NERVOUS SYSTEM AND BEHAVIOUR 417 



NERVOUS TRANSMISSION 



Nerve cells (neurones) consist of a cell body and fibres, the latter sometimes 

 separable as dendrites and axons (Fig. 10.2). Dendrites conduct impulses 

 towards the cell, axons away from the cell. Some types of neurones lack 

 dendrites, e.g. primary sensory cells in which the impulse originates in the 

 cell body, and certain ganglion cells where axons terminate directly on the 

 cell body. Unipolar neurones are widespread in invertebrates: each cell 

 gives rise to a single process which then subdivides; synapses occur be- 

 tween nerve fibres, and the cell body functions as a trophic centre outside 

 the main path of transmission (Figs. 10.12, 10.14). In nervous systems 

 neurones are arranged in complex patterns. From the receptors sensory 

 impulses are fed into the nervous system; impulses pass from neurone to 

 neurone across interneural boundaries (synapses), and proceed along 

 efferent axons towards the effector organs (129, 130). 



When a nervous impulse proceeds along a nerve fibre it is accompanied 

 by a wave of surface depolarization, which is revealed as a transitory rise 

 of electrical potential known as the action potential. This takes the form of 

 a negative spike potential which rises rapidly to a maximum and then 

 decays at a decreasing rate. The action potential is all-or-none in nature, 

 i.e. the height of the spike is unaffected by the intensity of the stimulus 

 above threshold. Immediately after excitation or the passage of an impulse 

 the fibre is inexcitable and refractory to another impulse. This phase is 

 followed by a relative refractory period of reduced excitability, and later 

 by a stage of heightened excitability, the supernormal phase. 



A quiescent nerve fibre possesses a resting potential which depends on 

 differences in the distribution of ions across the fibre-membranes. Analyses 

 of axoplasm have revealed high internal concentrations of K , amounting 

 to about 360 mM in cuttlefish giant axons, and low levels of Na + and 

 Cl~ (Table 2.8). The low internal chloride is balanced by an accumulation 

 of large organic ions within the fibre. The interior of the fibre is negative 

 to the exterior, and this potential difference is due to the high concentration 

 gradient of potassium across the membrane. By the use of micro-electrodes 

 inserted into squid giant axons, direct measurements have been made of 

 resting and action potentials. At rest the potential is some 60 mV negative; 

 during activity the inside of the membrane becomes about 50 mV positive 

 (the action potential) and consequently reverses in sign (Fig. 10.3). 



When the fibre-membrane is depolarized a brief change occurs in mem- 

 brane permeability. There is initially a large increase of sodium permea- 

 bility, and sodium ions rapidly penetrate into the fibre. Near the peak of 

 the action potential a change occurs from high sodium to potassium 

 permeability. According to recent hypothesis the first stage of the action 

 potential consists of a flow of capacity current. The rising phase of the 

 action potential is produced by an inward surge of sodium ions, the falling 

 phase by an accelerated outward movement of potassium ions, which 

 restores the resting potential. The immediate source of energy for the 



M.A. — 14 



