Muscular System Flashcards
three types of muscular tissue
skeletal, cardiac, and smooth
the scientific study of muscles
myology
skeletal muscle tissue
- move the bones of the skeleton
- striated (alternating light and dark protein bands called striations)
- works in a voluntary manner (controlled by neurons/nerve cells that are part of the somatic/voluntary division of the nervous system)
cardiac muscle tissue
- contained only in the heart (heart wall)
- striated but involuntary
- alternating contraction and relaxation of the heart is not consciously controlled; heart beats because it has a natural pacemaker that initiates contraction (autorhythmicity)
autorhythmicity
built-in rhythm that makes the heart beat because of its natural pacemaker that initiates the contraction
several hormones and neurotransmitters can adjust heart rate by speeding or slowing the pacemaker
smooth muscle tissue
- located in the walls of hollow internal structures (i.e. blood vessels, airways, and most organs in the abdominopelvic cavity)
- nonstriated = smooth
- usually involuntary; some smooth muscle tissue like muscles that propel food through GI tract has autorhythmicity
TRUE OR FALSE: both cardiac muscle and smooth muscle are regulated by neurons that are part of the somatic voluntary division of the nervous system and by hormones released by endocrine glands.
FALSE. Cardiac and smooth muscles are regulated by neurons in the AUTONOMIC INVOLUNTARY DIVISION of the nervous system.
four functions of muscular tissue
- producing body movements
- stabilizing body positions
- storing and moving substances within the body
- generating heat
a function of the muscular tissue that allows movement of the whole body using the integrated functioning of skeletal muscles, bones, and joints
producing body movements
a function of the muscular tissue that stabilizes joints and helps maintain body positions (sitting or standing)
stabilizing body positions
e.g. postural muscles that contract continuously when one is awake
a function of the muscular tissue that sustains the contractions of ringlike bands of smooth muscles called sphincters, which prevent the outflow of the contents of a hollow organ
storing and moving substances within the body
examples of the muscle tissue storing and moving substances within the body
- temporary storage of food in the stomach or urine in the urinary bladder due to smooth muscle sphincters closing off the outlets of the organs
- cardiac muscle contractions of the heart pump blood through the blood vessels of the body (contraction and relaxation of smooth muscle in the walls of blood vessels help adjust blood vessel diameter and thus regulate the rate of blood flow)
- smooth muscle contractions also move food and substances such as bile and enzymes through the GI tract
- push gametes (sperm and oocytes) through passageways of the reproductive system
- propel urine through the urinary system
- skeletal muscle contractions also promote the flow of lymph and aid the return of blood in veins to the heart
a function of the muscular tissue that involves the process of thermogenesis to maintain normal body temperature
generating heat
involuntary contractions of skeletal muscles that can increase the rate of heat production
shivering
four properties of muscular tissue that contributes to homeostasis
- electrical excitability
- contractility
- extensibility
- elasticity
a property of the muscular tissue (and nerve cells) that enables them to respond to certain stimuli through the production of electrical signals called action potentials (impulses)
electrical excitability
term for action potentials in the muscles; term for action potentials in the nerve cells
muscle action potential; nerve action potentials
two types of stimuli trigger action potentials in muscle cells
- autorhythmic electrical signals arising in the muscular tissue itself, as in the heart’s pacemaker
- chemical stimuli, such as neurotransmitters released by neurones, hormones distributed by the blood, or even local changes in pH
a property of the muscular tissue that involves the ability to contract forcefully when stimulated by an action potential
contractility
when a skeletal muscle contracts, it generates tension (force of contraction) while pulling on its attachment points
what indicates that the tension generated in a contraction is great enough to overcome the resistance of the object to be moved?
when the muscle shortens and movement occurs
a property of the muscular tissue that involves the ability to stretch, within limits, without being damaged
extensibility
what tissue limits the range of extensibility and keeps muscle within the contractile range of the muscle cells?
