<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> <P> The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual . This plateau is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated . </P> <P> The cardiac action potential plays an important role in coordinating the contraction of the heart . The cardiac cells of the sinoatrial node provide the pacemaker potential that synchronizes the heart . The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles . Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers . Conversely, anomalies in the cardiac action potential--whether due to a congenital mutation or injury--can lead to human pathologies, especially arrhythmias . Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, and verapamil . </P>

Where do action potentials occur in a neuron