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Autonomic Nervous System

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Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan

Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used.

Mappings of the lecture slides to the 12th and 13th editions are provided in the supplements.

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Outline

• Basic principles• More detail on motor control• Neurotransmitters and receptors• Physiological responses• Autonomic integration and control• Two medical conditions

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Basic Principles

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Autonomic Nervous System

• The autonomic nervous system (ANS) responds to certain visceral sensations, and excites or inhibits effectors.

• Effectors include smooth muscle, cardiac muscle, and endocrine and exocrine glands.

• The ANS consists of sensory neurons, integrative centers, and motor neurons.

• The ANS operates via lower- and higher-level reflex arcs, and almost always without conscious control.

Visceral = pertaining to organs or tissue coverings of organs.Effector = a muscle, gland, or organ capable of responding to

a stimulus, especially a nerve impulse (action potential).

Chapter 15, page 523

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Divisions

• The output or motor components of the ANS are its sympathetic and parasympathetic divisions.

• Most organs receive nerve impulses (action potentials) from both ANS divisions—this is known as dual innervation.

• The divisions often, but not always, work in opposition to one another.


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Divisions (continued)

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Divisions (continued)

• Nerve impulses from one division stimulate the organ to increase its activity (excitation).

• Nerve impulses from the other division decrease the organ’s activity (inhibition).

• For example, sympathetic activation increases and parasympathetic activation decreases heart rate.

• In some instances, the two divisions work together, such as the male sexual response.

Nerve impulses = action potentials.


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Control

• The ANS was originally named autonomic because it was thought to function autonomously.

• Nuclei in the hypothalamus and brainstem, however, are involved in regulating ANS functions.

Autonomous = self-governing; free of external influence or control.


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Conscious Control

• Due to the lack of sensory awareness, very few autonomic responses can be consciously altered.

• Consider, however:

- Practitioners of yoga and other meditative techniques can learn from long and diligent practice how to regulate some autonomic functions, such as heart rate.

- Biofeedback using electronic monitoring can provide sensory feedback to enhance the ability to exert some conscious control of ANS functions.


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Biofeedback

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Sensory Input

• Most sensory input to the ANS is from autonomic sensory neurons.

• Many of these neurons are interoreceptors located in blood vessels, visceral organs, muscles, and nervous system.

• Interoreceptors are sensory receptors that monitor the body’s internal environment.

• They include chemoreceptors to monitor blood CO2 level, and mech-anoreceptors to detect stretch in the walls of hollow organs and blood vessels.


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Sensory Input (continued)

• Sensory signals from interoreceptors are usually not consciously per-ceived since they generally don’t reach the level of the cerebral cortex.

• Intense activation of interoreceptors, however, can produce conscious sensations.

• For example, inadequate coronary blood flow can result in chest pain known as angina pectoris.

• Somatic (body) sensations can also affect the ANS—intense pain can produce changes in autonomic activity.


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Other Sensory Inputs

• The special senses acting through the limbic system can also affect autonomic responses.

• For example, part of the reaction to an unexpected loud noise can include increased heart rate in a physiological response known as sympathetic arousal.


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Motor Output

• Autonomic motor neurons regulate visceral activities by either increas-ing (exciting) or decreasing (inhibiting) ongoing activities in effectors.

• ANS motor responses include changes in the diameter of the pupils, changes in heart rate, and dilation and constriction of blood vessels.

• Unlike skeletal muscle tissue, tissues innervated by the ANS can func-tion autonomously to some extent when their nerve supply is damaged.

ANS effectors = cardiac muscle, smooth muscle, and endocrine and exocrine glands.

Innervation = to supply an organ or a body part with nerves.


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Motor Pathways

• An autonomic motor pathway has two motor neurons positioned in series.

• The cell body of the first neuron is located in the CNS—its myelinated axon extends to an autonomic ganglion located outside of the CNS.

• The cell body of the second neuron is located in the autonomic gang-lion—its unmyelinated axon extends to an effector.

• An exception to this rule is the axon of the first motor neuron extends directly to chromaffin cells in the adrenal medulla—there is no second neuron.

Ganglion (plural, ganglia) = collection of cell bodies of neurons outside of the CNS.


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Motor Pathways (continued)

Schematic diagram = a drawing intended to explain how something works.

