CH. 12 Skeletal Muscle Flashcards
What connects skeletal muscle to two or more bones?
tendons
What is continuous with tendon connective tissue?
epimysium: tissue surrounding muscle
What extends into the muscle body, dividing the muscle into fascicles?
perimysium
What is the triad composed of?
T tubule + two lateral sacs
What do filaments form?
Sarcomeres
What gives skeletal and cardiac muscle its striated appearance?
the orderly arrangement of thick and thin filaments of the myofibrils
How is the structure of a sarcomere broken down? (6 parts total)?
A band I band H zone Z line M line Sarcomere
What kind of protein is actin, and what does it have a binding site for?
a contractile protein
myosin
What kind of protein is tropomyosin and where does it overlap?
Regulatory protein
overlaps binding sites on actin for myosin
What kind of protein is troponin?
A regulatory protein
What are the 3 complexes in which troponin binds to?
- attaches to actin
- attaches to tropomyosin
- binds Ca2+ reversibly
What does Ca2+ binding to troponin regulate?
skeletal muscle contraction
What is titin?
- an elastic protein that anchors thick filaments between the M line and the Z line
- provides structural support and elasticity in muscle
What initiates excitation?
When a nerve impulse reaches the axon terminal of the motor neuron?
What two things return a cell to its resting state?
- potassium ions diffuse out of the cell
2. sodium-potassium pump moves sodium and potassium ions back to their original positions
What is excitation?
action potential comes down a lower motor neuron that releases acetylcholine and binds to an acetylcholine ligand-gated channel which opens up a sodium channel that allows for sodium to come in that initiates a motor end plate potential that scatters across the surface of the muscle cell down the t-tubule
What is coupling?
motor end plate potential goes down the t tubule and activates DHP which activates ryanodine in the smooth ER which will then activate the release of Ca2+ into the cytoplasm
What is contraction?
- calcium binds to troponin which causes troponin to change shape moving tropomyosin off of the thin filament so that it could interact with the thick filament
- myosin binds ATP, breaks down ATP and now has ADP and inorganic phosphate in the pocket and myosin now has high affinity for the thin filament and binds to the thin filament and
- when it does that it releases inorganic phosphate which causes the myosin head to change shape it cocks and that pulls the thin filament with it, at that point it releases ADP and then Is stuck to the thin filament and thats called rigor.
- the only way that it can be released from the thin filament is if ATP sits in the pocket and the thick filament could release the thin filament and it starts all over again.
What is the sliding-filament model of contraction?
neither thick nor thin filaments shorten but rather they slide past each other
According to the sliding-filament model, what occurs during contraction?
- A band stays the same length
- I band shortens
- H zone shortens
- Thus the sarcomere shortens
What is the cross bridge cycle?
Filament sliding is due to cyclical formation and breaking of cross bridges
What is the high energy form of the myosin head?
- ADP and inorganic phosphate bound to myosin
- High affinity for actin
What is the low energy form of the myosin head?
- ATP bound to myosin
- Low affinity for actin
What does the cross-bridge cycle rely on?
hydrolysis of ATP
What needs to happen for excitation-contraction coupling to occur?
- dependent on neural input from the motor neuron
- requires calcium release from the sarcoplasmic reticulum
What are the 6 roles of the neuromuscular junction?
- each motor neuron innervates several muscle cells
- each muscle fiber receives input from a single motor neuron (similar to ordinary synapse)
- where acetylcholine is released
- motor end plate (high density of acetylcholine receptors and highly folded)
- end-plate potential
- motor neuron AP always creates a muscle cell AP
What occurs if no calcium is present during the process of excitation-contraction coupling?
troponin holds tropomyosin over myosin binding sites on actin
- no cross bridges would form between actin and myosin
- muscle would be relaxed
What occurs if calcium is present during excitation-contraction coupling?
binds to troponin, causing movement of troponin, causing movement of tropomyosin, exposing binding sites for myosin on action
- cross bridges form between actin and myosin
- cycle occurs; muscle contracts
How are calcium channels of the sarcoplasmic reticulum gated?
They are voltage gated
- coupled to T tubules by ryanodine and DHP receptors
- calcium induced opening and closing
How is contraction terminated?
calcium must leave troponin, allowing tropomyosin to cover myosin binding sites on actin
How is calcium removed from the cytosol?
calcium-ATPase pumps in the sarcoplasmic reticulum transports calcium from the cytosol into the sarcoplasmic reticulum
What is the twitch of a muscle?
contraction produced in a muscle fiber in response to a single action potential
- can be defined for a muscle fiber, for a motor unit, or at the whole muscle level
What are the phases of a twitch?
- latent period
- contraction period
- relaxation phase
What kind of force is exerted by contracting muscles?
tension force
What kind of force opposes contraction?
load
What does coupling start with?
motor end point potential
What are the components of an isometric twitch contraction?
- the length of the muscle fiber stays stretched
- contractile elements contract and tension is generated
What occurs when the load is greater than the tension?
- the elastic elements are stretched
- muscle does not shorten, load not lifted
What is an isotonic twitch contraction?
constant tension and muscle shortens as load is lifted when tension is greater than load
What two factors are graded muscle contractions dependent on?
- tension produced by each fiber
- number of active cross bridges that bind to actin
- more cross bridges that bind = more force - number of fibers contracting = more work that can be done
In what two ways does an increase in frequency of action potentials in muscle fibers increase tension?
- treppe
2. summation
What factors affect the force generated by individual muscle fibers?
