What are the main elements of a reflex, and how do somatic and autonomic reflexes differ?
Lower centre > efferent > effector > sensor > afferent (higher centre involvement)
Autonomic reflexes are different to somatic reflexes due to their potential to have 2 efferent limbs: sympathetic and parasympathetic.
Describe how continence is maintained. Even when bladder is full?
Sympathetic control of micturition - Continence
Continence is maintained by sympathetic tone
Sympathetic activation imposes:
o Inhibition of the detrusor muscle
o Excitation (constriction) of the internal sphincter
combined actions permit bladder filling and holding
1. As the bladder empties, inputs from afferents in the trigone and detrusor drop off and send less signals to the spinal cord
2. The relay to the brainstem PMC is reduced:
- Activation to the parasympathetic preganglionics are reduced
- Activation to the sympathetic preganglionics are restored
- Inhibition of Onuf's nucleus is removed
What are the steps involved in the micturition cycle? How is the cycle different from the micturition reflex?
1. Decision from cortex to void
2. PMC inhibits Onuf's nucleus ( relaxes pelvic floor muscles and external urethral sphincter)
3. *Bladder descends and urine trickles into the urethra and triggers afferents
4. *Afferents activate PMC which :
- Inhibits the sympathetic preganglionics
- Activates the parasympathetic preganglionics
- Inhibits pelvic floor and external sphincter muscles
5. Afferents can also activate interneurons that
- Activate the parasympathetic pregangionics
- Inhibit Onuf's nucleus to inhibit other pelvic flood and external sphincter muscles
6. *The detrusor contracts in waves (interneuron mediated) until bladder is emptied
7. *Afferent signal an empty bladder to PMC
8. *Inhibition on Onuf's nucleus is removed pelvic floor muscles resume their supportive role, and autonomic tone maintaining continence is restored to sympathetic and parasympathetic pathways.
** micturition reflex includes these steps, but step 3 also includes strong afferent inputs from stretch receptors.
What are the consequences of spinal damage?
Spinal cord injury
Acute Response to spinal cord injury:
No ascending inputs to the pontine micturition centre = no micturition reflex possible during spinal shock stage
No inhibition of sympathetic tone to allow the bladder to void = overfull bladder
Requires catheterisation to allow micturition = infection
Chronic response to spinal cord injury:
Stronger interneuron contribution and reorganisation to compensate
Requires days - weeks to regain function
Function is at the spinal level only, but voiding is relearned (but without the ability to consciously control)