the connective tissue within the muscle limits
examples of extensibility
- each time your stomach fills with food, the smooth muscle in the wall is stretched
- cardiac muscle being stretched each time the heart fills with blood
a property of the muscular tissue to return to its original length and shape after contraction and extension
elasticity
what is the separate organ in the skeletal muscles that is composed of hundreds to thousands of cells?
muscle fibers or muscle cell
two terms for the same structure
subcutaneous layer or hypodermis
- separates muscle from skin
- composed of areolar connective tissue and adipose tissue
- provides a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles
adipose tissue of the subcutaneous layer
- stores most of the body’s triglycerides
- serves as an insulating layer that reduces heat loss
- protects muscles from physical trauma
fascia
- dense sheet or broad band of irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs of the body
- hold muscles with similar functions together
- allows free movement of muscles
- carries nerves, blood vessels, and lymphatic vessels
- fills spaces between muscles
three layers of connective tissue extend from the fascia to protect and strengthen skeletal muscle
- epimysium
- outer layer encircling the entire muscle
- consists of dense irregular connective tissue - perimysium
- dense irregular connective tissue
- surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles - endomysium
- penetrates the interior of each fascicle and separates individual muscle fibers from one another
- mostly reticular fibers
TRUE OR FALSE: Many fascicles are large enough to be seen with the naked eye.
TRUE. They give a cut of meat its characteristic “grain” - if you tear a piece of meat, it rips apart along the fascicles.
tendons
attaches a muscle to the periosteum of a bone
all three connective tissue layers may extend beyond the muscle fibers to form a ropelike tendon
example is the calcaneal (Achilles) tendon of the gastrocnemius (calf) muscle, which attaches the muscle to the calcaneus (heel bone)
fibromyalgia
- chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons, and ligaments
- pain results from gentle pressure at specific “tender points”; even without pressure, there is pain, tenderness, and stiffness of muscles, tendons, and surrounding soft tissues
reports:
- severe fatigue, poor sleep, headaches, depression, irritable bowel syndrome, and inability to carry out their daily activities
- no specific identifiable cause
treatment:
- stress reduction, regular exercise
- application of heat, gentle massage
- physical therapy
- medication for pain, and a low-dose antidepressant to help improve sleep
TRUE OR FALSE: Generally, an artery and two or three veins accompany each nerve that penetrates a skeletal muscle.
FALSE. Generally, an artery and ONE OR TWO veins accompany each nerve that penetrates a skeletal muscle.
somatic motor neurons
- neurons that stimulate the skeletal muscle to contract
- has a threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibers
TRUE OR FALSE: Microscopic blood vessels called capillaries are barely found in muscular tissue.
FALSE. Microscopic blood vessels called capillaries are PLENTIFUL in muscular tissue. Each muscle fibers n close contact with one or more capillaries.
what brings in oxygen and nutrients and removes heat and the waste products of muscle metabolism?
blood capillaries
what does the muscle fiber do to ATP during contraction?
muscle fibers synthesize and use considerable ATP
these reactions require oxygen, glucose, fatty acids, and other substances that are delivered to the muscle fiber in the blood
what is the most important component of the skeletal muscle?
muscle fibers
where does the muscle fiber arise during embryonic development?
from the fusion of a hundred or more small mesodermal cells called myoblasts
what is the result of the fusion of myoblasts?
each mature skeletal muscle fiber will have a hundred or more nuclei
what happens to the muscle fiber when the fusion has occurred?
muscle fiber loses its ability to undergo cell division; thus the number of skeletal muscle fibers is set before you are born and most of these cells last a lifetime
where are the multiple nuclei of a skeletal muscle fiber located?
beneath the sarcolemma
T tubules
- transverse (T) tubules
- tiny invaginations of the sarcolemma
- thousands of them tunnel in from the surface toward the center of each muscle fiber
- filled with interstitial fluid
how does muscle action travel? why?
along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber
this arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant
what can be found within the sarcolemma?
