Intro to skeletal muscle Flashcards
What is skeletal muscle tissue?
- Skeletal : Attach and move skeleton
- 40% of body weight
- Fibres are multinucleate cells
- Cells with obvious striations
- Contractions are voluntary
What is cardiac tissue?
- Wall of the heart
- Striated walls
-Involuntary contractions
What is smooth tissue?
- Walls of hollow organs
- Lack striations
- Involuntary contractions
What are the functions of skeletal muscle tissue?
- Designed to contract
- Generate mechanical force
- Heat generation (shivering, involuntary contractions etc)
- Locomotion/ external movements
Descrive how skeletal muscle is connected to the skeleton
- connected via tendons
- attached to immobile bone at one end (origin)
- Other end attached to insertion (moveable bone)
- Non-contractile parts (elastic elements) of the muscle and tendons are stretched before tension where attachment occurs
- Contracting muscles - typically pull on attachment points to move insertion toward origin
- Elastic elements allow muscle to elastically recoil back to its original resting length when contraction is completed
- Antagonistic (Synergistic) pairs
- Contraction of one does the opposite to counterpart (biceps and triceps)
What are myofibres?
- Elongated cells
- Arranged parallel to one another and bundled by connective tissue into fascicles
What are sacrolemma?
cell membrane
- Striated (banded)
What are the different components of skeletal muscle in connective tissue (layers) ?
- Superficial fascia = loose connective tissue underlying skin
- Deep fascia = dense connective tissue around muscle
- Epimysium - surrounds whole muscle
- Perimysium - surrounds bundles (fascicles) of muscle cells
- Endomysium - separates individual muscle cells
- Connective tissue layers extend beyond muscle bellly to form the tendon = aponeuroses and MTJ
What are myofibrils?
- Long bundles of protein, made up of thick and thin filaments
- myofilaments arranged in sacromeres
- Repeated units
- From one Z disc to the next
- Thick filaments ar center, thin filaments at either end attached to the z-discs
- I band near either end - only thin myofilaments
- A band at midsection - thick filaments
- H zone at centre - only thick fialments (no overlap with thin)
What is myosin?
- composes thick filaments
- held in place by the M line proteins
What is Actin?
- Composed thin filaments alongside troponin and tropomyosin
- Myosin-binding site on each actin molecule - covered by tropomyosin in relaxed muscle
- Thin filaments held in place by Z lines. From one Z line to the next = sacromere
What is the sliding filament mechanism?
- Movement of thin filaments over thick
- sacromere shorterning
- Thick filaments = stationary; thin are dragged over thick
- length of the filaments do not change
What is the anatomy of a nerve-muscle
- extensive blood supply for every muscle
- Specialised nerve cell = motor neuron = associated with muscles- every muscle fibre receieves innervation from a nerve
What events link muscle excitation / action potentials to muscle contraction (cross bridge cycling)?
- Action potential - propagates down the sacrolemma
- Transverse tubules conduct AP into the cell’s interior
- Ca 2+release channels open in Sarcoplasmic reticulum
How are muscle contractions activated by action potentials?
- APs induce the release of Ca 2+ into the sarcoplasm
- When Ca 2+ binds to troponin on thin filament, troponins changes
- Shifts tropomyosin off myosin binding sites
- Enables myosin to bidn to actin
What is Cross-bridge cycling?
Cross-bridge
Cycling
Myosin head binds to actin
Power Stroke
— Globular head bends
toward center of
sarcomere
— thin filaments pulled
toward center of sarcomere
- Cross bridge link broken and head unbends
- Myosin binds to next actin molecule on the thin filament
- Repeats, shortening length of the sarcomere
What are contractile properties?
- Twitch = single contraction and relaxation of a muscle in response to a single bried stimulus
- Strength of twitch contraction can be modifed
In vitro (isolated) - increasing stimulus strength induces large numbers of myofibres to contract
In vivo ( in the body) - activation of additional motor units (motor unit recruitment) increases contraction strength - Henneman’s size principle motor units - genrally recruited in order of smallest to largest (fewest fibres to most fibres ) as contraction increases
What is the length-tension relationship?
