438 THE BIOLOGY OF MARINE ANIMALS 



pared with slow neuropile conduction. A second common feature of giant- 

 axon systems is that one or a few fibres serve large muscle masses and, 

 because of fast conduction speeds, they produce nearly synchronous muscu- 

 lar contractions. An interesting adaptation is described in the squid where 

 the giant fibres to the mantle are graded in diameter. The arrangement is 

 such that the largest fibres generally form the longest pathways, and con- 

 duction rates and transmission distances are so adjusted that contraction 

 of the whole mantle sac takes place nearly simultaneously (80, 132). 



JUNCTIONAL TRANSMISSION 



There is now convincing evidence, both histological and physiological, 

 that the junctions between neurones, and between neurones and effectors, 

 are regions of protoplasmic and functional discontinuity. This concept is, 

 in fact, necessary to explain the complex and selective nature of nervous 

 activity. The transmission of an impulse along a nerve fibre results from 

 self-propagation of electrotonic potential and, on arriving at the terminals 

 of the fibre, the excitatory state in some way activates the next unit in 

 series, either another neurone or an effector cell. 



Synaptic Organization 



Synapses are found in all nervous systems. In the nerve-nets of coelen- 

 terates axons intertwine or run alongside each other for short distances, 

 forming functional junctions. These synapses are structurally non- 

 polarized since equal axonal areas are in contact with each other (Fig. 

 10.16). Experiments show that conduction is diffuse in these nerve-nets 

 and that two-way transmission must take place across the synapses, i.e. 

 they are functionally non-polarized as well. 



Corresponding types of junctions are found in other groups, e.g. anne- 

 lids and crustaceans. The two giant axons of sabellids and serpulids form 

 an unpolarized synapse in the brain, where they cross over and make con- 

 tact with each other. The synapse is unpolarized, allowing two-way trans- 

 mission, and is of the relay type, an impulse in one fibre (pre-synaptic) 

 evoking a corresponding impulse in the other. When electrical activity at 

 the junctional region is recorded, pre- and post-synaptic action potentials 

 are seen, as well as a local synaptic potential (Fig. 10.17). Synaptic delay 

 is of the order of 0-8 msec (Protula). The local response is propagated as an 

 electrotonic potential for short distances : in Protula it shows decrement to 

 half amplitude in 3 mm. From the local response a propagated all-or- 

 nothing post-synaptic potential arises. In the nereid Neanthes virens large 

 overlapping septa or macrosynapses occur at regular intervals in the lateral 

 giant fibres of the nerve cord, and these junctions allow conduction in 

 either direction. Oblique partitions are also found in the lateral giant fibres 

 of the prawn (Palaemon) and other decapod crustaceans. These synapses 

 are non-polarized and permit two-way conduction with very slight delay 

 (about 0T msec). 



