Autonomic Nervous System

• Short-term functions of ANS

  • Constant fluid composition (water and salt)
  • Gas exchange (airway resistance, pulmonary blood flow)
  • Ingestion and digestion of nutrients
  • Excretion of wastes
  • Transport of gases and nutrients to needy tissues
  • Body temperature regulation
  • Reproduction behavior (mechanics)
  • Defensive behaviours
• Longer-term functions of ANS

  • Recovery of the body (circadian rhythms; eg sleep-wake cycles)
  • Development and maintenance of organs
  • Protection (inflammation, immunity)
• Ganglion

  A collection of nerve cell bodies found outside the CNS
• Sympathetic organisation


  • Sympathetic system comes from pre-ganglion neurons and synapse onto post-synapse and all reside in the spinal cord
  • They come from the paravertebral/ sympathetic chain
• Paravertebral organisation

  • Pre-ganglionic neurons of the sympathetic system reside in intermediate lateral cell column
  • Projects to the paravertebral ganglia and synapse onto the post-ganglionic neuron
• Exception

  • Directly innervates adrenal medulla and has not post synapse to release adrenaline from the glands
  • The adrenal medulla is similar to postganglionic neurons. It releases adrenaline and noradrenaline into the blood instead of sending axonal projections
• Parasympathetic organisation

  • Located within the CNS
  • Long projects mainly via the Vagus nerve
  • Innervate near to the tissues itself
• Sympathetic function

  • Blood vessel constriction
  • Increase HR
  • Relax airways
  • Inhibit digestion
  • Stimulate glucose production
  • Stimulate adrenaline secretion
  • Stimulate salvation/ sweating
• Parasympathetic function

  • Decrease HR
  • Constrict airways
  • Stimulate digestion
• Arteriole diameter

  • Controlled by tonic release of Noradrenaline (NA)
  • Increase of NA signal (indicated by spike frequency) on alpha-receptors constricts blood vessels
  • Decrease of NA signal (less spike frequency) on alpha-receptors dilates the blood vessels
• Neurotransmitters of the ANS

  • The autonomic nervous system consists of two neurons connected in series
  • The neuron innervating the target organ occurs outside the central nervous system
  • Pre-ganglionic neurons use the same neurotransmitter (ACh)
  • Post-ganglionic neurons use different neurotransmitters (ACh and NA)
• Parasympathetic

  • Preganglionic neurone = acetylcholine (Ach)
  • postganglionic neuron = acetylcholine (Ach)
• Sympathetic

  • Preganglionic neurone = acetylcholine (Ach)
  • postganglionic neuron = noradrenaline (NA)
• Pupil Dilation vs Constriction

  1. Decrease light intensity --> increase SNS --> pupils dilate

  2. Increased light intensity --> Increased PNS --> pupillary constrictor muscles activate --> pupils diameter decreases

  3. Sympathetic activity contracts the pupillary dilator muscles causing pupil to dilate

  4. Pupillary constrictor is a sphincter
• Stimulation of SA node (HR)

  1. M2 - Muscarinic receptors

  2. Na and Ca influx depolarise the cell, leading to increase in frequency of APs in SNS
• Lower stretch sense on the receptors detect lower blood pressure

  SNS increases and decreases PNS to increase CO