Chapter 12: Muscle Physiology Flashcards
what is the somatic motor division?
- supplies motor impulses to the skeletal muscles.
- voluntarily controls skeletal muscles
- ALWAYS excitatory
- is functionally and anatomically different from the autonomic pathways
anatomy of the somatic nervous system
- a single motor neuron travels from the CNS to innervate skeletal muscle cells
- a motor neuron originates in the ventral horn of the spinal cord
- a single motor neuron innervates several muscle fibres (cells) via branching axons
- a single muscle fibre can only be innervated by one neuron
what is a motor unit?
a motor neuron plus all the muscle fibres that it innervates
what is a neuromuscular junction?
the location of a synapse between a motor neuron and a skeletal muscle fiber in the somatic nervous system
what is a terminal bouton?
the axon terminal of a motor neuron in the somatic nervous system that stores and releases acetylcholine
what is a motor end plate?
- a specialized region of the muscle fibers plasma membrane
- has a large number of nicotinic cholinergic receptors (respond to acetylcholine)
what are the components of the neuromuscular junction?
- axon terminal (terminal bouton)
- motor end plates (on muscle membrane)
- schwann cell sheaths
how does signal transmission occur at neuromuscular junctions in the somatic nervous system?
- when a motor neuron is activated by converging synaptic input, action potentials are propagated to the terminal boutons of all muscle fibers in the motor unit located at the neuromuscular junctions
- the arrival of the AP causes voltage-gated Ca2+ channels in the boutons to open
- calcium enters the cytosol and triggers synaptic vesicles with ACh to fuse with the pre-synaptic membrane
- acetylcholine is released via exocytosis and diffuses across the synaptic cleft
- ACh binds to nicotinic cholinergic receptors at the motor end plate on post-synaptic membrane, causing cation channels to open
- the open channel allows Na+ to flow into and K+ to flow out of the the muscle fibre simultaneously
- the net influx of Na+ produces a depolarization called an end-plate potential (EPP)
- if the end-plate potentials depolarizes the muscle fiber to threshold, an action potential in the muscle cell plasma membrane occurs
- this triggers contraction of the muscle fiber
comparison between the somatic and autonomic motor divisons
- somatic has 1 neuron in efferent path, autonomic has 2 neurons in efferent path
- somatic has only ACh/nicotinic receptor/transmitter but autonomic has ACh/cholinergic and NE/adrengenic
- somatic targets skeletal muscle, autonomic targets smooth and cardiac muscle, endocrine/exocrine glands and adipose tissue
- somatic only has excitatory effect but autonomic has excitatory and inhibitory effects on target tissue
- somatic only has axons in peripheral but autonomic has pre-ganglionic neurons, ganglia and post-ganglionic neurons
3 types of muscles
- skeletal muscle
- cardiac muscle
- smooth muscle
what does skeletal muscle look like?
- large, multi-nucleated cells
- appear striped or striated under the microscope
- is attached to the skeletal system (bones)
what are the functions of muscle tissue?
- produce skeletal movement
- maintain body position
- support soft tissues
- maintain body temperature
- store nutrient reserves
what are the three layers of the muscle fibers?
- epimysium
- perimysium
- endomysium
–> the ends of these muscles come together to form tendons(bundle) or aponeurosis (sheet)
what is epimysium?
- connective tissue that surrounds the entire body of the muscle
- it defines the muscles volume
what is perimysium?
- connective tissue that extends into the body
- divides muscle into numerous bundles (called fascicles) of individual muscle cells
what is endomysium?
- a shin sheath of connective tissue that encases muscle fibres
- separates single muscle fibres from one another
what is a fasisicle?
- bundles of individual muscle cells
- contains 100’s to 1000’s of muscle cells called muscle fibres (due to their elongated shape)
what is a muscle fiber?
- individual/single muscle cells
- wrapped by a layer of endomysium
how large are skeletal muscle fibers?
- 3 to 4 inches long (attach at a tendon)
- 0.05mm to 0.15mm wide (diameter)
- contain hundreds of nuclei, not just one
- develop through the fusion of mesodermal cells (myoblasts)
what are the major structural features of a skeletal muscle cell?
- sarcolemma
- myofibrils
- sacroplasmic reticulum
- transverse tubules (T-tubules)
what is the sarcolemma?
- is the plasma/cell membrane of muscle fiber cells
- surrounds the sarcoplasm (cytoplasm in muscle fibers)
what are transverse tubules?
