Chapter 9: Muscles and Muscle Tissue



Chapter 10: Muscles and Muscle Tissue

Chapter Objectives

OVERVIEW OF MUSCLE TISSUE

1. Describe the three key functions of muscle.

2. Describe the four special properties of muscle tissue.

SKELETAL MUSCLE TISSUE

3. Describe the difference between a fascicle, a myofiber, a myofibril and a myofilament.

4. Explain what is meant by muscle atrophy and hypertrophy.

5. Describe and give the functions of the four kinds of proteins found in myofibrils.

6. Describe the different bands found in a sarcomere.

7. Explain how the arrangement of the thick and thin myofilaments forms the observed sarcomere structural patterns of the myofibril.

8. Discuss the connection between the sarcolemma and the Transverse tubules.

9. Describe and give the function and location of the sacroplasmic reticulum and the terminal cisternae.

10. List the three layers of connective tissue that surrounds skeletal muscle and give a function for the tissue.

THE NEUROMUSCULAR JUNCTION

11. Show the general features of the neuromuscular junction that allows signals coming from the brain to be conveyed across the gap between the neuron motor cell and the sarcolemma of the muscle cell.

12. Describe the effects of certain drugs, bacterial toxins and disorders on the function of the neuromuscular junction (NMJ).

CONTRACTION AND RELAXATION OF SKELETAL MUSCLE FIBERS

13. Describe the conditions inside a skeletal muscle cell at rest in regards to ATP, Calcium, and the state of the myofilaments.

14. Explain why the process of muscle contraction is called Excitation-Contraction Coupling.

15. Describe what is happening in a skeletal muscle cell during the Sliding Filament Mechanism in regards to ATP, Calcium and myofilament function.

16. Illustrate the progressive overlap of the thick and thin filaments as they pull the Z disc toward the center of the sarcomere, and the result on the length of the fibril, fiber, and muscle.

17. Define a power stroke cycle and explain the steps of the cycle. Include the factors that maintain continuous cycling of muscle contraction.

18. Describe the condition called rigor mortis and its cause.

MUSCLE METABOLISM

19. Describe energy use in muscle cells and list the three sources for ATP regeneration in muscle cells.

20. Discuss the relative durations of time that each form of ATP production provides for sustained activity.

21. Describe the use of creatine phosphate in muscles to produce ATP.

22. Define oxygen debt and discuss the purpose of the elevated use of oxygen after exercise.

CONTROL OF MUSCLE TENSION

23. Define the All or None Principal

24. Define a motor unit and indicate the relationship between a motor neuron, the number of muscle fibers it innervates, and the strength of contraction by the motor unit.

25. Discuss the effects of increasing the number of active motor units on contraction force.

26. Define muscle tone and note how it normally works in body posture maintenance.

27. Discuss the activities that occur in each period of a twitch contraction as shown in a myogram.

28. Define Refactory period.

29. Define twitch, treppe and tetanus and explain the conditions that cause them.

30. Compare and contrast isotonic and isometric contractions.

TYPES OF SKELETAL MUSCLE FIBERS

31. Discuss the slow oxidative fibers based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate.

32. Discuss the fast glycolytic based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate.

33. Discuss the intermediate fatigue resistant fibers based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate.

34. Examine how various types of exercise can induce changes in the fibers in skeletal muscle.

SMOOTH MUSCLE TISSUE

35. Compare skeletal and smooth muscle cells with respect to organelles, cytoskeleton, and contractile filaments.

36. Contrast the source and nature of differences in contraction between smooth and skeletal muscles.

CARDIAC MUSCLE TISSUE

37. Compare and contrast differences in cardiac and skeletal muscle cell structure and physiology.

Chapter Lecture Notes

Functions of Skeletal Muscle

Movement of body

Posture maintenance

Heat production - 85% of body heat is generated by skeletal muscle

25 - 40 % of energy from nutrients is converted to ATP by cellular respiration

60 - 75 % of energy from nutrients is converted to heat

Properties of Skeletal Muscle

Excitability - ability to receive and respond to a stimulus

stimulus = change in environment strong enough to initiate an electrical signal, action potential

In skeletal muscle, stimulus is usually acetylcholine (ACh, neurotransmitter)

