Muscle Physiology Flashcards
EPPs differ from EPSPs in that ___________
EPPs are normally of sufficient magnitude to create an action potential leading to muscle contraction.
Whereas hormonal regulation of skeletal muscle metabolism occurs, only ______________ controls skeletal muscle contraction.
innervation by a motor neuron
All neural communication to skeletal muscle is _____
excitatory
In order for a skeletal muscle to relax what must occur?
The neural stimulation must decrease.
The toxin latroxin stimulate the release of acetylcholine at the neuromuscular junctions. What effect does this have on the body?
The respiratory muscles (which are skeletal muscles) are effected. The venom can cause respiratory failure and death by inducing spastic contractions of these muscles.
Rattlesnake venom contains crotoxin, a toxin that inhibits the release of acetylcholine. How would this affect the body.
Muscles would be unable to contract, causing paralysis of skeletal muscle.
Skeletal muscle is an effector muscle of which nervous system?
Somatic
Smoth muscle and cardiac muscle are effector organs of which nervous system?
Autonomic
Skeletal muscles are connected to at least two bones, with the following exceptions:
connected to skin (facial muscles)
connected to cartilage (muscles in larynx)
connected to other muscles (sphincter)
Muscles connect to bones by
tendons
Fascicles
bundles of individual muscle cells, as well as connective tissue, blood vessels, and nerves. Each fascicle contains nundreds to thousands of muscle cells, called muscle fibers because of their elongated shape.
How do muscle fibers differ from most cells?
They have more than one nucleus because each muscle fiber is formed during embryonic life from the fusion of several cells.
Sacrolemma
muscle fiber’s plasma membrane
Sacroplasm
a muscle fiber’s semifluid cytoplasm, which is packed with mitochondria and hudreds of myofibrils
Myofibrils
banded, rod-like elements whithin the sarcoplasm which contain the fiber’s contractile machinerly. Each myofibril is a bunddle of overlapping thick and think filaments made of the proteins myosin and actin.
Sacroplasmic reticulum
a saclike membranous network that surrounds each of the myfibrils and is closely associtated transvers tubules (T tubules).
Transverse tubules (T tubules)
structures that transmit action potentials from the sacrolemma into the cell’s interior, triggering the release of calcium from the sacroplasmic reticulum
The functions of the sacroplasmic reticulum are to________.
store calcium ions and to release them into the cytosol when the muscle cell is stimulated to contract.
Thick and thin filaments exist in a ___ ratio.
2:1
Sarcomere
fundamental unit of myofibrils that repeat over and over
A band
the dark striation observed under the microscope due to the presence of thick filaments.
H Zone
The region of the A band of a sarcomere where only thick filaments are present. No thin filaments are overlapping.
I band
The light striation of a sacromere comprises of thin filaments with no overlapping thick filaments.
Actin
most common microfilament; found in thin filaments in muscle fibers; provides structural support for microvilli
Myosin
The contractile protein found in thick filaments in striated muscle
Myosin kinase
An enzyme involved in excitation-contraction coupling in smooth muscle; phosphorylates myosin crossbridges; also know as mysoin chan kinase (MLCK)
Tropomyosin
One of the two regularoty proteins in striated muscle; a long fibrous molecule that acts to block myosin-binding sites on thin filaments when a muscle is not contracting.
Troponin
One of the 2 regulatory proteins in striated muscle. It binds calcium reversibly and is responsible for starting the crossbridge cycle by moving tropmyosin out of its blocking position
How do tropomyosin and troponin work together in regards to regulating muscle contraction?
Tropomyosin extends over actin monomers, blocking myosin-binding sites in muscles at rest. Troponin is a comlex of 3 proteins - one attaches to the actin strand, another binds to tropomyosin and a thrid containing a site to which calcium ions can bind reversibly. The binding of calcium to this site triggers muscle contraction by causing troponin to move topomysoin aside, exposing the myosin-binding sites on the actin molecules.
Crossbridges
protusion on both ends of the thick filament that bind to actin and are responsible for generating the motion that causes muscle contraction.
