Chapter 09 ANS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I. Neural Control of Involuntary Effectors

Autonomic Neurons Innervate organs not under voluntary control Effectors include: Cardiac muscle Smooth muscle of visceral organs and blood vessels Glands Part of the PNS

Differences between somatic and autonomic Somatic motor neurons have cell bodies in the spinal cord and just one neuron traveling from spinal cord to effector. Autonomic motor system has two sets of neurons in the PNS. The preganglionic neuron has cell bodies in the brain or spinal cord and synapses in an autonomic ganglion The postganglionic neuron has cell bodies in the ganglion and synapses on the effector

Autonomic Neurons Preganglionic neurons: originate in the midbrain or hindbrain or from the thoracic, lumbar, or sacral spinal cord Postganglionic neurons: originate in ganglion Autonomic ganglia are located in the head, neck, and abdomen as well as in chains along either side of the spinal cord

The ANS has pre- and postganglionic neurons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Autonomic ganglion CNS Involuntary effector Smooth muscle Preganglionic neuron Postganglionic neuron

Visceral Effector Organs Somewhat independent of innervation and will not atrophy if a nerve is cut (unlike skeletal muscle) Target may become even more sensitive to destimulation; called denervation hypersensitivity Cardiac muscle and some smooth muscle contract rhythmically without nerve stimulation. Autonomic innervation can speed up or slow down intrinsic contractions. Autonomic motor neurons can stimulate or inhibit, depending on the organ and the receptors

Neurotransmitters Somatic motor neurons release only acetylcholine which is always excitatory. Autonomic neurons release mainly acetylcholine and norepinephrine but may be excitatory or inhibitory

Somatic vs. Autonomic System

II. Divisions of the Autonomic Nervous System

Sympathetic Division Preganglionic neurons orginate from the thoracic and lumbar regions of the spinal cord. Also called the thoracolumbar division Preganglionic neurons synapse in sympathetic ganglia that run parallel to the spinal cord. These are called the paravertebral ganglia. These ganglia are connected, forming a sympathetic chain of ganglia (sympathetic trunk).

Sympathetic Division Myelinated axons of the preganglionic neurons exit the spinal cord at ventral roots and travel in white rami communicantes and synapse in autonomic ganglia at multiple levels. Unmyelinated axons of the postganglionic neurons form the gray rami communicantes, which return to the spinal nerve and travel with other spinal nerves to their effectors.

Sympathetic Chain of Paravertebral Ganglia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Spinal cord Posterior (dorsal) root Sympathetic chain of paravertebral ganglia Anterior (ventral) root Rami communicantes Sympathetic ganglion Spinal nerve Vertebral body Rib

Convergence and Divergence Because preganglionic neurons can branch and synapse in ganglia at any level, there is: Divergence: One preganglionic neuron synapses on several postganglionic neurons at different levels. Convergence: Several preganglionic neurons at different levels synapse on one postganglionic neuron. Allows the sympathetic division to act as a single unit through mass activation and to be tonically active

Sympathetic Neuron Pathways Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Visceral effectors: Smooth muscle of blood vessels, arrector pili muscles, and sweat glands 1. Preganglionic axons synapse with postganglionic neurons 2. Postganglionic axons innervate target organs Sympathetic chain ganglion Dorsal root ganglion Dorsal root Spinal nerve Sympathetic chain White ramus Splanchnic nerve Ventral root Gray ramus Visceral effector: intestine Collateral ganglion (celiac ganglion) Preganglionic neuron Spinal cord Postganglionic neuron

Collateral Ganglia Many of the sympathetic neurons that exit the spinal cord below the diaphragm do not synapse in the sympathetic chain of ganglia. Instead, they form splanchnic nerves, which synapse in collateral ganglia. Collateral ganglia include celiac, superior mesenteric, and inferior mesenteric ganglia. Postganglionic neurons innervate organs of the digestive, urinary, and reproductive systems.

Collateral Sympathetic Ganglia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diaphragm Superior mesenteric ganglion Celiac ganglion Adrenal gland Renal plexus First lumbar sympathetic ganglion Aortic plexus Inferior mesenteric ganglion Pelvic sympathetic chain

Adrenal Glands The adrenal medulla secretes epinephrine and norepinephrine when stimulated by the sympathetic nervous system as a part of mass activation Chromaffin cells of the adrenal medulla are modified postganglionic neurons that have lost their axons Preganglionic sympathetic neurons innervate directly on these cells.

Summary of the Sympathetic Division

Parasympathetic Division Preganglionic neurons originate from the brainstem or sacral region of the spinal cord. Also called the craniosacral division They synapse on ganglia located near or in effector organs; called terminal ganglia Preganglionic neurons do not travel with somatic neurons (as sympathetic postganglionic neurons do). Terminal ganglia supply very short postganglionic neurons to the effectors

Cranial Nerves and the Parasympathetic Division The occulomotor, facial, glosso-pharyngeal, and vagus nerves carry parasympathetic preganglionic neurons. Occulomotor (III) nerve Preganglionic fibers exit midbrain and synapse on the ciliary ganglion. Postganglionic fibers innervate the ciliary muscle of the eye.

