Chapter 12 Flashcards
Define the term “motor unit” as it is used in the somatic motor system.
A somatic motor neuron plus all skeletal muscles it innervates.
What is the sarcoplasmic reticulum (SR)?
A form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace.
Describe the functions of sarcoplasmic reticulum. For each function name the protein responsible for the function and describe what it does and how it does what it does (mechanism)
Stores and releases Ca+2
Pumps in Ca+2 through the Ca+2 pump (1 ATP pumps 1 Ca+2)
Releases Ca+2 through the Ca+2 release channel which is physically/chemically connected to a voltage-sensitive Ca+2 channel that opens when brought to threshold by an AP.
What is a “t-tubule”? Describe the function of t-tubules, and explain why t-tubules are important.
T-tubules (transverse tubules) are invaginations of the plasmic membrane. They allow action potentials to move rapidly from the cell surface into the interior of the fiber so that they reach the terminal cisternae nearly simultaneously.
Large powerful skeletal muscle cells store large amounts of what (visible microscopically) used to power ATP synthesis?
Glycogen
The regulatory set of proteins in the sarcomere are what? They bind which what ion which permits sarcomere shortening to occur.
tropomin and tropomyosin
bind to Ca+2
Outline the basic steps in a crossbridge cycle. Be sure to explain where ATP is used and what it is used for.
- Somatic motor neuron releases ACh at neuromuscular junction.
- Net entry of Na+ through ACh receptor-channel initiates a muscle action potential
- AP in t-tubule activates a voltage sensitive Ca+2 channel
- Voltage sensitive Ca+2 channel opens the Ca+2 release channel in the sarcoplasmic reticulum releasing Ca+2 in to the cytoplasm
- Ca+2 binds to troponin, allowing actin-myosin binding.
- Myosin heads execute power stroke (Cost: 1 ATP to detach and recock)
- Actin filament slides toward center of sarcomere.
- Sarcoplasmic Ca+2 pump pumps Ca+2 back into the SR. (Cost: 1 ATP/1 Ca+2)
- Decrease in free cytosolic Ca+2 causes Ca+2 to unbind from troponin.
- Tropomyosin re-covers binding site. (When myosin heads release, elastic elements pull filaments back to their relaxed position.)
The neurotransmitter released at a neuromuscular junction is what?
ACh (Acetylcholine)
Describe the fate of the neurotransmitter that is released at a neuromuscular junction. What would be the result if this did not occur?
The enzyme acetylcholinesterase removes the ACh from the junction. Without it the ACh would continue to activate the ACh receptors as long as the ACh persists in the junction.
Name and describe the activity of the receptor present on skeletal muscle cells at the neuromuscular junction.
ACh binds to nicotinic ACh receptors in the neuromuscular junction opening a channel and allowing Na+ to enter the cell and depolarize the motor end plate.
Explain how an action potential can initiate cross bridge cycling in sarcomeres. Be sure to name the proteins involved and their activities.
- The AP depolarizes membrane in axon terminal
- the Voltage gated Ca+2 channels open.
- Ca+2 diffuses in and initiates exocytosis
- Synaptic vesicles release ACh in to the cleft and diffuses.
- ACh binds to nicotinic ACh receptors, opens the channel, and allows Na+ to enter the muscle cell and depolarize the motor end plate.
- Voltage gated Na+ channels are brought to threshold and the AP is created that begins cross bridge cycling.
Define “latent period.” Why does it exist? What is happening during the latent period?
The delay between the muscle action potential and the beginning of muscle tension development.
It is the time required for calcium release and binding to troponin.
Outline the primary ways that skeletal muscle cell has to regenerate its ATP pool.
Glycolysis:
Advantages - 1. Anaerobic, 2. Fast,
Disadvantages - 1. Yields only 2 ATP per glucose, 2. Requires glucose which is limited.
Kreb Cycle & Oxidation phosphorilation:
Advantages - 1. Requires fatty acids which are in unlimited supply, 2. yields 30 ATP per glucose
Disadvantages - 1. Requires oxygen, 2. Slow
Phosphocreatine:
Advantages - 1. really fast, 2. three times more creatin phosphate than ATP, 3. anaerobic
Disadvantages - 1. produces CO2
What are the two different types of muscle fibers/cells we are considering?
Fast twitch fibers/cells
Slow twitch fibers/cells
What explains the differences in power generated (force) by the two types of muscle fibers/cells (fast twitch/slow twitch)?
Fast twitch are larger in diameter so it contains lots of myofibrils and uses fast myosin which has more crossbridge cycles per second.
Slow twitch are smaller in diameter and use slow myosin which has fewer crossbridge cycles per second.
