Anatomy Lecture Exam 3: Muscular System
Effect of Exercise on Skeletal Muscles: Aerobic or endurance exercises
(jogging, biking, swimming) result in changes that occur in the skeletal muscles such as: increase in capillaries around the slow oxidative muscle fibers increase in mitochondria available to perform more aerobic respiration and produce more ATP for the oxidative muscle fibers increase in myoglobin content via blood delivery to bind, store and release more oxygen for aerobic respiration of the oxidative muscle fibers Overall, endurance improved = increase in stamina
Effect of Exercise on Skeletal Muscles: Resistance exercise or isometric exercises
(weightlifting, isometric exercises where muscle are pitted against immovable objects) result in changes that occur in the skeletal muscles such as: Based on Wolff's Law where if you challenge a skeletal muscle, it will grow Increase in the size (growth) of skeletal muscles = hypertrophy of skeletal muscles occurs due to increase in the number of self-replicating myofibrils within each muscle fiber Increase in glycogen content (stored glucose) to further support the anaerobic respiration taking place via the glycolytic muscle fibers Overall, bulky skeletal muscles (mainly glycolytic muscle fibers) generate more force/tension during skeletal muscle contraction leading to an increase in the strength of those skeletal muscles contractions during the proper exercises However, once again, bulky muscles mainly use anaerobic respiration due to mainly being glycolytic muscle fibers and therefore can have buildup of lactic acid that can cause muscle fatigue, thus they don't have high endurance
Thick filaments
16nm in diameter; composed of the protein MYOSIN. Each thick filament consists of 300 myosin molecules. Thick filament is also known as an A band Each myosin consists of a tail and 2 globular heads. The myosin globular heads contain the enzyme ATPase and acts as the binding sites for actin which act as the binding sites for ATP
Thin filaments
8nm in diameter anchored by the Z lines Thin filaments contain 3 different proteins: - Actin - Tropomyosin - Troponin
PERIMYSIUM (inner connective tissue layer)
A course connective tissue membrane that wraps around a group of endomysium-covered muscle fibers forming what is called a FASCICLE
Troponin
A three-polypeptide complex: TnC, TnT and TnI
EPIMYSIUM (outer connective tissue layer)
A tough connective tissue membrane that wraps around a group of fascicles forming what is known as the skeletal muscle (the organ) itself
Sources of ATP to support skeletal muscle contraction: Creatine Phosphate (CP)
ATP produced from direct phosphorylation of ADP by CP using the enzyme Creatine Kinase: CP + ADP --> ATP + creatine ATP produced via this mechanism supports the next 10 seconds of skeletal muscle activities
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 4
Acetylcholine will diffuse and bind to acetylcholine receptors on the motor end plate of the sarcolemma of the muscle fiber to cause depolarization which leads to the generation of action potential (electrical current) at the motor end plate of the sarcolemma of the muscle fiber
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 6
Activated MLCK converts ATP into ADP + Pi and phosphorylates (adds ONLY the inorganic phosphate (Pi)) the myosin heads of the A bands (thick filaments), activating them, to which they bind to accessible myosin binding sites on actin myofilaments (thin filaments) Remember, in smooth muscle, although tropomyosin is present in thin filaments, it never blocks the myosin binding sites on actin myofilaments (thin filaments), even when the smooth muscle is relaxed
Sources of ATP to support skeletal muscle contraction: Anaerobic catabolism of glucose
Anaerobic catabolism of glucose is the same as glycolysis and thus it doesn't require oxygen, however this is inefficient because you only net 2 ATP per glucose molecule Produces 30 seconds to 60 seconds of skeletal muscle activities. If anaerobic respiration is extended past about 60 seconds, then increase in LACTIC ACID production occurs via left over pyruvic acid from glycolysis which reduces blood pH, which then reduces the activity of the glycolytic enzymes and causes you to only net one ATP per glucose molecule during glycolysis, therefore you will start to create an ATP deficit environment and lead to muscle fatigue
After skeletal muscle contracts as described above, how does it then relax?
As NEW ATP attaches to the myosin globular head, the link between myosin and actin weakens resulting in the cross bridge detachment and subsequently muscle relaxation (Step 3 in the cross bridge cycle) The ATP will remain attached to the myosin globular head all throughout the relaxed state until skeletal muscle contraction is going to occur again The skeletal muscle (the organ) will always have to relax (undergo cross bridge detachment) before it can go into another cross bridge cycle and contract again
large motor units
As an example, large sized motor units make contact with roughly 100,000 skeletal muscle fibers through the axon terminals of the motor neuron innervating that skeletal muscle (the organ) If you activate the large sized motor unit, you may cause the skeletal muscle (the organ) to contract
medium-sized motor units
As an example, medium sized motor units make contact with roughly 10,000 skeletal muscle fibers through the axon terminals of the motor neuron innervating that skeletal muscle (the organ)
small motor units
As an example, small sized motor units make contact with roughly 100 skeletal muscle fibers through the axon terminals of the motor neuron innervating that skeletal muscle (the organ) If you activate a small sized motor unit, the skeletal muscle (the organ) may not contract because you're not making contact with a lot of skeletal muscle fibers within that skeletal muscle (the organ)
The way Skeletal Muscles Attach to Bones in the Skeletal System:
Attachment of Skeletal muscles to bones (whether at the origin or insertion) occurs 2 ways: Direct and Indirect attachment
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 2
Axon of motor neuron generates and transmits action potential to the axon terminals which causes calcium channels on the axon terminal to open and allow calcium to flow into the axon terminal thereby activating docking proteins that move vesicles containing the neurotransmitter acetylcholine (Ach) to the axon terminal membrane facing the neuromuscular cleft
2 Types of Smooth Muscle:
Based on smooth muscle fiber arrangement, innervation and responsiveness to stimuli 1) Single-Unit Smooth Muscle 2) Multiunit Smooth Muscle
Rigor Mortis
COMPLETE lack of ATP results in skeletal muscle contracture termed RIGOR MORTIS which occurs when an individual dies and ATP synthesis ceases Actin and myosin are irreversibly cross linked and skeletal muscles remain contracted
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 4
Calcium binds to and activates Calmodulin to form a Calcium-Calmodulin complex
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 7
Calcium ions (Ca2+) then bind to TnC (Troponin C) which results in a conformational change of the troponin and the movement of tropomyosin away from blocking the myosin-binding sites on actin myofilaments (thin filaments) (tropomyosin blockade ended) Remember, troponin T is what is binding tropomyosin here Thus, the function of calcium here is to end the tropomyosin blockade of the myosin-binding sites on actin myofilaments
How is the ATP Generated used in Skeletal Muscle Contraction?
