MSM - Muscle Cell Contraction Lesson
Muscle Cell Contraction
Muscle contraction starts with a signal from the nervous system.
A signal from your brain goes through your nervous system to your muscle to stimulate a contraction.
But how do muscles contract?
Structure of Muscle Fibers
Each muscle fiber contains hundreds of organelles called myofibrils.
Each myofibril is made up of bundles of contractile proteins called myofilaments. There are two main types of myofilaments:
- Actin- thin filaments
- Myosin - thick filaments
These myofilaments are arranged into a series of functional units called sarcomeres along with the muscle fiber.
The basic unit of muscle contraction is the sarcomere.
The striations in skeletal and cardiac muscle cells are caused by the alignment of bands in the sarcomere (smooth muscle has a similar contractile mechanism but is not as highly organized).
The most prominent are the A and I-bands and Z line.
The area between two Z lines is the sarcomere.
The thin filament actin is attached to the Z line. Actin makes up the I-band of the sarcomere.
The A band is where the myosin and actin overlap in the sarcomere. When a muscle contracts, the sarcomere shortens and the Z lines move closer together.
Sliding Filament Theory
Muscles contract when sarcomeres shorten.
The most widely accepted theory explaining how this occurs is called the sliding filament theory.
According to this theory, myosin filaments use energy from ATP to walk along the actin filaments with their cross bridges. This pulls the actin filaments closer together.
The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere.
When all of the sarcomeres in a muscle fiber shorten, the fiber contracts.
A muscle fiber either contracts fully, or it doesn't contract at all.
The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.
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