Introduction

The autonomic nervous system (ANS), also known as the visceral or involuntary nervous system, functions below the level of consciousness. Because it innervates cardiac muscle, smooth muscle, and various endocrine and exocrine glands, this nervous system influences the activity of most of the organ systems in the body. Therefore, it is evident that the ANS makes an important contribution to the maintenance of homeostasis. Regulation of blood pressure; gastrointestinal responses to food; contraction of the urinary bladder; focusing of the eyes; and thermoregulation are just a few of the many

Table 9.1 Distinguishing Features of Autonomic and Somatic Nervous Systems

Autonomic nervous system

Somatic nervous system

Unconscious control; involuntary All innervated structures except skeletal muscle (e.g., cardiac and smooth muscles; glands) Visceral functions (e.g., cardiac activity, blood flow, digestion, etc.) Peripheral ganglia located outside cerebrospinal axis Preganglionic and postganglionic neurons Nonmyelinated

Neurotransmitters: acetylcholine, norepinephrine

Cell bodies in brainstem, lateral horn of spinal cord No discrete innervation of individual effector cells Axon terminal with multiple varicosities releases neurotransmitter over wide surface area affecting many tissue cells Gap junctions allow spread of nervous stimulation throughout tissue

Conscious control; voluntary Skeletal muscle

Movement; respiration; posture

No peripheral ganglia; synapses located entirely within cerebrospinal axis Alpha motor neuron

Myelinated

Neurotransmitter: acetylcholine only

Cell bodies in ventral horn of spinal cord

Axon divides; each axon terminal innervates single muscle fiber directly Motor end-plate or neuromuscular junction = axon terminal in apposition to specialized surface of muscle cell membrane No gap junctions between effector cells; no spread of electrical activity from one muscle fiber to another homeostatic functions regulated by the ANS. Several distinguishing features of the ANS and the somatic nervous system, which innervates skeletal muscle, are summarized in Table 9.1.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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