Autonomic Nervous System Overview

Overview of the Autonomic Nervous System (ANS)

• Division of visceral motor neurons in the body.

• Divided into two primary divisions: Sympathetic and Parasympathetic.

Sympathetic Division

• Often referred to as the fight or flight system.

• Prepares the body for stressful situations:

  • Increases heart rate.

  • Increases breathing rate.

  • Redirects blood flow to skeletal muscles, enhancing physical performance.

• Anatomical name: Thoracolumbar division.

  • Neurons are located in lateral gray horns from T1 to L2.

• Neurons comprise:

  • Preganglionic fibers (short)

  • Postganglionic fibers (long)

• Neurotransmitters:

  • Preganglionic: Acetylcholine (ACh)

  • Postganglionic: Norepinephrine (NE, also known as epinephrine/adrenaline).

Parasympathetic Division

• Referred to as the rest and digest system.

• Promotes relaxation and maintenance functions:

  • Decreases heart rate.

  • Increases digestive activity.

• Anatomical name: Craniosacral division.

  • Neurons located in cranial nerves (III, VII, IX, X) and sacral region (S2 to S4).

• Neurons comprise:

  • Preganglionic fibers (long)

  • Postganglionic fibers (short)

• Neurotransmitters:

  • Preganglionic: Acetylcholine (ACh)

  • Postganglionic: Acetylcholine (ACh)

Neural Pathways

• Ganglia: collections of nerve cell bodies outside the central nervous system (CNS).

  • Sympathetic ganglia (close to CNS): chain ganglia and collateral ganglia.

  • Parasympathetic ganglia (close to target organ).

Neuronal Structure

• Preganglionic Neurons: originate in CNS (preganglionic neurons in T1-L2 for sympathetic and cranial/sacral for parasympathetic).

• Postganglionic Neurons: originate in ganglia and transmit signals to effector organs.

• Neuronal divergence:

  • Sympathetic: 1 preganglionic neuron can synapse with up to 32 postganglionic neurons.

  • Parasympathetic: 1 preganglionic neuron synapses with about 5 postganglionic neurons.

Differences Between Sympathetic and Parasympathetic Systems

• Preganglionic Location:

  • Sympathetic: lateral gray horn T1-L2

  • Parasympathetic: cranial nerves III, VII, IX, X and S2-S4

• Postganglionic Location:

  • Sympathetic: ganglia near spinal column (chain and collateral)

  • Parasympathetic: ganglia near the effector organs

• Fibers Length:

  • Sympathetic: short preganglionic, long postganglionic.

  • Parasympathetic: long preganglionic, short postganglionic.

Neurotransmitter Mechanics

• Acetylcholine (ACh): used by both divisions at the ganglionic synapse.

• Epinephrine (Adrenaline): primarily released from the sympathetic division, stimulates target organs effectively.

  • Affects organs based on receptor type (adrenergic receptors).

• Different receptors for the same neurotransmitter can lead to various responses (activation/inhibition based on receptor type).

Receptor Types

1. Nicotinic Receptors:

   • Accepts ACh in autonomic ganglia and at neuromuscular junctions.

   • Found in both sympathetic and parasympathetic systems.

2. Muscarinic Receptors:

   • Found on effector organs in the parasympathetic system.

   • Can either activate or inhibit, depending on the subtype of the receptor.

3. Adrenergic Receptors:

   • Found on target organs in the sympathetic system.

   • Respond to epinephrine/norepinephrine, leading to activation or inhibition.

Physiological Responses

• Fight-or-flight response leads to:

  • Increased energy availability (e.g., heart, lungs).

  • Decreased energy to non-essential systems (e.g., digestion).

• Rest-and-digest response optimizes bodily energies for digestion and recovery, slowing down the heart and diverting energy to digestive activities.

Clinical Application

• Epinephrine in Emergencies:

  • Used in EpiPen to manage allergic reactions fast by stimulating the sympathetic nervous response without activating postganglionic neurons directly.

• Physical Activity Post-Meal:

  • Difficult due to diverting blood from digestion to muscles, which may lead to adverse effects (e.g., cramps, digestive distress).

Summary of Anatomy and Structure

• Autonomic nervous system vital for regulating visceral functions.

• Key differences between sympathetic and parasympathetic divisions in terms of anatomy, neurotransmitters, and functions.

• Understanding these differences is crucial for application in physiology and medicine, especially in response to stress and physiological changes in the body.

Important Terms and Definitions

• Preganglionic Neuron: Neuron originating in the CNS.

• Postganglionic Neuron: Neuron located in the peripheral ganglia.

• Ganglia: Collection of nerve cell bodies in the peripheral nervous system.

• Visceral Effectors: Organs innervated by the autonomic nervous system.

• Neuronal Divergence: Refers to how many different postganglionic neurons one preganglionic neuron can synapse to, influencing the speed and breadth of physiological responses.