Ch. 10 Muscular Tissue Notes

Muscular Tissue

Learning Objectives

• Compare and identify the 3 major muscle groups: skeletal, cardiac, and smooth.

• Describe the general anatomy of muscles.

• Describe the ultrastructure of skeletal muscle and its role in muscle contraction.

• Describe muscle proteins and their functions.

• Explain the structure of a sarcomere.

• Describe nerve-muscle relationship, the motor unit, and the neuromuscular junction.

• Explain the mechanism of muscle contraction and relaxation by sliding filament theory.

• Explain the role of ATP in muscle contraction

• Define the physiologic fiber types of muscles.

• Correlate the effector exercise with muscle performance.

• Describe muscle degenerating conditions like Paralysis, Polio, Tetanus, Botulism, Myasthenia gravis, Multiple Sclerosis

Types of Muscular Tissue

• Three types:

  • Skeletal

  • Cardiac

  • Smooth

Functions of Muscular Tissue

• Movement:

  • Skeletal

  • Cardiac

  • Smooth muscles

• Cardiac muscle:

  • Exclusively in the heart.

  • Cardiac muscle cells coordinate heart pumping.

• Stability:

  • Skeletal muscle

• Communication:

  • Skeletal muscle

• Control of body openings and passages:

  • Skeletal and smooth muscles

• Heat production:

  • Skeletal muscles

Anatomy Overview

• Skeletal muscle

• Cardiac muscle

• Smooth muscle

Properties of Muscular Tissue

1. Excitability (responsiveness):

   • Responds to chemical, mechanical, or electrical stimuli.

2. Conductivity:

   • Initiates events leading to contraction.

3. Contractility:

   • Ability to shorten substantially.

4. Extensibility:

   • Able to stretch between contractions.

5. Elasticity:

   • Ability to return to original length after stretching.

Skeletal Muscle Tissue

• Skeletal muscle cells

  • Long and cylindrical

  • Referred to as muscle fibers (or myofibers).

  • Large cells: up to 100 µm in diameter and 30 cm (11.8 in) long (e.g., Sartorius).

  • Many nuclei: Needed for protein and enzyme production for cell function.

• Contain cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum.

• Specialized smooth endoplasmic reticulum called the sarcoplasmic reticulum (SR) stores, releases, and retrieves calcium ions (Ca^{++}).

Structure of Muscle Cell

• Skeletal

  • Striated, voluntary

• Smooth

  • Involuntary (digestive)

• Cardiac

  • Heart

The Myocytes

• Muscles consist of long slender cells (fibres), each of which is a bundle of finer fibrils.

• Within each fibril are relatively thick filaments of the myosin and thin ones of actin and other proteins.

• Muscle fibre lengthens or shortens, the filaments remain essentially constant in length but slide past each other.

• Muscles differ in the arrangement of their myofilaments.

• The principal types of muscles:

  • Striated muscle

    • Filaments are organized in transverse bands.

  • Obliquely striated muscle

    • Filaments are staggered, making the bands oblique.

  • Smooth muscle

    • Filaments are arranged irregularly.

Muscle Types

• Cardiac Muscle

• Skeletal Muscle

• Smooth Muscle

Hierarchy of Muscle Structure

• Fascicles

• Myofibrils

• Myofilaments

  • Actin

  • Myosin

Muscle

• Skeletal muscle fiber

  • Single cylindrical muscle cell.

  • Muscle fiber is surrounded by connective tissue called the endomysium.

• Skeletal muscle

  • Made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering.

• Muscle is surrounded by a connective tissue sheath called the epimysium.

• Fascia

  • Connective tissue outside the epimysium surrounds and separates the muscles.

• Epimysium projects inward to divide the muscle into compartments.

• Compartment contains a bundle of muscle fibers.

• Muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium.

Structure of Skeletal Muscle

• The plasma membrane of muscle fibers is called the sarcolemma

  • Sarco means “flesh”

• The cytoplasm is referred to as sarcoplasm.

