Muscle Tissue

Chapter 10

Muscle Tissue

• Muscle is one of the 4 primary types of tissue.

• It is subdivided into skeletal, cardiac and smooth muscle

Skeletal Muscle Tissue

The 5 functions of skeletal muscles are:1. To produce skeletal movement.

2. To maintain posture and body position.

3. To support soft tissues.

4. To guard the entrances and exits of the body.

5. To maintain body temperature.

Functional Anatomy of Skeletal Muscle

• Organization of Connective Tissues

•

Organization of Connective Tissues

• Muscles have 3 layers of connective tissues

1. epimysium2. perimysium3. Endomysium

• the endomysium, perimysium and epimysium come together to form a tendon (a bundle) or an aponeurosis (a sheet).

Skeletal Muscle Fibers

• Long fibers develop through the fusion of mesodermal cells (myoblasts) until they become very large and contain hundreds of nuclei

• The 2 types of myofilaments are:1. thin filaments: made of the protein actin, and

2. thick filaments: made of the protein myosin

Sarcoplasmic Reticulum

• involved in transmitting the action potential to the myofibril

• Triad – Terminal cisternae – T tubules

Striated

• thin filaments (I bands)

• thick filaments (A bands )

Thin filaments contain 4 proteins

1.F actin (2 twisted rows of globular G actin. Active sites on G actin strands bind to myosin.)

2.nebulin (holds F actin strands together)

3.tropomyosin (a double strand, prevents actin-myosin interaction)

4.troponin (a globular protein, binds tropomyosin to G actin, controlled by Ca++)

Thick Filaments

• Twisted myosin subunits.

• The tail binds to other myosin molecules.

• The free head, made of 2 globular protein subunits

Sliding Filaments and Muscle Contraction

The thin filaments of the sarcomere slide toward the M line, in between the thick filaments. This is called the sliding filament theory. The width of the A zone stays the same, but the Z lines move closer together.

The Control of Skeletal Muscle Activity

• Neural stimulation occurs at the neuromuscular junction (NMJ)

• Acetylcholine (ACh)

• Acetylcholinesterase or AChE

Excitation - Contraction Coupling

• The Contraction Cycle has 5 steps:1. 1. Exposure of active sites

2. 2. Formation of cross-bridges

3. 3. Pivoting of myosin heads

4. 4. Detachment of cross-bridges

5. 5. Reactivation of myosin

Movie

Relaxation

• Since AChE quickly breaks down ACh, the duration of a contraction depends on:1. the duration of the neural stimulus

2. the number of free calcium ions in the sarcoplasm

3. the availability of ATP

Key

• Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs.

• The trigger for contraction is the appearance of free calcium ions in the sarcoplasm; the calcium ions are released by the sarcoplasmic reticulum when the muscle fiber is stimulated by the associated motor neuron.

• Contraction is an active process; relaxation and return to resting length is entirely passive.

Tension Production by Muscle Fibers

• All-or-none principal

• Tension produced by the contraction of an individual muscle fiber can vary

Length-Tension Relationships

• There is an optimum amount of overlap to produce the greatest amount of tension

Frequency of Stimulation

• A single neural stimulation produces a single contraction or twitch which lasts about 7-100 milliseconds.

• Sustained muscular contractions require many repeated stimuli

Twitches are divided into 3 phases

1. The latent period before contraction. The action potential moves through the sarcolemma, causing calcium ions to be released.

2. The contraction phase: Calcium ions bind to troponin, tension builds to a peak.

3. The relaxation phase: Calcium levels fall, active sites are covered, and tension falls to resting levels.

Twitch types

• Treppe

• Summation of twitches

• Incomplete tetanus

• Complete tetanus

Tension Production by Skeletal Muscles

The amount of tension a whole muscle can produce depends on:

1. The internal tension produced by the muscle fibers

2. The external tension the muscle fibers exert on their elastic extracellular fibers (series elastic elements such as tendons)

3. The total number of muscle fibers stimulated

2 basic patterns of muscle tension:

Isotonic contraction Isometric contraction.

