1. A method of isolation of individual afferent fibers from muscle has yielded a representative sample of the fibers which comprise groups 1 (12 to 20 µ) and II (4 to 12 µ) of the afferent fiber diameter distribution of muscle nerves in cat.
2. Afferent fibers from muscle stretch receptors account for groups I and II of the afferent diameter spectrum of muscle nerves to soleus and medial gastrocnemius. Fibers from tendon organs are largely confined to the diameter range above 12 µ. This fiber group, which has a simple one-peak diameter distribution, is termed group IB. Fibers from muscle spindles show a bimodal diameter distribution and account for the remainder of fibers in the 12 to 20 µ group (termed IA) and substantially all of group II (4 to 12 µ).
3. No significant difference has been found in the receptor characteristics of the large (group IA) and intermediate sized (group II) spindle afferent fibers other than a slightly higher threshold of the latter to steady external stretch.
The analysis of the behaviour of individual sensory fibres was begun by Adrian over twenty years ago and was extended by Bronk, Zotterman, Matthews, and others. A requisite to such analysis was the recording from single fibre preparations. By this technique, Matthews made a detailed study of stretch receptors in muscles of the frog (1931 a, b) and of the cat (1933). In the latter paper he described the response patterns of different types of receptors to external stretch and to contraction of the muscle. This classical paper provides the background for the present study. Matthews (1933) found that the individual stretch receptors in the cat fell into two principal categories: (1) the A fibres (A1 and A2) which under most conditions showed a slowing or cessation of their discharge rate during contraction; and (2) the B fibres which exhibited an acceleration of their discharge rate during contraction. He concluded that A fibres come from endings located in the muscle spindles, their decreased response during contraction being due to a lessened stretch on the spindle when the surrounding muscle fibres contracted. The B fibres he regarded as originating in the tendon organs, their behaviour indicating that they were further stretched during contraction. Fulton and Pi-Suiner (1928) had predicted such an 'in series' and 'in parallel' behaviour of receptors from tendon organs and from muscle spindles on the basis of the 'silent period' of the muscle during the stretch reflex.In a recent study (Kuffler, Hunt & Quilliam, 1951) of the function of small diameter efferent fibres in the lumbosacral ventral roots of cats, it was found that stimulation of these fibres increased the afferent discharge from A-type receptors in muscle, and it was concluded that these efferent fibres excited contractile elements within the muscle spindle, the intrafusal muscle fibres. Accordingly, the muscle spindle discharge is influenced by external stretch and the various modifications of stretch during contraction, as well as by the nervous mechanism of the efferent small-nerve fibres.
A B S T R A C T The effect of changing the ionic composition of bathing fluid on the receptor potential of primary endings has been examined in isolated mammalian spindles whose capsule was removed in the sensory region. After impulse activity is blocked by tetrodotoxin, ramp-and-hold stretch evokes a characteristic pattern of potential change consisting of a greater dynamic depolarization during the ramp phase and a smaller static depolarization during the hold phase. After a high-velocity ramp there is a transient post-dynamic undershoot to below the static level. On release from hold stretch, the potential shows a postrelease undershoot relative to base line. The depolarization produced by stretch is rapidly decreased by the removal of Na + and Ca 2+. Addition of normal Ca 2+ partly restores the response. Stretch appears to increase the conductance to Na + and Ca 2+ in the sensory terminals. The postdynamic undershoot is diminished by raising external K + and blocked by tetraethylammonium (TEA). It apparently results from a voltage-dependent potassium conductance. The postrelease undershoot is decreased by raising external K +, but is not blocked by TEA. It is presumably caused by a relative increase in potassium conductance on release. Substitution of isethionate for CI-or the addition of ouabain does not alter the postdynamic and postrelease undershoots.
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