EFFECTOR MECHANISMS 381 



and rapidly diminishes with distance from the end-plate; when two or 

 more impulses reach the end-plate, the graded e.p.p's can fuse with one 

 another. 



The muscle impulse is a wave of excitation which is propagated with 

 measurable velocity over the fibre; it is all-or-nothing in character, and 

 evokes a contractile response or twitch. The action potential is produced 

 by transitory depolarization of the fibre membrane ; following the spike 

 potential there is a marked negative after-potential. Since the muscle 

 membrane recovers (repolarizes) more quickly than the muscle fibre can 

 relax, it is able to conduct impulses at high frequencies, thus maintaining 

 a state of contraction in the muscle (clonus or tetanus). 



The nerve impulse excites the neighbouring end-plate through the inter- 

 mediation of a chemical transmitter. When the impulse arrives at the axon 

 terminals it releases a certain amount of transmitter, which diffuses across 

 the short distances between axon and end-plate, and excites the latter. 

 In vertebrate skeletal muscles the transmitter is acetylcholine; this is 

 destroyed by acetylcholinesterase, which is concentrated in the vicinity of 

 the end-plate. The drug eserine, which inactivates acetylcholinesterase, 

 enhances the e.p.p. ; and curare, which competes preferentially with acetyl- 

 choline for the receptors in the end-plate, reduces or abolishes the e.p.p. 

 Nerve fibres releasing acetylcholine at their terminals are termed choliner- 

 gic; among invertebrates, echinoderms, lamellibranchs, annelids and 

 sipunculoids appear to possess cholinergic motor axons (p. 440). Other 

 recognized chemical transmitters are adrenaline and 5-hydroxytryptamine. 

 Many post-ganglionic sympathetic fibres of vertebrates are adrenergic, 

 e.g. sympathetic fibres to the intestine and blood vessels. Adrenaline- 

 sensitive muscles among invertebrates include : crop ofAplysia, proboscis of 

 Arenicola, intestine of sea cucumber (Thy one) and crayfish. 5-hydroxy- 

 tryptamine is released at the terminals of cardio-acceleratory fibres of 

 molluscs (p. 112) and has an inhibitory action on lamellibranch smooth 

 muscle (32, 34, 62, 88, 102, 104, 119). ' 



Contraction 



The characteristic response of many striped muscles to a stimulus, either 

 nervous or electrical, is a single twitch. Following the arrival of the nervous 

 impulse there is a brief latent period before contraction begins, a rising 

 contractile phase or shortening, then relaxation or extension. Part of the 

 latency is occupied by the transmission of a self-propagating action po- 

 tential over the surface of the muscle fibre. In the anterior byssal retractor 

 of Mytilus, for example, this occurs at a rate of 13-29 cm/sec. Lamelli- 

 branch smooth muscle is unusual in that conduction is decremental. 

 The muscle impulse, like the nerve-action spike, is a wave of surface ac- 

 tivity, and it is not clear how it spreads inwards to activate the contractile 

 myofibrils in the interior of the fibre. 



The contractile characteristics of muscles, like the conduction velocities 

 of nerves, represent a balance between functional requirements and 



