Tortora Chapter 10: Muscular Tissue

myology

the scientific study of the structure, function, and diseases of skeletal, cardiac, and smooth muscular tissue

Differences between muscular tissues

the three types of muscular tissue differ in 1) microscopic anatomy, 2) location, and 3) how they are controlled my nervous/endocrine system

skeletal muscle tissue

the function of skeletal muscles is to move bones of the skeleton

- striated

- voluntary

striations

alternate dark and light bands found on skeletal and cardiac muscle

cardiac muscle tissue

muscle tissue found only in the heart's walls

- striated

- involuntary

- autorhythmicity

autorhythmicity

a feature of cardiac muscle tissue that allows the generation of action potentials without an external stimulus

smooth muscle tissue

muscle tissue located in the walls of hollow internal structures (visceral organs, blood vessels, airways; also attached to skin hair follicles)

- nonstriated (smooth)

- involuntary

- some autorhythmicity (GI tract)

involuntary muscle

regulated by neurons that are part of the autonomic (involuntary) division of the nervous system and by hormones released by endocrine glands

Functions of muscular tissue

1. Produce body movements: total and local body movements by skeletal muscles

2. Stabilizing body positions: skeletal muscle contractions stabilize joints and maintain body positions (constantly working)

3. Storing and moving substances within the body: smooth muscle sphincters prevent outflow of contents of hollow organs; cardiac muscle pumps blood through vessels; skeletal muscles promote flow of lymph

4. Producing heat: muscular tissue contractions produce heat and maintain body temperature

thermogenesis

the production of heat in the body as a result of muscle contrations

Properties of muscular tissue

1. Electrical excitability: ability to respond to certain stimuli by producing electrical signals (action potentials)

2. Contractility: ability of muscular tissue to contract forcefully when stimulated by action potentials

3. Extensibility: ability of muscular tissue to stretch without being damaged

4. Elasticity: ability of muscular tissue to return to its original length and shape after contraction or extension

action potential

the ability to respond to certain stimuli by producing electrical signals

- muscle action potentials

- nerve action potentials (nerve impulses)

Two types of stimuli that trigger action potentials

- Electrical signals: by nerve cells or by autorhythmicity

- Chemical stimuli: neurotransmitters released by neurons, hormones, pH changes

muscle fibers

skeletal muscle cells (myocytes)

- surrounded by connective tissue

Gross anatomy of Skeletal muscle

- skeletal muscle consists of a muscle belly

- tendons that connect muscle belly to skeleton

- rope-like tendons or flat aponeurosis

muscle belly

fleshy well-vascularized central portion of muscle

tendons

tough, glistening white dense regular connective tissue structures that attach muscle to bones

aponeuroses

flat sheets of connective tissue that attach muscle to muscle

Connective tissue coverings of skeletal muscle

epimysium, perimysium, endomysium

endomysium

a thin wrapping of reticular fibers surrounding each muscle fiber (cell)

fascicle

bundle of muscle fibers wrapped in perimysium connective tissue

perimysium

dense irregular connective tissue covering muscle fasicles (bundles of muscle fibers)

epimysium

thicker covering of dense irregular connective tissue surrounding muscle belly (bundles of muscle fascicles)

Structure of tendons

tendons are continuations of all connective tissue coverings (endomysium, perimysium, and epimysium) that run through the muscle and emerge at the ends of the muscle belly as muscle fiber cells taper off

- This gives muscles their strength

fascia

dense irregular connective tissue sheets that encircle groups of skeletal muscles (organs)

Microscopic anatomy of a Skeletal muscle fiber (cell)

- Sarcolemma

- Transverse tubules (T tubules)

- Sarcoplasm

- Myofribrils

- Sarcoplasmic reticulum

- Sarcomere

- Myofilaments

hypertrophy

increase in muscle fiber (cell) size

hyperplasia

increase in number of muscle cell fibers (cells)

fibrosis (scarring)

the replacement of muscle fibers by fibrous scar tissue

- regeneration of skeletal muscle tissue is limited

sarcolemma

the plasma membrane of a muscle fiber (cell)

