Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS), along with the endocrine system, regulates and integrates bodily functions, with autonomic drugs mimicking or altering ANS functions by either stimulating or blocking autonomic nerves.

Nervous System Divisions

• The nervous system is divided into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system, which includes neurons outside the CNS.
• The peripheral nervous system is further divided into efferent neurons, carrying signals from the CNS to peripheral tissues, and afferent neurons, relaying information from the periphery to the CNS.
• Afferent neurons provide sensory input crucial for modulating the efferent division through reflex arcs.

Functional Divisions of Efferent Peripheral Nervous System

• The efferent portion of the peripheral nervous system includes the somatic and autonomic nervous systems.
• Somatic efferent neurons control voluntary functions like skeletal muscle contraction for locomotion.
• The ANS regulates involuntary vital bodily functions, controlling digestion, cardiac output, blood flow, and glandular secretions.

Anatomy of the ANS

• The ANS carries nerve impulses from the CNS to effector organs via preganglionic and postganglionic neurons.
• Preganglionic neurons, located in the CNS, connect in ganglia to postganglionic neurons, which terminate on effector organs like smooth muscle, cardiac muscle, and exocrine glands.
• Afferent neurons in the ANS are important for reflex regulation, sensing pressure and signaling the CNS to modulate efferent responses.
• The efferent ANS is divided into the sympathetic, parasympathetic, and enteric nervous systems.
• Sympathetic preganglionic neurons originate in the thoracic and lumbar regions (T1 to L2) of the spinal cord, synapsing in ganglia chains near the spinal cord, with short preganglionic and long postganglionic fibers.
• Sympathetic preganglionic nerve endings are highly branched, allowing one neuron to interact with many postganglionic neurons, enabling the activation of multiple effector organs simultaneously.
• The adrenal medulla receives sympathetic preganglionic fibers and releases epinephrine and norepinephrine into the bloodstream upon stimulation by acetylcholine.
• Parasympathetic preganglionic fibers arise from cranial nerves III, VII, IX, and X, as well as the sacral region (S2 to S4) of the spinal cord, synapsing in ganglia near or on effector organs, featuring long preganglionic and short postganglionic fibers.
• Parasympathetic connections are typically one-to-one between preganglionic and postganglionic neurons, enabling discrete responses.
• The enteric nervous system, located in the gastrointestinal tract, pancreas, and gallbladder, functions independently of the CNS, and is modulated by the sympathetic and parasympathetic systems.

Functions of the Sympathetic Nervous System

• The sympathetic division adjusts in response to stressors like trauma, fear, hypoglycemia, cold, and exercise.
• Stimulation of the sympathetic division increases heart rate and blood pressure, mobilizes energy stores, and increases blood flow to skeletal muscles and the heart.
• It also dilates pupils and bronchioles and affects GI motility, bladder function, and sexual organs.
• The "fight-or-flight" response is triggered by direct sympathetic activation and the release of epinephrine and norepinephrine from the adrenal medulla, promoting responses in organs with adrenergic receptors.
• The sympathetic nervous system functions as a unit, often discharging completely during exercise or fear reactions and is involved in a wide array of physiologic activities.

Functions of the Parasympathetic Nervous System

• The parasympathetic division maintains homeostasis and is essential for bodily functions like digestion and waste elimination.
• It opposes or balances sympathetic actions, predominating in "rest-and-digest" situations, with fibers innervating specific organs activated separately.

Role of the CNS in Autonomic Function Control

• The ANS requires sensory input from peripheral structures, with afferent impulses traveling to integrating centers in the CNS (hypothalamus, medulla oblongata, and spinal cord).
• These centers respond to stimuli by sending out efferent reflex impulses via the ANS.

Innervation by the ANS

• Most organs are innervated by both sympathetic and parasympathetic divisions, with one system usually predominating.
• The vagus nerve is the primary controller of heart rate.
• Some effector organs, like the adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system.

Somatic Nervous System

• The efferent somatic nervous system involves a single myelinated motor neuron originating in the CNS and traveling directly to skeletal muscle without ganglia mediation.
• The somatic nervous system is under voluntary control, unlike the involuntary ANS, and responses are generally faster.

Anatomical Arrangement of Neurons

• Major differences in the anatomical arrangement of neurons lead to variations of the functions in each division.
• The sympathetic nervous system is widely distributed, while the parasympathetic division is more limited.
• Sympathetic preganglionic fibers have a broader influence and synapse with more postganglionic fibers, permitting a diffuse discharge, while the parasympathetic division is more circumscribed, with mostly one-to-one interactions.

Chemical Signaling Between Cells

• Neurotransmission in the ANS exemplifies chemical signaling between cells, including hormone secretion and local mediator release.
• Endocrine cells secrete hormones into the bloodstream, affecting broadly distributed target cells.
• Most cells secrete local mediators, like histamine and prostaglandins, that act locally and are rapidly destroyed, thus not distributed throughout the body.
• Communication between nerve cells and effector organs involves neurotransmitters released from nerve terminals, triggered by action potentials and Ca2+ influx.
• Neurotransmitters diffuse across the synapse and combine with specific receptors on the postsynaptic cell.

Membrane Receptors

• Neurotransmitters, hormones, and local mediators bind to specific receptors on target cells due to their hydrophilic nature.
• Norepinephrine, epinephrine, acetylcholine, dopamine, serotonin, histamine, glutamate, and γ-aminobutyric acid are commonly involved in the actions of therapeutically useful drugs, each binding to a specific receptor family.
• Acetylcholine and norepinephrine are the primary chemical signals in the ANS, while the CNS uses a wider variety of neurotransmitters.
• Cholinergic neurons mediate transmission via acetylcholine at autonomic ganglia, the adrenal medulla, parasympathetic postganglionic nerves, and the neuromuscular junction.
• Adrenergic neurons mediate transmission via norepinephrine and epinephrine in the sympathetic system from postganglionic nerves to effector organs.
• Epinephrine (80%) and norepinephrine (20%) are released from the adrenal medulla.

Signal Transduction in the Effector Cell

• Binding of chemical signals to receptors activates enzymatic processes, resulting in cellular responses like phosphorylation of intracellular proteins or changes in ion channel conductivity.
• Neurotransmitters are signals, and receptors are signal detectors and transducers.
• Second messenger molecules translate extracellular signals into intracellular responses, often through G proteins.
• Adenylyl cyclase and calcium/phosphatidylinositol systems are common second messengers.

Membrane Receptors Affecting Ion Permeability

• Some receptors, like postsynaptic nicotinic receptors, are directly linked to membrane ion channels, rapidly affecting ion permeability upon neurotransmitter binding and known as ionotropic receptors.

Membrane Receptors Coupled to Second Messengers

• Many receptors signal neurotransmitter binding by initiating a series of reactions that result in a specific intracellular response.
• Second messenger molecules intervene between the neurotransmitter and the cell's ultimate effect via G proteins.
• Receptors coupled to the second messenger system are known as metabotropic receptors, including muscarinic and adrenergic receptors.