1. Unit 2 Covering, Support, and Movement
Announcements
Today in Lecture:
Bone Tissue (Chapter 6)
Muscular Tissue (Chapter 10)
Due This Week:
Ex 10 UYK
Ex 11 UYK
This Week in Lab:
Ex 12 Skeletal Muscle Structure
Ex 13 Contraction of Skeletal Muscle
Ex 14 Skeletal Muscles and their Action
Ex 15 Surface Anatomy –FYI
I will respond to your requests for appointments today
I will update WebCT this week
Mid-term Evaluations today or Friday
PART 4 MUSCULAR TISSUE
Unit 2 Covering, Support, and Movement

2. Skeletal Muscle Tissue
Description
Long, cylindrical, multinucleate cells; obvious striations
Function
Voluntary movement; locomotion; manipulation of the environment; facial expression; voluntary control
Location
In skeletal muscles attached to bones or occasionally to skin
Introduce and then move to next slide

3. Cardiac Muscle Tissue Description
Branching, striated, generally uninucleate cells that interdigitate at specialized junctions (intercalated disks)
Function
As it contracts, it propels blood into the circulation; involuntary control
Location
The walls of the heart
Introduce and then move to next slide

4. Smooth Muscle Tissue Description
Spindle-shaped cells with central nuclei; no striations; cells arranged closely to form sheets
Function
Propels substances or objects along internal passageways; involuntary control
Location
Mostly in the walls of hollow organs
Introduce and then move to next slide

5. Muscle Functions Producing movement
Maintaining posture and body position
Stabilizing joints
Generating heat

6. Physical Characteristics of Muscle
The ability to receive and respond to a stimulus
Electrical excitability
The ability to shorten forcefully when adequately stimulated
Contractility
The ability to be stretched or extended
Extensibility
The ability to recoil after being stretched
Elasticity

7. ANATOMY OF SKELETAL MUSCLE
Describe the gross structure of a skeletal muscle
Describe the microscopic anatomy of a skeletal muscle fiber
Describe the sliding filament model of muscle contraction

8. Skeletal Muscle Organs
A skeletal muscle is an organ because it is made up of several kinds of tissues
Skeletal muscle fibers
Muscle cell = muscle fiber
100s – 1000s of cells = skeletal muscle
Blood vessels
Nerve fibers
Connective tissue
SKELETAL MUSCLE TISSUE
Each skeletal muscle is a separate organ composed of muscle fibers (or muscle cells) as well as blood vessels, nerves and connective tissue components (Figure 10.1)

9. Nerve and Blood Supply
Each muscle is served by one nerve, an artery, and one or more veins.
Enter / exit near center of muscle
Lots of branching through 3 CT sheaths

10. Nerve Supply of Muscle Muscle has a rich nerve supply
Each muscle fiber is innervated by its own nerve ending
Allows for precise neural control

11. Blood Supply of Muscle Muscle has a rich blood supply
Contracting muscle fibers use huge amounts of energy and require continuous delivery of oxygen and nutrients
Capillaries are long and winding
Accommodates for changes in muscle length

12. Connective Tissue Sheaths
Individual muscle fibers are wrapped and held together by connective tissue sheaths
Sheaths support each cell and reinforce the muscle as a whole
The sheaths are:
Epimysium – surrounds whole muscle
Perimysium – surrounds fascicles
Endomysium – surrounds each fiber
Introduce and then move to next slide

13. Muscle Attachments
Most skeletal muscles span joints and are attached to bones in at least two places
When muscle contracts, insertion (moveable bone) moves towards origin (immoveable/less moveable bone)
In limb muscles, origin typically lies proximal to insertion

14. Types of Muscle Attachments
Direct attachments
muscle’s epimysium is fused to bone’s periosteum
Indirect attachments
muscle’s connective tissue sheaths are attached to bone either as a
ropelike tendon
sheetlike aponeurosis

