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 fibers contributes to the sense of proprioception.

Function of muscle spindles

For the body to keep moving properly and with finesse, the nervous system has to have a constant input of sensory data coming from areas such as the muscles and joints. In order to receive a continuous stream of sensory data, the body has developed special sensory receptors called proprioceptors. Muscle spindles are a type of proprioceptor, and they are found inside the muscle itself. They lie parallel with the contractile fibers. This gives them the ability to monitor muscle length with precision.

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 with respect to the kind of potentials they generate:

Type Primary/secondary Response
Type Ia primary Respond to the rate of change in muscle length, as well as to change in velocity, rapidly adapting
Type Ib N/A In Golgi tendon organ, responds to muscle tension changes
Type II secondary Provide position sense of a still muscle, fire when muscle is static [2]

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 Ia fibers connect to both nuclear bag fibers and nuclear chain fibers. These connections are also called "annulospiral endings", deriving from the Latin word annulus which means "a ring-shaped area or structure".[3]

Efferent innervation

In addition, the spindle also has a motor efferent innervation carried by the efferent nerve fibers of gamma motor neurons, which is used by the nervous system to modify the spindle's sensitivity.

Termination of afferents

Proprioceptive afferents send central axons through the dorsal root of spinal nerve bifurcating into ascending and descending branches, which in turn send branches to multiple spinal segments. Some branches synapse at the dorsal horn and some at the ventral horn (where the motor neurons are) which mediate different responses including stretch reflex. Ascending pathways to the brain have some similarities with the cutaneous afferents, but are different because the proprioceptive information also has to reach the cerebellum which controls the timings of muscle contractions for voluntary movements.[4]

See also


  1. ^ Boyd, I. A. (1980). "The isolated mammalian muscle spindle". Trends in Neurosciences. 3 (11): 258–277. doi:10.1016/0166-2236(80)90096-X. S2CID 53199331.
  2. ^ Michael-Titus, Adina T (2007). Nervous System: Systems of the Body Series. Churchill Livingstone. ISBN 9780443071799.
  3. ^ "annulus - Wiktionary". en.wiktionary.org. Retrieved October 22, 2018.
  4. ^ Purves, Dale; Augustine, George J; Fitzpatrick, David; Hall, William C; Lamantia, Anthony Samuel; Mooney, Richard D; Platt, Michael L; White, Leonard E, eds. (2018). "Chapter 9 - The Somatosensory System: Touch and Proprioception". Neuroscience (6th ed.). Sinauer Associates. Central Pathways Conveying Proprioceptive Information from the Body, pp. 204-205. ISBN 9781605353807.