Studies on the site of termination of static and dynamic fusimotor fibres within muscle spindles of the tenuissimus muscle of the cat.

1. The site of termination of static and dynamic fusimotor fibres has been mapped by finding which intrafusal muscle fibres have been depleted of glycogen as a result of tetanic stimulation of single gamma fibres. Long periods of stimulation coupled with occlusion of the blood supply were necessary to cause glycogen depletion.2. In cat tenuissimus muscle, dynamic gamma motor fibres always activated bag intrafusal muscle fibres, and occasionally chain fibres. Static gamma fibres always activated chain fibres and frequently activated bag fibres as well.3. It is argued that these results can be fitted into the hypothesis of the mechanism of internal functioning of the spindle originally proposed by Jansen & Matthews (1962). It is also pointed out that the results raise problems concerning the mechanism of development of the spindle motor innervation.