NUR326: Basics of the Autonomic Nervous System

Nervous System

Aids in collecting information from the environment, interpreting findings, and forming a response to stimuli

Central Nervous System

Brain and spinal cord

Peripheral Nervous System

the sensory and motor neurons that connect the central nervous system to the rest of the body

Motor neurons

neurons that carry outgoing information from the brain and spinal cord to the muscles and glands

Sensory neurons

neurons that carry incoming information from the sensory receptors to the brain and spinal cord

Somatic nervous system

Controls voluntary movements. Targets skeletal muscle. Is responsible for the reflex arc

Autonomic nervous system

Controls involuntary functions. Targets the heart, lungs, eyes, smooth muscle, and glands. Is divided into the sympathetic nervous system (SNS) and parasympathetic nervous system (PSNS). SNS is active during stress. PSNS is active when not under stress.

Brain

Functions include sensation, responses, thought processing, movement, communication, emotions, and memory, as well as life-sustaining processes.

Spinal cord

Tracts: The spinal cord has a descending tract and an ascending tract. Regions: The four regions are cervical, thoracic, lumbar, and sacral, which include spinal nerves. Function: The spinal cord sends and receives information to and from designated areas of the body.

The ANS uses a

a two-neuron system to transmit messages to target organs and back. The nerves are referred to as pre- and postganglionic as the nerves will synapse at an area called a ganglion and neurotransmission occurs. These processes provide unconscious control of cardiac and smooth muscle and exocrine and endocrine glands.

SNS: Preganglionic neurons release

Acetylcholine (ACh)

SNS: postganglionic neurons release

norepinephrine (NE) or epinephrine

when the SNS is active

Target organ cell receptors will take up NE or epinephrine

Target organs of the SNS

the heart and vascular system, eyes, lungs, GI organs, bladder, sex organs, and immune system.

PSNS: Both pre- and postganglionic neurons release

acetylcholine (ACh)

Target organs of the PSNS

the heart, eyes, salivary glands, lungs, bladder, liver, sex organs, and immune system

Homeostasis

achieved through many feedback loops controlled by the brain in areas such as the hypothalamus that result in activation of the ANS. When disease mechanisms are present, this may challenge the ANS. The result can be overstimulation of one division while the other is suppressed.

Neurotransmitters of the SNS

ACh, NE and epinephrine

Acetylcholine (ACh)

This is an excitatory neurotransmitter released in the neuromuscular junctions of the SNS to effect movement. It is only used by preganglionic nerves to synapse within the ganglia and stimulate postganglionic nerve action, so it does not influence target organs of the SNS.

NE and epinephrine

Postganglionic nerves in the SNS use NE and epinephrine as neurotransmitters of SNS stimulation. These chemicals influence target organs of the SNS to produce the fight-or-flight response.

Receptors of the SNS

alpha and beta receptors. are on cells of the target organs that belong to the system. Not every organ of the SNS has all receptor types. (adrenergic receptors)

Alpha receptors

Include alpha1 and alpha2 receptors. Bind with NE and epinephrine when the body is experiencing SNS activation Medications either: Antagonize alpha1 receptors, resulting in vasodilation and lowered blood pressure, or Agonize alpha2 receptors, resulting in vasodilation and lowered blood pressure

Beta receptors

Include beta1 and beta2 receptors. Beta1 receptors: Increased cardiac output, including electrical conduction, which increases heart rate Increased renin production, which will contribute to an elevation in blood pressure. Beta2 receptors: Relaxation of smooth muscle and glycogenolysis

Neurotransmitters of the PSNS

ACh: Stimulates pre- and postganglionic nerves to synapse in ganglia and target organs to influence a rest-and-digest response

Receptors of the PSNS

take up ACh to make skeletal muscles move and restore body functions during the rest-and-digest response. They also exist in the brain to assist in cognitive functions (cholinergic receptors)

Nicotinic (PSNS)

N1 activation: mediates voluntary muscle movement. N2 activation: stimulates the release of epinephrine and NE in the SNS and mediates the pre- and postsynaptic stimulation of the SNS and PSNS, as well as the release of other neurotransmitters, including dopamine, NE, ACh, glutamate, and gamma-aminobutyric acid.

Muscarinic (PSNS)

Include M1 to M5 receptors. Stimulation results in rest-and-digest functions.

ACh is released

when an action potential travels along the neuron and causes depolarization, resulting in an influx of calcium into the neuron. Calcium then binds with enzymes and proteins to prepare synaptic vesicles for release of ACh into the synapses. ACh binds to nicotinic and muscarinic receptors. It is used and then rapidly degraded.

NE release

In the PNS, preganglionic release of ACh stimulates the chromaffin cells of the adrenal medulla to release NE into the bloodstream. NE binds to alpha1, alpha2, and beta1 SNS receptors. Its primary action is to cause arterial vasoconstriction to elevate blood pressure.

