The Autonomic Nervous System

• The autonomic nervous system (ANS), along with the endocrine system, regulates and integrates bodily functions.
• The endocrine system uses blood-borne hormones to send signals to target tissues.
• The nervous system transmits electrical impulses via nerve fibers to effector cells, which respond to neuromediator substances.
• Drugs that mimic or alter the ANS are called autonomic drugs, which act by stimulating or blocking portions of the ANS.
• The nervous system has two anatomical divisions: the central nervous system (CNS) composed of the brain and spinal cord, and the peripheral nervous system, which includes neurons outside the CNS.
• The peripheral nervous system is divided into efferent neurons that carry signals away from the CNS, and afferent neurons that bring information to the CNS.
• Afferent neurons modulate the efferent division through reflex arcs.

Functional Divisions

• The efferent portion of the peripheral nervous system is divided into the somatic nervous system and the ANS.
• Somatic efferent neurons control voluntary functions like skeletal muscle contraction for locomotion.
• The ANS regulates everyday bodily functions without conscious input.
• The ANS is also known as the visceral, vegetative, or involuntary nervous system.
• The ANS consists of efferent neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature, and exocrine glands, controlling digestion, cardiac output, blood flow, and glandular secretions.

Anatomy of ANS

• The ANS carries nerve impulses from the CNS to effector organs through preganglionic and postganglionic neurons.
• The cell body of the preganglionic neuron is located within the CNS.
• Preganglionic neurons emerge from the brainstem or spinal cord and connect in ganglia.
• Ganglia are relay stations between preganglionic and postganglionic neurons.
• The cell body of the postganglionic neuron originates in the ganglion, is usually nonmyelinated, and terminates on effector organs.
• Afferent neurons (fibers) regulate the system through reflex, such as sensing pressure in the carotid sinus and aortic arch, signaling the CNS to influence the efferent branch.

Sympathetic Neurons

• The efferent ANS is divided into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system.
• Sympathetic and parasympathetic neurons originate in different spinal cord regions.
• Sympathetic preganglionic neurons come from the thoracic and lumbar regions (T1 to L2) of the spinal cord, synapsing in two cord-like chains of ganglia.
• Preganglionic neurons are short compared to postganglionic ones.
• Postganglionic neuron axons extend from ganglia to tissues they innervate.
• Sympathetic preganglionic nerve endings are branched, enabling one neuron to interact with many postganglionic neurons, activating multiple effector organs.
• The adrenal medulla receives preganglionic fibers from the sympathetic system.
• The adrenal medulla, stimulated by acetylcholine, secretes epinephrine (adrenaline) and norepinephrine directly into the blood.

Parasympathetic Neurons

• Parasympathetic preganglionic fibers arise from cranial nerves III, VII, IX, and X, and from the sacral region (S2 to S4) of the spinal cord, synapsing in ganglia near or on effector organs.
• Parasympathetic preganglionic fibers are long, and postganglionic ones are short.
• Ganglia are located close to or within the innervated organ.
• There is a one-to-one connection between preganglionic and postganglionic neurons, enabling discrete responses.

Enteric Neurons

• The enteric nervous system is the third division of the ANS.
• It contains nerve fibers that innervate the gastrointestinal (GI) tract, pancreas, and gallbladder
• It is the "brain of the gut".
• It functions independently of the CNS, controlling motility, exocrine and endocrine secretions, and microcirculation of the GI tract.
• It is modulated by the sympathetic and parasympathetic systems.

Functions of the Sympathetic Nervous System

• The sympathetic division adjusts the body in response to stressful situations like trauma, fear, hypoglycemia, cold, and exercise.

Effects of Stimulation

• Sympathetic output increases heart rate and blood pressure, mobilizes energy stores, and increases blood flow to skeletal muscles and the heart, while diverting flow from the skin and internal organs.
• Sympathetic stimulation dilates pupils and bronchioles and affects GI motility, bladder function, and sexual organs.

Fight-or-Flight

• The body's changes during emergencies are the "fight or flight" response.
• Reactions are triggered by direct sympathetic activation of effector organs and by adrenal medulla stimulation to release epinephrine and norepinephrine.
• Hormones from the adrenal medulla enter the bloodstream and promote responses in organs with adrenergic receptors.
• The sympathetic nervous system functions as a unit, discharging as a complete system during severe exercise or in response to fear.
• The system has a diffuse distribution of postganglionic fibers and involves in a wide array of physiological activities.
• It prepares the body to handle uncertain situations and unexpected stimuli, but is not essential for survival.

