Lesson 7: Skeletal Muscle Part I
The myosin-actin crossbridge cycle consists of 4 main steps in which ATP is altered, resulting in the movement of a myosin head in relation to an actin filament. State what the myosin head does in response to each of the following: (1) ATP binds to the myosin head. (2) ATP is hydrolyzed, forming ADP and Pi. (3) ADP and Pi dissociate from the myosin head. Bonus: Be sure to use the terms "cocked" and "powerstroke"
(1) ATP binds to the myosin head. - The myosin head dissociates from the actin filament. (2) ATP is hydrolyzed, forming ADP and Pi. - The myosin head cocks forward into its higher energy "cocked" conformation, binding to actin one rung higher than before. (3) ADP and Pi dissociate from the myosin head. The myosin head does its powerstroke, pulling the actin filament. The myosin head remains attached to the actin filament as it waits for another molecule of ATP to bind and restart the cycle. Note: This likely feels like a lot to memorize, but the AAMC will test you on it!
Myocytes have special terminology for their cellular parts. State what each of the following terms refer to: (1) Sarcolemma (2) Sarcoplasm (3) Sarcoplasmic Reticulum
(1) Sarcolemma - The cell membrane of the myocyte. (2) Sarcoplasm - The cytoplasm of the myocyte. (3) Sarcoplasmic Reticulum - the specialized endoplasmic reticulum of the myocyte.
*CRB* The neuromuscular junction (NMJ) consists of which of the following? I. Motor Neuron II. Motor End Plate III. T-tubules (A) I and II Only (B) I and III Only (C) II and III Only (D) I, II, and III
(A) I and II Only The neuromuscular junction (NMJ) consists of the motor neuron and the motor end plate.
*CRB* Which of the following important proteins in muscles have ATPase activity? I. Actin II. Myosin III. Titin (A) II only (B) I and II only (C) I and III only (D) I, II and III
(A) II only Only Myosin has ATPase activity.
Which region correctly describes a sarcomere? (A) Region between two Z-lines (B) Region between two H zones (C) Region between two A bands (D) None of the above
(A) Region between two Z-lines The sarcomere is the region of the myofibril between two Z lines.
*CRB* In the Troponin-complex, which of the following components will bind the calcium? (A) Troponin-C (B) Troponin-I (C) Troponin-D (D) Troponin-T
(A) Troponin-C Troponin-C will bind calcium. Think of that C standing for calcium-binding! Troponin-I and Troponin-T are the two other components of the Troponin Complex
Damage to which of the following neuron types can lead to spastic contractions? (A) Upper Motor Neurons (B) Lower Motor Neurons (C) Both Upper and Lower Motor Neurons (D) None of the above
(A) Upper Motor Neurons Upper Motor Neurons are responsible for telling the Lower Motor Neurons when to stop contracting. For this reason, if an Upper Motor Neuron is damaged, it won't be able to tell the muscle to stop contracting, resulting in one contraction after another.
The role of titin is to anchor _____________ to the ____________. (A) myosin, Z-line (B) myosin, M-line (C) actin, Z-line (D) actin, M-line
(A) myosin, Z-line The role of titin is to anchor myosin to the Z-line.
Skeletal muscle's most basic unit of contraction is known as the: (A) Myosin-actin Crossbridge (B) Sarcomere (C) Myofibril (D) Striation
(B) Sarcomere Skeletal muscle's most basic unit of contraction is known as the sarcomere.
*CRB* Which of the following is the proper name for the theory where actin slides over the myosin for contraction to occur? (A) Sarcomere Theory (B) Sliding Filament Theory (C) I-band Theory (D) None of the Above
(B) Sliding Filament Theory Sliding Filament theory describes how actin slides over the myosin for contraction to occur.
