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Muscle Physiology - AIAS

Human Physiology from cells to systems 8th edition by Lauralee Sherwood AIAS
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Describe the structure of skeletal muscle
SM is attached to bones and it is striated (with lines) and voluntary. It is made of muscle fibers (cells), myofibrils (intracellular structure), thick and thin filaments (cytoskeletal elements)
Describe skeletal muscle fiber
skeletal muscle fiber lying parallel to one another held together by connective tissue. A single skeletal muscle cell (myocyte) is a muscle fiber: with multiple nuclei, large elongated and cylindrical shape, fibers extend entire length of muscle. It also has many mitochondria.
Describe the 3 types of skeletal muscle connective tissue
Epimysium: surrounds each whole muscle, Perimysium: surrounds fascicles (a bundle of muscle fiber), Endomysium: surrounds each individual muscle fiber
Describe the structure of cardiac muscle
Cardiac muscle is striated and involuntary. It?s found only in the heart. The cells are quadrangular and usually contain a single, centrally placed nucleus. The fibers branch to from 2 continuous networks, each contract as a functional unit. Intercalated discs provide strength and aid impulse conduction. It is myogenic.
Describe the structure of single unit smooth muscle
smooth visceral muscle tissue is located in viscera and it is non-striated (smooth) and involuntary. Fibres are arranged in a network. They are spindle shaped, with a single nucleus and arranged in sheets. Dense bodies are held together by intermediate filaments. Cross bridge binding happens through phosphorylated light chains. It is self-excitable and does not require nervous stimulation for contraction. (myogenic)
Describe Multiunit smooth muscle
It is found in blood vessels, lung airways, hair follicles and the eye. Multiple units function independently of each other and must be separately stimulated to contract. Contraction is initiated in response to stimulation by the nerves supplying the muscle (neurogenic)
What is the difference between cardiac fiber and smooth muscle
Cardiac muscle has more sarcoplasm, more mitochondria, a less developed sarcoplasmic reticulum and larger T tubules located at Z lines than skeletal muscle.
Identify the microanatomy of skeletal muscle
A skeletal muscle fiber contains numerous Myofibrils which are composed of thick and thin filaments and the protein molecules actin and myosin.
Describe the structure of myofibrils
Myofibrils make up 80% of muscle fiber and contain cytoskeletal microfilaments: Myosin forms the thick filaments . Actin is the main structural protein of the thin filament.
Describe how a myofibril looks under a microscope
light and dark stripes
Describe the A band
It's dark and is composed of thick filaments together with thin filaments that overlay each end of the thick filament.
Describe the I band
It's light and consists of the remaining portion of the thin filaments that do not project in the A band
Describe the Z line
is in-between each I band. It connects the thin filaments of 2 adjoining sarcomeres.
Describe the Sarcomere
is the area between the two Z lines. It is the functional unit of the skeletal muscle
Describe the H Zone
is a lighter area within A band thin filaments do not reach.
Describe the M line
represents the proteins that actually hold the thick filaments vertically down the A band within the H Zone
Describe Titin
It is the largest protein in the body, extends in both directions from the M line along the length of the thick filament to the Z line
What is a functional unit?
The smallest component that can perform all functions of that organ
Describe the structure of myosin molecules and their organisation within a thick filament
Each thick filament is made up of hundreds of myosin molecules. Each myosin is made up of 2 subunits with long intertwined tails (looks like golfclubs). The myosin head forms the cross bridge that takes place between the thick and 6 surrounding thin filaments.
Name the 2 important binding sites of the cross bridge
actin binding site and myosin ATPase site (ATP splitting)
Describe the composition of a thick filament
is made up of myosin molecules aligned lengthwise. Half point in one direction and half in the other. The globular heads of the myosin pertrude in small intervals and form the cross bridges
Describe the composition of a thin filament
The thin filament is made up of 3 proteins: actin as well as the regulatory proteins: tropomyosin and troponin. Actin is most abundant and forms 2 strands twisted together. Each actin has a specific binding site for attaching with the myosin cross bridges. The trompomyosin is like a long ribbon that covers most of the binding sites of the actin molecules (off position)
What is the role of the 3 polypeptides in Troponin?
One binds to tropomyosin, one binds to actin and one binds to Ca+
Explain the sliding filament mechanism
The thin filaments on each side of a sarcomere slide inward over the stationary thick filaments toward the A bands centre. As they slide inward, the thin filaments pull the Z lines to which they are attached closer together. As all sarcomeres throughout the muscle fibers length shorten simultaneously the entire fiber shortens.
