Mednotes

L9 Nervous System CNS control of ANS

Properties2
tagsnervoussystem, lecture, physiology, autonomicnervoussystem
aliasesCNS control of ANS

11 min read

Overview

This lecture reviews the autonomic nervous system (ANS): what it does, how its two divisions (sympathetic and parasympathetic) are organised anatomically, the neurotransmitters used at each stage of the efferent pathway, and how the ANS is controlled centrally, from spinal reflexes up through the brainstem and cerebrum. It closes with two applied examples of the general anatomy: the autonomic consequences of spinal cord injury, and the detailed sympathetic/parasympathetic/somatic control of bladder continence and micturition.

ANS overview and functions

  • The ANS controls body functions and actions without conscious action by the individual.
  • It regulates: homeostasis, metabolism, communication, immune response, and reproduction.

Divisions: sympathetic vs parasympathetic

  • The ANS has two divisions: sympathetic division (SNS) and parasympathetic division (PSNS).
  • Outflow origin: SNS fibres leave the CNS from the thoracic and lumbar regions of the spinal cord (thoracolumbar outflow). PSNS fibres leave the CNS from the brain and the sacral portion of the spinal cord (craniosacral outflow).
  • Transmitter class: SNS is adrenergic; PSNS is cholinergic.
  • Functional role: SNS drives metabolism and the emergency “fight and flight” response; PSNS drives restoration and waste, “rest and digest.”
  • Most regions of the body receive input from both divisions, acting reciprocally or synergistically. Skin and blood vessels receive sympathetic innervation only.

Overview diagram (repeated across the lecture as a reference figure):

  • Sympathetic: thoracolumbar spinal cord -> paravertebral (sympathetic chain) ganglia (superior cervical ganglion = A, middle cervical ganglion = B, inferior cervical ganglion = C) -> postganglionic fibres to the eye, lacrimal gland, submandibular/sublingual glands, parotid gland, heart, trachea, and lung. Via the greater and lesser splanchnic nerves, fibres also reach prevertebral ganglia (celiac ganglion = 1, superior mesenteric ganglion = 2, inferior mesenteric ganglion = 3), which supply the liver, gallbladder, stomach, small intestine, adrenal gland, kidney, large intestine, bladder, and genitalia. A further sympathetic outflow leaves the paravertebral chain directly to supply skin and the musculoskeletal system.
  • Parasympathetic: cranial nerves III, VII, IX, and X arise from the midbrain/medulla and synapse in the ciliary, pterygopalatine, submandibular, and otic ganglia, supplying the eye, lacrimal gland, submandibular/sublingual glands, parotid gland, heart, trachea, lung, liver/gallbladder, stomach, small intestine, and kidney. Sacral outflow supplies the large intestine, bladder, and genitalia.

ANS anatomy — general organisation

  • The ANS efferent pathway is a two-neuron pathway: a preganglionic neuron with its cell body in the spinal cord or brainstem synapses in a ganglion, and a postganglionic neuron continues from the ganglion to the effector.
  • SNS ganglia lie close to the spinal cord (in the paravertebral chain). PSNS ganglia lie at or in the effector organ.
  • Sensory input: visceral afferents mediate autonomic reflexes.
  • Postganglionic ANS axons end in terminal varicosities, contacting multiple effector cells along their course, rather than a single discrete ending.
  • Comparison with the somatic motor system:
    • ANS: preganglionic axon leaves the intermediolateral horn cell of the spinal cord, synapses in a ganglion, and continues as a postganglionic axon that branches into varicosities contacting multiple smooth muscle or gland cells.
    • Somatic motor: a single motor axon leaves the anterior horn cell of the spinal cord and travels directly to skeletal muscle fibres, ending in a neuromuscular junction, with no intervening ganglion or synapse.

Neurotransmitters

  • Acetylcholine (ACh): used at all autonomic ganglia (nicotinic receptors), at PSNS effector organs (muscarinic receptors), and — as an exception to the general SNS pattern — at sympathetic fibres supplying sweat glands.
  • Norepinephrine: released at SNS effector organs, acting on alpha and beta receptors.
  • Epinephrine: released from the adrenal medulla.
  • Other: cotransmitters and nitric oxide (NO) are also involved [slide does not elaborate further].

Transmitter pattern by pathway level (Fig 8-46, Vander, 8th ed, p. 220):

  1. Somatic nervous system: CNS -> effector organ directly, single synapse releasing ACh.
  2. ANS parasympathetic: CNS -> ganglion (ACh) -> effector organ (ACh).
  3. ANS sympathetic: CNS -> ganglion (ACh) -> effector organ (norepinephrine).
  4. Adrenal medulla pathway: CNS -> adrenal medulla (ACh) -> epinephrine (also norepinephrine, dopamine, peptides) released via the bloodstream -> effector organ.

Sympathetic division anatomy

  • Preganglionic fibres originate from the intermediolateral neuronal column of the thoracic/lumbar spinal cord (visible histologically as paired columns within the grey matter).
  • These fibres exit via the ventral horn/root to the paravertebral sympathetic ganglia (the sympathetic chain), which runs vertically alongside the vertebral column and great vessels, with ganglia at intervals (illustrated adjacent to the first, second, and fourth ribs).
  • At the paravertebral ganglion, preganglionic fibres enter the sympathetic chain from the spinal nerve via the white ramus communicans; postganglionic fibres rejoin the spinal nerve via the gray ramus communicans.
  • From the paravertebral ganglia, fibres take one of several routes:
    • synapse and send postganglionic fibres to the body;
    • ascend to the cervical ganglia to supply the head;
    • pass through to prevertebral ganglia to supply the viscera;
    • continue to the adrenal medulla (which releases epinephrine directly into the bloodstream).
  • Sympathetic tone: baseline ongoing sympathetic activity [slide does not elaborate further].
  • Divergence: listed as a property of the SNS [slide does not elaborate further].