Put the following in order from largest to smallest: I. Fascicle II. Myofiber III. Myofibril IV. Myofilaments V. Muscle (A) V > I > III > IV > II (B) V > I > II > III > IV (C) I > V > II > III > IV (D) I > V > III > IV > II
(B) V > I > II > III > IV The following is in order from largest to smallest: V. Muscle I. Fascicle II. Myofiber (aka muscle fiber, muscle cell, or myocyte) III. Myofibril IV. Myofilaments
*CRB* Which of these steps of myosin-actin crossbridge cycle could not occur if there was no actin? (A) ATP binds to the myosin head. (B) ATP is hydrolyzed, forming ADP and Pi. (C) ADP and Pi dissociate from the myosin head. (D) None of the above would fail to occur
(C) ADP and Pi dissociate from the myosin head. This is the step that cannot occur if there is no actin, since the ADP dissociates due to conformational changes during the power-stroke that occurs when the myosin head is bound to actin.
*CRB* Which of the following statements does NOT actively describe what occurs at the Neuromuscular Junction? (A) The postsynaptic membrane contains Acetylcholine Receptors, and the motor neuron releases Acetylcholine. (B) Binidng of ACh to its receptor leads to inward sodium currents. (C) Acetylcholinesterase will hydrolyze Acetylcholine into a choline group and two acetyl groups. (D) The depolarization in the muscle is called an End Plate Potential.
(C) Acetylcholinesterase will hydrolyze Acetylcholine into a choline group and two acetyl groups. Acetylcholine is hydrolyzed to form 1 choline group and 1 acetyl group.
Damage to which of the following neuron types can lead to muscle weakness? (A) Upper Motor Neurons (B) Lower Motor Neurons (C) Both Upper and Lower Motor Neurons (D) None of the above
(C) Both Upper and Lower Motor Neurons Damage to either the Upper or Lower Motor Neurons can lead to muscle weakness.
Put the following in order from outermost layer to innermost layer: I. Perimysium II. Epimysium III. Endomysium (A) I > II > III (B) III > I > II (C) II > I > III (D) III > II > I
(C) II > I > III The following are in order from outermost layer to innermost layer: II. Epimysium I. Perimysium III. Endomysium
Muscles are attached to bones via: (A) ligaments (B) fascia (C) tendons (D) cartilage
(C) tendons Muscles are attached to bones via tendons.
*CRB* Not only are there End Plate Potentials, but there are also Miniature End Plate Potentials (MEPPs). Which of the following statements about these are true? I. MEPPs are the smallest possible depolarization at the Neuromuscular Junction. II. MEPPs are caused by the exocytosis of a single vesicle of ACh. III. MEPPs can happen spontaneously (without depolarization of the motor neuron). (A) I and II only (B) I and III only (C) II and III only (D) I, II and III
(D) I, II and III Each of the following statements about MEPPs are true: I. MEPPs are the smallest possible depolarization at the Neuromuscular Junction. II. MEPPs are caused by the exocytosis of a single vesicle of ACh. III. MEPPs can happen spontaneously (without depolarization of the motor neuron).
Via what mechanism does Ca2+ get back into the sarcoplasmic reticulum when it is time for muscle relaxation? (A) Passive diffusion through the membrane (B) Secondary active transport (C) Passive diffusion through the ryanodine channels (D) Primary active transport
(D) Primary active transport When it is time for muscle relaxation, Ca2+ is transported back into the sarcoplasmic reticulum via a pump that utilizes ATP and is known as the Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA).
Each layer of connective tissue directly encases what part of a muscle (i.e. Muscle, Fascicle, Myofiber, Myofibril, or Myofilaments)? - Epimysium - Perimysium - Endomysium
- Epimysium directly encases the muscle. - Perimysium directly encases the fascicles. - Endomysium directly encases the myofibers.
Draw a sarcomere and label it with the following: A-band, I-band, H-zone, Z-line, M-line, Actin, Myosin
A-band - Contains both myosin and actin (hint: All). I-band - Contains only actin, the thin filament (hint: I is a thin letter). H-zone - Contains only myosin, the thick filament (hint: H is a thick letter). Z-line - Located at the end of the sarcomere (hint: Z is the last letter in the alphabet) M-line - Located in the center of the sarcomere (hint: Middle)
Actin is known as the thin or thick filament? Myosin is known as the thin or thick filament?
