How it works
The first drawing opens one spindle so you can label the muscle spindle structures: its fibers and the four nerve fibers that reach it. The second shows what it does, the stretch reflex.
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Fibers first, then the nerves from below
Number 1 is the extrafusal fibers, the ordinary muscle along the top. Number 2 is the bracket round the intrafusal fibers, inside the spindle: one nuclear bag fiber, 3, and two nuclear chain fibers, 4. Four nerve fibers come up from below: the primary ending, 5, coils round the middle of all three; the secondary ending, 6, sits beside it; the gamma motor neuron, 7, goes to their ends; the alpha motor neuron, 8, goes to the extrafusal fibers. In the reflex, 9 to 12: the spindle, the sensory fiber, the synapse and the spinal cord.
- Bag and chain are named for their nuclei: bunched in a swollen middle, or lined up in a row.
- Gamma goes to the fibers inside the spindle and alpha to the ordinary fibers outside it.
Questions and answers
What are the parts of a muscle spindle?
A few small intrafusal fibers, of two kinds: nuclear bag and nuclear chain. Two sensory endings sit on them, the primary (type Ia) and the secondary (type II), and gamma motor neurons reach their ends. The ordinary fibers round the spindle are extrafusal.
What does a muscle spindle do?
It senses stretch. It lies beside the ordinary muscle fibers, so it is stretched when the muscle is. Its sensory fibers signal the length of the muscle and how fast the length is changing.
How does the stretch reflex work?
A stretched spindle fires its Ia sensory fiber. In the spinal cord that fiber synapses directly on an alpha motor neuron of the same muscle, and the muscle contracts against the stretch.
Is this the answer key to "place the labels into the diagram of muscle spindle function"?
No. That sentence is the title of a publisher's homework activity. This is our own drawing of the same subject, with each fiber and ending named and the reflex in a second view.