Cranial Nerves and the Parasympathetic Division Facial (VII) nerve: Preganglionic fibers exit the pons and synapse in: Pterygopalatine ganglion: Postganglionic fibers synapse on nasal mucosa, pharynx, palate, and lacrimal glands. Submandibular ganglion: Postganglionic fibers synapse on salivary glands.

Cranial Nerves and the Parasympathetic Division Glossopharyngeal (IX) nerve: Preganglionic fibers exit medulla and synapse on otic ganglion. Postganglionic fibers innervate the parotid gland. Vagus (X) nerve: Preganglionic fibers exit medulla, branch into several plexi and nerves, and travel to terminal ganglia within effector organs (heart, lungs, esophagus, stomach, pancreas, liver, intestines).

Path of the Vagus Nerve Hyoid bone Vagus nerve Thyroid cartilage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hyoid bone Vagus nerve Thyroid cartilage of larynx Trachea Right pulmonary plexus Right cardiac branch Left pulmonary plexus Left cardiac branch Right gastric nerve Left gastric nerve Stomach Celiac plexus Liver Superior mesenteric nerve

Sacral Nerves Preganglionic nerves originate in lateral gray horn of spinal cord segments S2,3,4 and synapse on terminal ganglia Postganglionic nerves provide innervation to the descending colon, sigmoid colon, rectum, urinary organs and reproductive organs.

Summary of Parasympathetic Division

Comparison of the Sympathetic and Parasympathetic Divisions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cranial nerve III Ciliary muscle and pupil of eye Midbrain Cranial nerve VII Hindbrain Lacrimal gland and nasal mucosa Cranial nerve IX Cranial nerve X Submandibular and sublingual glands T1 T2 T3 Parotid gland T4 T5 Lung T6 T7 Sympathetic chain ganglion Celiac ganglion T8 Heart Greater splanchnic nerve T9 T10 Liver and gallbladder T11 Spleen Lesser splanchnic nerve T12 Stomach Pancreas L1 L2 Superior mesenteric ganglion Large intestine Small intestine Adrenal gland and kidney S2 S3 Inferior mesenteric ganglion S4 Urinary bladder Pelvic nerves Reproductive organs

III. Functions of the Autonomic Nervous System

General functions Sympathetic Functions The sympathetic division activates the body for “fight or flight” through the release of norepinephrine from postganglionic neurons and the secretion of epinephrine from the adrenal medulla. Prepares the body for intense physical activity in emergencies by increasing heart rate and blood glucose levels and by diverting blood to skeletal muscles Tonically regulates heart, blood vessels, and other organs

General functions Parasympathetic Functions The parasympathetic division is antagonistic to the sympathetic division. Allows the body to “rest and digest” through the release of ACh from postganglionic neurons Slows heart rate, and increases digestive activities

Summary of Autonomic Functions

Cholinergic Synaptic Transmission Acetylcholine (ACh) is the neurotransmitter secreted by all preganglionic neurons (sympathetic and parasympathetic) It is also the neurotransmitter released from most parasympathetic postganglionic neurons. Some sympathetic postganglionic neurons (those that innervate sweat glands and skeletal muscle blood vessels) release ACh. These synapses are called cholinergic.

Adrenergic Synaptic Transmission Norepinephrine is the neurotransmitter released by most sympathetic postganglionic neurons. These synapses are called adrenergic.

Response to Adrenergic Stimulation Can be epinephrine in the blood or norepinephrine from sympathetic nerves Can stimulate or inhibit, depending on receptors Stimulation: heart, dilatory muscles of the iris, smooth muscles of many blood vessels (causes vessel constriction) Inhibition: Bronchioles in lungs, other blood vessels; inhibits contraction and causes dilation of these structures

Neurotransmitters of the Autonomic Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cranial parasympathetic nerves Terminal ganglion ACh Visceral effectors ACh Paravertebral ganglion NE Visceral effectors ACh Adrenal medulla ACh Sympathetic (thoracolumbar) nerves E, NE (hormones) Circulation NE Visceral effectors ACh Sacral parasympathetic nerves Collateral ganglion Visceral effector organs ACh ACh

Varicosities Axons of postganglionic neurons have various swellings called varicosities that release neurotransmitter along the length of the axon. They form “synapses en passant” - in passing. Sympathetic and parasympathetic neurons innervate the same tissues but release different neurotransmitters

Sympathetic and Parasympathetic Release of Different Neurotransmitters Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sympathetic neuron Varicosity Synapses en passant Smooth muscle cell Parasympathetic neuron (a) Axon of Sympathetic Neuron Synaptic vesicle with norepinephrine (NE) NE Adrenergic receptors Antagonistic effects Smooth muscle cell Cholinergic receptors ACh Axon of Parasympathetic Neuron Synaptic vesicle with acetylcholine (ACh) (b)

α and β Adrenergic Receptors Two types of α(alpha) - α1 and α2 Two types of β(beta) - β1 and β2 All act using G-proteins and second messenger systems. β receptors use cAMP. α receptors use a Ca2+ second messenger system. Alpha receptors are more sensitive to norepinephrine Beta receptors are more sensitive to blood epinephrine

