Henneman's size principle  Henneman’s size principle describes relationships between properties of motor neurons and the muscle fibers they innervate and thus control, which together are called motor units. Motor neurons with large cell bodies tend to innervate fast-twitch, high-force, less fatigue-resistant muscle fibers, whereas motor neurons with small cell bodies tend to innervate slow-twitch, low-force, fatigue-resistant muscle fibers. In order to contract a particular muscle, motor neurons with small cell bodies are recruited (i.e. begin to fire action potentials) before motor neurons with large cell bodies. It was proposed by Elwood Henneman. At the time of Henneman’s initial study of motor neuron recruitment, it was known that neurons varied greatly in size, that is in the diameter and extent of the dendritic arbor, size of the soma, and diameter of axon. However, the functional significance of neuron size was not yet known. In 1965, Henneman and colleagues published five papers describing the firing patterns of motor neurons innervating two muscles in a cat's leg, the soleus muscle and the gastrocnemius muscle (the "calf" in a cat's hindleg). The soleus muscle is composed of "red" muscle which was revealed to indicate that muscle fibers were fatigue-resistant but created small forces when contracting. The gastrocnemius muscle is heterogeneous, composed of both "red" and "pale" muscle, and thus containing fast-twitch high force fibers. Henneman's and colleagues took advantage of the differences between the soleus and gastrocnemius muscles to show that the neurons innervating the soleus muscle:  Together, these relationship were termed the "size principle". Decades of research elaborated on these initial finding on motor neuron properties and recruitment of motor units (neuron + muscle fibers), and the relationship between neuron excitability and its size became a central focus of neurophysiology. A neuron's size is related to its electrical excitability, and so it was hypothesized that neuron size was the causal mechanism for the recruitment order. An alternative hypothesis is that the structure of spinal circuits and inputs to motor neurons controls recruitment. Both likely contribute and reflect the astounding coordinated development of neural circuit and cellular properties in motor neurons and muscle. A relationship between force production and recruitment order is a common feature across motor systems (vertebrates examples: human, cat, zebrafish; invertebrate examples: stick insect, drosophila, crayfish).