Muscle Relaxation and Return to Resting Length

1. Elastic forces are the pull of the elastic elements returning to normal length.

2. Opposing muscle contractions reverse the direction of the original motion, the work of opposing muscle pairs.

3. Gravity can take the place of opposing muscle contraction to return a muscle to its resting state.

ATP and CP Reserves

• ATP is the active energy molecule

• Creatine phosphate (CP) stores of energy in cell. – energy in creatine phosphate is used to

recharge ADP to ATP

ATP Generation

1. Aerobic metabolism of fatty acids in the mitochondria:- the primary energy source of resting muscles- 34 ATP molecules produced per glucose molecule

2. Anaerobic glycolysis in the cytoplasm:- the breakdown of glucose from glycogen- primary energy source for peak muscular activity- 2 ATP molecules produced per molecule of glucose- skeletal muscles store glycogen

Energy Use and the Level of Muscular Activity

• At peak levels of exertion, muscles can’t get enough oxygen to support mitochondrial activity. The muscle then relies on glycolysis for ATP.

• The pyruvic acid produced by glycolysis, which would normally be used up by the mitochondria, starts to build up and is converted to lactic acid.

Muscle Fatigue

• Muscle fatigue is associated with:1. depletion of metabolic reserves

2. damage to the sarcolemma and sarcoplasmic reticulum

3. low pH (lactic acid)

4. muscle exhaustion and pain

Recovery Period

• It can take hours or days for muscles to return to their normal condition. • Removal of Lactic Acid to Glucose

• This removal and recycling of lactic acid by the liver is called the Cori cycle.

• Oxygen debt

Key

• Skeletal muscles at rest metabolize fatty acids and store glycogen.

• During light activity, muscles can generate ATP through the anaerobic breakdown of carbohydrates, lipids or amino acids.

• At peak levels of activity, most of the energy is provided by anaerobic reactions that generate lactic acid as a byproduct.

• Heat Production and Loss: The more active muscles are, the more heat they produce. During strenuous exercise, up to 70 percent of the energy produced can be lost as heat, raising body temperature

Hormones and Muscle Metabolism

• Growth hormone

• Testosterone

• Thyroid hormones

• Epinephrine

Types of Skeletal Muscle Fibers

1. Fast Fibers

2. Slow Fibers

3. Intermediate Fibers

Muscle Performance and the Distribution of Muscle Fibers

• Different muscles have different percentages of fast, slow and intermediate fibers

• Muscles with mostly fast fibers are pale (white muscle) like chicken breast.

• Muscles with mostly slow fibers are dark (red muscle) like chicken legs.

• Most human muscles have mixed fibers and are pink

Muscle Hypertrophy and Atrophy

• Atrophy

• Hypertrophy

Physical Conditioning

• Aerobic endurance

• Anaerobic endurance

Structural Characteristics of Cardiac Tissue

1. are small

2. have a single nucleus

3. have short, wide T tubules and no triads

4. have SR with no terminal cisternae

5. are aerobic (high in myoglobin and mitochondria)

6. have specialized contact points called intercalated discs

Functional Characteristics of Cardiac Tissue

1. automaticity

2. variable contraction tension

3. prevention of wave summation and tetanic contractions

4. extended contraction time

Structural Characteristics of Smooth Muscle Tissue

1. are long and slender2. are spindle shaped, with a single, central

nucleus3. have no T tubules, myofibrils or sarcomeres4. have scattered myosin fibers, with more heads

per thick filament5. have thin filaments attached to dense bodies6. transmit contractile force from cell to cell

through dense bodies7. have no tendons or aponeuroses

Functional Characteristics of Smooth Muscle Tissue

1. Excitation-Contraction Coupling

2. Length-Tension Relationships

3. Control of Contractions

4. Smooth Muscle Tone