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Flashcards in 3-Muscle Physiology Deck (14):
1

What are the types of muscles?

There are three types of muscles:
Skeletal Muscle which are striated, meaning striped with a banded appearance and are voluntary.
Cardiac Muscle which are also striated and involuntary.
Smooth Muscle have Smooth appearance.

2

Describe the skeletal muscle structure.

Bone - Tendon - Muscle body - fascicle - muscle fiber (cell) -myofibril - protein filaments. Muscle fibers contain a nucleus and the sarcolemma. Sarcolemma contain the myofibril, which is made up of protein filaments. Muscle body is made up of fascicles, which are made up of several muscle fibers, which are made up of myofibrils, composed of protein filaments. One muscle fiber is one cell. One muscle fiber has many nuclei – they are multinucleate. Muscle fibers contain myofibrils – rod-like elements that contain contractile machinery. Myofibrils are where the contraction actually occur. Each comprises of overlapping thick and thin filaments made up of the protiens myosin and actin. Each is surrounded by a sac-like membranous network – the Sarcolema Reticulum. Closely associated with T-Tubules.

3

Describe the muscle fiber structure.

Within myofibrils are sarcomeres. Anything that you can voluntarily move is skeletal muscle. Reason we can do this is because our brains send APs down motor neurons, and muscle fibers tell muscles to contract.

4

What is a Sarcoplasmic Reticulum.

Sarcoplasmic Reticulum [SR] – sac-like membranous network that surrounds each of the myofibrils and is closely related to T-Tubules. Has enlargements called lateral sacs or terminal cisternae that store calcium. Function – to store calcium ions, which are released in response to electrical signals that travel from the sarcolemma to the T-tubules, they serve as chemical messengers that carry these signals to the myofibrils to initiate contraction.

5

What is a T-Tubule?

T-Tubules are continuous with the sarcolemma and penetrate into the interior of the cell. APs need to make their way to each myofibril to tell it to contract. Without them, APs stay on the membrane. They make sure all myofibrils receive an AP. Each is associated with two lateral sacs, forming a triad. SR and T-Tubules play an important role in activation of muscle contraction. They help transmit signals from the sarcolemma to the myofibrils, enabling a muscle cell to respond to neural input.

6

What are neuromuscular junctions?

Neuromuscular Junctions are synapse between motor neuron and skeletal muscles. The motor neuron [Presynaptic cell] transmits an action potential and secrets the neurotransmitter Acetylcholine upon its arrival at the axon terminal. Although a motor neuron typically branches and innervates more than one muscle cell, each muscle fiber receives input from only one motor neuron. After release, the ACh diffuses into the plasma membrane of the muscle cell [Postsynaptic cell], where it binds to specific receptors, triggering a change in ion permeability that results in depolarization. Any AP in the motor neuron triggers the release of ACh from each of its many terminal buttons, which causes many ACh receptors to become activated. As a consequence, the resulting depolarization – in the end plate potential, is much larger than an ordinary
postsynaptic potential, so large that it is always above threshold and triggers an AP in the muscle cell. If an AP occurs in a motor neuron, it is always followed by an AP in the muscle cell that it innervates. Once an AP is initiated in a muscle, it propagates through the entire sarcolemma and down the T-Tubules, and as it does this, it triggers the release of calcium from the lateral sacs of the SR. Opens channels and allows Na to come in and K to leave. The enzyme, acetylcholinesterase breaks down ACh to avoid muscle being constantly contracted – gets reuptaken.

7

What are sarcomere components?

Sarcomeres components are individual units in myofibrils. Whenever a muscle contracts, the overall muscle gets smaller and shorter. Sarcomere is the area between the Z lines [anchors the thin filaments at one end]. Remember – Z is at the end of the alphabet, anything between the z lines is what contracts. There are proteins inside the sarcomere. Thin filaments – made up of two strands of F actin that form double helix. Thick filaments – made up of myosin dimers bound together at tails, binding sites on heads [cross bridges] for actin, The ATPase site. Titin the spring at the end, makes muscle elastic and helps prevent them for tearing. Tropomyosin, extends along the thin filaments, masks the myosin binding site in the absence of calcium. Troponin is the calcium binding site. What essentially happens is the thick filaments, they attach to actin, and with a rowing motion, pull action towards the sarcomere and thin and thick filaments slide on top of each other. Actin and myosin do not ever get any shorter – just pull on top of each other [overlap]. Sliding filament theory – trying to get z lines closest to each other. Muscles contract because the thick and thin filaments of the myofibrils slide past each other.

8

What is myosin?

