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FastestThings

Nerve Conduction

Aα sensory fiber

The fastest sensory class in the shared textbook table: primary muscle-spindle and Golgi-tendon afferents, conducting at 80–120 metres per second.

Also known as Group Ia, Group Ib, A-alpha afferent
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Conduction velocity

80–120m/s

Sensory afferent class. Textbook range 80–120 m/s. The rank uses the top of the range. The row shows the whole range.

OfficialFundamental Neuroscience for Basic and Clinical Applications (opens source in a new tab)

Mechanism

Why it's so fast

These fibers are fast because they are thick and myelinated, built to report muscle length and tendon force with little delay.

  1. Myelin and a wide axon

    The impulse jumps between nodes of myelin instead of creeping along the whole membrane. A diameter of 13–20 µm also keeps internal resistance low, which is why this class sits at the top of the sensory table.

  2. What the class carries

    Group Ia endings are on muscle spindles and Group Ib endings are on Golgi tendon organs. The three tables share 80–120 m/s for that sensory class. Motor Aα axons in the Haines table are a different row, 72–120 m/s.

  3. A band, not one axon

    The page ranks the top of the published range and prints both ends. Nothing in the tables says every axon in the class runs at 120 metres per second.

The next sensory class is the Aβ fiber, at 35–75 m/s.

Provenance

Every measurement

Claimed, measured and official figures side by side, each with its source. The ranked figure is marked.

  1. 80–120m/sRanked figure

    Sensory afferent class. Textbook range 80–120 m/s. The rank uses the top of the range. The row shows the whole range.

    The same Haines table prints motor Aα axons at 72–120 m/s and 12–20 µm. This row is the sensory Ia/Ib range, 80–120 m/s, which the three afferent tables share. Copies of the Henley chapter on other open-textbook sites are the same book.

    Official
    Fundamental Neuroscience for Basic and Clinical Applications (opens source in a new tab)

Reference

Specs

Class
Aα, Group Ia and Ib
Diameter
13–20 µm
Myelin
Yes
Range
80–120 m/s

Context

Background

An Aα sensory fiber is a large myelinated afferent. In the classification the three tables share, Group Ia endings sit on muscle spindles and Group Ib endings sit on Golgi tendon organs. The impulse jumps between nodes of myelin, and the wide axon keeps resistance low, so this class is the top of the sensory table.

Henley's Table 23.1, River Publishers' Table 2.1, and Haines Table 17.3 each print 80–120 m/s for a diameter of 13–20 µm. The rank uses 120 m/s, the top of that range. The page shows 80–120 m/s. Motor Aα fibers in the Haines table are 72–120 m/s, so they are named in the note and not mixed into this row.

Spot a wrong number?

Every report is read by a person, and corrections are dated on the page.