Muscle contraction

Muscle Contractions

Guided Notes

Introduction to Muscle Contraction

Explain how a person is able to bend their arm: relaxation and contraction of the arm

Complete the chart with information about the characteristics of muscles:

Excitability

Contractibility

Extensibility

Elasticity

Respond to stimuli by transmitting electrical impulses along their membranes

Enables the generation of force as they shorten and produce movement

Allows them to stretch within limits

The ability of an object to return to its normal shape after being stretched or compressed

Muscle Cells

Focus Question: What are the components of a muscle fiber (cell)?

Complete the sentences:

Muscle fascicles are

Muscle fiber are

Check the interactive and complete the chart with information about the components of muscle fibers:

Myofibrils

Myosin

Sarcomeres

Actin

The cells of our skeletal muscles (also called muscle fibers) are made up of complex proteins called myofibrils.

Within each sarcomere, there are layers of protein strands called myofilaments. Myosin is a myofilament with golf-club-shaped “heads” attached to them. They are located at the center of a sarcomere.

Myofibrils divided into sections called sarcomeres. All of the exciting chemistry that creates muscle movement occurs here in the sarcomere. Each muscle has thousands of sarcomeres that contract together to produce movement.

Within each sarcomere, there are layers of protein strands called myofilaments. Actin is a myofilament made of thinner proteins. They surround the thicker myosin strand in the sarcomere. The actual number of actin strands encircling myosin depends upon the specific muscle. Larger muscles would have more actin, and smaller muscles would have fewer.

Complete the Check Your Understanding and write the answers here:

Which myofilaments are part of a sarcomere? Myosin and Actin

What part of the muscle contracts to produce movement? Sacromeres

Muscle fibers (cells) are made of smaller complex proteins called Myofibrils

Sliding Filament Theory

Focus Question: What are the steps of the sliding filament theory?

Explain the sliding filament theory:

ATP and ADP

Complete the sentences:

ATP stands for adenosine triphosphate

ATP is a molecule that that stores energy for cells to use in the body

Muscles need ATP to provide energy to fuel the sliding movement that occurs when the muscle contracts

When ATP releases energy, it becomes ADP

When ATP loses a phosphate atom, it releases energy

When ADP gains a phosphate atom, it recharges to ATP

Theory in Action

Complete the chart with information about the seven steps in the sliding filament theory:

Steps

Information

Muscles at Rest

When a muscle is at rest, there are two tiny proteins called tropomyosin and troponin that are wrapped around the actin. There are also ADP molecules stuck to each one of the myosin heads when the muscle is in a relaxed state. It is the tropomyosin, troponin, and ADP that keep the myosin heads from extending and attaching to the protected actin strand. Notice the muscle is in a flexed state in the image because the myosin heads are extended and bonded to the actin strands. When the muscle cell is in a resting state, the myosin and actin strands are not in contact with one another.

Waiting Muscles

During rest periods, the muscle builds up an abundance of calcium ions that will be used when it is time for the muscle to contract. The cell gets the Ca²⁺ ions through the calcium pumps of the sarcoplasmic reticulum (SR), which is wrapped around each sarcomere.

Excitation

When it's time for muscle movement, the neurons surrounding the muscle send signals to stimulate action. When the signal reaches the SR, the calcium pumps open wide, and the calcium ions begin to flow.

Ready for Bonding

At this point, the calcium ions bind to the troponin. This causes the troponin to change shape and rotate around the actin, which moves the tropomyosin out of the way. With tropomyosin out of the way, binding sites on the actin molecule are open and ready to bind with myosin.

The breakdown of ATP to ADP releases energy used to release the heads of the myosin. Now myosin is ready for bonding to actin.

Building a Cross-Bridge

With the binding sites open, the myosin heads extend and attach to the actin. This attachment is called a cross-bridge. The contact causes the myosin heads to bend toward the center of the sarcomere, shortening the overall length. We call this sarcomere shortening a muscle contraction.

The Slide

As the sarcomeres shorten, the myosin will release ADP but remains attached to actin. This occurs throughout each contraction until ADP is released, leaving actin in a fixed state until another ATP binds to myosin, causing the movement again.

The steps of muscle contraction and relaxation repeat as long as ATP and calcium are present.

