Lecture 3, Skeletal Muscle Structure and Function Flashcards
Muscle Function
the ability to move is essential for basic survival:
* to obtain energy
◦ hunt for food
◦ eat and digest food
* run from predators
* to pump blood through the body and allow for breathing (muscles to control breathing allowing for respiration)
* keep warm in the winter
◦ thermogenesis (the generation of heat)
◦ muscles help produce heat
* to communicate with others
◦ sign language, body language, texting etc.
* there are several muscles that control the movement from the mouth down
* collect nutrients from the air into our bodies
Types of Muscle (muscle cells)
- muscle tissue makes up a large proportion of body weight (~40% in males, 32% in females)
- cardiac and smooth muscle, combined, make up around 10% of body weight
-> cardiac muscle, skeletal muscle and smooth muscle (details)
Muscle Properties
muscle has four defining properties (plus a bonus 5th property)
1. electrical excitability - ability to respond to (electrical) stimuli, and to conduct electrical impulses along the tissue
2. contractility - ability to contract/shorten
A. this property is what allows for the generation of force, and the production of movement
B. contractions occurs in response to electrical impulses
3. extensibility - ability to be stretched without being damaged (eg. tearing)
4. elasticity - ability to return to its original length and shape after being stretched or after a contraction
5. * adaptability - ability to change in response to how it is used
Supporting Structures - Fascia
fascia is sheet/band of fibrous connective tissue that lies deep to the skin, and surrounds muscle and other organs of the body
superficial fascia (sub-cutaneous)
- separates skin from muscle
- contains adipose tissue (fat), nerves, blood vessels and lymph vessels
- purpose: insulation and physical protection
deep fascia (lies within the muscles)
- holds similar muscles together
- occupies space between muscles
- contains nerves and blood vessels
Supporting structure - extension of deep fascia
each skeletal muscle is a separate organ composed of hundreds to thousands of muscle cells (they may have similar functions but are all separate organs)
* muscle cell = muscle fiber
* called muscle fiber because of their elongated shape (thin but elongated)
* muscle fibers lie in parallel to one another to make up the muscle
one muscle fiber can be 10-100 micro metres in diameter, and up to 2.5 feet in length
* fibers can extend the entire length of the muscle
* fiber can be as long as your thigh
endomysium, perimysium and epimysium
deep fascial tissues have layered extensions that serve to further protect and strengthen the muscle:
endomysium: thin layer of areolar connective tissue that covers individuals muscle fibers
perimysium: a fibrous layer surrounding groups of muscle fibers to form a fascicle
* fascicle: a group of 10-100 fibers (there are little passage ways that give signal to the brain in order for it to contract)
* contains collagen and elastin fibers, blood vessels and nerves
epimysium: a dense layer of collagen fibers that covers the entire muscle
* separates the muscle from the surrounding tissues and organs
* protects it from abrasion and physical damage
Supporting Structure - CT components
tendons attach muscle to bone. ligaments attach bone to bone
aponeurosis: a flat extension of a tendon
* all 3 come together to form a tendon
* fascia to tendon to bone
* tendons help with transmission of force
* aponeurosis is flat extension of a tendon (transmitted across a whole sheath to a larger area)
* tendon become continous with the matrix of the bone when they attach (become one)
◦ if you pull a tendon, a part of the bone may come off with it
Hierarchical Structure of Muscle
skeletal muscle -> fascicle -> muscle fiber (cells) -> myofibrils
each muscle fiber contains 100s-1000s or organelles called myofibrils
* allow for muscle contraction
* have the ability to shorten , thereby providing the mechanism for muscles to contract and produce force
Sarcolemma, SR
homologous structures in muscle cells:
- plasma membrane = sarcolemma (plasma membrane is the outside of the cell)
◦ “sarc” = greek for “flesh”
- cytosol (intra-cellular fluid) = sarcoplasm
endoplasmic reticulum = sarcoplasmic reticulum (SR)
the sarcolemma forms structures known as Transverse tubules or T tubules
- T tubules are continuous with the sarcolemma (circle myafilaments)
- electral signal to T tubules (continous with sarcolemma) allows for release of calcium which allows for contraction
SR forms a sleeve-like network around each myofibril
- terminal cisternae are enlarged regions on either end of the SR segment, framing the T tubules
- serve as a storage site for Ca2+
Myofibril - actin and myosin
- each myofibril is 2 micro metres in diameter, and extends the entire length of the muscle (can be 7m in length)
- myofibrils have the ability to shorten in length, which is what allows muscles to contract
◦ this is done by the action of filaments within the myofibril
actin (titin) filaments are 8nm in diameter and 1-2 micro metres long
myosin (thick) filaments are 16nm in diameter, and 1-2 micro metres long - filaments are arranged in functional contractile units known as sarcomeres
- movements of the filaments within the myofibrils allow for muscle to contract
- thin filaments mainly contain actin, thick ones are mainly composed of protein myosin
- shortening or lengthening within sarcomere allowing muscle to contract (all along the length of myofibril )
Actin Filaments
- thin filaments primarily consist of the protein Actin, but also contains the supporting regulatory proteins Troponin and Tropomyosin
- actin is a globular protein (each circle is one protein) that coordinates with several other actin molecules to create two, intertwined helical chains
- each individual actin molecule contains a binding site for myosin
Myosin Filaments
- thick filaments primarily consist of the protein Myosin, which are made up of two polypeptide “heavy” chains and four polypeptide “light” chains
- myosin forms two globular heads that serve as binding sites to the neighbouring actin filaments
- the binding of actin to the myosin tails results in the formation of a cross-bridge
Cellular Structure - The Sarcomere
Sarcomere: the contractile unit of skeletal muscle - give striated appearance to muscle
* the region between Z disks
* a “chain” of several consecutive sarcomeres make up the myofibril
the actin and myosin filaments are arranged in a specific, ordered fashion to create the striated (striped) appearance
The Sarcomere - Bands and Disks
A Band: the zone covering the length of the myosin filaments - forms band of stacked ligaments
* A stands for anisotrophy (like looking at a glass surface with polarized sunglasses - change in iridescence depending on the angle of your view
I Band: the zone covering the thin (actin) filaments only
* I stands for isotropic
H Zone: the light area in the middle of the A band where the thin filaments do not reach
* consists only of myosin filaments
M Line: runs down the middle of H zone
* consists of proteins that hold the thick filaments in place in their vertical stack
Z disk: the middle of I band
* contains proteins that connect thin filaments together and keep them in their vertical position
The Sarcomere - region and zone of overlap
a sarcomere is the region between Z-disks = 1 A-band + 1/2 I-band on each side
zone of overlap: where actin and myosin filaments actually overlap each other
* is a part of the A band
* as the muscle contracts, the two Z lines come together and the zone of overlap gets larger