sarcoplasm - the cytoplasm of a muscle fiber
sarcoplasm
- found within the sarcolemma
- includes a substantial amount of glycogen, which is a large molecule composed of many glucose molecules
*glycogen can be used for the synthesis of ATP
the red-colored protein contained in the sarcoplasm
myoglobin
myoglobin
- protein found only in muscle
- binds oxygen molecules that diffuse into muscle fibers from interstitial fluid
- releases oxygen when it is needed by the mitochondria for ATP production
where are the mitochondria located in the muscular system? why?
the mitochondria lie in rows throughout the muscle fiber, strategically close to the contractile muscle proteins that use ATP during a contraction so that ATP can be produced as quickly as needed
myofibrils
- little threads
- contractile organelles of skeletal muscle
- 2 micrometer in diameter and extend the entire length of a muscle fiber
sarcoplasmic reticulum
- fluid-filled system of membranous sacs
- encircles each myofibril
- similar to smooth ER in nonmuscular cells
dilated sacs of sarcoplasmic reticulum
terminal cisterns
*butt against T tubule from both sides
muscle hypertrophy
- muscle growth that occurs after birth through the enlargement of existing muscle fibers
- due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles
- results from very forceful, repetitive muscular activity (i.e. strength training)
TRUE OR FALSE: Hypertrophied muscles contain fewer myofibrils, hence they are capable of more forceful contractions.
FALSE. Hypertrophied muscles contain MORE myofibrils, hence they are capable of more forceful contractions.
what hormones stimulate an increase in the size of skeletal muscle fibers?
human growth hormone and other hormones during childhood
what hormone promotes further enlargement of muscle fibers?
testosterone
satellite cells
- a few myoblasts do persist in mature skeletal muscle as satellite cells
- retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers
fibrosis
happens when the number of new skeletal muscle fibers that can be formed by satellite cells is not enough to compensate for significant skeletal muscle damage or degeneration
known as the replacement of muscle fibers by fibrous scar tissue
muscular atrophy
- decrease in size of individual muscle fibers as a result of progressive loss of myofibrils
two types of muscular atrophy
- disuse atrophy
- atrophy that occurs because muscles are not used
- e.g. bedridden individuals and people with casts experience because the flow of nerve impulses to inactive skeletal muscle is greatly reduced, but the condition is reversible - denervation atrophy
- nerve supply is disrupted or cut
- over a period of 6mos to 2yrs, the muscle shrinks to about 1/4 its original size, and its fibers are irreversibly replaced by fibrous connective tissue
triad
transverse tubule + two terminal cisterns on either side of it
what happens to the sarcoplasmic reticulum in a relaxed muscle fiber?
the sarcoplasmic reticulum stores calcium ions (Ca2+)
what triggers muscle contraction in the sarcoplasmic reticulum?
through the release of Ca2+ from the terminal cisterns of the sarcoplasmic reticulum
filaments or myofilaments
- located within myofibrils; smaller protein structures
thin filaments vs thick filaments
thin filaments are 8 nm in diameter and 1-2 micrometer long composed of the protein actin
thick filaments are 16 nm in diameter and 1-2 micrometer long composed of the protein myosin
BOTH thin and thick filaments are directly involved in the contractile process
TRUE OR FALSE: There are two thin filaments for every thick filament in the regions of filament overlap.
TRUE.
sarcomeres
- present because the filaments inside a myofibril do not extend the entire length of a muscle fiber
- instead, they are arranged in compartments called sarcomeres
- basic functional units of a myofibril
Z discs
narrow, plate-shaped regions of dense protein that separates one sarcomere from the next
THUS, a sarcomere extends from one Z disc to the next Z disc
components of a sarcomere organized into variety of bands and zones
- A band
- darker middle part of the sarcomere
- extends the entire length of the thick filaments - zone of overlap
- toward each end of the A band
- where the thick and thin filaments lie side by side - I band
- lighter, less dense area
- contains the rest of the thin filaments but no thick filaments
- a Z disc passes through the center of each I band - H zone
- narrow; in the center of each A band
- contains thick but not thin filaments - M line
- supporting proteins that hold the thick filaments together at the center of the H zone form the M line
- middle of the sarcomere
what creates the striations that can be seen in both myofibrils and in the whole skeletal and cardiac muscle fibers?
the alternating dark A bands and light I bands
what are the three kinds of proteins that build myofibrils?
