1. Chapter 6
The Muscular System

2. Learning Objectives
List the four traits that all muscle types have in common.
Demonstrate and explain the use of antagonistic muscle pairs.
Describe the attachment of muscle to bone and how it leads to movement.
Define the terms origin and insertion as they apply to muscle anatomy.
Compare the terms muscle fiber and fascicles.
Discuss the banded appearance of striated muscles using the terms myofibril, myofilaments, actin, and myosin.
Explain muscle contraction at the molecular level by describing the sliding filament model.
Explain why ATP is essential to muscle contraction and relaxation.
Describe the role of the tropomyosin-troponin complex in muscle contraction.
Draw a neuromuscular junction, and explain the role of acetylcholine in muscle contraction and the release of calcium ions.
Describe the ways to vary the contraction of whole muscles.
Explain the process of motor unit recruitment.
Differentiate between a muscle twitch, a stronger contraction resulting from wave summation, tetanus, and fatigue.
List the sources of ATP for muscle contraction, and describe in detail where and how the ATP is generated.
Compare and contrast slow-twitch and fast-twitch muscles, including where they are located in the body and when they are utilized in different physical activities.
Describe the best way to build muscle endurance and the requirements for building larger muscle mass.

3. Function and Characteristics of Muscles
All muscles are
Excitable (they respond to stimuli)
Contractile (they can shorten)
Extensible (they can stretch)
Elastic (they can return to their original length after being shortened or stretched)
Three types of muscle
Skeletal
Cardiac
Smooth

4. Function and Characteristics of Muscles
Skeletal muscles are voluntary muscles responsible for
Moving our body
Maintaining posture
Supporting internal organs
Pushing against veins and lymphatic vessels to move blood and lymph along
Generating heat

5. Skeletal Muscles Working in Pairs
The body has more than 600 skeletal muscles
Synergistic muscles
Muscles that must contract at the same time to cause movement
Antagonistic muscles
Movement is produced when one muscle of the pair contracts and the other relaxes
Example: the biceps muscle and triceps muscle of the upper arm

6. Figure 6.1 Skeletal muscle.

7. Figure 6.2 Some major muscles of the body.7

8. Damage to Skeletal Muscles or Tendons
Most of the major muscles we use for locomotion, manipulation, and other voluntary movements are attached to bones
Tendon
Band of connective tissue that attaches a muscle to a bone
Origin of a muscle
The end attached to the bone that remains relatively stationary during movement
Insertion of a muscle
The end attached to the bone that moves

9. Damage to Skeletal Muscles or Tendons
Tendinitis
Condition of having an inflamed tendon
Caused by overuse, misuse, or age
Healing is slow because tendons have a poor blood supply
Most effective treatment is rest

10. Damage to Skeletal Muscles or Tendons
Muscle pull
Also called a muscle strain or tear
Caused by overstretching that damages the muscle or tendon
Treatment includes ice to reduce swelling and keeping the muscle stretched

11. Contraction of Muscles
An entire, intact muscle is formed from individual muscle fibers grouped in increasingly larger bundles, each wrapped in a connective tissue sheath
Fascicle: a bundle of muscle cells
A skeletal muscle has many fascicles
Each fascicle is surrounded by its own connective tissue sheath
The connective tissue sheaths of fascicles merge at the ends of muscles to form tendons that attach the muscle to bone

12. Contraction of Muscles
A muscle cell  a muscle fiber
When skeletal muscle cells are viewed under a microscope, they have distinct bands called striations formed by the arrangement of myofibrils within the cell
Myofibrils are specialized bundles of proteins
Each myofibril contains two types of myofilaments
Myosin (thick) filaments
Actin (thin) filaments are more numerous

13. Contraction of Muscles
Each myofibril has tens of thousands of contractile units, called sarcomeres. The ends of each sarcomere are marked by dark protein bands called Z lines. Within each sarcomere the actin and myosin filaments are specifically arranged
One end of each actin filament is attached to a Z line
Myosin filaments lie in the middle of the sarcomere, and their ends partially overlap with surrounding actin filaments. The degree of overlap increases when the muscle contracts

14. Figure 6.3 The structure of a skeletal muscle.14

15. Contraction of Muscles
Muscle contraction occurs at the molecular level
According to the sliding filament model, a muscle contracts when actin filaments slide past myosin filaments, shortening the sarcomere
Myosin molecules are shaped like two-headed golf clubs. The club-shaped myosin heads are key to moving actin filaments

16. Contraction of Muscles
The myosin head, also known as a cross-bridge, attaches to a nearby actin filament
Then the head bends and swivels, pulling the actin filament toward the midline of the sarcomere
The myosin head disengages from the actin filament
The movements of myosin require ATP
The cycle begins again

