Muscle Contraction & Movement

Muscles

Muscles are attached to bones by tendons

Muscles work in antagonistic pairs

? Ex. Biceps and triceps

? One muscle contracts while the other relaxes

Contractile apparatus

Skeletal muscle

? Muscle cell = muscle fiber

? Muscle fibers are made of myofibrils (striated)

? Myofibrils are made of units called sarcomeres

? Sarcomeres are made of thick and thin filaments

? Z line is the end of the sarcomere

? Thick and thin filaments slide over one another to shorten the muscle during contraction

The Thick Filament (Myosin)

Consists of the protein called myosin. A myosin molecule is shaped a bit like a golf club, but with 2 heads. The heads stick out to form the cross bridge. Many of these myosin molecules stick together to form a thick filament

Thin Filament (Actin)

The thin filament consists of a protein called actin. The thin filament also contains tropomyosin. This protein is involved in the control of muscle contraction

Sliding filament theory

? Links the structure of a sarcomere to its function

? During contraction thin filaments slide over thick filaments

? Thick filaments= myosin and have “heads”

? Thin filaments = actin, these slide

? Ca and ATP required for sliding and attachment

? ATP binds to a myosin head, which is released from an actin filament

? Hydrolysis of ATP cocks the myosin head

? The myosin head attaches to an actin binding site with the help of Calcium

? The power stroke slides the thin filament when ADP and Pi are released from it

? 350 myosin heads per thick filament

? Can bind and unbind to thin filament up to 5 times per second

Motor neurons and muscle contraction

Motor neurons stimulate muscle contraction

Motor neurons are branched and can stimulate more than one muscle fiber

Motor unit = motor unit and all the muscle fibers it controls

Neuromuscular junctions = the synapse between a motor neuron and a muscle fiber

The strength of a muscular contraction is controlled by the number of motor units activated. More motor units = stronger contractions

Muscles requiring precise control have one motor neuron per muscle fiber

Mechanism of stimulation:

An action potential releases acetylcholine into the neuromuscular junction

Acetylcholine depolarizes the muscle cell channels inside on the sarcoplasmic reticulum release Ca so it can reach the contractile apparatus

Mechanism of relaxation

Motor neuron stops firing

Ca pumped back into the SR

Muscle injuries

The Injury: The term 'pulled muscle' comes from the description of how the injury takes place. Usually the muscle is forcibly stretched beyond its limits and the muscle tissue becomes torn. Depending upon its severity it is classified as a first, second or third degree strain:

? A first degree strain is damage to a few muscle fibers.

? A second degree strain is damage to a more extensive number of muscle fibers.

? A third degree strain is a complete rupture of the muscle itself.

Signs and Symptoms

Grade 1

With a grade 1 the signs may not be present until after the activity is over. There may be a sensation of cramp or tightness and a slight feeling of pain when the muscles are stretched or contracted.

Grade 2

With a grade 2 there is immediate pain which is more severe than the pain of a grade one injury. It is confirmed by pain on stretch and contraction of the muscle. A grade 2 is usually sore to touch.

Grade 3

A grade 3 is a catastrophic injury. There is an immediate burning or stabbing pain and the athlete is unable to walk. The muscle is completely torn and there may be a large lump of muscle tissue above a depression where the tear is.