Muscle Relaxation

What gets the contraction to stop? When there is no longer a nerve stimulus, the calcium ions diffuse back inside the SR. The troponin and tropomyosin return to their resting positions. ATP molecules attach to the heads of myosin. It is the energy that is released by the ATP molecule on the myosin heads that fuels muscle relaxation.

Voluntary Muscle Contractions

Focus Question: How do voluntary muscles work?

Answer:

Why are voluntary muscles attached to bones considered voluntary? Because the brain can actively decide to move them

What’s the goal of the skeletal muscle? to create torque and tension to move a bone.

What is torque compared to tension? Use the example of the biceps and triceps to explain.

Torque is the rotational force exerted by muscles to support joints and ligaments, but at the same time pick something up. In contrast, tension is the generation of force by a single muscle fiber (cell) by the interactions between actin and myosin.

Skeletal Muscle Fibers

Complete the sentence:

The two types of skeletal muscle fibers are slow twitch and fast twitch

Those two types differ in their speed of contraction and endurance

Most of a person’s muscles are made up of both slow twitch and fast twitch muscle fibers

Answer:

Why do the muscles in a person’s back contain mostly slow-twitch muscle fibers?

To maintain posture

Why are the muscles in a person’s eyes mostly made up of fast-twitch muscle fibers?

Write S for Slow-Twitch Fibers and F for Fast-Twitch Fibers next to each statement. The first one is done for you:

S	Most active during endurance activities like long-distance running or cycling
F	Pale in color
F	Contain fewer mitochondria
S	Can contract for long periods before stopping
S	Are red in color
F	Ideal for activities like jumping to catch a ball, powerlifting, or running short sprints
S	Contain increased ATP
F	Contract quickly but stop after a short time
F	Produce less ATP at one time
S	Contain a lot of mitochondria

Involuntary Muscle Contractions

Focus Question: How do involuntary muscles work?

Answer:

Where can we find the involuntary muscles? the intestines, stomach, uterus, blood vessels, and heart

What are involuntary muscles broken into? Smooth and cardiac muscles

Where are the cardiac muscles? Only found in the heart

How are involuntary muscles different from skeletal muscles? in that the brain does not have to make a conscious effort to move them

What makes involuntary muscles contraction possible? They are constantly contracting as long as the needed molecules are available for the contraction to happen.

Complete the charts with information about contraction and about voluntary and involuntary muscles:

Name of contraction: smooth muscle contraction

Explanation: The involuntary movement of smooth muscles means that they consistently push fluids or other substances through them. This keeps blood and digestion flowing throughout all activities, including sleeping.

Name of contraction: cardiac muscle contraction

Explanation: Unlike smooth muscles, cardiac muscle uses strong, strategic contractions of the muscles in unison to propel blood throughout the heart's chambers and the body's blood vessels.

Write V for Voluntary Muscle (Skeletal) and I for Involuntary Muscle (Smooth and Cardiac) next to each statement. The first one is done for you:

I	Spindle-shaped, fairly small cells
V	Involved in body part movement
V	Get fatigued
I	Involved in internal organ movement
V	Cylindrical, unbranched, and fairly long cells
V	Cells with multiple nuclei
I	Mostly low energy
V	Unique examples: diaphragm, pharynx, abdominal muscles, middle ear muscles, muscles under the skin
V	Mostly high energy
I	Cells with one nucleus each
I	Do not get fatigued
I	Unique examples: ducts of glands, urogenital tracts, respiratory tract

Vital Signs

Answer:

What are vital signs helpful for?

After you complete the interactive, number these steps of the stages of the sliding filament theory from top to bottom, The first one is completed for you:

5	The flexing of the cross-bridge pulls the actin filament toward the center of the sarcomere.
1	Calcium ions are released by sarcoplasmic reticulum into the sarcoplasm.
3	The breakdown of ATP releases energy, releasing the head of the myosin.
6	An ATP molecule reattaches to the ATP binding site on the myosin head.
4	The myosin head attaches to the exposed binding site on the actin filament, forming a cross-bridge.
2	Calcium ions bind to troponin, exposing the binding site on the actin filament.
8	With the ATP molecule in place on the myosin head and calcium ions present, the cycle can continue.
7	The myosin head is released from the actin filament’s binding site and the binding site is covered up again.