17. Figure 6.4 When a muscle contracts, actin filaments slide past myosin filaments.17

18. Contraction of Muscles
Muscle contraction is controlled by the availability of calcium ions
Muscle cells contain the proteins troponin and tropomyosin
The troponin-tropomyosin complex and calcium ions regulate muscle contraction at the actin-myosin binding sites

19. Contraction of Muscles
When a muscle is relaxed, the troponin- tropomyosin complex covers the actin-myosin binding sites
Muscle contraction occurs when calcium ions bind to troponin, causing it to change shape
This change in shape moves tropomyosin, exposing the actin-myosin binding sites

20. Figure 6.5 Calcium ions initiate muscle contraction.20

21. Calcium Ions and Regulatory Proteins
Calcium ions are stored in the sarcoplasmic reticulum, a form of smooth endoplasmic reticulum found in muscle cells
Transverse tubules (T tubules)
Pockets in the plasma membrane of a muscle cell
Carry signals from motor neurons deep into the muscle cell to every sarcomere

22. Calcium Ions and Regulatory Proteins
Rigor mortis
Muscle contraction will occur as long as ATP is present
Without ATP, cross-bridges cannot be broken
Within 3 to 4 hours after death, the muscles become stiff  rigor mortis
Actin and myosin gradually break down and muscles relax again after 2 to 3 days

23. Role of Nerves Neuromuscular junction
Junction between the tip of a motor neuron and a skeletal muscle cell
A nerve impulse travels down a motor neuron to the neuromuscular junction, where it causes the release of acetylcholine (a neurotransmitter) from the motor neuron
Acetylcholine diffuses across a small gap and binds to receptors on the plasma membrane of the muscle cell

24. Role of Nerves
The acetylcholine causes changes in the permeability of the muscle cell, resulting in an electrochemical message similar to a nerve impulse
The message travels along the plasma membrane into the T tubules and then to the sarcoplasmic reticulum, releasing calcium ions for muscle contraction

25. Role of Nerves
Web Activity: Muscle Structure and Function

26. Figure 6.6 Neuromuscular junction.26

27. Muscular Dystrophy (MD)
A group of inherited conditions in which muscles weaken: if too many calcium ions enter a muscle cell, proteins may be destroyed, eventually causing the cell to die; on a large scale, muscles weaken
Duchenne muscular dystrophy
One of the most common forms
The gene for production of the protein dystrophin is defective
Lack of dystrophin allows excess calcium ions to enter muscle cells, eventually killing the cells

28. Voluntary Movement
Motor unit: a motor neuron and all the muscle cells it stimulates
All the muscle cells in a given motor unit contract together
The number of muscle cells in a motor unit is highly variable
Muscles responsible for precise movements have fewer muscle cells in each motor unit than do muscles responsible for less precise movements
On average, there are 150 muscle cells in a motor unit

29. Figure 6.7 A motor unit. 29

30. Motor Units and Recruitment
Increasing the number of motor units that are stimulated increases the strength of muscle contraction. This process, performed by the nervous system, is called recruitment.
Muscle twitch
Contraction of a muscle in response to a single stimulus
Twitches are very brief and typically not part of normal movements
If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to summation

31. Muscle Twitches, Summation, and Tetanus
A sustained, powerful contraction caused by very frequent stimuli
Fatigue sets in when a muscle is unable to contract even when stimulated
Changing the frequency of stimulation is another way to vary the contraction of muscles

32. Figure 6.8 Muscle contraction shown graphically.32

33. Energy for Muscle Contraction
Muscle contraction requires an enormous amount of energy
ATP for muscle contraction comes from many sources, typically used in sequence
ATP stored in muscle cells
Creatine phosphate stored in muscle cells
Anaerobic metabolic pathways
Aerobic respiration

34. Figure 6.9 Energy sources for muscle contraction.34

35. Slow-Twitch and Fast-Twitch Muscle Cells
Slow-twitch muscle cells
Contract slowly, with great endurance
Abundant mitochondria
Packed with myoglobin (oxygen-binding pigment)
Dark, reddish appearance
Myoglobin
Rich blood supply

36. Slow-Twitch and Fast-Twitch Muscle Cells
Contract rapidly and powerfully but with much less endurance
Can make and break cross-bridge attachments more rapidly
Have more actin and myosin
Rely on anaerobic metabolic pathways to generate ATP and therefore tire quickly

37. Figure 6.10 Slow- and fast-twitch muscle cells.37

38. Building Muscle Aerobic exercise
Enough oxygen is delivered to the muscles to keep them going for long periods
Increases endurance and coordination
Promotes development of new blood vessels
Increases the number of mitochondria
Typically does not increase size of muscles
Examples: walking, jogging, swimming

39. Building Muscle Resistance exercise Builds strength
Muscles increase in size when they are repeatedly made to exert more than 75% of their maximum force
Increases in muscle size reflect increases in the diameter of existing muscle cells
Example: weight lifting