Reflexes

The thermoregulatory system we used as an example in the previous section, and many of the body's other homeostatic control systems, belong to the general category of stimulus-response sequences known as reflexes. Although in some reflexes we are aware of the stimulus and/or the response, many reflexes regulating the internal environment occur without any conscious awareness.

In the most narrow sense of the word, a reflex is a specific involuntary, unpremeditated, unlearned "built-in" response to a particular stimulus. Examples of such reflexes include pulling one's hand away from a hot object or shutting one's eyes as an object rapidly approaches the face. There are also many responses, however, that appear to be automatic and stereotyped but are actually the result of learning and practice. For example, an experienced driver performs many complicated acts in operating a car. To the driver these motions are, in large part, automatic, stereotyped, and unpremeditated, but they occur only because a great deal of conscious effort was spent learning them. We term such reflexes learned, or acquired. In general, most reflexes, no matter how basic they may appear to be, are subject to alteration by learning; that is, there is often no clear distinction between a basic reflex and one with a learned component.

The pathway mediating a reflex is known as the reflex arc, and its components are shown in Figure 7-3.

A stimulus is defined as a detectable change in the internal or external environment, such as a change in temperature, plasma potassium concentration, or blood pressure. A receptor detects the environmental change; we referred to the receptor as a "detector" earlier. A stimulus acts upon a receptor to produce a signal that is relayed to an integrating center. The pathway traveled by the signal between the receptor and the integrating center is known as the afferent pathway (the general term "afferent" means "to carry to," in this case, to the integrating center).

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Mechanism Reflex Arc

FIGURE 7-3

General components of a reflex arc that functions as a negative-feedback control system. The response of the system has the effect of counteracting or eliminating the stimulus. This phenomenon of negative feedback is emphasized by the minus sign in the dashed feedback loop.

FIGURE 7-3

General components of a reflex arc that functions as a negative-feedback control system. The response of the system has the effect of counteracting or eliminating the stimulus. This phenomenon of negative feedback is emphasized by the minus sign in the dashed feedback loop.

An integrating center often receives signals from many receptors, some of which may be responding to quite different types of stimuli. Thus, the output of an integrating center reflects the net effect of the total afferent input; that is, it represents an integration of numerous bits of information.

The output of an integrating center is sent to the last component of the system, a device whose change in activity constitutes the overall response of the system. This component is known as an effector. The information going from an integrating center to an effector is like a command directing the effector to alter its activity. The pathway along which this information travels is known as the efferent pathway (the general term "efferent" means "to carry away from," in this case, away from the integrating center).

Thus far we have described the reflex arc as the sequence of events linking a stimulus to a response. If the response produced by the effector causes a decrease in the magnitude of the stimulus that triggered the sequence of events, then the reflex leads to negative feedback and we have a typical homeostatic control system. Not all reflexes are associated with such feedback. For example, the smell of food stimulates the secretion of a hormone by the stomach, but this hormone does not eliminate the smell of food (the stimulus).

To illustrate the components of a negativefeedback homeostatic reflex arc, let us use Figure 7-4 to apply these terms to thermoregulation. The temperature receptors are the endings of certain nerve cells in various parts of the body. They generate electric signals in the nerve cells at a rate determined by the temperature. These electric signals are conducted by the nerve fibers—the afferent pathway—to a specific part of the brain—the integrating center for temperature regulation. The integrating center, in turn, determines the signals sent out along those nerve cells that cause skeletal muscles and the muscles in skin blood vessels to contract. The nerve fibers to the muscles are the efferent pathway, and the muscles are the effectors. The dashed arrow and the © indicate the negative-feedback nature of the reflex.

Almost all body cells can act as effectors in homeostatic reflexes. There are, however, two specialized classes of tissues—muscle and gland—that are the major effectors of biological control systems. The physiology of glands is described in Chapter 6, that of muscle in Chapter 11.

