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In all such actions, the impulses are carried to and from the spinal cord  In other words a reflex action is an involuntary action and there is no choice as to whether the action should take place or not.

Many of the reflexes are innate, and are called unconditional or inherited reflexes. These reflexes are few in number and do not depend on a prior experience. Previous experiences establish conditioned or acquired reflexes. They are much more in number and develop due to environmental changes. The conditioned reflex is one way in which animal and humans learn.

In all reflex actions, the spinal cord forms the main path for the impulses, from and to the brain. Reflex actions have survival value.


Figure 23.9 Knee jerk reflex


SUMMARY

(1) The nervous system together with the endocrine system coordinates body activities. (2)The control center of the nervous system is the brain which has three divisions, i.e. forebrain, midbrain and hindbrain. (3) The forebrain consists of cerebrum thalamus and hypothalamus. The midbrain has decreased in size during vertebrate evolution, and the forebrain has gradually acquired much of its function, while the hindbrain consists of medulla, cerebellum and the pons. (4) The spinal cord is the main path for conduction of nervous and is a reflex center. (5) Of the 12 cranial nerves, 11 cranial nerves control the head, and 1 cranial nerve communicates between the brain and the internal organs. There are 31 pairs of spinal nerves. (6) The sympathetic nerves produce the fight-or-flight response during times of stress. The parasympathetic nerves help to maintain homeostasis, to recover from stress. (7) The reflex action is automatic, a sudden response to a stimulus. The stimulus follows a path from the receptor to the effector organ called a reflex arc.

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Table of Contents


23.0 Introduction
23.1 Central nervous system
23.2 The automatic nervous system
23.3 Receptors and effects
23.4 Reflex action - mechanism of nervous action

Chapter 24

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