skeletal muscle Flashcards
muscle fiber
synonym for muscle cell
sarcolemma
the plasma membrane of a skeletal muscle fiber that encloses cytoplasm (sarcoplasm)
associated with (immediately outside of) the sarcolemma is a coat of polycaccharide material in which are embedded collagen fibrils that at the fiber’s ends, fuse with the ____ of the muscle
tendon
a muscle is composed of subunits called
fasciculi
each fasciculus is a bundle of
cells
each muscle fiber is a bundle of
myofibrils
sarcomere
the portion of a myofibril between two adjacent Z-discs
thick filament
myosin
thin filament
actin
each sarcomere is composed of several thousand myofilaments called _____ filaments and _____ filaments
actin
myosin
each myosin filament is made of many myosin molecules; each myosin molecule has a tail or body (that includes two arms) and two
heads
what three proteins make up actin filaments
F-actin
tropomyosin
troponin complex
F-actin:
the helical backbone of the actin filament; contains many “active sites” (binding sites for myosin)
tropomyosin
lies atop F-actin and covers up the active sites when contraction is not desired
troponin complex (attached intermittently along tropomyosin)
troponin T binds tropomyosin
troponin I binds actins
troponin C binds calcium ions
actin filaments attach to
both Z discs of a sarcomere and interdigitate with myosin filaments
the sarcomere is made up of a complete
A band and the 2 halves of the I bands adjacent to it
I band
all actin (thin filament) in a sarcomere
A band
all the myosin (thick filament)
titin
a protein that maintains alignment between actin and myosin filaments and also acts like a spring between a Z disc and the end of a myosin filament (may be important in some diseases)
stimulus –> CNS –>
movement
the synapse between a motor neuron and a skeletal muscle fiber is called a
neuromuscular junction (NMJ)
the tiny fluid-filled space between cells at a synapse is the
synaptic cleft
molecules of transmitter in the presynaptic axon terminal are contained within
many synaptic vesicles, the walls of which are made of membrane
if the synapse is a NMJ, the transmitter is always
acetylcholine (ACh)
acetylcholine receptors are embedded in the plasma membrane of the
postsynaptic cell
steps of traveling wave of electricity in the motor neuron leading to muscle fiber shortening
EVENTS IN THE NEURON
- action potential arrives at axon terminal
- calcium enters terminal
- acetylcholine is released from terminal
EVENTS IN THE SYNAPTIC CLEFT
4. acetylcholine diffuses across synaptic cleft, binds to receptors on sarcolemma, and is broken down by acetylcholinesterase
EVENTS IN THE MUSCLE FIBER
- cation channels open and cations enter
- fiber produces end plate potential
- fiber produces action potential
- action potential travels into interior of the fiber along transverse tubules
- calcium is released from sarcoplasmic reticulum
- troponin removes tropomyosin’s inhibition of actin-myosin binding
- myosin pulls on actin, shortening the sarcomere
synaptic transmission
the process whereby a signal is passed between neurons or other excitable cells
events in the neuron
- an action potential arrives at the axon terminal of the motor neuron
- the segment of axon terminal membrane in which the action potential is occurring is electrically positive on the inside with respect to the outside
- each synaptic vesicle contains about 5000 ACh molecules
- the arriving action potential will function to cause the release of ACh into the synaptic cleft from synaptic vesicles
- the depolarization of the axon terminal, that is the action potential, opens voltage-gated calcium channels and thus increases the permeability of the axon terminal membrane to Ca++
- calcium ions enter the axon terminal, where they allow synaptic vesicles to move to and fuse with the inside surface of the axon terminal plasma membrane; this allows the vesicle contents to be released via exocytosis
events in the synaptic cleft
- each arriving action potential causes the discharge of about 300 vesicles (300 * 5000 = 1,500,000)
- the ACh binds to ACh receptors embedded in the muscle fiber sarcolemma
- most of the ACh is quickly removed from the synaptic cleft by being broken down by the enzyme acetylcholinesterase
events in the muscle fiber
- the binding of 2 ACh molecules to a single ACh receptor opens an ion channel specific for cations
- the influx of Na+ makes the cell less negative on the inside (depolarizes the cell), thus producing a graded potential in the muscle fiber
- the graded potential is called an excitatory postsynaptic potential (EPSP), aka an end plate potential (EPP)
- the action potential travels along the sarcolemma in both directions
- when an action potential gets to a T tubule, it travels inward into the fiber in addition to moving along the sarcolemma; thus T tubules carry the electrical signal into the interior of the fiber
- T-tubule protien (dihydropyridine receptor) senses the voltage change due to the action potential and interacts with nearby sarcoplasmic reticulum protein (ryanodine receptor/calcium release channel)
- opening of the calcium release channels allows many calcium ions to move out of the sarcoplasmic reticulum and into the sarcoplasm surrounding the myofibrils
bernard katz recorded miniature end plate potential (mEPPs) from the muscle fiber in the
absence of stimulation of the motor neuron
each miniature end plate potential (mEPP) is caused by
the spontaneous discharge of 1 synaptic vesicle
virtually every end plate potential is large enough to elicit an
action potential in the sarcolemma
transverse (T) tubules
invaginations of the sarcolemma into the interior of the muscle fiber that lie at right angles to the myofibrils; they are made of membrane and filled with ECF
sarcoplasmic reticulum (SR)
membranous structure that is adjacent and mostly parallel to the myofibrils, in contact with the T tubules, and a storage depot for calcium
a T tubule protein called a dihydropyridine receptor “senses”
the voltage change due to the action potential, becomes activated, and interacts with a protein in the nearby sarcoplasmic reticulum called a ryanodine receptor/calcium release channel, which is also an ion channel specific for calcium
excitation-contraction (EC) coupling- the traid
the junction between two terminal cisternae and a T-tubule
sliding filament theory of contraction
- ADP is released from the myosin head, which clears the way for another ATP to bind
- ATP binds to the myosin head, which causes actin and myosin to dissociate; myosin head is “sprung” or “uncocked”
- ATP is hydrolyzed to ADP plus P (both remain attached to the myosin head); this causes a conformational change in myosin that “cocks” the head
- If and only if calcium ions are present, calcium binds to troponin C, causing troponin to “tug on” tropomyosin, which moves tropomyosin away from the active sites on myosoin
- the myosin head binds to actin; the head when attached to myosin is called a “cross bridge”
- the phosphate group is released from the myosin head, causing pivoting (that is “springing” or “uncocking” of the myosin head
- because myosin is attached to actin, it pulls on actin as it changes shape, and the Z disc attached to the actin myofilament is pulled along the actin
- this happens at both ends of the sarcomere, and thus the two Z discs are pulled toward each other, and the sarcomere shortens (ADP remains attached to myosin)
- myosin and actin remain attached to each other until the ADP is released from myosin and ATP binds
- many cycles of myosin head springing occur, ratcheting the “right” and “left” actin filaments closer and closer together
