100 MUSCLE 



Exercise XIX 



impulses at about 30 m/sec. Striated muscle 

 conducts at only 2 to 5 m/sec; heart muscle at 

 less than I m/sec, and smooth muscle at only 

 5 to 20 cm/sec. The more slowly the excitatory 

 impulse is conducted over a muscle fiber, the 

 more slowly it responds. 



The excitability of such cell membranes can be 

 drastically altered by chemical reagents. Nerve 

 cells transmit their excitation across a synapse 

 with another nerve or with a muscle by releasing 

 at the synapse excitatory or inhibitory substances 

 (neurohumors, "nerve hormones"). The usual 

 excitatory neurohumor is acetylcholine, though 

 at the first line of synapses in the sympathetic 

 nervous system it is epinephrine (adrenaline). 

 In such cases the neurohumor released by an 

 excited nerve cell at the synapse locally de- 

 polarizes the membrane of the succeeding nerve 

 or muscle cell, so exciting it in turn. 



In addition to such substances which de- 

 polarize nerve and muscle cell membranes, ex- 

 citing them or lowering their thresholds to other 

 stimuli, there are also hyperpolarizing substances, 

 which increase the polarization of the membrane, 

 raising the threshold of the cell and making it 

 more difficult to excite. These are therefore not 

 excitatory, but inhibitory substances, and may 

 be released at nerve endings which are concerned 

 with inhibition, rather than with excitation. 

 7-amino butyric acid, 



(CH2NH2 • CH2 • CH2 • COOH), 



acts in this way; but, as we shall see, acetyl- 

 choline also may at times, as in the heart, inhibit 

 rather than excite. 



The characteristics of the excitable membranes 

 of nerves and muscles can be altered and con- 

 trolled by the application of such substances, 

 released naturally at nerve endings, or applied 

 artificially. The rate of the heart beat, for ex- 

 ample, as well as its amplitude, are controlled in 

 this way. The heart muscle beats automatically, 

 as a result of an intrinsic cycle of excitation and 

 recovery. The heart rate is regulated through the 

 activity of two nerves, which are opposed in their 

 effects: a nerve from the sympathetic system, 

 which releases epinephrine at its synapse with the 



heart muscle and excites the heart to beat more 

 quickly; and the vagus nerve from the parasym- 

 pathetic nervous system, which inhibits the heart 

 by releasing acetylcholine, slowing the beat. 



During this period we shall examine the struc- 

 ture of the various types of muscle, the con- 

 tractile effects of ATP upon the muscle proteins, 

 and the effects of excitatory and inhibitory sub- 

 stances on the frog heart. 



EXPERIMENTAL PROCEDURE 

 Types of muscle 



First examine the prepared sections of striated, 

 smooth, and cardiac muscle. Striated and car- 

 diac muscle may be recognized by their cross- 

 banding or striations. Smooth muscle has no 

 visible banding. Each of the long, spindle- 

 shaped cells of smooth muscle contains a promi- 

 nent, elongated nucleus. The fibers of striated 

 muscle seen under the microscope are distinct 

 cells, lying parallel to one another. The similarly 

 striated fibers of cardiac muscle, however, 

 branch widely with one another, forming an 

 intercommunicating network that contains many 

 nuclei, but no apparent cell boundaries. Until 

 recently the entire ventricle of the heart was 

 thought to constitute a single, multinucleate 

 cell (a so-called syncitium). Recently, however, 

 membranes that divide cardiac muscle cells have 

 been found with the electron microscope. 



Using Huxley's article as guide, study the 

 banding of a microscopic section of striated 

 muscle. Identify the A and / bands and the Z 

 and H lines. What arrangement of molecules 

 accounts for the A and / bands? 



Contraction of glycerinated muscle 



Next, we shall study the contraction of muscle 

 fibers on addition of ATP, using the famous and 

 extraordinarily important preparation devised 

 by Albert Szent-Gyorgyi. The psoas muscle of 

 a rabbit contains exceedingly little connective 

 tissue, being composed almost entirely of long, 

 parallel muscle fibers. It is the lack of connec- 

 tive tissue that makes this the "tenderloin." A 



