GENERAL ZOOLOGY 



spinal cord. Ihe neuron over which the impulse enters the spinal cord is a 

 sensory or afferent neuron. The cytosome of such a sensory neuron is located 

 in the dorsal-root ganglion of the dorsal or sensory root of a spinal nerve. One 

 of the processes of the aflferent neuron enters the centrally located gray matter 

 of the spinal cord, and its terminal filaments end in contact with other nerve 

 cells found there. In the hypothetical case under consideration, the impulse 

 would pass through a synapse between a process of a sensory neuron and a 

 dendrite or the cell body of a motor or efferent neuron and leave the spinal cord 

 by way of the axon of the same eflferent neuron. This nerve fiber passes into 

 the ventral or motor root of the spinal nerve and continues to a skeletal muscle, 

 on which it terminates. The place of contact between a nerve-cell process 

 and a muscle cell is known as a neuromuscular junction or motor end plate (see 

 Fig. 4.22). It is at such a place of contact that the impulse is discharged and 

 a specific chemical compound, acetylcholine, is liberated (p. 115). I'he 

 muscle is stimulated by the acetylcholine and contracts. This reaction is the 

 effect produced by the change in the environment. A muscle is known, there- 

 fore, as the effector in a reflex arc. The parts of the simplest type of reflex 

 arc are the receptor, where the stimulus is received; the afferent neuron, over 

 which the impulse is conducted to the spinal cord; the efferent neuron, over 

 which the impulse is conducted away from the spinal cord and from which it 

 is discharged; and the effector, where the reaction to the stimulus occurs. 



In the great majority of reflex actions the effect is produced in some part 

 of the body other than that at which the stimulus is received. If the skin of 

 a dog's back is rubbed with a pointed instrument, the animal will respond by 

 attempting to scratch the place of stimulation with its hind leg. The receptors 

 in this instance are located at the roots of the hairs of that region of the back 

 which is stroked. The afferent neurons conduct the impulses to the spinal 

 cord over the dorsal root of the spinal nerve whose fibers extend to that region 

 of the skin. Within the gray matter of the cord each sensory neuron has 

 synapses with neurons of which both the cytosome and processes lie entirely 

 inside the spinal cord. Over the processes of such neurons the nerve impulse 

 is conducted posteriorly along the spinal cord to the level of exit of the nerves 

 that extend to the hind legs. Here synapses occur with efl^erent neurons, and 

 impulses leave the spinal cord over the ventral root of a spinal nerve and pass 

 to muscles (eflfectors) that produce the scratching motion. 



In this type of reflex arc three kinds of neurons are concerned (Fig. 4.10.4). 

 The neuron along which the impulse passes within the spinal cord is known 

 as an adjustor neuron or interneuron. Adjustor neurons are very numerous in 

 the central nervous system and make possible the varied reactions that a single 

 stimulus can produce. For example, when acid is applied to the skin of a 

 frog's back, the first reaction is a contraction of muscles of the body wall in 

 the region stimulated. Very soon, however, this is followed by other reactions 

 which can be observed best in a frog from which the brain has been removed 

 (Fig. 4.105). The fore leg on the side stimulated moves toward the location 

 of the acid, and this reaction is followed quickly by movements of the hind 



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