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Autonomic Nervous System
Dr. Michael P. Gillespie
Autonomic Nervous SystemAuto = self; nomic = lawThe autonomic nervous system (ANS) includes autonomic
sensory neurons, integrating centers in the central nervous system (CNS), autonomic motor neurons, and the enteric division.
The ANS is also referred to as the visceral nervous system or involuntary nervous system.
It acts as a control system. It operates largely without conscious control; however,
centers in the hypothalamus and brain stem do regulate ANS reflexes.
Dr. Michael P. Gillespie 2
Flow of nerve impulses in the ANS(1) Autonomic sensory neurons in visceral
organs and blood vessels propagate into (2) integrating centers in the CNS. Then, impulses in (3) autonomic motor neurons propagate to various effector tissues, thereby regulating the activity of smooth muscle, cardiac muscle, and many glands. (4) The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal tract.
Dr. Michael P. Gillespie 3
Functions of ANSThe ANS affects heart rate, digestion,
respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal.
Most autonomous functions are involuntary.Some ANS actions can work with some
degree of conscious control:BreathingSwallowingSexual arousalHeart rate
Dr. Michael P. Gillespie 4
Divisions of the ANSThe ANS is classically divided into two
subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS).
For some functions these systems operate independently and for others they operate co-operatively.
In many cases the PSNS and SNS have opposite actions with one activating a physiological response and the other inhibiting it.
The enteric nervous system is also considered to be a part of the ANS.
Dr. Michael P. Gillespie 5
Afferent and EfferentANS functions can be divided into sensory
(afferent) and motor (efferent) subsystems.
Dr. Michael P. Gillespie 6
Somatic Nervous System The somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from receptors for somatic senses (tactile,
thermal, pain, proprioceptive) and special senses (sight, hearing, taste, smell, equilibrium).
These sensations are normally consciously perceived. Somatic motor neurons innervate skeletal muscles to produce both voluntary
and involuntary movements. When a somatic motor neuron stimulates a muscle, it contracts; the effect is
excitation. If it fails to stimulate a muscle it becomes paralyzed. A few skeletal muscles, such as those in the middle ear, are controlled by
reflexes and cannot be contracted voluntarily.
Dr. Michael P. Gillespie 7
Autonomic Sensory NeuronsThe main input to the ANS comes from autonomic
(visceral) sensory neurons. These neurons are primarily associated with interoceptors (monitor the internal environment).
Most of the time, these signals are not consciously perceived; although, intense activation of interoceptors may produce conscious sensation.
Pain sensations from damaged viscera and angina pectoris (chest pain) form inadequate blood flow to the heart can produce conscious sensation.
Dr. Michael P. Gillespie 8
Autonomic Motor NeuronsAutonomic motor neurons regulate visceral activities by
either increasing (exciting) or decreasing (inhibiting) activities in the effector tissues (cardiac muscle, smooth muscle, and glands).
Examples of autonomic motor responses include: changes in the diameter of the pupils, dilation and constriction of blood vessels, adjustment of the rate and force of the heartbeat.
Unlike skeletal muscle, tissue innervated by the ANS often continue to function to some extent even if their nerve supply is damaged.
The heart continues to beat when it is removed for transplantation, smooth muscle lining the GI tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control.
Dr. Michael P. Gillespie 9
Two Divisions of ANS OutputUnlike the somatic output (motor), the output
portion of the ANS has two divisions: sympathetic division and parasympathetic division.
Most organs have dual innervation (both PSNS and SNS innervation).
In some organs, nerve impulses from one division stimulate the organ to increase its activity (excitation) and impulses from the other division decrease its activity (inhibition).
Dr. Michael P. Gillespie 10
Sympathetic Nervous SystemThe sympathetic division is often called the fight-or-flight
division.Sympathetic activities result in increased alertness and
metabolic activities in order to prepare the body for an emergency situation.
Physical activity and emotional stress can trigger sympathetic activities.
