RESPONSIVENESS IN VERTEBRATES 



Depending on the diameter of a nerve and the amount of insulating 

 material it has, conduction velocities vary from 1 to 120 meters per second. 

 Some of the large motor nerves to skeletal muscle in mammals conduct im- 

 pulses at a rate of approximately 120 meters per second, which is about 

 one-third as fast as sound travels in air. 



Discharge and Response. When the impulse arrives at the end of a 

 nerve fiber, a chemical compound known as a neurohumor is released. Such 

 a compound is responsible for conduction of the nerve impulse across the 

 synapse or junction between two nerve fibers and, also, across the neuro-effector 

 junction between the nerve fiber and the effector cell. Depending on the kind 

 of nerve fiber concerned, the neurohumor released will be acetylcholine, 

 epinephrine (adrenaline), or norepinephrine (noradrenaline). The so-called 

 discharge of the nerve impulse occurs at the neuro-effector junction. Release 

 of the neurohumor changes the environment of the effector cell in the region 

 of the junction. Then the effector cell reacts in its characteristic fashion; 

 the change in the environment finally evokes a response of the organism. 



Let us consider the sequence of events when a nerve impulse is' discharged 

 from the terminal branches of an efferent nerve fiber at neuromuscular 

 junctions, or motor end plates, of the striated muscle cells which such a fiber 

 serves (Fig. 4.22). In responding to the acetylcholine liberated by the motor 

 nerve endings, the permeability of the membrane of the muscle cell is in- 

 creased; ions move across the membrane into the muscle cells generating an 

 electric current. This muscle impulse is passed along the length of the muscle 

 cell in a manner similar to nerve conduction. Sodium ions move in, potas- 

 sium ions move out, and the energy requirements are met by the metabolism 

 of the muscle cell. The muscle cell, in addition to its capacity to conduct 

 an impulse, is able to contract or change its shape. Muscular contraction 

 consists of a shortening and thickening of the muscle cell and the develop- 

 ment of tension. Under resting conditions, some nerve fibers are continually 

 conducting impulses which have been set up in the deep receptors. In 

 response to these, the skeletal muscles are always in a state of slight tension; 

 they exhibit what is called muscle tone. The number of nerve fibers conduct- 

 ing impulses to a muscle is mediated by centers in the brain and depends on 

 the intensity of the stimulus. Analyze for yourself the circumstances under 

 which you shift your position in a chair or race to answer the telephone. 

 Physiologically, a muscle, which is a bundle of muscle cells, is activated by a 

 volley of nerve impulses arriving in rapid succession and eliciting a fusion of 

 twitches, or responses of the individual muscle cells. This results in the 

 smooth, sustained contractions that are responsible for the daily muscular 

 work of the body. 



The mechanism of contraction involves two characteristic proteins of mus- 

 cle, myosin and actin. Under the influence of the muscle impulse, myosin 

 and actin unite to form actomyosin. Myosin also acts as an enzyme which 

 can release the energy in ATP. This powers the shortening and thickening 

 of the actomyosin fibers and, so, of the muscle cell. To provide additional 



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