. Animal biology. Zoology; Biology. 244 METAZOAN PHYLA are carried from the center to the effector cells is known as the efferent path. 281. Reflex Acts.—In a typical and ideally simple reflex act a receptor neuron situated in the hypodermis is stimulated from without (Fig. 152), the stimulus being received directly by the cell body or by a short den- drite. An afferent impulse follows the axon of this cell to a synapse in a central ganglion where it is passed to the dendrite of a motor neuron. This sends out an efferent impulse along its axon to a muscle cell, the effector, causing contractio
. Animal biology. Zoology; Biology. 244 METAZOAN PHYLA are carried from the center to the effector cells is known as the efferent path. 281. Reflex Acts.—In a typical and ideally simple reflex act a receptor neuron situated in the hypodermis is stimulated from without (Fig. 152), the stimulus being received directly by the cell body or by a short den- drite. An afferent impulse follows the axon of this cell to a synapse in a central ganglion where it is passed to the dendrite of a motor neuron. This sends out an efferent impulse along its axon to a muscle cell, the effector, causing contraction and resulting in a movement appropriate to the stimulation which originated the act. This entire mechanism is Motor neuron Efferent nerve fiber 6/anf fibers venfrcrf ganglion Synapse 'Afferent nerve fiber. Sejnsory n^urort Effectors Venfrai longitudinal muscles Receptor Fig. 152.—Diagram illustrating reflex action in an earthworm. {From Parker, in Popular Sci. Mon., vol. 75, after Retzius.) called a reflex arc. Actually such a simple reflex does not occur, since in any action several receptors are stimulated at the same time and several effectors participate in the action. Also one or more connective neurons are usually involved, and they form chains of neurons through which conduction takes place. Since each of these connective neurons is in communication with others, spreading of the impulse also occurs. In the earthworm not only may several connective neurons in the same ganglion be involved in a reflex act, but it is also possible for impulses to pass from one ganglion to another. This transmission of an impulse from one metamere to another is due to connective neurons the axons of which are known as association fibers. The association fibers are contained in three tracts known as gfan/fibers—though they are really bundles of fibers —which he in the dorsal part of the ventral nerve cord. These fibers put the cell of which they are a part into communication with ce
Size: 2370px × 1055px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcoll, booksubjectbiology, booksubjectzoology
