Physiology of the Autonomic Nervous System
Regulation of Autonomic Nervous System Activity
The ANS functions to maintain a relatively stable internal body environment, that is, to maintain a state of homeokinesis (homeostasis).
it does so by regulating the activity of cardiac muscle, smooth muscle, and glands. The distribution of ANS nerves is widespread, including all viscera. Normally, the regulation of ANS activity occurs below the level of consciousness. However, emotional reactions (such as fear or excitement) and input from the cerebral cortex also affect ANS activity.Most organs that are innervated by the ANS have both sympathetic and parasympathetic innervations, and in most cases the effects of the two divisions are antagonistic. For example, parasympathetic stimulation of the heart reduces heart rate, while sympathetic stimulation increases heart rate. The antagonistic actions are controlled to bring about appropriate overall regulation of the innervated organ. in the case of the heart, relatively high parasympathetic nerve activity to the heart in an animal at rest maintains a low resting heart rate. During rest there is little sympathetic nerve activity to the heart. To increase heart rate, such as during exercise, parasympathetic nerve activity is first reduced to permit an increase in heart rate, and sympathetic nerves to the heart are activated if a further increase in heart rate is necessary for more intense exercise.
Changes in ANS nerve activity can occur as a result of discrete reflexes. For example, the diameter of the pupil of the eye is controlled by an ANS reflex initiated by changes in the amount of light detected by the retina. The circular smooth muscle fibers of the iris constrict the pupil and are under the control of parasympathetic nerves. Changes in the amount of light reaching the retina initiate a reflex response to achieve a precise and proper pupil diameter.
Arterial blood pressure, salivary secretion, secretion of hydrochloric acid by the stomach, urination, and defecation are among functions regulated in part by ANS reflexes. The reflex centers for various ANS reflex arcs are found throughout the CNS. For example, the brainstem has several centers regulating blood pressure, and the reflex center for urination is in the sacral region of the spinal cord. ANS reflexes may use sympathetic nerves, parasympathetic nerves, or both. For example, the ANS reflex regulation of blood pressure uses sympathetic nerves to blood vessels and both parasympathetic and sympathetic nerves to the heart.
The sympathetic division of the ANS is primarily responsible for the fight-or-flight response that is associated with fear, anxiety, rage, and other strong emotions. The outcomes of this response can be predicted by imagining what changes would favor skeletal muscle activity for either fighting or running away.
Outcomes include increases in heart rate, blood pressure, blood glucose, and blood flow to skeletal muscle; dilation of airways in the lungs (e.g., bronchi); dilation of the pupils; and decreased activity of the digestive tract.
The fight-or-flight response is a short-term event characterized by high levels of sympathetic nerve activity throughout the body. This widespread sympathetic activation is not the result of a discrete reflex but is a more general sympathetic activation initiated in response to fear, anxiety, stress, and so on. The hypothalamus and amygdala in the brain appear to be especially important sites in the initiation of the sympathetic response, but it is not clear how feelings of fear, stress, and so on affect these centers to initiate the response.
in addition to widespread increases in sympathetic nerve activity, the fight-or-flight response includes an increase in the release of epinephrine and norepinephrine from the adrenal medullae. Chromaffin cells of the adrenal medullae are innervated by preganglionic sympathetic neurons (see Chapter 9), and these cells release their catecholamines when stimulated.
in most species the primary catecholamine released by chromaffin cells is epinephrine. Epinephrine and norepinephrine in the circulation bind to adrenergic receptors throughout the body to amplify the general effects of increased sympathetic nerve activity. The blood levels of these catecholamines are relatively low and functionally insignificant when animals are not undergoing a strong sympathetic response.The list of events in the fight-or-flight response includes dilation of the pupil of the eye. The control of pupil size was earlier described as also being controlled by a parasympathetic reflex based on light reaching the retina. This is an example of how circumstances may affect which division of the ANs dominates the regulation of an organ or gland. if an animal is not undergoing a strong sympathetic response, the parasympathetic nerves are the primary regulator of pupil size, but during times of intense stress, sympathetic stimulation may dilate the pupil in spite of the animal being in a brightly lit environment.