Functions of the Parasympathetic Nervous System

• The parasympathetic division maintains homeostasis, essential bodily functions, such as digestion and elimination of wastes.
• It opposes or balances sympathetic actions and predominates in "rest-and-digest" situations.
• The parasympathetic system never discharges as a complete system, and fibers innervating specific organs are activated separately.

Role of the Central Nervous System

• The ANS requires sensory input from peripheral structures, provided by afferent impulses.
• These impulses travel to integrating centers in the CNS, such as the hypothalamus, medulla oblongata, and spinal cord, which respond by sending efferent reflex impulses.

Innervation by ANS

Dual

• Most organs are innervated by both divisions of the ANS.
• Vagal parasympathetic innervation slows heart rate, and sympathetic innervation increases it.
• One system usually predominates.
• In the heart, the vagus nerve controls heart rate.
• Antagonism is dynamic and controls homeostatic organ functions.

Sympathetic

• Some effector organs, such as the adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system.

Somatic Nervous System

• The efferent somatic nervous system differs from the ANS, using a single myelinated motor neuron from the CNS directly to skeletal muscle without ganglia.
• It is under voluntary control, whereas the ANS is involuntary.
• Responses in the somatic division are generally faster.
• The sympathetic nervous system is widely distributed, innervating virtually all effector systems, while the parasympathetic division is more limited.
• Sympathetic preganglionic fibers have a broader influence than parasympathetic fibers, synapsing with more postganglionic fibers, leading to a diffuse discharge.
• The parasympathetic division is more circumscribed, with one-to-one interactions, and ganglia close to or within organs, limiting branching.

Chemical Signaling between Cells

• Neurotransmission is a form of chemical signaling between cells.
• Other types of chemical signaling include hormone secretion and local mediator release.

Hormones

• Endocrine cells secrete hormones into the bloodstream, impacting broadly distributed target cells.

Local Mediators

• Most cells secrete chemicals that act locally on cells in the immediate environment, not entering the blood or distributing throughout the body (histamine & prostaglandins).

Neurotransmitters

• Communication between nerve cells and effector organs occurs through neurotransmitter release from nerve terminals.
• Release is triggered by an action potential, causing depolarization.
• Increased intracellular Ca2+ initiates fusion of synaptic vesicles with the presynaptic membrane.
• Neurotransmitters rapidly diffuse across the synaptic cleft and combine with postsynaptic receptors.

Membrane Receptors

• Neurotransmitters and most hormones/local mediators are too hydrophilic to penetrate lipid bilayers.
• Signals are mediated by binding to specific receptors on the cell surface.

Neurotransmitters

• Over 50 signal molecules have been identified in the nervous system.
• Acetylcholine, dopamine, serotonin, histamine, glutamate, and y-aminobutyric acid (GABA) are commonly involved in drug actions.
• Acetylcholine and norepinephrine are the primary chemical signals in the ANS, while a wider variety function in the CNS.

Acetylcholine

• Autonomic nerve fibers are divided into cholinergic or adrenergic based on the neurotransmitter released.
• Cholinergic neurons transmit impulses via acetylcholine.
• Acetylcholine mediates nerve impulses across autonomic ganglia in both the sympathetic and parasympathetic systems and is the neurotransmitter at the adrenal medulla.
• In the parasympathetic system, transmission from postganglionic nerves to effector organs also releases acetylcholine.
• In the somatic nervous system, transmission at the neuromuscular junction is cholinergic.

Norepinephrine and Epinephrine

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

Signal Transduction in the Effector Cell

• Binding of chemical signals to receptors activates enzymatic processes in the cell membrane, leading to cellular responses like phosphorylation of intracellular proteins or changes in ion channel conductivity.
• A neurotransmitter is a signal, and a receptor is the signal detector and transducer.
• Second messenger molecules translate extracellular signals into responses that may be further propagated or amplified.

Membrane Receptors Affecting Ion Permeability

• Some neurotransmitter receptors are membrane proteins that provide a binding site for neurotransmitter molecules.
• Some receptors, like postsynaptic nicotinic receptors in skeletal muscle cells, are directly linked to membrane ion channels.
• Neurotransmitter binding rapidly affects ion permeability.
• Known as ionotropic receptors.

Membrane Receptors Coupled to Second Messengers

• Many receptors are not directly coupled to ion channels.
• The receptor signals recognition of a bound neurotransmitter by initiating reactions that lead to a specific intracellular response.
• Second messenger molecules intervene between the original message and the ultimate cellular effect.
• The two widely recognized second messengers are the adenylyl cyclase system and the calcium/phosphatidylinositol system.
• Receptors coupled to the second messenger system are known as metabotropic receptors.
• Muscarinic and adrenergic receptors are examples.