Question

Choose the correct statement w.r.t. micturition reflex
A. It is initiated by stretch receptors of ureters.
B. Sympathetic supply causes relaxation of internal urethral sphincter.
C. Parasympathetic supply causes relaxation of urinary bladder muscles.
D. CNS passes on motor messages to initiate relaxation of urethral sphincters.

Answer 1

Hint: Micturition also known as urination is the act of releasing urine from the urinary bladder to the urethra and ultimately to the outside of the body. The process of micturition is controlled by the brain and is a voluntary process. This voluntary action is controlled by the brain in a well-regulated process called the micturition reflex. The micturition reflex helps in the contraction and voiding of the bladder. Since, the question is based on the correctness of the statements about micturition reflex, answering this question is based on the detailed knowledge of the steps and organs involved in the micturition reflex.

Complete answer:
Option $$1$$: If we look at the statement properly, and based on our knowledge of micturition, we can easily find that the process of micturition is initiated by the stretch receptors on the urinary bladder. But in Option $$1$$ instead of Urinary bladder the location of stress receptors is written wrong i.e., Ureters.
Thus, Option $$1$$ is wrong.
Option $$2$$: The opening of the internal urethral sphincter (IUS) is controlled by the autonomic nervous system (ANS) and is a type of involuntary response. The ANS consists of two types of neural responses: the sympathetic response and the parasympathetic response. The sympathetic supply helps in the contraction of the IUS whereas the parasympathetic supply relaxes the IUS. This particular point is reversed in Option $$2$$.
Thus, Option $$2$$ is incorrect.
Option $$3$$: The urinary bladder walls have detrusor muscles which are again controlled by the ANS but the contraction and relaxation reflexes are controlled differently. The parasympathetic supply contracts the detrusor muscles while the sympathetic supply relaxes the detrusor muscles. Again, if we have a glance at the statement provided in the option, we can easily see that the original fact is reversed in it making it wrong.
Thus, Option $$3$$ is also incorrect.
Option $$4$$: The micturition reflex is a neurologically controlled process where one can trace the smooth interplay between the Central Nervous System (CNS) and the Autonomic Nervous System (ANS). The stretch receptors on the bladder send sensory signals to the CNS which then gives the motor response which helps in the relaxation of the urethral sphincters.
Thus, Option $$4$$ is correct.

Hence, Option $$4$$ CNS passes on motor messages to initiate relaxation of urethral sphincters is the correct answer.

Note:
The process of voiding the bladder to pass urine out of the body is called micturition which is a voluntary process and is controlled by the micturition reflex. Once the bladder is filled with urine the stretch receptors of the bladder are stimulated and a sensory signal is sent to the CNS. The parasympathetic response contracts the bladder and the sympathetic response triggers bladder relaxation. The normally closed internal urethral sphincter is relaxed by the parasympathetic supply while the external urethral sphincter is relaxed by the pudendal nerve. All these steps ultimately lead to urination.