Contractility - ability to shorten (can shorten up to 50% of resting length)

Extensibility - ability to stretch (can stretch up to 20% of resting length)

Elasticity - ability to return to original length after contraction

Histology

Common root words for muscle

sarco = fleshy

myo = muscle

Large → Small: fascicle → myofiber → myofibril → myofilament (Table 10.3)

Muscle consists of elongated cells called muscle fibers or myofibers (Fig 10.1 & 10.2)

A bundle of these fibers is called a fascicle

Inside of a muscle cell there are small cylinders called myofibrils which may number several 100 to several 1000/cell (Fig 10.3)

Exercise increases myofibril production (hypertrophy); lack of exercise decreases myofibrils (atrophy)

Each myofibril consists of myofilaments (proteins) (Fig 10.3, 10.4 and Table 10.2 & 10.3)

thick myofilaments = myosin

thin myofilaments = actin, troponin, tropomyosin

Sarcomere - myofilaments don't extend entire length of muscle fiber; they are stacked into compartments called sarcomeres (Fig 10.3 & Table 10.1)

Sarcomeres are the functional unit of a skeletal muscle (contractile unit)

Parts of a sarcomere

A band - myosin + overlapping actin

I band - only actin, troponin, tropomyosin

Z disc – through center of I band

H zone - only myosin

M line – middle of sacromere

Sarcomere extends from Z disc to Z disc - 2 I bands / sarcomere

Sarcoplasm – cytoplasm

Sarcolemma - cell membrane (Fig 10.2)

T tubule - tubular invagination of sarcolemma that surrounds each myofibril at the A band - I band junction

Sarcoplasmic reticulum (SR) - smooth endoplasmic reticulum that stores Ca2+

Each myofibril is surrounded by SR

Terminal cisternae – dilated end sacs of the SR

Triad - 2 terminal cisternae + 1 T tubule located at the A band - I band junction

Entire muscle is wrapped in fibrous connective tissue which is continuous with tendons that insert skeletal muscle into periosteum of bone

Connective tissue holds muscle together as well as serving to transmit blood vessels and nerves to inner muscle cells

Connective tissue layers of skeletal muscle (Fig 10.1)

Epimysium - outside of entire muscle

Can be called deep fascia (sheet of fibrous connective tissue) as compared to superficial fascia also known as subcutaneous layer

Perimysium - divides muscle cells into fascicles (bundles)

Endomysium - covers individual muscle cells

Neuromuscular Junction

A synapse or site of communication between a neuron and muscle is usually named a neuromuscular junction (NMJ) (Fig 10.9)

Structure

Neuromuscular junction = axon terminal (synaptic end bulb) of a motor neuron + synaptic cleft + motor end plate (sarcolemma under motor neuron)

One per muscle fiber and usually in middle

Function

Acetylcholine (Ach) is released from the axon terminal (synaptic end bulb) of the motor neuron by exocytosis

Ach diffuses across the synaptic cleft to the motor end plate

Ach binds to receptors on the motor end plate

Ach binding initiates a change in the electrical state of the muscle cell called an action potential

Problems at the NMJ

Curare - binds to ACh receptors in skeletal muscle membrane

Competes with ACh but does not stimulate the ACh receptor

Muscle paralysis

Myasthenia gravis - antibodies destroy ACh receptors

Muscle paralysis

Botulism (from the bacteria Clostridium botulinum) - toxin inhibits ACh release

Muscle paralysis

A dilute solution of botulinum toxin (Botox) can be injected into a muscle that is in spasm to help it relax

Tetanus - (from the bacteria Clostridium tetani) - this anaerobic bacteria produces a toxin that blocks an inhibitory neurotransmitter in the central nervous system

Causes muscle spasms and painful convulsions

Tetanus shots immunize against the toxin

Organophosphates (in some pesticides) - inhibits acetylcholinesterase

Acteylcholinesterase – enzyme that inactivates acetylcholine

Muscle spasms

Sliding Filament Mechanism

Sliding Filament Mechanism means: myosin (thick myofilaments) cross bridges pull actin (thin myofilaments) toward H zone during contraction (Fig 10.5)

I band and H zone may disappear in a contracting muscle

A band stays same length during contraction

At rest

Calcium in SR (terminal cisternae)