15. Microscopic Anatomy of Muscle Fiber
Most important component of skeletal muscle
Diameter μm
Typical length ~ 4inches (can be up to 12 inches)
Embryonic dev  fusion of 100+ myoblasts (mesodermal cells)  fibers with 100+ nuclei
Fusion prevents cell division
Skeletal muscle fiber # set before birth
Most last entire lifetime

16. Anatomy of a Skeletal Muscle Fiber
Sarcolemma
T tubules
Sarcoplasm
Glycosomes
Myoglobin
Usual organelles
Specialized organelles
Myofibrils
Sarcoplasmic reticulum

17. Figure 10.2 Microscopic Organization of Skeletal Muscle

18. Myofibrils Each muscle fiber contains thousands of myofibrils
Account for about 80% of cell volume
Myofibrils are composed of even smaller myofilaments
Understanding how myofilaments are organized is key to understanding how muscle cells contract

19. Striations, Sarcomeres, and Myofilaments
Repeating light and dark bands are evident along length of each myofibril
A bands are dark
Lighter midsection region called H zone
H zone bisected by dark M line
I bands are light
Bisected by dark Z disk
Dark A bands and light I bands are nearly perfectly aligned with each other
Gives the cell as a whole its striated appearance

20. Striations, Sarcomeres, and Myofilaments
Smallest contractile unit of a muscle fiber
Functional unit of skeletal muscle
Region between two successive Z disks
Contains an A band flanked by half an I band at each end
Aligned end-to-end like boxcars in a train

21. Striations, Sarcomeres, and Myofilaments
Sarcomere’s banding pattern is due to myofilaments
Thick filaments extend entire length of A band
Thin filaments extend across I band and partway into A band
Thick and thin filaments overlap at A band ends
NOTE: The amount of overlap of thick and thin filaments varies
Contracted, relaxed, stretched

22. Figure 10.3 Arrangement of Filaments Within a Sarcomere

23. Figure 10.3 Zones and Bands of a Sarcomere

24. Molecular Composition of Myofilaments
Myofibrils are made of 3 types of protein:
Contractile Proteins
Generate force during contraction
Myosin and actin
Regulatory Proteins
Switch contraction processes on and off
Tropomyosin and troponin
Structural Proteins
Keep filaments aligned
Give elasticity and extensibility
Link myofibrils to sarcolemma and ECM
Titin and dystrophin

25. Molecular Composition of Myofilaments
Thick filament composed of myosin protein
Motor protein
Rodlike tail with flexible hinge
2 globular heads
Actin and ATP binding sites; ATPase enzymes
Orientation within A band
Tails point towards M line
Heads face Z disk
Each skeletal thick filament has approx. 300 myosin molec bundled

26. Molecular Composition of Myofilaments
Thin filament composed of actin protein
Kidney-shaped with a myosin binding site
Thin filament composed of two strands of actin molecules twisted together like a helix
One end is firmly attached to Z disk
Other end extends into A band

27. Molecular Composition of Myofilaments
Two regulatory proteins in thin filaments:
Tropomyosin
Rod shaped protein
Spirals around each thin filament
Helps stiffen and stabilize it
Blocks the myosin binding sites on actin (relaxed)
Troponin
Part one binds to actin
Part two binds to tropomyosin
Part three binds calcium ions
Lifts tropomyosin off thin filament to expose myosin binding sites (contraction)

28. Sarcoplasmic Reticulum
Elaborate network of smooth endoplasmic reticulum wrapped around each myofibril
Regulates intracellular levels of ionic calcium
Stores calcium and releases it on demand
Provides the final “go” signal for contraction

29. T Tubules Specialization at each A band-I band junction
Sarcolemma protrudes deep into cell interior
Forms T tubule
Comes into close contact with sarcoplasmic reticulum
Triad
T Tubule flanked by 2 terminal cisterns (dilated end sacs of SR)

30. Sliding Filament Theory of Contraction
The theory:
During contraction, thin filaments slide past thick filaments
Filament lengths don’t change
Occurs when muscle fibers are stimulated by nerves
Myosin heads latch onto myosin-binding sites of actin molecules and sliding begins
Filament sliding shortens sarcomeres, which shortens muscle fiber, which shortens entire muscle
Sir Hugh Huxley

