Innervation of muscles and sarcoplasmic reticulum

In the previous section we saw how two regulatory proteins prevent the myosin heads from completing the force stroke. Only the increase of Calcium ions in the sarcoplasm allows the release of this "safety", by placing the switch in the "on" position. It is precisely the presence of calcium in the intracellular environment that determines the onset of the complex chemo-mechanical events underlying muscle contraction.

The increase in sarcoplasmic calcium is the end result of fine nerve control. The trigger for contraction occurs only when the skeletal muscle receives a signal from its motor nerve.

In addition to nerve structures, the presence of the so-called sarcoplasmic reticulum is very important. Inside we find a "high concentration of calcium ions.

The sarcoplasmic reticulum

The sarcoplasmic reticulum is a networked canalicular structure, which completely envelops every muscle fiber, sneaking into the internal spaces between one myofibril and the other. Examining it carefully, it is possible to notice two particular structures:

RETICLES: they are formed by longitudinal canaliculi (which accumulate Ca2 + ions) which, anastomosing with each other, flow into larger tubular structures, called terminal cisterns, which concentrate and sequester Ca2 +, and then release it when an adequate stimulus arrives.

TRANSVERSE TUBULES (T-tubules): invaginations of the cell membrane (sarcolemma), closely associated with the terminal cisterns. The membrane that covers them, being in direct contact with the sarcolemma, is free to communicate with the extracellular fluid (external to the cell).

The TRANSVERSE TUBE + TERMINAL TANKS complex (placed at its sides) constitutes the so-called FUNCTIONAL TRIAD.

The particular structure of the transverse tubules allows the rapid transmission of the action potential, without latencies, inside the muscle fiber.

The transverse tubule is regulated by a voltage-dependent receptor protein, whose activation upon reaching the action potential stimulates the release of Ca2 + from the terminal cisterns. The increased concentration of these ions represents the initial event of muscle contraction.

The basics of muscle contraction

The nerve impulse, originating centrally and transported by the motoenurons, reaches the motor plate level and spreads inside the muscle fiber thanks to the membranous tubular system. The action potential and the consequent depolarization of the sarcolemma, determine the release of Ca2 + from the cisterns of the sarcoplasmic reticulum. These ions, interacting with the troponin-tropomyosin regulation system, cause the release of the active site on the actin and the consequent formation of actomyosin bridges (see dedicated article).

Once the stimulus that gave rise to the contraction is exhausted, muscle relaxation occurs through an active ATP-dependent process, which has the purpose of bringing calcium ions back into the sarcoplasmic reticulum (restoring the inhibitory effect of the troponin-tropomyosin system) and favor the dissolution of the actomyosin bridge.

Muscle innervation

The contraction of muscle fibers is the result of a nerve stimulus that runs through an alpha motor neuron until it reaches the motor plate. The cell body of this motor neuron is located in the ventral horn of the gray matter of the spinal cord.

Several muscle fibers, sharing similar anatomical-physiological characteristics, are innervated by a single motor neuron. Each of these fibers receives afferents from only one motor neuron.

The number of fibers controlled by the motor neuron is inversely proportional to the degree of fineness and precision of the movement required by the muscle that contains them. The extraocular muscles, for example, support the motility of the bulb with extreme precision; for this reason each motor neuron innervates very few muscle fibers. In other body regions, where as much finesse is not required, the ratio can go from 1: 5 to 1: 2000 - 1: 3000. Generally speaking, the smaller the muscle, the smaller the motor unit.

The complex consisting of the alpha-spinal motor neuron, its efferent fiber (which exits and goes to the periphery transmitting the impulse) and the controlled muscle fibers, constitutes the simplest neurofunctional unit of the muscle, called:

NEUROMOTOR UNIT.

The neuromotor unit is the smallest functional entity of the muscle that can be controlled by the nervous system.

Contrary to what one might think, the nerve fibers of a motor unit are not all directed to neighboring fibers. In fact, muscle fibers belonging to a given unit are mixed with fibers belonging to other motor units. This particular arrangement allows a wider spatial distribution of the force generated by the motor units and a lower tension between the bundles of fibers.

Furthermore, not all neuromotor units are the same. They are classified on the basis of the contraction time, the peak force generated, the relaxation time and the fatigue time. This allows to distinguish the motor units in:

- type I lens (or S from "Slow" or SO from "Slow Glycolitic")

- fast type IIb (or FF from "Fast Fatiguing" or FG "Fast Glycolitic")

- type IIa intermediates (or FR from "fast fatigue resistent" or FOG "Fast Oxidative Glycolitic").

Each motor unit is made up of muscle fibers with homogeneous characteristics. Resistant fibers, for example, all refer to slow motor units, vice versa for fast ones.