• Within a muscle fiber, proteins are organized into organelles called myofibrils that run the length of the cell and contain sarcomeres connected in series.

• Myofibrils are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one muscle fiber.

• The sarcomere is the smallest functional unit of a skeletal muscle fiber.

  • Highly organized arrangement of contractile, regulatory, and structural proteins.

  • Shortening of these individual sarcomeres leads to the contraction of individual skeletal muscle fibers (and ultimately the whole muscle).

Structure of Muscle Cell

• Muscles are composed of many fibers that are arranged in bundles called fascicles.

• These fibers are found within muscle cells, called myocytes.

Anatomy of Skeletal Muscle

• Each muscle is composed of bundles of muscle fibers.

• Each muscle fiber (cell) has many nuclei and is a cluster of myofibrils

• Myofibrils contain two types of protein filaments that are arranged in a regular, over-lapping pattern:

  • Myosin – thicker filament

  • Actin – thinner filament

Muscle Tissue Anatomy

• Epimysium

  • Outer covering of muscles

• Fascicle

  • A bundle of muscle fibers

• Perimysium

  • Each fascicle is covered by the perimysium

• Endomysium

  • Thin covering around each muscle fiber

• Both perimysium & endomysium contain blood vessels and nerve endings

Microscopic Anatomy of a Muscle

• Myofibrils

• Nucleus

• Filaments

• Sarcoplasmic reticulum

• Sarcolemma

Myofibrils are made of

• Actin

  • Thin filaments

• Myosin

  • Thick filaments

Anatomy of the Muscle Fiber

• A = Sarcolemma

• B = Sarcoplasm

• C = Myofibrils

• D = Myofilaments (Actin / Myosin)

• E = Light (I) Band

• F = Dark (A) Band

Muscle Proteins

• Contractile:

  • Myosin

  • Actin

• Regulatory:

  • Troponin

  • Tropomyosin

• Structural:

  • Titin

  • Nebulin

  • Alpha-actin

  • Myomesin

  • Dystrophin

Sarcomere

• The functional unit of contraction in skeletal muscle myofibrils.

• Located between two Z lines.

• One end of each actin filament is attached to the Z line.

• Myosin filaments are located between two actin filaments and overlap them on each end.

Components of a Sarcomere

• Contractile proteins

  • Proteins that generate force during muscle contractions.

  • Myosin

    • Contractile protein that makes up thick filament

    • Molecule consists of a tail and two myosin heads, which bind to myosin-binding sites on actin molecules of thin filament during muscle contraction.

  • Actin

    • Contractile protein that is the main component of thin filament

    • Each actin molecule has a myosin-binding site where myosin head of thick filament binds during muscle contraction.

• Regulatory proteins

  • Proteins that help switch muscle contraction process on and off.

  • Tropomyosin

    • Regulatory protein that is a component of thin filament.

    • When skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin.

  • Troponin

    • Regulatory protein that is a component of thin filament.

    • When calcium ions (Ca^{2+}) bind to troponin, it changes shape.

    • This conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.

• Structural proteins

  • Proteins that keep thick and thin filaments of myofibrils in proper alignment, give myofibrils elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix.

  • Titin

    • Structural protein that connects Z disc to M line of sarcomere, thereby helping to stabilize thick filament position.

    • Can stretch and then spring back unharmed, and thus accounts for much of the elasticity and extensibility of myofibrils.

  • α-Actinin

    • Structural protein of Z discs that attaches to actin molecules of thin filaments and to titin molecules.

  • Myomesin

    • Structural protein that forms M line of sarcomere

    • Binds to titin molecules and connects adjacent thick filaments to one another.

  • Nebulin

    • Structural protein that wraps around entire length of each thin filament

    • Helps anchor thin filaments to Z discs and regulates length of thin filaments during development.

