Autonomic Nervous System

• The autonomic nervous system (ANS), along with the endocrine system, regulates and integrates bodily functions.
• The nervous system influences bodily functions through rapid transmission of electrical impulses, while the endocrine system uses blood-borne hormones.
• Autonomic drugs work by either stimulating or blocking the actions of the ANS.

Divisions of the Nervous System

• The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system.
• The CNS consists of the brain and spinal cord.
• The peripheral nervous system includes all neurons outside the brain and spinal cord, specifically nerves entering or leaving the CNS.
• The peripheral nervous system is divided into efferent and afferent divisions.
• Efferent neurons carry signals from the brain and spinal cord to peripheral tissues.
• Afferent neurons bring information from the periphery to the CNS.
• Afferent neurons provide sensory input that modulates the efferent division through reflex arcs.

Functional Divisions of the Efferent Nervous System

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

Anatomy of the ANS

Efferent Neurons

• The ANS uses two types of efferent neurons: preganglionic and postganglionic neurons.
• Preganglionic neuron cell bodies are located within the CNS.
• Preganglionic neurons emerge from the brainstem or spinal cord and connect in ganglia.
• Ganglia act as relay stations between preganglionic and postganglionic neurons.
• Postganglionic neuron cell bodies originate in the ganglia and terminate on effector organs.
• Postganglionic neurons are generally nonmyelinated.

Afferent Neurons

• Afferent neurons are important for reflex regulation, sensing pressure, and signaling the CNS to influence the efferent branch.

Sympathetic Neurons

• 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.
• Sympathetic preganglionic neurons synapse in ganglia located near the spinal cord.
• Sympathetic preganglionic neurons are short compared to postganglionic neurons.
• Postganglionic neuron axons extend from the ganglia to innervated tissues.
• Sympathetic preganglionic nerve endings are highly branched, allowing one neuron to interact with many postganglionic neurons.
• This arrangement enables the sympathetic division to activate multiple effector organs simultaneously.
• The adrenal medulla receives preganglionic fibers from the sympathetic system.
• In response to acetylcholine, the adrenal medulla secretes epinephrine and norepinephrine directly into the bloodstream.

Parasympathetic Neurons

• Parasympathetic preganglionic fibers arise from cranial nerves III, VII, IX, and X, and the sacral region (S2 to S4) of the spinal cord.
• Parasympathetic preganglionic fibers synapse in ganglia near or on effector organs.
• Parasympathetic preganglionic fibers are long, and postganglionic fibers are short.
• There is typically a one-to-one connection between preganglionic and postganglionic neurons in the parasympathetic system.
• This enables discrete responses.

Enteric Neurons

• The enteric nervous system is the third division of the ANS.
• It innervates the gastrointestinal (GI) tract, pancreas, and gallbladder.
• It can function independently of the CNS.
• The enteric nervous system controls motility, exocrine and endocrine secretions, and microcirculation of the GI tract.
• It is modulated by both the sympathetic and parasympathetic nervous systems.

Functions of the Sympathetic Nervous System

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

Stimulation Effects

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

Fight-or-Flight Response

• The body's responses during emergencies are referred to as the “fight or flight” response.
• This is triggered by direct sympathetic activation and stimulation of the adrenal medulla.
• The adrenal medulla releases epinephrine and norepinephrine into the bloodstream.
• Sympathetic nervous system often discharges as a complete unit.
• The sympathetic nervous system prepares the body to handle uncertain situations and unexpected stimuli.

Functions of the Parasympathetic Nervous System

• The parasympathetic division maintains homeostasis.
• It is essential for life, maintaining bodily functions like digestion and waste elimination.
• The parasympathetic division opposes or balances the actions of the sympathetic division.
• It predominates in "rest-and-digest" situations.
• Parasympathetic fibers innervating specific organs are activated separately.
• The parasympathetic system functions to affect organs individually.

Role of the CNS

• The ANS requires sensory input from peripheral structures to provide information on the current state of the body.
• Afferent impulses from the viscera and other autonomically innervated structures travel to integrating centers in the CNS.
• Integrating centers include the hypothalamus, medulla oblongata, and spinal cord.
• These centers respond to stimuli by sending out efferent reflex impulses via the ANS.

