Autonomic Nervous System Neuroanatomy Autonomic Nervous System Dr. Michael P. Gillespie

Autonomic Nervous System Auto = self; nomic = law The 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

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

Functions of ANS The 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: Breathing Swallowing Sexual arousal Heart rate Dr. Michael P. Gillespie

Divisions of the ANS The 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

Afferent and Efferent ANS functions can be divided into sensory (afferent) and motor (efferent) subsystems. Dr. Michael P. Gillespie

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

Autonomic Sensory Neurons The 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

Autonomic Motor Neurons Autonomic 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

Two Divisions of ANS Output Unlike 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

Sympathetic Nervous System The 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). Islets of Langerhans – Insulin / Glucagon Glycogen stores Dr. Michael P. Gillespie

Parasympathetic Division The 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

Comparison of Somatic and Autonomic Nervous Systems Dr. Michael P. Gillespie

Comparison of Somatic and Autonomic Nervous Systems Dr. Michael P. Gillespie

Two-neuron Efferent Pathway The 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

Preganglionic Neuron Sympathetic Division Parasympathetic 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

Structure of the Sympathetic Division Dr. Michael P. Gillespie

Structure of the Parasympathetic Division Dr. Michael P. Gillespie

Adrenergic and Cholinergic Receptors Dr. Michael P. Gillespie

Autonomic Ganglia Sympathetic ganglia Parasympathetic ganglia Dr. Michael P. Gillespie

Sympathetic Ganglia Sympathetic 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

Parasympathetic Ganglia Parasympathetic 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

Postganglionic Neuron Axons 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

Comparison of Sympathetic And Parasympathetic Divisions of ANS Dr. Michael P. Gillespie

Effects of Sympathetic And Parasympathetic Divisions of ANS Dr. Michael P. Gillespie

Effects of Sympathetic And Parasympathetic Divisions of ANS Dr. Michael P. Gillespie

Autonomic Plexuses Thorax Abdomen and Pelvis 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

Postganglionic Neurons in the Sympathetic Division Dr. Michael P. Gillespie

Autonomic Plexuses in the Thorax, Abdomen, and Pelvis Dr. Michael P. Gillespie

Sympathetic Trunk Ganglia Cervical Portion Thoracic Portion Lumbar Portion Sacral Portion Dr. Michael P. Gillespie

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

Sacral Parasympathetic Outflow Dr. Michael P. Gillespie

Parasympathetic Division of the ANS Preganglionic 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

Enteric Division Plexuses Myenteric plexus Submucosal plexus This system controls motility and secretory functions of the gastrointestinal tract. Dr. Michael P. Gillespie

Cholinergic Neurons Cholinergic neurons – acetylcholine Ach Cholinergic neurons include: All sympathetic and parasympathetic preganglionic neurons Sympathetic postganglionic neurons that innervate most sweat glands All parasympathetic postganglionic neurons Dr. Michael P. Gillespie

Cholinergic Receptors Cholinergic receptors bind with acetylcholine Two types Nicotinic receptors Muscarinic receptors Dr. Michael P. Gillespie

Adrenergic Neurons and Receptors Release noripinephrine (noradrenalin) Most sympathetic postganglionic neurons are adrenergic. Two types: Alpha receptors Beta receptors Dr. Michael P. Gillespie

Cholinergic and Adrenergic Neurons in the Autonomic Nervous System Dr. Michael P. Gillespie

Autonomic Tone Autonomic tone is a balance between sympathetic and parasympathetic activity. Autonomic tone is regulated by the hypothalamus. Dr. Michael P. Gillespie

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

Parasympathetic Responses Rest-and-digest response Conserve and restore body energy ↑digestive and urinary function ↓Body functions that support physical activity Dr. Michael P. Gillespie

Integration and Control of Autonomic Functions Direct innervation – brain stem and spinal cord Hypothalamus is the major control and integration center of the ANS It receives input from the limbic system Dr. Michael P. Gillespie