0526 - ANS and Bladder Control - RM

Question 1

Outline the main features and organisation of the autonomic nervous system.

Answer 1

Autonomic – divided into sympathetic and parasympathetic systems. 2-neuron pathway (pre and post-ganglionic), plus visceral afferent (sensory) neurons necessary for reflexes.

Symp – Carries afferent pain information - Preganglionics = T1-L3 – Intermediolateral Cell Column, poorly or non-myelinated. Postganglionics = paravertebral ganglia (sympathetic trunk), prevertebral ganglia (abdo. cavity).
Adrenal medulla is a special postganglion that releases adrenaline into blood (as target organ)

Parasymp – Carries afferent reflex control/homeostasis information. Preganglionics = Brainstem (cranial nn), sacral (S2-S4 - pelvic splanchnic nn). Postganglionics = Cranial = in ganglia (except X) Sacral and X = on or near target organ.

Question 2

Outline the main features and organisation of the enteric nervous system.

Answer 2

Controls intestines. Multiple interneurons, but acts as a postganglia (receives Symp and Parasymp inputs). Has own processing ability, and could operate independently if removed from CNS.

Divided into Myenteric plexus (gut motility), and submucosal plexus (gut secretions).

Inhibited by Sympathetic, stimulated by parasymp (rest and digest).

Question 3

Define a reflex. What are its key elements?

Answer 3

Reflex – involuntary and relatively stereotyped response to a specific sensory stimulus.

Elements –
Receptor and afferent pathway (nociceptor, mechano receptor, chemoreceptor)
Interneurons or synapse(s)
Efferent pathway (motor neurons)
Effector (typically muscle, organ, or gland)

Question 4

What are the key differences between somatic and autonomic reflexes

Answer 4

(Key = Area (somatic/autonomic))

Processing (low level/higher level)
Motor Neurons (1/2 (symp and parasymp))
Effector (single skeletal m/one or more cardiac m, smooth mm, glands)
Effect (stimulate/stimulate one and inhibit other (symp and parasymp))**

** - be aware that somatic reflexes stimulate an inhibitory neuron for the antagonist muscle.

Question 5

What are the six key differences between effector action in somatic and autonomic reflexes?

Answer 5

(Key = Area (autonomic/somatic))

Motor Neuron (lateral horn or outside spinal cord/ventral horn)
Speed (slow/fast)
Access (varicosities/direct access to muscle – Neuromuscular junction)
Vesicle release (Excitatory Junction Potential/End Plate Potential)
Contraction (slow rise/fall over several seconds/single twitch)
Coordination (uncoordinated/reliable and coordinated).

Question 6

Explain the elements involved in bladder control

Answer 6

Initiation of reflex – response to urge in the Pontine Micturition Centre (PMC) or consciousness of full bladder (mechano-stretch afferents, trigone pressure). Higher-order systems amplify the ANS signal and make the ultimate decision.

Micturition reflex – relaxation of sphincters (internal - para. Smooth and external (levator ani) - skeletal), and waves of contraction of bladder wall (detrusor m - para. Smooth). This relies on synchronisation of autonomic and CNS.

Continence – sphincters remain constricted and detrusor m relaxed. Maintained by sympathetic system.

Question 7

Describe how the networks switch between emptying of bladder and interim continence

Answer 7

Decision relies on afferents from trigone and detrusor through pontine micturition centre (PMC). If they are strong (full bladder), and higher-level systems initiate voiding, parasympathetic system is activated, sympathetic preganglions are inhibited, and Onuf’s nucleus is inhibited (inhibits pelvic floor muscles).

When afferents drop off, the opposite happens to resume continence.

Question 8

Briefly describe the micturition reflex

Answer 8

Bladder is full and urine enters urethra, triggers afferents that:

1 - activate PMC and interneurons (inhibit sympathetic, activate parasympathetic, inhibit Onuf’s nucleus (pelvic floor/ external sphincter))
2 – contract the detrusor to empty bladder.

Afferents signal an empty bladder, and inhibition on Onuf’s nucleus is removed (pelvic floor resumes supporting bladder).

Question 9

Briefly describe the micturition cycle and outline how it differs from the micturition reflex

Answer 9

Cortical decision to void based on afferents, and PMC inhibits Onuf’s nucleus. Urine enters urethra, triggers afferents that 1 - activate PMC and interneurons (inhibit sympathetic, activate parasympathetic, inhibit pelvic floor/ external sphincter) 2 - contract the detrusor to empty bladder. Afferents signal an empty bladder, and inhibition on Onuf’s nucleus is removed (pelvic floor resumes supporting bladder).

Differs from reflex primarily due to the conscious decision to void.

Question 10

Explain the consequences of spinal damage on bladder control

Answer 10

Initially, due to a lack of ascending inputs, there is no communication with PMC, thus no micturition reflex to inhibit sympathetic (continence). Thus, bladder overflows and requires catheterisation (risk of infection).

Longer term, there is a stronger interneuron connection that can re-learn voiding at the spinal level only.