Muscular System

Big Picture

The muscular system is responsible for allowing us to move. Muscles make up quite a large part of the body and make up more than one-third of our body mass. Muscles are able to contract and relax which allows the body to move and performs all bodily actions. In addition, muscles provide the force that push substances such as food through the body.

Key Terms

Muscular System: The organ system that includes all the muscles of the body.

Muscle Fiber: A muscle cell.

Tendon: Tough connective tissues which attach skeletal muscles to the skeleton.

Myofibrils: Bundles of threadlike structures that are found in muscle fiber.

Sarcomere: Basic functional unit of the muscle.

Sliding Filament Theory: Theory explaining how muscle fibers contract

Three types of muscles:

Skeletal Muscle: Muscle tissue that is attached to bone.

Cardiac Muscle: Muscle that is found only in the walls of the heart.

Smooth Muscle: Muscle tissue in the walls of internal organs such as the stomach.

Types of Muscels

The muscular system is made up of all of the muscles of the body.

  • Muscles are made up of muscle fibers that can contract
  • There are three types of muscle: skeletal, cardiac, and smooth.
  • Muscle contraction can be either voluntary (under conscious control–such as flexing your bicep) or involuntary (not under conscious control–such as a heart beat) depending on the muscle type
Image Credit: Histology Dep., Jagiellonian Univ., GNU-FDL 1.2
  • striated (striped) because muscle fibers are arranged in bundles
  • allow the body to move
  • contracts in short, strong bursts
Attached to bones

Image Credit: Polarlys, GNU-FDL 1.2

  • not striated - muscle fibers are arranged in sheets
  • move food and other substances through the body
  • helps organs carry out their functions
  • contracts slowly but steadily
Lines inside of internal organs (stomach, intestines, etc)


Muscular system cont.

Types of Muscels (Cont.)


Image Credit: Nathanael Reveal, GNU-FDL 1.2

  • striated
  • responsible for heartbeats
  • contain lots of mitochondria to provide ATP
  • electrical impulses are sent so that all muscle fibers contract at the same time
Only in the heart
  • There are over 600 skeletal muscles in the human body that vary in size.
  • Tendons connect skeletal muscles to bones
  • Many skeletal muscles are attached to the ends of bones on opposite sides of a joint
  • Bone moves when muscles contract
  • Skeletal muscles work in pairs. When one muscle contracts to bend a joint, the other muscle needs to contract in order to straighten the joint.
  • Exercise is needed to maintain big, strong muscles.

Muscle Contraction

Muscles are made up of muscle fibers, which each contain hundred of myofibrils. Myofibrils are made up of repeating sections of sarcomeres.

Sacromeres are what gives the striated appearance to muscles.

  • Each sarcomere contains two protein filaments: actin and myosin.
  • Actin filaments are anchored to structures called Z lines. The region between two Z lines is the sacromere.
  • Myosin filaments overlap the actin filaments. They have tiny structures called cross bridges that attach to the actin filaments.

The sliding filament theory explains how actin and myosin interact to contract muscle fibers:

  • Myosin slides along actin, pulling the actin filaments and Z lines together and shortening the sacromere
  • ATP is required to fuel this process
Muscle Contraction
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0