Your Own Personal Functioning Limb

I have constructed a model working arm with an elbow joint. I created this model using: cardboard, wire hanger, acrylic paints and paper.




The bicep and tricep are a muscle pair that work together to either bend the arm at the elbow or extend it. The origin of each is on the humerus. The insertion of the tricep is on the ulna and the insertion of the bicep is on the radius (at least on the elbow side.) The elbow itself is a hinge joint, where ligaments, cartilage and synovial cavities with fluid make the "hinge" movable. The reason it is a hinge joint is because it moves in only one direction. Motor neurons stimulate the contraction of muscles making movement possible. In the upper arm, most of these neurons come off of three nerves: the radial, the ulnar and the median nerves.



Muscle fibers are very unique cells. They contain tons of myofibrils. These segments are what makes a muscle fiber contract.


The Myofibrils are bundles of myofilaments. These strands of protein are the cause of contraction. The thick myofilaments (made of mosin protein) and the thin filaments (actin) slide past each other to make a muscle contract. This happens in the area between two "Z bands" in a myobril. This section is called a sarcomere. The filaments themselves do not shorten but the sarcomere does as the filaments slide and become more compact.


The cause of this is function is motor neurons on a muscle fiber. When the impulse for contraction reaches a axon terminal on the fiber the neurotransmitter ACh is released. These Ach molecules are carried in vesicles out of the axon terminal and into the synaptic cleft. There they are picked up by receptors on the sarcolemma (plasma membrane of a muscle fiber.)


This stimulates the myofilaments to slide which contracts the sarcomeres in a muscle fiber. As a muscle contracts, it pulls on the bone (in this case the ulna and the radius) to draw the forearm upward. This is how a joint is moved.