when the a motoneurons fire, which causes the intrafusal muscle fibers also to shorten and again pick up some degree of tension, restoring the output of the spindle and its sensitivity to small length changes. The CNS can modulate the y fusimotor fiber activity and thus adjust the spindle sensitivity to Ax and dx/dt at a given muscle length (xo). The ys activity alters the sensitivity to Ax on the group II afferent, and yd input affects the type Ia fiber response to dx/dt. More will be said of this control system below.

The number of spindles per muscle volume varies considerably with the type of muscle. For the cat, Granit (1955) gives 45 spindles for the big gastrocnemius medalis, vs. 56 and 57 in the considerably smaller soleus and tibialis anterior muscles, respectively. The number of spindles may be determined by the fineness of motor control required by the muscles considered. If this is true, then muscles associated with fine hand movements and movements of the tongue ought to have a larger density of spindles than skeletal muscles associated with maintaining posture. Primate and ungulate extraocular muscles have very high densities of spindles, as do neck muscles. The role spindles in extraocular muscles and in neck muscles in controlling the tracking of visual objects has yet to be described.

Two events stimulate the output of the primary, Ia, afferent fiber of a spindle: (1) an increase in the length of the extrafusal muscle, causing stretch of the sensory endings; and (2) at constant extrafusal muscle length, stimulation of the fusimotor (y) motor efferent fibers, which causes the intrafusal muscle fibers to shorten, creating tension. Both events stretch the sensory endings in the centers of the intrafusal fibers. Spindles only fire for increasing intrafusal fiber tension. The type II afferents fire with a frequency proportional to extrafusal muscle length; type Ia fiber frequency strongly depends on stretch velocity, with only a small length component.

There are many experimental scenarios possible in the study of spindle dynamics. Note that a spindle is a three-input [x(t), yd and ys activity], two output [Ia and II afferent fiber frequencies], nonlinear system. To begin with, one can do an open-loop experiment with an isolated spindle.The extrafusal muscle is stretched while keeping yd and ys activity at zero and recording from the Ia and II afferent fibers. Such a scenario is shown schematically in Figure 2.3-5A and B. The stretch is repeated with only the dynamic, yd, motoneuron stimulated. Note that the increased tension on the dynamic nuclear bag fiber produces a significantly enhanced response on the Ia sensory neuron for the same stretch. There is also a slightly enhanced type II sensory fiber response as well.

A second experiment on the isolated (open-loop) spindle involves stimulation of the nuclear chain fibers via the ys intrafusal motoneuron, while recording from the type Ia and type II afferents and stretching the extrafusal muscle. The results of this protocol are illustrated schematically in Figure 2.3-6A and B. The chain fibers bias the discharge of the type Ia and II afferent fibers, alter length sensitivity of the type Ia fibers, and increase the length sensitivity of the type II fibers.

In lower vertebrates such as amphibians, the y fusimotor fibers are simply branches from the a motoneurons. In this basic scheme, the intrafusal muscle fibers are stimulated and shorten along with the skeletal muscles, maintaining their sensitivity to Ax and dx/dt; there is no feedback per se. In mammals, as has been seen,

Secondary ending (n) response with fi stimulation r (instantaneous frequency) -100 pps

Secondary ending (n) response with fi stimulation

Secondary ending (Q) response without yd stimulation

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