Muscles and Movement

Movement is made more flexible with joints. Here, ligaments hold bones together. They limit the movement thus preventing dislocation. The joints move due to the force of muscles acting on them.

Muscles and Movement

Muscles are attached to bones by tendons that are made of collagen fibres. When a muscle contracts, the tendon and its attached bone are pulled towards the contracting muscle.

Many joints work due to the action of antagonistic muscles; one set causes the joint to move one way, the other set causes it to return. When one muscle in the pair is contracting, the other is relaxing (not stretching). An example of this arrangement is the elbow joint controlled by the biceps and triceps muscle.

There are three types of muscle, skeletal, cardiac and smooth...

  Skeletal: Cardiac: Smooth:
Function: Locomotion Pumping blood through heart Line blood vessels, digestive tract (peristalsis), uterus, etc.
Cells: Striated Specialised striated Unstriated
Control of contraction: Voluntary - conscious Involuntary Involuntary
Arrangement: Regular arrangement so the muscle contracts in one direction Cells branch and interconnect allows efficient transfer of impulses so brings about simultaneous contraction No regular arrangement (different cells can contract in different directions)
Speed of contraction: Rapid Intermediate Slow
Length of time contracted: Short Intermediate Can remain contracted for a relatively long time

Each muscle is called a fibre. Each fibre made up of a bundle of myofibrils.

Each myofibril is made of myofilaments - actin and myosin.

A muscle fibre:

A muscle fibre

The myofilaments are arranged so that each myosin is surrounded by 6 actins:

Myofilaments

The actin, being thinner, shows up under a microscope as a light band. The myosin, being thicker, shows up as a dark band.

Actin:

It consists of 2 threads wrapped around each other. At each twist there is a binding site for myosin. In a relaxed state, a molecule called tropomyosin covers these sites.

Actin

Myosin:

The filament consists of many myosin molecules. Each molecule has a tail and a double globular head.

Myosin

The head attaches to the myosin binding sites on the actin where the actin and myosin filaments overlap.

These attachments are called cross-bridges. The heads contain ATPase enzyme which releases energy from ATP to power muscle contraction.

Contraction occurs when an impulses from a motor neurone reaches the synapse at the junction with the muscle. If it is stronger than a threshold stimulus (see Nervous and Hormonal Control Learn It) contraction will occur.

What follows is the Sliding Filament Hypothesis:

  1. Acetylcholine, a neurotransmitter substance, is released into the synapse, diffuses across and attaches to specific receptors on the sarcolemma (the outer membrane of the muscle fibre).
  2. The muscle sarcolemma is depolarised.
  3. Depolarisation spreads along the fibre.
  4. This causes calcium to be released from the sarcoplasmic reticulum into the sarcoplasm.
  5. Calcium displaces tropomyosin, thus uncovering the myosin binding sites on the actin filaments.
  6. ATP attached to the myosin heads cause them to flex and attach to the actin in the overlapping areas.
  7. ATP is hydrolysed to ADP + P. The energy released causes the heads to alter their angle to their tails. This pulls the actin filament past the myosin filament.

The cross-bridges detach and reattach, this time further along the actin filament.

 

The areas of overlap therefore lengthen and the light bands shorten. The sarcomere becomes relatively short. The shortening of sacromeres along the length of the muscle is the cause of contraction.

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