Autonomic Nervous System: Sympathetic & Parasympathetic

• The autonomic nervous system (ANS) innervates internal organs, including blood vessels, stomach, intestines, liver, kidneys, bladder, genitals, lungs, pupils, heart, and sweat, salivary, and digestive glands
• The autonomous nervous system has parasympathetic and sympathetic divisions
• The SNA, also known as the visceral, vegetative, or involuntary nervous system, is distributed throughout the body
  • It regulates autonomous functions that occur without conscious control.
• In the periphery, the ANS consists of nerves, ganglia, and plexuses that innervate the heart, blood vessels, glands, other visceral organs, and smooth muscle of various tissues.

Sympathetic Division

• The somas of pre-ganglionic neurons are located in the inter-medio-lateral horn of the spinal cord
• They are located between vertebrae T1 and L4
• The sympathetic division is also called thoracolumbar

Parasympathetic Division

• Pre-ganglionic neurons are located in the brain stem and in the sacral medulla (S3 and S4)
• The parasympathetic division is also called craneosacro

Neurotransmitters

• Acetylcholine is neurotransmitter released by pre-ganglionic fibers
  • It interacts with nicotinic receptors
• Post-ganglionic neurons use noradrenaline
  • It acts on adrenergic receptors, with some exceptions
• Acetylcholine is the neurotransmitter released at the ganglionic synapse and at the termini of post-ganglionic neurons
  • It activates muscarinic receptors

System Balance

• The ultimate goal of ANS innervation is to achieve a visceral tone of equilibrium between the sympathetic and parasympathetic systems
• The sympathetic system is related to emergency situations
• The parasympathetic system governs states of rest and digestion for the control of homeostasis.

Functional Considerations

• Sympathetic and parasympathetic neurotransmitters can be seen as physiological or functional antagonists
• Most viscera are innervated by both divisions of the ANS
  • Their activities in specific structures can be discrete, independent, integrated, or interdependent

Sympathetic vs Parasympathetic Effects

• Sympathetic Stimulation: Fight response, Adrenergic agonist is "on", blocks rest functions
• Parasympathetic Stimulation: Parasympathetic agonist is "on", causing sludge effect, blocks dry functions

Specific Autonomic Nervous System Functions

• Iris of the eye: sympathetic dilates the pupil, parasympathetic constricts the pupil
• Ciliary muscle of the eye: sympathetic, relaxes, parasympathetic, constricts
• Salivary glands: sympathetic reduces secretion, parasympathetic increases secretion
• Lacrimal gland: sympathetic reduces secretion, parasympathetic increases secretion
• Heart: sympathetic increases frequency and strength of contraction parasympathetic decreases frequency and strength of contraction
• Bronchi: sympathetic dilates, parasympathetic constricts
• Digestive system: sympathetic decreases motility, parasympathetic increases motility
• Sweat glands: sympathetic increases secretion
• Piloerector muscles: sympathetic contracts

Heart and Iris Effects

• Sympathetic and parasympathetic stimulation exhibit a pattern of functional antagonism in the control of heart rate and pupillary opening
• Actions in male sex organs are complementary
  • They are integrated to promote sexual function

Actions of the ANS

• SNA actions depends on the type of innervation and the receptor population of the stimulated organ
• Some organs have both sympathetic and parasympathetic innervation
• Sympathetic and parasympathetic actions are usually antagonistic, such as in the heart and intestinal musculature

Specific Organ Effects

• Sympathetic stimulation increases heart rate and decreases intestinal motility
• Parasympathetic stimulation decreases heart rate and increases intestinal motility

Arteriole Considerations

• Structures that only receive sympathetic innervation include the arteriolar musculature
  • Depending on the neurotransmitter and the receptor that is activated, vasoconstriction or vasodilation will occur
• Vasomotor tone is only controlled by the sympathetic system