- nuclei located just underneath

Transverse tubules (T-tubules)

invaginations of the muscle fiber's (cell's) sarcolemma, therefore filled with interstitial fluid

- transmit action potential through cell (allow entire muscle fiber to contract simultaneously)

sarcoplasm

the cytoplasm of a muscle fiber (cell)

- contains myoglobin only present in muscle cells

myoglobin

a protein only found in muscle that binds oxygen molecules that diffuse into the muscle fiber(inside) from intersitial fluid (outside)

- Release the oxygen when cell needs it for ATP production

myofibrils

small thread-like structures which are the contractile elements of skeletal muscle

- have prominent striations that make the whole muscle fiber (cell) look striated

sarcoplasmic reticulum (SR)

fluid-filled system of membranous sacs encircling each myofibril

- specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca++ (calcium)

Triad of SR and T-Tubule

(2) dilated end sacs of the sarcoplasmic reticulum called terminal cisterns surround (1) T-tubule on both sides, forming a triad (3)

myofilaments

smaller protein structures (filaments) within myofibrils

- Actin (thin filament)

- Myosin (thick filament)

Two types of myofilaments

- Thin filaments: composed mostly of the protein actin

- Thick filaments: composed mostly of the protein myosin

- Both filaments are directly involved in the contractile process

- Overlap of thick and thin filamens creates striations

sarcomere

basic functional unit of a myofibril

- filaments (actin and myosin) inside a myofibril are arranged in these compartments

Structure of sarcomere

Z discs, I band, A band, H zone, M line

Z discs

narrow plate-shaped regions of dense protein material that separate one sarcomere from the next

- distance from one Z disc to the other is one sarcomere

A band

Darker middle part of the sarcomere, which extends the entire length of the thick filaments (myosin)

- Towards the ends of the band, thick and thin filaments overlap

I band

a lighter, less dense area that contains thin filaments (actin) but no thick filaments

- Z disc passes through center of each band

H zone

narrow zone in the center of each A band that contains thick filaments (myosin) but no thin filaments (actin)

M line

supporting proteins that hold the thick filaments together at the center of the H zone form the M line (middle)

Three types of proteins in Myofibrils

1. contractile proteins: generate force during contraction; myosin and actin

2. regulatory proteins: help switch the contraction process on and off

3. structural proteins: keep thick and thin filaments in the proper alignment, give myofibril elasticity and extensibility, and link myofibrils to the sarcolemma and ECM

Structure and Function of Myosin (contractile protein)

- motor protein in all muscular tissue

- converts ATP into mechanical energy

- myosin tail points towards M line

- each myosin has 2 heads

- each head has 2 binding sites (actin binding site and ATP binding site)

Structure and Function of Actin (contractile protein)

- thin filaments are anchored to Z discs

- actin molecules join to form actin filament twisted into helix

- each actin molecule has a myosin binding site

- Two regulatory proteins (tropomyosin and troponin) are part of each actin filament

Regulatory proteins and function

- tropomyosin and troponin are part of the thin (actin) filament

- regulatory proteins block myosin from binding to actin

- Tropomyosin blocks myosin binding site of actin

- Troponin holds tropomyosin in place

- calcium binds to troponin causing to change shape and allow myosin to bind to actin (muscle contraction)

Structural proteins of skeletal muscle fibers

titin, myomesin, nebulin, dystrophin

titin (structural protein)

anchors a thick filament to both a Z disc and the M line, helping stabilize the position of the thick filament

myomesin (structural protein)

forms the M line; binds to Titin; connects thick filaments to one another at the M line

nebulin (structural protein)

anchors thin filaments to Z discs and regulates the length of thin filaments

dystrophin (structural protein)

links thin filaments of sarcomere to integral membrane proteins of the sarcolemma; reinfoces sarcolemma;

transmits tension generated by sarcomere to the tendons

sliding filament model

skeletal muscle shortens during contraction because the thick and thin filaments slide past one another and overlap

Muscle contraction and sliding filament model

- Thick (myosin) filament binds to thin (actin) filament after calcium binds to troponin and tropomyosin uncovers myosin binding sites of thin filaments

