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A muscle spindle, with γ motor and Ia sensory fibers. A type Ia sensory fiber, or a primary afferent fiber, is a type of afferent nerve fiber. [1] It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch changes. The information carried by type Ia ...
When a muscle is stretched, primary type Ia sensory fibers of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials. Likewise, secondary type II sensory fibers respond to muscle length changes (but with a smaller velocity ...
The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated. [1] [2] The other classification is a sensory grouping that uses the terms type Ia and type Ib, type II, type III, and type IV, sensory fibers. [1]
These fibers are relatively large, myelinated, and quickly conducting. Muscle spindles monitor the length within muscles and send information via faster Ia afferents. These axons are larger and faster than type Ib (from both nuclear bag fibers and nuclear chain fibers) and type II afferents (solely from nuclear chain fibers).
Sensory fiber types Myelination Diameter Speed From muscles From skin Receptors; Thick 12-20 72-120 I Aα Proprioceptors (muscle spindle, Golgi tendon organ) Medium 6-12 35-75 II Aβ Merkel nerve ending, tactile corpuscle, lamellar corpuscle, Bulbous corpuscle: Thin 1-6 4-36 III Aδ Free nerve ending: None 0.2-1.5 0.4-2.0 IV C Free nerve ending
The pyramidal tracts include both the corticobulbar tract and the corticospinal tract.These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in the control of motor functions of the body.
In each muscle, we have 10-100 tiny muscle-like pockets called muscle spindles. The type II fibers (aka secondary fibers) connect to nuclear chain fibers and static nuclear bag fibers in muscle spindles, but not to dynamic nuclear bag fibers. The typical innervation to muscle spindles consists of one type Ia fiber and 2 type II fibers. [6]
The capsule is connected in series (along a single path) with a group of muscle fibers (10-20 fibers [3]) at one end, and merge into the tendon proper at the other. Each capsule is about 1 mm long, has a diameter of about 0.1 mm , and is perforated by one or more afferent type Ib sensory nerve fibers ( Aɑ fiber ), which are large (12-20 μm ...