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 for signaling, while the nervous system uses rapid electrical impulses and neuromediators.
• Autonomic drugs work by either stimulating or blocking the actions of the ANS.
• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system.
• CNS comprises the brain and spinal cord.
• The peripheral nervous system includes neurons outside the brain and spinal cord.
• The peripheral nervous system has efferent (signals away from the CNS) and afferent (signals to the CNS) divisions.
• Afferent neurons provide sensory input for modulating efferent function through reflex arcs.

Functional Divisions

• 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.
• The ANS regulates involuntary vital functions such as digestion, cardiac output, blood flow, and glandular secretions.
• The ANS is also known as the visceral, vegetative, or involuntary nervous system.

Anatomy of the ANS

• The ANS carries nerve impulses via preganglionic and postganglionic neurons.
• Preganglionic neuron cell bodies are located within the CNS and synapse in ganglia.
• Postganglionic neuron cell bodies originate in the ganglia and terminate on effector organs.
• Afferent neurons regulate the ANS through reflexes by sensing conditions like pressure and signaling the CNS.
• The efferent ANS includes the sympathetic and parasympathetic nervous systems, as well as the enteric nervous system.
• Sympathetic preganglionic neurons originate in the thoracic and lumbar (T1 to L2) regions.
• These sympathetic preganglionic neurons synapse in ganglia near the spinal cord.
• Sympathetic preganglionic neurons are short, while postganglionic neurons are long.
• Sympathetic preganglionic nerve endings are highly branched, affecting multiple postganglionic neurons.
• The adrenal medulla receives sympathetic preganglionic fibers and releases epinephrine (adrenaline) and norepinephrine into the bloodstream upon stimulation by acetylcholine.
• Parasympathetic preganglionic fibers arise from cranial nerves (III, VII, IX, X) and the sacral region (S2 to S4) of the spinal cord.
• Parasympathetic fibers synapse in ganglia near or on effector organs.
• Parasympathetic preganglionic fibers are long, and postganglionic fibers are short.
• Usually, there is a one-to-one connection between preganglionic and postganglionic parasympathetic neurons.
• The enteric nervous system innervates the gastrointestinal tract, pancreas, and gallbladder.
• This system controls motility, exocrine and endocrine secretions, and microcirculation of the GI tract.
• The enteric nervous system functions independently but is modulated by the sympathetic and parasympathetic systems.

Functions of the Sympathetic Nervous System

• The sympathetic division adjusts in response to stress like trauma, fear, hypoglycemia, cold, and exercise.

Effects of Stimulation

• Sympathetic stimulation increases heart rate and blood pressure.
• It mobilizes energy stores.
• It increases blood flow to skeletal muscles and the heart.
• It diverts blood flow from the skin and internal organs.
• Sympathetic stimulation dilates pupils and bronchioles.
• It affects GI motility, bladder function, and sexual organs.
• The “fight or flight” response involves direct sympathetic activation and adrenal medulla stimulation, releasing epinephrine and norepinephrine.
• The sympathetic nervous system tends to function as a unit.
• 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 life, maintaining functions such as digestion and waste elimination.
• The parasympathetic opposes or balances the sympathetic system in “rest-and-digest” situations.
• The parasympathetic system acts on specific organs individually, not as a complete system.

Role of the CNS

• The ANS requires sensory input from peripheral structures.
• Afferent impulses from viscera travel to integrating centers in the CNS, like the hypothalamus, medulla oblongata, and spinal cord.
• These centers send out efferent reflex impulses via the ANS.

Innervation by the ANS

• Most organs are innervated by both sympathetic and parasympathetic divisions.
• One system usually predominates, like the vagus nerve in controlling heart rate.
• This antagonism is dynamic.
• Some organs like the adrenal medulla, kidney, pilomotor muscles, and sweat glands only receive sympathetic innervation.

Somatic Nervous System

• The somatic nervous system differs from the ANS.
• A single myelinated motor neuron travels directly to skeletal muscle without ganglia.
• The somatic nervous system is under voluntary control and is faster than the ANS.
• The sympathetic nervous system is widely distributed.
• The parasympathetic division is more limited in distribution.
• Sympathetic preganglionic fibers have a broader influence, synapsing with more postganglionic fibers.
• The parasympathetic division has mostly one-to-one interactions, and its ganglia are close to or within organs.

Chemical Signaling Between Cells

• Neurotransmission is a general process of chemical signaling.
• Other types include hormone secretion and release of local mediators.

Hormones

• Endocrine cells secrete hormones into the bloodstream.
• Hormones affect broadly distributed target cells.

Local Mediators

• Most cells secrete chemicals that act locally.
• These local chemical signals are rapidly destroyed or removed.
• Histamine and prostaglandins are examples of local mediators.

Neurotransmitters

• Communication between nerve cells and effector organs occurs through neurotransmitter release.
• Action potential arrival at the nerve ending leads to neurotransmitter release.
• Increased intracellular Ca2+ initiates fusion of synaptic vesicles and neurotransmitter release.
• Neurotransmitters diffuse across the synaptic cleft and combine with receptors on the postsynaptic cell.

Membrane Receptors

• Neurotransmitters, hormones, and local mediators bind to specific receptors on the cell surface.

Types of Neurotransmitters

• Norepinephrine, epinephrine, acetylcholine, dopamine, serotonin, histamine, glutamate, and γ-aminobutyric acid are commonly involved in drug actions.
• Each chemical signal binds to a specific family of receptors.
• Acetylcholine and norepinephrine are the primary chemical signals in the ANS.
• A wide variety of neurotransmitters function in the CNS.

Acetylcholine

• Neurons are termed cholinergic if transmission is mediated by acetylcholine.
• Acetylcholine mediates nerve impulse transmission across autonomic ganglia in both sympathetic and parasympathetic systems.
• It is the neurotransmitter at the adrenal medulla.
• Transmission from parasympathetic postganglionic nerves to effector organs involves acetylcholine release.
• In the somatic nervous system, transmission at the neuromuscular junction is cholinergic.

Norepinephrine and Epinephrine

• Fibers are termed adrenergic when norepinephrine and epinephrine are the neurotransmitters.
• In the sympathetic system, norepinephrine mediates nerve impulse transmission from postganglionic nerves to effector organs.
• The adrenal medulla releases epinephrine (80%) and norepinephrine (20%).

Signal Transduction in the Effector Cell

• Chemical signal binding to receptors activates enzymatic processes.
• This leads to a cellular response like protein phosphorylation or changes in ion channel conductivity.
• A neurotransmitter is a signal, and a receptor is a signal detector and transducer.
• Second messenger molecules translate extracellular signals into intracellular responses.

Membrane Receptors Affecting Ion Permeability

• Some receptors are directly linked to membrane ion channels.
• Neurotransmitter binding rapidly affects ion permeability.
• These receptors are known as ionotropic receptors.

Membrane Receptors Coupled to Second Messengers

• Many receptors signal neurotransmitter recognition by initiating a series of reactions.
• Second messenger molecules intervene between the neurotransmitter and the cellular effect.
• G proteins are often involved, especially with the adenylyl cyclase system and the calcium/phosphatidylinositol system.
• Receptors coupled to the second messenger system are known as metabotropic receptors.
• Muscarinic and adrenergic are examples of metabotropic receptors.