α2 Receptors Located on presynaptic axons When stimulated, result in inhibition of norepinephrine release in the synapse May be a negative-feedback system Some drugs to lower blood pressure act on these α2 receptors to inhibit presynaptic neurons in the brain, inhibiting the whole sympathoadrenal system. There are different subtypes that will give different responses

*Adrenergic effects in different organs

Examples of Adrenergic and Cholinergic Agonists and Antagonists

Response to Cholinergic Stimulation ACh released from preganglionic neurons of both the sympathetic and parasympathetic division is stimulatory. ACh from postganglionic neurons of the parasympathetic division is usually stimulatory, but some are inhibitory, depending on receptors. In general, sympathetic and parasympathetic effects are opposite

Cholinergic Receptors Nicotinic: found in autonomic ganglia Stimulated by Ach from preganglionic neurons Serve as ligand-gated ion channels for Na+ & K+ Blocked by curare Muscarinic: found in visceral organs and stimulated by release of Ach from postganglionic neurons Five types identified; can be stimulatory or inhibitory (opening K+ or Ca2+ channels) Use G-proteins and second messenger system Blocked by atropine

Cholinergic Receptors & Responses to ACh

Comparison of Nicotinic & Muscarinic ACh Receptors Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nicotinic ACh receptors Muscarinic ACh receptors Postsynaptic membrane of • All autonomic ganglia • All neuromuscular junctions • Some CNS pathways • Produces parasympathetic nerve effects in the heart, smooth muscles, and glands • G-protein-coupled receptors (receptors influence ion channels by means of G-proteins) Na+ Na+ or Ca2+ ACh ACh ACh Ligand-gated channels (ion channels are part of receptor) β β α α γ γ K+ K+ K+ Depolarization Hyperpolarization Depolarization (K+ channels opened) (K+ channels closed) Excitation Inhibition Excitation Produces slower heart rate Causes smooth muscles of the digestive tract to contract

Receptor Activity in Autonomic Regulation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parasympathetic division Sympathetic division Preganglionic neurons Nicotinic ACh receptors ACh Postganglionic neurons ACh Norepinephrine Stimulates muscarinic ACh receptors Stimulates α1-adrenergic receptors Stimulates β1-adrenergic receptors Stimulates β2-adrenergic receptors Parasympathetic nerve effects Vasoconstriction in viscera and skin Increased heart rate and contractility Dilation of bronchioles (of lung) and blood vessels

Other Autonomic Neurotransmitters Some postganglionic autonomic neurons do not release ACh or norepinephrine. Called “nonadrenergic, noncholinergic fibers” Proposed neurotransmitters include ATP, vasoactive intestinal peptide (VIP), and nitric oxide (NO).

Nonadrenergic, Noncholinergic Fibers Important for erection of the penis. Parasympathetic neurons innervate blood vessels, causing relaxation and vasodilation using NO. NO can also produce smooth muscle relaxation in the stomach, intestines, urinary bladder, and the brain.

Organs with Dual Innervation Most visceral organs are innervated by both sympathetic and parasympathetic neurons. Most of the time these systems are antagonists: Heart rate – sym increases, para decreases Digestive functions – sym decreases, para increases Pupil diameter – sym dilates, para constricts

Complementary Effects Occur when both divisions produce similar effects on the same target Example - Salivary gland secretion: Parasympathetic division stimulates secretion of watery saliva; sympathetic constricts blood vessels so the secretion is thicker.

Cooperative Effects Occur when both divisions produce different effects that work together to promote a single action. Example - Erection and ejaculation: Parasympathetic division causes vasodilation and erection; sympathetic causes ejaculation Example - Urination: Parasympathetic division aids in urinary bladder contraction; sympathetic helps with bladder muscle tone to control urination.

Summary of Autonomic Functions

Organs Without Dual Innervation The following organs are innervated by the sympathetic division only: Adrenal medulla Arrector pili muscles in skin Sweat glands in skin Most blood vessels Regulated by increase and decrease in sympathetic nerve activity Important for body temperature regulation through blood vessels and sweat glands

Control of ANS by Higher Brain Centers Many visceral functions are regulated by autonomic reflexes. Sensory input is sent to brain centers (usually by the vagus nerve), which integrate the information and modify the activity of preganglionic neurons. Medulla oblongata controls many cardiovascular, pulmonary, urinary, reproductive, and digestive functions.

Regulation of the Medulla Higher brain regions regulate the medulla. Hypothalamus: major regulatory center of the ANS – body temperature, hunger, thirst, pituitary gland Limbic system: responsible for autonomic responses during emotional states (blushing, pallor, fainting, cold sweating, racing heart rate) Cerebellum – motion sickness nausea, sweating, cardiovascular changes Frontal & temporal lobes – emotion and personality

Aging Associated with increased levels of sympathetic activity Increased sympathetic tone Increased risk for hypertension and cardiovascular diseases

Autonomic Reflexes