They have a neurogenic heart, which has rhythmic depolarization that is responsible for initiating heartbeats. Heartbeats originate in nervous tissue; innervated muscle cells cause the heart to contract when stimulated by nerve impulses. The cardiac ganglion, which consists of nine neurons, attaches to the dorsal wall of the heart. The anterior neurons innervate the heart, whereas the other posterior neurons make synaptic contact with those anterior neurons. The posterior neuron acts as the pacemaker but also functions as the cellular oscillator and the central pattern generator. This posterior neuron produces a train of impulses, which excites the other posterior neurons. The heart contracts when the posterior neurons activate the five anterior neurons, which send impulses to the muscle cells. This is how the Frank–Starling mechanism works within crustaceans. The Frank-Starling mechanism refers to the vitally important intrinsic control of the heart; mainly, the stretching of the cardiac muscle tends to increase the force of its contraction by an effect at the cellular level. This mechanism is important as it allows the organism to match its output of blood with its input of blood. Because of the stretching between beats, the Frank-Starling mechanism allows the heart to then naturally contract more forcefully, allowing greater flow of blood, which results in the matched heart output to the increased blood received. The Frank-Starling mechanism is a little different in crustaceans, as it involves the cardiac ganglion as described previously. The stretching of the heart induces the ganglion to fire more regularly and powerfully.