- Thin filaments slide inward and meet at the M line

- Sarcomere shortens (Z discs pulled closer) and so do muscle fibers (muscle contracts)

Lengths of the filaments do not change (only degree of overlap)

-- H zone: gets smaller and disappears in fully contracted muscle (full overlap of thick and thin filaments)

-- A band: no change in length

-- I band: gets smaller and disappears in fully contracted muscle (full overlap of thick and thin filaments)

neuromuscular junction

the synapse between a somatic motor neuron and a skeletal muscle fiber from which muscle action potentials are created

synapse

a region where communication occurs between two neurons (or between a somatic motor neuron and a muscle fiber)

synaptic cleft

the small gap that separates the two cells (neuron and muscle fiber)

neurotransmitter

chemical used by a neuron to transmit an impulse (chemical signal) across a synapse to another cell (muscle fiber)

axon terminal

the end of an axon (of a nerve cell)

synaptic end bulbs

found at end of axon terminal, contain synaptic vesicles filled with neurotransmitters

synaptic vesicles

membrane-bounded sacs in which synthesized neurotransmitters are stored

acetylcholine (ACh)

the neurotransmitter released by the neuron at the NMJ (neuromuscular junction)

motor end plate

the region of the sarcolemma (plasma membrane of the muscle fiber) opposite of the synaptic end bulbs

acetylcholine receptors

integral transmembrane proteins that bind specifically to ACh (acetylcholine)

junctional folds

deep grooves of the motor end plate in which ACh receptors are found, that provide a large surface area for ACh

motor unit

a somatic motor neuron plus all the skeletal muscle fibers it stimulates

Four steps of a nerve impulse (nerve action potential)

1. Release of Acetylcholine (ACh):

happens after a nerve impulse reaches the synaptic end bulb and stimulates its channels to open, allowing Ca2+ (calcium) to enter the bulb; calcium enters and stimulates exocytosis of synaptic vesicles that contain ACh, and acetylcholine diffuses into the synaptic cleft

2. Activation of ACh Receptors:

acetylcholine binds to the receptors on the motor end plate and opens an ion channel that allows Na+ (sodium) across the membrane into the muscle fiber

3. Production of muscle action potential:

the inflow of Na+ (sodium) triggers a muscle action potential; the muscle action potential travels along the sarcolemma to T-tubules, and comes into contact with Sarcoplasmic Reticulum (via triads); muscle action potential causes SR to release Ca2+ into the muscle fiber cell sarcoplasm and muscle contracts

4. Termination of ACh activity:

ACh in the synaptic cleft is broken down by enzymes (acetylcholinesterase) that are attached to collagen fibers in ECM; ACh receptors can no longer be activated; Ca2+ returns to the cell's SR

The Contraction Cycle

the repeating sequence of events that causes filaments of the sarcomeres to slide across each other (causes contraction, shortening of the muscle cell);

as SR releases Ca2+ into sarcoplasm, calcium binds to troponinand causes tropomyosin to uncover myosin-binding sites on actin, contraction cycle begins

Four steps of the Contraction Cycle

1. ATP Hydrolysis:

myosin head has ATP binding site and ATPase, an enzyme that breaks down ATP into ADP + phosphate group (stays attached to myosin); causes myosin to energize and reorientate

2. Attachment of Myosin to Actin to form cross-bridges:

the energized myosin head attaches to myosin-binding site on actin and releases phosphate group

3. Power Stroke:

after cross-bridges form, myosin head releases ADP which generates force, sliding thin filament (actin) past thick filament (myosin) towards M line

4. Detachment of Myosin from Actin: myosin head binds to another ATP molecule and detaches from actin's myosin binding site

cross-bridges (contraction cycle)

attachment of myosin heads to actin during contraction

excitation-contraction coupling

the sequence of events that links excitation (a muscle action potential) to contraction (sliding of the filaments)

7 steps of excitation-contraction coupling

1. T-tubules have integral membrane proteins called voltage-gated Ca2+ channels

2. muscle action potential travels along T-tubule and the Ca2+ channels detect it and change