Myosin is the thick filament. Its the head -- where actual interaction with actin takes place. Actin binding site is where it makes contact with thin filament. The ATPase site -- all muscle contraction requires ATP (energy), no energy -- cannot make any muscle contraction (none of them). All muscle contraction needs two things: ATP for energy and Calcium.

9

What is Actin?

Actin is the thin microfilament. Up close it is spherical. Made of tiny spheres. Site where actin is supposed to bind with myosin – can only bind in that one site, lock and key. Two more proteins associated with actin. Troponin and tropomyosin which are regulatory proteins. These proteins determine if that muscle can contract at all, they regulate muscle contraction. Skeletal muscle doesn’t contract until these two tell it to. Tropomyosin-- pink rope, in relax muscle, it covers every one of the binding sites for myosin on actin. Cross bridges cannot make contact while this is covered because no connection and can’t move closer. When we don’t want a contraction its covers the sites. Troponin-- moves tropomyosin around, controls tropomyosin its a Calcium sensitive protein. So when calcium levels are high in your muscle, calcium binds troponin and turns it on, and Troponin moves tropomyosin out of the way.

10

What are the steps muscle contraction excitation contraction coupling.

1. Motor Neuron AP signals from brain saying motor neuron move my leg
2. End plate potential (Excitation of muscle cell)-Depolarization and Calcium levels in the cell increase when you produce end plate potential
3. Increase in muscle cell calcium levels. Binds to troponin, turning it on, and troponin moves tropomyosin out of the way allowing for active sites to be bound to.
4. Troponin and Tropomyosin conformational changes.
5. Crossbridge cycling -- sliding filaments (CONTRACTION).

11

Describe muscle contraction?

Motor neuron and AP releases ACh, Binding a ton of ACh receptors on motor end plate (spot where motor neuron binds). AP is produced in a muscle cell and it spreads everywhere. Idea is to contract all myofibrils, in order to do work or move things, all need to be contracted. Just one contracted is a twitch. Acetylcholine (ACh) is released from the axon terminal of a motor neuron and binds to receptors in the motor end plate. This binding elicits an end-plate potential, which triggers an action potential in the muscle cell. Action potential propagates along the sarcolemma and down T tubules which go deep down into the muscle fiber to contract all of the muscle fibers.

12

What is muscle cell calcium elevation.

Where does calcium come from? The AP goes down T Tubules (part of muscle cell membrane) And when it does, it gets detected on the membrane by DHP receptors (all this does is detect a change in voltage). When it gets detected, its attached to a calcium receptor on SR receptor. Sarcoplasmic reticulum (SR) is full of calcium, so whenever that AP makes its way down the T tubule, and the DHP receptors detects it, they pull open Ca channels on SR, since way more Ca on SR than in sarcoplasm. when Ca channels open, Ca comes rushing out binding to troponin, turning it on, which moves tropomyosin allowing active sites on actin to be available to myosin to connect (cross-bridge).

13

What is the purpose of the DHP receptors?

The purpose of the DHP receptor is to detect action potentials on the T tubule. DHP responds to change in voltage changing shape and pulls on Ryanodine receptors to let calcium into the cell from the SR which stores Ca.

14

Describe the Crossbridge cycling?

Crossbridge cycle: thick and thin filaments go through this cycle when muscle contracts.
Step 1: formation of the crossbridge: where myosin binds to actin. Making physical contact with actin.
ATP was attached and got hydrolyzed into ADP + i.
Myosin found actin and hydrolyzed ATP attached to it.
Myosin head is flattened out.
Myosin is bound to ADP, which has high affinity for actin.

Step 2: inorganic phosphate gets released. Provides energy for power stroke.
Myosin head binds to actin and rows.
Thick filaments grabs thin filament and pulls toward middle of sarcomere. This is power stroke. Energy for this comes from inorganic phosphate that gets released

Step 3: ADP gets released.
Myosin is in low-energy form. Right now myosin head is bound to active molecule and has no energy attached (rigor). The muscles contract at this point, if a new ATP molecule does not bind then this myosin head with stay attached forever. – this is why dead bodies are so still [rigor mortis]
Very high affinity for actin. Needs more energy, ADP, for actin to be released and muscle head to relax

Step 4: ATP binding lowers myosin’s affinity for actin.
o It means it is not attracted to actin and does not want to bind to actin. When ATP binds, lowers actin’s affinity to myosin and allows muscle to relax. o Binding of ATP to myosin head: lowers affinity to actin.

Step 5: ATP gets hydrolyzed, broken in half, unbinds myosin and actin. It gets split to ADP and inorganic phosphate. It increases myosin’s affinity for actin. Also really important, it cocks the myosin head, flattens it out. Now can bind actin molecule.