MSM - Muscle Cell Contraction Lesson

AandP_Lesson_TopBanner.png 

Muscle Cell Contraction

The innervation of skeletal muscle explains the way muscles contract and generate movement. In this diagram, skeletal muscle fibers receive signals from the nervous system.

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.

Myofilaments include actin, which are thin filaments, and myosin, which are thick filaments.

Each myofibril is made up of bundles of contractile proteins called myofilaments. There are two main types of myofilaments:

  1. Actin- thin filaments
  2. 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 basic unit of a 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 (A) a sarcomere (B) contracts, the Z-lines move closer together and the I-band gets smaller. The A-band stays the same width and, at full contractions, the thin filaments overlap.

 

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.

 

AandP_BottomBanner.png 

IMAGES CREATED BY GAVS OR OPENSOURCE