- t-tubules are invaginations of the sarcolemma
- they’re the structures that transmit action potentials rapidly from the sarcolemma to the myofibrils
- electrical signals that reach the T-tubule trigger the release of Ca2+ from the sarcoplasmic reticulum causing depolarization
- T-tubules ensure that the entire length of the muscle fiber is depolarized so it can contract simultaneously
what are myofibrils?
- made up of bundles of protein filaments called myofilaments
- a result of a collection of 2 protein myofilaments called myosin and actin
- these myofilaments are responsible for muscle contraction (shortening of a muscle fibre)
- composed of a fundamental unit called a sarcomere that repeats over and over
what is the sarcoplasmic reticulum?
- the sac-like, membranous structure surrounding each myofibril
- helps transmit action potentials to myofibrils
- form chambers (called cisternae) that attach to T-tubules which store calcium
what is a triad in skeletal muscle?
- arrangement of three components: 2 terminal cisternae of the sarcoplasmic reticulum and one t-tubule
- responsible for the regulation of excitation-contraction coupling
- the cisternae concentrate Ca2+ via ion pumps and release Ca2+ into sarcomere to begin muscle contraction
why are muscles striped?
- appearance is refered to as “striated”
- caused by the orderly arrangement of protein fibers called thick and thin filaments (actin and myosin)
- the fibers run parallel to the long axis of the muscle
what are sarcomeres?
- the basic contractile unit of muscle fiber
- they’re the repeating units that make up myofibrils
- each sarcomere is composed of two main protein filaments (actin and myosin) responsible for muscle contraction
- sarcomeres are bordered by Z lines on either side
what are thick and thin filaments?
actin = THIN
myosin = THICK
- referred to as contractile proteins because they are the machinery that generates contractile forces in muscles
what are the regions that appear on a sarcomere?
- A band
- I band
- H zone
- Z line
- M line
what do the regions of the sarcomere represent?
- Z line = anchor thin filaments (actin) and define the borders of sarcomeres at either end
- M lines = connect thick filaments (actin) together and runs down the middle of the sarcomere
- I band = light band with only thin filaments (actin)
- A band = dark band represents the length ofthick filaments (myosin), and where it overlaps with thin (actin) filaments
- H zone = centre of the sarcomere band where only thick filaments (myosin) are present
what is the structure of a thin filament (actin)?
- basic component of thin filaments are actin monomers called G-actin which each have an active myosin binding site
- G-actins link together end-to-end to form F-actin (filamentous actin)
- F actin is composed of 2 rows of 300-400 G-actin molecules to form a twisted double helix arrangement
- 2 regulatory proteins (tropomyosin and troponin) are found in thin filaments
what proteins are found in thin filaments? what do they do?
–> tropomyosin and troponin are regulatory proteins that enable muscles to start/stop contracting
- tropomyosin is a long fibrous muscle that extends over numerous actin monomers (G-actin) and blocks myosin-binding sites (prevents binding from occuring)
- troponin is a complex of 3 proteins:
(1) attaches to actin
(2) attaches to tropomyosin protein
(3) binds to Ca2+ reversibly - Ca2+ binding triggers skeletal muscle contraction
- occurs when Ca2+ binds to troponin, and a conformational change occurs. This influences the position of tropomyosin and as a result, tropomyosin shifts its position. This exposes the myosin-binding sites on the actin filaments and they can interact
what is the structure of a thick filament (myosin)?
- thick filament is made of 100’s of myosin molecules (look like golf clubs)
- a myosin molecule is composed of 2 intertwined subunits, and each have a long tail and fat head
- has an additional protein called titin
- myosin has 2 binding sites located on the head
–> the head is referred to as the “crossbridge” because they sometimes bridge the gap between thick and thin filaments
–> the middle of the filament lacks heads (crossbridges), it is called the bare area
what is function of the 2 binding sites on thick filaments?
(1) actin binding site
- capable of binding to the actin monomers in the thin filament (forming cross-bridges)
(2) nucleotide binding site for ATP and ATPase
- ATP can attach
- ATPase activity means it can hydrolyze ATP into ADP and inorganic phosphate (Pi).
- this provides the energy required for the myosin to undergo a conformational change, allowing it to bind to actin
what proteins are found in thick filaments? what do they do?
- titin is found in thick filaments
- it’s a very elastic protein that provides elastciity to the sarcomere
- that extends along the thick filament from M line to Z line
- works to anchor thick filaments in their proper position relative to thin filaments
- helps return muscles back to original state after being subjected to a stretching force
how does titin contribute to the response of muscles when subjected to stretching forces?
