Neurological Control of Movement

 I'm studying for the NSCA-CSCS exam in June, it's comprised of two parts. One part scientific foundations and the other part is applied. I studied Political Science in college, not biological sciences; in fact I avoided higher level science because math scares me. In retrospect I should have gone for it because now I'm self studying subjects that are really nice to have a teacher for.  While studying and reviewing my materials I realized I learn best from teaching others so today we're going to talk about neurological control of movement (how are brain tells our muscles to move).

The body has more than 430 skeletal muscles and each skeletal muscle is an organ that contains muscle tissue, connective tissue, nerves and blood vessels. Epimysium covers all of our skeletal muscles and is continuous with our tendons at the ends of the muscle. Tension is developed in a muscle because of a signals sent from the spinal cord. Here's my understanding of how muscle contraction works.

Nerve Impluse:

Step 1:

A signal is sent from the brain or spinal column.

Step 2: Motor neuron in spinal column (ventral horn) is activated, and action potential passes outward in the ventral root of the spinal cord from the nerves to effectors (muscle)

Step 3: The action potential in conveyed through a motor end plate on each muscle fiber of a motor unit. 

Step 4: The action potential causes a release of acetylcholine (ACh) from the axon terminal in to the synaptic clefts on the surface of the muscle fiber which increases the permeability of Na+ (Sodium) in to the sarcoplasm, if there's enough ACh an action potential will occur.

Depolarization:

Step 5: Na+ enters muscle fiber, rapid depolarization of sarcolemma occurs= action potential

Step 6: The action potential spreads away from the end plate in all directions and depolarizes the Tubules where it continues down in to the sarcoplasm where is depolarizes the sarcoplasmic reticulum (SR) membranes.

Step 7: The SR responds to the action potential by opening Ca++ (Calcium) release channels which floods the surrounding sarcoplasm located between the thin (actin) and thick (myosin) filaments with Ca++.

Step 8: Ca++ binds with Troponin, Troponin changes shape and exposes the myosin binding sites on actin.

Step 9: Myosin heads (cross bridges) attach to actin binding sites, the myosin head flexes drawing actin filaments of sarcomeres towards each other. The ATP binding site is exposed and ATP binds to the head.

Step 10: Under the influence of ATP the myosin head detaches from actin binding site.

Steps 9 & 10 are repeated over and over again during a single contraction event as long as ATP and Ca++ are available.

Relaxation:

Step 11: Ca++ is returned to SR

Step 12: Troponin again covers actin-myosin binding sites and muscle relaxes.

More on Muscles.....

Muscle cells or muscle fibers are long and cylindrical; they often run the entire length of the muscle and have the approximate diameter of a human hair. These fibers have many nuclei on the edges of the cells or fibers. Under the epimysium are in bundle groups or muscle fibers called fasciculi, each fasciculi is covered with a connective tissue called perimysium and each muscle fiber is surrounded by endomysium. The endomysium is encircled by the sarcolemma or the cells membrane.

For muscle contraction to occur there has to be a neuromuscular junction, this is the junction between a motor neuron (nerve cell) an the muscle fiber it innervates. One motor neuron or nerve cell can innervate several hundred muscle fibers, but each fiber only has one neuromuscular junction. A motor neuron or nerve cell and all of the fibers it innervates is collectively called a motor unit.

All of the muscle fibers of a motor unit contract when they are stimulated by the motor neuron The extent of control a muscle has is determined by the number of muscle fibers in a motor unit. The fewer the muscle fibers per motor unit the more neuromuscluar control we have of those particular movements. 

The action potential that flows along the motor neuron is not directly responsible for creating muscle excitation, but the motor neuron creates excitation in the muscle fiber its innervates by a chemical transmission. When a motor unit is effected by the action potential all of the muscle fiber with in the unit will contract. This is known as the all or nothing principle.

There we have it, my long winded explanation for how muscles contract, if any science people out there want to help me out, please feel free! Thanks!