______________ are responsible for generating the motion that causes muscle contraction.
Crossbridges
Titin
an extremely elastic protein, located alon each thick filament from the M line to each Z line, anchoring the thick filaments in their proper positions relative to the thin filaments.
These strands act as springs. They can stretch with external force and apply opposing force to shorten the sacromere back to its original position.
Sliding-filament model
process of muscle contraction whereby thick and thin filaments slide past each other
- I bands shorten, sliding past thick filaments, moving deeper into the H zone & decreasing its width
- Adjacent a bands move closer together which decreases the width of the I bands
- Z lines at either end of a sarcomere move closer together, shortening the sacromer, myofibrils, muscle fibers and ultimately whole muscles
Name the 5 Steps of the Crossbridge Cycle
- Binding of myosin to actin
- Power stroke
- Rigor
- Unbinding of myosin and actin
- Cocking of the mysoin head
Explain the first step of the crossbridge cycle
Binding of myosin to actin. Myosin starts in energized form (ADOP & Pi are bound to the ATOase sit of the myosin head). In this state myosin has a high affininty for actin, and the myosin head binds to an actin monomer in the adjacent thin filament. This step can only occur in the presence of calcium.
Explain the second step of the crossbridge cycle
Power stroke: The binding of myosin to actin triggers the release of the Pi and ADP from the ATPase site. During this porcess, the myosin head pivots toward the middle of the sarcomere, pulling the thin filament along with it and the myosin molecule goes into its low-energy state.
Explain the 3rd step of the crossbridge cycle
Rigor: In its low-energy form, myosin and actin are tightly bound together, a condition called rigor
Explain the 4th step of the crossbridge cycle
Unbinding of myosin and actin: A new ATP enters the ATOase site on the myosin head, triggering a conformation change in the head, which decreases the myosin’s affinity for actin, so the myosin detaches from the actin.
Explain step 4 of the crossbridge cycle
Cocking of the myosin head: Soon after it binds to myosin’s ATPase site, ATP is split by hydrolysis into ADP and Pi, which releases energy. some of the enrgy is captured by the myosin molecule as it goes into its high-energy conformation. Although ATP has been hydrolyzed at this point, the end-products of the reaction (ADP, Pi) remain bound to the ATPase site. If calcium is present, the cycle will start over at step 1.
Explain rigor mortis
The stiffening of the body that occurs after death because the crossbirdge cycle gets stuck at the rigo step due to a lack of ATP. Rigor mortis will continue until enzymes leaked by disintegrating cellular components begin to break down the myofibrils.
Excitation-contraction coupling
in a muscle cell, the sequence of events that links the action potential to the contraction
What is the name of the receptors found in the membranes of the T tubules that function as voltage sensors?
Dihydropyridine receptors (DHP receptors)
Summation happens whenever muscle twitches occur at such high frequencies that calcium cannot be removed from the cytosol as rapidly as it is released. This leads to a condition known as ______. This term also refers to a condition in which bacterial toxins cause inappropriate motor neuron stimulation.
Tetanus
When larger stimulatory forces are needed, the nervous system can activate extra fibers, increasing the number of active motor units. This is known as ____________.
recruitment
Because of the existence of a correlation between the size of a motor unit and the type of fiber it contains, recruitment of fibers happen in a specific order. Which of the following fibers are recruited first?
Slow oxidative fibers
In what order are muscle fibers recruited and why?
- Slow oxidative - they are located in the smaller motor units
- Fast oxidative - they are found in intermediate motor units
- Fast glycolytic fibers - they are found in large motor units
What is the protein component of the thin filament that binds to calcium thereby initiating skeletal muscle contraction?
Troponin
The repeated, oscillating interaction between actin and myosin that results in the generation of force by a skeletal muscle cell is called ________.
Crossbridge cycling
The contractile portion of the thin filament is composed of what protein?
Actin
Contraction of skeletal muscle fibers is stimulated by ________.
motor neurons
