Innervation and Function of the Detrusor Muscle

For successful urination to occur, the detrusor must contract while the internal sphincter simultaneously relaxes. If the detrusor contracts while the sphincter remains tightly closed, it can lead to increased pressure and discomfort without successful voiding.

Neurological Control: The Autonomic Nervous System

The movement of the detrusor muscle is governed by the autonomic nervous system (ANS). The ANS is the part of the nervous system responsible for regulating involuntary physiological processes, such as heart rate, digestion, and bladder contraction.

Within the broader framework of the autonomic nervous system, the detrusor muscle is specifically targeted by the parasympathetic nervous system.

The Role of Parasympathetic Stimulation

The parasympathetic nervous system is often referred to as the "rest and digest" or "feed and breed" system, but in the context of the urinary tract, it is the "voiding" system. When the bladder reaches a certain level of fullness, sensory signals are sent to the central nervous system, which responds by activating parasympathetic pathways.

This activation results in:

• Increased Detrusor Contraction: Parasympathetic signals trigger the smooth muscle fibers of the detrusor to shorten, squeezing the bladder.
• Facilitation of Micturition: This contraction increases the pressure within the bladder, driving urine toward the urethra.

💡 Study Tip: The "Dual Label" Concept

In many medical examinations, you may encounter questions regarding the innervation of the detrusor. It is important to remember that while parasympathetic is the most specific and primary answer, it is also technically a part of the autonomic nervous system. If a question asks for the type of nervous system that innervates the detrusor, both "parasympathetic" and "autonomic" are considered correct descriptions of its control mechanism.

Summary Table for Quick Review

Worked Example: The Micturition Reflex

To apply these concepts, let's trace a single physiological event:

1. Stimulus: The bladder fills with urine, stretching the bladder walls.
2. Detection: Stretch receptors in the detrusor muscle detect this tension.
3. Signal: Sensory information travels to the spinal cord and brain.
4. Response (The Reflex): The brain initiates a response via the parasympathetic nervous system.
5. Action: The detrusor muscle contracts, and the internal urethral sphincter relaxes.
6. Result: Urine is expelled from the body.

By understanding this chain of command—from the autonomic nervous system to the specific smooth muscle of the bladder wall—you can master the complex mechanics of urinary physiology.