- Fiber diameter: more actin and myosin allows for more force to be generated
- Fiber length
- Optimal length: resting length of muscle at which the fiber can develop the greatest amount of tension (due to maximum overlap of thick filament crossbridges and thin filaments)
What is force-generating capacity dependent on?
the number of cross bridges in each sarcomere and the geometric arrangement of sarcomeres
- more crossbridges/sarcomere = more force
- more sarcomeres in parallel = more force
What occurs when muscle fibers are at non-optimal lengths?
- Greater than optimum: decrease cross bridge overlap
- Less than optimum: thin filaments overlap each other and Z lines make contact with thick filaments
Why would a second stimulation produce a higher contraction than the first?
There are higher calcium levels present
What causes summation and tetanus?
- amount of tension developed depends on amount of calcium bound to troponin
- at high frequencies, the release exceeds reuptake
- calcium increases in cytosol
- this eventually saturates system causing maximum tetanic contraction (all troponin bound to calcium and crossbridge cycling maxed out)
What does recruitment refer to?
- muscle fibers stimulate more fibers to contract leading to a greater overall muscle tension
- recruitment occurs at the level of the motor unit
How does motor unit recruitment occur?
- activation of the motor neuron activates all muscle fibers in the motor unit
- increases in tension occur in proportion to the size of the motor unit
What muscles are recruited for delicate movements?
small motor units
What muscles are recruited for strength?
large motor units
What are small diameter fibers recruited for?
weaker movements
What are large diameter fibers recruited for?
stronger movements
What are the size of the motor unit and fiber diameter often related to?
motor neuron cell bodies and axon diameter
Why are large motor units recruited last?
larger neurons are more difficult to depolarize to threshold because they require greater synaptic input
What occurs when a muscle contracts isotonically in relation to velocity of shortening when increasing load is present?
- latent period of shortening increases
- duration of shortening decreases
- velocity of shortening decreases
What sources of ATP do muscle cells pull from?
- phosphorylation of ADP by creating phosphate
- oxidative phosphorylation of ADP in mitochondria
- anaerobic glycolysis
What are the two basis in which skeletal muscle are classified?
- velocity of contraction: fast versus slow
- “how quickly does myosin interact with thin filament - primary energy source: oxidative vs. glycolytic
- “how it ATP generated”
How do fibers differ in speed of contraction?
- fast-twitch fibers: myosin with fast ATPase activity
- contract 2-3 times more rapidly than slow fibers
- relax more rapidly - slow-twitch fibers: myosin with slow ATPase activity
- last 10 times longer than fast fiber contractions
How do glycolytic fibers differ from oxidative fibers?
Glycolytic:
- anaerobic glycolysis
- fewer mitochondria
- many glycolytic enzymes
- high glycogen stores
- use little oxygen (anaerobic)
- large diameter
- quick to fatigue
Oxidative:
- oxidative phosphorylation
- many mitochondria
- myoglobin (red)
- small diameter
- resistant to fatigue
- many capillaries
What are the three types of skeletal muscle fibers?
- slow oxidative
- fast oxidative
- fast glycolytic
What are the properties of slow oxidative fibers?
- low myosin ATPase
- high oxidative capacity (aerobic)
- small diameter
- fatigue slowly
What are the properties of fast glycolytic fibers?
- high myosin ATPase activity
- high glycolytic capacity
- no myoglobin
- large diameter
- fatigue rapidly
What are the properties of fast oxidative fibers?
- intermediate myosin ATPase fibers
- high oxidative capacity (aerobic)
- myoglobin
- slow to fatigue, but more rapid than slow oxidative fibers
- intermediate diameter
What is the recruitment order of muscle fibers during exercise?
- slow oxidative fibers
- fast oxidative fibers
- fast glycolytic fibers (sprinting)
What are the long term responses of muscles to exercise in high intensity exercise?
- decreased oxidative capacity
- some fast oxidative fibers can be converted to fast glycolytic fibers
- decrease in size and number of mitochondria
- increase in fiber diameter
- reduced resistance to fatigue
What are the long term responses of muscles to aerobic exercise?
- increased oxidative capacity
- some fast glycolytic fibers can be converted to fast oxidative fibers
- increase in size and number of mitochondria
- decrease in fiber diameter
- increase in number of capillaries surrounding muscle fibers
What are intrafusal fibers?
- contractile cells of the muscle spindle
- adjust the sensitivity of the muscle to stretch
- innervated by gamma motor units
What are extrafusal fibers?
- contractile cells fo the muscle
- responsible for skeletal muscle contraction
- innervated by alpha motor neurons
What are muscle spindles?
detect change in muscle length
What are the types of sensory endings of the muscle spindles?
- Annulospiral endings: type 1A afferent fibers
2. Flower-spray endings: type 2 afferent fibers
What are golgi tendon organs and how are they activated?
- sensory capsules within tendons
- tendon stretch activates the golgi tendon organ
- reflex inhibition of muscle via type 1B afferent neurons
- protection against overactivity of muscle
How is the sliding filament mechanism of contraction different in smooth muscle than in skeletal muscle?
- actin and myosin are longer in smooth muscle than in skeletal muscle
- myosin heads whole length
- longer range of contraction
How does smooth muscle relaxation occur?
- phosphatase removes phosphate from myosin
2. calcium is removed from the cytoplasm by either calcium ATPase or Ca/Na countertransport
How does contraction time in smooth muscle differ from skeletal muscle?
myosin ATPase contraction is 10-100 times slower in smooth muscle than in skeletal muscle
How are single-unit smooth muscle fibers activated? Where are they found?
- fibers are connected by gap junctions and contract synchronously as a single unit
- intestinal tract, blood vessels, and respiratory tract
How are multi-unit smooth muscle fibers activated?
Where are they located?
- each fiber acts individually
- receives own innervation
- recruitment but no tone occurs - located in large airways and arteries, muscles of the eye
What is tone referring to?
level of contraction without stimulation
