Properties of motor units in the transversus abdominis muscle of the garter snake.

Abstract

1. The organization of motor units in the single‐fibre‐thick transversus abdominis muscle of the garter snake has been studied. This small segmental muscle (60‐100 fibres) contains three distinct fibre types (faster twitch, F; slower twitch, S; and tonic, T) which are predominantly arranged in the repeating pattern F, T, S, T, F, T, etc. 2. Motor‐unit maps were obtained by activating an individual motor axon and identifying all of the muscle fibres innervated by that axon, using either the activity‐induced uptake of extracellular marker molecules to label presynaptic terminals of the stimulated axon, or systematic intracellular recording to identify muscle fibres activated by the axon. 3. Each muscle contained three types of motor units (F, T and S) that corresponded to the three types of fibres. All of the muscle fibres in a motor unit were of the same type. Each segmental muscle contained approximately the same number of motor units: one to two faster twitch, three to four slower twitch, and three tonic. 4. Each motor unit was dispersed widely throughout the muscle. Fibres within a motor unit were neither clustered nor anticlustered. This suggests that despite the fact that axons are constrained to innervate fibres of the appropriate type, the distribution of each motor unit does not generate the alternating pattern of fibre types. 5. In several experiments, all of the twitch motor units in one segmental component of the muscle were mapped. The projection of any one axon appeared random not only with respect to the muscle's fibre type pattern, but also with respect to the innervation supplied by other axons. 6. Twitch motor units were arranged according to a hierarchy of sizes. In each muscle examined, the largest motor unit was faster twitch; a single faster twitch motor axon usually innervated all of the faster twitch fibres in the muscle (fourteen to twenty‐four fibres). This was followed by three to four slower twitch motor units which varied in size from eight to ten fibres to very small motor units containing only four to five fibres. 7. Each tonic motor axon innervated an average of ninety‐three end‐plates per segmental muscle. The relatively large size of tonic motor units compared to twitch motor units is related to the ability of tonic muscle fibres to retain polyneuronal innervation into adulthood, both by providing five to seven end‐plate sites per fibre, and by allowing terminal boutons from different tonic motor axons to co‐innervate the same end‐plate.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle.

“…In the present study, we have utilized a means of motor unit identification totally independent of glycogen depletion. The thin transversus abdominis muscle of the garter snake provides the opportunity to directly identify motor unit fibres and to study several motor units independently in one muscle (Wilkinson & Lichtman, 1985;Lichtman & Wilkinson, 1987). Furthermore, the preparation allows physiological study of the individual fibres within motor units.…”

Metabolic capacity and myosin expression in single muscle fibres of the garter snake.

“…The muscle's approximately twenty-five type S fibres comprise four to five motor units which vary systematically in size, similar to the motor unit arrangement of mammalian muscle (Lichtman & Wilkinson, 1987; see also Hammond & Ridge, 1978). Enzyme content and twitch time-to-peaks are similar among fibres belonging to one motor unit, and differ systematically among different size motor units in one muscle (Nemeth et al 1991).…”

“…Three contiguous transversus abdominis muscles were dissected from the animal and placed in reptilian saline solution (see Lichtman & Wilkinson, 1987, for details of the dissection procedure). Identification of fibres was performed at room temperature; muscles were then quick-frozen for subsequent biochemical analyses (see below).…”

“…Thus, although the intrinsic functional capacity of S fibres is limited by the range of metabolic and contractile proteins expressed, properties of a particular fibre within this range are determined by the innervating motoneuron. In contrast, the approximately twentyfive type F fibres form either a single large motor unit or two units with similar contraction times (Lichtman & Wilkinson, 1987;R. S. Wilkinson, unpublished (Lichtman & Wilkinson, 1987 only the S fibre population is subdivided into motor units of systematically differing properties.…”

“…In contrast, the approximately twentyfive type F fibres form either a single large motor unit or two units with similar contraction times (Lichtman & Wilkinson, 1987;R. S. Wilkinson, unpublished (Lichtman & Wilkinson, 1987 only the S fibre population is subdivided into motor units of systematically differing properties. Interestingly, only S fibres contained two myosin heavy chains, providing them with a simple mechanism for neural control of contractile properties.…”