1. The Muscular System
Chapters 9 & 10

2. Muscle Types: General Characteristics
There are three types of muscle tissue: skeletal, smooth & cardiac
Most muscle cells are elongated and called muscle fibers (not true of cardiac cells)
Muscle contraction depends on two kinds of myofilaments, thin actin and thicker myosin containing microfilaments.
Prefixes myo and mys (“muscle”) and sarco (“flesh”) always refer to muscles.
Ex) Sarcolemma (“muscle husk”) is the plasma membrane of muscle fibers

3. Muscle Types: Skeletal Muscle Tissue
Attached to bones
Makes up 40% of body weight
Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
Under conscious (voluntary) control; controlled by somatic motor neurons
Microscopically, tissue appears striated; Cells are long, cylindrical and multi-nucleate

4. Muscle Types: Smooth Muscle Tissue
Makes up walls of organs and blood vessels; Propel urine, mix food in digestive tract, dilate/constrict pupils, regulate blood flow
Involuntary control by endocrine and autonomic nervous system
Tissue is non-striated; Cells are short, spindle-shaped and mononucleated
Extremely extensible but maintains ability to contract

5. Muscle Types: Cardiac Muscle Tissue
Makes up myocardium of heart
Autorhythmic, generates movement of blood
Unconciously (involuntarily) controlled by endocrine and autonomic nervous systems
Microscopically appears striated; cells are short, branching and mono-nucleated
Cells connected to each other at intercalated disks

6. Functional Characteristics of Muscle
Excitability – can receive and respond to stimuli
Contractility – can shorten/thicken and generate a pulling force
Extensibility – can be stretched and lengthen
Elasticity – after contracting or lengthening, will recoil to original resting length

7. Muscle Functions Producing Movement - (both voluntary and involuntary)
- Respiration (diaphragm contractions)
- Constriction of organs & vessels (peristalsis, vasoconstriciton, pupils)
- Heartbeat
- Communication (non-verbal & facial)
Maintaining Posture
- Also support soft tissues within body cavities
Maintaining body temperature (muscle contractions generate heat, “thermogenesis”)
Stabilizing Joints

8. Gross Anatomy of a Skeletal Muscle
Tendon: connects the muscle to bone
Endomysium: connective tissue sheath that wraps each muscle fiber
Perimysium: collagenic sheath surrounding bundles, or fascicles, of muscle
Epimysium: Coarse sheath that wraps and strengthens the entire muscle
Normal activity is dependent on rich supply of nerves and blood

9. Basic Features of a Skeletal Muscle
Most skeletal muscles span joints and are attached to bones in at least two places
When a muscle contracts, the movable bone (insertion) moves toward the immovable/less movable bone (origin)
Attachments may be direct or indirect (anchored by tendons or an aponeurosis; more common)

10. Nerve & Blood Supply
Each muscle is usually served by one nerve, an artery and one or more veins that enter/exit near the center of the muscle
Muscle capillaries are long & winding to accommodate changes in muscle length.

11. Organizational Levels of Skeletal Muscles

12. Microanatomy of a Skeletal Muscle

13. Microanatomy of a Muscle Fiber

14. Structure of Myofilaments

15. Sarcoplasmic Reticulum & T tubules

16. Sliding Filament Mechanism of Contraction
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

17. Role of Ionic Calcium in Contraction

18. Sequence of Events in Sliding Filament

19. The Neuromuscular Junction

20. Motor Unit: Nerve/Muscle Functional Unit
Muscles that control fine movement (fingers, eyes) have small motor units
Large weight bearing muscles (thighs, hips) have large motor units
Muscle fibers from a motor unit are spread throughout muscle. Contraction of single motor unit causes a weak contraction of the entire muscle
Stronger and stronger contraction require more motor units being recruited

21. Wave Summation and Tetanus
Twitch: a single stimulus is delivered; the muscle contracts and relaxes
Wave summation: stimuli are delivered more frequently, so that the muscle does not have adequate time to relax completely and contraction force increases
Unfused (incomplete) tetanus: more complete twitch fusion occurs as stimuli are delivered more rapidly
Fused (complete) tetanus: a smooth continuous contraction without any evidence of relaxation