  • Dystrophin

    • Structural protein that links thin filaments of sarcomere to integral membrane proteins in sarcolemma, which are attached in turn to proteins in connective tissue matrix that surrounds muscle fibers

    • Thought to help reinforce sarcolemma and help transmit tension generated by sarcomeres to tendons.

From muscles to the sarcomere

• The skeletal muscle cells are organized from contractile units know as sarcomeres, which are the structural subunits arranged in a repeated pattern, along the length of the cells.

• The sarcomere holds properties that are crucial for its function:

  1. fast and efficient shortening,

  2. millisecond activation/inactivation, and

  3. precise structural self-assembly.

Sarcomere

• I band

• Z line

• Thin filaments

• Thick filaments

• A band

• Titin

• Actin

• Myosin

Contraction and Relaxation of Skeletal Muscle Fibers

Skeletal Muscle Contractions

• Controlled voluntarily by the nervous system

• Motor Unit

  • A motor neuron (nerve cell) and all of the muscle fibers it controls

The sliding filament theory (1)

• Actin myofilament:

  • An actin myofilament is made up of actin molecule, tropomyosin and troponin complex.

  • Tropomyosin form two helical strand which are wrapped around actin molecules.

  • Each G-actin is attached with an ATP molecule.

  • The whole assembly of actin molecules is known as F-actin (Fibrous actin).

  • Tropomyosin switches ON or OFF the muscle contraction mechanism.

  • Troponin complex is a globular protein which binds to tropomyosin and calcium ions.

• Myosin myofilament:

  • A myosin myofilament consists of two distinct region, a long rod-shaped tail called myosin rod and two globular intertwined myosin head.

  • The globular head appear at interval along the myosin myofilament, projecting from the sides of the filament.

  • The myosin head can attach to the neighboring acting filament where actin and myosin filaments overlaps.

The Myofilaments

• Actin and Myosin

Sliding Filament Theory

• The arrangement of actin and myosin myofilament within a sarcomere is crucial in the mechanism of muscle contraction.

• Muscle contracts by the actin and myosin filaments sliding past each other.

• Sarcomere is the unit of muscle contraction, its length contracts resulting in whole muscle contraction.

• During contraction, length of A-band (Dark band) remains same whereas length of I-band (Light band) and H-zone gets shorter.

Neuromuscular junction

• The Neuromuscular Junction (NMJ) or Neuromuscular Synapse (NMS)

• Events at the NMJ (or NMS) produce a muscle action potential:

  • Voltage-gated calcium channels in a neuron’s synaptic end bulb open, resulting in calcium influx.

  • This causes exocytosis of a neurotransmitter (NT) into synaptic cleft

  • NT binds to ligand-gated Na^+ channels on the motor endplate, which causes an influx of Na^+ into muscle

  • This depolarizes the muscle and results in Ca^{2+} release from the sarcoplasmic reticulum

  • NT gets broken down by acetlycholinesterase

The sliding filament theory (3)

1. Blocking of myosin head:

   • Actin and myosin overlaps each other forming cross bridge.

   • The cross bridge is active only when myosin head attached like hook to the actin filament.

   • When muscle is at rest, the overlapping of actin filament to the myosin head is blocked by tropomyosin.

   • The actin myofilament is said to be in OFF position.

2. Release of calcium ions:

   • Nerve impulse causing depolarization and action potential in the sarcolemma trigger the release of calcium ions from sarcoplasmic reticulum.

   • The calcium ion then binds with the troponin complex on the actin myofilament causing displacement of troponin complex and tropomyosin from its blocking site exposing myosin binding site.

   • As soon as the myosin binding site is exposed, myosin head cross bridge with actin filament.

   • Now, the actin myofilament is said to be in ON position.

The sliding filament theory (4)

3. Active Cross-bridge formation:

   • When myosin head attached like hooks to the neighboring actin filament, active cross bridge is formed.

