AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) is a subdivision of the efferent (motor) division of the peripheral nervous system (PNS), and includes the sympathetic (sim-pa-THET-ik; sympatho, feel suffering with) and parasympathetic (par-a-sim-pa-THET-ik; para, alongside) divisions. The effectors stimulated by the ANS are glands, smooth muscle, and cardiac muscle. Motor impulses from the ANS travel through ganglia in the PNS on their way to these effectors (Figure 19-1).
In all cases except one, the ANS impulse passes through two motor neurons before reaching the effector. The preganglionic neuron (prē-gang-lē-ON-ik; pre, before) is the motor neuron that conducts the impulse out of the CNS (Figure 19-2). It has a class B axon (thin diameter, but myelinated). The preganglionic neuron synapses with a postganglionic neuron (post-gang-lē-ON-ik; post, after) inside a ganglion in the PNS. The postganglionic neuron conducts the impulse along a class C axon (thin diameter and unmyelinated) to the effector. The one exception to the “two motor neuron” rule is the stimulation of the adrenal gland. In this case, the adrenal gland receives stimulation from only a “preganglionic” neuron in the sympathetic division.
In some cases, an effector receives impulses from only one division of the ANS, but other effectors receive dual innervation; that is, they receive impulses from the sympathetic and parasympathetic divisions. When an effector is dually innervated, if the effect of one division is excitation, the effect of the other division is inhibition. Under normal conditions, both divisions of the ANS are active but at different levels. During fight-or-flight situations, the sympathetic division is highly active while the parasympathetic division is less active. In contrast, when the parasympathetic division is most active, the sympathetic division is less active.
Figure 19-1. Divisions of the autonomic nervous system
Figure 19-2. Pathways for sympathetic impulses
SYMPATHETIC DIVISION
The sympathetic division has two other names based on its major associated functions and the location where its preganglionic neurons exit the dorsal cavity. Functionally, the sympathetic system is the nervous system’s fight-or-flight division because its activity increases during times of physical and emotional stress, including exercise, anxiety, trauma, etc. It is also the thoracolumbar division (thorak-ō-LUM-bar) because its impulses exit the dorsal cavity only in the thoracic and lumbar regions (Figure 19 3).
Figure 19-3. Comparison of somatic and autonomic innervation
The cell bodies of sympathetic, preganglionic neurons reside in the lateral horns of the spinal cord. The axons of these neurons exit the cord through thoracic or lumbar spinal nerves and pass through paravertebral ganglia (para-VER-te- bral), so-named because they are located alongside the vertebrae. There are 23 paravertebral ganglia aligned like a “chain of beads” on each side of the vertebral column. A preganglionic axon may synapse with a postganglionic neuron in a paravertebral ganglion, or it may pass through the paravertebral ganglia and synapse in a prevertebral ganglion (prē-VER-te-bral), so- named because it is anterior to (pre-, before) the vertebrae. Whether or not a sympathetic impulse passes through more than one paravertebral or prevertebral ganglion, it will pass along no more than two axons before reaching its effector.
The axons of preganglionic neurons in the sympathetic system are relatively short, while the postganglionic fibers are long. All preganglionic neurons in the sympathetic division release the neurotransmitter acetylcholine (Ach; as- e-til-KŌ-lēn) from their axon terminals. Most postganglionic neurons in this division release the neurotransmitter norepinephrine (NE; nōr- ep-i-NEF-rin), which is a catecholamine chemical described in chapter 3. On the other hand, a few postganglionic neurons in the sympathetic division release Ach onto their effectors. Effectors stimulated by Ach from sympathetic neurons
include the sweat glands, some blood vessels coursing through skeletal muscles, and the adrenal medulla (the inner core part of the adrenal gland). However, the adrenal gland receives Ach stimulation directly from a preganglionic neuron. In response, the adrenal gland secretes the hormones epinephrine and norepinephrine; thus, the adrenal gland functions much like a typical postganglionic neuron in the sympathetic division. Epinephrine and norepinephrine usually promote similar effects on the same target organs.
It is often important for autonomic signals to reach a great number of effectors in a short amount of time. To facilitate this, one preganglionic neuron may synapse with more than one postganglionic neuron within a ganglion; this pattern of impulse flow is called divergence (dī-VER-jens) (Figure 19-4). While divergence is a part of both divisions of the ANS, it is more extensive and common in the sympathetic division. Divergence allows sympathetic impulses to “spread out” to numerous effectors more rapidly, which is an obvious advantage when one considers certain fight-or- flight responses.
PARASYMPATHETIC DIVISION
Like the sympathetic division, the parasympathetic division has two other names based on its major associated functions and the location where its preganglionic neurons exit the dorsal cavity. Functionally, the parasympathetic system is the nervous system’s rest-and-digest division because its activity increases during times of rest, relaxation, and digestion. It is also the craniosacral division (krā-nē-ō-SĀ-kral) because its impulses exit the dorsal cavity only in the cranium (which houses the brain) and the sacral regions (Figure 19-5).
The cell bodies of parasympathetic preganglionic neurons reside in gray matter of the brain and lateral horns of the spinal cord. Preganglionic fibers exit the dorsal cavity and pass through ganglia located close to the target organ (effector). Compared to the sympathetic division, the axons of the preganglionic neurons in the parasympathetic division are relatively long, while the postganglionic axons are short. All neurons (preganglionic and postganglionic) in the parasympathetic division release acetylcholine from their axon terminals.