- when a muscle is stretched, the titin strands elongates as the sarcomere lengthens.
- eventually, the strands acts like a spring and resists further stretching.
- when the stretching force is removed, titin pulls the muscle fibers back to their original length by bringing Z lines and thick filaments closer together and causes the sarcomeres to shorten.
- this helps the muscle return to its normal state after being stretched
what is the sliding filament model?
- the original theory was that when a muscle contracts, the thin and thick protein filaments shorten
- the new theory is that when contraction occurs, thick and thin filament slide past each other and overlap, and do not actually change in length
how does the sliding filament model work?
when a muscle contracts, the sarcomere within a myofibril undergoes the following changes:
- the A band stays the same length (length of myosin)
- the I band shortens (where actin overlaps myosin)
- the H zone shortens (area of only myosin)
- overall, the entire sarcomere shortens BUT the filaments themselves don’t shrink
–> sliding of thin filaments toward the M line
how do the filaments slide during contraction?
- the sliding of filaments and shortening of a sarcomere is due to the cyclical formation and breaking of cross bridges (links between actin and myosin)
- is powered by ATP hydrolysis (into ADP and Pi)
- the cross bridge cycle!
what are the 5 steps to the cross-bridge cycle?
1) binding of myosin to actin
- myosin begins in its high energy form = ADP + Pi is bound to the ATPase site of myosin head
- the ATP has been hydrolyzed already
- myosin has high affinity for actin
- the myosin head binds to the actin monomer in the adjacent thin filament
- only occur in presence of Ca2+
2) power stroke
- the binding of myosin to actin triggers the release of Pi and ADP from the ATPase site on myosin
- at this time, the myosin head pivots toward the middle of the sarcomere and pulls the thin filament along with it
3) rigor
- after the powerstroke ends, and ADP and Pi are gone, myosin goes back to a low energy state
- myosin and actin are tightly bound together (called rigor) and are stuck.
4) unbinding of actin and myosin
- a new ATP enters the ATPase site on myosin head and causes a conformational change of the myosin head
- this causes affinity for actin to decrease
- therefore, myosin detaches from the actin binding site
5) cocking of myosin head
- soon after a new ATP binds to myosin ATPase, it is split via hydrolysis into ADP and Pi
- this process releases energy
- some of it is captured by the myosin molecule and goes into it’s high energy state
- if calcium is present, the cycle will continue
how does the crossbridge cycle relate to the sliding filament theory?
–> the crossbridge cycle is a series of events that underlies the sliding filament theory
- in the resting state, myosin heads are in a low-energy configuration with ATP bound.
- using energy via ATP hydrolysis (ADP + Pi), the myosin heads repeatedly bind and form cross bridges with actin
- this initiates a power stroke movement which slides the actin (thin) filaments past the myosin (thick) filaments toward the center of a sarcomere
- these cross bridges repeatedly break and reform which cause myosin filaments repeatedly attach to and pull on thin actin filaments
- this causes the sarcomere to shorten and cause a muscle contraction.
how long could the cross-bridge cycle continue for?
- could continue indefinitely as long as their is enough ATP and Ca2+ available to bind troponin
- however, to stop it, regulatory proteins (troponin and tropomyosin) interact with calcium to control the availability of myosin-binding sites on actin
what is excitation-contraction coupling?
the process by which the action potential of the motor neuron leads to the synchronous contraction of the myofibrils
what are the steps to excitation-contraction coupling
1) the motor neuron synapses with the muscle cell at the neuromuscular junction
2) the motor neuron transmits an action potential and secretes neurotransmitter acetylcholine from the axon terminal
3) ACh diffuses to the sarcolemma of the muscle cell and binds to the nicotinic receptors on the motor end plate. the binding elicits an end plate potential (EPP) which triggers an action potential in the muscle fiber cell
2) the action potential propagates along the sarcolemma and down the t-tubules
3) DHP receptors in the t-tubule membrane undergo a conformational change, which transmits a signal to ryanodine receptors in sarcoplasmic reticulum to open Ca2+ channels in the lateral cisternae sacs.
4) Ca2+ leaves the sarcoplasmic reticulum and enter the cytosol, which increases cytosolic concentration of Ca2+
5) the Ca2+ binds to troponin causing a conformational change and shifts tropomyosin out of normal resting position, which exposes the myosin-binding sites on actin.
6) the myosin heads can now bind to actin, which allows the cross bridge cycle to begin and sarcomere (muscle fiber) to contract.
7) Ca2+ is actively transported via ATPase into the lumen of SR following the action potential