   • The cross bridge between actin and myosin filament acts as an enzyme (Myosin ATPase).

   • The enzyme Myosin ATpase hydrolyses ATP stored into ADP and inorganic phosphate and release energy.

   • This released energy is used for movement of myosin head toward actin filament.

   • The myosin head tilts and pull actin filament along so that myosin and actin filament slide each other.

   • The opposite end of actin myofilament within a sarcomere move toward each other, resulting in muscle contraction.

Cross bridge formation

• After sliding the cross bridge detached and the actin and myosin filament come back to original position.

• The active cross bridge form and reform for 50-100 time within a second using ATP in rapid fashion.

• Therefore, muscle fiber consists of numerous mitochondria

• In muscle contraction, sarcomere can contracts by 30-60% of its length

Summary of Contraction and Relaxation in Skeletal Muscle

• The shortening of the sarcomere occurs along the entire length of the muscle fiber

• The strength of a muscle contraction depends on:

  • How often the individual muscle fibers are stimulated to contract

  • How many muscle fibers contract within a given muscle

Muscle Metabolism

Muscles have 3 ways to produce ATP:

• Creatine phosphate

• Anaerobic glycolysis

• Aerobic respiration

Energy for Muscle Contractions

• Energy for muscle contractions comes from ATP (adenosine triphosphate)

• Glucose is converted into ATP by mitochondria during cellular respiration.

Creatine Phosphate (CP)

• Creatine kinase catalyzes the transfer of a phosphate group from CP to ADP to rapidly yield ATP

Cellular Respiration Pathways

• Anaerobic Respiration occurs when available oxygen has been depleted

• Produces only 2 ATP’s per glucose molecule

• Also produces Lactic Acid – causes muscle soreness & fatigue

• Typically occurs during short periods of intense exercise

Anaerobic Glycolysis

• When CP stores are depleted, glucose is converted into pyruvic acid to generate ATP

Cellular Respiration Pathways

• Aerobic Respiration requires a supply of oxygen in order to take place

• Produces the maximum number of ATP molecules (36-38 ATP’s for each glucose molecule converted)

• ATP is used in long continuous exercise (distance running)

Aerobic Respiration

• Under aerobic conditions, pyruvic acid can enter the mitochondria and undergo a series of oxygen-requiring reactions to generate large amounts of ATP

Oxygen Supply

• Oxygen is carried to the muscle cells by red blood cells through the circulatory system

Fatigue

• Decrease in the strength of muscle contractions due to repeated stimulation without periods of rest

• If continued, muscle will lose ability to contract

• Occurs when ATP supply is depleted and oxygen is not replenished fast enough – lactic acid builds up in the muscle fibers

Oxygen Debt & Recovery Period

• Oxygen debt

  • Amount of oxygen needed to restore pre-exertion oxygen levels

• During recovery (rest) period, oxygen is replenished along and more ATP is produced while lactic acid is broken down

Rigor Mortis

• State of rigidity in muscles that occurs after 3-4 hours after death

  • Calcium leaks out of sarcoplasmic reticulum

  • Myosin heads to bind to actin forming cross-bridge

  • Cross-bridge can’t detach since ATP synthesis has ceased

  • After 24 hours, proteolytic enzymes digest the cross-bridge

Excitation-Contraction Coupling

• This concept connects the events of a muscle action potential with the sliding filament mechanism

Muscle Fatigue

• Muscle fatigue is the inability to maintain force of contraction after prolonged activity

• The onset of fatigue is due to:

  • Inadequate release of Ca^{2+} from SR

  • Depletion of CP, oxygen, and nutrients

  • Build up of lactic acid and ADP

  • Insufficient release of ACh at NMJ

Central Fatigue

• Central fatigue occurs due to changes in the central nervous system and generally results in cessation of exercise

Oxygen Consumption After Exercise

• Why do you continue to breathe heavily for a period of time after stopping exercise?

