Autonomic Nervous System

Describe the autonomic nervous system, including anatomy, receptors, subtypes and transmitters (including their synthesis, release and fate)

The ANS is the section of the nervous system which regulates involuntary and visceral functions. These include:

• Haemodynamics
• Digestion
• Urination and defecation
• Thermoregulation
• Sexual function

The autonomic nervous system can be divided into

• Central ANS
  Control occurs in the hypothalamus, brainstem, and spinal cord.
• Peripheral ANS
  Divided anatomically and functionally into the:
  • Sympathetic nervous system
  • Parasympathetic nervous system

GRAPH FROM PAGE 258 of GANONG

Central Control

The hypothalamus controls autonomic functions by neural and endocrine mechanisms. It is subdivided anatomically into four regions:

• Anterior hypothalamus
  Controls the PNS and thermoregulation. It also releases ADH in response to increased plasma osmolality, and oxytocin.
• Medial hypothalamus
  Inhibits appetite in response to increase in blood glucose.
• Lateral hypothalamus
  Contains the thirst centre and drive to seek food.
• Posterior hypothalamus
  Controls vasomotor centres, modulating sympathetic vasoconstriction, as well as positive and negative inotropy and chronotropy. Also modulates wakefulness in response to sympathetic stimuli.

Signals from the hypothalamus have a tonic output to:

• All smooth muscle
• Heart
• Exocrine organs
• Endocrine organs
• GIT
• GU

Central Anatomy

In the grey matter of the spinal cord, efferent nerves synapse with two other nerves connected in series. This maintains tonic autonomic outflow.

FIGURE FROM PAGE 67 - POWER AND KAM

Efferent nerves exit the spinal root anteriorly, and form the ventral root.

Conversely, afferent nerves enter posteriorly, forming the dorsal root and then dorsal root ganglion, before synapsing in the spinal cord.

Sympathetic Nervous System

The sympathetic nervous system optimises the body for short-term survival.

Sympathetic innervation is from the sympathetic trunks. These:

• Are a paired bundle of sympathetic neurons which run lateral to the vertebral bodies from T1 to L2
  The trunk is subdivided into four parts:
  • The cervical part innervates the head, neck, and part of the thorax
  • The thoracic part is further subdivided into:
    • Upper thoracic from T1-T5, which innervates the aorta, heart, and lungs
    • Lower thoracic from T6-T12, which innervates the foregut and midgut
  • The lumbar part forms the coeliac plexus
  • The pelvic part innervate the pelvic visceral and lower limb vasculature
• Contain the sympathetic ganglion, which is a synapse between the:
  • Short pre-ganglionic fibre
    Cell body is located in the lateral horn of the spinal cord, and connects to the sympathetic ganglion.
    • Releases ACh to stimulate the post-ganglionic fibre.
  • Long post-ganglionic fibre
    Cell body is located in the sympathetic ganglion, and stimulates the effect site.
    • Has a nicotinic ACh receptor
    • Releases NA at the effect site
    • Sensitivity (for ACh) and activity (for NA release) is modulated by a number of other substances:
      • Enkephalin
      • Neuropeptide Y
      • Dopamine
      • Adrenaline
      • Prostaglandin
      • GABA
      • Neurotensin

There are three exceptions to the above structure:

• The adrenal gland is a modified sympathetic ganglion. It is:
  • Directly innervated by preganglionic neurons releasing ACh
• Sweat glands have muscarinic receptors, and are stimulated by ACh rather than noradrenaline
• Skeletal muscle arterioles also have muscarinic ACh receptors, and are stimulated by ACh

Effect

Sympathetic stimulation has a number of effects by either direct neural innervation or adrenaline release. They are consistent with a 'fight or flight' response, and optimise the body for short-term stress conditions.

Parasympathetic Nervous System

Parasympathetic innervation arises from the:

• Cranial nerves
  From CN III, VII, IX, and (mostly) X.
  • The vagus is the major cranial parasympathetic, innervating the:
    • Heart via the cardiac plexus
      • The SA node is innervated by the right vagus
      • The AV node is innervated by the left vagus
        The ventricles are also sparsely innervated from the left vagus.
    • Lungs via the pulmonary plexus
    • Stomach, liver, spleen, and pancreas, and gut proximal to the splenic flexure via the gastric plexus.
• Hypogastric plexus
  Arises from S2-S4, and innervates the bladder, uterus, and gut distal to the splenic flexure.

The parasympathetic nervous system ganglia sit close to the target organ. This means that the:

• Pre-ganglionic fibre is long
  • Preganglionic cell body sits within the brainstem (cranial nerves) or sacral grey matter (hypogastric plexus)
  • Releases ACh to stimulate the post-ganglionic neurone at a nicotinic ACh receptor
• Post-ganglionic fibre is short
  • Releases ACh to stimulate the target organ at a muscarinic ACh receptor

Effect

Ganglion Blockade

Blockade of the ganglion (at the nicotinic ACh receptor) blocks transmission and reduces sympathetic and parasympathetic impulse transmission. Clinical effect of ganglion blockade depends on which part of the ANS is dominant in that organ system:

• SNS dominant organ systems
  Effective sympatholysis:
  • Vasculature
    Vasodilation, hypotension.
  • Sweat glands
    Anhydrosis.
• PNS dominant organ systems
  Effective parasympatholysis:
  • Heart
    Tachycardia.
  • Iris
    Mydriasis.
  • GIT
    Decreased tone.
  • Bladder
    Urinary retention.
  • Salivary
    Reduced secretions.

Enteric Plexus

The enteric plexus is a system of autonomic nerves in the GIT which is free of CNS control. It consists of sensory and integrative neurons as well as excitatory and inhibitory motor neurons which generate coordinated muscular activity.

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
3. Klabunde, RE. Neural Activation of the Heart and Blood Vessels. Accessed 2016.