The influence of extrafusal muscle activity on discharge patterns of primary muscle spindle endings

The influence of extrafusal muscle activity in anaemically decerebrate cats upon discharge patterns of primary spindle endings was ascertained by simultaneously recording spike trains from several Ia afferents and muscle tension fluctuations of the triceps-surae muscle. 1. Tension fluctuations were averaged with respect to spikes from primary endings yielding tension "trajectories" of specific shape for each spindle and probably reflecting frequently recurring mechanical events in the spindles' surroundings. 2. Spindles situated in close vicinity and influenced by similar mechanical events as evidenced by similar average tension trajectories are correlated in their discharge patterns to a degree depending on the strength of their mechanical coupling. 3. The modulation of spindle discharge frequency in response to average tension changes at the muscle tendon is very different in amplitude for different spindles; this response may show a high sensitivity. It is usually phase advanced by 90-180 degrees with respect to the "internal length" changes; between spindles ther may be phase differences of up to 180 degrees. 4. It is concluded that primary endings react very sensitively to local extrafusal events. The CNS receives much more accurate information about these events in the correlation of several Ia afferents than in the discharge of a single fibre.