Effects of sympathetic stimulation: rapid heart rate, faster breathing rate, dilation of pupils, dry mouth, sweaty but cool skin, dilation to blood vessels to organs involved in combating stress (heart and skeletal muscles), constriction of blood vessels to organs not involved in combating stress (GI tract and kidneys), and release of glucose from the liver).
Dr. Michael P. Gillespie 11
Parasympathetic DivisionThe parasympathetic division is often
referred to as the rest-and-digest division because its activities conserve and restore body energy during times of rest or digesting a meal.
The majority of the nerve output is directed towards smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts.
The parasympathetic division conserves energy and replenishes nutrients.
Dr. Michael P. Gillespie 12
Comparison of Somatic and Autonomic Nervous Systems
Dr. Michael P. Gillespie 13
Comparison of Somatic and Autonomic Nervous Systems
Dr. Michael P. Gillespie 14
Two-neuron Efferent PathwayThe ANS differs from the somatic nervous system in that it
requires a sequential two-neuron efferent pathway.The preganglionic neuron will synapse with a
postganglionic neuron before innervating the target organ.The first of the two motor neurons is called the
preganglionic neuron. The cell body is located in the brain or spinal cord. It exits the CNS as part of a cranial or spinal nerve.
It synapses with a postganglionic neuron in an autonomic ganglion, which is the second neuron in the autonomic pathway. The postganglionic neuron is located entirely in the PNS.
Dr. Michael P. Gillespie 15
Preganglionic NeuronSympathetic Division
Thoracolumbar division [cell bodies in the lateral horns of the 12 thoracic segments and the first two (sometimes three) lumbar segments]
Thoracolumbar outflow [axons]Parasympathetic Division
Craniosacral division [cell bodies in the nuclei of four cranial nerves (III, VII, IX, & X) and the lateral gray matter of the second through fourth sacral segments]
Craniosacral outflow [axons]
Dr. Michael P. Gillespie 16
Structure of the Sympathetic Division
Dr. Michael P. Gillespie 17
Structure of the Parasympathetic Division
Dr. Michael P. Gillespie 18
Adrenergic and Cholinergic Receptors
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Autonomic GangliaSympathetic gangliaParasympathetic ganglia
Dr. Michael P. Gillespie 20
Sympathetic GangliaSympathetic ganglia – sites of synapse between
sympathetic preganglionic and postganglionic neurons.Sympathetic trunk ganglia (also called vertebral chain
ganglia or paravertebral ganglia). Lie in a vertical row on either side of the vertebral column. Extend from the base of the skull to the coccyx. Innervate organs above the diaphragm (head, neck and heart). Superior, middle, and inferior cervical ganglia.
Prevertebral (collateral) ganglia. Lie anterior to the vertebral column. Innervate organs below the diaphragm. Celiac ganglion, superior mesenteric ganglion, inferior
mesenteric ganglion, aorticorenal ganglion, and renal ganglion.
Dr. Michael P. Gillespie 21
Parasympathetic GangliaParasympathetic ganglia – sites of synapse
between preganglionic parasympathetic neurons and postganglionic neurons in the terminal (intramural) ganglia.
Terminal ganglia in the head have specific names: ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, and otic ganglion.
The remaining terminal ganglia do not have specific names.
Dr. Michael P. Gillespie 22
Postganglionic NeuronAxons of preganglionic neurons pass to sympathetic trunk
ganglia and synapse with postganglionic neurons.A single sympathetic preganglionic fiber has many axon
collaterals and may synapse with 20 or more postganglionic neurons. This divergence explains why many sympathetic responses affect almost the entire body simultaneously.
After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors.
Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector.
