Intrafusal muscle fibers

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Intrafusal muscle fibers are muscle fibers that comprise the muscle spindle. These fibers are walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal." While the intrafusal fibers are wrapped with sensor receptors, their counterpart, extrafusal fibers are the ones responsible for the power-generating component of muscle and are innervated by motor neurons.

It is by the sensory information from the two types two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.

[edit] Type Ia

The first of the two main group of stretch receptors wrapping the intrafusal fibers are the Ia fiber, which are the largest and fastest fibers, and they fire when the muscle is stretching. They are characterized by their rapid adaption, because as soon as the muscle stops changing length, the Ia stop firing and adapt to the new length. Ia fibers essentially supply proprioceptive information about the rate of change of its respective muscle: the derivative of the muscle's length (or position).

[edit] Type II

The second of the two main groups of stretch receptors are the II fibers, and they are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. Their firing rate is directly related to the muscle's instantaneous length, or position. This information would indicate the position one's leg once it has stopped moving.

[edit] External links

• Histology at uwa.edu.au
• Histology at umdnj.edu

Nervous system - Sensory system - Somatosensory system - edit
Spinal pathway: Somatosensory information
Medial lemniscus: Touch (Pressure & Vibration) | Proprioception Spinothalamic tract: Pain | Temperature
Receptors
Touch: Pacinian corpuscles | Meissner's corpuscles | Merkel's discs | Ruffini endings | Free nerve endings | Hair follicle receptors Proprioception: Golgi organ | Muscle spindle (Intrafusal muscle fiber) Pain: Nociceptors    Temperature: Thermoreceptors

Muscle tissue v • d • e

skeletal muscle/general: epimysium, fascicle, perimysium, endomysium, muscle fiber, myofibril sarcomere (a, i, and h bands; z and m lines), myofilaments (thin filament/actin, thick filament/myosin, elastic filament/titin), tropomyosin, troponin neuromuscular junction, intrafusal muscle fibers, extrafusal muscle fiber, motor unit, muscle spindle, sliding filament mechanism myoblast, satellite cells, sarcoplasm, sarcolemma, sarcoplasmic reticulum, T-tubule cardiac muscle: myocardium, intercalated disc smooth muscle: calmodulin, vascular smooth muscle