

• Spinal cord injury.
• The S2 segment.