Neural Pathways

The ANS has components in both the central and peripheral nervous systems. It includes control nuclei in the hypothalamus and other regions of the brainstem, motor neurons in the spinal cord and peripheral ganglia, and nerve fibers that travel through the cranial and spinal nerves you have already studied.

The autonomic motor pathway to a target organ differs significantly from somatic motor pathways. In somatic pathways, a motor neuron in the brainstem or spinal cord issues a myelinated axon that reaches all the way to a skeletal muscle. In autonomic pathways, the signal must travel across two neurons to get to the target organ, and it must cross a synapse where these two neurons meet in an autonomic

Homeostasis And Baroreceptor Reflex Arc

Baroreceptors sense increased blood pressure

Common carotid artery

Figure 15.1 Autonomic Reflex Arcs in the Regulation of Blood Pressure. In this example, a rise in blood pressure is detected by baroreceptors in the carotid artery. The glossopharyngeal nerve transmits signals to the medulla oblongata, resulting in parasympathetic output from the vagus nerve that reduces the heart rate and lowers blood pressure.

Terminal ganglion

Heart rate decreases

Baroreceptors sense increased blood pressure

Common carotid artery ■

Terminal ganglion

Heart rate decreases

Saladin: Anatomy & I 15. The Autonomic Nervous I Text I © The McGraw-Hill

Physiology: The Unity of and Visceral Reflexes Companies, 2003 Form and Function, Third Edition

566 Part Three Integration and Control ganglion (fig. 15.2). The first neuron, called the preganglionic neuron, has a soma in the brainstem or spinal cord whose axon terminates in the ganglion. It synapses there with a postganglionic neuron whose axon extends the rest of the way to the target cells. (Some call this cell the ganglionic neuron since its soma is in the ganglion and only its axon is truly postganglionic.) The axons of these neurons are called the pre- and postganglionic fibers.

In summary, the autonomic nervous system is a division of the nervous system responsible for homeostasis, acting through the mostly unconscious and involuntary control of glands, smooth muscle, and cardiac muscle. Its target organs are mostly the thoracic and abdominal viscera, but also include some cutaneous and other effectors. It acts through motor pathways that involve two neurons, pregan-glionic and postganglionic, reaching from CNS to effector. The ANS has two divisions, sympathetic and parasympa-thetic, that often have cooperative or contrasting effects on the same target organ. Both divisions have excitatory effects on some target cells and inhibitory effects on others. These and other differences between the somatic and autonomic nervous systems are summarized in table 15.1.

Somatic And Autonomic Neuron Pathways

Figure 15.2 Comparison of Somatic and Autonomic Efferent Pathways. The entire distance from CNS to effector is spanned by one neuron in the somatic system and two neurons in the autonomic system. Only acetylcholine (ACh) is employed as a neurotransmitter by the somatic neuron and the autonomic preganglionic neuron, but autonomic postganglionic neurons can employ either ACh or norepinephrine (NE).

Figure 15.2 Comparison of Somatic and Autonomic Efferent Pathways. The entire distance from CNS to effector is spanned by one neuron in the somatic system and two neurons in the autonomic system. Only acetylcholine (ACh) is employed as a neurotransmitter by the somatic neuron and the autonomic preganglionic neuron, but autonomic postganglionic neurons can employ either ACh or norepinephrine (NE).

Table 15.1

Comparison of the Somatic and Autonomic Nervous Systems

Feature

Somatic

Autonomic

Effectors

Skeletal muscle

Glands, smooth muscle, cardiac muscle

Efferent pathways

One nerve fiber from CNS to effector; no ganglia

Two nerve fibers from CNS to effector; synapse at a ganglion

Neurotransmitters

Acetylcholine (ACh)

ACh and norepinephrine (NE)

Effect on target cells

Always excitatory

Excitatory or inhibitory

Effect of denervation

Flaccid paralysis

Denervation hypersensitivity

Control

Usually voluntary

Usually involuntary

Saladin: Anatomy & I 15. The Autonomic Nervous I Text I © The McGraw-Hill

Physiology: The Unity of and Visceral Reflexes Companies, 2003 Form and Function, Third Edition

Chapter 15 The Autonomic Nervous System and Visceral Reflexes 567

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. How does the autonomic nervous system differ from the somatic motor system?

2. How do the general effects of the sympathetic division differ from those of the parasympathetic division?

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Responses

  • michael
    How does the ANS and motor pathways "differ"?
    7 years ago
  • Brigitte
    What are the differences between the somatic and autonomic nervous systems?
    7 years ago

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