Introduction to Autonomic Pharmacology

Introduction to Autonomic Pharmacology

• The autonomic nervous system (ANS) controls involuntary processes like heart rate, blood pressure, respiration, and digestion.
• It works alongside the somatic nervous system, which controls voluntary movements.
• The ANS is divided into two main divisions: the sympathetic (SNS) and parasympathetic (PNS) nervous systems.
• The SNS is often associated with the "fight-or-flight" response, while the PNS is linked to the "rest-and-digest" response.
• Figure 1 shows the organization of the ANS, illustrating the pathways, neurotransmitters, and target organs of the sympathetic & parasympathetic systems.

Objectives

• Identify and describe the functions of the components of the nervous system.
• Explain the role of the sympathetic and parasympathetic nervous systems.
• Describe the synthesis, release, and degradation of autonomic neurotransmitters.
• Distinguish between cholinergic and adrenergic receptors.
• Recognize the implications of autonomic drugs on the body.

Organization of the Nervous System

• The central nervous system (CNS) includes the brain and spinal cord.
• The peripheral nervous system (PNS) contains neuronal tissues outside the CNS.
• The PNS is further divided into the autonomic nervous system (ANS) and somatic nervous system, controlling involuntary and voluntary functions, respectively.
• Figure 2 shows an illustrative diagram of the CNS, PNS, and their subdivisions.

Autonomic Nervous System

• The autonomic nervous system regulates various involuntary functions.
• The sympathetic nervous system (SNS) is associated with the "fight or flight" response.
• The parasympathetic nervous system (PNS) is associated with the "rest and digest" response.

Organization of Autonomic Nervous System

• The sympathetic division originates in the thoracic and lumbar spinal cord.
• The sympathetic division utilizes acetylcholine (ACh) at preganglionic synapses, and norepinephrine (NE) at postganglionic synapses. - The parasympathetic division originates in the brainstem and sacral spinal cord.
• The parasympathetic division utilizes acetylcholine (ACh) at both preganglionic and postganglionic synapses.
• Figure 4 illustrates differences in neuron lengths and neurotransmitters used in sympathetic and parasympathetic pathways.

Neurotransmitter Chemistry of the ANS

• Cholinergic fibers release acetylcholine (ACh), including all preganglionic fibers, some postganglionic parasympathetic fibers, and at neuromuscular junctions.
• Adrenergic fibers release norepinephrine (NE), primarily from postganglionic sympathetic fibers.
• Other neurotransmitters, like dopamine and epinephrine, also play a role in the ANS.
• Figure 5 details neurotransmitters and their targets.

Catecholamine Neurotransmitters

• Catecholamine synthesis begins with tyrosine, producing dopamine, norepinephrine, and epinephrine.
• Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) degrade catecholamines.
• Figure 6 illustrates the pathway of catecholamine synthesis.

Receptors

• Cholinergic receptors include nicotinic (N) receptors in ganglia and synapses and muscarinic (M) receptors in target organs.
• Adrenergic receptors include alpha (α) and beta (β) receptors, each impacting different bodily functions.
• Figure 7 shows receptor types and their functions.

Pharmacotherapeutic Implications

• Sympathomimetic drugs mimic sympathetic activity.
• Sympatholytic drugs block sympathetic activity.
• Parasympathomimetic drugs enhance parasympathetic activity.
• Parasympatholytic drugs inhibit parasympathetic activity.

Off-Target Effects

• Some drugs affecting the autonomic nervous system can induce secondary, often unwanted, effects (e.g., hypertension, anxiety, blurred vision).

Summary

• ANS is crucial for homeostasis and bodily functions.
• The ANS involves neurotransmitters for signaling.
• Receptor specificity is critical for targeted drug actions.

Review Questions

• What are the primary neurotransmitters of the autonomic nervous system?
• How do adrenergic and cholinergic receptors differ in function?
• What is the function of the adrenal medulla in autonomic pharmacology?
• How do sympathomimetic and sympatholytic drugs differ?
• What are some common side effects of drugs affecting the ANS?