Identify the intricate physiological processes involved in musculature contraction
Signals that come from the CNS via the motor neurons causes the release of calcium. Calcium binds to troponin, which pulls the tropomyosin complex aside and puts the actin in the on position. This allows the myosin cross bridges from the thick filament to bind with the actin molecules surrounding the thin filament. Than the power stroke can happen and this cycle will happen as long as the binding site is in the on position
Explain the power stroke
The power stroke is when the myosin cross bridge is bound to the actin and it bends in order to pull the thin filament in towards the thick filament. After the power stroke the Myosin swings back and binds to a new actin molecule. This process is repeated many times as the muscle fiber contracts
Identify the intricate physiological processes involved in musculature relaxation
For relaxation to happen the amount of CA in the muscle needs to decrease . Acetylcholinesterase breaks down Ach at the neuromuscular junction and muscle fiber action potential stops. CA can unbind from the troponin. The troponin/trompomyosin complex returns to the off position. The Ca+ pump returns Ca+ back into the lateral sac of the sarcoplasmic reticulum. Relaxation occurs.
Explain excitation-contraction coupling
refers to the events linking muscle excitation to muscle contraction.
Explain muscle contraction
Muscle contraction refers to the cross bridge activity that allows the thin filaments to be drawn inward by the thick filaments.
Explain muscle excitation
Muscle excitation refers to the presence of an action potential in a muscle fiber. A somatic motor neuron releases ACh at neuromuscular junction promotes Ca+ release into the cytosol leading to an action potential.
What is the difference between concentric and eccentric contraction
Concentric is a contraction in which the muscle shortens, eccentric it lengthens
Explain hydrolysis
When a larger molecule is broken down into a smaller one by the edition of water.
Identify the sarcoplasmic reticulum
It is a modified endoplasmic reticulum. It consist of a fine network of interconnected compartments that surround each myofibril A-band and I-band with lateral sacs at the end, that store Ca+
Identify Transverse Tubules (T tubules)
They run perpendicularly from surface of muscle cell membrane to central portions of the muscle fiber.The action potential on the surface membrane spreads down into the t tubule and releases Ca+ from sarcoplasmic reticulum into the cytosol.
Identify the physiological importance of calcium in muscle dynamics
When a nerve impulse reaches the motor end plate the neuron releases ACh which transmits the impulse to the mort end plate and then into the T tubules and sarcoplasmic reticulum. This releases Ca+ into the cytosol. Ca+ binds to troponin on the thin filaments. This causes tropomyosin to change shape, physically moving it away from its blocking position and uncovers the binding sites on actin for the myosin cross bridges. When action potential stop Ca+ is taken up by the sarcoplasmic reticulum again.
Identify the physiological importance of ATP in muscle dynamics
ATP is broken down by the myosin ATPase (hydrolysed) to ADP & inorganic Phosphate. ADP and Phosphate remains bound to the myosin and the energy created is stored within the cross bridge. Phosphate is released during the power stroke and ADP after the power stroke. Now a new ATP can attach to the myosin and be broken down.
Name the 4 steps that require ATP in muscle dynamics
- splitting of ATP by myosin ATPase provides the energy for the power stroke
- binding of ATP to myosin to detach cross-bridge from the actin filament at end of power stroke
- active transport of Ca+ back into sarcoplasmic reticulum during relaxation
- activity of the Na+K+ pump during action potential production
Identify the key components involved in skeletal muscle mechanics
- Whole muscles: are groups of muscle fibers bundled together and attached to bones
- connective tissue covering muscle: divides muscle internally into columns
- connective tissue extends beyond ends of muscle to form tendons
- Tendons attach muscle to bone
Identify the key components involved in skeletal muscle metabolism
ATP, creatine phosphate, oxidative phosphorylation, glycolosis.
Name the three metabolic pathways that supply ATP during muscle contraction/relaxation
1) transfer of a high-energy phosphate from creatine phosphate to ADP. When the muscle is active this high energy phosphate group is transferred to ADP and turned into ATP.
2) oxidative phosphorylation, when O2 is present
3) glycolysis, when O2 is not present
Identify the key components involved in fibre type
Three types of skeletal muscle fiber based on differences in ATP hydrolysis and synthesis. slow-oxidative (type I) fiber.
fast-oxidative (type IIa) fibers.
fast-glycolytic (type IIx) fibers
Name the 3 types of muscle contraction
Isotonic, Isokinetic, Isometric
Explain Isotonic
constant tension. In which the load remains constant as the muscle changes length. E.g. lifting bucket
Explain Isokinetic
constant velocity. In which the velocity (speed) of shortening remains constant as the muscle changes length. E.g. special exercise machines
Explain Isometric
constant length. In which the muscle length does not change as the tension increases. E.g. keeping legs stiff while standing or holding an object in fixed position
Explain concentric and eccentric contractions
In concentric contractions the muscle shortens. in eccentric contractions the muscle lengthens
Explain Tetanus in muscle contraction
if a muscle fiber is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli, a maximal sustained contraction occurs, known as tetanus
Explain asynchronous recruitment
The body alternates motor unit activity, like shifts at a factory, to give motor units that have been active an opportunity to rest while others take over.
Explain the process of twitch summation
If a muscle fiber is restimulated before it has completely relaxed the second twitch is added onto the first twitch, resulting in summation