Effector

Cell body and dendrites

(dendrites are not shown)

AxonNeuron 1 Neuron 2

Synapse

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Effector Smooth muscle

Cardiac muscle Endocrine and exocrine glands

Autonomic ganglion

Central nervous system

Preganglionic neuron

Postganglionic neuron

Myelinatedaxon

Unmyelinated axon

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Neurotransmitters of the ANS

• Somatic motor neurons release acetylcholine (ACh) at the neuromus-cular junctions (synapses) with skeletal muscles.

• Autonomic motor neurons release either ACh or norepinephrine (NE) at their synapses.

• More detail will be provided as we proceed with this lecture material.


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More Detail on Motor Control

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Motor Pathways

• The cell body of an ANS preganglionic neuron is located in the brain or spinal cord.

• Its axon—a small-diameter, myleninated type B fiber—exits the CNS as part of a cranial nerve or spinal nerve.

• The axon extends to an autonomic ganglion outside of the CNS, where it synapses with a postganglionic neuron.

Chapter 15, page 526 Figure 15.1

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• The axon of the postganglionic neuron is a small-diameter, unmyleni-nated type C fiber.

• The axon synapses with an effector (smooth muscle, cardiac muscle, or gland).


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Parasympathetic Preganglionic Neurons

• In the parasympathetic division, cell bodies of preganglionic neurons are found in the nuclei of cranial nerves III, VII, IX, and X, and sacral segments 2 through 4.

• The parasympathetic division is also known as the craniosacral divi-sion.


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Sympathetic Preganglionic Neurons

• The cell bodies of the preganglionic neurons are located in the lateral horns of the 12 thoracic segments and the first 2-to-3 lumbar segments of the spinal cord.

• The sympathetic division is therefore also known as the thoracolumbar division.


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Lateral Horn of Spinal Cord

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WM = white matter; GM = gray matter.

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Autonomic Ganglia

• The autonomic ganglia differ in location and structure in the two ANS divisions.

• Parasympathetic division—the ganglia are located close to or in the walls of visceral organs.

• Sympathetic division—the ganglia form an interconnected chain of cell bodies and axons (known as the ganglionic chain), which is in close proximity to the spinal cord.

Chapter 15, page 529 Figure 15.2 Figure 15.3

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Axon Lengths

• Parasympathetic division—the preganglionic axons are long and the postganglionic axons are short.

• Sympathetic division—the converse generally holds true: short pre-ganglionic axons and long postganglionic axons.

Chapter 15, page 529 Figure 15.2 Figure 15.3

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Comparison of Axon Lengths

Preganglionic neuronPostganglionic neuron

Effector

Sympathetic division:

Short Long

Effector

Parasympathetic division:

Long Short

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Parasympathetic Postganglionic Neurons

• One parasympathetic preganglionic neuron can synapse with up to 4-to-5 postganglionic neurons.

• Each postganglionic axon, however, only innervates only one effec-tor.

• The arrangement enables parasympathetic responses to be localiz-ed to one or possibly a few organs.


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Sympathetic Postganglionic Neurons

• A sympathetic preganglionic neuron can synapse with 20 or more post-ganglionic neurons.

• A postganglionic axon (actually, fiber) can innervate many different effectors.

• The divergence helps explain why sympathetic activation affects much of the body simultaneously.


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Neurotransmitters and Receptors

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Cholinergic and Adrenergic Neurons

• Autonomic neurons are cholinergic or adrenergic based on the neuro-transmitter synthesized and released at their synapses.

• Receptors, which consist of proteins, are located in the plasma mem-brane of the postsynaptic neuron or effector cell.

Cholinergic = acetylcholine (ACh) is the neurotransmitter.Adrenergic = norepinephrine (NE) is the neurotransmitter.


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Cholinergic and Adrenergic Neurons (continued)

Preganglionic neuronPostganglionic neuron

Effector

Sympathetic division:

Cholinergeric Adrenergic (usually)

Effector

Parasympathetic division:

Cholinergeric Cholinergeric

Cholinergic = acetylcholineAdrenergic = norepinephrine

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Cholinergic Neurons

• Cholinergic neurons in the autonomic nervous system include:

- All preganglionic neurons in the parasympathetic and sympathetic divisions.