Troponin-tropomyosin prevents myosin from binding to sites on actin

ATP bonded to myosin cross bridges (concentration of ATP is high in relaxed muscle)

Excitation-Contraction Coupling (Fig 10.10)

Muscle action potential will be generated at the NMJ due to neural stimulus - Ach binds to its receptor on the motor end plate

Action potential - rapid change in electrical polarity at the plasma membrane of a muscle or neuron

Will travel along the sarcolemma down into the T tubules

Production of AP in muscle leads to contraction; Excitation-Contraction Coupling

The action potential running through the T tubules causes the sarcoplasmic reticulum’s terminal cisternae to release calcium into the sacroplasm

The calcium will then bind to troponin

Tropomyosin changes shape so myosin binding sites on actin are uncovered (Fig 10.7)

Myosin then can bind to actin

The calcium will also activate ATPase ability of myosin

The ATPase activity of myosin activates its power stroke movement (Fig 10.6)

Pulls actin inward (H zones and I bands narrow and may disappear; A band does not change its length)

At end of power stroke, a new ATP binds to ATP binding site on the myosin cross bridge, resulting in detachment of myosin from actin

Attach, pull, detach (release) = steps in contraction

If lack ATP - rigor mortis

To relax following contraction

ACh is inactivated by acetycholinesterase (from sarcolemma surface)

Calcium is actively transported back into SR

ATP is required to pump calcium back into the SR

ATP attaches to myosin cross bridge and it releases from actin

Troponin-tropomyosin covers myosin binding sites on actin preventing binding of myosin cross bridges

ATP and Muscle Function

ATP required for

Power stroke - movement of myosin cross bridges pulls actin inward in a sarcomere

At end of power stroke, ATP binds to ATP binding site, resulting in detachment of myosin from actin (ATP needed to release cross bridges)

Pumping Ca2+ back into terminal cisternae of the SR

Sources of ATP (Fig 10.11)

Stored ATP - lasts only 6 seconds during bursts of muscle contraction

ATP generated from creatine phosphate (CP) (CP + ADP → creatine + ATP)

Together ATP that is stored and CP provide muscle power for 10-15 sec

CP replenished during resting periods

Even as ATP and CP are being used, ATP is generated by aerobic respiration and anaerobic respiration

Resting and slowly contracting muscles obtain bulk of ATP via aerobic respiration of fatty acids (enters as acetyl-CoA)

Aerobic pathway: glucose + O2 → CO2 + H2O + 36ATP

In actively contracting muscles, glucose (from blood and breakdown of glycogen) is primary fuel supply

Anaerobic pathway: glucose → lactic acid + 2 ATP

Aerobic pathway produces 20X more ATP than anaerobic respiration but takes 2 1/2 times longer

Anaerobic respiration causes oxygen debt to occur

Oxygen Debt

Oxygen Debt: Amount of oxygen needed to metabolize the accumulated lactic acid and to restore ATP levels

Muscle fatigue is result of ATP depletion and accumulation of lactic acid

80% of lactic acid diffuses from skeletal muscle to blood and is transported to liver where it is converted back to glucose

20% remains in muscle - processed to CO2 and H2O + ATP

Oxygen debt results in labored breathing in order to pay back the O2 debt

Lactic acid decreases pH of blood which stimulates increased respiration

All or None Principal

All or None Principle - individual muscle fibers of a motor unit will contract to its fullest extent of its immediate ability when stimulated by a nerve impulse (action potential) of threshold level

The principle does not apply to the entire muscle but only to motor units

Motor unit - motor neuron + all the skeletal muscle fibers it services (5 fibers to 2000 muscle fibers) (Fig 10.12)

One entire muscle has many motor units

Not all are stimulated at same time

The smaller the number of muscle fibers/motor unit, the more precise the control of the muscle fibers

So the strength of a contraction of an entire muscle depends on

Size of load - heavier the load, stronger the contraction (pencil vs book)

Number of motor units activated

Initial length of fibers - more a muscle is stretched, to a point, the greater the contraction (Fig 10.8)

Muscle tone - partial sustained contraction important in maintenance of posture

Only a fraction of motor units are activated at any one time

Produces tautness instead of recognized contraction

Maintain tone without fatigue because there is a system of rotation; at any one moment some fibers are contracted and others are relaxed & resting, ready to take up the work next (motor units innervate scattered muscle cells and not clustered)

