NERVOUS SYSTEM AND BEHAVIOUR 



441 



and muscle membranes, and depolarizes the muscle end-plate. The 

 transient local depolarization of the latter is manifest as an end-plate 

 potential (e.p.p.). This is not itself propagated, but it electrically discharges 

 or stimulates a small surrounding region of muscle fibre, and so gives rise 

 to a propagated wave of muscular excitation. Crustacean muscle fibres 

 show a different functional pattern: here the end-plates are distributed 

 widely over each muscle fibre, and the numerous end-plate potentials often 

 produce local contractions, without propagated action potentials. 



Acetycholine exists in bound form at vertebrate motor nerve endings 

 and is released on the arrival of a nervous impulse. It is rapidly destroyed 

 (hydrolysed) by a specific enzyme, acetylcholinesterase, concentrated at 



c. br. m. 



com. ant. m 



n. Inf. ant 



Fig. 10.18. First-Order Giant Cell in the Brain of the Squid (Loligo pealei) 



ant. dendr., anterior dendrite of giant cell; ax., axon of giant cell; b, interaxonic 

 bridge; com. ant. m., commissura anterior magnocellularis ; c. br. m., brachio-magno- 

 cellular connective; lat. dend., lateral dendrites of giant cell; n, nucleus; n. inf. ant., 

 nervus infundibuli anterior; v. dend., ventral dendrites of giant cell. (From Young (132).) 



the end-plate, thus preparing the neuromuscular junction for the arrival 

 of a new impulse. Nerve fibres which release acetylcholine at their terminals 

 are designated cholinergic fibres. Apart from the end-plate of vertebrate 

 striated muscle, transmitter action by acetylcholine occurs in the heart 

 (vagus fibres), smooth muscle, peripheral ganglia and glands (autonomic 

 fibres), and in central synapses (spinal cord) (31, 32, 33, 34, 36). 



Adrenergic nerve fibres form another category and their chemical trans- 

 mitter is adrenaline or noradrenaline. Adrenergic nerve fibres supply 

 heart, smooth muscle, glands, chromatophores, etc. Still other transmitters 

 are suspected in invertebrates, e.g. 5-hydroxytryptamine as a neuro-cardiac 

 transmitter in molluscs (94#, 122). 



