Autonomic Nervous System (ANS)

Overview and Divisions

The autonomic nervous system (ANS) is a critical component of the peripheral nervous system responsible for involuntary regulation of internal organs, smooth muscle, cardiac muscle, and glands. It maintains homeostasis and responds to stress through two main branches: the sympathetic and parasympathetic divisions.

• Sympathetic Division: Prepares the body for 'fight-or-flight' responses during stressful situations.

• Parasympathetic Division: Promotes 'rest-and-digest' activities during relaxed states.

• Target Tissues: Smooth muscle, cardiac muscle, exocrine and some endocrine glands, lymphoid tissue, liver, and adipose tissue.

Anatomical and Cellular Differences

The sympathetic and parasympathetic divisions differ in their anatomical organization and neurotransmitter usage.

• Sympathetic:

  • Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord.

  • Ganglia are located in chains alongside the vertebral column (paravertebral ganglia).

  • Short preganglionic and long postganglionic neurons.

• Parasympathetic:

  • Preganglionic neurons originate in the brainstem (cranial nerves) and sacral spinal cord.

  • Ganglia are located near or within target organs.

  • Long preganglionic and short postganglionic neurons.

  • Cranial nerve X (vagus) contains about 75% of all parasympathetic fibers.

Autonomic Pathways

Autonomic pathways typically consist of two neurons in series: a preganglionic neuron and a postganglionic neuron, with the exception of the adrenal medulla.

• Preganglionic Neuron: Cell body in the CNS; releases acetylcholine (ACh) onto nicotinic receptors in the ganglion.

• Postganglionic Neuron: Cell body in the autonomic ganglion; releases neurotransmitter onto target tissue.

• Adrenal Medulla: Functions as a modified sympathetic ganglion, releasing catecholamines (mainly epinephrine) directly into the bloodstream.

Neuroeffector Junction and Varicosities

The neuroeffector junction is the site where autonomic neurons communicate with their target tissues. Neurotransmitters are released from varicosities—swellings along the axon—rather than from a single synaptic terminal.

• Varicosities: Allow widespread and diffuse release of neurotransmitters over the target tissue.

• Neurotransmitter release and termination mechanisms are similar to those in other neurons.

Neurotransmitters of the ANS

The primary neurotransmitters in the ANS are acetylcholine (ACh) and norepinephrine (NE), with epinephrine (E) released by the adrenal medulla.

• Sympathetic:

  • Preganglionic: ACh (nicotinic receptors)

  • Postganglionic: NE (adrenergic receptors)

  • Adrenal medulla: Epinephrine (adrenergic receptors)

• Parasympathetic:

  • Preganglionic: ACh (nicotinic receptors)

  • Postganglionic: ACh (muscarinic receptors)

Neurotransmitter Synthesis

• Acetylcholine: Synthesized from choline and acetyl CoA by choline acetyltransferase.

• Norepinephrine: Synthesized from tyrosine via a series of enzymatic steps.

Receptors and Signaling Pathways

Autonomic receptors are classified based on their ligand and signaling mechanism.

• Adrenergic Receptors (Sympathetic):

  • All are G-protein coupled (metabotropic).

  • Two main types: Alpha (α) and Beta (β), each with subtypes (e.g., α1, α2, β1, β2, β3).

  • Mediate effects of NE and E on target tissues.

• Muscarinic Receptors (Parasympathetic):

  • G-protein coupled receptors (M1–M5 subtypes).

  • Mediate effects of ACh on target tissues.

  • Can activate or inhibit second messenger pathways (e.g., cAMP, phospholipase C).

• Nicotinic Receptors:

  • Ligand-gated ion channels found in autonomic ganglia and neuromuscular junctions.

Receptor Signaling Pathways

• G-protein coupled receptors can interact with ion channels or membrane-bound enzymes, leading to changes in intracellular signaling molecules such as cAMP or Ca2+.

• Example: Activation of β1 adrenergic receptors increases cAMP, enhancing cardiac contractility.

Antagonistic Control and Homeostasis

Most organs receive dual innervation from both sympathetic and parasympathetic branches, which often exert opposite effects to maintain homeostasis.

• Antagonistic Control:

  • Sympathetic stimulation increases heart rate; parasympathetic stimulation decreases it.

  • Sympathetic dilates bronchioles; parasympathetic constricts them.

• Homeostatic Reflexes:

  • ANS works with endocrine and behavioral systems to regulate body temperature, blood pressure, and other vital functions.

  • Central control centers include the hypothalamus, pons, and medulla.

Summary Table: Sympathetic vs. Parasympathetic Responses

The following table summarizes the antagonistic effects of the sympathetic and parasympathetic divisions on various organs and tissues:

Example: Fight-or-Flight vs. Rest-and-Digest

• Fight-or-Flight (Sympathetic): Increased heart rate, dilated pupils, bronchodilation, decreased digestive activity.

• Rest-and-Digest (Parasympathetic): Decreased heart rate, constricted pupils, bronchoconstriction, increased digestive activity.

Additional info:

• The image labeled "Neuro 10" depicts a neuron with extensive dendritic branching, illustrating the complexity of neural networks involved in autonomic control.

• Central integration of autonomic responses occurs in the hypothalamus, pons, and medulla, which receive sensory input and coordinate endocrine and behavioral responses.