Autonomic and Somatic Nervous Systems Overview

Autonomic Nervous System Overview

• The autonomic nervous system (ANS) is involved in involuntary bodily functions.
• The ANS controls visceral organs, smooth muscle, and glands.
• It operates via subconscious control.
• Key neurotransmitters are acetylcholine (ACh) and noradrenaline (norepinephrine).
• The two main branches of the ANS are the sympathetic and parasympathetic divisions.

Somatic Nervous System

• The somatic nervous system (SNS) controls voluntary movements.
• SNS controls skeletal muscles.
• The neurotransmitter for the SNS is acetylcholine (ACh).

ANS vs. SNS Efferent Pathways

• SNS axons are heavily myelinated, extending directly from the central nervous system (CNS) to the effector organ.
• ANS axons are a two-neuron chain:
  • Preganglionic neurons (lightly myelinated)
  • Ganglionic neuron (extends to the effector organ)
• The two neurons synapse in a ganglion.

ANS Neurotransmitters

• Preganglionic fibers release acetylcholine (ACh).
• Postganglionic fibers release either norepinephrine or acetylcholine.

ANS Divisions (Sympathetic)

• The sympathetic nervous system (SNS) is activated under stress, producing the "fight-or-flight" response.
• It increases heart rate, breathing rate, and blood flow to muscles.
• It inhibits digestion and elimination.
• Neurotransmitters: Acetylcholine (preganglionic) and norepinephrine (postganglionic).
• The sympathetic pathway has a short preganglionic axon and a long postganglionic axon.
• Ganglion location is close to the spinal cord (thoracolumbar).
• There are specialized connections to the adrenal medulla. This results in a systemic release of epinephrine and norepinephrine.

ANS Divisions (Parasympathetic)

• The parasympathetic nervous system (PNS) is activated in relaxed states.
• It conserves energy and promotes digestion.
• It slows heart rate, breathing rate, and increases digestion and elimination
• Neurotransmitters: Acetylcholine (preganglionic and postganglionic).
• The parasympathetic pathway has a long preganglionic axon and a short postganglionic axon.
• Ganglion location is close to the target organ (craniosacral).

Interaction of ANS Divisions

• Most visceral organs receive input from both sympathetic and parasympathetic divisions, creating dynamic antagonism.
• Sympathetic and parasympathetic divisions often have opposing effects on the same target organ. For instance, sympathetic fibers increase heart rate, while parasympathetic fibers decrease it.
• Cooperative effects are observed in external genitalia control. Parasympathetic fibers cause vasodilation, leading to erection. Sympathetic fibers control ejaculation and peristalsis.

ANS Peripheral Receptors

• Cholinergic receptors respond to acetylcholine (ACh).
• Muscarinic receptors are one type of cholinergic receptor.
• Adrenergic receptors respond to epinephrine and norepinephrine.
• Alpha (α) and beta (β) receptors are two types of adrenergic receptors.

Sympathetic Neurotransmitters

• Norepinephrine is released at the effector organ.
• Synthesized in the presynaptic terminal.
• Stored in vesicles.
• Released during action potentials.
• Binds to receptors. It binds to presynaptic receptors a2 (autoreceptors). It binds to postsynaptic α1 or β1 receptors.
• Reuptake by the presynaptic neuron. Some is broken down by Monoamine Oxidase (MAO).

Sympathomimetics/Sympatholytic Agents

• Some drugs mimic the effects of the sympathetic nervous system, called sympathomimetics.
• Examples include: Salbutamol (β2 agonist) and Epinephrine (stimulates α & β).
• Drugs that block these effects are called sympatholytics.
• Examples include: Propranolol (non-selective β-antagonist) and Metoprolol (β1 selective antagonist).

Parasympathetic Nervous System

• The parasympathetic nervous system (PNS) is responsible for maintaining homeostasis and restorative activities.
• Parasympathetic activity is often referred to as the “Rest-and-Digest” system.
• ACh is the primary neurotransmitter.
• Cholinergic agonists mimic the actions of acetylcholine. Pilocarpine is an example used in glaucoma treatment.
• Cholinergic antagonists block the effects of acetylcholine. Atropine is an example.

Summary

• The autonomic nervous system regulates visceral functions, including heart rate, digestion, and blood pressure.
• The peripheral nervous system (PNS) includes sensory and motor pathways. The sensory pathway transmits information to the CNS, while the motor pathways transmit signals out to effectors.
• The central nervous system (CNS) processes information and makes decisions. The sensory nervous system and the motor nervous system are collectively referred to as the peripheral nervous system. The sympathetic and parasympathetic systems are part of the autonomic nervous system, which regulates visceral organs and systems.