31. Figure 10.5 The Sliding Filament Mechanism

32. CONTRACTION OF A SKELETAL MUSCLE FIBER
Explain how events at the neuromuscular junction stimulate a skeletal muscle fiber to contract
Describe how an action potential is generated
Explain excitation-contraction coupling
Describe cross bridge cycling
Explain the length-tension relationship in a skeletal muscle fiber

33. Overview of Skeletal Muscle Contraction
For a skeletal muscle fiber to contract, 3 events have to occur:
The fiber must be activated by a nerve – changing membrane potential
The fiber must generate and propagate an action potential along its sarcolemma and down its T tubules
Ca2+ must be released from the SR to trigger contraction 
Now let’s look at the details…

34. The Neuromuscular junction
The synapse between a somatic motor neuron and a skeletal muscle fiber
Neurotransmitter is acetylcholine (ACh)
Causes a change in membrane permeability, which leads to a change in membrane potential
Fig 10.9 a
The Neuromuscular junction

35. Figure 10.9b The Neuromuscular Junction
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

36. Figure 10.9c The Neuromuscular Junction
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

37. Figure 10.9d The Neuromuscular Junction
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

38. Recipe for Muscle Fiber Activation
Ingredients:
Neuron
Skeletal muscle cell
Acetylcholine
Ion channels
Ca2+
Directions:
Action potential arrives at motor neuron’s axon terminal
Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal
Ca2+ entry causes some synaptic vesicles to release acetylcholine by exocytosis

39. Recipe for Muscle Fiber Activation
Ingredients:
Neuron
Skeletal muscle cell
ACh
ACh receptors
Ion channels
Na+ K+
Ca2+
Directions:
ACh diffuses across synaptic cleft and binds to sarcolemma receptors
ACh binding opens ion channel - Na+ diffuses into cell (more), K+ diffuses out (less), producing a local change in membrane potential (depolarization)
Muscle action potential
ACh broken down by acetylcholinesterase

40. Generation and Propagation of Action Potential
Sarcolemma is polarized
Potential difference across the membrane
The resting membrane potential
Action potential
A reversal of resting membrane potential
Released electrical energy
Involves 2 steps
Depolarization
Repolarization

41. Recipe for Generating and Propagating an Action Potential
Ingredients:
Sarcolemma
ACh
ACh receptor
Ion channels
Na+ K+
Ca2+
Directions:
ACh binds to sarcolemma receptors
Na+ channels open
Na+ diffuses into cell (less K+ leaves)
Sarcolemma depolarizes (inside more +)
Depolarization in NMJ  depolarization of adjacent sarcolemma segments (AP = dominoes)
Areas of depolarization quickly repolarize
K+ channels open
K+ diffuses into cell
Sarcolemma repolarizes (inside more -)

42. Phases of an Action Potential in Skeletal Muscle
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

43. Excitation – Contraction Coupling
Connects nervous excitation of a skeletal muscle fiber to contraction

44. Recipe for Excitation – Contraction Coupling
Ingredients:
Sarcolemma
T tubules
Action Potential SR
Voltage-gated Ca2+channels
Ca2+
Troponin
Tropomyosin
Directions:
Action potential propagates along sarcolemma, into T tubules, towards SR
Voltage-gated Ca2+ channels in SR open
Ca2+ pours into sarcoplasm; binds to troponin
Troponin moves tropomyosin away from myosin-binding sites on actin - Myosin now free to bind with actin - contraction cycle begins
Contraction cycle continues until Ca2+ active transport pumps return Ca2+ to SR
Calsequestrin

45. Figure 10.7 Excitation – Contraction Coupling
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

46. Cross Bridge Cycling
The series of events during which myosin heads pull thin filaments toward the sarcomere’s center

47. Figure 10.6 The Contraction Cycle
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber (Figure 10.10).
A synapse is a region of communication between two neurons or a neuron and a target cell (e.g., skeletal muscle cell).
Synapses separate cells from direct physical contact.
Neurotransmitters bridge that gap.
The neurotransmitter at a NMJ is acetylcholine (ACh).