Innervation by the ANS

Dual Innervation

• Most organs are innervated by both divisions of the ANS, parasympathetic and sympathetic.
• Vagal parasympathetic innervation slows the heart rate, and sympathetic innervation increases it.
• One system usually predominates in controlling the activity of a given organ.
• In the heart, the vagus nerve is the predominant factor for controlling rate.

Sympathetic Innervation

• 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 differs from the ANS.
• A single myelinated motor neuron originates in the CNS and travels directly to skeletal muscle without ganglia.
• The somatic nervous system is under voluntary control, whereas the ANS is involuntary.
• Responses in the somatic division are generally faster than those in the ANS.

Variations in Anatomical Arrangement

• The sympathetic nervous system is widely distributed, innervating practically all effector systems, whereas the parasympathetic division is more limited.
• Sympathetic preganglionic fibers have a broader influence and synapse with a larger number of postganglionic fibers.
• This permits a diffuse discharge of the sympathetic nervous system.
• The parasympathetic division is more circumscribed, with mostly one-to-one interactions, and the ganglia are close to or within the organs they innervate.

Chemical Signaling Between Cells

• Neurotransmission in the ANS exemplifies chemical signaling between cells.
• Other types of chemical signaling include hormone secretion and the release of local mediators.

Hormones

• Specialized endocrine cells secrete hormones into the bloodstream.
• These hormones travel throughout the body, affecting broadly distributed target cells.

Local Mediators

• Most cells secrete chemicals that act locally on cells in the immediate environment.
• These chemical signals are rapidly destroyed or removed.
• They do not enter the blood and are not distributed throughout the body.
• Histamine and prostaglandins are examples of local mediators.

Neurotransmitters

• Communication between nerve cells and effector organs occurs through the release of neurotransmitters.
• Neurotransmitter release is triggered by the arrival of an action potential at the nerve ending, leading to depolarization.
• An increase in intracellular Ca2+ initiates fusion of synaptic vesicles with the presynaptic membrane and release of their contents.
• Neurotransmitters diffuse across the synaptic cleft and combine with specific receptors on the postsynaptic cell.

Membrane Receptors

• Neurotransmitters, hormones, and local mediators are hydrophilic and cannot penetrate cell membranes.
• Their signal is mediated by binding to specific receptors on the cell surface of target organs.

Types of Neurotransmitters

• There are over 50 signal molecules in the nervous system.
• Norepinephrine, epinephrine, acetylcholine, dopamine, serotonin, histamine, glutamate, and γ-aminobutyric acid are commonly involved in the actions of therapeutically useful drugs.
• Each chemical signal binds to a specific family of receptors.
• Acetylcholine and norepinephrine are the primary signals in the ANS, while a wide variety of neurotransmitters function in the CNS.

Acetylcholine

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

Norepinephrine and Epinephrine

• When norepinephrine and epinephrine are the neurotransmitters, the fiber is termed adrenergic.
• In the sympathetic system, norepinephrine mediates the transmission of nerve impulses from autonomic 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 within the cell membrane.
• This results in a cellular response, such as the phosphorylation of intracellular proteins or changes in the conductivity of ion channels.
• A neurotransmitter is a signal, and a receptor is a signal detector and transducer.
• Second messenger molecules translate the extracellular signal into a response.
• Each component serves as a link between extracellular events and chemical changes within the cell.

Membrane Receptors Affecting Ion Permeability (Ionotropic Receptors)

• 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.
• Binding of the neurotransmitter occurs rapidly and directly affects ion permeability.
• These receptors are known as ionotropic receptors.

Membrane Receptors Coupled to Second Messengers (Metabotropic Receptors)

• Many receptors are not directly coupled to ion channels.
• The receptor signals its recognition of a bound neurotransmitter by initiating a series of reactions that result in a specific intracellular response.
• Second messenger molecules intervene between the original message and the ultimate effect on the cell.
• They translate neurotransmitter binding into a cellular response, usually through the intervention of a G protein.
• The most 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 of metabotropic receptors.