The Efferent Regulation of the Muscle Spindle in the Frog

ABSTRACT
Matthews’s experiments (1931a, b) have established that the muscle spindle of the frog contains a sense organ which responds to stretch and ceases to respond when the surrounding muscle fibres shorten. In addition, the spindle is known to possess a motor supply of its own, a bundle of small striated ‘intrafusal’ muscle fibres. These fibres run through the spindle capsule (Barker, 1948 ; Katz, 1949, fig. 1) where they establish intimate contact with the sensory nerve, while being separated by a lymphatic space from the ‘extrafusal’ tissue. These muscle fibres are said to lose their striations at the region of contact with the sensory terminals. The anatomical arrangement suggests that the intrafusal muscle bundle pulls directly on the stretch receptor, in contrast with the ordinary muscle fibres which will release the tension on the sense organ if they shorten. A contraction of the intrafusal muscle fibres, therefore, must affect the response of the stretch receptor in a manner very different from that of the surrounding mass of ‘extrafusal’ fibres. The function of the intrafusal muscle fibres is believed to be a regulation of mechanical ‘bias’ on the sense organ, but little is known concerning their motor innervation and the physiological conditions under which they are thrown into activity.