Terminals of Single Ia Fibers: Location, Density, and Distribution Within a Pool of 300 Homonymous Motoneurons LORNE M. MENDELL AND ELWOOD HENNEMAN Department of Physiology, Harvard Medical School, Boston, Massachusetts 02115 IT IS WELL KNOWN that the afferent fibers from primary endings in muscle spindles make direct connections with motoneurons which innervate the muscle of origin (21). In the case of the medial gastrocnemius (MG) muscle of the cat there are about 60 group Ia fibers (5) projecting to approxi- mately 300 alpha motoneurons (7). The precise distribution of the afferent termi- nals within this pool of motoneurons has never been established. In particular, the number of motoneurons receiving termi- nals from a single afferent fiber and the spatial extent of the terminal branching are unknown. The location and distribu- tion of terminals on the surface of indi- vidual motoneurons are equally uncertain. Limitations in existing anatomical tech- niques make it difficult to follow all the axonal ramifications of a single neuron in the central nervous system, especially if the branches are widely distributed. The num- ber of Ia fibers converging on a single moto- neuron has been estimated in anatomical studies (38, 39) but, as we have noted in a preliminary report (25), these estimates are much too low. We explored several approaches to this problem before adopting the one described below. In brief, we recorded the excitatory postsynaptic potentials (EPSPs) produced in motoneurons of the gastrocnemius mus- cle by impulses in a single afferent fiber (Ia) from that muscle. Since these individ- ual EPSPs were usually too small to dis- tinguish among the other spontaneously occurring responses of the motoneuron, an ;iveraging computer was used to summate Received for publication July 22, 1970. a few hundred of them. The presence or absence of a monosynaptic EPSP in a moto- neuron was used to determine whether or not the afferent fiber under study sent terminals to that particular cell. The am- plitude and time course of the EPSPs per- mitted inferences regarding the location of the active synapses. TERMINOLOGY Several kinds of EPSPs have been de- scribed by previous investigators. In order to prevent confusion in this paper we shall distinguish three types which may be re- corded in tracellularly in spinal motoneu- rons. 1. Unit EPSP will refer to an all-or-none potential resulting from release of one quantum of transmitter. The size of the unit EPSP may vary with the site of the synapse and the properties of the moto- neuron as Kuno and Miyahara (19, 20) demonstrated. 2. Individual EPSP will designate the response to a single afferent impulse in a group Ia fiber. An individual EPSP con- sists of a variable number of unit EPSPs. The term “unitary EPSP,” used in some recent publications, does not refer to unit EPSPs and should therefore be avoided. 3. Aggregate EPS,P will designate an EPSP evoked by a synchronous volley in all of the Ia fibers of a muscle nerve (7). It represents the sum of many individual EPSPs. METHODS Except as noted, experiments were carried out on adult cats anesthetized with pentobar- bital sodium (35 mg/kg intraperitoneally and 171 ownloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (130.063.180.147) on December 23, 201