Traditionally, the term "reflex" was restricted to situations in which the receptors, afferent pathway, integrating center, and efferent pathway were all parts of the nervous system, as in the thermoregulatory reflex. Present usage is not so restrictive, however, and recognizes that the principles are essentially the same when a blood-borne chemical messenger known as a hormone, rather than a nerve fiber, serves as the efferent pathway, or when a hormone-secreting gland (termed an endocrine gland) serves as the integrating center. Thus, in the thermoregulation example, the integrating center in the brain not only sends signals by way of nerve fibers, as shown in Figure 7-4, but also causes the release of a hormone that travels via the blood to many cells, where it produces an increase in the amount of heat produced by these cells. This hormone therefore also serves as an efferent pathway in thermoregulatory reflexes.

Accordingly, in our use of the term "reflex," we include hormones as reflex components. Moreover, depending on the specific nature of the reflex, the integrating center may reside either in the nervous system or in an endocrine gland. In addition, an endocrine gland may act as both receptor and integrating center in a reflex; for example, the endocrine-gland cells that secrete the hormone insulin themselves detect changes in the plasma glucose concentration.

In conclusion, many reflexes function in a homeo-static manner to keep a physical or chemical variable of the body relatively constant. One can analyze any such system by answering the questions listed in Table 7-2.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

TABLE 7-2 Questions to Be Asked About Any Homeostatic Reflex

1. What is the variable (for example, plasma potassium concentration, body temperature, blood pressure) that is maintained relatively constant in the face of changing conditions?

2. Where are the receptors that detect changes in the state of this variable?

3. Where is the integrating center to which these receptors send information and from which information is sent out to the effectors, and what is the nature of these afferent and efferent pathways?

4. What are the effectors, and how do they alter their activities so as to maintain the regulated variable near the set point of the system?

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Essentials of Human Physiology

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Responses

  • Franziska
    What is a example of the body reflex response?
    7 years ago
  • PERVINCA
    What is an example of integrating center in the body?
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  • juha m
    What is a negative homeostatic reflex?
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  • marja
    What are the components of a negative feedback control system in general?
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  • hugo
    Does the efferent pathway of a reflex arc carry information to the integrating center?
    7 years ago
  • Leon
    What type of feedback loop is a reflex arc?
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  • ismo
    Does effectors act as integration center?
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  • benilde boni
    What are the functions in the reflex arc stimulus?
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  • tammy taylor
    How reflexes function?
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  • sarah eisenberg
    What are three body functions that reflexes control?
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  • Auli Lenho
    Is the reflex arc a feedback mechanism?
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  • anna
    What are the reflexes we learned?
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  • diamond
    How the hormones function in a reflex arc?
    7 years ago
  • Ermias
    How can a reflex be explained by a stimulus response model?
    7 years ago
  • costante
    What component of the reflex arc determines the response to a stimulus?
    7 years ago
  • annett
    What are the components of a reflex?
    7 years ago
  • calimero
    What are the 3 basic components of the reflex?
    7 years ago
  • Rihanna
    What the function of the integrating center of a negative feedback loop?
    7 years ago
  • CLAUDIA PICCIO
    WHAT ARE THE PARTS AND FUNCTIONS OF A REFLEX ARC?
    7 years ago
  • natalia
    What are components of reflex pathway?
    7 years ago
  • selam
    What are the basic components of reflexes in humans?
    6 years ago
  • LAIBA
    What type of reflex response loop is the thermoregulatory pathway?
    6 years ago
  • mariano
    What are the sequence of events which leads to the withdrawl of the hand away from a hot object?
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  • Clara
    What are the hormones related to function of reflex arc?
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  • ulpu
    Is temperature a reflex arc?
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  • harris wright
    What is the basic function of reflex?
    5 years ago
  • callan
    Is gastrointestinal reflex pathway a negative feedback?
    5 years ago
  • eugene
    What type of reflex response loop is the thermoregulatory pathway as summarized above?
    4 years ago
  • lily-rose
    Why driving car is reflex action?
    1 year ago
  • Rolando
    What is the function of an integrating centre in a homeostatic reflex?
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