Ch 13

Question 1

autonomic nervous system (ANS) is a subdivision of the efferent (motor) division of the peripheral nervous system (PNS) and includes the sympathetic (sim-puh-THET-ik; sympatho, feel suffering with) and parasympathetic (pair-uh-sim-puh-THET-ik; para, alongside) divisions.

Question 2

In all cases except one, the ANS impulse passes through two motor neurons before reaching the effector. The preganglionic neuron (pree-gang-lee-ON-ik; pre, before) is the motor neuron that conducts the impulse out of the CNS (Figure 13-11). 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

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.

Cell bodies of sympathetic, preganglionic neurons reside in the lateral horns of the spinal cord

All preganglionic neurons in the sympathetic division release the neurotransmitter acetylcholine (Ach; as-EE-tull-KOE-lene) from their axon terminals

Most postganglionic neurons in this division release the neurotransmitter norepinephrine (NE; nor ep-ih-NEF-rin), which is a catecholamine chemical described in chapter 2. 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).

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

parasympathetic division has two other names based on associated functions and the location where its preganglionic neurons exit the dorsal cavity. Functionally, the parasympathetic system is the rest-and-digest division because its activity increases during times of rest, relaxation, and digestion. It is also the craniosacral division (KRAY-nee-oh-SAY-krul) because its impulses exit the dorsal cavity only in the cranium and the sacral regions

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

embrane receptors that bind Ach are called cholinergic receptors

Nicotinic receptors (nik-oh-TIN-ik) bind Ach and nicotine

Muscarinic receptors (mus-kuh-RIN-ik) bind Ach and muscarine (MUS-ka-reen), a chemical produced in a certain species of mushroom

The membrane-bound receptors that bind norepinephrine and epinephrine are called adrenergic receptors (ad-rih-NER-jik), so named for adrenaline (uh-DREN-uh-lin, the former name for epinephrine)
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 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 effector.
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 metabolism.
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 blood.

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.

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).