II. Skeletal Muscle Overview

A. Skeletal Muscle Distinguishing Characteristics

• Striated

• Voluntary

• Multi-nucleated

B. Functions

• Movement

• Maintain Posture

• Stabilize Joints

• Generate Heat

III. Skeletal Muscle AnatomyMacroscopic Anatomy

• Fascia surrounds each muscle.

Deep layer called epimysium.

• Bundles of muscle cells (fascicles)are

surrounded by perimysium

• Each muscle cell (fiber)

surrounded by endomysium

These connective tissues allow each

structure to remain separate and move

independently. Blood vessels and nerves pass through these tissues.

Figure 8.1

III. Skeletal Muscle AnatomyA. Microscopic Anatomy

Each muscle cell has:

- membrane (sarcolemma)

- cytoplasm (sarcoplasm)

- many nuclei

- mitochondria

- many myofibrils made of thick (myosin) and thin (actin) protein filaments. Myofibrils are organized in repeating units called sarcomeres: these are the units that contract.

Figure 8.1

III. Skeletal Muscle Anatomy

B. Microscopic Anatomy

• Surrounding each myofibril is a network of channels called the sarcoplasmic reticulum and transverse tubules.

Figure 8.4

III. Skeletal Muscle Anatomy

C. Sarcomere Anatomy

• Sarcomeres consists of 2 overlapping myofilaments with a repeated pattern.

• Thin filament-actin

• Thick filament-myosin

Figure 8.2

Muscle contraction video

II. Skeletal Muscle AnatomyC. Sarcomere Anatomy

Figure 8.3

III. Skeletal Muscle Anatomy

D. Anatomy of Filaments

1. Myosin• Myosin molecules have long, rod-shaped tails with globular

heads. • The heads form cross bridges between myosin and actin.

III. Skeletal Muscle Anatomy

D. Anatomy of Filaments

2. Actin• 2 proteins, troponin and tropomyosin, help to control the

myosin-actin interactions in muscle contraction.

Troponin Tropomyosin

III. Skeletal Muscle Anatomy

D. Anatomy of Filaments

II. Skeletal Muscle Anatomy

E. Summary

Summary Animation

IIII. Skeletal Muscle Contraction

A. Overview of the Sliding Filament Theory

1. When a muscle cell contracts, individual sarcomeres shorten.

2. Thin filaments slide past thick filaments, so that they overlap to a greater degree.

3. I bands shorten, H zones disappears, and A bands move closer together.

Simplified Animation Figure 8.8

IIII. Skeletal Muscle Contraction

B. ATP Background Information

1. ATP consists of an adenine nucleotide attached by high-energy bonds to three phosphate groups.

2. When the terminal phosphate group is off, energy is released.

3. ATPADP + Pi + energy

4. This energy is available toperform cellular work.

III. Skeletal Muscle Contraction

C. Detailed Sequence of Events

1. Cross bridge attachment

• Myosin heads bind to actin

2. The working stroke

• Myosin head pivots, propelling the actin

Figure 8.7

IIII. Skeletal Muscle Contraction

C. Detailed Sequence of Events

3. Cross bridge detachment

• ATP binds to myosin head, allowing it to release actin

4. Cocking of myosin head

• Breakdown of ATP ADP + Pi provides energy to cock myosin head

Figure 8.7Myosin Head Animation

IIII. Skeletal Muscle Contraction

D. Role of Calcium in Muscle Contraction

1. A nervous impulse triggers the release of Ca2+ from the sarcoplasmic reticulum.

2. Ca2+ binds to troponin, altering the shape and position of troponin.

3. This causes movement of the attached tropomyosin molecules, exposing the myosin binding site.

4. When calcium levels drop, the tropomyosin blockade is reestablished and contraction ends.

Cross Bridge/ATP Animation

III. Skeletal Muscle Contraction

Summary

Figure 8.7Summary Animation