- All postganglionic neurons in the parasympathetic division.- Postganglionic neurons in the sympathetic division that innervate

sweat glands in the skin.


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Cholinergic Neurons (continued)

• ACh, stored in the synaptic vesicles of the end buttons of axons, is released via exocytosis in response to action potentials.

• ACh diffuses across the synaptic cleft and binds to the cholinergic receptors in the postsynaptic membrane to produce graded poten-tials.

• Cholinergic receptors are classified as either nicotinic or muscarinic based on their functional properties.


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Synapse

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Tobacco Plant

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Nicotinic Receptors

• Nicotinic receptors respond to nicotine, a substance not naturally found in the human body.

• Nicotine, when introduced at nicotinic synapses, binds to the post-synaptic receptors, and mimics the action of ACh.

• Activation of nicotinic receptors produces depolarizing graded po-tentials.

• The result is excitation of the postsynaptic cell so it is more likely to produce a response.


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Nicotinic Receptors (continued)

• Nicotinic receptors are found in:

- Preganglionic neurons of the parasympathetic and sympathetic divisions.

- Chromaffin cells of the adrenal medulla innervated by the sym-pathetic division.

- Neuromuscular junctions in skeletal muscle (which are innervated by somatic motor neurons).


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Amanita muscaria

Muscarine is found in some species of mushrooms, including Amanita muscaria. Its ingestion can result in intense parasympathetic

responses, convulsions, and death.

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Muscarinic Receptors

• Muscarinic receptors are named for a mushroom toxin known as muscarine that mimics the action of ACh in binding to post-synap-tic receptors.

• These receptors are found in smooth muscle, cardiac muscle, and glands innervated by postganglionic axons of the parasympathetic division.

• Most sweat glands innervated by postganglionic axons of the sym-pathetic division also have muscarinic receptors.


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Muscarinic Receptors (continued)

• Stimulation of muscarinic receptors results in depolarizing (excita-tory) or hyperpolarizing (inhibitory) graded potentials based on the cell type.

• For example, the binding of ACh to the muscarinic receptors in the digestive tract inhibits (relaxes) the smooth muscle sphincters.

• In contrast, ACh excites the muscarinic receptors in the smooth muscle fibers of the iris of the eye, causing the smooth muscles to contract and decrease pupil size.


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Adrenergic Neurons

• Adrenergic neurons release norepinephrine, sometimes called noradrenalin.

• Most postganglionic neurons in the sympathetic division are adrenergic—except for those that innervate sweat glands in the skin.

• Norepinephrine is stored in synaptic vesicles and is released via exocytosis in response to action potentials from postganglionic neurons.


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Adrenergic Neurons (continued)

• Norepinephrine diffuses across the synaptic cleft and binds to the adrenergic receptors in the postsynaptic membrane of the effector cell.

• A depolarizing or hyperpolarizing graded potential results, depend-ing on the cell type.


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Norepinephrine and Epinephrine

• Adrenergic receptors in the postsynaptic membrane bind norepineph-rine and epinephrine (a closely-related molecule of the catecholamine family).

• Norepinephrine is released as a neurotransmitter in most postgan-glionic neurons of the sympathetic division

• Epinephrine (and small amounts of norepinephrine) are released as hormones from the chromaffin cells of the adrenal medulla into gen-eral blood circulation.


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Neurotransmitter Inactivation

• The action of norepinephrine is terminated when it is inactivated by an enzyme (COMT or MAO) and is then reabsorbed by the end buttons of the neuron.

• Norepinephrine persists in the synaptic cleft for a longer period of time than ACh because COMT and MAO are relatively slow-acting compared to acetylcholine esterase (AChE).

• Thus, the effects triggered by norepinephrine are generally longer-lasting than those triggered by ACh.

COMT = catechol-O-methyl transferaseMAO = monoamine oxidase inhibitor


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Alpha and Beta Receptors

• Adrenergic receptors are classified as alpha () or beta () types.

• The receptors are found on the postsynaptic membranes of effectors innervated by most postganglionic axons in the sympathetic division.


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Alpha and Beta Receptors (continued)

• Cells of most sympathetic effectors have either or receptors, but some cells have both types.

• Norepinephrine stimulates receptors more strongly than it stim-ulates receptors.

• Epinephrine, in comparison, is a strong stimulator of both and receptors.


