Skeletal muscle

Striated skeletal muscle, Extrafusal muscle cells, Intrafusal muscle cells

This is the striated muscle that joins different skeletal parts (and in some places also pieces of cartilage; e.g. in the larynx) with each other and produces volitionally modifiable changes in position - movements - between the joined bone/cartilage parts.

It also happens that striated muscles start from the skeleton and attach to the skin (some facial muscles) or form ring-shaped units, so-called shutter muscles (sphinktrar), e.g. around the opening and around the neck part of the bladder.

The elongated, clearly striated, muscle cells that form skeletal muscle are usually divided into two main groups:

1/ The extrafusal muscle cells/ fibers/ threads (extrafusals ) build up the muscle mass itself and are responsible for the muscles' power development. The extrafusals are striated throughout their length and contain a large number of cell nuclei more or less evenly distributed inside the cell.
The extrafusals vary in thickness between different muscles ranging from 10-20 micrometers in our finest, thinnest muscles (eg eye muscles) up to approx. 100 micrometers in our coarsest and largest muscles (eg gluteal muscles).
The extrafusals vary in length between different muscles, ranging from a few millimeters up to more than 1 decimeter (here we move among the body's largest cells).
The extrafusals of a muscle run between the muscle's original tendon and its attachment tendon and are surrounded by the muscular membrane (fascia).
A muscle contains, depending on size, anything from a few thousand up to just over a million extrafusals.
The extrafusals are directly affected by the alpha motor neuron via synapse-likecontact areas: the motor end plates.

2/ The intrafusal muscle cells/ fibers/ threads (intrafusals ) have nothing to do with the muscle power development. They are no more than 10 millimeters long and only approx. 10 microns thick. The striation is absent in the middle part of the intrafusal. The striation, and thus the contraction, is limited to the end areas of the intrafusal.
The intrafusals are directly affected by the gamma motor neuron via synapse-likecontacts that connect to the striated end areas. If the gamma-efference is activated, it causes the outer ends of the intrafusal to contract and the non-striated middle part to stretch.

The intrafusals are collected in bundles of 5-15 pieces. Each such bundle is surrounded by a special connective tissue capsule, is distinctly spindle-shaped (coil = lat. fusal) and is called a muscle spindle.
The spindle muscle is a particularly complicated sensory organ and records proprioceptive information.