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Related Concept Videos

Smooth Muscle Contraction01:25

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Smooth muscle contraction is a complex process vital for various bodily functions, from maintaining blood vessel tension to facilitating the movement of food through the digestive tract. Unlike striated muscles, smooth muscle contraction begins more slowly and lasts longer.
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Somatic spinal reflexes are rapid, involuntary muscular responses to external stimuli that involve the somatic musculature and the spinal cord.
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When the neuron of a motor unit fires an action potential, it triggers a series of events, leading to a twitch contraction in the muscle fibers. The process of excitation-contraction coupling is crucial in relaying the action potential to the muscle fibers.
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Skeletal muscle cells, also called muscle fibers, are distinctly elongated, multi-nucleated, slender biological units. They are packed with specialized structures designed to facilitate their primary function, which is contraction.
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Fascicles are bundles of muscle fibers in a skeletal muscle. Muscle fascicle arrangement is directly associated with the power and range of motion of various muscles. The configuration of these fascicles can vary, leading to different functional outcomes.
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Related Experiment Video

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The evolution of muscle spindles.

Robert W Banks1, Uwe Proske2

  • 1Department of Biosciences, and Biophysical Sciences Institute, Durham University, Durham, UK.

Experimental Physiology
|November 4, 2024
PubMed
Summary

Muscle spindles, crucial for movement and sensation, likely evolved in early amniotes, with similar structures later appearing in amphibians and birds independently. Their complex functions in mammals evolved through convergent evolution, particularly in endothermic species.

Keywords:
collateral motor innervationfusimotor innervationintrafusal fibresmyotatic reflexprimary endingsecondary endingspindle capsulestretch receptorstetrapods

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Area of Science:

  • Neuroscience
  • Evolutionary Biology
  • Comparative Anatomy

Background:

  • Muscle spindles are stretch-sensitive sensory receptors in skeletal muscles.
  • They uniquely regulate their sensitivity via intrafusal muscle fiber contraction.
  • Spindles have diverse roles in posture, locomotion, proprioception, and wound healing.

Purpose of the Study:

  • To investigate the evolutionary origins and diversification of muscle spindles.
  • To explore the functional significance of spindle structure and innervation across vertebrate groups.
  • To understand the role of convergent evolution in the development of sophisticated motor control.

Main Methods:

  • Comparative analysis of muscle spindle structure and innervation across vertebrates.
  • Phylogenetic analysis to infer evolutionary relationships and origins.
  • Review of existing literature on muscle spindle function and evolution.

Main Results:

  • Muscle spindles likely first appeared in early amniotes, with simpler forms evolving later in amphibians and reptiles.
  • Amphibian and reptile spindle sensitivity is controlled by motor axons supplying extrafusal muscle.
  • Birds, like mammals, possess dedicated fusimotor innervation, indicating convergent evolution.
  • Endothermy may have facilitated the development of complex motor control and spindle sophistication in birds and mammals.

Conclusions:

  • The evolution of muscle spindles involved early origins in amniotes and subsequent convergent evolution in other lineages.
  • Convergent evolution, particularly in endothermic vertebrates, has led to sophisticated motor control mechanisms.
  • Understanding spindle evolution provides insights into the development of complex sensory-motor systems.