1. Muscular System: Histology and Physiology
Chapter 9
Muscular System: Histology and Physiology

2. Functions of the Muscular System
Body movement (skeletal muscles attached to bones)
Maintenance of posture
Respiration (skeletal muscles of thorax are responsible for the movement necessary for respiration)
Production of body heat (when skeletal muscles contact, heat is given off as a by-product)
Communication (speaking, writing)
Constriction of organs and vessels (contraction of smooth muscle)
Heart beat (contraction of cardiac muscle)

3. General Functional Characteristics of Muscle
Contractility: ability of a muscle to shorten with force
Excitability: capacity of muscle to respond to a stimulus (by nerve or hormone)
Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree
Elasticity: ability of muscle to recoil to original resting length after stretched

4. Types of Muscle Tissue Skeletal Smooth Cardiac
Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
Voluntary
Smooth
Walls of hollow organs, blood vessels, eye, glands, skin
Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow
In some locations, autorhythmic
Controlled involuntarily by endocrine and autonomic nervous systems
Cardiac
Heart: major source of movement of blood
Autorhythmic

6. Skeletal Muscle Structure
Composed of muscle cells (fibers), connective tissue, blood vessels, nerves
Fibers are long, cylindrical, multinucleated
Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length
Develop from myoblasts (they are converted to muscle fibers as contractile proteins accumulate within their cytoplasm); numbers remain constant (# of muscle fibers remain constant after birth----so, enlargement of muscles is an increase in size rather than #)
Striated appearance due to light and dark banding

7. Connective Tissue Coverings of Muscle
Layers
External lamina. Delicate, reticular fibers. Surrounds sarcolemma (P.M.)
Endomysium. Loose C.T. with reticular fibers.
Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculus
Epimysium. C.T. that surrounds a whole muscle (many fascicles)
Fascia: connective tissue sheet
Forms layer under the skin
Holds muscles together and separates them into functional groups.
Allows free movements of muscles.
Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics.
Continuous with connective tissue of tendons and periosteum.

8. Nerves and Blood Vessel Supply
Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves
Axons branch so that each muscle fiber is innervated
Capillary beds surround muscle fibers

9. Muscle Fibers Nuclei just inside sarcolemma
Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm without myofibrils)
Threadlike (extends from one end of muscle fiber to the other)
Composed of protein threads called myofilaments: thin (actin 8nm) and thick (myosin 12nm)
Sarcomeres: actin & myosin myofilaments form highly ordered units called sarcomeres. They are joined end to end to form the myofibrils.

10. Parts of a Muscle

11. Actin and Myosin Myofilaments

12. Actin (Thin) Myofilaments
Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.
Composed of G actin monomers each of which has an active site
Actin site can bind myosin during muscle contraction.
Tropomyosin: an elongated protein winds along the groove of the F actin double helix.
Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands.
The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

13. Myosin (Thick) Myofilament
Many elongated myosin molecules shaped like golf clubs.
Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.
Myosin heads
Can bind to active sites on the actin molecules to form cross-bridges.
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

14. Sarcomeres: Z Disk to Z Disk
Z disk: filamentous network of protein. Serves as attachment for actin myofilaments
Striated appearance
I bands: from Z disks to ends of thick filaments
A bands: length of thick filaments
H zone: region in A band where actin and myosin do not overlap
M line: middle of H zone; delicate filaments holding myosin in place
In muscle fibers, A and I bands of parallel myofibrils are aligned.
Titin filaments: elastic chains of amino acids; make muscles extensible and elastic

15. Sliding Filament Model
Actin myofilaments sliding over myosin to shorten sarcomeres
Actin and myosin do not change length
Shortening sarcomeres responsible for skeletal muscle contraction
During relaxation, sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity.
- agonist = muscle that accomplishes a certain movement, such as flexion.
- antagonist = muscle acting in opposition to agonist.

16. Sarcomere Shortening

17. Physiology of Skeletal Muscle Fibers
Nervous system controls muscle contractions through action potentials
Resting membrane potentials
Membrane voltage difference across membranes (polarized)
Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out (through leak channels) but not completely because negative molecules hold some back.
Outside cell more positive and more Na+ on outside than inside.
Na+ /K+ pump maintains this situation.
Must exist for action potential to occur

18. Ion Channels Types Each is specific for one type of ion
Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site
Example: neurotransmitters
Gate is closed until neurotransmitter attaches to receptor molecule. When Ach (acetylcholine) attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient
Voltage-gated
Open and close in response to small voltage changes across plasma membrane
Each is specific for one type of ion

19. Action Potentials Phases
Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occurs
Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential
All-or-none principle: like camera flash system
Propagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location.
Frequency: number of action potential produced per unit of time

20. Gated Ion Channels and the Action Potential

21. Action Potential Propagation

22. Neuromuscular Junction
Synapse: axon terminal resting in an invagination of the sarcolemma
Neuromuscular junction (NMJ):
Presynaptic terminal: axon terminal with synaptic vesicles
Synaptic cleft: space
Postsynaptic membrane or motor end-plate

23. Function of Neuromuscular Junction
Synaptic vesicles
Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane.
Acetylcholine
Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh

25. Excitation-Contraction Coupling
Mechanism by which an action potential causes muscle fiber contraction
Involves
Sarcolemma
Transverse (T) tubules: invaginations of sarcolemma
Terminal cisternae
Sarcoplasmic reticulum: smooth ER
Triad: T tubule, two adjacent terminal cisternae
Ca2+
Troponin

26. Action Potentials and Muscle Contraction

27. Cross-Bridge Movement