- contractile proteins which generate force during contraction
- regulatory proteins which help switch the contraction process on and off
- structural proteins which keep the thick and thin filaments in the proper alignment, give the myofibril elasticity and extensibility, and link the myofibrils to the sarcolemma and extracellular matrix
contractile proteins in muscle
- myosin and actin
- components of thick and thin filaments
myosin
- main component of thick filaments
- functions as a motor protein in all three types of muscle tissue
- about 300 molecules of myosin form a single thick filament
motor proteins
pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion that is, the production of force
each myosin molecule is shaped like two golf clubs twisted together - expound
myosin tail
- twisted golf club handles that points toward the M line in the center of the sarcomere
- tails of neighboring myosin molecules lie parallel to one another, forming the shaft of the thick filament
myosin heads
- two projections of each myosin molecule (golf club heads)
- each myosin head has two binding sites
- heads project outward from the shaft in a spiraling fashion, each extending toward one of the six thin filaments that surround each thick filament
two binding sites of myosin heads
- actin-binding site
- ATP-binding site
- ATP-binding site also functions as an ATPase (an enzyme that hydrolyzed ATP to generate energy for muscle contraction)
actin
- main component of the thin filament
- individual actin molecules join to form an actin filament that is twisted into a helix
myosin-binding site
located on each actin molecule where a myosin head can attach
two regulatory proteins that are part of the thin filament
- tropomyosin
2. troponin
what happens to myosin in relaxed muscle?
myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin
what holds the tropomyosin strands in place?
troponin molecules
what happens to troponin when calcium ions (Ca2+) bind to troponin?
troponin undergoes a conformational change (change in shape)
this change moves tropomyosin away from myosin-binding sites on actin, and muscle contraction subsequently begins as myosin binds to actin
structural proteins
- a dozen in the muscle
- contribute to the alignment, stability, elasticity, and extensibility of myofibrils
key structural proteins
titin, alpha-actinin, myomesin, nebulin, and dystrophin
means gigantic; a structural protein that is the third most plentiful protein in skeletal muscle (after actin and myosin)
titin
how big is titin protein?
- molecular mass of about 3 million daltons
- 50 times larger than an average-sized protein
- each titin molecule spans half a sarcomere, from a Z disc to an M line
- a distance of 1 to 1.2 micrometer in relaxed muscle
how is titin relevant in the muscular tissue?
- connects a Z disc to the M line of the sarcomere, thereby helping stabilize the position of the thick filament
- the part of the titin molecule that extends from the Z disc is very elastic (can stretch to at least four times its resting length after a muscle has contracted or been stretched)
- help prevent overextension of sarcomeres
- maintains the central location of the A bands
alpha-actinin
- composes the dense material of Z discs that bind to actin molecules of the thin filament and to titin
myomesin
- molecules that form the M line
TRUE OR FALSE: The M line proteins bind to titin and connect adjacent thick filaments to one another. Myomesin holds the thick filaments in alignment with the H zone.
FALSE. The M line proteins bind to titin and connect adjacent thick filaments to one another. Myomesin holds the thick filaments in alignment with the M LINE.
nebulin
- long, nonelastic protein wrapped around the entire length of each thin filament
- helps anchor the thin filament to the Z discs and regulates the length of thin filaments during development
dystrophin
- links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached in turn to proteins in the connective tissue extracellular matrix that surrounds the muscle fibers
- with its associated proteins, they are thought to reinforce the sarcolemma and help transmit the tension generated by the sarcomeres and tendons
related diseases to dystrophin / muscular dystrophy
homeostatic imbalances
explain the sliding filament mechanism
- skeletal muscles shorten during contraction because the thick and thin filaments slide past one another
Muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. As a result, the thin filaments slide inward and meet at the center of a sarcomere. They may even move so far inward that their ends overlap. As the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted. However, the width of the A band and the individual lengths of the thick and thin filaments remain unchanged. Since the thin filaments on each side of the sarcomere are attached to Z discs, when the thin filaments slide inward, the Z discs come closer together, and the sarcomere shortens. Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle.