- Normal resting length for skeletal muscles ( sacromere ~ 2.0 micrometeres) - for generation of max tension
- If length decreased ( sacromere length < 2.0 micrometers ) - tension reduced as fluid pressure builds, thin filaments from either end of the sacromere collide
Wnat are the types of muscle contraction?
- Summation : when a muscle is stimulated multiple times in rapid succession
- Complete tetanus - muscle stimulates at high frequency so that no relaxation occurs (Steady state of tension)
- In vitro - high rate of stimulation prevents muscle fibres from relaxing
- In vivo - different motor units activated in rapid succession to generate sustained contraction
What is an isometric contraction?
- Muscle does not shorten,remains at the same length
- Force generated by the contraction is equal to the load applied
What are isotonic contractions?
- Muscle changes length
What are concentric contractions?
- Force generated is greater than load applies
- Muscle shortens as it contracts
What are eccentric contractions?
- Force generated is less than load applied
- Muscle lengthens even though it is contracting
What do muscle contraction require?
- ATP
- Cross bridge cycling
-Ca 2+ pump activity
Sources available: - cytosolic ATP
- Phosphocreatine (PCr)
- Aerobic respiration (oxidative phosphorylation)
- Anaerobic respiration (fermentation)
Describe ATP and Creatine Phosphate in muscle
ATP - limited stores (used up in a few contractions)
- Phosphocreatine (creatine phosphate)
- Storage of high energy phosphate bonds
- Used to quickly regenerate ATP from ADP
- Limited supply in cells
What is anaerobic respiration?
- Glycolysis + lactate fermentation
- breakdown of glucose
- Stored as glycogen in muscle cells
- Does not require O2
-Generates ATP quickly (faster than aerobic respiration) - Used during intense exercise - O2 supply cannot keep up with demand
- Lactate produced - associated with muscle soreness and fatigue
What is aerobic respiration?
- Occurs in mitochondria
- Requires O2 to form ATP
- Fatty acids = primary nutrient source
- Contain lots of energy, but requires O2 to release it
- Aerobic exercise - light to moderate exercise (walking, jogging, swimming)
- Max O2 uptake
- Max rate of O2 delivery to the muscles
- Max level of aerobic activity
How is skeletal muscle developed and regenerated (via postmitotic nuclei) ?
- where are they located?
-Adult SKM fibres have 100-1000s of postmitotic nuclei that are located at the periphery of the fibres
What are satellite cells?
Active in young growin.. and … in older muscles
Donate … into growing…
Repair…
- active in young growing muscle and quiescent in older muscles
- donate myonuclei into growing muscle fibres
- Satellire cells - repair muscle damage
What is myogenesis?
- Reg by … reg factors
- Appear at different.. during… development
- … MRFs regulate ….. {MYOD-r Myf-5),
- Secondary MRFS regulate fusion of … and terminal differentiation (MRF4)
- The muscle making process
- Regulated by myogenic regulatory factors (MRFS)
- They appear at different times during muscle development
- Primary MRFs regulate determination ( MyoD or Myf-5), Secondary MRF’S regulate fusion of myoblasts and terminal differentiation (MRF4)
Somite cells to Myoblasts then Proliferation then Fusion = muscle fibres
What are the two ways that muscle tissue can grow?
- Cells can double their nuclei/DNA and contents in a process called cell cycle and then split - HYPERPLASIA - most common form of muscle growth during development
- Adult muscle cells can also grow in size = HYPERTROPHY. Hyperplasia does not occur in adulthood. Cellular hypertrophy limited because the DNA concentration within a cell with one nucleus will be diluted due to their post-mitotic nature
What are the two possible outcomes of hypertrophy?
Hypertrophy with … dilution
(increase in nuclear number)
DNA: … is kept … during hypertrophy
- Hypertrophy with DNA dilution (only protein synthesis)
- Mechanism for increase in nuclear number
- DNA - to muscle volume is kept constant during hypertrophy
What is the role of satellite cells in hypertrophy?
- Satellite cell proliferation and differentation
- Hypertrophy stimulus
- Nucleus donated by satellte cell
- Fusion of some satellite cells with muscle fibres
