Muscle Spindles
Search Strategy[edit | edit source]
The following databases have been used to found information about 'Muscle Spindles':
- Pubmed.
- Web of Knowledge.
- PEDro.
The following search terms have been used:
- Muscle spindles.
- Proprioception.
- Kineasthetic.
- Muscle spindles AND proprioception.
Definition[edit | edit source]
Muscle spindles are skeletal muscle sensory organs that contribute to fine motor control and provide axial and limb position information to the central nervous system. They are involved in the sensation of position and movement of the body, this is called proprioception
Composition[edit | edit source]
Muscle spindles are small sensory organs, with an elongated shape. A muscle spindle contains intrafusal fibers, those are several small, specialized muscle fibers. Intrafusal muscle fibers are oriented parallel to the regular, power-producing extrafusal muscle fibers. Intrafusal muscle fibers are at both ends connected to either tendinous ligaments or extrafusal fibers, namely contractile proteins. So, intrafusal fibers are stretched or shortened correspondingly, when extrafusal fibres change length. The central part of the muscle spindle is covered with a capsule of connective tissue. The sensory dendrites of the muscle spindle afferent wrap the central region. The muscle spindle is stretched when the muscle lengthens increase, this opens mechanically-gated ion channels in the sensory dendrites. This leads to a receptor potential that triggers action potentials in the muscle spindle afferent. [2,5]There can be found two types of sensory endings in muscle spindles: the primary and secondary endings of spindles, they are located in the middle of the spindle. The primary endings respond to its speed and the size of a muscle length change. They belong to the fastest axons, because they are myelinated. They contribute both to movement and the sense of limb position. Secondary endings are only sensitive to length and not to velocity, so contribute only to the sense of the position. These endings have smaller axons and thus slower conduction speed. Both endings in muscle spindles are very sensitive to low-amplitude changes in muscle length, especially if these changes occur at a high frequency. A spindle ending is located at the end of an neuron or an axon whose body is in spinal ganglion. [3,6]