Dr. Michael P. Gillespie 23
Comparison of Sympathetic And Parasympathetic Divisions of ANS
Dr. Michael P. Gillespie 24
Effects of Sympathetic And Parasympathetic Divisions of ANS
Dr. Michael P. Gillespie 25
Effects of Sympathetic And Parasympathetic Divisions of ANS
Dr. Michael P. Gillespie 26
Autonomic Plexuses Thorax
Cardiac plexus – innervates the heart Pulmonary plexus – innervates the bronchial tree
Abdomen and Pelvis Celiac (solar) plexus [largest plexus]– innervates the
stomach, spleen, pancreas, liver, gallbladder, kidneys, adrenal medulla, testes, & ovaries
Superior mesenteric plexus – innervates the small and large intestines
Inferior mesenteric plexus – innervates the large intestine Hypogastric plexus – innervates the pelvic viscera Renal plexus – innervates the renal arteries within the
kidneys and ureters
Dr. Michael P. Gillespie 27
Postganglionic Neurons in the Sympathetic Division
Dr. Michael P. Gillespie 28
Autonomic Plexuses in the Thorax, Abdomen, and Pelvis
Dr. Michael P. Gillespie 29
Sympathetic Trunk GangliaCervical PortionThoracic PortionLumbar PortionSacral Portion
Dr. Michael P. Gillespie 30
Pathways from Sympathetic Ganglia to Visceral Effectors Spinal nerves (31 pairs) – innervate visceral effectors of the skin of
the neck, trunk, and limbs including sweat glands, smooth muscle in blood vessels, arrector pili muscles. Gray rami communicantes – structures containing sympathetic
postganglionic axons that connect the ganglia of the sympathetic trunk to the spinal nerves.
Cephalic periarterial nerves – innervate visceral effectors of the skin of the face (sweat glands, smooth muscle of blood vessels, arrector pili muscles. Innervates visceral effectors of the head (smooth muscle of the eye, lacrimal glands, pineal gland, nasal mucosa, and salivary glands.
Sympathetic nerves – innervates visceral effectors in the thoracic cavity including the heart and lungs.
Splanchnic nerves – innervates the organs of the abdominopelvic cavity.
Dr. Michael P. Gillespie 31
Sacral Parasympathetic Outflow
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Parasympathetic Division of the ANSPreganglionic neurons originate from the
cranial nerves III, VII, IX, & X as well as the sacral spinal nerves S2-4.
The presynaptic neuron usually synapses with 4-5 postsynaptic neurons, all of which supple a single visceral effector.
Dr. Michael P. Gillespie 33
Enteric DivisionPlexuses
Myenteric plexusSubmucosal plexusThis system controls motility and secretory
functions of the gastrointestinal tract.
Dr. Michael P. Gillespie 34
Cholinergic NeuronsCholinergic neurons – acetylcholine AchCholinergic neurons include:
All sympathetic and parasympathetic preganglionic neurons
Sympathetic postganglionic neurons that innervate most sweat glands
All parasympathetic postganglionic neurons
Dr. Michael P. Gillespie 35
Cholinergic ReceptorsCholinergic receptors bind with acetylcholine Two types
Nicotinic receptorsMuscarinic receptors
Dr. Michael P. Gillespie 36
Adrenergic Neurons and ReceptorsRelease noripinephrine (noradrenalin)Most sympathetic postganglionic neurons are
adrenergic.Two types:
Alpha receptorsBeta receptors
Dr. Michael P. Gillespie 37
Cholinergic and Adrenergic Neurons in the Autonomic Nervous System
Dr. Michael P. Gillespie 38
Autonomic ToneAutonomic tone is a balance between
sympathetic and parasympathetic activity.Autonomic tone is regulated by the
hypothalamus.
Dr. Michael P. Gillespie 39
Sympathetic Responses Stress ↑sympathetic system ↑fight-or-flight response. ↑ production of ATP Dilation of pupils ↑heart rate and blood pressure Dilation of airways Constriction of blood vessels that supply the kidneys and
gastrointestinal tract ↑blood supply to the skeletal muscles, cardiac muscle, liver and
adipose tissue ↑glycogenolysis ↑blood glucose ↑lipolysis
Dr. Michael P. Gillespie 40
Parasympathetic ResponsesRest-and-digest responseConserve and restore body energy↑digestive and urinary function↓Body functions that support physical
activity
Dr. Michael P. Gillespie 41
Integration and Control of Autonomic FunctionsDirect innervation – brain stem and spinal
cordHypothalamus is the major control and
integration center of the ANSIt receives input from the limbic system
Dr. Michael P. Gillespie 42