<P> These time and length - scales can be used to understand the dependence of the conduction velocity on the diameter of the neuron in unmyelinated fibers . For example, the time - scale τ increases with both the membrane resistance r and capacitance c . As the capacitance increases, more charge must be transferred to produce a given transmembrane voltage (by the equation Q = CV); as the resistance increases, less charge is transferred per unit time, making the equilibration slower . In a similar manner, if the internal resistance per unit length r is lower in one axon than in another (e.g., because the radius of the former is larger), the spatial decay length λ becomes longer and the conduction velocity of an action potential should increase . If the transmembrane resistance r is increased, that lowers the average "leakage" current across the membrane, likewise causing λ to become longer, increasing the conduction velocity . </P> <P> In general, action potentials that reach the synaptic knobs cause a neurotransmitter to be released into the synaptic cleft . Neurotransmitters are small molecules that may open ion channels in the postsynaptic cell; most axons have the same neurotransmitter at all of their termini . The arrival of the action potential opens voltage - sensitive calcium channels in the presynaptic membrane; the influx of calcium causes vesicles filled with neurotransmitter to migrate to the cell's surface and release their contents into the synaptic cleft . This complex process is inhibited by the neurotoxins tetanospasmin and botulinum toxin, which are responsible for tetanus and botulism, respectively . </P> <P> Some synapses dispense with the "middleman" of the neurotransmitter, and connect the presynaptic and postsynaptic cells together . When an action potential reaches such a synapse, the ionic currents flowing into the presynaptic cell can cross the barrier of the two cell membranes and enter the postsynaptic cell through pores known as connexons . Thus, the ionic currents of the presynaptic action potential can directly stimulate the postsynaptic cell . Electrical synapses allow for faster transmission because they do not require the slow diffusion of neurotransmitters across the synaptic cleft . Hence, electrical synapses are used whenever fast response and coordination of timing are crucial, as in escape reflexes, the retina of vertebrates, and the heart . </P> <P> A special case of a chemical synapse is the neuromuscular junction, in which the axon of a motor neuron terminates on a muscle fiber . In such cases, the released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor, an integral membrane protein in the membrane (the sarcolemma) of the muscle fiber . However, the acetylcholine does not remain bound; rather, it dissociates and is hydrolyzed by the enzyme, acetylcholinesterase, located in the synapse . This enzyme quickly reduces the stimulus to the muscle, which allows the degree and timing of muscular contraction to be regulated delicately . Some poisons inactivate acetylcholinesterase to prevent this control, such as the nerve agents sarin and tabun, and the insecticides diazinon and malathion . </P>

Which of the following is not a general characteristic of an action potential