Additional Material (Testable on Midterm) Flashcards
What is muscle tissue?
Specialized cells that use ATP in the generation of force
3 Types of Muscle Tissue
- Skeletal
- Smooth
- Cardiac
Muscle Tissue (Functions)
Body movement, Substance Movement, Control of Substance Movement, Thermogenesis
What are the characteristics of muscle tissue?
- Electrical Excitability
- Contractility
- Extensibility
- Elasticity
Electrical Excitability
-The ability to respond to certain stimuli by producing electrical signals
-Electrical signals produced are called action potentials
Contractility
-Ability of muscle tissue to generate tension (force) when stimulated by an action potential
Extensibility
-Ability of muscle to stretch (lengthen) without being damaged
-Muscle can still contract when stretched
Elasticity
-Ability of muscle tissue to return to it’s original shape after contraction or stretch
Skeletal Muscle
-a.k.a. striated muscle
-Striations are alternating light and dark bands that are characteristic of this muscle type
-Voluntary/Conscious control (also subject to involuntary control)
Hierarchy of Skeletal Muscle Organization
- Muscle
- Fascicle
- Muscle Fibre (Muscle Cell)
- Myofibril
Muscle
-Size: cm
-Named
-Subdivided into bundles of fascicles
Fascicles
-Size: mm
-Each fascicle is made up of many muscle fibres
Muscle fibres
-a.k.a. the muscle cell
-Size: Small
-Cylindrical in shape
-Multinucleated
-Filled with myofibrils
-Within muscle fibres: Sarcolemma, Transverse Tubules (T-tubules), Sarcoplasm, Myoglobin, Mitochondria
Sarcolemma
The cell (plasma) membrane of the muscle cell
Transverse Tubules
-a.k.a. T-tubules
-Tiny invaginations tunnel in from the sarcolemma towards the centre of the muscle fibre
Sarcoplasm
The cytoplasm of the muscle fibres - contains lots of glycogen
Myoglobin
A protein that binds oxygen that has diffused into the muscle fibre and delivers it to the mitochondria
Mitochondria
Lots of them
Myofibrils
-Specialized contractile organelles of the muscle cell
-Size: extend the length of the muscle cell
-Held in place by cytoskeleton proteins
-Composed of a number of sarcomeres arranged in series (end to end)
-Within Myofibrils: sarcoplasmic reticulum
Sarcoplasmic Reticulum
Fluid filled tubes an sacs running along and surrounding each myofibril - they store and release calcium into the cell (when it’s needed)
Sarcomere
-Functional unit of a myofibril (of contraction)
Contain 2 contractile proteins: actin and myosin (a.k.a. myofilaments)
-Thick and thin filaments overlap and their interaction is what generates force/contraction
-Their overlap also creates light and dark strips which gives skeletal muscle it’s striated appearance
Actin
Makes up thin fillaments
Myosin
Makes up thick fillaments
Motor Neuron
Neuron/nerve cell that conducts action potentials to muscle cells
Neurological Control
- To generate tension, skeletal muscle cells must be stimulated by a nerve signal from a motor neuron
- Axon connects with muscle - when it reaches the muscle, it branches out into a number of axon terminals
- Each axon terminal forms a junction called neuromuscular junction (NMJ) with the sarcolemma of a number off different muscle cells
- Axon terminal and the sarcolemma never actually touch - there is a gap between them called the synaptic cleft
- When the signal arrives, it releases a neurotransmitter (acetylcholine (ACh)) which crosses the synaptic cleft
Sliding Filament Mechanism
- When signal crosses the synaptic cleft it stimulates the sarcolemma (cell membrane)
- Signal is continued by the muscle fibres and spreads out across the sarcolemma
- Signal travels down the transverse tubules and stimulates the sarcoplasmic reticulum to release calcium
- Calcium allows myosin (of thick filament) to connect with the actin (of the thin filament)
- Myosin pulls actin/thin filaments together
- Disengages and starts cycle again
- This reattaching of filaments used ATP
- With repetition, it shortens the sarcomere, myofibril, and the muscle
- When APs stops, sarcoplasmic reticulum pumps calcium back inside (uses ATP)
- Without sufficient calcium, thick filaments cannot continue their reattaching of thin filaments
- Tension generation stops
Production of ATP in Muscle Fibres
-Skeletal muscle fibres need to vary the levels at which they consume ATP
-Muscle fibres store enough ATP to last for ~3sec of activity
What are the 3 energy pathways through which more ATP can be generated?
- Creatine phosphate
- Anaerobic glycolysis
- Aerobic cellular respiration
Creatine Phosphate
- Is a molecule that stores high amounts of energy in its chemical bonds
- Pcr is split by an enzyme, the energy released is used to reform ATP
- Happens very fast therefore PCr is the first source of energy used when muscle contraction begins
- Provides energy for ~3-15 sec of maximal contraction
- No oxygen needed
- No lactic acid produced (hence anaerobic alactic)
Anaerobic Glycolysis
- In context of muscles: when muscle activity continues and PCr is depleted, glucose is used to make ATP
- Cell breakdown glycogen stored in their cytoplasm/sarcoplasm or glucose from the blood and the energy release in breaking them down is used to reform ADP + PI
- Process of making ATP from glucose occurs in the cell cytoplasm and is called glycolysis
- Through glycolysis, a molecule of glucose is broken into 2 molecules of pyruvic acid and 2-3 ATP
- Normally, pyruvic acid enters mitochondria where it undergoes a series of reactions (that requires oxygen) called aerobic cellular respiration.
