Autonomic Nervous System (ANS)

Overview of the Autonomic Nervous System

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It is essential for maintaining homeostasis and is divided into two main branches: the sympathetic and parasympathetic nervous systems.

• Dual Innervation: Most organs receive input from both sympathetic and parasympathetic fibers, which often have opposing effects.

• Homeostasis: The ANS maintains a dynamic balance between its branches to regulate bodily functions.

Sympathetic vs. Parasympathetic Nervous Systems

The sympathetic and parasympathetic systems are antagonistic, meaning one branch excites while the other inhibits target organs. Their actions are summarized as follows:

• Sympathetic Nervous System: Responsible for the "fight-or-flight" response, preparing the body for stressful or emergency situations.

• Parasympathetic Nervous System: Responsible for "rest-and-digest" activities, promoting relaxation and recovery.

Key Differences

• Anatomical Origin:

  • Sympathetic: Thoracic and lumbar spinal cord

  • Parasympathetic: Brainstem and sacral spinal cord

• Neurotransmitters:

  • Sympathetic: Preganglionic neurons release acetylcholine (ACh); postganglionic neurons release norepinephrine (NE) or epinephrine (E).

  • Parasympathetic: Both preganglionic and postganglionic neurons release acetylcholine (ACh).

• Receptors:

  • Sympathetic: Adrenergic receptors (α, β subtypes)

  • Parasympathetic: Muscarinic cholinergic receptors

Examples of Organ Responses

Neurotransmitters and Receptors

Neurotransmitters are chemical messengers released by neurons to communicate with target cells. In the ANS:

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE) and Epinephrine (E): Released by sympathetic postganglionic neurons and the adrenal medulla.

• Receptors:

  • Cholinergic receptors (nicotinic and muscarinic)

  • Adrenergic receptors (α1, α2, β1, β2, β3)

Neuroeffector Junction

The neuroeffector junction is the synapse between an efferent neuron and its target effector (e.g., muscle or gland). Neurotransmitter release at this junction determines the physiological response.

Autonomic Pathways

• Parasympathetic Pathway: Preganglionic neurons originate in the brainstem or sacral spinal cord and synapse near or within target organs.

• Sympathetic Pathway: Preganglionic neurons originate in the thoracic and lumbar spinal cord and synapse in sympathetic ganglia close to the spinal cord.

Adrenal Medulla

The adrenal medulla acts as a modified sympathetic ganglion, releasing epinephrine and norepinephrine directly into the bloodstream during sympathetic activation.

The Efferent Nervous System: Autonomic and Somatic Divisions

The efferent division of the nervous system consists of the autonomic (involuntary) and somatic (voluntary) branches. The autonomic branch controls smooth muscle, cardiac muscle, and glands, while the somatic branch controls skeletal muscle.

Summary Table: Sympathetic vs. Parasympathetic Features

Key Equations

• Nerve Signal Transmission: Where is voltage, is current, and is resistance (Ohm's Law, relevant for understanding nerve conduction).

Additional info:

• Concept check questions and answers are provided at the end of the chapter for self-assessment.

• Preparation for exams should include reviewing both the summary tables and concept check questions.