Chapter Outline

• 11.1 The Autonomic Nervous System

• 11.2 The Somatic Nervous System

This chapter explores the structure and function of the autonomic and somatic nervous systems, focusing on their roles in regulating involuntary and voluntary bodily functions. Key concepts include neural pathways, neurotransmitters, and the physiological effects of nervous system activity.

The Autonomic Nervous System

Introduction to the Autonomic Nervous System

The autonomic nervous system (ANS) innervates most effector organs and tissues in the body, such as cardiac muscle, smooth muscle, and glands. It regulates involuntary physiological processes, including heart rate, digestion, respiratory rate, and glandular secretion.

• Key Point: The ANS operates largely below the level of consciousness and is responsible for the body's rapid adjustments to changes in the internal and external environment.

• Example: During a stressful event, the ANS increases heart rate and redirects blood flow to muscles (fight-or-flight response).

Dual Innervation in the Autonomic Nervous System

Most organs receive input from both branches of the ANS: the sympathetic and parasympathetic divisions. This dual innervation allows for precise regulation of organ function.

• Sympathetic Division: Prepares the body for emergency situations (fight-or-flight).

• Parasympathetic Division: Promotes rest, digestion, and energy conservation (rest-and-digest).

• Key Point: The balance between sympathetic and parasympathetic activity determines the overall state of organ function.

Anatomy of the Autonomic Nervous System

The ANS consists of two-neuron chains connecting the central nervous system (CNS) to effector organs. These neurons communicate via synapses located in peripheral structures called autonomic ganglia.

• Preganglionic Neuron: Originates in the CNS and synapses in an autonomic ganglion.

• Postganglionic Neuron: Originates in the ganglion and innervates the effector organ.

• Key Point: The arrangement of neurons and ganglia differs between the sympathetic and parasympathetic divisions.

Anatomy of the Sympathetic Nervous System

Sympathetic preganglionic neurons emerge from the thoracic and lumbar regions of the spinal cord. The sympathetic division is also called the thoracolumbar division.

• Sympathetic Chain Ganglia: Paired structures running parallel to the spinal cord; most preganglionic fibers synapse here.

• Collateral Ganglia: Located in the abdominal cavity; innervate organs such as the stomach and intestines.

• Adrenal Medulla: Acts as a modified sympathetic ganglion, releasing hormones (e.g., epinephrine) into the bloodstream.

• Key Point: Sympathetic activation produces widespread effects due to hormone release and multiple organ innervation.

Anatomy of the Parasympathetic Nervous System

Parasympathetic preganglionic neurons originate in the brainstem and sacral spinal cord, known as the craniosacral division.

• Cranial Nerves: Vagus nerve (cranial nerve X) is a major parasympathetic pathway to thoracic and abdominal organs.

• Terminal Ganglia: Parasympathetic ganglia are located near or within the target organs, resulting in short postganglionic fibers.

• Key Point: Parasympathetic effects are more localized and specific compared to the sympathetic division.

The Mixed Composition of Autonomic Nerves

Recent studies show that autonomic nerves are generally mixed, containing both efferent (motor) and afferent (sensory) fibers. This allows for bidirectional communication between the CNS and peripheral organs.

• Key Point: Sensory input from organs helps regulate autonomic output.

Autonomic Neurotransmitters and Receptors

The main neurotransmitters in the ANS are acetylcholine and norepinephrine. These chemicals bind to specific receptors on effector organs to produce physiological responses.

• Cholinergic Neurons: Release acetylcholine; found in all preganglionic neurons and parasympathetic postganglionic neurons.

• Adrenergic Neurons: Release norepinephrine; found in most sympathetic postganglionic neurons.

• Receptor Types: Nicotinic and muscarinic receptors for acetylcholine; alpha and beta receptors for norepinephrine.

• Key Point: The type of neurotransmitter and receptor determines the effect on the target organ.

The Somatic Nervous System

Introduction to the Somatic Nervous System

The somatic nervous system (SNS) controls voluntary movements by innervating skeletal muscles. It consists of a single neuron pathway from the CNS to the muscle fiber.

• Key Point: The SNS is responsible for conscious control of body movements.

• Example: Moving your arm to pick up an object is mediated by the SNS.

Neurotransmitters and Receptors in the Somatic Nervous System

The neurotransmitter released at the neuromuscular junction is acetylcholine, which binds to nicotinic receptors on skeletal muscle fibers to initiate contraction.

• Key Point: The somatic pathway is always excitatory, leading to muscle contraction.

Disorders of the Somatic Nervous System

• Myasthenia Gravis: An autoimmune disorder where antibodies attack nicotinic acetylcholine receptors, leading to muscle weakness and fatigue.

• Curae: A plant-derived toxin that blocks nicotinic receptors, causing paralysis.

Systems Integration

Interaction Between Autonomic and Somatic Nervous Systems

Both systems work together to maintain homeostasis and respond to environmental changes. The autonomic system regulates involuntary functions, while the somatic system controls voluntary actions.

• Key Point: Integration ensures coordinated responses, such as increased heart rate during exercise.

Key Terms and Concepts

• G proteins

• cAMP second messenger system

• Phosphatidylinositol second messenger system

• Acetylcholine

• Norepinephrine

• Catecholamines

• Cholinergic receptors

• Adrenergic receptors

• Synapses

Table: Comparison of Sympathetic and Parasympathetic Divisions

Key Equations

• Resting Membrane Potential (RMP):

• Neurotransmitter Release (Generalized):

Additional info: Some details, such as the specific arrangement of ganglia and neurotransmitter mechanisms, were expanded for clarity and completeness.