Anatomy/Muscular System

Muscular System
<Anatomy

Antagonistic Pairs
Muscles work in antagonistic pairs. This basically means that there are always at least two muscles working in opposite functions for any joint. One will, for example, extend the joint, while the other flexes it. A good example is the elbow joint. The Triceps brachii extends the joint when it contracts, and the Biceps brachii flexes the joint when it contracts.

Interactive animation to see this concept in action

Of course, there are never really only two muscles acting on a joint. In the scenario I just described, the Brachialis and Brachioradialis would be acting on the elbow in flexion with the Biceps, and the Anconæus would act with the Triceps, although it has a very minor function.

Three Types of Muscle
As the title of this section implies, there are three types of muscle: Skeletal, Cardiac, and Smooth.



Skeletal Muscle
Despite the fact that there are three types of muscle, skeletal muscle is the kind most stressed by the event. Science Olympiad requires you to know 50 skeletal muscles for the event. A comprehensive list can be found here. The skeletal muscle is the kind most commonly thought of when one hears the word "muscle". Examples include Vastus muscles, the Rectus abdominis, and the Biceps brachii.




 * A Tendon attaches muscle to bone. The Epimysium (plural epimysia) is a layer of dense connective tissue that surrounds the entire muscle. Its function is to protect muscles from friction that occurs between other muscles and bones.  In tendons, it is thicker and contains more collagen.
 * The muscle consists of many Fascicles (aka bundles) of muscle fibers. Each fasicle is wrapped in a connective tissue covering called the Perimysium.
 * The Endomysium consists mostly of reticular fibers. It is the sheath of connective tissue surrounding a fiber.

Anatomy of a Skeletal Muscle Fiber



 * The skeletal muscle fiber is a cell.
 * The Sarcolemma is the plasma membrane.
 * It has multiple inward extensions which form a set of T Tubules (the T stands for transverse).
 * The Sarcoplasm is the cytoplasm & the Sarcoplasmic Reticulum is the endoplasmic reticulum.
 * Myofibrils are the cylindrical organelles found inside a muscle fiber.
 * Myofilaments are the filaments of a myofibril.
 * Myofilaments are organized into repeating units called Sarcomeres.



When muscles contract, the I band and H zone decrease in length but the A band stays the same length.
 * Above is a picture of the structure of sarcomere.
 * There are two types of myofilaments. Myosin filaments are thick and Actin filaments are thin.
 * Z lines separate myofibrils into the compartments called sarcomeres.
 * I bands are where there are only thin filaments.
 * H zones are where there are only thick filaments.
 * A bands are all along the thick filaments. (some overlapping)

The NMJ and Muscle Contraction
The neuromuscular junction is the point where a motor neuron meets the muscle fiber. One motor neuron can form many NMJ's. The surface of the muscle fiber forms small ridged folds for the end of axon to rest in. Inside these folds are depressions with acetylcholine receptors. The folds are known as synaptic clefts.

Acetylcholine is necessary for life. It is the only neurotransmitter that is used in the motor division of the somatic nervous system, part of the peripheral nervous system that controls voluntary actions.

Muscle contraction generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Although the term 'contraction' implies shortening, it means muscle fibers generating tension with the help of motor neurons.

The process is defined by the Sliding Filament Model. The many steps are as follows (note: the bolded terms are essential ones that you will need to know):

1. A nerve impulse sent voluntarily or involuntarily travels through motor neurons to the sarcolemma.

2. The nerve impulse travels along the sarcolemma and down the T-tubules.

3. From the T-tubules, the impulse travels to the sarcoplasmic reticulum, which releases Calcium in response.

4. Calcium fills the binding sites in the troponin molecules, altering their shape and position. This allows movement of the attached Tropomyosin molecule.

5. The movement of the tropomyosin permits the Myosin head to contact Actin.

6. Contact with Actin causes the Myosin head to swivel.

7. The Myosin head remains firmly attached to Actin while swiveling. So when the head swivels it pulls the Actin with it. (And, therefore, the entire myofilament.)

8. At the end of the swivel, ATP fits into the binding site on the cross-bridge, breaking the bond between the cross-bridge (Myosin) and Actin. The Myosin head then swivels back; the ATP breaks down to ADP and Phosphate, and the cross-bridge again binds to an Actin molecule.

9. As a result, the Myosin head is once again firmly bound to Actin. However, because the head was not attached to Actin when it swiveled back, the Myosin head will bind to a different Actin molecule (i.e one further back on the myofilament). Once the head is bound, the cross-bridge will swivel; step 7 is repeated.

As long as Calcium is present and attached to Troponin, steps 7--9 will repeat. As they do, the thin myofilament is being pulled by the Myosin heads of the thick myofilament. Thus, the thick and thin myofilaments are actually SLIDING PAST EACH OTHER. This is why it is called the Sliding Filament Model.

Muscular System Diseases
Muscles have many important functions in the body. They:
 * Enable movement
 * Aid in respiration (diaphragm)
 * Aid in digestion
 * Protect internal organs
 * Help move blood throughout the body