Muscle and Nerve

Muscles and Nerves

Overview

Here, let's address muscle tone, the muscle spindle, and peripheral nerve classification.

Muscle Tone

Alpha motor neuron loop

Re-draw our alpha motor neuron loop:

Draw the spinal cord and lower extremity, and, then, show the muscle spindle project via a type 1a sensory afferent to excite the extensor motor neuron, which projects via an A alpha motor nerve to excite the extrafusal muscle fiber.

Gamma motor neuron

Next, label a gamma motor neuron in the anterior gray matter, and show that it projects to the muscle spindle.

Supraspinal projection

Finally, draw a supraspinal projection to the gamma motor neuron to show that supraspinal centers send descending excitatory inputs to the gamma motor neurons.

Spasticity

The gamma motor neurons serve to produce muscle tone.
When the supraspinal input to the gamma motor neurons is disrupted, initially, muscle tone becomes flaccid; it is not until days or weeks later that spasticity develops.

Muscle Spindle

Now, let's draw the muscle spindle anatomy.
Begin with the connective tissue-enclosed muscle spindle capsule;
next label the skeletal muscle fibers outside of the capsule as extrafusual muscle fibers.

These fibers produce limb movement and are innervated by the alpha motor nerves.

Nuclear bag fiber

Then, within the central non-contractile portion of the muscle spindle, draw nuclei clustered together like marbles in a bag, and label this the nuclear bag fiber.

Nuclear chain fiber

Next, draw a longitudinal row of nuclei, like pearls on a chain, and label this the nuclear chain fiber.
Both fiber types exist within the muscle spindle, and they each are attuned to different aspects of muscle tone.

Muscle spindle sensory afferents

Next, let's address the type 1a and type 2 sensory afferents of the muscle spindle.
Show an annulospiral sensory nerve ending around the non-contractile central portion of both the nuclear bag and nuclear chain fibers, which connects to the type 1a sensory afferent fiber.
Then, show a flower-spray sensory nerve ending connect the type 2 fiber to the nuclear chain fiber, only.
However, two forms of nuclear bag fibers actually exist: bag 1 and bag 2; the latter shares many similarities with nuclear chain fibers.
Indicate, now, that in addition to attaching to chain fibers, the type 2 sensory afferents also attach to bag 2 fibers.

Muscle spindle Efferents

Now, let's address the efferent innervation of the muscle spindle.
The gamma motor nerves either terminate in plate or trail endings in the polar region of the muscle spindle, along the intrafusal muscle fiber.
Indicate that, generally, the plate endings lie along nuclear bag fibers, and, that the trail endings lie along nuclear chain fibers, and the bag 2 fibers.
The bag 1 fibers act when there is a change in the muscle fiber length, during the dynamic phase.
Whereas, the chain fibers and bag 2 fibers act when the muscle length is unchanged, during the static phase.

Peripheral Nerve Classification

Two classification schemes are commonly used.

The Gasser scheme, which applies to all nerve types, motor sensory and autonomic.
And the Lloyd scheme, which applies to sensory nerves, only.
The two schemes are fairly redundant, however, and can be learned together.
We will only label the largest and smallest fibers here, but the table will be completed for reference.
Label the top row of our table as Nerve Class, Diameter, and Speed.
Make a notation that the diameter units are micrometers, and the speed units are meters/second.

Large Fibers

List the largest fiber type as Gasser class A-alpha, and Lloyd class Ia and Ib.
Indicate that they are 12-22 micrometers in diameter, and, conduct at 70-120 m/second.

Small Fibers

Myelin & Conduction Speed

See: Myelin & Nerve Conduction Speed

Impulses slowly ascend the small peripheral nerve axons, whereas they quickly ascend the larger peripheral nerve fibers, because the myelin sheaths suppress action potential firing.
The impulses of large fibers only fire in the interspaces between segments of myelin (the nodes of Ranvier), which is called saltatory conduction.