Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb 14 The Autonomic Nervous System

Autonomic Nervous System (ANS)  Motor neurons that:  Innervate smooth & cardiac muscle & glands  Subconscious control

ANS differs from the SNS  Effectors  SNS – skeletal muscle  ANS – non-skeletal muscle & gland cells  Efferent pathways  SNS – single PNS neuron  ANS – 2 PNS neurons  Target organ responses  SNS – contraction of muscle  ANS – contraction or relaxation, excretion  Neurotransmitters used  SNS – acetylcholine  ANS – acetylcholine, norepinephrine & epinephrine

Distinctions of Efferent Pathways  SNS motor neurons  Single neuron extends from CNS to effector  Heavily myelinated axons  ANS motor neurons  Two-neuron PNS chain  Preganglionic neuron & postganglionic neuron  Lightly myelinated preganglionic axon from CNS to ganglion  Unmyelinated postganglionic axon extends to effector

Neurotransmitter Differences  SNS neurons release acetylcholine (ACh), which has an excitatory effect  In the ANS:  Preganglionic fibers release ACh  Postganglionic fibers  release norepinephrine or ACh  effect is stimulatory or inhibitory  effect depends on neurotransmitter receptor in cells of effector tissue

Comparison of Somatic & Autonomic Systems Figure 14.2

Anatomy of ANS Figure 14.3 Short preganglionic Long postganglionic Ganglia close to spinal cord Long preganglionic Short postganglionic Ganglia on/in target organ

Divisions of the ANS  Sympathetic (SANS)  mobilizes the body during stressfull situations  Parasympathetic (PANS)  stimulates maintenance activities & conserves body energy  The two divisions counterbalance each other’s activity  SANS signals usually override PANS

Examples of ANS Effects  PANS  Lowers BP, heart & respiratory rates  Increases gastrointestinal tract activity  Superficial arterioles open (smooth muscle relaxed)  Pupils are constricted  SANS  Blood flow to organs/skin reduced, flow to muscles increased  Heart & respiratory rates increased  Iris contracts - Pupils dilate

Parasympathetic Division Outflow Cranial OutflowCranial NerveGanglionEffector Organ(s) Occulomotor (III)CiliaryEye Facial (VII)Pterygopalatin Submandibular Salivary, nasal, & lacrimal glands Glossopharyngeal (IX) OticParotid salivary glands Vagus (X)Located within the walls of target organs Heart, lungs, & most visceral organs Sacral OutflowS 2 -S 4 Located within the walls of the target organs Large intestine, urinary bladder, ureters, & reproductive organs

Parasympathetic Division Outflow  Longer preganglionic axons  Ganglion near/on target organ  Short postganglionic axons  Vagus nerve (CN X) innervates all visceral organs

Sympathetic Outflow  Sympathetic neurons in lateral horns of spinal cord segments T 1 through L 2  T 1 -T 4 preganglionic fibers pass through the white rami communicantes & synapse in sympathetic chain ganglia  T 5 -L 2 preganglionic fibers pass through the gray rami communicantes & chain ganglia to form splanchnic nerves & synapse in collateral ganglia around abdominal aorta  Postganglionic fibers innervate the numerous organs of the body

Sympathetic Outflow  Sympathetic neurons in lateral horns of spinal cord segments T 1 - L 2  T 1 -T 4 preganglionic fibers synapse in sympathetic chain ganglia  T 5 -L 2 preganglionic fibers form splanchnic nerves & synapse in collateral ganglia on abdominal aorta

Sympathetic Trunks & Pathways

Pathways to the Head  T 1 -T 4 preganglionic axons synapse in the superior cervical ganglion  Serve skin & blood vessels of the head  Stimulate dilator muscles of the iris  Inhibit nasal & salivary gland secretions

Pathways to the Thorax  T 1 -T 6 preganglionic axons synapse in cervical chain ganglia  Postganglionic axons from middle & inferior cervical ganglia enter spinal nerves C 4 -C 8 to innervate the heart, thyroid & skin of neck  Other T 1 -T 6 preganglionic axons synapse in nearest chain ganglia to directly serve the heart, aorta, lungs, & esophagus

Pathways with Synapses in Collateral Ganglia  T 5 -L 2 preganglionic axons exit sympathetic chain ganglia & form splanchnic nerves  Splanchnic nerves form aortic plexus & numerous ganglia  Postganglionic axons from abdominal ganglia innervate viscera

Pathways with Synapses in the Adrenal Medulla  Axons of the thoracic splanchnic nerve go directly to the adrenal medulla  Upon stimulation, medullary cells secrete norepinephrine & epinephrine into the blood greater thoracic splanchnic nerve

Visceral Reflexes  Visceral reflexes have the same elements as somatic reflexes  Afferent fibers are found in spinal & autonomic nerves

ANS Neurotransmitters  PANS  Acetylcholine (ACh) released by pre- & postganglionic axons  SANS  ACh released by preganglionic axons  ACh or norepinephrine (NE) released by postganglionic axons  Cholinergic fibers – ACh-releasing axons  Adrenergic fibers –NE-releasing postganglionic SANS axons  Excitatory or inhibitory effects depend upon the receptor type

Cholinergic Receptors  Bind ACh  Nicotinic receptors  Muscarinic receptors  Named & distinguished by interaction w/ agonists  Nicotine  Muscarine  Agonist – stimulates effect  Antagonist – blocks effect

Cholinergic Receptors  Nicotinic Receptors  Locations:  Skeletal muscle motor end plates, CNS neurons  SANS & PANS ganglionic neurons  Adrenal medulla cells  Ion channels  ACh always stimulatory  Muscarinic Receptors  Locations  Cells stimulated by postganglionic PANS fibers, CNS  ACh inhibition or excitation depends on receptor subtype  subtypes – M1, M2, M3

Adrenergic Receptors  Receptors that bind to norepinephrin & epinephrine  In cells innervated by SANS postganglionic axons  Alpha  subclasses -  1,  2,  NE is stimulatory  Beta  Subclasses -  1,  2,  3  NE is generally inhibitory  Exception – NE binding to  receptors of the heart is stimulatory

Drugs that Influence the ANS Table

Levels of ANS Control Figure 14.9

Interactions of the Autonomic Divisions  Most visceral organs innervated by both sympathetic & parasympathetic fibers  results in dynamic antagonisms that precisely control visceral activity  Sympathetic fibers increase heart & respiratory rates, & inhibit digestion & elimination  Parasympathetic fibers decrease heart & respiratory rates, & allow for digestion & the discarding of wastes