1. The Muscular System

2. There are four characteristics associated with muscle tissue:
Excitability
Contractility
Extensibility
Elasticity
- Tissue can receive & respond to stimulation
- Tissue can shorten & thicken
- Tissue can lengthen
- After contracting or lengthening, tissue always wants to return to its resting state

3. The characteristics of muscle tissue enable it to perform some important functions, including:
Movement – both voluntary & involuntary
Maintaining posture
Supporting soft tissues within body cavities
Guarding entrances & exits of the body
Maintaining body temperature

4. Types of muscle tissue:
Skeletal
Cardiac
Smooth (Visceral)

5. Types of muscle tissue:
Skeletal muscle tissue
Associated with & attached to the
skeleton
Under our conscious (voluntary) control
Microscopically the tissue appears striated
Cells are long, cylindrical & multinucleate

6. Cardiac muscle tissue
Makes up myocardium of heart
Unconsciously (involuntarily) controlled
Microscopically appears striated
Cells are short, branching & have a single nucleus
Cells connect to each other at intercalated discs

7. Makes up walls of organs & blood vessels
Smooth (visceral) muscle tissue
Makes up walls of organs & blood vessels
Tissue is non-striated & involuntary
Cells are short, spindle-shaped & have a single nucleus
Tissue is extremely extensible, while still retaining ability to contract

8. Connective Tissue of a Muscle
Epimysium. Dense regular connective tissue surrounding the entire muscle
Separates muscle from surrounding tissues and organs
Connected to the deep fascia
Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle
Contains b.v and nerves
Endomysium. Loose connective tissue that surrounds individual muscle fibers
Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue)
Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

9. Anatomy of skeletal muscles
epimysium
tendon
perimysium
Muscle Fascicle
Surrounded by perimysium
endomysium
Skeletal muscle
Skeletal muscle fiber (cell)
Surrounded by epimysium
Surrounded by endomysium
Play IP Anatomy of Skeletal muscles (IP p. 4-6)

10. Basic Features of a Skeletal Muscle
Muscle attachments
Most skeletal muscles run from one bone to another
One bone will move – other bone remains fixed
Origin – less movable attachment
Insertion – more movable attachment

11. Microanatomy of a Muscle Fiber (cell)

12. Muscle Fiber Anatomy Sarcolemma - cell membrane
Surrounds the sarcoplasm (cytoplasm of fiber)
Punctuated by openings called the transverse tubules (T-tubules) - Narrow tubes that extend into the sarcoplasm that are filled with extracellular fluid
Contains many of the same organelles seen in other cells
An abundance of the oxygen-binding protein myoglobin
Myofibrils -cylindrical structures within muscle fiber
Are bundles of protein filaments (=myofilaments)
Two types of myofilaments
Actin filaments (thin filaments)
Myosin filaments (thick filaments)
When myofibril shortens, muscle shortens (contracts)

13. Microanatomy of a Muscle Fiber (Cell)
transverse (T) tubules
sarcoplasmic reticulum
sarcolemma
terminal cisternae
myoglobin
mitochondria
thick myofilament
thin myofilament
myofibril
triad
nuclei

14. Sarcomere - repeating functional units of a myofibril
*Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.
A bands: a dark band; full length of thick (myosin) filament
M line - protein to which myosins attach
H zone - thick but NO thin filaments
I bands: a light band; from Z disks to ends of thick filaments
Thin but NO thick filaments
Extends from A band of one sarcomere to A band of the next sarcomere
Z disk: filamentous network of protein. Serves as attachment for actin myofilaments
Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

16. Thin Myofilament
(myosin binding site)

17. Thick myofilament
(has ATP & actin binding site)
*Play IP sliding filament theory p.5-14 for overview of thin & thick filaments

18. Sarcomere A band Z line Z line I band Zone of overlap M line
H zone
I band
Zone of overlap
Zone of overlap
M line
Thin myofilaments
Thick myofilaments

19. Sarcoplasmic Reticulum (SR)
SR is an elaborate, smooth endoplasmic reticulum
runs longitudinally and surrounds each myofibril
Form chambers called terminal cisternae on either side of the T-tubules
A single T-tubule and the 2 terminal cisternae form a triad
SR stores Ca++ when muscle not contracting
When stimulated, calcium released into sarcoplasm
SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

20. Sarcoplasmic Reticulum (SR)

21. Myosin (Thick) Myofilament
Many elongated myosin molecules shaped like golf clubs.
Single filament contains roughly 300 myosin molecules
Myosin heads
Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)
Attached to the rod portion by a hinge region that can bend and straighten during contraction.
Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

22. Actin (Thin) Myofilaments
Thin Filament: composed of 3 major proteins
F (fibrous) actin
Tropomyosin
Troponin

23. Sliding Filament Theory – Ca2+
Muscle contraction occurs when the muscle receives an impulse from the nerve cell in the form of an action potential.
The nerve impulse penetrates the muscle fiber via the transverse tubules
This causes calcium (found in the terminal cisternae) to be released.
Calcium ions bind to troponin (on actin)

24. Sliding Filament Theory – Ca2+
During binding, troponin changes shape, causing troponin molecules to move out the way of the myosin binding site
Extended myosin heads bind to actin
Bound ADP and phosphate are released, causing the myosin heads to return to their original position
Returning to their original position allows actin filaments to be pulled into the sarcomere

25. Sliding Filament Theory – Ca2+
ATP binds to myosin heads causing them to let go
Process repeats itself so long as calcium is present
Overview of Sliding Filament Theory Animation

26. Sliding Filament Theory
Myosin heads attach to actin molecules (at binding (active) site)
Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere
Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

27. Once a muscle fiber begins to contract, it will contract maximally
As sarcomeres shorten, myofibril shortens. As myofibrils shorten, so does muscle fiber
Once a muscle fiber begins to contract, it will contract maximally
This is known as the “all or none” principle
Sliding Filament Theory Animation