Anatomy and Physiology I
The Muscular System
20 May 2013
Professor: Dr. Koronika
Q1. Describe normal anatomy/physiology of muscle contraction.
Anatomy of Skeletal Muscle: A single skeletal muscle, such as the triceps muscle, is attached at its
• Origin to a large area of bone; in this case, the humerus.
• At its other end, the insertion, it tapers into a glistening white tendon which, in this
case, it’s attached to the ulna, one of the bones of the lower arm.
As the triceps contracts, the insertion are pulled toward the origin and the arm is straightened or extended at the elbow. Thus the triceps is an ...view middle of the document...
(Cytokines secreted by a cell type that inhibit proliferation of that same type of cell are called chalones.) Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene. These discoveries have already led to the growth of an illicit market in drugs supposedly able to suppress myostatin.
In adults, increased muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue. In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.
Because a muscle fiber is not a single cell, its parts are often given special names such as
• sarcolemma for plasma membrane
• sarcoplasmic reticulum for endoplasmic reticulum
• sarcosomes for mitochondria
• sarcoplasm for cytoplasm
Although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells.
The Sliding Filament Theory
For a contraction to occur there must first be a stimulation of the muscle in the form of an impulse (action potential) from a motor neuron (nerve that connects to muscle).
Note that one motor neuron does not stimulate the entire muscle but only number of muscle fibres within a muscle. The individual motor neuron plus the muscle fibres it stimulates, is called a motor unit. The motor end plate (also known as the neuromuscular junction) is the junction of the motor neurons axon and themuscle fibres it stimulates.
When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction and the sliding filament theory.
The reaction, created from the arrival of an impulse stimulates the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres.
Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments, as seen in the image below. When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges (heads), thus preventing contraction.
When the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum (which is effectively a storage house for calcium within the muscle) and release calcium into the sarcoplasm (fluid within the muscle cell). Some of this calcium attaches to troponin which causes a...