Excitation Contraction Coupling Flashcards
How does the NEURONAL AP travel down the motor neuron?
In a saltatory fashion
-skips over nodes of ranvier
What occurs once the neuronal AP reaches the bouton?
It depolarizes the bouton
- Voltage gated ca2+ channels in the plasma membrane open
- ca2+ rushes into the bouton
What is caused by an elevated level of ca2+ in the bouton?
Vesicles of ACh fuse with the membrane
After ACh is dumped into the synaptic terminal, what channels are able to open?
ACh binds to receptors on MUSCLE –motor end plate
-allows Na+ to rush in and depolarize
The rushing in of Na+ ions at the motor endplate creates what?
- an Endplate action potential
- propagates along the muscle fiber
What is the result of the endplate potential?
-nearby voltage gated Na+ in MUSCLE membrane open leading to a MUSCLE AP
What inactivates ACh in the synaptic cleft?
Acetylcholinesterase on the motor end plate
What is myasthenia gravis?
Antibodies created to nicotinic ACh receptors at the neuromuscular junction
- muscle fatigability
- reduced excitation
What are some possible sites for blocking neuromuscular transmission in skeletal muscle?
Nerves 1. Motor neuron disease 2. Demyelinating disease Muscle 1. Neuromuscular junction disease 2. Muscle disease 3. Sarcomere disease
What is ALS?
- Motor neuron death in the spinal cord
- loss of neuronal action potential
- reduced excitation
What is guillan barre syndrome?
autoimmune response against myelin
- impaired neuronal AP
- reduced excitation
What is muscular dystrophy?
- reduced attachment of muscle to ensheathing membrane
- reduced contraction
What is malignant hyperthermia?
- Mutation in Ryr1 - excessive ca2+ release in muscle
- triggered by inhaled anesthetics
- excessive contraction
What occurs as the muscle action potential travels down the T-tubules?
- opens ca2+ channels -ryanodine receptors in SR
- Ca2+ floods sarcoplasm
Can skeletal muscle contract in the absence of calcium extracellularly?
YES - calcium comes from the SR in muscle contraction (cycling is intracellular)
What is calcium’s role in the sarcoplasm?
It allows actin to interact with myosin cross-bridges – development of tension
What is the skeletal muscle triad?
1-Tubule surrounded by 2 sarcoplasmic reticulum cisternae
How are the tubules and cisternae attached?
- Direct coupling
- L type ca2+ channels in T tubule
- Ryanodine receptors in cisternae of SR
What are the three steps of ca2+ release at the skeletal muscle triad?
- Membrane depolarization opens L-type ca2+ channel or DHP receptors (t-tub)
- Coupling causes ca2+ release channel in SR to open (ryanodine receptors)
- Ca2+ exits SR and activates troponin C → muscle contraction
What is calcium induced calcium release?
In addition to being stimulated by DHP receptors, ryanodine receptors are also triggered by the presence of ca2+ in the cytoplasm (released by other ryanodine receptors)
What is the major mechanism of skeletal muscle relaxation?
SERCA pumps
-hydrolyze ATP to pump ca2+ into SR
What are the two minor methods of skeletal muscle relaxation?
- PMCA - pumps 1 ca2+ out of cell in exchange for 1 ATP
- NCX - Lets 3 Na2+ into cell to pump 1 ca2+ out of the cell
What is the temporal relationship between AP, ca2+ release and tension in skeletal muscle?
- NA+ dependent AP is <5 msec - rapid
- triggers transient inc in ca2+
- leads to slower development of tension
What is the result of a single action potential?
-twitch
What is the result of high frequency APs with oscillation?
-Unfused tetanus
What is the result of high frequency APs without oscillation?
-Fused tetanus
What is the term for an increase in muscle tension from successive action potentials?
Summation
What is the term for mantained contraction in response to repetitive stimulation?
Tetanus
What two things control the strength of contraction in skeletal muscle?
1) rate coding - frequency of stimulation
2) recruitment of additional motor units
What is treppe?
The steady increase in tension in successive twitches
Is treppe the same thing as summation?
NO - each twitch relaxes to zero tension before the next twitch initiates
How does treppe occur?
- repeated stimulation of the muscle at low frequency
- ca2+ release from previous twitches exceeds ca2+ uptake
- Increases the number of crossbridges formed
- May warm up the muscle - inc enzymatic rate
What is a motor unit?
A somatic motor neuron and all the myofibers it innervates
Why is tetanus able to occur?
the functional refractory period of the AP in a twitch is much shorter than contraction time
How does recruitment occur?
Large motor units are recruited after small
-as more force is required
What is involved in fine motor control?
Small motor units
What is the energy source of slow twitch fibers?
Oxidative
What is the energy source of Fast twitch type IIa fibers?
Oxidative
What is the energy source of fast twitch type IIb fibers?
Glycolytic (anaerobic metabolism of glucose)
List in order the energy sources for contraction (5)
- ATP
- Phosphocreatine
- Anaerobic metabolism of glucose
- Oxidative metabolism of glucose
- Oxidative metabolism of fats, carbs, proteins
What is muscle pain or burn?
Lactic acid build up
-anaerobic glycolysis (Fast twitch IIb)
What is Delayed onset muscle soreness?
- not correlated with lactic acid
- muscle damage, inflammatory response
What is Muscle Fatigue
-decline in muscle tension as result of previous contractile activity
- dec shortening velocity
- slower rate or relaxation
What causes muscle fatigue? Is there a correlation with ATP?
build up of lactic acid and inorganic phosphates
-NOT due to low ATP (has high levels)
What is high frequency fatigue?
- high intensity, short duration exercises
- failure of AP to conduct down T-tubule
- fast recovery
What is low frequency fatigue?
- Low intensity, long duration exercises
- build up of lactic acid and phosphates
- changes the conformation of muscle proteins
- slow recovery
