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PinkMonkey Online Study Guide-Biology

23. 4 Reflex Action - Mechanism of Nervous Action

The term reflex action refers to a rapid, automatic unlearned response to a stimulus. These actions are involuntary as opposed to voluntary actions, which are governed by willful or conscious control of the animal.


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Figure 23.8 Reflex arcs


A familiar example of the reflex action in man is the "knee jerk" reflex or the "hand withdrawal" after pin prick. It is common understanding that if suddenly one is pricked by a sharp object such as a pin one immediately retracts from the object. In such actions, the impulses are carried by the sensory fibers from the receptor organ to the central nervous system and back to the effector organ. The path or the route through which the impulses travel from the receptor organ (via the Central Nervous System) is called a reflex arc.

Reflex hand withdrawal - Stimulus ® Receptor organ (skin)

(pin prick) ¯

Effector organ ¬ Motor fibers ¬ C.N.S. ¬ Sensory fibers

(Skeletal muscles)

If the controlling center of the reflex arc is located in the brain it is called cerebral reflex. If it is located in the spinal cord, it is called spinal reflex.

The reflex arcs are of two types.

1) Somatic reflex arc involving effector organs located in (soma) body structures, e.g. skeletal muscles.

2) Visceral reflex arc involves effectors located in the visceral organs; e.g. glands or smooth muscles.

Components of the reflex arcs

Somatic reflex arc (Figure 23.8) Visceral reflex arc
1. Receptor organ (e.g.) skin 1. Receptor organ (e.g. Epithelial lining of the gut)
2. Somatic sensory or afferent neuron (unipolar) 2. Visceral sensory or afferent neuron (unipolar)
3. Intermediate neuron (Multipolar) 3. Visceral motor of efferent (Pre-ganglionic)
4. Somatic motor or efferent neuron (Multipolar) 4. Visceral motor or efferent neuron (Post-ganglionic)
5. Effector organ (e.g. Skeletal muscles) 5. Effector organ (e.g. gland or smooth muscles)

A simple reflex arc consists of only one sensory neuron and one motor neuron (monosynaptic reflex). But in a majority of reflexes, a sensory neuron influences several motor neurons through intermediate (association) neurons of the spinal cord. This forms a complex reflex arc, or polysynaptic reflexes.

Some common examples of reflex actions are knee jerk reflex, contraction of iris diaphragm (i.e. narrowing of pupil in response of intense light) blinking of eye lids when aimed at, withdrawal of hand on pin prick, sneezing, and secretion of saliva at the sight or smell of food.

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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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