Muscles II: Microscopic Anatomy and Contraction

October 29 -30 2015

Muscle Structure

Muscle

Fascicle (bundle of fibers)

Muscle Fiber (single cell)

Myofibril (organelle)

Sarcomere (unit of contraction)

Microscopic Anatomy of Skeletal Muscle

• Large, cylindrical, multinucleate cells• Contain many mitochondria; nearly filled with

myofibrils• Some organelles have unique vocabulary:

– Sarcolemma: cell membrane– Sarcoplasm: cytoplasm– Sarcoplasmic reticulum: modified ER, surrounds each myofibril; store Ca2+

• Each myofibril can be divided into contractile units called sarcomeres.

• Sarcomeres consist of overlapping protein filaments of actin and myosin.

• Regular arrangement of dark and light bands. Dark bands occur where myosin is present.

Microscopic Anatomy of Skeletal Muscle

• The M line is where the myosin attaches

• Z discs (a membrane) mark the edge of each sarcomere; serve as attachment site for actin

Microscopic Anatomy of Skeletal Muscle

Use the picture to come up with a definition of the following:

I band

A band

H zone

Microscopic Anatomy of Skeletal Muscle

Use the picture to come up with a definition of the following:

I band – area without myosin fibers; aka light band

A band – area with myosin fibers; aka dark band

H zone – area without actin fibers

Microscopic Anatomy of Skeletal Muscle

First match the words …

actin cell

myofibril group of cells

sarcomere cell membrane

fascicle protein

muscle fiber organelle

sarcolemma contractile unit Then, write a paragraph that uses all the words in both

columns above and explains that structure of the muscle.

Turn & Talk

Contraction Overview• Globular heads of

myosin filaments attach to actin filaments.

• Myosin pulls actin filaments : “Sliding filament theory”

• Causes sarcomere to shorten, particularly the light bands

Contraction Overview• Globular heads of

myosin filaments attach to actin filaments.

• Myosin pulls actin filaments : “Sliding filament theory”

• Causes sarcomere to shorten, particularly the light bands

light dark light

light dark light

Contraction Overview

Which shows contracted muscle fibers?How can you tell?

Contraction Overview

Relaxed muscle Contracted muscle (large light bands) (small light bands)

Contraction Overview

What are these?

See animation!

Mitochondria

Contraction Details1. A motor neuron stimulates the muscle cell by

releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.

Note:A motor unit is a single motor neuron and all the muscle fibers it activates

Contraction Details1. A motor neuron stimulates the muscle cell by

releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.

2. ACh causes an electric current called an action potential to move through the muscle cell.

Contraction Details1. A motor neuron stimulates the muscle cell by

releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.

2. ACh causes an electric current called an action potential to move through the muscle cell.

3. The action potential causes the release of Ca2+ from the sarcoplasmic reticulum.

Contraction Details4. Ca2+ exposes myosin-binding sites on actin

filaments.

5. Myosin heads (& ADP) attach to actin binding sites, forming cross-bridges.

Muscle relaxed. Ca2+ present. No Ca2+ present. Cross-bridge formed.

myosin

actin

ADP + P

myosin head

Contraction Details6. Myosin heads release ADP, move the actin filament

in “power stroke”

Power stroke,ADP + P released

myosin

actin

Contraction Details6. Myosin heads release ADP, move the actin filament

in “power stroke”

7. ATP binds to myosin head. The crosslink between actin and myosin breaks.

8. ATP becomes ADP + P, readying the myosin head to reattach to actin.

Power stroke, ATP binds,ADP + P released cross-links break

myosin

actin

Contraction Details• If Ca2+ is still present, cycle will repeat, with myosin

heads reattaching and contracting the muscle even more.

• Once the action potential is over, the Ca2+ is reabsorbed into the sarcoplasmic reticulum. Without Ca2+, myosin cannot attach to actin.

Watch me!

Contraction DetailsNOTE:

ATP is required to breakcross-links, not to form them.

Explains rigor mortis

Why then do muscles need

ATP?

To reset head so it can contract further -- contraction is a series of sliding motions.

Turn & TalkDescribe the role of each of the following in muscle

contraction

Scholar with more siblings….• ACh• Ca2+

Scholar with less siblings … • ATP• Action potential

Exit Ticket1. In comparing electron micrographs of a

relaxed skeletal muscle fiber and a fully contracted muscle fiber, which would be seen only in the relaxed fiber?

a) Z discsb) Triadsc) I bandsd) A bandse) H zones

Exit Ticket2. Which word describes the unit of contraction of a muscle?

a) Myofibrilb) Sarcomerec) A bandd) H band

Exit Ticket3. Which of the following correctly lists the order of structure of the muscle from

largest to smallest?

a) fascicle, myofibril, sarcomere, muscle fiberb) myofibril, fascicle, sarcomere, muscle fiberc) fascicle, muscle fiber, myofibril, sarcomere d) muscle fiber, fascicle, myofibril, sarcomere

Exit Ticket4. Which of these stores calcium ion?

a) Sarcoplasmic reticulumb) Sarcomerec) Sarcolemmad) mitochondria

Exit Ticket5. Which of these best describe the

process of muscle contraction?

a) The actin filaments shortenb) The myosin filaments shortenc) The light bands shortend) The dark bands shorten

Exit Ticket6. Which of these best describe the process of muscle contraction?

a) Myosin heads attach to actin filaments that are exposed by the presence of ATP

b) Myosin heads attach to actin filaments that are exposed by the presence of Ca2+

c) Actin heads attach to myosin filaments that are exposed by the presence of ATP

d) Actin heads attach to myosin filaments that are exposed by the presence of Ca2+