22. Providing Energy for Contraction

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24. The Cori Cycle

25. Types of Muscle Fibers
Muscle fibers can be classified based on speed of contraction & pathway of ATP formation
Slow v. Fast (based on efficiency of ATPase)
Oxidative (rely mainly on aerobic path) v. Glycolytic (rely mainly on anaerobic pathway)
Based on these characteristics muscle fibers can categorized as:
Slow oxidative
Fast oxidative
Fast glycolytic
Most muscles have a mixture of fiber types, giving a range of contractile speeds and fatigue resistance

26. Influence of Exercise Endurance Exercise Increases: Improves:
Resistance Exercise
Increases:
# of capillaries
# of mitochondria
Amount of myoglobin
Improves:
Overall metabolism
Efficiency of NM coordination
GI mobility
Skeletal strength
Stroke volume of heart
Fatty acid deposits in blood vessels (removes them)
Increases size of muscle fibers (not number of fibers) amount of connective tissue between cells (more protection from injury)
Glycogen stores are increased.
Important to focus on both parts of an antagonistic pair
Cross-training yields the benefits of both leading to muscles with more mitochondria, more myofilaments, more glycogen reserves etc.

27. Smooth Muscle – Layers & Innervation

28. Peristalsis & Diffuse Junctions

29. Contraction of Smooth Muscle
Still stimulated by release of Ach from nerve endings which generates an action potential; T-tubules are absent, but there are membrane infoldings called caveoli to allow rapid influx of calcium ins.
SR is less developed, although it does release some calcium ions, most comes from intracellular space.
Smooth muscle does not contain sarcomeres but it does contain alternating thick and thin filaments.
The filaments are arranged on a diagonal, causing smooth muscle to contract in a cork-screw manner.
Adjacent cells generally exhibit slow, synchronized contractions (entire sheet contracts together)

30. Skeletal Muscle Interaction in the Body
Body muscles work either together or in opposition to achieve wide variety of motion
Muscles can only pull, never push. Therefore muscles or muscle groups usually work in pairs.
As a muscle shortens, the insertion usually moves toward the origin; There is one muscles or muscle groups to pull the insertion toward the origin and a second muscle or muscle group to undo the action and pull the insertion away.

31. Four Functional Groups
Prime movers (agonist): Provides the major force for producing a specific movement
Antagonists: Muscles that oppose or reverse a particular movement; Usually stretched or relaxed when prime mover is active, can provide resistance to prevent overshoot or help slow or stop the movement
Synergists: help prime movers by adding extra force and reducing undesirable or unnecessary movements (stabilize joints)
Fixators: category of synergists that help immobilize a bone or muscles origin (contribute to maintaining upright posture)

32. Naming Skeletal Muscles
Multiple descriptive criteria can be used to name muscles:
Location of the muscle: ex) temporalis, intercostal
Shape of the muscle: ex) deltoid (triangle), trapezius
Relative size of the muscle: ex) maximus, minimus, longus, brevis
Direction of muscle fibers: ex) rectus, transversus, oblique
Number of origins: ex) biccep, tricep, quadricep
Location of attachments: ex) sternocleoidalmastoid
Action: ex) flexor, extensor, adductor

33. Importance of Fascicle Arrangement
Fascicle arrangement determines the range of motion and power of a muscle
Skeletal muscles shorten up to 70% of resting length when they contract, the longer and more parallel the fibers are to the long axis of the muscle, the more the muscle can shorten (this does not equate to power)
Power depends on the total number of muscle cells; Bipennate & multipennate muscles pack in a lot of cells and are very powerful despite relatively minimal shortening.

34. Types of Fascicle Arrangement
Circular: fascicles arranged in concentric rings; “sphincters”, close openings when contracting
Convergent: muscle has a broad origin and converges toward a single tendon or insertion
Parallel: long axes of fascicle run parallel to long axis of muscle; strap-like
Fusiform: parallel, but spindle shaped rather than strap-like
Pennate: fasicles are short and attach obliquely to a central tendon that runs length of the muscle
Unipennate: fascicles insert into only one side of the tendon
Bipennate: fascicles insert from opposite sides (feather-like)
Multipennate: multiple bipennate fused into central tendon

35. Muscle Mechanics: Fascicle Arrangement
(b)
(f)
Circular
(orbicularis oris)
(b) Convergent
(pectoralis major)
(c)
(e)
(d) Unipennate
(extensor
digitorum
longus)
(d)
(c) Parallel
(sartorius)
(e) Bipennate
(rectus femoris)
(f) Fusiform
(biceps brachii)
(g) Multipennate
(deltoid)

36. Lever Systems
A lever is a rigid bar that moves on a fixed point (fulcrum) when force is applied to it.
The applied force (effort) is used to move a resistance (load)
In the human body, joints are fulcrums, bones act as levers and muscle contraction provides the effort which is applied at it’s insertion point.
The load that is moved is the insertion bone and overlying tissues and anything else associated.

