1.19 - ANS & Bladder Control Flashcards
Location of sympathetic preganglionics?
In the CNS
Thoracic and Upper lumbar levels
Location of sympathetic postganglionics?
Paravertebral ganglia: chain parallel to spinal cord
Prevertebral ganglia: abdominal cavity
Location of parasympathetic preganglionics?
Cranial and sacral regions
Location of parasympathetic postganglionics?
On or near target organ
Describe the enteric nervous system
Like a big fat postganglia receiving both sympathetic and
parasympathetic inputs, but with its own processing power to control and coordinate apporopriate motility and secretions
The parasympathetic NS (vagus and splanchnic nerves) provides a stimulatory drive on the ENS
The sympathetic NS imposes an inhibitory drive on the ENS
Describe autonomic reflexes
Autonomic reflexes are different to somatic reflexes due to their potential to have 2 efferent limbs: sympathetic and parasympathetic
The effects of the 2 efferent limbs of the ANS are typically opposite or reciprocal in nature. One efferent limb of the ANS acts as an “accelerator” of the effectors. The other efferent limb acts the “brakes” of the effectors. Distinct to somatic reflexes which only receive one type of command (i.e. “acceleration”)
Describe some of the major differences between autonomic and somatic efferent action
Motor neurons in the lateral horn Motor neuron in ventral horn and ganglion outside the spinal cord
(postganglionic neuron)
Slow AP conduction (poorly myelinated, thin)
Single AP results in a long lasting contraction (several seconds duration)
No discernible twitch
Slow rise with slow decay
Describe Micturition
Micturition = ejection of urine from the bladder (urination or voiding)
Continence = the voluntary control (holding) of urination (or defecation)
In between urination sessions, continence is required for bladder filling and holding until an appropriate opportunity for micturition arises
Both the autonomic and central nervous systems synchronise to accomplish efficient bladder emptying
Higher-order control systems (brainstem, hypothalamus, cortex) amplify/synchronise response to ANS
What are the three components of micturition and continence?
Initiation of reflex: e.g. in response to urge of consciousness of full bladder Micturition reflex (an emptying reflex): relaxation of sphincters & contraction of the bladder wall (detrusor muscle) Continence: sphincters remain constricted (filling and storing of urine)
Describe compliance of the bladder
Compliance = change in volume / change in pressure
Greater compliance –> volume can increase drastically with only a small pressure change (more elastic walls)
Describe the filling phase
At the commencement of filling, pressure rises little with significant volume changes (relatively large compliance)
After volumes of >350mL, pressure rises exponentially (compliance reduces as bladder wall becomes more rigid due to distension)
Describe the voiding phase
Contractions during emptying are short and repetitive until bladder fully emptied
Residual volume is pathological and can lead to infections
Bladder Anatomy: What do the mechano-stretch afferents do?
Respond to stretch in the bladder (bladder fullness)
Bladder Anatomy: What is the uretic orifice?
Entrance of urine from the kidney
Bladder Anatomy: What is the Trigone?
Triangle that joins ureteric orifices to the urethra
Contains specialised sensory ending that set the pressure threshold (set sympathetic tone)
Bladder Anatomy: What are the musical afferents?
In the epithelium of the trigone and urethra
When urine trickles in urethral part, discharges to S2-S4
Bladder Anatomy: What is the detrusor muscle?
Smooth muscle of the bladder wall (involuntary)
Contraction –> increase intravesicle pressure to expel urine
Bladder Anatomy: Levator ani
Pelvic floor muscles skeletal muscle (voluntary) Relaxation --> removes support of bladder, allowing passive descending of bladder and assists opening of urethra
Bladder Anatomy: What is the Internal Urethral sphincter?
Smooth muscle (involuntary) Relaxation --> opening of urethra
Bladder Anatomy: What is the external urethral sphincter?
