Skeletal and Muscular Systems: Skeletal Muscle Contraction

Skeletal Muscle Structure

Skeletal muscles are responsible for voluntary movement as they are connected to the skeleton.
Each muscle is made up of many fibres known as myofibrils, which are further composed of two filamentous proteins: actin (thin filaments) and myosin (thick filaments).
The repeat units of actin and myosin within a myofibril form a sarcomere, which is the functional unit of a muscle.
The outer membrane of a muscle fibre is referred to as the sarcolemma. It conducts electric impulses to stimulate muscle contraction.
On the inside of muscle fibres, sarcoplasmic reticulum stores calcium ions, which are essential for muscle contraction.

Contraction of a muscle fibre occurs as a result of a process known as the Sliding Filament Theory.
During a contraction, myosin heads or “cross bridges” pull the actin filaments inwards, causing the sarcomere to shorten.
The shortening of many sarcomeres within a myofibril and across many myofibrils leads to the contraction of the muscle itself.

An electrical impulse, or action potential, travels from the brain or spinal cord along a motor neuron to a skeletal muscle.
At the end of the motor neuron, the electric signal results in the release of a neurotransmitter called acetylcholine at the neuromuscular junction, a specialised synapse between the neuron and muscle fibre.
Acetylcholine binds to receptors on the sarcolemma, triggering an action potential, which propagates through the muscle fibre and causes the release of calcium ions from the sarcoplasmic reticulum.

Released calcium ions bind to a protein in the actin filaments called troponin, which causes the protein tropomyosin to move and reveal binding sites for myosin heads on the actin filament.
This allows the myosin heads to connect with the actin filament and form cross bridges, which is when contraction begins.

Contraction of a muscle requires energy in the form of adenosine triphosphate (ATP).
ATP binds to the myosin heads and provides the energy for them to flex and pull the actin filaments inwards.
After the power stroke, ATP is again needed to detach the myosin head from the actin filament, completing the cycle of contraction and relaxation.