If muscle tone is not maintained, possibly due to nerve damage, the muscle has less than normal tone, or is flaccid

If flaccidity is maintained for an extended period of time the muscle will atrophy, a decrease in muscle mass by a decrease in myofibrils

Leg in cast

Bedridden

The opposite of atrophy is hypertrophy, an increase in diameter of muscle from forceful muscle activity or repetitive muscle activity; there is an increase in number of myofibrils, mitochondria, SR, blood vessels, connective tissue

Kinds of Contractions

Muscle twitch - rapid jerky response to single stimulus

Can be traced on a myogram (Fig 10.13)

Three phases: latent, contraction, relaxation

Length of twitch varies for different muscles

Short in eye muscle

Long in leg muscle

Refractory period - period of lost irritability (excitability) - short period of time during which there is no response to additional stimuli

Short in skeletal muscle (.005 sec)

Long in cardiac (.3 sec)

Wave Summation (Treppe) - staircase phenomenon - reoccurring stimuli results in increasingly stronger twitch contractions (Fig 10.14)

Increased heat in warmed up muscle increases speed of all reactions

Increased calcium in sarcoplasm activates sliding filament mechanism faster

Practical application: warming up of athletes

Tetanus - (not to be confused with bacterial disease of same name) - fusion of twitches (Fig 10.14)

Relaxation is not allowed to occur

Body normally delivers volleys of impulses in rapid succession, giving us smooth sustained contractions

Voluntary contractions (normal movements) are tetanic contractions

Isotonic contraction - muscle shortens and can move a load (Fig 10.15)

Tension, force exerted by a contracting muscle on some object, has to exceed load to move it

Tension remains constant as the muscle shortens

Isometric contractions - develops increased tension in muscle but muscle does not shorten or there is minimal shortening (Fig 10.15)

Cross bridges formed but unsuccessful in moving thin myofilaments (cross bridges spinning wheels on same cross bridge binding site)

Length remains about the same

Types of Skeletal Muscle Fibers

Slow twitch - fatigue resistant – red (SO) (Table 10.4)

Contract more slowly

Slower myosin ATPase

Fuel source: fatty acids

Aerobic respiration

More mitochondria

Low glycogen

High myoglobin

Myoglobin - similar to hemoglobin, hence red in color; binds O2 and acts as a reservoir for O2 until needed by mitochondria

More capillaries

Small diameter

ex. - large postural muscles, long distance runners

Fast twitch – fatigable – white (FG)

Contract more rapidly

Faster myosin ATPase

Fuel source: glucose

Anaerobic respiration

Fewer mitochondria

High glycogen

Low myoglobin

Fewer capillaries

Large diameter

ex. – muscles of arm and legs

Intermediate – intermediate fatigue resistant – pink (FOG)

Contract more rapidly

Faster myosin ATPase

Fuel source: glucose

Anaerobic and aerobic respiration

More mitochondria

High glycogen

High myoglobin

More capillaries

Intermediate diameter

Most muscles have a combination of both types - all fibers of a motor unit are of same type

Endurance type exercises (running, swimming) can transform fast twitch - fatigable into intermediate by producing more mitochondria and increased myoglobin

Cannot transform slow twitch to fast twitch fibers

Smooth Muscle

Smooth – Involuntary, nonstriated, visceral (Fig 10.16)

Located in walls of organs and blood vessels

Has actin and myosin but not arranged orderly into sarcomeres

Contract and relax more slowly than skeletal muscle due to arrangement of actin and myosin and a different calcium regulation molecule (calmodulin instead of troponin-tropomyosin)

Contracts in waves as impulse spreads from one cell to another

ex. - propulsion of food through digestive system

Cardiac Muscle

Cardiac – Involuntary, striated (Table 10.5)

Same arrangement of actin and myosin as skeletal muscle but not organized into discrete myofibrils

Has own intrinsic rhythm

Own electrical system

Cardiac cells can contract without nerve stimulation

Source of stimulation is specialized cells that initiate contraction, pacemaker

Nerve stimulation merely speeds up or slows down rate of contraction

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