Parasympathetic division anatomy

  • PSNS ganglia are near or in the effector organs, with cranial and sacral outflow.
  • Cranial outflow (from the brainstem):
    • CN III — eye (lens/iris).
    • CN VII — lacrimal, nasal, and salivary glands.
    • CN IX — parotid gland; also carries sensory fibres.
    • CN X (vagus) — accounts for 75% of PSNS activity; supplies heart, lungs, and GI tract.
  • Sacral outflow: preganglionic fibres from S2, S3, S4, supplying pelvic organs and pelvic function.

Central control of the ANS

  • Sensory input drives reflexes, including local axon reflexes.
  • Autonomic ganglia themselves participate in autonomic control.
  • The spinal cord coordinates autonomic reflexes.
  • Brainstem structures involved: periaqueductal gray (PAG), parabrachial region, nucleus of the tractus solitarius (NTS, a sensory relay), and ventrolateral medulla.
  • Descending pathways: possibly reticulospinal tracts — marked with a question mark on the slide, indicating this is uncertain or proposed rather than established.
  • Cerebrum: hypothalamus, insula, medial prefrontal cortex, amygdala.
  • Hierarchical schematic, from cerebrum down to spinal cord:
    1. Cerebral hemisphere level: hypothalamus, insular cortex, amygdala.
    2. Midbrain level: periaqueductal gray matter.
    3. Pons level: parabrachial region.
    4. Medulla level: nucleus of the tractus solitarius, dorsal motor nucleus of X, nucleus ambiguus, ventrolateral medulla.
    5. Spinal cord level: intermediolateral horn.
    • Together these depict the descending chain of autonomic control centres from cerebrum to spinal cord.
  • The insula is shown in a dissection photograph, exposed by opening the lateral (Sylvian) fissure.

ANS and spinal cord injury

Spinal cord injury usually causes sympathetic (SNS) dysfunction, affecting:

  • temperature control
  • blood pressure control
  • autonomic dysreflexia
  • bowel/bladder dysfunction

[Slide lists these points without further elaboration on mechanism.]

ANS control of the bladder

Relevant structures: detrusor muscle, internal sphincter, external sphincter.

  • Innervation:
    • SNS, via the hypogastric nerve: inhibits the detrusor muscle and inhibits PSNS activity.
    • PSNS, via the pelvic nerves: contracts the detrusor muscle and relaxes the internal sphincter.
    • Somatic, via the pudendal nerve (from Onuf’s nucleus): controls the external sphincter.
    • Sensory: visceral and somatic afferents.

Continence (bladder filling):

  • SNS is dominant.
  • The detrusor relaxes; both sphincters are closed.
  • Somatic (voluntary) activity keeps the external sphincter closed.
  • This is enhanced by descending input from the Lateral Pontine Micturition Centre (LPMC).

Micturition (voiding):

  • PSNS is activated: the detrusor contracts and the internal sphincter relaxes.
  • Somatic (Onuf’s nucleus) activity decreases, relaxing the external sphincter.
  • Voluntary activation of the (medial) pontine micturition centre [named “MPMC” on the slide, not otherwise expanded] inhibits the LPMC, reducing descending inhibition and permitting micturition.
  • Bowel activity is described as very similar to bladder control [slide does not elaborate further].

Self-test

  1. Define the autonomic nervous system in terms of the type of control it exercises.
  2. List the five functions the ANS regulates.
  3. Distinguish the spinal/CNS origin of sympathetic outflow from that of parasympathetic outflow.
  4. Distinguish the sympathetic and parasympathetic divisions by transmitter class and overall physiological role.
  5. Which body structures receive sympathetic innervation only?
  6. Describe the two-neuron efferent pathway that characterises the ANS, including where the ganglia lie for each division.
  7. Explain how a postganglionic ANS axon terminates on its effector cells, and how this differs from a somatic neuromuscular junction.
  8. Distinguish the ANS efferent pathway from the somatic motor pathway in terms of neuron number and route.
  9. Describe the neurotransmitter used at each stage of the somatic, parasympathetic, sympathetic, and adrenal-medulla pathways.
  10. Identify the exception to the rule that sympathetic effector organs use norepinephrine.
  11. Describe the route of a sympathetic preganglionic fibre from its cell body to the paravertebral ganglion, including the rami communicantes involved.
  12. List the possible destinations of fibres synapsing in or passing through the paravertebral sympathetic ganglia.
  13. List the cranial nerves carrying parasympathetic outflow and the structure(s) each supplies.
  14. Which single cranial nerve accounts for the majority of parasympathetic activity, and what proportion?
  15. Describe the sacral parasympathetic outflow, including spinal levels and target.
  16. List the levels of the neuraxis involved in central autonomic control, from cerebrum to spinal cord, with the key structure(s) at each level.
  17. What is uncertain about the descending pathway linking higher autonomic centres to the spinal cord?
  18. Describe the main autonomic consequences of spinal cord injury.
  19. Describe the innervation controlling the detrusor muscle, internal sphincter, and external sphincter during continence.
  20. Describe how this innervation pattern changes during micturition, compared with continence.
  21. Application: a patient’s bladder is filling but has not yet reached the threshold for voiding. Which division dominates, and what is happening to the detrusor and sphincters?
  22. Integrative: relate the general anatomical rule “SNS ganglia lie close to the cord, PSNS ganglia lie near/in the effector” to the specific nerves supplying the bladder (hypogastric vs pelvic nerves).

Answers

Graph View

Backlinks