the contraction cycle
At the onset of contraction, the sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm. There, they bind to troponin. Troponin then moves tropomyosin away from the myosin-binding sites on actin.
Once the binding sites are “free,” the contraction cycle—the repeating sequence of events that causes the filaments to slide—begins.
consists of four steps
- ATP hydrolysis
- attachment of myosin to actin
- power stroke
- detachment of myosin from actin
the contraction cycle - ATP hydrolysis
- a myosin head includes an ATP-binding site that functions as an ATPase—an enzyme that hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group
- energy generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle
- myosin head is said to be energized when it contains stored energy
- energized myosin head assumes a “cocked” position, like a stretched spring; in this position, the myosin head is perpendicular (at a 90° angle) relative to the thick and thin filaments and has the proper orientation to bind to an actin molecule
- notice that the products of ATP hydrolysis—ADP and a phosphate group—are still attached to the myosin head
the contraction cycle - attachment of myosin to actin
- energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group
- when a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a cross-bridge
- although a single myosin molecule has a double head, only one head binds to actin at a time
the contraction cycle - power stroke
- after a cross-bridge form, the myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments
- as the myosin head changes to its new position, it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension (force) in
the process - energy required for the power stroke is derived from the energy stored in the myosin head from the hydrolysis of ATP (see step 1 )
- once the power stroke occurs, ADP is released from the myosin head
the contraction cycle - detachment of myosin from actin
- at the end of the power stroke, the cross-bridge remains firmly attached to actin until
it binds another molecule of ATP - as ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin
what happens after the four steps of the contraction cycle have taken place?
- repeats as the myosin ATPase hydrolyzes the newly bound molecule of ATP
- continues as long as ATP is available and the Ca2+ level near the thin filament is sufficiently high
what happens to the cross-bridges after the four steps of the contraction cycle have taken place?
- cross-bridges keep rotating back and forth with each power stroke, pulling the thin filaments toward the M line
- each of the 600 cross-bridges in one thick filament attaches and detaches about five times per second
what happens to myosin heads during the contraction cycle?
- at any one instant, some of the myosin heads are attached to actin, forming cross-bridges and generating force
- other myosin heads are detached from actin, getting ready to bind again
what happens as the contraction cycle continues?
- movement of cross-bridges applies the force that draws the Z discs toward each other and the sarcomere shortens
- during a maximal muscle contraction, the distance between two Z discs can decrease to half the resting length
what happens to the Z disc as the contraction cycle continues?
Z discs pull on neighboring sarcomeres and the whole muscle fiber shortens
what are the elastic components of the muscle and what happens to them during the contraction cycle?
elastic components include
- titin molecules
- connective tissue around the muscle fibers (endomysium, perimysium, and epimysium)
- tendons that attach muscle to bone
they stretch slightly before they transfer the tension generated by the sliding filaments
what happens to the cells of the skeletal muscle that start to shorten?
- they pull on their connective tissue coverings and tendons
- the coverings and tendons stretch and then become taut, and the tension passed through the tendons. pulls on the bones to which they are attached
generally, the result is a movement of a part of the body
TRUE OR FALSE: The contraction cycle always results in the shortening of the muscle fibers and the whole muscle.
FALSE. The contraction cycle DOES NOT always result in the shortening of the muscle fibers and the whole muscle.
In some contractions, the cross-bridges rotate and generate tension, but the thin filaments cannot slide inward because the tension they generate is not large enough to move the load on the muscle (e.g. trying to lift a whole box of books with one hand).