48. Figure 10.10 Summary of the Events of Contraction and Relaxation in a Skeletal Muscle Fiber

49. Summary Video
Muscle Contraction Process: Molecular Mechanism [3D Animation]
A few points missing but a good visualization

50. CONTRACTION OF A SKELETAL MUSCLE ORGAN
Define motor unit and muscle twitch
Explain how smooth, graded skeletal muscle contractions are produced
Differentiate between isometric and isotonic contractions

51. Muscle mechanics
Skeletal muscle fiber contraction pretty much the same as skeletal muscle organ contraction
Muscle tension
The force exerted by a contracting muscle on an object
Load
The force exerted on a muscle by an object’s weight
A contracting muscle doesn’t always shorten and move the load (isometric)
When a muscle does generate enough tension, it shortens to move the load (isotonic)
A skeletal muscle contracts with varying force and for different periods of time in response to stimuli of varying frequencies and intensities

52. The Motor Unit The functional unit of skeletal muscle
A motor neuron and the muscle fibers it stimulates
Each fiber only has one NMJ
The axon branches out to form NMJs with many fibers
May innervate as few as 10 or as many as 2,000 (avg = 150)
All innervated muscles fire simultaneously
Muscle fibers of a motor unit are dispersed throughout a muscle (not clustered)
Biceps brachii/gastrocnemius  up to 3000 fibers in motor unit
Muscles controlling precise movements have many small motor units
Larynx  2-3 fibers per motor unit
Eye  fibers per motor unit

53. Figure 10.13 Motor Units and Recruitment
This image shows 2 different motor units – green and purple

54. Twitch Contraction
A brief contraction of all the muscle fibers in a motor unit in response to a single action potential
Skeletal muscle twitches
msec
AP lasts only 1-2 msec
Has three distinct phases:
Latent period
Contraction period
Relaxation period

55. Figure 10.13 Myogram  Twitch Contraction
Note: Refractory period  amount of time where an AP will not stimulate a muscle response. Skeletal muscle (~5 msec). Cardiac muscle (~300 msec).

56. Graded Muscle Responses
Our muscles don’t usually work by twitching.
Muscle contractions are relatively smooth and vary in strength as different demands are placed on them
Variations are called graded muscle responses
Variations due to:
Changes in stimulation frequency
Produces smooth muscle contractions
Changes in stimulation strength
Produces more or less muscle tension

57. Neural Regulation of Contraction Frequency
Nervous system produces smooth muscle contractions by regulating the frequency of stimulation to a motor unit.
Three examples:
Wave Summation
Incomplete (unfused) tetanus
Complete (fused) tetanus

58. Wave Summation
When two identical electrical stimuli are delivered to a muscle in rapid succession, second twitch is stronger than the first
Produces smooth, continuous muscle contractions by rapidly stimulating all of the muscle fibers of a motor unit

59. Incomplete Tetanus
Sustained muscle contraction with partial relaxation between stimuli
Also produces smooth, continuous muscle contractions

60. Complete Tetanus
Sustained muscle contraction with no relaxation between stimuli
Prolonged tetanus inevitably leads to muscle fatigue – the muscle is unable to contract and tension drops to zero

61. Figure 10.15 Frequency of Stimulation

62. Neural Regulation of Muscle Tension
Nervous system regulates muscle tension by regulating motor unit activation
The more motor units activated at the same time, the stronger the muscle contraction
Process is called recruitment
Weakest motor units recruited first (precise movements), progressively stronger ones added as needed (large amt of tension).

63. Other Factors Affecting Muscle Tension
Size of the muscle fibers
Resistance exercise muscle force by muscle cells size  hypertrophy
Degree of muscle stretch (fig 10.8)
The length-tension relationship
Based on the zone of overlap
µm = optimal
Under vs. overstretched

64. Figure 10.15 Isotonic and Isometric Contractions