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Receptor Subtypes

• Alpha and beta receptors have subtypes—1, 2, 1, 2, and 3.

• The classification is based on the responses they elicit, and the selective binding of drugs that activate or block the receptors.

1 and 1 receptors generally produce excitation, and 2 and 2 receptors produce inhibition of effector cells.

3 receptors are limited to brown adipose cells where their activa-tion produces thermogenesis.

Thermogenesis = the production of heat, especially within an animal body.


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Receptor Agonists

• Some drugs and natural substances selectively activate or block cholinergic and adrenergic receptors.

• An agonist is a substance that binds to and activates receptors in the postsynaptic membrane to mimic the effect of the neurotrans-mitter or hormone.

• Phenylephrine, a common ingredient in cold and sinus medications, is an agonist of 1 receptors.

• The drugs constricts blood vessels in the nasal mucosa to reduce the production of mucus and relieve nasal congestion.


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Receptor Antagonists

• An antagonist is a substance that binds to and blocks receptors to prevent a neurotransmitter or hormone from exerting its effect.

• Atropine blocks mucarinic (ACh) receptors—it dilates the pupils, reduces glandular secretions, and relaxes smooth muscle of the digestive tract.

• Atropine is used to dilate the pupils during eye exams.

• It is also used as an antidote for some chemical warfare agents (such as nerve gas) that trigger massive amounts of ACh release.


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Physiological Responses

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Autonomic Tone

• Most body organs are innervated by the parasympathetic and sym-pathetic divisions that usually, but not always, work in opposition to one another.

• The balance between the two divisions, known as autonomic tone, is regulated by the hypothalamus.


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• The two divisions can affect the same body organs differently because:

- The postganglionic neurons release different neurotransmitters.- Effector organs have different adrenergic and cholinergic recep-

tors.

Autonomic Tone (continued)


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Sympathetic Responses

• The sympathetic division dominates during physical activities and emotional stress.

• Sympathetic activation favors body functions that support physical activities, including fight-or-flight (to be discussed in the upcoming slides).

• It also reduces body functions involved in the storage of potential energy from food—sympathetic activation slows down the digestive process.


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Sympathetic Responses (continued)

• Emotions such as fear, embarrassment, and rage can stimulate the sympathetic division.

• Sympathetic activation and the release of norepinephrine and epi-nephrine from the adrenal medulla set in motion a complex series of physiological responses.

• The changes collectively are known as the fight-or-flight response.


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Fight-or-Flight

http://www.uh.edu

http://darwin-online.org.uk

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Fight-or-Flight Response

• The sympathetic fight-or-flight response to a stressor results in many physiological changes, including:

- Dilation of the pupils.- Increases in heart rate, force of cardiac contraction, and blood

pressure.- Dilation of the bronchioles (a type of airway passageway).- Constriction of blood vessels to the kidneys and digestive tract.- Dilation of blood vessels to the skeletal muscles, cardiac muscle,

and liver.

Stressor = a condition or agent that causes physiological or psychological stress to an organism.


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Fight-or-Flight Response (continued)

• Other physiological changes in the fight-or-flight response include:

- Breakdown of glycogen to glucose in the liver (glycogenolysis).- Release of glucose from the liver to serve as an immediate

source of energy for anaerobic respiration in other body tissues.- Breakdown of trigylcerides to glycerol and fatty acids (lipolysis).- Slowing-down of non-essential processes, including a reduction

in smooth muscle activity in the digestive tract.


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• The effects of sympathetic activation, such as in the fight-or-flight response, are more widespread and longer-lasting than the effects of parasympathetic activation.

• The postganglionic axons of the sympathetic division diverge more extensively, and therefore many different tissues are activated simultaneously.

• Epinephrine and norepinephrine are also secreted by the adrenal medulla into general blood circulation, which serve to intensify and prolong sympathetic responses.

• The enzymes, MAO and COMT, are slower to break-down norepi-nephrine and epinephrine than the action of AChE on acetylcholine.

Sympathetic Persistence


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Sympathetic Inactivation

• Norepinephrine and epinephrine are eventually inactivated by en-zymes in the liver.

• The metabolites are recycled for the re-synthesis of catecholam-ines.

Catecholamine = an amine derived from the amino acid tyrosine that act as a neurotransmitter or hormone;

includes norepinephrine, epinephrine, and dopamine.