  • To “pay back” your oxygen debt

• The extra oxygen goes toward:

  • Replenishing CP stores

  • Converting lactate into pyruvate

  • Reloading O_2 onto myoglobin

Control of Muscle Tension

• The strength of a muscle contraction depends on how many motor units are activated

  • A motor unit consists of a somatic motor neuron and the muscle fibers it innervates

  • Activating only a few motor units will generally result in a weak muscle contraction

  • Activating many motor units will generally result in a strong muscle contraction

Motor Unit Recruitment

• Motor unit recruitment is the process in which the number of active motor units increases

  • Weakest motor units are recruited first, followed by stronger motor units

  • Motor units contract alternately to sustain contractions for longer periods of time

Muscle Contraction

• Muscle contractions can be termed twitch, summation or tetanus.

• Twitch contraction is the period of contraction and relaxation of a muscle after a single stimulation.

• Summation is the occurrence of additional twitch contractions before the previous twitch has completely relaxed.

• Summation can be achieved by increasing the frequency of stimulation, or by recruiting additional muscle fibers within a muscle.

• Tetanus occurs when the frequency of muscle contraction is such that the maximal force is tension is generated without any relaxation of the muscle.

Twitch

• When stimulated by a single action potential a muscle contracts and then relaxes.

• The time between the stimulus and the initiation of contraction is termed the latent period, which is followed by the contraction period.

• At peak contraction the muscle relaxes and returns to its resting position.

• Taken all together these three periods are termed a twitch.

Twitch Contraction

• The brief contraction of all muscle fibers in a motor unit in response to a single action potential

  • Latent period

  • Contraction period

  • Relaxation period

  • Refractory period

Muscle Tone

• Even when at rest, a skeletal muscle exhibits a small amount of tension, called tone

• Tone is established by the alternating, involuntary activation of small groups of motor units in a muscle

Types of Skeletal Muscle Fibers

• Skeletal muscle fibers can be classified based on two criteria:

  1. How fast do fibers contract relative to others?

  2. How do fibers regenerate ATP ?

• Type 1: Slow oxidative (SO) fibers

  • Contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP.

  • They produce low power contractions over long periods and are slow to fatigue.

  • These fibers have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin.

  • Myoglobin is a red pigment, similar to the hemoglobin in red blood cells, that improves the delivery of oxygen to the slow-twitch fibers.

  • Because of their high myoglobin content, slow-twitch fibers are also called red fibers.

• Type 2A: Fast oxidative (FO) fibers

  • Have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers.

  • They are called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers.

• They produce ATP relatively quickly, and can produce relatively high amounts of tension.

• They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly.

• However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers.

• FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting.

• Type 2B: Fast glycolytic (FG) fibers

  • Have fast contractions and primarily use anaerobic glycolysis.

  • The FG fibers fatigue more quickly than the other.

  • They are largest fibres, called into action when all-out effort is required (fight or flight).

  • They contract many times faster than slow-twitch fibres and with much greater force, but they fatigue quickly.

  • Fast-twitch type-2B fibres are responsible for producing high force, strength, power, and speed.

  • With the largest glycogen storage capacity in the body, they play an important role in processing the carbohydrates that we eat.

  • Lose them, and you become more susceptible to developing insulin resistance, which often results in weight gain and potentially serious health complications.

Skeletal Muscle Fiber Types

Skeletal Muscle Fiber Types

Skeletal Muscle Fiber Types

Exercise and Skeletal Muscle Tissue

• What fiber type does a marathoner use primarily?

• What fiber type does a shot putter use primarily?

• What fiber type does a soccer player use primarily?

Anabolic Steroids

• Synthetic variations of testosterone that increase muscle size and strength

  • May be prescribed for cancer or AIDS patients, among others

  • Often abused by athletes trying to gain advantage

  • Side effects impact cardiovascular, liver, musculoskeletal, integumentary and reproductive systems as well as influencing behavior

Cardiac Muscle

• Cardiac muscle has the same arrangement as skeletal muscle, but also has intercalated discs

• Characterized by:

  • cross-striations

  • intercalated discs

  • uni-nucleate cells

  • automaticity

  • composed of the same contractile proteins as skeletal muscle.