Actin is known as the thin filament. Myosin is known as the thick filament.
*CRB* True or false? The larger the region between two Z-lines, the stronger the possible contraction.
False. There is actually a length-tension relationship, whereas there is an optimal length for the maximal contractile force, which looks like a bell curve peaking around 2 microns.
*CRB* Increasing the sensitivity of troponin-C has what effect on the activity of the myosin-actin crossbridge cycle?
Increasing the sensitivity of troponin-C allows calcium to bind more easily to it. This results in more tropomyosin being moved out of the way, allowing more myosin to bind to actin, increasing the activity of the myosin-actin crossbridge cycle.
Describe role of sarcoplasmic reticulum
Kinda acts like a storage area. It has calcium ion pumps on membrane, that pumps calcium ions into reticulum. Resting muscle has a lot of calcium ions on inside of reticulum, but when muscle needs to contract, these calcium ions dump out into cytoplasm and then bind to troponin
Explain in detail how the following interact in such a way that will result in the actin-myosin crossbridge cycle: Axon Terminal of Motor Neuron, Ryanodine Channels, Sarcolemma, Dihydropyridine Receptor, Sodium Channels, Motor End Plate, Acetyl Choline, Sodium Ions, Calcium Ions, Sarcoplasmic Reticulum, T-tubule, and Sarcoplasm.
The axon terminal of the motor neuron will release acetyl choline in response to an action potential travelling down its axon. The acetyl choline will bind to sodium channels embedded in the sarcolemma located within the motor end plate of the skeletal muscle cell (myocyte). The sodium channels will open, allowing sodium ions to rush into the cell, creating an action potential that will then travel along the sarcolemma and down into the T-tubules. This action potential will cause the dihydropyridine receptor to change confromations, which results in the opening of the ryanodine channels embedded in the membrane of the sarcoplasmic reticulum. Calcium will then leave the sarcoplasmic reticulum through the ryanodine channels, entering the sarcoplasm (the cytoplasm of a myocyte). From there it will diffuse and bind to troponin.
What is the relationship between the connective tissue layers (epimysium, perimysium, endomysium) and the tendon?
The connective tissue layers are continuous with the tendon. They are all interconnected.
When a sarcomere contracts, what happens to the length of each of the following bands/zones? (1) A-band (2) I-band (3) H-zone
The length of the myosin cannot change, so the A-band stays the same length. As the sarcomere contracts, the actin will slide over the myosin, causing increased overlap and a shorter I-band and H-zone.
What organelle is responsible for the bumpy nature of a myofiber?
The nuclues is located in the periphery of the myofiber, creating a bump.
What is the purpose of the actin-myosin crossbridge cycle?
The purpose of the actin-myosin crossbridge cycle is to pull actin filaments closer together, resulting in muscle contraction. This is why muscles move on the most basic level!
Why doesn't the actin filament slip back into its starting position each time the myosin head detaches from the actin?
This is due to the fact that there are many myosin heads interacting with the actin filament at a given time.
Describe the relationship between Ca2+, tropomyosin, troponin, myosin, and actin.
Tropomyosin is wrapped around the actin filament, covering up the mysosin binding sites. Troponin is what holds the Tropomyosin in place on the actin, and when Ca2+ binds to troponin, it will pull tropomyosin away from the binding sites, allowing myosin to bind to the actin filament, allowing the cross-bridge cycle to begin.
Difference between upper and lower motor neuron with muscle contraction:
Upper motor neuron send signal to LMN when to start and stop contraction LMN sends signals to muscles to stop contraction
When calcium levels in the myocytes of a muscle are high, the muscle will contract or relax? When calcium levels in the myocytes of a muscle are low, the muscle will contract or relax?
When calcium levels in the myocytes of a muscle are high, the muscle will contract. When calcium levels in the myocytes of a muscle are low, the muscle will relax.