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Parasympathetic Responses

• The parasympathetic division enhances rest-and-digest activities.

• These activities support body functions that conserve and restore body energy during periods of rest and recovery.


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Parasympathetic Activation (continued)

• The ungainly acronym, SLUDD can be used in recalling five major parasympathetic responses.

• The responses are: 1) salivation, 2) lacrimation, 3) urination, 4) digestion, and 5) defecation.

• Other parasympathetic responses include decreased heart rate, decreased diameter of the airways, and constriction of the pupils.

• Parasympathetic activation is also involved in sexual arousal.

Lacrimation = secretion of tears.


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Autonomic Integration and Control

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Autonomic Reflexes

• Autonomic reflexes serve a major role in regulating body functions including (among others):

- Blood pressure, by regulating heart rate, force of ventricular contraction of the heart, and diameter of blood vessels.

- Digestion, by regulating smooth muscle tone and motility of the digestive tract.

- Defecation and urination, by regulating the opening and closing of smooth muscle sphincters.

Motility = motion or movement.


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Components of an Autonomic Reflex Arc

• Receptor—distal end of a sensory neuron (often an interoreceptor)

• Sensory neuron—cell body and axon.

• Integrative center in the spinal cord—interneurons

• Motor neurons

• Effector—smooth muscle, cardiac muscle, or endocrine or exocrine gland


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Autonomic Reflex Arcs

Sympatheticdivision

Parasympatheticdivision

http://www.medical-look.com

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Hypothalamic Control

• The hypothalamus is the key integrative and control center for the autonomic nervous system.

• It receives sensory input from visceral functions, smell (olfaction), and taste (gustation).

• It also receives input for body temperature, osmolarity, and con-centration of various substances in the blood including glucose.

• And, the hypothalamus receives input from the limbic system for emotional states.

Osmolarity = a measure of the concentration of a solution.


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Hypothalamic Control (continued)

• Nuclei in the anterior and medial areas control the parasympathetic division.

• Nuclei in the posterior and lateral areas of the hypothalamus control the sympathetic division.

• Output from these autonomic centers is to the brainstem and spinal cord.


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Two Medical Conditions

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Raynaud’s Phenomenon

• In Raynaud’s phenomenon, the fingers and toes become ischemic in response to the thermal sensations of cold or to emotional stress.

• The symptoms result from excessive sympathetic stimulation of the smooth muscle in the arterioles of the digits, thereby reducing blood flow.

• People with Raynaud’s phenomenon often have low blood pressure.

Ischemia = a decrease in the blood supply and therefore oxygen to a body organ or tissue due to constriction or obstruction of the

blood vessels.Arteriole = small branch of an artery that connects with a capillary.

Digits = fingers and toes.


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Raynaud’s Phenomenon (continued)

http://www.nlm.nih.gov

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Raynaud’s Phenomenon (continued)

• Some people who have the phenomenon have an increased number of -adrenergic receptors.

• The phenomenon is most common in young woman, and is observed most often in cold climates.

• Treatment includes use of calcium channel blockers and alpha recep-tor blockers to relax the smooth muscles in the arteriole walls.

• Smoking, alcohol, and some illicit drugs can exacerbate the symptoms.


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Autonomic Dysreflexia

• Autonomic dysreflexia is an exaggerated response of the sympa-thetic division in persons who have spinal cord injuries at or above T6 (the sixth thoracic segment).

• The condition is due to interruption of ANS control by the hypothala-mus.


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Autonomic Dysreflexia (continued)

• Sensory nerve impulses, such from a full urinary bladder, stimulate the sympathetic nerves inferior to the injured level of the spinal cord.

• Other triggering events (or stimuli) include:

- Stimulation of pain receptors below the spinal cord injury- Smooth muscle contractions from sexual stimulation- Labor and delivery- Bowel distension


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Autonomic Dysreflexia (continued)

• Autonomic dysreflexia triggers a complex chain of events and feedback mechanisms in the parasympathetic and sympathetic divisions.

• The condition is characterized by:

- Pounding headache- Flushed, warm skin with profuse sweating above the level of

injury- Pale, cold, and dry skin below the injury level- Anxiety

• Because the condition can be life-threatening, it requires imme-diate medical intervention, including identifying and removing the stimulus.


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