A comparative analysis of the encapsulated end‐organs of mammalian skeletal muscles and of their sensory nerve endings

Abstract

SummaryThe encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end-organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle-spindle capsules; however, information on the other types of end-organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle-specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver-impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle-spindle afferents in hind-limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind-foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike-initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first-order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind-foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo-distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle-specific variability. Finally, we present some comparative data on muscle-spindle afferent complements of rat, rabbit and guinea pig, on...

Permanent central synaptic disconnection of proprioceptors after nerve injury and regeneration. II. Loss of functional connectivity with motoneurons

“…We estimate that the number of IA afferents, i.e., primary-ending afferents (see METHODS), in rat hindlimb muscles equals the number of spindle receptors (20) found in the MG muscle of normal Wistar rats (Haftel et al 2005;Sekiya et al 1986), although the IA afferent-to-spindle receptor ratio may be greater in rodents (Banks et al 2009). Among the single *IA afferent-motoneuron pairs sampled from control rats in the present study 93% produced STA-EPSPs, so we estimate that each one of 20 IA afferents transmits synaptic excitation to nearly every one of about 100 ␣-motoneurons in the MG motor pool (Hashizume et al 1988;Vanden Noven et al 1993).…”

The Proprioceptive Senses: Their Roles in Signaling Body Shape, Body Position and Movement, and Muscle Force

“…All these tissues are innervated by mechanically sensitive receptors, and their density varies across muscles and regions of the body (e.g., Ref. 16). The question arises, which are the principal kinesthetic receptors?…”

Innervation of Partially Inserted Human Tendons Used to Reconstruct Anterior Cruciate Ligament

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