Question

Describe the urination reflex in terms of stimulus, part of the CNS involved, effector muscle, internal urethral sphincter, and voluntary control. (p. 429)

Short answer

Answer: The key components of the urination reflex are the stimulus of the bladder wall stretching, the involvement of the spinal cord (sacral region S2-S4) and cerebral cortex, the effector muscle (detrusor muscle), the internal urethral sphincter, and voluntary control through the external urethral sphincter. The spinal cord and cerebral cortex work together to regulate the reflex by either allowing or inhibiting it based on the stretch receptors' signals and social contexts.

Step-by-step solution

Define the stimulus

The stimulus for the urination reflex is the stretching of the bladder wall as it fills with urine. When the bladder fills up to a certain capacity, the stretch receptors in the wall trigger the reflex to initiate the process of urination.

Identify the part of the CNS involved

The micturition (urination) reflex is regulated by the spinal cord, specifically the sacral region S2-S4. When the stretch receptors in the bladder wall sense the fullness, they send signals to the spinal cord via afferent (sensory) nerves. In addition, the cerebral cortex, being the higher center for voluntary control, also plays a role in the process by either allowing or inhibiting the reflex based on social contexts and conveniences.

Describe the effector muscle

The effector muscle in the urination reflex is the detrusor muscle. The detrusor muscle is a smooth muscle that forms the inner layer of the bladder wall. When stimulated by the parasympathetic nervous system, the detrusor muscle contracts to expel urine from the bladder.

Explain the role of the internal urethral sphincter

The internal urethral sphincter is a smooth muscle located at the junction of the bladder and urethra. It is under involuntary control and prevents urine leakage by keeping the urethra closed. During the urination reflex, the parasympathetic nervous system causes the internal urethral sphincter to relax, allowing urine to flow from the bladder into the urethra.

Discuss the role of voluntary control

Voluntary control of urination is mainly through the external urethral sphincter, which is a skeletal muscle under the control of the somatic nervous system. A person can consciously contract the external urethral sphincter to inhibit the urination reflex until he or she can void their bladder in an appropriate and socially acceptable manner. In summary, the urination reflex involves the stimulus of the bladder wall stretching, the involvement of the spinal cord and cerebral cortex, the effector muscle (detrusor muscle), the internal urethral sphincter, and voluntary control through the external urethral sphincter.