Structure and Function of the MSK System – Muscles Flashcards
Characteristics of skeletal muscles
Primarily involved in locomotion
Also involved in maintaining posture
Under voluntary control
Composed of muscle cells (muscle fibres)
Often known as striated muscle due to the appearance of the contractile proteins
Characteristics of smooth muscle
Lines structures including the stomach, intestines, and blood vessels
Primary function is to influence movement of material into, out of, and within the body
Involuntary muscle type
Non-striated – different arrangement of muscle cells compared to skeletal muscle
Characteristics of cardia muscle
Only found in the heart
Involuntary muscle type – moves blood through the circulatory system
Contraction of cardiac muscle regulated by the sinoatrial node
Striated structure – similar to skeletal muscle
Where is the origin of skeltal muscle found
Origin of a muscle is the end of the muscle that is attached closest to the trunk or to the more stationary bone
Composition of skeletal muscles
Skeletal muscle is a collection of muscle cells/muscle fibres
Each fibre is a long, cylindrical cell with up to several hundred nuclei near the surface
Skeletal muscle fibres are the largest cells in the body, formed by fusion of many individual embryonic muscle cells
Fibres lie in parallel arrangement, sheathed in connective tissue
Adjacent muscle fibres bundled together into units known as fascicles
How does growth and repair of skeletal muscle occur
Satellite (stem) cells activate & differentiate into muscle when needed for growth & repair
What do myofibrils consist of
several types of proteins organised into repeating contractile structures known as sarcomeres
Composed of thin and thick filaments
Proteins include:
Actin (thin filaments)
Myosin (thick filaments)
Tropomyosin & troponin (regulatory proteins)
What is actin
Actin makes up thin filaments of the muscle fibre
One actin molecule is a globular protein – usually multiple G-actin molecules polymerise to form long chains/filaments known as F-actin
In skeletal muscle, two F-actin polymers twist together like DNA creating thin filaments
Each G-actin molecule has a single myosin binding site and vice versa
Allows for cross-bridging of actin and myosin
What is myosin
Myosin makes up thick filaments of the muscle fibre
One myosin molecule is composed of two identical protein chains, each containing one large heavy chain and two smaller light chains
Phosphorylation of the light chains in skeletal muscle enhances contractile force
Heavy chains of myosin form the motor domain that uses ATP to create movement
What does a sarcomere consist of
Consists of thin & thick filaments in a repeating pattern of alternating light & dark bands
Z disks – zigzag protein structures, act as attachment sites for thin filaments
I bands – region occupied by thin filaments
A band – runs along the entire length of a thick filament, outer edges thick/thin overlap
H zone – region occupied by thick filaments
M line – proteins that act as attachment sites for thick filaments
What is muscle tension, load, contraction and relaxation
Muscle tension – force created by a contracting muscle
Load – weight or force that opposes muscle contraction
Contraction – creation of tension in a muscle, requires ATP
Relaxation – release of tension created by a contraction
How does muscle contraction occur at neuromuscular junction
Conversion of ACh signal from a somatic motor neuron into an electrical signal in MF
Explain excitation-contraction coupling
Muscle action potentials are translated into calcium signals
Calcium signals in turn initiate a contraction-relaxation cycle
Contraction-relaxation cycle
Sliding filament theory of contraction
One cycle is called a muscle twitch
4 major events in EC coupling
Ach is released from somatic motor neuron
Ach initiates action potential in muscle fibre
Triggers calcium release from sarcoplasmic reticulum
Calcium combines with troponin to initiate contraction
How does relaxation occur in EC coupling
Relaxation occurs following removal of calcium from the cytosol
As cytosolic calcium concentration decreases, it releases from troponin allowing tropomyosin to slide back and block actin’s myosin-binding site leading to release of crossbridges and fibre relaxation
What is the sliding filament theory of contraction
Overlapping actin and myosin filaments of fixed length slide past each other in an energy-requiring process, resulting in muscle contraction
Myofibrils at resting length have slight overlapping of actin and myosin
During contraction actin and myosin slide past each other
How does calcium initiate contraction
In resting skeletal muscle, tropomyosin wraps around actin filaments and partially covers actin’s myosin-binding sites (“off” position)
Prior to contraction, tropomyosin must be moved to uncover the remainder of actin’s myosin-binding site (“on”) position
The “off” position of tropomyosin is regulated by troponin
When contraction begins in response to calcium, troponin C binds reversibly to Ca2+
Calcium-troponin C complex pulls tropomyosin away from binding sites (“on”)
Enables myosin heads to bind to actin
How does myosin and actin cause contraction
- ATP binds to myosin and actin is released
- ATP hydrolysed to ADP and Pi. Newly formed actin-myosin crossbridge is weak as TM is partially blocking actin’s binding site. Myosin has stored potential energy, like a stretched spring
- Ca2+ binds to troponin to uncover rest of the myosin-binding site. Crossbridges become strong as Pi is released. Myosin heads move towards M line, sliding actin with them
- ADP released, myosin now back tightly bound to actin in rigor state
What is the rigor state
No ATP or ADP is bound to myosin – very brief in living muscle
Following death, muscles are unable to bind more ATP so they remain in rigor state
In rigor mortis, the muscles “freeze” due to immovable crossbridges
This lasts for around 24hrs after death until enzymes begin breaking down muscle fibres
What is the purpose of creatine kinase
enzyme responsible for transfer of the phosphate group from phosphocreatine
How is energy tranfered form covalent bonds to ATP
Fibres must use metabolism of biomolecules to transfer energy from covalent bonds to ATP
Carbs most efficient and rapid source of ATP
How much ATP is produced per Molecule of glucose
Aerobic – 30 ATP for each molecule of glucose
Anaerobic – 2 ATP for each molecule of glucose
What are the two types of skeltal msucles
Type I (MyHC I) – slow-twitch fibres
Type II (MyHC II) – fast-twitch fibres
two types of fast twich fibres
Type IIa – fast-twitch oxidative glycolytic fibres
Type IIb – fast-twitch glycolytic fibres
Why are fast twich fibres quicker
Fast-twitch fibres can pump Ca2+ into the sarcoplasmic reticulum faster than slow-twitch fibres, this results in quicker twitches in FT fibres
How is ability to resist fatigue related to each fibre
Type I / IIa – rely on oxidative phosphorylation for ATP production, so do not fatigue as quick
Type IIb – rely on anaerobic glycolysis for ATP production resulting in quicker fatigue due to H+ accumulation from ATP hydrolysis
What does myoglobin do
Helps bring oxygen to interior of fibres quickly Oxidative fibres (I / IIA) contain more so receive oxygen quicker than glycolytic fibres
What is isotonic and isometric contractions
Isotonic – muscle changes length and creates enough force to move a load
Isometric – muscle does not change length and force created is not enough to move a load