Epinephrine release

synthesized from NE. It binds to alpha1, alpha2, beta1, and beta2 receptors. Its primary effects are widespread and affect the arteries, heart, lungs, and skeletal muscle.

Fight-or-flight response

Neurotransmitters involved: NE and epinephrine influence target organs that cause the fight-or-flight response. ACh stimulates the adrenal medulla to produce NE and epinephrine.

Physiological changes during fight or flight (increases)

Oxygenation: Increased cardiac output, lung capacity, heart rate, and contractility. Vasoconstriction of large vessels. Bronchodilation. Energy: Stimulation of the liver to increase gluconeogenesis (synthesis) and glycogenolysis (breakdown) of glucose. Breakdown of adipose tissue. Visual acuity: Pupillary dilation

Physiological changes during fight or flight (decreases)

GI motility: Inhibited blood flow. Urine output: Relaxation of bladder detrusor muscles. Constriction of the urethral sphincters. Immune system function

Rest-and-digest response:

Neurotransmitters involved: ACh

Changes during rest and digest

Cardiovascular: Slowing of heart rate and electrical conduction. Vasodilation of vessels. Pulmonary: Stimulation of bronchial smooth muscle. GI: Increased gastric and intestinal peristalsis and relaxation of all sphincters. Increased production and release of bile (gallbladder). Secretion of insulin and digestive enzymes (pancreas). Increased saliva production. Genitourinary: Contraction of the bladder muscles and ureters. Relaxation of all sphincters. Eyes:

Contraction of the sphincter muscle of the iris, which also constricts the pupil

Autonomic tone

Neurotransmitters involved: ACh, NE, epinephrine

Autonomic tone Physiological response:

consistently adjusts to optimize functions based on sensory inputs and the changing internal environment. Autonomic dysfunction can threaten the balance of tone and homeostasis.

ACh, NE, and epinephrine are produced and released

as part of the ANS control to balance the SNS and PSNS.

An imbalance can be either a cause of or the result of pathophysiological processes.

PSNS relies exclusively on

ACh

SNS uses

NE at most postganglionic synapses.

Indicators of ANS functioning include:

Blood pressure, Blood pH, Glucose, Temperature regulation

Baroceptors

sense changes in blood pressure, and hence, a negative feedback loop is initiated. If blood pressure is high, the NTS will stimulate the PSNS nerves to increase outflow and inhibit the SNS nerves to lower outflow.

Chemoreceptors

sense changes in pH, partial pressure of oxygen, and partial pressure of carbon dioxide of arterial blood to maintain expected acid-base balance so cells can function.

Glucose homeostasis:

Eating a meal triggers the CNS, activating parasympathetic nerves. This increases the pancreatic output of insulin and stimulates the liver to promote glycogen synthesis and prevent glycogenolysis and gluconeogenesis.

Temperature regulation

are sensed by thermoreceptors in the skin, spinal cord, visceral organs, and brain. Afferent signals are sent to the hypothalamus, which will then relay efferent responses to the areas that need to respond.

Drug selectivity

refers to a drug's ability to bind with a receptor.

Nonselective medications

affect multiple receptor types and therefore cause more widespread effects.

Selective medications

affect one receptor type, which may lessen the potential for side effects.

Receptors on cells

Inhibition of cellular processes occurs when the cell's action is no longer required for homeostasis. The chemicals of the body can also stimulate receptors to increase cellular activity.

Types of medications

Agonists increase cellular activity. Antagonists inhibit cellular activity.

Partial agonists

display characteristics of both agonists and antagonists. Their ability to bind to receptors is weaker, and the receptors have less affinity for these medications. Therefore, the therapeutic and side effects are lesser.

Adrenergic agonists

Albuterol, Clonidine

Adrenergic antagonists

Propranolol, Metoprolol

Adrenergic agonists cause

CNS stimulation and should be avoided, if possible, in clients who have severe cardiovascular disease. "sympathomimetics"

Adrenergic antagonists are

blockers of the SNS influence and are also known as sympatholytics. They lower heart rate, decrease overall cardiac output, and reduce peripheral vascular resistance, which results in lower blood pressure.

Cholinergic agonists

Bethanechol, Pilocarpine, Nicotine

Cholinergic antagonists

Atropine, Scopolamine

Cholinergic agonists are

a small group of medications that either mimic (direct acting) or increase the availability of ACh by preventing its enzymatic breakdown (indirect acting).

Cholinergic antagonists work by

binding to the muscarinic and nicotinic receptors on target organs to result in the opposite of parasympathetic effects. Used for a wide range of conditions, including bradycardia, overactive bladder, motion sickness, GI spasms and cramping, and bronchoconstriction.