Unit 3 - Muscle - (Course Notes) Important Terms and Concepts Flashcards
What is muscle?
Tissue specialized to convert biochemical reactions into mechanical work
Muscle Function
Two main functions:
i. Motion
ii. Force
- Can only contract
- Cannot expand - except when physically pulled by antagonist muscle group
- Generate heat & contribute to body temperature homeostasis
Muscle Types
Three types of muscle in the human body:
i. Skeletal
ii. Smooth
iii. Cardiac
i. Skelatal:
-attached to bones→ control movement
-contract in response to signal from somatic motor neuron→ cannot contract on its own, no hormonal influence
ii. Smooth: no striations
-primary muscle of internal organs & tubes (bladder, stomach, blood vessels)
-influences movement of materials through body
iii. Cardiac: has striations
-found only in the heart→ pump to move blood around the body
Skeletal Muscle
Characteristics
~40% of body weight, responsible for positioning and movement of skeleton
- attached to bones via tendons
- composed of dense regular connective tissue → collagen (cable-like fibres made of protein)
Skeletal Muscle
Structure
-outer connective tissue → epimysium
-contains bundles of muscle tissue called “fascicles”
-fascicles contain muscle fibres
fascicles are covered by perimysium → a connective tissue sheath
Skeletal Muscle
Muscle Fibres
(muscle cells)
-muscle fibres are found within each fascicle
-covered by an innermost connective sheath → endomysium
-within the muscle fibres are a large number of myofibrils → the functional units of skeletal muscle
-muscle cells contain many glycogen granules (energy storage), and mitochondria (ATP synthesis) → in the cytosol
Structure of a Muscle Fibre
-long, cylindrical cell
-several hundred nuclei on the surface
-cell membrane called “sarcolemma”
-majority of space is taken up by myofibrils → contractile and elastic protein bundles
-have a special ER (sarcoplasmic reticulum, stores calcium)
Sarcoplasmic Reticulum
-series of branching tubes called T-tubules (transverse tubules)
-transverse tubules have a lumen continuous with the ECF
-closely associated with terminal cisternae (sequester Ca 2+)
-one T-tubule with flanking terminal cisternae (called a “triad”)
-T-tubule allow for rapid action potential diffusion into muscle fibre
sequester- isolate or hide away
General Term
- Muscle cell
- Cell membrane
- Cytoplasm
- Modified ER
Muscle Equivalent:
1. Muscle Fibre
2. Sarcolemma
3. Sarcoplasm
4. Sarcoplasmic Reticulum
Organization of the Myofibril
Characteristics
-myofibrils occupy most of the space in a muscle
-highly organized→ consist of bundles of contractile elastic proteins
A. Contractile proteins→actin & myosin
B. Regular proteins→ troponin & tropomyosin
C. Accessory proteins→ nebulin (aligns this filament) & titin (elastic protein that returns stretched muscle to relaxed)
Titin’s primary functions are→ stabilize the thick filament, center it between the thin filaments, prevent overstretching of the sarcomere, and recoil the sarcomere like a spring after being stretched.
Organization of the Myofibril
Contractile Proteins
i. Myosin
ii. Actin
i. Myosin
-motor protein→ two coiled protein molecule with a head and tail (chains)
-thick filament→ 250 myosin molecules joined
-arranged with heads at the ends and tails together
Organization of the Myofibril
Contractile Proteins
i. Myosin
ii. Actin
ii. Actin
-subunits G-actin (globular actin)
-G-actin subunits polymerize, form a chain (F-actin, “filamentous actin”)
-two “F-actin” chains twist together (forms basis of thin filament)
-coiled F-actin associates with regulatory proteins→ Troponin and Tropomyosin (regulate muscle contraction, forms completed thin filament)
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
-a sarcomere is made up of thick and thin filaments→ has striations
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
Z-line (disks)→ site of attachment for thin filaments
-one sarcomere→ made of two Z discs & filaments between them
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
I band (isotropic band) → region containing only thin filaments
-Z disc runs through the middle of the I band → each half of I band is part of a different sarcomere
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
A band (anisotropic band) → region contains thick and thin filaments
-thick and thin filaments overlap at outer edges of A band
-center is occupied by thick filaments only
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
H zone (part of A band) → region sontaining only thick filaments
-central region is lighter than outer edges
Organization of the Myofibril
The Sarcomere
myofibrils have striations
(skeletal muscle = striated muscle)
Sarcomere- one repeated pattern of myofibril unit
M line - the center of the sarcomere → site of attachment for thick filaments
What is the effect of not having T-tubules?
Smooth muscle
- No direct coupling of AP (action potential) to calcium release from SR through DHP receptor-ryanodine receptor coupling (skeletal muscle)
- Calcium entering through cell membrane causes calcium release from the SR
How do the properties of myosin contribute to the characteristics of smooth muscle as a whole?
- contract slow & for longer periods of time (than skeletal or cardiac muscle)
- due partly to slower myosin ATPase activity
- longer actin and myosin filaments allow longer contractions and allow smooth muscle to be stretched yet still be able to contract
