Chapter 9 Human Physiology

The Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS) is a part of the nervous system that regulates key autonomic functions of the body, including the activities of the heart and blood vessels; the smooth muscles, including the muscles of the intestinal tract; and the glands of the endocrine system.

Divisions of the Autonomic Nervous System (ANS)

• Functions to regulate the body’s unconscious actions. It is composed of two divisions: the sympathetic division and the parasympathetic division which often act in a seemingly opposing manner.

• Sympathetic Division - responds to emergency stimuli

• Parasympathetic Division - Returns the body to baseline conditions.

Nervous System Organization

• Central Nervous system - Brain and spinal Cord

• Peripheral Nervous System 

1. Spinal nerves going to or coming out of the spinal cord.

2. Cranial nerves going into or out of the brain

Peripheral nervous System

• The PNS consists of sensory input information going to the central nervous system and motor output information going to the body muscles and glands.

• Motor output is further divided into the somatic division which serves skeletal muscles and the autonomic division which serves autonomous functions of the body such as heart rate and smooth muscle contraction

The Autonomic Nervous System

• Autonomic motor nerves innervate organs whose functions are not usually under voluntary control. The effectors that respond to autonomic regulation include cardiac muscle, smooth muscles and glands. These include many internal organs as well as the blood vessels that supply and drain them.

Autonomic Pathways

• Autonomic impulses are conducted from the CNS by an autonomic neuron that synapses with a second autonomic neuron. It is the axon terminals from the second neuron in this pathway that actually innervates the involuntary effector such as smooth muscles (intestine) in this case

• Unlike somatic neurons, which conduct impulses along a single axon from the spinal cord to the muscle fiber, autonomic motor control involves two neurons in a series. As shown in the bottom figure the first of these neurons has its cell body in the gray matter of the brain or spinal cord. However, the axon of this neuron does not directly innervate the effector organ but instead synapse with a second neuron within an autonomic ganglion

Autonomic Ganglia

• An autonomic ganglion is a collection of cell bodies outside the CNS. The first neuron is thus called a preganglionic neuron and the second neuron in the pathway is called a postganglionic neuron. The postganglionic neuron has an axon that extends and innervates the effector organ such as smooth muscle, cardiac muscle and glands.

Anatomical Organization of the Two Autonomic Nervous System DIvisions

• The Sympathetic division is associated with fight or flight responses. The parasympathetic division predominates in the body’s relaxed state. The nerves of the sympathetic division arise in the thoracic and lumbar segments of the spinal cord. The nerves of the parasympathetic division arise from the cranial and sacral segments.

The Sympathetic Chain Ganglia

• Preganglionic neurons of the the sympathetic division originate in the thoracolumbar regions of the spinal cord and send axons to sympathetic ganglia which parallel the vertebrae of the spinal column.

• The sympathetic ganglia (right) parallels both sides of the spinal column and anatomically is referred to as the sympathetic chain of ganglia.

Pathway of Sympathetic Neurons

• Other preganglionic neurons pass through the ganglia without synapsing . these ultimately synapse in a collateral ganglion such as the celiac ganglion which innervates the intestine

Examples of Collateral Ganglia

• Collateral Ganglia of the sympathetic division of the ANS include the celiac ganglion and the superior and inferior mesenteric ganglia. 

Functions of the Sympathetic Division

• The sympathetic division of the ANS activates the body to either deal with a threat “fight” or escape from it “flight” largely through the release of norepinephrine from postganglionic neurons and the secretion of epinephrine (adrenalin) from the adrenal glands.

• Stimulation of the sympathetic nerves can result in increasing the heart rate, constriction of visceral blood vessels and decreased activity of the digestive tract.

Functions of the parasympathetic Division

• Unlike the fight-or-flight model of the sympathetic division, there is no catch phrase to describe the action of the parasympathetic division. However, since many actions are opposed in nature to the sympathetic division some have described the theme of the parasympathetic division as “rest and digest” or “rest and repose”.

Parasympathetic Regulation

• Stimulation of the parasympathetic nerves can result in slowing the heart dilation of visceral blood vessels and increase activity of the digestive tract. Visceral organs respond differently to sympathetic and parasympathetic nerve activity because the postganglionic axons of these two divisions release different neurotransmitters.

Neurotransmitters in the Autonomic Nervous System

• Acetylcholine (ACh) is the neurotransmitter of all preganglionic axons sympathetic and parasympathetic. ACh is also the neurotransmitter released by most parasympathetic postganglionic axons upon their effectors. Sympathetic postganglionic axons release NE upon their effectors.

ANS Control of Heart Rate

• Most visceral organs such as the heart receive dual innervation meaning they are innervated by both sympathetic and parasympathetic fibers. Most of the time the effects of the two divisions are antagonistic or in 

• The effect of dual innervation of the pacemaker region of the heart is the best example of the opposing effects of these two systems. In this case, sympathetic and parasympathetic fibers innervate the same cells in the sinoatrial node or, peacemaker, located in the right atrium

Example of Antagonistic Neurotransmitters

• Visceral organs respond differently to the sympathetic and parasympathetic nerve activity because the postganglionic axons of these two divisions release different neurotransmitters. Pacemaker cells which are modified cardiac muscle fibers possess receptors for both NE and ACh.

Comparing Effects of Neurotransmitters on Their Receptors

• Nicotinic acetylcholine receptors are ligand-gated, meaning that the ion channel running through the receptor is opened by binding of ACh. These always act to depolarize the target cell. Muscarinic ACh receptors are G-protein-coupled receptors which can open up different ion channels to either hyperpolarize the target cell or depolarize (excite) it.

The Effect of Norepinephrine (NE) on the Heart

• In the case of the heart norepinephrine  secreted by sympathetic fibers and acting upon adrenergic receptors speeds up the rate and strength of contraction by  exciting pacemaker cells and causes them to increase their firing rate.

1. Ne binds to its receptor. This receptor is a typical G-protein coupled receptor.

2. The receptor exchanges GDP for GTP and releases the alpha subunit.

3. 3.Alpha subunit activates adenylate cyclase enzyme (2ndSecond Messenger).

4. 4.Intracellular cAMP increases and activates a protein kinase.

5. 5.The activated kinase phosphorylates a Ca2+ channel which opens it. 

6. 6.Intracellular Ca2+ increases 

The effect of Norepinephrine of the Heart

• Remember in physiology all concepts are related. In the heart the effect of NE upon the pacemaker cells is immediately translated to all the cells of the atria and ventricles. In other words a wave of depolarization beginning at the SA node immediately spread throughout the heart.

The Effect of Acetylcholine (ACh) on the Heart

• In the case of the heart, acetylcholine secreted by parasympathetic fibers and acting on muscarinic ACh receptors reduces heart rate and contraction strength by hyperpolarizing pacemaker cells, causing them to decrease their firing rate

• When ACh binds to muscarinic receptors an activated G-protein coupled receptor indirectly opens up K+ channels and the cells hyperpolarizes in other words is inhibited. Heart rate decrease back to normal