PHARMACOLOGY OF AUTONOMIC NERVOUS SYSTEM

Autonomic Cholinergic and Adrenergic pathways

These pathways differ in terms of their neurotransmitters, receptor subtypes, neurotransmitter synthesis, storage, release, and the specific drugs that target each pathway.

1. Cholinergic Pathway:

• Neurotransmitter: The cholinergic pathway primarily uses acetylcholine (ACh) as its neurotransmitter.
• Receptor subtypes: Cholinergic receptors are further divided into two main subtypes: nicotinic receptors and muscarinic receptors. Nicotinic receptors are found in the ganglia of both the sympathetic and parasympathetic systems, as well as in the adrenal medulla. Muscarinic receptors are located in the effector organs innervated by the parasympathetic system.
• Neurotransmitter synthesis: Acetylcholine is synthesized from acetyl-CoA and choline by the enzyme choline acetyltransferase (ChAT) within the cholinergic nerve terminals.
• Neurotransmitter storage and release: Acetylcholine is stored in vesicles within cholinergic nerve terminals and is released upon nerve stimulation.
• Drugs affecting cholinergic pathway: Drugs that stimulate the cholinergic pathway are called cholinergic agonists, while drugs that inhibit the cholinergic pathway are called cholinergic antagonists.

2. Adrenergic Pathway:

• Neurotransmitter: The adrenergic pathway primarily uses norepinephrine (noradrenaline) as its neurotransmitter. Epinephrine (adrenaline) is also released from the adrenal medulla into the bloodstream, acting as a hormone.
• Receptor subtypes: Adrenergic receptors are further divided into two main subtypes: alpha-adrenergic receptors and beta-adrenergic receptors. Each subtype is further divided into multiple subtypes (e.g., alpha-1, alpha-2, beta-1, beta-2, beta-3).
• Neurotransmitter synthesis: Norepinephrine is synthesized from tyrosine through a series of enzymatic reactions, including the conversion of tyrosine to L-DOPA and the subsequent conversion of L-DOPA to dopamine by the enzyme aromatic L-amino acid decarboxylase. Dopamine is then converted to norepinephrine by dopamine beta-hydroxylase.
• Neurotransmitter storage and release: Norepinephrine is stored in vesicles within adrenergic nerve terminals and is released upon nerve stimulation. Epinephrine is stored in vesicles within the adrenal medulla and is released into the bloodstream in response to sympathetic stimulation.
• Drugs affecting adrenergic pathway: Drugs that stimulate the adrenergic pathway are called adrenergic agonists, while drugs that inhibit the adrenergic pathway are called adrenergic antagonists.

In terms of drug specificities, the drugs that stimulate or inhibit the cholinergic pathway are different from those that target the adrenergic pathway. Some commonly used drugs that affect these pathways include:

Cholinergic pathway drugs:

• Cholinergic agonists (e.g., muscarinic agonists like pilocarpine)
• Cholinergic antagonists (e.g., muscarinic antagonists like atropine)

Adrenergic pathway drugs:

• Adrenergic agonists (e.g., alpha-agonists like phenylephrine, beta-agonists like isoproterenol)
• Adrenergic antagonists (e.g., alpha-blockers like prazosin, beta-blockers like propranolol).

 

Major systems or organs innervated by the autonomic cholinergic and adrenergic systems

The autonomic nervous system (ANS) is divided into two main branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). These branches utilize different neurotransmitters to innervate various organs and systems in the body.

The sympathetic nervous system primarily releases the neurotransmitter norepinephrine (noradrenaline), while the parasympathetic nervous system primarily releases the neurotransmitter acetylcholine. Here is a list of some major systems or organs innervated by the autonomic cholinergic and adrenergic systems:

Autonomic Cholinergic System (Parasympathetic Nervous System):

1. Heart: Cholinergic innervation slows down heart rate (negative chronotropic effect) and reduces the force of contractions (negative inotropic effect).
2. Smooth muscles of the respiratory system: Cholinergic innervation causes bronchoconstriction and increased secretion of mucus in the airways.
3. Gastrointestinal tract: Cholinergic innervation promotes increased motility, increased secretion of digestive enzymes, and increased blood flow to the digestive organs.
4. Urinary bladder: Cholinergic innervation causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter, leading to bladder emptying.
5. Eye: Cholinergic innervation causes constriction of the pupil (miosis) and contraction of the ciliary muscle, which allows for accommodation for near vision.
6. Salivary glands: Cholinergic innervation stimulates salivary gland secretion.

