Journal of Neurophysiology, Vol 52, Issue 3 410-420, Copyright © 1984 by APS

ARTICLES

Recruitment order of motoneurons in stretch reflexes is highly correlated with their axonal conduction velocity

P. Bawa, M. D. Binder, P. Ruenzel and E. Henneman

Motor units of soleus and medial gastrocnemius (MG) muscles were studied in pairs during stretch reflexes in the decerebrate cat to determine the relation between their recruitment orders and axonal conduction velocities. In 97% of soleus pairs, the motor unit with the lower axonal conduction velocity was recruited first. Since the soleus is a homogeneous muscle in the cat, differences in motor-unit type are, therefore, not a sine qua non for orderly recruitment nor is recruitment random within homogeneous populations of motor units, as recently proposed (28). In the medial gastrocnemius, a heterogeneous muscle, the same high correlation (97%) between recruitment sequence and conduction velocity was observed. Thus, the factors that determine recruitment order in heterogeneous muscles are as closely correlated with axonal diameter as they are in homogeneous muscles. Comparison of axonal conduction velocities in our sample of MG units with those in three samples of type-identified MG units studied by other investigators also suggests that motor-unit type is not the critical factor controlling the sequence of activation in heterogeneous muscles. It is concluded that the combined effects of all presynaptic and postsynaptic factors that determine susceptibility to discharge in motoneurons during stretch reflexes are strictly correlated with their axonal conduction velocities, as predicted by the size principle.

This article has been cited by other articles:

				E. F. Hodson-Tole and J. M. Wakeling Motor unit recruitment patterns 1: responses to changes in locomotor velocity and incline J. Exp. Biol., June 15, 2008; 211(12): 1882 - 1892. [Abstract] [Full Text] [PDF]

				A. A. V. Hill and D. Cattaert Recruitment in a heterogeneous population of motor neurons that innervates the depressor muscle of the crayfish walking leg muscle J. Exp. Biol., February 15, 2008; 211(4): 613 - 629. [Abstract] [Full Text] [PDF]

				A Kuppuswamy, S Theodorou, M Catley, P H Strutton, P H Ellaway, A H McGregor, and N J Davey Motor neurone excitability in back muscles assessed using mechanically evoked reflexes in spinal cord injured patients J. Neurol. Neurosurg. Psychiatry, September 1, 2005; 76(9): 1259 - 1263. [Abstract] [Full Text] [PDF]

				A. M. Pastor and D. Gonzalez-Forero Recruitment Order of Cat Abducens Motoneurons and Internuclear Neurons J Neurophysiol, October 1, 2003; 90(4): 2240 - 2252. [Abstract] [Full Text] [PDF]

				A. J. Sokoloff, S. G. Siegel, and T. C. Cope Recruitment Order Among Motoneurons From Different Motor Nuclei J Neurophysiol, May 1, 1999; 81(5): 2485 - 2492. [Abstract] [Full Text] [PDF]

				R. H. Lee and C. J. Heckman Bistability in Spinal Motoneurons In Vivo: Systematic Variations in Rhythmic Firing Patterns J Neurophysiol, August 1, 1998; 80(2): 572 - 582. [Abstract] [Full Text] [PDF]

				T. C. Cope, A. J. Sokoloff, S. M. Dacko, R. Huot, and E. Feingold Stability of Motor-Unit Force Thresholds in the Decerebrate Cat J Neurophysiol, December 1, 1997; 78(6): 3077 - 3082. [Abstract] [Full Text] [PDF]