Muscular System

Muscular System

Middle School

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Learning Objectives

• Describe the main functions of the muscular system in the human body.

• Compare and contrast the three types of muscle tissue: skeletal, smooth, and cardiac.

• Explain how skeletal muscles work in pairs to produce movement.

• Summarize the process of muscle contraction using the sliding filament theory.

Key Vocabulary

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Tendon

Tough connective tissues that attach the various skeletal muscles of your body to the skeleton.

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Sarcomere

The basic functional and structural unit of a muscle, which is primarily composed of actin and myosin.

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Muscle Fiber

A single muscle cell that contains myofibrils and acts as the basic building block of muscle.

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Sliding Filament Theory

A theory that explains how muscle fibers slide over each other to cause muscle contraction and generate force.

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Skeletal Muscle

This is the muscle tissue that is attached to your bones and is under your voluntary control.

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Myofibrils

These are the bundles of threadlike structures that are found within each of the muscle fibers.

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The Muscular System: An Overview

• The muscular system enables all body movement and makes up about 30–40% of body mass.

• It also maintains your posture, stabilizes joints, and helps generate body heat.

• Muscles are made of special cells called muscle fibers that can contract.

• This ability to contract and relax is what produces force for movement.

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Multiple Choice

What are the primary functions of the muscular system?

• Body movement, maintaining posture, stabilizing joints, and generating heat.

• Pumping blood, filtering waste, and producing hormones.

• Sending nerve signals and protecting internal organs.

• Only producing movement and maintaining body shape.

The Three Types of Muscle

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Skeletal Muscle

• Attaches to bones, allowing for voluntary, or conscious, body movements.

• ​​Appears striped, or striated, and contracts in short and strong bursts.

• ​There are over 600 of these muscles throughout the human body.

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Smooth Muscle

• ​Found in the walls of internal organs like the stomach.

• ​​This involuntary muscle works automatically without you thinking about it.

• ​It has a smooth, non-striated appearance and contracts slowly and steadily.

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Cardiac Muscle

• ​This specialized, striated muscle is found only in your heart’s walls.

• ​​As an involuntary muscle, it is responsible for pumping your blood.

• ​It is designed to never get tired and works your entire life.

Multiple Choice

Which type of muscle is found only in the heart and is considered involuntary?

• Skeletal muscle

• Smooth muscle

• Cardiac muscle

• Voluntary muscle

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How Skeletal Muscles Work

Bending the Elbow

• To bend your elbow, the biceps muscle on the front of your upper arm contracts.

• As the biceps muscle contracts and shortens, it pulls your forearm bone upwards.

• Meanwhile, the triceps muscle on the back of your upper arm relaxes to allow the movement.

Straightening the Elbow

• To straighten your elbow, the triceps muscle on the back of your upper arm contracts.

• As the triceps muscle pulls on the forearm, the arm begins to straighten out.

• While the triceps contracts, the biceps muscle on the front of the arm relaxes.

Multiple Choice

Why must skeletal muscles work in pairs?

• Because one muscle is voluntary and the other is involuntary.

• Because muscles can only contract and pull, not extend and push.

• To ensure both muscles get equal exercise.

• To connect to two different bones at the same time.

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The Sliding Filament Theory

• Muscles are made of muscle fibers, which contain hundreds of smaller myofibrils.

• Myofibrils have repeating sections called sarcomeres, which contain actin and myosin filaments.

• Myosin filaments pull actin filaments, causing them to slide past each other.

• This sliding action shortens the sarcomere and requires energy from ATP.

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Multiple Choice

According to the Sliding Filament Theory, what happens during muscle contraction?

• Actin and myosin filaments both get shorter.

• Myosin filaments slide along actin filaments, shortening the sarcomere.

• The Z lines move farther apart, extending the muscle.

• ATP is created by the movement of the filaments.

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General Characteristics of Muscles

• Excitability is the ability to receive and respond to stimuli like nerve impulses.

• Contractibility is the ability to shorten forcefully when stimulated, which causes movement.

• Extensibility is the ability of a muscle to be stretched without being damaged.

• Elasticity is the ability to recoil and return to its original resting length.

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Multiple Choice

What is the term for a muscle's ability to be stretched without being damaged?

• Excitability

• Contractibility

• Extensibility

• Elasticity

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Common Misconceptions

Misconception	Correction
Muscles push bones to create movement.	Muscles only pull, not push. Opposing muscles cause opposite movement.
All muscle movement is voluntary.	Only skeletal muscles are voluntary. Smooth and cardiac muscles are involuntary.
You stop using muscles when you are resting.	Involuntary muscles like the heart are always working, even during rest.

Multiple Choice

How do the characteristics of smooth muscle make it suited for its function in internal organs like the stomach?

• It contracts quickly and powerfully to grind food.

• It is voluntary, allowing you to consciously control digestion.

• It contracts slowly and steadily without tiring easily, perfect for sustained processes like digestion.

• It is striated, which allows it to expand and contract rapidly.

Multiple Choice

Why is a constant supply of ATP crucial for the process described in the sliding filament theory?

• ATP is required for the muscle to relax and extend.

• ATP provides the energy for myosin to pull on the actin filaments, causing the contraction.

• ATP forms the Z lines at the end of the sarcomere.

• ATP is the primary protein that makes up the actin filaments.

Multiple Choice

A person bends their arm to lift a box. Describe the roles of the biceps and triceps in this action.

• The triceps contracts, the biceps relaxes, and the tendons push the bone up.

• The biceps contracts, while the triceps relaxes.

• Both biceps and triceps contract, and the tendons decide which way the bone moves.

• The tendons contract, pulling the biceps and triceps, which then moves the bone.

Multiple Choice

Predict what would happen if a person's cardiac muscle tissue was replaced with skeletal muscle tissue.

• The heart would beat more slowly and efficiently.

• The heart would function normally as both are striated muscles.

• The person would have to consciously control their own heartbeat, and the muscle would tire quickly.

• The heart would become an involuntary organ.

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Summary

• The muscular system's main functions are movement, posture, and generating heat.

• The three muscle types—skeletal, smooth, and cardiac—are all excitable and elastic.

• Skeletal muscles attach to bones with tendons and work in pairs for movement.

• The Sliding Filament Theory explains how muscles contract using energy from ATP.

Poll

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