EFFECTOR MECHANISMS 



TABLE 9.1 

 Estimates of Tension Developed by some Invertebrate Muscles 



383 



Animal 



Tensions 



Metridium senile 



As te rias rubens 



Holothwia grisea 

 Pinna fragilis 

 Venus verrucosa 

 Ostrea edulis 

 Pecten magellanicus 

 Mytilus edulis 



Circular muscle of the column. 3-5 g/cm of body wall trans- 

 verse to the muscle, corresponding to 40 kg/cm 2 of muscle 

 fibre cross-section. 



Tube foot. Force of a downward push, ca. 200 mg; of an 

 upward pull, ca. 50 mg. 



Five longitudinal muscles. 13 g/cm 2 of body cross-section. 



Fast posterior adductor. 1-5 kg/cm 2 cross-section. 



Slow adductor. 35-4 kg/cm 2 cross-section. 



Slow adductor. 12 kg/cm 2 cross-section. 



Slow adductor. 4 kg/cm 2 cross-section. 



Slow adductor. 11-3 kg/cm 2 cross-section. 



Pedal retractor. Twitch tension 2-2-5 kg/cm 2 cross-section. 

 Twitch/tetanus ratio 1:3. 



Anterior byssus retractor. Twitch tension 3-5-4-5 kg/cm 2 

 cross-section. Twitch/tetanus ratio 1:8. 



Posterior adductor. Twitch tension 0-5 kg/cm 2 cross-section. 

 Twitch /tetanus ratio 1 : 14. 



TABLE 9.2 

 Forces Needed to Open Lamellibranch Valves 



Animal 



Ostrea circumpicta 

 Mytilus crassitesta 

 Cardium edule 

 Chlamys senatorius 



Force 



7,880 g/cm 2 muscle cross-section 

 5,386 g/cm 2 ditto 



2,856 g/cm 2 ditto 



1,272 g/cm 2 ditto 



The Muscle Twitch and Fused Contractions. The response of 

 muscles of most animals to a stimulus delivered through the motor nerves 

 is a single contraction or twitch. There are exceptions, notably the longi- 

 tudinal parietal retractors and the disc sphincter muscles of sea-anemones, 

 where several nervous impulses are required to evoke contraction: this 

 condition is an instance of neuromuscular facilitation (discussed on pp. 

 385, 424). With direct electrical stimulation even these muscles respond 

 to a single pulse with a contraction (91). 



When a muscle is excited by a train of nervous impulses it responds by a 

 series of contractions which succeed one another at a rate depending on 

 the frequency of excitation. When the frequency is sufficiently fast the 

 separate contractions undergo some degree of fusion; a complete fusion 

 of twitches, producing a steady contraction throughout the period of 

 stimulation, is tetanus (Fig. 9.9). The ability of a muscle to be tetanized 

 depends on its refractory period. Some muscles possessing long refractory 

 periods cannot develop fused contractions; such a muscle remains in- 

 excitable to a second stimulus until its contraction is spent or completed, 

 e.g. the subumbrella circular muscles of medusae and the cardiac muscle of 

 vertebrates. The refractory period of circular muscle of jellyfish (Aurelia, 

 etc.) is 0-7 sec, whereas the twitch reaches maximal tension in 0-4-0-6 sec. 