Figure 19-4. Divergence among autonomic neurons
RECEPTORS FOR ANS NEUROTRANSMITTERS
To be stimulated by the ANS, an effector must display receptors to which the ANS neurotrans- mitter can attach. Two major groups of receptors respond to ANS neurotransmitters: cholinergic and adrenergic receptors.
Cholinergic Receptors
Membrane receptors that bind Ach are called cholinergic receptors (kō-lin-ER-jik). There are two major classes of cholinergic receptors: nicotinic and muscarinic.
- Nicotinic receptors (nik-o-TIN-ik) bind Ach and nicotine (a chemical found in cigarette smoke). Nicotinic receptors are displayed on all postganglionic neurons in the sympathetic and parasympathetic divisions, (2) cells in the adrenal medulla (bind Ach released from sympathetic “preganglionic” neurons), and (3) skeletal muscle cells, which bind Ach released from somatic motor neurons, not from autonomic neurons.
- Muscarinic receptors (mus-ka-RIN-ik) bind Ach and muscarine (MUS-ka-rēn), a chemical produced in a certain species of mush-Muscarinic receptors are displayed on all muscles and glands stimulated by the parasympathetic division. In addition, sweat glands and some blood vessels in skeletal muscles display muscarinic receptors that bind Ach released from postganglionic neurons of the sympathetic division.
While Ach can bind to both nicotinic and muscarinic receptors, nicotine cannot bind to muscarinic receptors, and muscarine cannot bind to nicotinic receptors. Certain drugs may bind to only one type of cholinergic receptor, or it may bind to both types. In this way, specific drugs can have either local effects or affect structures throughout the body. Nicotine at low doses is excitatory to postganglionic neurons and neuromuscular junctions of muscle fibers that bind Ach; however, at high doses, nicotine can inhibit these structures.
Figure 19-5. Pathways for parasympathetic impulses
Adrenergic Receptors
The membrane-bound receptors that bind norepinephrine and epinephrine are called adrenergic receptors (ad-re-NER-jik), so named for adrenaline (a-DREN-a-lin, the former name for epinephrine). There are two major classes of adrenergic receptors: alpha (α) and beta (β). Norepinephrine (NE) normally binds more often to α receptors than to β receptors, while epinephrine (E) binds to α and β receptors.
- Alpha (α) receptors are the most common adrenergic receptor and are displayed on most cells in the body except cardiac muscle cells. There are two subclasses of alpha receptors: α1 and α2. Binding of NE to α1 receptors usually results in excitation of the effector, whereas binding of NE to α2 receptors usually results in inhibition of the
- Beta (β) receptors are displayed on cardiac, smooth, and skeletal muscle cells and cells in the liver, kidneys, and adipose tissue. There are three classes of beta receptors:
- β1 receptors are found on the cardiac muscle cells, liver cells, and kidney cells. The binding of epinephrine to β1 receptors usually results in excitation. β1 receptors are also found on skeletal muscle cells but binding of E on these cells does not cause depolarization; instead, it causes an increase in cellular
- β2 receptors are found on smooth muscle cells located in (1) blood vessels of the heart and skeletal muscles, (2) walls of bronchioles—air passageways in the lungs, and (3) walls of the Binding of E with β2 receptors causes inhibition, or relaxation of smooth muscle in the locations listed. As a result, E causes vasodilation in the heart and skeletal muscles, dilation of bronchioles, and relaxation of intestines.
- β3 receptors are found on adipocytes (fat cells) in adipose tissue. Binding of E to these receptors results in excitation, whereby the adipocytes break down their stored lipids (a process called lipolysis) and release fatty acids into the
Note: To associate α1 and β1 receptors with excitation, and to associate α2 and β2 receptors with inhibition, remember that numerically, 1 comes before 2, and alphabetically excitation comes before inhibition.
The effects of different neurotransmitters on selected effectors are summarized in Table 19-1.
Table 19-1. Effects of sympathetic and parasympathetic neurotransmitters on selected effectors
*Released from a “preganglionic” neuron.
**Released from a postganglionic neuron.
CONTROL OF THE ANS
The diencephalon and brain stem control various aspects of the ANS. Involuntary control centers involving heart regulation, urination, defecation, pupil diameter, and breathing exist in the medulla and pons. The hypothalamus regulates aspects of the sympathetic and parasympathetic divisions. About three-fourths of all parasympathetic activities involve the paired vagus nerves (cranial nerve X), which exit from the medulla oblongata.
VISCERAL (AUTONOMIC) REFLEXES
Visceral reflexes involve smooth and cardiac muscle tissues. Moreover, the impulse pathway in a visceral reflex arc follows the same general pattern as that in somatic arcs with one major exception: a visceral reflex arc always involves two motor neurons (a preganglionic neuron and a postganglionic neuron).
TOPICS TO KNOW FOR CHAPTER 19
(Autonomic Nervous System)
acetylcholine (Ach)
adrenergic receptors
alpha receptors
autonomic nervous system
beta receptors
cholinergic receptors
control of the ANS
craniosacral division
divergence
dual innervation
epinephrine (E)
fight-or-flight division
muscarinic receptors
nicotinic receptors
norepinephrine (NE)
parasympathetic division
parasympathetic stimulation on selected target organs (tables)
paravertebral ganglia
postganglionic neuron
preganglionic neuron
prevertebral ganglion
receptors for ANS neurotransmitters
rest-and-digest division
sympathetic division
sympathetic stimulation on selected target organs (tables)
thoracolumbar division
visceral (autonomic) reflexes
α1 receptors
α2 receptors
β1 receptors
β2 receptors
β3 receptors
EOC Questions
