Module 5: Muscles Flashcards
What are the three types of muscles?
Muscles are biological machines that utilize chemical energy from the breakdown and metabolism of food to perform useful work.
- Skeletal muscle is used primarily for voluntary motion
- Smooth muscle is found within the walls of blood vessels, airways, various ducts, urinary bladder, uterus, and the digestive tract
- Cardiac muscle is found in the heart
What are the functions of the body’s 600 different muscles?
- Movement
- Heat production
- Body support and posture
What is the structure of a muscle?
- Whole muscles are made up of bundles of fasciculi. Groups of fasciculi are surrounding by a white connective tissue called perimysium
- Each fascicle is made up of groups of muscle cells (fibers). Each fiber is a elongated cell with many nuclei
- Each muscle cell (fiber) contains many cylindrical bundles of myofibrils and is surrounded by the sarcolemma (the muscle cell membrane over which the action potential is transmitted)
- Each myofibril contains thin and thick myofilaments. The functional unit is called the sarcomere. The myofibrils are surrounded by the sarcoplasmic reticulum
- Thin myofilaments (the contractile elements of the cell) contain mostly the protein actin along with troponin and tropomyosin
- Thick myofilaments contain the protein myosin
- The interaction of thin and thick myofilaments results in muscle contraction
What is the sarcolemma?
The sarcolemma is the muscle cell membrane, over which the action potential is transmitted. Muscle cells are surrounded by the sarcolemma. The sarcolemma has small transverse (T) tubules that extend down into the cell. These T tubules conduct the action potential deep into the cell where the contractile proteins are located.
What is the sarcoplasmic reticulum?
The myofibrils are surrounded by the sarcoplasmic reticulum. The SR is a network of tubes containing Ca++ which are essential for contraction. At either end of the SR and continuous with the SR are the terminal cisternae. The terminal cisternae are a membranous enlargement of the SR, which is close to the T tubule.
What are thin myofilaments?
The thin myofilaments are composed predominantly of the globular protein actin. Each actin molecule contains a special binding site for the other contractile protein myosin. Also found on the thin myofilaments are long protein strands called tropomyosin. When the muscle is at rest, these proteins cover the binding sites on actin for myosin. A third regulatory protein is troponin, which has three subunits:
- Troponin A binds to acting
- Troponin T binds to tropomyosin
- Troponin C binds to Ca++
At rest, the troponin complex holds the tropomyosin over the myosin binding sites. When Ca++ binds to the troponin C unit, the tropomyosin is pulled off of the myosin binding sites on active by the troponin protein.
What are thick myofilaments?
The thick myofilament is made up of the protein myosin. The protein has a long, bendable tail and two heads that can each attach to the myosin binding sites on actin. The heads also have a site that can bind ATP, providing the energy for muscle contraction. Many myosin molecules are arrange to form one thick myofilament.
What is the relationship between actin and myosin?
Groups of thin myofilaments (actin) and groups of thick myofilaments (myosin) are arranged in a repeating pattern of thick, thin, thick, thin, etc. along the length of the myofibril from one end of the muscle cell to the other. Each group of thin myofilaments extends outward in opposite directions from a central Z disk (Z Line), where they are anchored. Similarly, groups of thick myofilaments extend outwards from the central M line, where they are attached. Each myofilament tis parallel to the length of the myofibril and the muscle cell. The region from one Z disk to another is called a sarcomere. This is the smallest functional contractile unit of the muscle cell.
The repeating pattern of thin and thick myofilaments gives the muscle cell a banded/striated appearance. The regions that contains the thick filaments appear as dark bands called A bands. The regions that contain only the thin filaments appear lighter and are I bands.
- A bands will contain the M line
- I bands will contain the Z disks
What is the sliding filament theory?
The interaction between actin and myosin leads to muscle contraction. When the head of a myosin molecule attaches to the binding site on actin and forms a crossbridge, the myosin undergoes a change in shape. This change in shape causes the myosin head to swing, producing the power stroke. This power stroke slides the actin past the myosin. Neither the thin nor the thick myofilaments shorten during a muscle contraction - the sarcomeres are shortening.
What is excitation-contraction coupling?
Excitation-contraction coupling refers to the process by which an action potential in the muscle cell membrane (sarcolemma) excites the muscle cell to produce a muscle contraction. The action potential that was generated near the NMJ will spread out over the sarcolemma and down the T-tubules into the core of the muscle cell. The action potential travels very close to the sarcoplasmic reticulum and will open Ca++ channels, causing the release of Ca++ from the terminal cisternae of the SR. The Ca++ will bind to troponin C on the thin myofilaments, causing tropomyosin to uncover the myosin binding sites found on the actin. Myosin will now be able to attach to the actin and a power stroke will occur.
What happens during the relaxation of muscles?
Once action potentials stop, Ca++ will no longer diffuse out of the SR. Special calcium pumps rapidly pump Ca++ back into the SR, up its concentration gradient; this requires ATP. Without the calcium present in the cytoplasm of the muscle cell, the tropomyosin will cover the myosin binding sites once again. Myosin will be unable to bind to the actin and power strokes will not occur. The muscle will relax.
The active transport of the Ca++ back into the SR can get saturated, therefore it may be the case that you cannot relax your muscles immediately after action potentials travelling to the muscles. As long as there is Ca++ floating around in the muscle cell cytoplasm, the muscle will contract.
What are the events that occur during a single crossbridge formation between actin and myosin?
- The splitting of ATP to ADP and Pi (which attach to the myosin head) releases energy to myosin and prepares the myosin head for activity. This is the activation and cocking of the myosin head stage.
- Formation of the crossbridges occur when Ca++, which have been released from the SR by an action potential, binds to troponin C. This rolls the tropomyosin off the myosin binding sites found on actin.
- The power stroke occurs when the myosin head bends and slides the thin myofilaments of actin over the thick myofilaments of myosin. The ADP and Pi molecules are shortly released from the myosin head.
- A new molecule of ATP will bind to the myosin heads and result in the breaking of the cross-bridge.
What is rigor mortis?
Rigor mortis begins 3-4 hours after death and the muscles become very stiff for about 12 hours. This stiffness slowly disappears over the next 24-48 hours. Rigor mortis results directly from the loss of ATP in the dead muscle cells, since dead cells do not produce ATP. Slow degradation of the SR releases Ca++. Myosin binds to actin, forming the crossbridge between the thin and thick myofilaments. These are rigid conditions of the muscle, since bonds cannot be broken without ATP and must wait until further cell degeneration (decomposition) to dissolve the linkage.
What are the two methods of altering the force of muscle contractions?
- Recruiting motor units
- Summation of twitch contractions
These methods can be combined to produce even larger contractions.
What is a motor unit?
A motor unit is a motor neuron and all of the muscle cells/fibers it causes to contract. In almost all situations, one motor neuron will contact several muscle cells, but each muscle cell is innervated by only one motor neuron. A large motor unit has a motor nerve in contact with up to 200 muscle cells. A small motor unit is a motor nerve in contact with a few muscle cells.
