Synpatic Transmission And EC Coupling Flashcards
What is synaptic transmission?
Site where electrical signal from one excitable cell is communicated to another
Can be electrical or chemical
Describe electrical synaptic transmission
Uncommon in mammals
Best example is between cardiac myocytes using gap junctions
Describe chemical synaptic transmission
Utilizes a NT diffusing across the synaptic cleft and binding to a receptor
Can be excitatory or inhibitory
Presynaptic cell can signal one or many post synaptic cells
Post synaptic cell can receive signals from one or many presynaptic cells
What chemical is used at the neuromuscular junction?
ACh
A presynaptic axon at the neuromuscular junction can innervate how many cells?
As few as 1 or many muscle cells
A post synaptic (muscle) cell at the neuromuscular junction can receive signals from how many cells?
Only one presynaptic axon
At the neuromuscular synapse an AP travels from what to what?
From spinal cord via myelinated motor neuron axon to synapse on one or more muscle fibers
One synapse per fiber
What is the specialized synaptic region of the neuromuscular synaptic transmission?
Motor end plate
After a synapse occurs what happens next in the neuromuscular synaptic transmission?
ACh is released in response to Ca entry
Binds to ACh (nicotinic) receptors on muscle membrane
Opens ion gates initiating an AP
Summarize excitation/contraction
AP travels to motor end plate causing Ca channels to open
Entry of Ca causes ACh vesicles to release contents into synaptic clefts
ACh causes opening of Na channels
Entry of Na causes depolarization and generation of an AP
In skeletal muscle the AP causes DHP receptor to physically alter the state of what?
Underlying ryanodine receptors (RyRs) causing the Ca channels to open
What do ryanodine receptors on skeletal muscle cause?
Ca channels of SR open and Ca diffuses to troponin component of actin
Ca/troponin complex causes tropomyosin to pull of of binding sites of actin
What is the thick/myosin filament comprised of?
A bundle of myosin threads
Two protein strings wind together to form the myosin molecule
Describe the structure of a myosin filament
Each myosin molecule has a heavy chain comprised of a protein chain which coils with its companion and ends in a bulbous hinged appendage
Bulbous end has two light chain molecules associated with it to complete the myosin molecule structure
Bulbous end of heavy chain + light chain form the head of myosin
What are thin filaments comprised of?
Four chains of two types of molecules
Two F actin chains comprised of G-actin molecules are wound together with two tropomyosin chains
When troponin binds with Ca what does this cause?
The tropomyosin to expose the binding sites of actin
Describe the association between actin and myosin in muscle
Actin molecules are attached at their ends and project from both directions towards each other
Myosin molecules are distributed b/w the actin
Myosin heads form potential crossbridges b/w actin and mysoin
Each mysoin is associated with how many actin molecules?
6 actin molecules
What are some binding proteins associated with a sarcomere?
Nebulin, titan and dystrophin
What is nebulin?
A long molecule which assists with aligning actin
What is titan?
A large, long and elastic protein associated with myosin and anchors it within the sarcomere
It also acts to return a stretched sarcomere to its resting length
What is dystrophin?
A large protein that connects the muscle cell cytoskeleton including actin to the ECM
Myosin will remain bound to actin until what occurs?**
Until ADP is released and ATP is bound to myosin
What is peripheral fatigue?
Contraction requires the availability of ATP
Stored sources mainly as phosphocreatine
Derived from glycolysis thus producing lectin acid derived from ox phos in mitochondria (requires constant supply of O2)
What initiates the power stroke?
Release of Pi from the myosin head causing myosin to pull actin
What is central fatigue?
The perception of fatigue or loss of motivation possibly brought on by increasing levels of byproducts of contraction
Ex. Hydrogen ion and lactic acid
What are the three types of skeletal muscle fibers?
Slow twitch oxidative, fast twitch glycolytic and oxidative
What are slow twitch oxidative fibers?
Red due to myoglobin and mitochondria
Small diameter
Uses O2
Fatigue resistance
What are fast twitch glycolytic fibers?
White
Large diameter
Use phosphocreatine and glucose
What are fast twitch oxidative fibers?
Characteristics of both slow oxidative and fast glycolytic fibers
What is the force of contraction dependent on?
Starting length of the sarcomere
How rapidly the fiber is stimulated by a nerve
When can maximum force be generated?
Near middle of sarcomere length extremes
Most efficient relationship between actin binding sites and myosin heads
Distance b/w actin and myosin is less
If a sarcomere is very short what occurs?
The myosin cannot pull the actin very far before it hits the Z line
The cross sectional area is also increased thus pushing the actin and myosin apart
If the sarcomere is very long what occurs?
Many myosin heads will have been pulled away from the proximity of the actin molecule — bigger H zone
If a second twitch is initiated before complete relaxation of muscle, the second twitch reaches what?
A greater force
The greater the frequency of stimulation, the greater the what?
Potential force
What is tetanus?
Overlap of twitches such that complete relaxation is not achieved between them
A neuron that innervates more fibers (motor unit) will generate what?
A greater force than one that innervates fewer fibers
A muscle with more glycolytic fibers will generate what?
A greater force but fatigue quicker
Activating more motor neurons and thus their motor units will generate what?
A greater force (recruitment)
Contractile force also depends on what?
Innervation and muscle types
Describe excitation contraction coupling
Myosin which has attached to it an ADP and an Pi binds to the actin site (come from a previously split ATP)
The Pi is released causing the myosin head to pull the actin past it
The remaining ADP is released and replaced with an ATP causing the myosin head to unbind from the actin
ATP is hydrolyzed to ADP + Pi causing the myosin head to re-cock
Cycle continues until Ca is re-sequestered into the SR
