Neuromuscular Anatomy and Physiology

There are 3 types of muscle cells that are found in the body. Skeletal muscle cells are cells that are bind together to form the skeletal muscles. Smooth muscle cells are found in the intestines and the arteries. Cardiac cells are found in the heart.

There are two types of skeletal muscle fiber types. There are slow twitch muscle fibers that contract slowly. They are dark in appearance because of their abundant blood supply.

Slow twitch muscle fibers are usually more resistant to fatigue due to the amount of mitochondia they contain, therefore they have a higher aerobic capasity. Another form of skeletal muscle fiber is the fast twitch muscle fiber. Fast twitch fibers are more white in appearance. There are two types of fast twitch muscle fibers. Type IIa are also called fast oxydative and Type IIb are called fast glycolotic.

Muscle contractions are conducted through motor neurons that send nerve impules from the brain to the muscle fiber. The impulses are chemically transmitted to the muscle fibers by acetylecholine, which is a neurotransmitter. A motor neuron meets a myofibril at a neuromuscular junction. Myofibrils are proteins that run the length of each muscle diber.

Two primary proteins in myofibrils are actin and myosin which are both contractile proteins. Several sarcomeres linked together make up muscle fiber. Each sarcomere is enveloped by a protective layer called the sarcolemma and a gel-like tissue called sarcoplasm. Mitochondria can be found in the sarcoplasm along with sarcoplasmic reticulum whose function is to store and release calcium.

The myofibrils run through the all sarcomeres. The actin and myosin within the sarcomere are located in different areas. In the middle is the myosinm the actin overlaps one end of the myosin, and runs through the boundary to the next sarcomere and overlaps the myosin.

Muscle Contraction
In order for muscle contraction to take place, there must be sufficient ATP near the contractile protiens and the central nervous system must send a nervous impulse. If that happens, the actin and myosin form a bridge according to the sliding filament theory.

The energy from the ATP causes the myosin to fo to the middle of the sarcomere which causes the actin to slide toward the center also which causes the length of the muscle to shorten. When a muscle contracts, it is basically the shortening of muscle fibers.

Summary of Muscle Contraction
The brain sends a nervous impulse that causes motor neurons to send acetylcholine to the neuromuscular junction.

• The nerve impulse then goes to the sarcolemma then to the sarcoplasmic reticulum.
  
  
• Calcium is released by the sarcoplasmic reticulum into the muscle fibers sarcoplasm.
  
  
• Proteins and actin filament binds with the calcium and exposes the sites for myosin filament.
  
  
• Cross bridges are formed by myosin binding with actin if there is enough ATP
  
  
• The sarcomere shortens as myosin pullsthe actin toward the center.
  
  
• The link between myosin and actin are broken when the nervous impulse stops. Calcium goes back to the sarcoplasmic reticulum.

A shortening of the muscle is concentric contraction, lengthening of the muscle is eccentric contraction. Contration of the fibers, but no change in length is isometric. The force of contraction depends on the size of fibers contracting and the number of fibers that contract simultaneously.

In general, the faster the speed of movement, the lower the force. A muscle generates maximum force when it's contraction begins at 1.5 times it's length.