Study Notes on Autonomic Reflexes

Learning Objectives


  • Describe the mechanisms that (1) maintain urinary continence, and (2) generate micturition

  • Describe the mechanisms that regulate defaecation

  • Describe the interaction between bladder and bowel reflexes, and explain the underlying mechanisms


Spinal Autonomic Circuitry — General Scheme of Organization


Inputs (Where Signals Come From): Multiple inputs (spinal + descending) integrate on shared interneuronal pools → coordinated autonomic output.

  • Spinal Afferents (1–4): Enter via dorsal roots → synapse on autonomic interneurons (excitatory or inhibitory) that in turn influence the preganglionic neuron.

  • Supraspinal Inputs (a–c): Descending fibers from brain nuclei → synapse on interneurons or directly on the preganglionic neuron for top-down control.


Note: This arrangement implements patterned, reflexive control of visceral functions.



Spinal and Supraspinal Regulation of Urinary System


Anatomy & Musculature:

  • Detrusor Muscle: The bladder wall contains smooth muscle (the detrusor) whose contraction expels urine from the bladder.

  • Internal Urethral Sphincter: A ring of smooth muscle forms the internal sphincter; its contraction prevents urine from leaving the bladder.

  • External Urethral Sphincter: A ring of striated (skeletal) muscle forms the external sphincter; its contraction, under somatic control, also prevents urine flow.


Innervation of the Bladder: 

  • Efferent Pathways (Motor Outflow):

    • Sympathetic Control (Hypogastric Nerve): Sympathetic fibers arising from the lumbar spinal cord travel via the hypogastric nerve to relax the detrusor muscle while contracting the internal sphincter.

    • Parasympathetic Control (Sacral Outflow): Parasympathetic fibers originating from the sacral spinal cord innervate the same targets but with opposite effects, contracting the detrusor muscle and relaxing the internal sphincter.

    • Somatic Control (External Sphincter): Somatic motor neurons from the sacral spinal cord innervate the external sphincter, allowing voluntary contraction to maintain continence.

  • Afferent Pathways (Sensory Inflow) - Stretch Receptors: Mechanoreceptors within the bladder wall detect filling; their afferents carry stretch signals to the dorsal horn of the sacral spinal cord, informing the CNS about bladder distension.

  • Supraspinal Regulation - Descending Inputs from Pons:

    • Pontine Micturition Center (PMC): A brainstem nucleus that sends descending commands to coordinate voiding, ultimately promoting detrusor contraction with sphincter relaxation (micturition).

    • Pontine Storage Center (PSC): A complementary nucleus that supports storage, reinforcing detrusor relaxation and sphincter contraction via descending inputs to the spinal circuits.


Functional Summary (Storage vs Voiding):

  • Storage Mode: Sympathetic outflow relaxes the detrusor and contracts the internal sphincter, somatic drive keeps the external sphincter closed, and the PSC biases the spinal circuits toward continence.

  • Voiding Mode: Rising afferent activity from bladder stretch engages supraspinal control; the PMC shifts the balance to parasympathetic dominance (detrusor contraction, internal sphincter relaxation) while reducing somatic drive to the external sphincter to permit urine flow.



Urinary Continence & Micturition — Reflexes That Maintain Storage and Enable Voiding


Continence During Bladder Filling:

  • Afferent Signaling: Gradual bladder filling activates low-level firing of stretch receptors in the detrusor; these afferents enter the sacral cord and synapse in the dorsal horn.

  • Sympathetic Reflex: Dorsal-horn interneurons relay to lumbar sympathetic preganglionic neurons; sympathetic outflow relaxes the detrusor and contracts the internal urethral sphincter.

  • Somatic Reflex: Parallel interneurons in the sacral cord increase somatomotor drive to the external urethral sphincter, producing tonic contraction.

  • Net Continence Effect: Detrusor relaxation + internal sphincter contraction + external sphincter contraction together maintain urinary continence during filling.



Transition to Voiding: Central Coordination:

  • Threshold & Awareness: As bladder volume rises, afferent firing reaches the level that generates conscious sensation of fullness, enabling a cortical decision to void.

  • Periaqueductal Gray (PAG): Bladder afferents are integrated in the midbrain PAG, a key autonomic hub that relays micturition-relevant signals to the pons.

  • Pontine Micturition Center (PMC): Convergent inputs from the PAG and cortex activate the PMC, which switches the system from storage to voiding.

    • Autonomic Shift: The PMC inhibits sympathetic outflow to the bladder while simultaneously activating parasympathetic outflow.

    • Detrusor Action: Parasympathetic activation contracts the detrusor, generating the pressure needed for urine expulsion.

    • Internal Sphincter Action: Reduced sympathetic tone plus parasympathetic influence relax the internal sphincter, lowering outlet resistance.

    • External Sphincter Action: Descending PMC pathways inhibit somatomotor neurons to the external sphincter, allowing it to relax.

  • Outcome: With detrusor contraction and both sphincters relaxed, outlet resistance falls and micturition proceeds efficiently.



One-Look Sequence (Storage → Void):

  • Storage Mode: Low-grade afferents → sympathetic ↑ (detrusor relax, internal sphincter contract) + somatic ↑ (external sphincter contract) → continence.

  • Voiding Mode: Fullness + decision → PAG → PMC → sympathetic ↓, parasympathetic ↑, somatic ↓ → detrusor contract + sphincters relax → urine flow.


Defecation Reflex

Circuit Anatomy:

  • Parasympathetic Efferents: Sacral outflow innervates sigmoid colon, rectum, and internal anal sphincter (smooth muscle).

  • Afferents: Stretch receptors in colon/rectum send signals to the dorsal horn of the sacral spinal cord.

  • Somatic Efferents: Ventral-horn motor neurons (sacral) innervate the external anal sphincter (striated, voluntary).

  • Descending Control: Cortical inputs descend to sacral motor pools to enable voluntary defecation or its suppression.

Reflex Sequence: Distension → Autonomic Response:

  • Rectal Distension: Fecal loading increases wall tension → stretch-receptor firing to sacral cord.

  • Parasympathetic Activation: Reflexly increases contractility of sigmoid/rectum and relaxes the internal anal sphincter.

  • External Sphincter Status: Unchanged by the autonomic reflex; remains under somatic, voluntary control.

  • Conscious Sensation: Rising afferent input generates the urge to defecate, enabling a cortical decision.

  • Voluntary Branching: Store or Void:

    • Decision—Delay: External sphincter contracts (somatic ↑) → reverse peristalsis reduces rectal pressure → urge subsides.

    • Decision—Defecate: External sphincter relaxes (somatic ↓) + ongoing parasympathetic drive (rectum contracts, internal sphincter relaxes) → low outlet resistance and stool expulsion.

  • Integrated Motor Pattern - Autonomic–Somatic Coordination: Effective defecation requires synchronous rectal contraction with relaxation of both sphincters; storage requires the opposite pattern (rectal pressure relief + external sphincter contraction).

One-Look Sequence (Storage → Defecation):

  • Storage Mode: Distension sensed → parasympathetic reflex starts, but voluntary External Sphincter: Contracted → reverse peristalsis → urge wanes.

  • Defecation Mode: Distension + decision → Rectum: Contracts, Internal Sphincter: Relaxes, External Sphincter: Relaxes → outlet resistance falls → defecation proceeds.

Bladder–Bowel Reciprocal Inhibition

Cross-Talk: Bladder afferent activation inhibits colorectal activity; colorectal afferent activation inhibits bladder activity.

Functional Gain: Interneuronal circuits linking these afferents to opposing efferent outflows prevent simultaneous urination and defecation.