Autonomic Nervous System Overview

Autonomic Nervous System (ANS)

• The ANS is primarily responsible for regulating and integrating physiological functions such as cardiac output, blood flow, digestion, and glandular secretions
• These functions are automatic (involuntary) and crucial for life
• The ANS is independent of conscious control
• Tissues may not be innervated by both sympathetic and parasympathetic branches; when they are, these branches usually have opposing effects (e.g., heart rate, GI motility) but there are exceptions
• The somatic nervous system controls consciously-controlled functions (e.g., movement, posture, respiration)
• Both the ANS and somatic systems have afferent "sensory" inputs that modify motor output via reflex arcs

ANS Objectives

• Understand the anatomical and pharmacological divisions of the ANS
• Compare and contrast neurotransmitter synthesis, storage, release, and termination of action at cholinergic and adrenergic synapses
• Understand the roles of pre- and postsynaptic receptors
• List mechanisms by which drugs influence neurotransmission and major drug classes used to manipulate autonomic function

Organization of the Nervous System

• The CNS comprises the brain and spinal cord
• The PNS connects the CNS to the periphery
• The PNS includes afferent and efferent divisions
• Afferent division transmits sensory stimuli to the CNS
• Efferent division transmits motor signals from the CNS
• The efferent division includes the somatic and autonomic nervous systems
• The somatic nervous system controls voluntary skeletal muscles
• The autonomic nervous system controls involuntary activities of visceral organs (e.g., glands, smooth muscles)
• The autonomic nervous system includes the sympathetic and parasympathetic branches

Effectors of the ANS

• The ANS innervates effectors that are not under voluntary control
• Examples include the heart, smooth muscles of bronchial tubes and GI tract, smooth muscles lining blood vessels, and glands

Subdivisions of the ANS

• Sympathetic NS: (thoracolumbar) originates from thoracic and lumbar spinal regions
• Parasympathetic NS: (craniosacral) originates from cranial nerves and sacral spinal roots
• These divisions typically have antagonistic effects, maintaining homeostasis

Efferent Neurons of the ANS

• Composed of two neuron chains:
  • Preganglionic neuron originates in the CNS
  • Postganglionic neuron synapses with the effector organ
  • Neurotransmitters can have variable effects, depending on the specific receptor and location

Neurotransmitters

• Neurotransmitters are signals released from nerve terminals, facilitating communication between nerve cells and effector organs.
• Important ANS neurotransmitters include acetylcholine (ACh) and norepinephrine (NE)

Cholinergic transmission

• Synthesis and release of ACh: ACh is synthesized in the nerve terminals, released by action potential arrival and diffuse across the synaptic cleft. Binding to the postsynaptic receptor triggers an intracellular response.

Adrenergic Transmission

• Key steps in norepinephrine neurotransmission: norepinephrine synthesis, storage vesicle uptake, release, binding to the receptor, subsequent removal through reuptake, and metabolism (e.g. through enzymes) .
• Catecholamines: Dopamine, norepinephrine, epinephrine. These are involved in various autonomic processes.

Other Transmitters (NANC)

• Other transmitters, termed NANC (non-adrenergic, non-cholinergic) play important roles in certain situations
• Examples include ATP, 5-hydroxytryptamine (serotonin), nitric oxide (NO) and specific neuropeptides (e.g. neuropeptide Y, vasoactive intestinal peptide (VIP), and substance P).

Receptors of the ANS

• Cholinergic receptors: respond to acetylcholine; include nicotinic and muscarinic subtypes
• Adrenergic receptors: respond to Norepinephrine /epinephrine; include alpha and beta subtypes with varying effects

Mechanisms of Action of Autonomic Mediators

• Autonomic mediators activate downstream pathways; these pathways involve second messengers (e.g. cyclic AMP) and have diverse effects.

Sympathetic vs. Parasympathetic Functions

• Sympathetic division: "fight-or-flight" response, e.g. increased heart rate, blood flow redirection during stress.
• Parasympathetic division: "rest-and-digest" response, e.g. increased digestive activity, slowing of heart rate.

Summary of ANS Characteristics

• Location of ganglia differs between sympathetic and parasympathetic systems: ganglia are typically close to, or located near, spinal cord for sympathetic and are located close to or on effector organs for parasympathetic. Preganglionic fiber branching differs significantly between sympathetic and parasympathetic systems : Sympathetic tends to be extensive, while parasympathetic is generally less extensive, with minimal branching.
• Distribution: Sympathetic fibers are widely distributed, while parasympathetic fibers tend to be more limited.
• Type of response: Sympathetic response is generally diffuse and parasympathetic response tends to be more discrete.