• Intercalated discs

  • Structures important in cardiac muscle contraction: gap junctions and desmosomes.

  • Gap junction

    • Forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next.

    • Electric coupling allows the quick transmission of action potentials and the coordinated contraction of the entire heart.

  • Syncytium

    • Network of electrically connected cardiac muscle cells creates a functional unit of contraction.

  • Desmosome

    • Cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting

• Pacemaker cells

  • Specialized cardiac muscle cells that directly control heart rate.

  • Respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate and hormones that modulate heart rate to control blood pressure.

  • Cardiac muscle cannot be consciously controlled.

Cardiac Muscles

• Cardiac muscle cells have more mitochondria and their contractions last 10 to 15 times longer than skeletal muscle contractions

• Cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell.

• Possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism.

• Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs.

• An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

Smooth Muscle

• Smooth muscle looks quite different than cardiac and skeletal muscle.

  • Thick in the middle, tapered on the ends, and is not striated

  • It can be arranged as either single-unit or multi-unit fibers

• Characterized by:

  • Spindle shaped cells

  • Uni-nucleate cells

  • Involuntary control

  • Found in walls of hollow organs, blood vessels and glands

• Smooth muscle contractions start more slowly and last longer than skeletal and cardiac muscle contractions

• Smooth muscle can shorten and stretch to a greater extent than skeletal and cardiac muscle

• Smooth muscle fibers shorten in response to stretch!

• Involuntary

• The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors.

• Stretching the muscle can trigger its contraction (the stretch-relaxation response)

• No striations present

• Found in the walls of:

  • Hollow organs like the urinary bladder, uterus, stomach, intestines

  • Passageways, such as the arteries and veins of the circulatory system

  • Tracts of the respiratory, urinary, reproductive systems

• Present in the eyes, where it functions to change the size of the iris and alter the shape of the lens

• In the skin where it causes hair to stand erect in response to cold temperature or fear.

Summary of Major Features of Three Types of Muscular Tissue

A Few More Facts About Muscle

• Mature skeletal muscle fibers cannot undergo mitosis

  • Hypertrophy

  • Hyperplasia

  • Smooth muscle and pericytes

Aging and Muscle Tissue

• Between 30–50 years of age, about 10% of our muscle tissue is replaced by fibrous connective tissue and adipose tissue.

• Between 50–80 years of age another 40% of our muscle tissue is replaced.

• Consequences are:

  • Muscle strength and flexibility decreases

  • Reflexes slow

  • Slow oxidative fiber numbers increase

Muscular Hypertrophy and Atrophy

• Muscular Hypertrophy

  • Enlargement of existing muscle fibers

  • Due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum and other organelles

• Muscular Atrophy

  • Decrease in size of muscle fibers due to loss of myofibrils

  • Occurs as a result of aging or disuse

Abnormal muscular conditions

• Paralysis

  • Loss of voluntary control of a muscle.

  • Can result from many conditions:

  • Polio

    • A virus infection that attacks the motor neurons to one or more muscles.

  • Tetanus

    • A bacterial infection (Clostridium tetani) that thrives in low oxygen environment (puncture wounds) and releases powerful toxins that interfere with motor neuron communications with muscle cells.

  • Botulism

    • A bacterial infection (Clostridium botulinum) that blocks the release of ACh into the neuromuscular junction. This is the toxin used in the highly popular Botox injections.

  • Myasthenia gravis

    • A progressive muscular weakness caused by the loss of ACh receptors at the neuromuscular junctions.

  • Multiple Sclerosis

    • An autoimmune disease that cause progressive muscle weakness and paralysis, cause