- During heavy excercise/demand, not enough oxygen is available (hence anaerobic)
- In absense of oxygen, pyruvic acid does not go through mitochondria - is converted into lactic acid/lactate
- Lactic acid diffuses out of the cell into the blood
- No oxygen required and producing lactic acid = anaerobic acid
- Capable of supplying energy for 30-40 sec
Lactic Acid/Lactate
- Metabolic by-product of anaerobic glycolysis
- At lower levels of activity, any lactate produced is consumed by other muscle fibres, less active nearby muscles, and heart so lactate does not accumulate
- Is converted back into glucose/glycogen in the liver
- Lactic acid has a 1/2 life of 15-25 minutes and is cleared in a matter of hours
Aerobic Cellular Respiration
- Pathway is actie when you are able to get oxygen into the cells (e.g. at rest or low-moderate intensity exercise)
- Oxygen is delivered by myoglobin or from oxygen diffusing from the blood
- In presence of oxygen, pyruvic acid enters the mitochondria and in a series of reactions (that uses oxygen), produces much more ATP (much more than glycolysis)
- Carbs, fats, and proteins can be used in this process to make ATP
- Carbohydrates yield relatively little APT
- Fats yield a lot of ATP
- Proteins aren’t used readily (often not even included)
- At rest, cells of the body use aerobic metabolism to generate their ATP
- In activities that last longer than 10min, most (90%) of the ATP generated comes from the aerobic system
Skeletal Muscle Fibres
-Not all skeletal muscle fibres are the same - differences include:
-The speed at which they generate tension
-How they use different energy substrates
-How they fatigue
What are the 3 main types of skeletal muscle fibres?
- Slow Oxidative (a.k.a. type I)
- Fast Oxidative-Glycolytic (a.k.a. type IIa)
- Fast glycolytic (a.k.a. type IIx)
Slow Oxidative
-a.k.a. type I, slow-twitch fibres
-Recruited 1st (i.e. before type II fibres)
-Fatigue resistant
-Used in endurance-type functions (e.g. maintaining posture, running a marathon)
-Lots of mitochondria, myoglobin, capillaries
-Generate ATP via aerobic cellular respiration (i.e. oxygen is available)
-With immobilization, they atrophy faster (than type II fibres)
Fast Oxidative-Glycolytic (FOG) Fibres
-a.k.a. type IIa fibres
-Recruited 2nd
-Moderately high resistance to fatigue
-Used in endurance (e.g. walking) and shorter-duration functions (e.g. sprinting)
-Intermediate amounts of mitochondria, myoglobin, capillaries
-Generate ATP via aerobic and anaerobic energy pathways
Fast glycolytic (FG) Fibres
-a.k.a. type IIx fibres
-Recruited 3rd
-Low resistance fatigue
-Used in high intensity, short duration activities (e.g. weight lifting, slap shot) and shorter-duration functions (e.g. sprinting)
-Relatively low amounts of mitochondria, myoglobin, capillaries
-Generate ATP via anaerobic energy pathways (i.e. glycolysis)
Distribution of Muscle Fibres
-Most muscles are a mix of SO, FOG, FG fibres
-Within a given motor unit, all fibre types are the same
Muscle Fibre and Motor Unit Recruitment
- When AP travels down motor neuron to the muscle fibres (i.e. the motor unit), all fibres in that motor unit will generate force
- Not all motor units are recruited with every contraction
- All motor units recruited for a given action do not contract at the same time
- Smallest/Weakest motor units (i.e. SO) are recruited first
- Precise movements require small changes in muscle contraction
- Muscles that perform fine movements will be made up of small motor units (few m. fibres/motor unit)
- Large (imprecise) movements dont require small changes in muscle contraction - they typically require large amounts of tension
- Muscles that perform gross movements will be made up of large motor units (many m. fibres/motor unit)
To increase the amount of force generated?
- Increase number of motor units recruited
- Increase the frequency of neuronal AP firing (wave summation)
*Length Tension Relationship
-The forcefulness of contraction (the ability to generate force) depends on length of the sarcomeres within a muscle before the contraction begins
-Optimal Overlap (~resting length) = greatest ability to generate tension
-Minimal Overlap (Lengthened) = decreased ability to generate tension
-Excessive overlap = decreased ability to generate tension
Isotonic
Muscle contraction through a range against a resistance that is not changing
Concentric
Shortening Contraction
Eccentric
Lengthening Contraction
Isometric
Muscle contraction in which the length of a muscle does not visibly change
Variable Resistance
Muscle contraction through a range in which the equipment varies the resistance to match the strength curve
Isokenetic
Muscle contraction through a range in which the equipment varies the resistance to match the strength curve keeps the velocity of movement constant