37. Power Levers
Speed levers operate at a mechanical advantage. In this type of system, the lever allows the given effort to move a heavier load or move a load farther and faster than otherwise possible.
A mechanical advantage exists if the load is close to the fulcrum and the effort is applied far from the fulcrum.
A small effort over a large distance is used to move a large load over a small distance

38. Speed Levers
Speed levers operate at a mechanical disadvantage because the force exerted by the muscle must be greater than the load moved or supported.
A mechanical disadvantage exists if the load is far from the fulcrum and the effort is applied near the fulcrum
These levers allow the load to move rapidly through a large distance

39. Classes of Lever Systems
Levers are classified based on the relative position of the three elements: effort, fulcrum & load
First-Class Levers: effort applied at one end of the lever and the load is at the other end with the fulcrum somewhere in the middle (E –F – L ); comparable to see-saws and scissors; can be power or speed
Second-Class Levers: effort applied at one end, with fulcrum at the other and load in the middle (E – L – F); comparable to a wheelbarrow, uncommon in the body
Third-Class Levers: Effort is applied between the load and the fulcrum (F – E – L); comparable to tweezers and forceps; always speed levers; most skeletal muscles operate this way

40. 1st Class Levers

41. 2nd Class Levers

42. 3rd Class Levers

43. Smooth Muscle Tissue
Cells are not striated and are narrower and much shorter than muscle cells
Lack coarse connective tissue sheaths, but there is a thin layer of connective tissue between the smooth muscle cells
Fibers smaller than those in skeletal muscle
Spindle-shaped; single, central nucleus
Lack highly structured neuromuscular junction; varicosities at the end of autonomic nerve fibers release NTs into wide synaptic cleft, “diffuse junction”
Sarcoplasmic reticulum is less developed & T-tubules are absent

44. Smooth Muscle Structure
Grouped into sheets (2 layers) in walls of hollow organs.
Longitudinal layer – muscle fibers run parallel to organ’s long axis; contraction causes organ to shorten
Circular layer – muscle fibers run around circumference of the organ; contraction causes organ to elongate
Both layers involved in peristalsis by alternating contraction and relaxation

45. Additional Differences Between Smooth & Skeletal Muscle Tissue
Sarcolemma does have small infoldings called caveoli that hold extracellular fluid & Ca2+ ions
Lack sarcomeres:
13x more actin than mysoin (v. 2x more in skeletal)
The myofilaments are arranged diagnolly
Tropomysosin is present but no troponin
Non-contractile intermediate filaments resist tension by attaching to dense bodies at regular intervals which act as anchors (correspond to z-discs)

46. Contraction of Smooth Muscle
Slow & synchronous contraction of entire sheet
Some similarities with skeletal muscular contraction
Sliding filament
Ca2+ trigger
ATP for energy
Contraction is slow, sustained & resistant to fatigue
Contraction/relaxation cycle is ~30x longer
Tension can be maintained at 1% the energy costx

47. Cardiac Muscle
Found only in heart; forms a thick layer called the myocardium
Striated fibers that branch
Each cell usually has one centrally located nucleus
Fibers joined by intercalated disks
Under ANS (involuntary) and Endocrine (hormones) control
Some cells are autorhythmic (pacemaker cells)

48. Cardiac Muscle Tissue

49. Developmental Aspects of Muscle Tissue
Cardiac and smooth muscle becomes amitotic but can lengthen and thicken
Myoblast-like satellite cells show very little regenerative ability
Cardiac cells lack satellite cells
Smooth muscle has good regenerative ability

50. Developmental Aspects: Male v. Female
Skeletal muscle makes up app. 35% of a woman’s body mass and 42% of a male’s.
Difference is primarily due to males hormone testosterone
With more muscle mass, men are generally stronger, however body strength per unit muscle mass is the same for both sexes

51. Developmental Aspects: Age
Connective tissue increases and a muscle fibers decrease with age
This results in muscles that become stringier and more sinewy
50% of muscle mass is lost by age 80 (sarcopenia)
Density of muscle capillaries also decreases, which reduced stamina and increases recovery time
Regular exercise reverses sarcopenia