Visceral Effector Organs

Since the autonomic nervous system helps to regulate the activities of glands, smooth muscles, and cardiac muscle, auto-nomic control is an integral aspect of the physiology of most of the body systems. Autonomic regulation, then, partly explains endocrine regulation (chapter 11), smooth muscle function (chapter 12), functions of the heart and circulation (chapters 13 and 14), and, in fact, all the remaining systems to be discussed. Although the functions of the target organs of autonomic innervation are described in subsequent chapters, at this point we will consider some of the common features of autonomic regulation.

Unlike skeletal muscles, which enter a state of flaccid paralysis and atrophy when their motor nerves are severed, the involuntary effectors are somewhat independent of their innervation. Smooth muscles maintain a resting tone (tension) in the absence of nerve stimulation, for example. In fact, damage to an autonomic nerve makes its target tissue more sensitive than normal to stimulating agents. This phenomenon is called denervation hypersensitivity. Such compensatory changes can explain why, for example, the ability of the stomach mucosa to secrete acid may be restored after its neural supply from the vagus nerve has been severed. (This procedure is called vagotomy, and is sometimes performed as a treatment for ulcers.)

In addition to their intrinsic ("built-in") muscle tone, cardiac muscle and many smooth muscles take their autonomy a step further. These muscles can contract rhythmically, even in the absence of nerve stimulation, in response to electrical waves of depolarization initiated by the muscles themselves. Autonomic innervation simply increases or decreases this intrinsic activity. Autonomic nerves also maintain a resting tone in the sense that they maintain a baseline firing rate that can be either increased or decreased. A decrease in the excitatory input to the heart, for example, will slow its rate of beat.

The release of acetylcholine (ACh) from somatic motor neurons always stimulates the effector organ (skeletal muscles). By contrast, some autonomic nerves release transmitters that inhibit the activity of their effectors. An increase in the activity of the vagus, a nerve that supplies inhibitory fibers to the heart, for example, will slow the heart rate, whereas a decrease in this inhibitory input will increase the heart rate.

Test Yourself Before You Continue

1. Describe the preganglionic and postganglionic neurons in the autonomic system. Use a diagram to illustrate the difference in efferent outflow between somatic and autonomic nerves.

2. Compare the control of cardiac muscle and smooth muscles with that of skeletal muscles. How is each type of muscle tissue affected by cutting its innervation?

Table 9.1 Comparison of the Somatic Motor System and the Autonomic Motor System

Feature

Somatic Motor

Autonomic Motor

Effector organs

Skeletal muscles

Cardiac muscle, smooth muscle, and glands

Presence of ganglia

No ganglia

Cell bodies of postganglionic autonomic fibers located in paravertebral, prevertebral (collateral), and terminal ganglia

Number of neurons from CNS to effector

One

Two

Type of neuromuscular junction

Specialized motor end plate

No specialization of postsynaptic membrane; all areas of smooth muscle cells contain receptor proteins for neurotransmitters

Effect of nerve impulse on muscle

Excitatory only

Either excitatory or inhibitory

Type of nerve fibers

Fast-conducting, thick (9-13 |m), and myelinated

Slow-conducting; preganglionic fibers lightly myelinated but thin (3 |m); postganglionic fibers unmyelinated and very thin (about 1.0 |m)

Effect of denervation

Flaccid paralysis and atrophy

Muscle tone and function persist; target cells show denervation hypersensitivity

222 Chapter Nine

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Responses

  • leea
    What is the difference between flaccid paralysis and denervation hypersensitivity?
    8 years ago
  • Leslie
    What organs are visceral effectors?
    8 years ago
  • Sakari
    Is skeletal muscle a visceral effector?
    8 years ago
  • Tomba
    What are examples of visceral effector organs?
    8 years ago
  • Aleksi M
    Is the heart an effector organ?
    8 years ago
  • vihtori hakkarainen
    What is a visceral effector organ?
    8 years ago
  • ruta berhane
    How is cardiac muscle affected by cutting its innervation?
    8 years ago
  • john
    What are somatic effector organs?
    8 years ago
  • SEMRET
    Where would the effectors/targets be located for the endocrine system?
    7 years ago
  • antonio
    What are the names of the effector organs?
    7 years ago
  • mackenzie
    Is cardiac muscle a visceral effector?
    7 years ago
  • Priscilla
    What are the effectors organs in somatic?
    7 years ago
  • michelle
    What are the effectors of the somatic motor system?
    6 years ago
  • Michael
    Is smooth muscle an examples of effector organs?
    5 years ago
  • Libby
    Is the heart muscle an effector in physiology?
    1 year ago

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