Exercise Physiology - Muscular System
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What do skeletal muscles do?
Move the bony levers of the skeletal system, enabling movement of the body
What does the muscular system include?
- Skeletal muscles;
- cardiac muscles
- smooth muscles
What do smooth muscles do?
Assist in regulation of blood flow to various parts of the body
Which type of muscle is controlled voluntarily and account for 50% of human body mass?
What is myofiber?
Skeletal muscle cells/fibers that are considered "excitable"
What are the three layers of connective tissue in a skeletal muscle?
What is epimysium and where is it located?
- Outer layer of connective tissue that surrounds the entire muscle and separates muscle from surrounding tissue and organs;
- Converges at the end of the muscle to form the tendon that attaches to the bone
What is perimysium?
Central layer of connective tissue w/in muscle that divides muscle into fascicles (singular = fasiculus)
What is a fasciculus?
Group of as many as 150 myofibers lying in parallel and bundled together
What is endomysium?
Inner layer of connective tissue w/in muscle that surrounds myofiber
What are motor neurons and where do you find them?
- Controls each skeletal myofiber;
- Central Nervous System
What is a motor unit?
A motor neuron and all the myofibers that it innervates.
How do motor units work with muscle contraction?
All myofibers of a motor unit contract simulatneously and at maximal force
What type of myofiber is contained in a motor unit?
All the same type (either Type II, Type IIa, or IIb)
How is a muscle able to regulate its force production during contraction? How does this work for maximal force output?
- By controlling the number of motor units that are activated;
- By varying the firing rate of neural impulses delivered by the motor neuron to its myofibers
- For maximal force output, all of the muscles motor units have been recruited and each one is firing at it's highest possible rate
What is a neuromuscular junction?
Junction where communication between a motor neuron and a skeletal muscle fiber occurs
Where is a synaptic knob located, and what is the purpose of it?
- Located at the end of an axon of the motor neuron;
- Contains the neurotransmitter acetylcholine (ACh)
What is the synaptic cleft?
Separates the synaptic knob from sarcolemma of the skeletal muscle fibers.
How many nuclei can a myofiber contain?
More than one, and up to 200-300/mm of fiber length
What is sarcolemma? What is it's purpose?
- The plasma membrane of the myofiber
- Contains chemically-gated sodium channels allow electrical stimulation of myofibers via action potentials
What content is substantial in myofibers and what does it provide?
- Mitochondrial content;
- Provides ATP via oxidative pathway
What is sarcoplasm?
Cytoplasm of the myofiber
What are T-tubules (transverse)?
Extensions of the sarcolemma that form a network of tubules
What is sarcoplasmic reticulum and what is it needed for?
- Richly developed in the myofiber to store calcium in special sacs called terminal cisterna;
- Needed to stimulate muscle contraction
What do myfibirils contain?
What do myofilaments consist of and how are they organized?
- Contractile proteins actin and myosin;
- Orgainzed in repeading functional units called sarcomeres;
Functions of actin and myosin
- Account for 60% of protein content of myofiber;
- Form crossbridge to slide past one another during muscle contraction, shortening the sarcomeres
Larger contractile protein sometimes called "thick filament"
Smaller contractile protein sometimes called "thin filament"
What is tropomyosin?
- Covers the actin bridging site during resting condition;
- Attached to troponin
What is the function of tropomyosin and troponin?
Regulate bridging of actin and myosin for muscle contraction and relaxation
What is the most common method used to classify myofibers?
Based on the isoform of myosin expressed by the cell
What are the 3 myosin isoforms? Describe them and when they used during exercise.
- Type I - slow twitch fibers w/ high oxidative capacity - exercise w/ low intensity and longer duration
- Type IIB - fast twitch fibers w/ low oxidative potential - exercise w/ high intensity and short duration
- Type IIA - intermediate fibers, both in twitch and oxidative capacity - exercise w/ high intensity and short duration
What are essential to myofiber twitch?
Regulatory and contractile filaments
What is the force generated by the whole musce a function of?
Function of both the number of myofibers within that muscle that are twitching and the rate at which the twitches occur
What effect does training (both endurance and resistance) have on myofibers?
- Cause conversion of type IIB fibers to type IIA;
- Doesn't cause conversion between type I and Type II myofibers
What does prolonged resistance training result in and what is in known as?
- Significant increase in myofiber size, particularty type II;
What is the Sliding-Filament theory? Describe it
- Explains how protein filaments interact to produce a twich of the myofiber
- 1) Nervous system excites myofiber's sarcolemma and T-Tubules
- 2) Calcium stored within sarcoplasmic reticulum releases into cell's cytosol
- 3) Calcium binds to troponin, causing associated tropomysin to change shape
- 4) Active sites of actin filament are exposed
- 5) Cross-bridge heads on myosin molecule bind to exposed active sites of actin
- 6) Adenosine triphosphatase (ATPase) cleaves ATP, resulting in "power stroke" that pulls actin towrad the center of the myosin molecule
- 7) Results in fiber shortening and force generation
- 8) When ATP binds to cross-bridge head, link between myosin and actin is broken, allowing process to repeat
What is the Calcium pump?
Responsible for delivering cytosolic calcium back into sarcoplamic reticulum, returning myofiber to state of relaxation
Describe muscle attachment locations
- - Begins at at proximal attachment (origin)
- - Ends at distal attachment (insertion)
- - Contracts to produce a specific action
What is an agonist?
A prime mover, responsible for producing a particular movement
What is an antagonist?
A primer mover that opposes the agonist
What is a synergist?
Assists the prime mover but is not the primary muscle responsible for the action
Why is identification of superficial landmarks of the skeletal muscles and other anatomic structures important?
Body composition and exercise testing
Upper body anatomic structures important for exercise testing
- Sternocleidomastoid -- radial pulse;
- Pectoralis major;
- Biceps brachii;
- Triceps brachii
Lower body anatomic structures important for exercise testing
- Gluteus maximus;
- Quadriceps femoris;
What is an important landmark for skinfold measurement and where is it located?
- Inguinal crease;
- Natural diagonal crease in the skin formed where the musculature of the thing meets the pelvic girdle
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