. Adventures with animals and plants. Biology. 278 Receptor Why Living Things Behave As They Do unit v Spinal cord Afferent (sensory) neuron. Fig. 264 The simplest type of reflex arc. No intermediary neuron is shown. Note that the eel! body of the afferent neuroji lies outside of the cord {in a ganglion). It is different fro?n the neurons you have seen before. It has one very long dendrite. Where are its ends? another nerve cell, but with a muscle cell in the arm. The muscle cell is the effec- tor. When the impulse reaches it the muscle cell contracts. In this reflex act, there were used, firs
. Adventures with animals and plants. Biology. 278 Receptor Why Living Things Behave As They Do unit v Spinal cord Afferent (sensory) neuron. Fig. 264 The simplest type of reflex arc. No intermediary neuron is shown. Note that the eel! body of the afferent neuroji lies outside of the cord {in a ganglion). It is different fro?n the neurons you have seen before. It has one very long dendrite. Where are its ends? another nerve cell, but with a muscle cell in the arm. The muscle cell is the effec- tor. When the impulse reaches it the muscle cell contracts. In this reflex act, there were used, first, a receptor; sec- ond, a transmitter, or nerve cell which carries the impulse to the nerve center (spinal cord); third, another transmit- ter which carries the impulse out of the nerve center (spinal cord); and, fourth, an effector which receives the impulse, and acts. The first neuron, which carries the impulse inward to the cord, is called the afferent, or sensory, neuron. Afferent means leading into. The second neuron, which carries the impulse out of the cord, is called an efferent, or motor, neu- ron. To help you understand what you have read do Exercise 5. This description and the diagram are oversimplified. What is shown happen- ing in one receptor and in single neurons happens in a group of receptors and neurons lying side by side. Therefore all the muscle cells in the muscle contract. And the diagram is oversimplified in another respect. In reality there are prob- ably several neurons lying in the cord between the afferent and efferent neu- rons. These relay the impulse from the afferent neuron to the efferent neuron. Such neurons are called inter?nediary neurons. But there is little relaying; in other words, there are few synapses and that, no doubt, is one reason why this im- pulse travels with great speed. You pull your hand away almost instantaneously. Now in all of us the impulse takes this path and it always takes the same path unless some of the cells have bee
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