Autonomic Adrenergic System (Sympathetic Nervous System):

1. Heart: Adrenergic innervation increases heart rate (positive chronotropic effect) and enhances the force of contractions (positive inotropic effect).
2. Blood vessels: Adrenergic innervation causes vasoconstriction in many vascular beds, leading to increased blood pressure.
3. Bronchi: Adrenergic innervation causes bronchodilation.
4. Gastrointestinal tract: Adrenergic innervation inhibits motility and reduces secretion in the digestive organs.
5. Urinary bladder: Adrenergic innervation relaxes the detrusor muscle and contracts the internal urethral sphincter, leading to urinary retention.
6. Eye: Adrenergic innervation causes dilation of the pupil (mydriasis) and relaxation of the ciliary muscle, allowing for distant vision.
7. Sweat glands: Adrenergic innervation stimulates the production of sweat.
8. Adrenal medulla: The sympathetic nervous system stimulates the release of epinephrine and norepinephrine from the adrenal medulla, which act as hormones to produce widespread sympathetic effects in the body.

It’s important to note that while these are some of the major systems or organs innervated by the autonomic cholinergic and adrenergic systems, there are other organs and tissues that receive autonomic innervation as well.

 

Effects of Cholinergic & Adrenergic

Cholinergic and adrenergic stimulation or antagonism can have significant effects on various organ systems in the body. These effects are mediated through the activation or blockade of specific receptors present in different tissues. Here’s a brief overview of the organ system effects associated with cholinergic and adrenergic stimulation or antagonism:

Cholinergic Stimulation:

1. Cardiovascular System: Cholinergic stimulation leads to a decrease in heart rate (negative chronotropic effect) and the force of cardiac contraction (negative inotropic effect). It also causes vasodilation, resulting in a decrease in blood pressure.
2. Respiratory System: Cholinergic stimulation causes bronchoconstriction and increased mucus secretion, leading to narrowed airways and increased respiratory secretions.
3. Gastrointestinal System: Cholinergic stimulation increases gastrointestinal motility, leading to enhanced peristalsis and increased secretion of digestive enzymes and fluids.
4. Urinary System: Cholinergic stimulation results in increased bladder contraction and relaxation of the sphincters, leading to increased urination.

Adrenergic Stimulation:

1. Cardiovascular System: Adrenergic stimulation leads to an increase in heart rate (positive chronotropic effect) and the force of cardiac contraction (positive inotropic effect). It also causes vasoconstriction, resulting in increased blood pressure.
2. Respiratory System: Adrenergic stimulation causes bronchodilation and decreased mucus secretion, leading to widened airways and reduced respiratory secretions.
3. Gastrointestinal System: Adrenergic stimulation decreases gastrointestinal motility, resulting in reduced peristalsis and decreased secretion of digestive enzymes and fluids.
4. Urinary System: Adrenergic stimulation relaxes the bladder and contracts the sphincters, leading to decreased urination.

Cholinergic Antagonism:

1. Cardiovascular System: Cholinergic antagonism results in an increase in heart rate (positive chronotropic effect) and can lead to an increase in blood pressure.
2. Respiratory System: Cholinergic antagonism causes bronchodilation and reduced mucus secretion, resulting in widened airways and decreased respiratory secretions.
3. Gastrointestinal System: Cholinergic antagonism decreases gastrointestinal motility, leading to reduced peristalsis and decreased secretion of digestive enzymes and fluids.
4. Urinary System: Cholinergic antagonism relaxes the bladder and contracts the sphincters, resulting in decreased urination.

Adrenergic Antagonism:

1. Cardiovascular System: Adrenergic antagonism leads to a decrease in heart rate (negative chronotropic effect) and can cause a decrease in blood pressure.
2. Respiratory System: Adrenergic antagonism causes bronchoconstriction and increased mucus secretion, leading to narrowed airways and increased respiratory secretions.
3. Gastrointestinal System: Adrenergic antagonism increases gastrointestinal motility, resulting in enhanced peristalsis and increased secretion of digestive enzymes and fluids.
4. Urinary System: Adrenergic antagonism relaxes the bladder and contracts the sphincters, leading to decreased urination.

It’s important to note that these effects can vary depending on the specific receptors involved, the location of these receptors, and the overall balance between cholinergic and adrenergic activity in different organ systems. Additionally, individual variations and the presence of any underlying medical conditions can also influence the responses to cholinergic and adrenergic stimulation or antagonism.

 

Cholinergic & Adrenergic Receptor Functions