Neural Control of Pelvic Function 3

Question 1

What are the spinal and local reflexes involved in the defecation reflex?

Answer 1

• Distension of the rectum initiates the defecation reflex
• Pelvic nerve afferents transmit distension signal to the spinal cord
  
  • leads to inhibition of sympathetic input to the colon
• Excitation of colonic motility by pelvic nerve efferents
• Internal anal sphincter is inhibited by withdrawal of sympathetic input and increase in pelvic nerve activity (parasympathetic)
• External sphincter and other striated pelvic muscles relax due to withdrawal of tonic pudendal nerve activity

Question 2

What are the brainstem mechanisms involved in the defecation reflex?

Answer 2

• Defecation is accompanied with closure of the glottis and thoracic and abdominal contractions (Valsalva maneuver)
• Responses probably mediated by areas in the **dorsolateral pontine tegmentum **and the ventrolateral medullary reticular formation
• Supraspinal input is the means by which voluntary control is maintained

Question 3

What spinal reflexes are involved in the micturition reflex?

Answer 3

• Bladder distension leads to relaxation of external and internal sphincters
  
  • afferents in pelvic nerve
  • relaxation of external sphincter mediated by withdrawal of pudendal activity
  • internl sphincter relaxed by withdrawal of sympathetic activity, increased by parasympathetic activity
• Contraction of bladder evoked by distension
  
  • can be demonstrated in chronically spinal cord transected animals and humans

Question 4

What is the brainstem control involved in the micturition reflex?

Answer 4

• Essential for micturition
• Distension of bladder leads to coordinated reflex
  
  • contraction of detrusor
  • relaxation of internal and external sphincters
• Afferents in pelvic nerve
  
  • ascend polysynaptically to pintine micturition center
  • leads to inhibition of pudendal and sympathetic activity