Innervation of the urinary bladder
For proper function, the urinary bladder receives three types of innervation and also has an extensive network of sensory fibers.
- Sympathetic innervation: Centers lie in the lateral horns at the Th11–L2 level. Preganglionic fibers synapse in the inferior mesenteric ganglion (ganglion mesentericum inferius), and postganglionic fibers pass through the inferior hypogastric plexus. Their task is to relax the detrusor muscle and contract the internal sphincter.
- Parasympathetic innervation: Centers are located in the sacral segments S2–S4. Fibers run in the pelvic splanchnic nerves (nn. splanchnici pelvini). Their activation causes detrusor contraction.
- Somatomotor innervation: Motor neurons of the anterior horns of S2–S4 send impulses via the pudendal nerve (n. pudendus) to the striated muscle of the external sphincter, providing voluntary control.
- Afferent pathways: Sensory impulses from stretch receptors go to the spinal cord, and then via tracts to the brainstem and cerebral cortex, forming a conscious urge to urinate.
Mechanisms of storage and emptying
The operation of the urinary bladder is divided into two alternating phases.
- Storage phase. The sympathetic system is activated. It stimulates $\alpha$-adrenergic receptors of the internal sphincter, closing it securely, and simultaneously inhibits detrusor activity. The external sphincter is also in tone, resisting the increase in intravesical pressure.
- Emptying phase. When the walls are stretched, visceral afferents are activated. The signal is processed in the pontine micturition center (near the locus coeruleus), which coordinates the process via reticulospinal pathways: the sphincters relax, and the parasympathetic system forces the detrusor to contract. Higher centers (frontal cortex, cingulate gyrus, basal ganglia) manage this process by removing the inhibitory influence on the brainstem.
Sympathetic innervation of the skin and extremities
Unlike pelvic organs, the skin receives only sympathetic innervation. Its stimulation causes vasoconstriction, sweating, and contraction of the arrector pili muscles (piloerection).
| Region | Preganglionic neuron | Postganglionic neuron |
|---|---|---|
| Head and neck | Th2–Th4 segments | Superior and middle cervical ganglia |
| Upper extremities | Th3–Th6 segments | Inferior cervical and superior thoracic ganglia |
| Lower extremities | Th10–L2 segments | Lower lumbar and upper sacral ganglia |
Neurogenic bladder dysfunctions
When the nervous system is damaged at various levels, specific syndromes occur:
- Detrusor hyperreflexia: The lesion is located above the sacral segments. The inhibitory influence of higher centers is lost. It manifests as imperative (urgent) urges and incontinence with a small residual urine volume. It is often seen in multiple sclerosis and Parkinson's disease.
- Detrusor-sphincter dyssynergia: The lesion is localized between the pons and the sacral region. The detrusor contracts, but the external sphincter does not relax. The result is incomplete emptying and a risk of infections.
- Detrusor areflexia: Occurs with damage to the sacral centers or peripheral nerves (e.g., in polyradiculoneuropathies, cauda equina tumors). The patient cannot initiate urination, the bladder becomes overfilled, and urine leaks mechanically (paradoxical ischuria).
Briefly: answers to topical questions
What neurotransmitters and receptor types are involved in the parasympathetic regulation of detrusor contraction?
The neurotransmitter acetylcholine, which acts on muscarinic receptors, is involved in the parasympathetic regulation of detrusor contraction.
- Acetylcholine — is released from parasympathetic nerve endings and causes contraction of the muscular coat of the urinary bladder.
- Muscarinic receptors — are located in the smooth muscle cells of the detrusor (specifically, $M_2$ and $M_3$ cholinoceptors); their stimulation leads to emptying of the urinary bladder.
Through which spinal cord tracts do afferent impulses from bladder stretch receptors pass to the brain?
Afferent impulses from bladder stretch receptors are conducted to the brain via the spinoreticular and spinothalamic tracts. Some afferent impulses are also conducted via the dorsal columns or spinal cord pathways.
The sympathetic system "stores" urine (relaxes the wall, constricts the sphincter — stress response), while the parasympathetic system "empties" it (contracts the wall, relaxes the sphincter — rest).
Class and exam questions
Where is the main suprasegmental micturition center located?
In the reticular formation of the brainstem, near the locus coeruleus.
Why does incontinence occur in detrusor areflexia?
So-called paradoxical ischuria develops. The urinary bladder loses its ability to contract and becomes overfilled (up to 2000 ml), after which urine begins to passively leak outward due to high pressure.
How does the innervation of the external sphincter differ from that of the internal sphincter?
The internal sphincter consists of smooth muscle and is controlled by the autonomic (sympathetic) system, whereas the external sphincter is formed by striated muscle tissue and is controlled consciously via the somatic pudendal nerve.
What else to study on this topic
- True (stress) urinary incontinence during exertion
- Non-neurogenic dysfunctions: bladder outlet obstruction
- Anatomy and topical diagnosis of cauda equina syndrome
- Tethered cord syndrome
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