muscle spindle

Examples of muscle spindle in the following topics:

• Muscle Tone

  • The main regulator of muscle tone is the muscle spindle, a small sensory unit that is closely associated with and lies parallel to a muscle.
  • Connecting to the endomysium of a muscle fiber, muscle spindles are composed of nuclear bag fibers and nuclear chain fibers.
  • Both cells of the muscle spindle contain sensory neurons.
  • If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction.
  • Smooth and cardiac muscles do not have specialized muscle spindles.

• Classification of Receptors by Location

  • While the cutaneous touch receptors found in the dermis and epidermis of our skin and the muscle spindles that detect stretch in skeletal muscle are both mechanoreceptors, they serve discrete functions.
  • In both cases, the mechanoreceptors detect physical forces that result from the movement of the local tissue, cutaneous touch receptors provide information to our brain about the external environment, while muscle spindle receptors provide information about our internal environment.
  • Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and an expanded schematic (right).
  • The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers.

• Proprioceptive Sensations

  • Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of a muscle.
  • The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles.
  • The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers onto the tendons of skeletal muscle.
  • Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right).
  • The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers.

• Hypotonia and Hypertonia

  • Hypertonia is the reduced ability of muscles to stretch due to increased muscle tension; hypotonia, due to chronic reduced muscle tension.
  • Hypertonia is a reduction in the ability of a muscle to stretch due to increased muscle tension; it is caused by lesions to upper motor neurons.
  • Hypotonia is the state of reduced muscle tone and tension, resulting in lessened ability to generate force from muscle contractions.
  • Hypotonia is thought to be associated with the disruption of afferent input from stretch receptors and/or lack of the cerebellum's facilitatory efferent influence on the fusimotor system (the system that innervates intrafusal muscle fibers thereby controlling muscle spindle sensitivity ).
  • A muscle spindle, with γ motor neurons, sensory fibers and proprioceptor that detect the amount and rate of change of length in a muscle.

• Classification of Nerves

  • Efferent nerves conduct signals away from the central nervous system to target muscles and glands.
  • Mixed nerves contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same nerve bundle.
  • A-alpha fibers are the primary receptors of the muscle spindle and golgi tendon organ.
  • A-beta fibers act as secondary receptors of the muscle spindle and contribute to cutaneous mechanoreceptors.
  • A-gamma fibers are typically motor neurons that control the intrinsic activation of the muscle spindle.

• Spinal Reflexes

  • The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle.
  • When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases.
  • The tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.
  • In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by a passive stretch or muscular contraction.
  • The tendon organ is a stretch receptor that signals the amount of force on the muscle and protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load.

• Components of a Reflex Arc

  • A reflex arc defines the pathway by which a reflex travels—from the stimulus to sensory neuron to motor neuron to reflex muscle movement.
  • The relay neuron in turn makes a synapse with one or more motor neurons that transmit the impulse to the muscles of the limb causing them to contract and pull away from the sharp object .
  • There are two types of reflex arcs:the  autonomic reflex arc, affecting inner organs, and the somatic reflex arc, affecting muscles.
  • In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in the contraction of the agonist or effector muscle.

• Types of Muscle Tissue

  • The function of muscles is movement, but the types of movement elicited differ between skeletal, cardiac, and smooth muscle.
  • Skeletal muscle fibers are the longest muscle fibers and have stripes on their surface.
  • The individual smooth muscle fibers are spindle shaped and contain a centrally located nucleus.
  • Although cardiac muscle is involuntary in nature, it is structurally different from smooth muscle.
  • Cardiac muscle is striated, similar to skeletal muscle, but beats involuntarily.

• How Skeletal Muscles Are Named

  • Most skeletal muscles in the body are parallel muscles; although they can be seen in a variety of shapes such as flat bands, spindle shaped, and some can have large protrusions in their middle known as the belly of the muscle.
  • Fusiform muscles are more spindle shaped (their diameter at the center is greater than at either end), whereas, non-fusiform muscles are more rectangular with a constant diameter.
  • In Pennate muscles, the tendon runs through the length of the muscle.
  • However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
  • If the central tendon branches within a pennate muscle, the muscle is called multipennate.

• Characteristics of Muscle Tissue

  • Some skeletal muscle can attach directly to other muscles or the skin, as seen in the face where numerous muscles control facial expression.
  • As with skeletal muscle cardiac muscle is striated, however it is not consciously controlled and so is involuntary.
  • Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
  • Cardiac and skeletal muscle are both striated in appearance, while smooth muscle is not.
  • Both cardiac and smooth muscle are involuntary while skeletal muscle is voluntary.