Type Ia Sensory Fiber
also called Primary Afferent Fiber
is a type of sensory fiber
. It is a component of a muscle fiber
's muscle spindle
which keeps track of how fast a muscle stretch changes (the velocity of the stretch).
Function of muscle spindles
In order to control movements, the nervous system
must receive continuous sensory information from muscles and joints
. For this purpose the body has specialized sensory receptors
. Muscle spindles are a type of proprioceptor, and they are located inside the muscle itself. They are sensitive to muscle length because they are in parallel with the contractile fibers.
Types of sensory fibers
This change in length of the spindle is transduced (transformed into electric membrane potentials
) by two types of sensory afferents, whose cell bodies are located in dorsal root ganglia
located next to the spinal cord
The two kinds of sensory fibers are different in respect to the kind of potentials they generate:
|| Response |
| Type Ia
|| Respond to the rate of change in muscle length, as well to change in length |
| Type Ib
|| in Golgi tendon organ |
| Type II
|| Respond only to changes in length |
The first of the two main groups 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 adaptation, 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).
Type 1a fibers connect to both nuclear bag fibers and nuclear chain fibers. These connections are also called "annulospiral endings ".
In addition, the spindle also has a motor efferent innervation carried by gamma motor neurons
, which is used by the nervous system to modify the spindle's sensitivity.
Termination of afferents
Ia afferents from the muscle spindle terminate on the proximal dendrites of motor neurones.
- Lecture notes from John D.C. Lambert on neurophysiology.