Micturition Reflex - Diagram and Process: Definition, Steps, Phases, Disorders

What is the Micturition Reflex?

The micturition reflex is how the body is controlled to allow urine to be released from the bladder. When the bladder is filled, the increased volume is detected by stretch receptors in the walls and will signal down the spinal cord and the brain that it requires emptying. Along with the decision to urinate, it can also be initiated by the relaxation of the internal and external sphincters of the urethra along with the contraction of the detrusor muscle of the bladder to allow urine out.

Visual Anatomy of the Reflex

It generally includes:

• The bladder

• The urethra

• The nerves

• The associated muscles

Stages of Micturition

There are two main stages of micturition:

Storage Phase

• The urine is stored inside the bladder

• The sphincter muscles are contracted not to let it be released.

• The sympathetic nervous system stimulates the detrusor muscle to be relaxed until storage is no longer appropriate and the internal urethral sphincter stays contracted.

Voiding Phase

• As the levels inside the bladder rise, the nerves get stimulated

• When urination is necessary, the brain sends nerve impulses to the bladder and the urethra.

• The external urethral sphincter is relaxed by the will.

• Urine flows from the bladder through the urethra and out of the body.

Steps of the Micturition Reflex

Micturition is the voiding of urine from the urinary bladder.

Filling of the Urinary Bladder

It is a condition where the urinary bladder is filled with urine. When there is a filling of urine in the bladder wall of the urinary bladder is stretched.

Activation of Stretch Receptors

Stretch receptors in the urinary bladder sense the rise in its volume and form impulses which are transmitted to the nervous system. At the point of capacity of the bladder, the stretch receptors send afferent impulses through the pelvic nerve into the spinal. The stimulus in the stretch receptors produces sensory impulses which are transmitted to the central nervous system.

Afferent Impulses to Spinal Cord

The sensory impulses as a result of the stretch receptors of the bladder travel via the pelvic nerve and reach the sacral region of the spinal cord. The impulses that arrive in the bladder enter an integration centre, which is the spinal cord.

Efferent Impulses to the Bladder

Motor efferent impulses are sent from the spinal cord through the pelvic nerve back to the bladder. The efferent impulses stimulate the detrusor muscle to lead to its contraction.

Contraction of Detrusor Muscle

The contraction of the detrusor muscle increases pressure in the bladder. This pressure pushes the urine into the urethra and out of the body.

Relaxation of Internal Urethral Sphincter

There is also an opening of the internal urethral sphincter, which has an involuntary type of control. The relaxation of the involuntary sphincter will form a passage through which urine can flow from the bladder into the urethra.

Role of Pudendal Nerve & Voluntary Control

The role of pudendal nerve and voluntary control are:

Pudendal Nerve Inhibition

There shall be a stimulation of the stretch receptors within the urethra when urine flows. These stretch receptors send further afferent impulses back to the spinal cord, which also serves to reinforce the micturition reflex.

Through the relay in the spinal cord, the sensory stimulus can be transmitted further to the brain to issue commands and concomitantly inhibit signals transmitted by the pudendal nerve. Through this inhibition, it relaxes the external urethral sphincter and releases the urine.

Voluntary Control

Upon relaxation of the external sphincter, the bladder voids urine through the urethra to the external environment. This can be aided by the voluntary contractions of the musculature of the walls of the abdomen, which increase the intra-abdominal pressure that aids in the micturition.

Self-Regenerative Nature of The Reflex

The micturition reflex is self-renewable in a way that the initial contraction of the bladder causes greater stimulation of the stretch receptors. This goes on as a continuous process until the bladder has been emptied, and total micturition has been achieved.

Disorders & Conditions Affecting the Micturition Reflex

The common disorders related to micturition are:

Micturition Reflex NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Mechanism of Micturition

• Disorders related to Micturition

Practice Questions for NEET

Q1. Which specific action occurs in the urinary system during micturition?

1. Urethra relaxes

2. Ureter contracts

3. Ureter relaxes

4. Urethra contracts

Correct answer: 1) Urethra relaxes

Explanation:

When the bladder is full and ready to release urine, the relaxation of the urethra is crucial. It allows the urethral muscles to loosen and open up, creating a wider pathway for the urine to flow from the bladder to the outside of the body. This relaxation of the urethra is coordinated with the contraction of the bladder muscles to create a coordinated and controlled release of urine. Without the relaxation of the urethra, it would be difficult for urine to pass through and be expelled from the body.

Hence, the correct answer is option 1) Urethra relaxes.

Q2. What will happen if the stretch receptors of the urinary bladder wall are removed?

1. Micturition will continue

2. Urine will continue to collect normally in the bladder

3. There will be no micturition

4. Urine will not collect in the bladder

Correct answer: 1) Micturition will continue

Explanation:

The micturition signal is initiated by the stretching of the urinary bladder as it gets filled with urine. The stretch receptors in the bladder wall detect the increase in volume and send signals to the brain, signaling the need to urinate. The brain then sends signals to the detrusor muscle to contract, while the external sphincter muscle relaxes, allowing urine to be expelled. If the stretch receptors are removed from the bladder, the brain will not receive the signals to trigger urination, leading to a loss of control over the micturition process.

Hence, the correct answer is option 1) Micturition will continue.

Q3. The counter-current mechanism involves ____?

1. Vasa recta and PCT

2. Vasa recta and DCT

3. Vasa recta and loop of Henle

4. Vasa recta and collecting duct

Correct answer: 3) Vasa recta and loop of Henle

Explanation:

The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of the vasa recta is also in a counter-current pattern.

This mechanism helps to maintain a concentration gradient in the medullary interstitium, thereby concentrating the filtrate( urine).

Hence, the correct answer is option 3) Vasa recta and loop of Henle.

Also Read:

• MCQs on Micturition

• Human Excretory System

• Ultrafiltration

Recommended Video on Micturition