Autonomic Nervous System: Sympathetic Division and Pathways

Autonomic Nervous System (ANS)

Overview of Parasympathetic and Sympathetic Divisions

The Autonomic Nervous System (ANS) regulates involuntary physiological functions, including heart rate, digestion, respiratory rate, and glandular activity. It is divided into the parasympathetic and sympathetic divisions, which often have opposing effects on target organs.

• Parasympathetic Division: Promotes 'rest and digest' responses.

• Sympathetic Division: Promotes 'fight or flight' responses.

Sympathetic Division Structure

Sympathetic Preganglionic Neuron

Sympathetic preganglionic neurons originate in the central nervous system (CNS), specifically in the thoracolumbar region of the spinal cord (T1-L2).

• Origin: Lateral horns of thoracic and lumbar spinal cord segments (T1-L2).

• Length: Short axons.

• Myelination: Myelinated for rapid signal transmission.

• Neurotransmitter: Cholinergic (releases acetylcholine, ACh).

Sympathetic Division Pathway

Sympathetic pathways involve a short, branching preganglionic axon that synapses with a ganglionic neuron in an autonomic ganglion close to the vertebral column. The postganglionic axon is long and extends to the effector organ.

• Preganglionic neuron: Short, myelinated, cholinergic.

• Postganglionic neuron: Long, unmyelinated, usually adrenergic (releases norepinephrine, NE).

Sympathetic Trunk/Chain Ganglia

Location and Connections

The sympathetic trunk (chain ganglia) runs lateral to the spinal cord on both sides. Rami communicantes connect the sympathetic trunk to spinal nerves.

• White rami communicantes: Carry myelinated preganglionic fibers into the sympathetic trunk.

• Gray rami communicantes: Carry unmyelinated postganglionic fibers out to spinal nerves.

Options at Sympathetic Ganglia

Preganglionic fibers entering the sympathetic trunk have three possible pathways:

• Direct synapse: Synapse with postganglionic neuron at the same level.

• Ascend or descend: Travel within the trunk to synapse at a different level.

• Pass through: Continue through the trunk to synapse at collateral (prevertebral) ganglia via splanchnic nerves.

Collateral/Prevertebral Ganglia

Location and Function

Collateral ganglia are located anterior to the spinal cord, primarily in the abdomen and pelvic regions. Splanchnic nerves exit the sympathetic trunk and synapse with postganglionic neurons in these ganglia.

• Examples: Celiac ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.

• Function: Innervation of abdominopelvic viscera.

Sympathetic Pathways and Divergence

Divergence in Sympathetic Division

One preganglionic neuron can synapse with many (20+) postganglionic neurons, allowing widespread effects.

• Divergence: Amplifies the sympathetic response throughout the body.

• Receptor: ACh binds to nicotinic receptors on postganglionic neurons.

Sympathetic Postganglionic Neuron

Characteristics and Targets

Postganglionic neurons are long, unmyelinated, and synapse with various effectors.

• Targets: Smooth muscle, cardiac muscle, glands.

• Neurotransmitter: Most are adrenergic (release NE); some are cholinergic (release ACh, e.g., sweat glands).

Adrenal Medulla Pathway

Neuroendocrine Function

The adrenal medulla acts as a modified sympathetic ganglion. Preganglionic neurons synapse with chromaffin cells, which release hormones into the blood.

• Chromaffin cells: Neuroendocrine cells that release epinephrine and norepinephrine.

• Effect: Mimic and prolong sympathetic neural stimulation.

Varicosities of Postganglionic Neurons

Neurotransmitter Release

Varicosities are swollen regions along postganglionic axons where neurotransmitters are synthesized, stored, and released. NTs diffuse over a large area, affecting many target cells.

• Function: Allows broad activation of effector tissues.

• Difference from somatic system: No specific motor end plate; NTs diffuse in interstitial fluid.

Neurotransmitter Removal and Degradation

Mechanisms

The concentration of neurotransmitter (NT) in the synapse determines the effector response. Removal occurs by:

• Transport into cells

• Diffusion away from synapse

• Enzymatic degradation

Acetylcholine (ACh)

• Broken down by acetylcholinesterase in the synaptic cleft.

• Choline is actively transported back into the presynaptic neuron for recycling.

Norepinephrine (NE)

• Removed by active transport into the neuron or degraded by monoamine oxidase (MAO).

Receptors in the Sympathetic Division

Nicotinic Receptors

Nicotinic receptors are ligand-gated ion channels that open upon ACh binding, allowing Na+ influx and K+ efflux, resulting in depolarization and excitation of the postsynaptic membrane.

• Location: All postganglionic neurons, adrenal medullary cells, neuromuscular junctions of skeletal muscle.

• Effect: Excitation.

Muscarinic Receptors

Muscarinic receptors are G protein-coupled receptors. The effect of neurotransmitter binding depends on the receptor subtype and can be either inhibitory or excitatory.

• Location: All parasympathetic target organs; limited sympathetic targets (e.g., sweat glands).

• Effect: Activation, inhibition of cardiac muscle, or excitation of smooth muscle/glands.

Adrenergic Receptors

Adrenergic receptors are also G protein-coupled and respond to norepinephrine and epinephrine. Subtypes (α and β) determine the specific response.

• α1: Excitation (e.g., vasoconstriction).

• α2: Inhibition (e.g., inhibits NE release).

• β1: Excitation (e.g., increases heart rate).

• β2: Inhibition (e.g., bronchodilation).

• β3: Excitation (e.g., lipolysis in adipose tissue).

Table: Cholinergic and Adrenergic Receptors

Neurotransmitter	Receptor Type	Major Locations	Effect of Binding
Acetylcholine (ACh)	Cholinergic Nicotinic Muscarinic	All postganglionic neurons, adrenal medullary cells, neuromuscular junctions of skeletal muscle All parasympathetic target organs; limited sympathetic targets (e.g., sweat glands)	Excitation Activation, inhibition of cardiac muscle, excitation of smooth muscle/glands
Norepinephrine (NE) and Epinephrine	Adrenergic α1, α2, β1, β2, β3	Heart, blood vessels, lungs, adipose tissue, most sympathetic target organs	Excitation or inhibition depending on receptor subtype and organ

Autonomic Reflex Arc

Components and Function

The autonomic reflex arc is a neural pathway that controls involuntary responses in visceral organs. It consists of sensory receptors, afferent fibers, integration centers, efferent fibers, and effectors.

• Sensory receptor: Detects stimulus in viscera.

• Afferent fiber: Carries signal to CNS.

• Integration center: Processes information.

• Efferent fiber: Preganglionic and postganglionic axons transmit response to effector organ.

• Effector: Smooth muscle, cardiac muscle, or gland.

Key Equations

• Nicotinic receptor ion flow: $\text{Na}^+ \text{ influx} > \text{K}^+ \text{ efflux}$ Result: Depolarization of postsynaptic membrane

Example

• Fight or Flight Response: During stress, the sympathetic division increases heart rate, dilates bronchioles, and mobilizes energy stores.

Additional info: The notes focus on the sympathetic division, but the principles of neurotransmitter release, receptor types, and reflex arcs apply broadly to both divisions of the ANS.