Chapter 11 Autonomic Nervous System (ANS) Copyright 2010, John Wiley & Sons, Inc.

Introduction to the ANS Somatic nervous system (SNS) + ANS  peripheral nervous system (PNS) ANS Not under conscious control Is regulated by hypothalamus, brainstem The ANS supplies nerves to viscera Smooth muscle (stomach, blood vessels) Cardiac muscle (heart) Glands (sweat and digestive glands) Copyright 2010, John Wiley & Sons, Inc.

Comparison: SNS vs ANS SNS ANS Controls skeletal muscle Conscious, voluntary control Motor pathway: one neuron from CNS to effector Does include sensory neurons (from skin, skeletal muscles, and special sense organs) All release the neurotransmitter ACh ANS Controls viscera: smooth and cardiac muscle, and glands Unconscious, involuntary Motor pathway: series of two neurons from CNS to effector Does include sensory neurons (monitors viscera) Two divisions: sympathetic, parasympathetic Release either ACh or NE Copyright 2010, John Wiley & Sons, Inc.

Somatic Nervous System Copyright 2010, John Wiley & Sons, Inc.

ANS Motor Pathways Autonomic motor pathway includes two motor neurons Preganglionic neuron from CNS to neuron in autonomic ganglion Postganglionic neuron from cell body in ganglion to effector Copyright 2010, John Wiley & Sons, Inc.

ANS Motor Pathways Copyright 2010, John Wiley & Sons, Inc.

Divisions of the ANS Sympathetic (S) division + parasympathetic (P) division Most viscera supplied with nerves of both S and P divisions: dual innervation S and P have opposite (antagonistic) effects Heart rate: S stimulates, P inhibits Digestive organs: S inhibit, P stimulate S: “flight or flight,” P: “rest and digest” Some viscera receive only S (not P) nerves: Sweat glands, many blood vessels, hair muscles Copyright 2010, John Wiley & Sons, Inc.

Sympathetic (S) Division Sympathetic preganglionic neurons Have cell bodies located in lateral gray of spinal cord segments T1-T12 + L1-L2 So S division is called “thoracolumbar” Axons pass through ventral roots of spinal nerves May branch many times May ascend or descend to many levels of S trunk ganglia (from cervical to sacral) Can synapse with 20 or more postganglionic neuron cell bodies Results: widespread S effects (viscera respond “in sympathy with one another”) Copyright 2010, John Wiley & Sons, Inc.

Sympathetic (S) Division Sympathetic postganglionic neurons S postganglionic neurons cell bodies located In S “trunk ganglia” (2 long chains lateral to vertebrae) From cervical to sacral regions  widespread S effects Many axons from these cell bodies pass back into spinal nerves to reach viscera in skin (sweat glands, hair muscles, blood vessels) In S “prevertebral ganglia” anterior to 3 large abdominal arteries Named celiac, superior and inferior mesenteric ganglia Supply abdominal viscera: stomach, intestine, kidneys, liver, spleen Axons pass from ganglia to viscera in S nerves Copyright 2010, John Wiley & Sons, Inc.

Sympathetic (S) Division Copyright 2010, John Wiley & Sons, Inc.

Parasympathetic (P) Division P preganglionic neurons Cell bodies located in brainstem + in spinal cord segments S2-S4 Therefore P division is called “craniosacral” Axons in cranial nerves III, VII, IX and X and in pelvic nerves from S2-S4 Vagus nerves (cranial nerves X) carry 80% of all P nerve impulses. Vagus nerves carry both motor and sensory neurons to/from viscera within the thorax and most of the abdominal cavity. P preganglionic axons do not branch or pass though S trunk ganglia but pass directly almost to viscera Copyright 2010, John Wiley & Sons, Inc.

Parasympathetic (P) Division P postganglionic neurons Cell bodies lie in terminal ganglia Located within or near the innervated organ So P nerves cause precise, localized (not widespread) effects Because of anatomical arrangement, S nerves supply all viscera but P nerves do not reach some viscera. These include sweat glands, arrector pili muscles of hairs in skin, kidneys, spleen, adrenal medullae, and the walls of most blood vessels. Axons pass from ganglia to viscera in P nerves Copyright 2010, John Wiley & Sons, Inc.

Parasympathetic (P) Division Copyright 2010, John Wiley & Sons, Inc.

ANS Neurotransmitters: Comparison Acetylcholine (ACh) ACh more common; released by: All S and P preganglionic axons All P postganglionic axons Some S postganglionic axons (to sweat glands) ACh destroyed by enzyme ACh-ase so short-lived response Norepinephrine (NE) NE less common; released by: Almost all S postganglionic axons NE has longer lasting effects enhanced by epinephrine + NE from adrenal medullae Copyright 2010, John Wiley & Sons, Inc.

Sympathetic Effects Fight-or-flight activities Increase heart rate and contraction, and blood pressure (BP) Dilate pupils Dilate airways Dilate vessels to skeletal muscles, heart, liver and adipose tissue Constrict blood vessels to nonessential organs: skin, GI tract, kidneys Mobilize nutrients for energy: glucose and fats Copyright 2010, John Wiley & Sons, Inc.

Parasympathetic Effects Rest-and-digest activities SLUDD Salivation Lacrimation Urination Digestion Defecation Decrease heart rate, airway diameter, pupil diameter Copyright 2010, John Wiley & Sons, Inc.

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