Figure 2.

Single afferent responses from a muscle spindle in the extensor digitorum. The neural activity in a single primary afferent (type Ia afferent 5701) from the common finger extensor muscle during afferent-type classification maneuvers (A) and sinusoidal movements of the middle finger (B) in different contexts. In A, the unit is classed as a primary (type Ia) afferent. It responded to stretch and was silent during muscle shortening (left). It also responded during isometric contractions and with a burst of spikes during sudden relaxation (right; 2 repetitions shown). In B, the signals include the angle and angular velocity at the MCP joint, flexor, and extensor digitorum electromyography (EMG). Vertical gray bars in the background indicate periods of flexion (i.e., extensor digitorum stretch). The same viscous load was experienced across all active movement for damping any potential tremor at the finger and so promoting similar kinematics across conditions. Despite similar kinematics across the four contexts above, higher discharge rates were seen when the bias load induced higher levels of activity in the spindle-bearing extensor muscle (flexion assisted) compared with the no bias load condition (i.e., red condition in Fig. 1B) and vice versa when higher flexor activity was induced during extensor stretch (flexion resisted; far right column). Note that the extensor (i.e., spindle-bearing) muscle EMG levels in both the no bias load and flexor bias load condition (flexion resisted) were similar during periods of extensor stretch, indicated by the vertical gray bars.