3. change of the voltage-gated Ca2+ channels causes Ca2+ release channels on the SR terminal cisternae to open (are now unblocked) and release Ca2+

4. concentration of Ca2+ increases and calcium binds to troponin, muscle contraction occurs

5. terminal cisternae of the SR also have Ca2+ active transport pumps that use ATP to move Ca2+ from sarcoplasm into the SR

6. constant release of Ca2+ from inside SR into the sarcoplasm and transport of Ca2+ back into SR; influx of Ca2+ in to sarcoplasm is faster for duration of muscle action potential

7. after muscle action potential ceases, Ca2+ release channels close; Ca2+ active transport pumps move calcium back into SR; inside SR, calsequestrin protein molecules bind to Ca2+ allowing more storage of calcium in the SR

calsequestrin

calcium-binding protein within the sarcoplasmic reticulum which aids in storage of Ca2+

muscle tone

a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units

- keeps skeletal muscles firm (maintains posture)

2 types of muscle contractions

1. Isotonic

2. Isometric

isotonic contraction

the tension (force of contraction) in the muscle remains almost constant while the muscle changes its length;

PRODUCES BODY MOVEMENTS AND MOVES OBJECTS

2 types of isotonic contractions

1. Concentric

2. Eccentric

concentric isotonic contraction

the tension generated is great enough to overcome the resistance of the object to be moved, muscle shortens

eccentric isotonic contraction

tension generated resists movement of load, muscle increases in length (but tension slows lengthening down)

isometric contraction

the tension generated is not enough to exceed the resistance of the object to be moved and the muscle does not change its length

Properties of Cardiac muscle tissue

- Striated (alternating light and dark bands formed by thick/thin filaments in sarcomeres)

- Involuntary contraction

- Autorhythmicity (ability to repeatedly generate spontaneous action potentials)

- Uninucleate (only one nucleus per cell)

- Branching muscle fibers

- Intercalated discs (joining muscle fibers)

- only endomysium covering each cell

- more numerous mitochondria

intercalated discs

irregular transverse thickening of the ends of cardiac muscle fiber's sarcolemma, which connects it to neighboring muscle fibers;

contain desmosomes and gap junctions that allow muscle action potentials to travel

Properties of Smooth muscle tissue

- Involuntary contraction

- Uninucleate (one nucleus per cell)

- No striations (thick and thin filaments are not arranged in sarcomeres)

- No T-tubules and little sarcoplasmic reticulum

- only endomysium covering each cell

- contain structural intermediate filaments (desmin)

- Contains dense bodies (similar to Z discs)

2 types of smooth muscle tissue

1. visceral (single unit)

2. multi-unit

visceral (single unit) smooth muscle tissue

found in skin, small arteries and veins, hollow viscera (stomach, intestines, uterus, urinary bladder)

- muscle fibers connected by gap junctions which allow fast propagation of muscle action potentials (from a single cell to many cells)

- one neuron for many muscle fibers (not every fiber connects to neuron)

- STIMULATION OF ONE VISCERAL MUSCLE FIBER CONTRACTS MANY OTHER FIBERS -

multi-unit smooth muscle tissue

consists of individual fibers, each fiber has its own motor neuron terminals;

few gap junctions between each muscle cell

- STIMULATION OF MULTI UNIT MUSCLE FIBERS CONTRACTS ONLY THAT FIBER -

intermediate filaments

filaments, which contain the protein desmin, appear to have a structural rather than contractile role;

tension generated by thick and thin filaments is transferred to intermediate filaments

dense bodies

structures attached to sarcolemma to which intermediate filaments attach;

during contraction, intermediate filaments pull on the dense bodies, causing shortening of the muscle

smooth muscle tone

a state of continuous partial contraction of smooth muscle tissue;

caused by prolonged presence of Ca2+ in sarcoplasm of muscle fiber

Which type of muscular tissue can contract/stretch the most?

Smooth muscle fibers can stretch considerably and still maintain their contractile function due to lack of striations