Autonomic Neurotransmitters and Their Receptors
The postganglionic neurons of the parasympathetic division of the ANs use acetylcholine as a neurotransmitter, whereas almost all postganglionic neurons of the sympathetic division use norepinephrine. Acetylcholine is also the neurotransmitter used by preganglionic neurons in both the sympathetic and parasympathetic divisions.
The organ response to ANs stimulation depends not only on the neurotransmitter being released but also on the type of cell membrane receptor on the cells of the organ. Muscarinic and nicotinic are the two general classes of acetylcholine receptors. Nicotinic receptors are found on skeletal muscle cells at the neuromuscular junction and in all autonomic ganglia (both sympathetic and parasympathetic), where acetylcholine is released by preganglionic neurons. Muscarinic receptors are found in most organs innervated by postganglionic parasympathetic neurons.
stimulation of muscarinic receptors brings about diverse cellular responses ranging from hyperpolarization of sinoatrial nodal cells to slow heart rate to contraction of urinary bladder smooth muscle for urination. Table 10-1 lists the organs where muscarinic receptors are found and the organ response to the stimulation of those receptors by parasympathetic nerves. More details on the precise mechanisms of these effects appear in the appropriate chapters on the different body systems. Parasympathetic stimulation increases salivary gland secretion, stimulates gastrointestinal motility, slows heart rate, and tends to reduce cardiac output. These are often undesirable during surgery. Muscarinic receptor antagonists (such as atropine) are often used as preanesthetic agents to block peripheral muscarinic receptors and reduce these potentially harmful effects of parasympathetic stimulation.Adrenergic receptors, which may be stimulated by either epinephrine or norepinephrine, also fall into two general classes, α-receptors and β-receptors. However, because of their physiologic and clinical importance, the sub-
Table 10-1. Location of Muscarinic Receptors and the Effects of Stimulation by Neurotransmitters of the Autonomic Nerves
| Location | Effect |
| Heart Sinoatrial node | Reduce heart rate |
| Atrioventricular node Salivary glands Gastrointestinal tract | Reduce impulse conduction velocity Increase secretion Increase motility of smooth muscle in wall and secretion of lining epithelium |
| Urinary bladder | Contract smooth muscle to empty bladder |
Circular muscle of iris of eye Constrict smooth muscle to reduce pupil
Ciliary muscle controlling lens of eye Contract muscle for lens accommodation
Endothelial cells lining blood vessels Stimulate release of nitric oxide to relax smooth muscle
Smooth muscle of lung airways (bronchiolar) Contract smooth muscle to shrink airways
Table 10-2.
Location of Adrenergic Receptors and the Effects of Stimulation by Neurotransmitters of the Autonomic Nerves| Receptor subtype | Locations Effect |
| α1 | Vascular smooth muscle Contracts muscle to constrict vessel Smooth muscle sphincters in Contracts muscle to constrict sphincters gastrointestinal tract Radial muscle of iris of eye Contracts muscle to enlarge pupil Smooth muscle sphincter of Contracts muscle to reduce opening into urinary bladder urethra |
| β1 | Heart: sinoatrial node Increase heart rate Heart: atrioventricular node Increase impulse conduction velocity Heart: ventricular muscle Increase force of contraction |
| β2 | Arterial vessels supplying blood to Relaxes smooth muscle to permit dilation skeletal muscle of vessels Smooth muscle of lung airways Relaxes muscle to permit airways to open (bronchiolar) Smooth muscle in wall of gastro- Relaxes muscle to reduce motility intestinal tract Liver Increases glycogenolysis, gluconeogenesis in some species |
types of α- and β-receptors also must be con- muscle relaxation. Epinephrine has a high
sidered. Table 10-2 lists the key subtypes affinity for β2-receptors, and it is released into
of adrenergic receptors, their sites, and the the circulation during a strong sympathetic
effects of their stimulation. Stimulation of α1- response. The β2 effects on smooth muscle of
receptors causes contraction of smooth muscle, airways and blood vessels supplying skeletal
and stimulation of β2-receptors causes smooth muscles are appropriate for fight-or-flight.