Chapter 15

I. INTRODUCTION

A. The autonomic nervous system (ANS) regulates the activity of smooth muscle, cardiac muscle, and certain glands.

B. Operation of the ANS to maintain homeostasis, however, depends on a continual flow of sensory afferent input, from receptors in organs, and efferent motor output to the same effector organs.

1. Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.

2. Functionally, the ANS usually operates without conscious control.

C. The ANS is regulated by centers in the brain, mainly the hypothalamus and medulla oblongata, which receive input from the limbic system and other regions of the cerebrum.

II. COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

A. The somatic nervous system contains both sensory and motor neurons.

1. The somatic sensory neurons receive input from receptors of the special and somatic senses.

2. These sensations are consciously perceived.

3. Somatic motor neurons innervate skeletal muscle to produce conscious, voluntary movements.

4. The effect of a motor neuron is always excitation.

B. The autonomic nervous system contains both autonomic sensory and motor neurons.

1. Autonomic sensory neurons are associated with interoceptors.

a. Autonomic sensory input is not consciously perceived.

b. The ANS also receives sensory input from somatic senses and special sensory neurons.

2. The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands.

3. Most autonomic responses can not be consciously altered or suppressed.

C. All autonomic motor pathways consists of two motor neurons in series.

1. The axon of the first motor neuron of the ANS extends from the CNS and synapses in a ganglion with the second neuron.

2. The second neuron synapses on an effector. Preganglionic fibers release acetylcholine and postganglionic fibers release acetylcholine or norepinephrine.

D. The output (efferent) part of the ANS is divided into two principal parts: the sympathetic and the parasympathetic divisions. Organs that receive impulses from both sympathetic and parasympathetic fibers are said to have dual innervation.

III. ANATOMY OF AUTONOMIC MOTOR PATHWAYS

A. Anatomical Components

1. The first of two autonomic motor neurons is called a preganglionic neuron.

a. Its cell body is in the brain or spinal cord.

b. Its myelinated axon, called a preganglionic fiber, passes out of the CNS as part of a cranial or spinal nerve, later separating from the nerve and extending to an autonomic ganglion where it synapses with the postganglionic neuron.

2. The postganglionic neuron, the second neuron in the autonomic motor pathway, lies entirely outside the CNS.

a. Its cell body and dendrites are located in an autonomic ganglion, where it makes synapses with one or more preganglionic fibers.

b. The axon of a postganglionic neuron, the postganglionic fiber, is unmyelinated and terminates in a visceral effector.

B. Preganglionic Neurons

1. The cell bodies of sympathetic preganglionic neurons are in the lateral gray horns of the 12 thoracic and first 2 or 3 lumbar segments.

a. This division is called the thoracolumbar division.

b. The axons of the sympathetic preganglionic neurons are known as the thoracolumbar outflow.

2. The cell bodies of parasympathetic preganglionic neurons are in cranial nerve nuclei (III, VII, IX, and X) in the brain stem and lateral gray horns of the second through fourth sacral segments of the cord.

a. The parasympathetic division is known as the craniosacral division.

b. The axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow.

C. Autonomic Ganglia

1. Sympathetic Ganglia

a. These ganglia include the sympathetic trunk (vertebral chain or paravertebral ganglia) that lie in a vertical row on either side of the vertebral column.

b. Other sympathetic ganglia are the prevertebral (collateral ganglia) that lie anterior to the spinal column and close to large abdominal arteries. They include the celiac, superior mesenteric, and inferior mesenteric ganglia.

2. Parasympathetic Ganglia

a. Parasympathetic ganglia are the terminal (intramural)l ganglia that are located very close to or actually within the wall of a visceral organ.

b. Examples include the ciliary, pterygopalatine, submandibular, and otic ganglia.

D. Autonomic Plexuses

1. These are tangled networks of sympathetic and parasympathetic neurons.

2. Major autonomic plexuses include the cardiac, pulmonary, celiac, and hypogastric.

E. Postganglionic Neurons

1. Sympathetic preganglionic neurons synapse with postganglionic neurons in ganglia of the sympathetic trunk or prevertebral ganglia.

2. Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia.

F. Structure of the Sympathetic Division

1. White rami communicans (plural - the white rami communicantes) connect the anterior ramus of the thoracic and first 2 or 3 lumbar spinal nerves with the ganglia of the sympathetic trunk.

2. The gray ramus communicans is the structure containing the postganglionic fibers that connect the ganglion of the sympathetic trunk to the spinal nerve.

3. Preganglionic fibers that pass through the sympathetic trunk without terminating in the trunk form nerves beyond the trunk known as splanchnic nerves.

4. Developmentally, the adrenal medulla is a modified sympathetic ganglion and its cells are similar to sympathetic postganglionic neurons.

G. Structure of the Parasympathetic Division

1. The cranial outflow consists of preganglionic axons that extend from the brain stem in four cranial nerves. The cranial outflow components are four pairs of ganglia and the plexuses associated with the vagus nerve.

2. The sacral outflow consists of preganglionic axons in anterior roots of the second through fourth sacral nerves.

IV. ANS NEUROTRANSMITTERS AND RECEPTORS

A. Cholinergic Neurons and Cholinergic Receptors

1. Cholinergic neurons release the neurotransmitter acetlycholine and include all sympathetic and parasympathetic preganglionic neurons, all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervate most sweat glands.

2. Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane. The two types of cholinergic receptors are nicotinic and muscarinic receptors.

a. Activation of nicotinic receptors causes excitation of the postsynaptic cell. These receptors are found in the dendrites and cell bodies of both sympathetic and parasympathetic post ganglionic neurons and in the motor end plate at the neuromuscular junction.

b. Activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors. These are present in all effectors (smooth muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic neurons as well as effectors (most sweat glands) innervated by cholinergic sympathetic postganglionic neurons.

B. Adrenergic Neurons and Adrenergic Receptors

1. The adrenergic neurons release norepinephrine and include most sympathetic postganglionic neurons.

2. The main types of adrenergic receptors are alpha and beta receptors.

a. These receptors are further classified into subtypes.

b. Depending on the subtype, activation of the receptor can result in either excitation or inhibition.

3. Effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons.

C. Receptor Agonists and Antagonists

1. An agonist is a substance that binds to and activates a receptor, mimicking the effect of a natural neurotransmitter or hormone; an antagonist is a substance that binds to and blocks a receptor, preventing a natural neurotransmitter or hormone from exerting its effect.

2. Different drugs can serve as agonists or antagonists to selectively activate or block ANS receptors.

V. PHYSIOLOGICAL EFFECTS OF THE ANS

A. Most body structures receive dual innervation, that is, fibers from both the sympathetic and parasympathetic divisions. Usually one division causes excitation and one causes inhibition.

B. The sympathetic responses prepare the body for emergency situations (the fight-or-flight responses).

1. The effects of sympathetic stimulation are longer-lasting and more widespread than those of the parasympathetic stimulation.

C. The parasympathetic division regulates activities that conserve and restore body energy (energy conservation-restorative system).

1. Parasympathetic responses stimulate salivation, lacrimation, urination, digestion, and defecation (“SLUDD”).

2. Parasympathetic responses also decrease heart rate, airway diameter, and pupil diameter.

VI. INTEGRATION AND CONTROL OF AUTONOMIC FUNCTIONS

A. Autonomic Reflexes

1. A visceral autonomic reflex adjusts the activity of a visceral effector, often unconsciously.

2. A visceral autonomic reflex arc consists of a receptor, sensory neuron, association neuron, autonomic motor neurons, and visceral effector.

B. Autonomic Control by Higher Centers

1. The hypothalamus controls and integrates the autonomic nervous system. It is connected to both the sympathetic and the parasympathetic divisions.

2. Control of the ANS by the cerebral cortex occurs primarily during emotional stress.