Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS) and the endocrine system coordinate bodily functions
• The endocrine system uses blood-borne hormones for signaling, while the nervous system uses electrical impulses and neuromediators
• Autonomic drugs mimic or alter ANS functions by either stimulating or blocking autonomic nerves
• The nervous system has a central (CNS) and peripheral (PNS) division
• The CNS includes the brain and spinal cord
• The PNS includes neurons outside the CNS
• PNS divides into efferent (signals away from CNS) and afferent (signals to CNS) divisions
• Afferent neurons provide sensory input that modulates efferent function through reflex arcs

Functional Divisions

• The efferent PNS divides into somatic and autonomic nervous systems
• Somatic neurons control voluntary actions like skeletal muscle contraction
• The ANS regulates involuntary, vital functions like digestion and blood flow
• The ANS is also known as the visceral, vegetative, or involuntary nervous system
• It consists of efferent neurons that innervate smooth muscle, cardiac muscle, vasculature, and exocrine glands

Anatomy of the ANS

• The ANS uses preganglionic and postganglionic efferent neurons to carry impulses
• Preganglionic neuron cell bodies are in the CNS, synapsing in ganglia in the PNS
• Ganglia act as relay stations between pre- and postganglionic neurons
• Postganglionic neuron cell bodies originate in the ganglion and terminate on effector organs
• Afferent neurons are important in reflex regulation and signaling the CNS
• The efferent division divides into sympathetic, parasympathetic, and enteric nervous systems
• Sympathetic preganglionic neurons originate in the thoracic and lumbar regions (T1-L2) of the spinal cord
• These synapse in ganglia chains alongside the spinal cord
• Sympathetic preganglionic neurons are short, and postganglionic neurons are long
• Sympathetic preganglionic nerve endings are highly branched, allowing activation of many postganglionic neurons
• The adrenal medulla receives preganglionic fibers, releasing epinephrine and norepinephrine into the blood upon stimulation

Parasympathetic neurons

• Parasympathetic preganglionic fibers arise from cranial nerves III, VII, IX, X and the sacral region (S2-S4)
• They synapse in ganglia near or on effector organs
• Parasympathetic preganglionic fibers are long and postganglionic fibers are short
• Connections are typically one-to-one, enabling discrete responses

Enteric neurons

• The enteric nervous system is the third division of the ANS
• It innervates the GI tract, pancreas, and gallbladder
• It functions independently of the CNS to control motility, secretions, and microcirculation
• The enteric system is modulated by sympathetic and parasympathetic systems

Functions of the Sympathetic Nervous System

• The sympathetic division adjusts in response to stress (trauma, fear, hypoglycemia, cold, exercise)
• Sympathetic stimulation increases heart rate and blood pressure
• It mobilizes energy stores, increases blood flow to skeletal muscles and the heart, and diverts flow from the skin and internal organs
• Sympathetic stimulation results in dilation of the pupils and the bronchioles
• It also affects GI motility and the function of the bladder and sexual organs

Fight-or-Flight Response

• The "fight or flight" response is triggered by direct sympathetic activation and adrenal medulla stimulation
• The adrenal medulla releases epinephrine and norepinephrine into the bloodstream
• These hormones promote responses in effector organs containing adrenergic receptors
• The sympathetic nervous system tends to function as a unit, discharging completely during exercise or fear
• It prepares the body for uncertain situations and unexpected stimuli

Functions of the Parasympathetic Nervous System

• The parasympathetic division maintains homeostasis
• It is required for essential bodily functions like digestion and waste elimination
• The parasympathetic division opposes the sympathetic division, predominating in "rest-and-digest" situations
• Parasympathetic system never discharges as a complete unit
• Parasympathetic fibers innervate specific organs separately

Role of the CNS

• The ANS requires sensory input from peripheral structures
• Afferent impulses from viscera travel to integrating centers in the CNS (hypothalamus, medulla oblongata, spinal cord)
• These centers respond by sending efferent reflex impulses via the ANS

Innervation

• Most organs receive dual innervation
• Vagal parasympathetic innervation slows heart rate, while sympathetic innervation increases heart rate
• One system usually predominates
• Some organs (adrenal medulla, kidney, pilomotor muscles, sweat glands) only receive sympathetic innervation

Somatic Nervous System

• A single myelinated motor neuron travels directly to skeletal muscle without ganglia
• The somatic nervous system is under voluntary control
• Somatic responses are generally faster than ANS responses

Anatomical Arrangement

• The sympathetic nervous system is widely distributed, innervating practically all effector systems
• The parasympathetic system's distribution is limited
• Sympathetic preganglionic fibers have a broader influence
• Parasympathetic divisions are circumscribed

Chemical Signaling Between Cells

• Neurotransmission in the ANS exemplifies chemical signaling between cells
• Other types of chemical signaling include hormone secretion and local mediators
• Endocrine cells secrete hormones into the bloodstream
• Most cells secrete local mediators that act on nearby cells
• Neurotransmitters transmit signals between nerve cells and effector organs
• Action potentials trigger neurotransmitter release from nerve terminals
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell

Membrane Receptors

• Neurotransmitters are hydrophilic, binding to specific receptors on the cell surface
• Signal is mediated via binding to specific receptors on the cell surface of target organs

Types of Neurotransmitters

• Norepinephrine, epinephrine, acetylcholine, dopamine, serotonin, histamine, glutamate, and γ-aminobutyric acid are common
• Each binds to a specific receptor family
• Acetylcholine and norepinephrine are primary in the ANS
• Many neurotransmitters function in the CNS

Acetylcholine

• Cholinergic neurons mediate transmission via acetylcholine
• Acetylcholine mediates nerve impulses across autonomic ganglia and at the adrenal medulla
• Parasympathetic postganglionic nerves also use acetylcholine
• The somatic nervous system uses acetylcholine at the neuromuscular junction

Norepinephrine and Epinephrine

• Adrenergic fibers use norepinephrine and epinephrine
• In the sympathetic system, norepinephrine mediates nerve impulses from postganglionic nerves to effector organs

Signal Transduction

• Binding of chemical signals to receptors activates enzymatic processes
• Neurotransmitters act as signals, and receptors act as signal detectors and transducers
• Second messenger molecules translate extracellular signals into intracellular responses

Membrane Receptors Affecting Ion Permeability

• Some receptors are directly linked to membrane ion channels
• Binding of neurotransmitters occurs rapidly, affecting ion permeability directly
• These are ionotropic receptors

Membrane Receptors Coupled to Second Messengers

• Many receptors signal recognition of neurotransmitters by initiating a series of reactions
• Second messenger molecules translate neurotransmitter binding into a cellular response
• G proteins are often involved
• Adenylyl cyclase and calcium/phosphatidylinositol systems are common second messengers
• These are metabotropic receptors
• Muscarinic and adrenergic receptors are examples