Slow twitch skeletal muscle (voluntary)
Relaxation –> opening of external end of urethra
Main role to secure water when intravesicle pressure is high
e.g. during cough or sneeze or heavy lifting
Describe the Sympathetic Control of Micturition
Continence is maintained by sympathetic tone
Sympathetic activation imposes:
o Inhibition (relaxation of the detrusor muscle
o Excitation (constriction) of the internal sphincter
o Inhibition of the parasympathetic ganglia
The combined actions permit bladder filling and holding (continence)
Describe the Parasympathetic Control of Micturition
Parasympathetic activity is necessary for Voiding
Parasympathetic activation imposed the opposite effects on the detrusor and internal sphincter: o Excitation (constriction) of the detrusor muscle (increases intravesicle pressure) o Inhibition (relaxation) of the internal sphincter (allows urine to pass)
The combined actions permit bladder emptying (Voiding)
Describe the somatic component of micturition
Conscious relaxation of Levator ani, other pelvic floor muscles and external urethral sphincter to assist with voiding. These muscles are controlled by Onuf’s Nucleus (a sacral motor neuron pool). Onuf’s nucleus activates the pelvic floor and external urethral sphincter
Onuf’s nucleus is commanded by higher centres, namely the pontine micturition centre
The pontine micturition centre is under indirect conscious control of the cortex
o When a decision to void (or hold) is made, the appropriate inhibition of Onuf’s nucleus is executed to relax to pelvic floor muscles and external sphincter
Describe the Visceral Afferent and Relays in Micturition
Parasympathetic afferents send stretch information from the detrusor wall, ascends to the pontine micturition centre via a relay in the dorsal horn
Under normal circumstances a decision is relayed back from higher centres if voiding is appropriate
If the pressure is very high, a micturition reflex will be initiated, commanded by the pontine micturition centre
Tricking urine is also relayed to the dorsal horn then to the micturition centre. When triggered, these afferents initiate the micturition reflex (hence hard to hold mid-stream)
What are the Descending Inputs in Micturition
If afferent signals are sufficient, the pontine micturition centre will:
o Inhibit bladder sympathetic preganglionics –> inhibit continence
o Inhibit Onuf’s nucleus –> permit voiding
o Activate bladder parasympathetics –> activate voiding
Describe the Sacral Interneurons in Micturition
During voiding, afferent signals can directly coordinate parasympathetic and somatic efferents at the level of the sacral spinal cord. Relays to interneurons:
o Increase parasympathetic tone to bladder
o Inhibit Onuf’s nucleus to permit voiding
Internuerons also important for switching between contraction and relaxation phases of detrusor during voiding
Output of parasympathetic preganglionic feeds back to inhibit itself –> cycle of contraction and relaxation
List the 8 stages of the micturition cycle
- Decision from cortex to void. E.g. from sensation of full bladder
- Pontine micturition centre (PMC) inhibits Onuf’s nucleus
- Bladder descends and urine trickles into urethra and triggers afferents
- Afferents activate PMC that:
• Inhibits the sympathetic preganglionics
• Activates the parasympathetic preganglionics
• Inhibit pelvic floor and external sphincter muscles - Afferents can also activate interneurons that:
• Activate the parasympathetic preganglionics
• Inhibit Onuf’s nucleus to inhibit other pelvic floor and external sphincter muscles - The detrusor contracts in waves (interneuron mediated) until bladder is emptied
- Afferents signal an empty bladder to PMC
- Inhibition of Onuf’s nucleus is removed –> pelvic floor muscles resume their supportive role, and autonomic tone restoring continence via sympathetic and parasympathetic pathways
Describe the Continence phase of Micturition
As the bladder empties, inputs from afferents in the trigone and detrusor drop off and send less signals to the spinal cord
The relay to the brainstem PMC is reduced
a. Activation of the parasympathetic preganglionics is reduced
b. Activation to sympathetic preganglionics is restored.
Sympathetic tone is important for continence
c. Inhibition of Onuf’s nucleus is removed
Describe acute and chronic response to spinal cord injury for micturition
Acute reponse to spinal cord injury:
o No ascending inputs to the pontine micturition centre –> no micturition reflex possible during spinal shock stage
o No inhibition of sympathetic tone to allows the bladder to void –> overflow bladder
o Requires catheterisation to allow micturitionàinfection
Chronic response to spinal cord injury:
o Stronger interneuron contribution and reorganisation to
compensate
o Requires days-weeks to regain function
o Function is at the spinal level only, but voiding is ‘re-learned’