1) Once again, ATP is hydrolyzed by the ATPase enzyme of the myosin globular heads to produce ADP + Pi to activate the myosin globular heads to initiate the cross bridge cycle in the steps leading up to skeletal muscle contraction 2) ATP is also required for cross bridge detachment and thus skeletal muscle relaxation and the ability to start another cross bridge cycle if desired 3) Finally, ATP is required for calcium sequestration which is the withdrawal of calcium ions against their concentration gradient via active transport back into the sarcoplasmic reticulums (SR) for storage
The sliding Filament Mechanism of Muscle Contraction
1) Sliding of the thin filaments in the sarcomeres into the H zone, toward the M line results in sarcomere length shortening/decreasing (the distance between the Z discs shortens) 2) Shortening of sarcomeres results in shortening of myofibrils 3) This then results in shortening of skeletal muscle fibers (skeletal muscle cells) 4) Thus, the length of skeletal muscle shortens (i.e. muscle contraction)
3 Skeletal Muscle Fiber Types:
1) Slow Oxidative Muscle Fibers (also known as red fibers) 2) Fast Oxidative Muscle Fibers (also known as intermediate fibers) 3) Fast Glycolytic Muscle Fibers (also known as white fibers)
Mitochondria
For aerobic respiration to produce energy
Eccentric contraction:
Force/tension generated by the skeletal muscle contraction develops as the skeletal muscle increases in length from an already shortened (contracted) length An example would be setting a heavy box back down moving with gravity You are still generating force/tension by the skeletal muscle as you are setting something down since you are still holding onto the weight of that object Eccentric contraction is not explained by the excitation-contraction coupling mechanism (sliding filament mechanism) since the muscle is not contracting because the increasing force is being absorbed by the connective tissue wrappings of the skeletal muscle (endomysium, perimysium and epimysium) and not being used to cause skeletal muscle length shortening (contraction)
Concentric contraction:
Force/tension generated by the skeletal muscle contraction exceeds the weight of the object and so the skeletal muscle shortens (contracts), and work is done, such as lifting a weight or picking up a heavy box against gravity Concentric contraction is therefore the only type of contraction explained by the excitation-contraction coupling mechanism (sliding filament mechanism) since we see shortening length of the skeletal muscle (contraction)
Major Pathway for ATP production: Anaerobic Respiration
Glycolytic muscle fibers are muscle fibers that stay in ANAEROBIC respiration longer and use it mainly for ATP production Remember, anaerobic respiration is very inefficient Since Glycolytic muscle fibers mainly use anaerobic respiration, they have a higher potential to cause muscle fatigue due to buildup of lactic acid
Inclusions
Glycosomes contain glycogen
According to this mechanism when a muscle contracts there is more overlap between the thin filaments and the A bands and thus:
H zone decreases and eventually disappears when the muscle contracts I bands decrease and eventually disappears when the muscle contracts Sarcomere length shortens (the distance between the Z discs shortens) meaning the skeletal muscle (the organ) shortens or in other words, contracts However, the length of the A bands and the length of the thin filaments remain the same in both a relaxed and contracted state (they do not shorten)
4) The length of the sarcomeres prior to skeletal muscle contraction
Sarcomeres at the optimum length generates the maximum force/tension of skeletal muscle contraction Sarcomere length below the optimum length (shortened sarcomeres prior to skeletal muscle contraction) results in decreased force/tension of skeletal muscle contraction since there is less H zone for thin filaments to slide into Sarcomere length greater than the optimum length (stretched sarcomeres prior to skeletal muscle contraction) results in decreased force/tension of skeletal muscle contraction since there is less opportunity for cross bridge cycles to occur due to less overlap of the thin filaments and the A bands (thick filaments), since remember, the myosin globular heads are located on the A bands and bind to the myosin binding sites located on the thin filaments
Actin
Several actin proteins for the structural framework of the thin filament. Actin molecules contain the binding sites for the myosin globular heads
Comparison of the 3 types of Muscle: Growth
Skeletal muscle and cardiac muscle grow by only hypertrophy Smooth muscle grows by BOTH hyperplasia and hypertrophy A good example of this occurring in smooth muscle is the growth of the uterus during pregnancy
Skeletal Muscle as an organ consists of (The reason why skeletal muscle is an organ)
Skeletal muscle cells that are elongated and are therefore referred to as skeletal muscle fibers Multiple connective tissue (CT) membranes present known as the endomysium, epimysium and perimysium Blood vessels Nerve endings
Microscopic Anatomy of a Skeletal Muscle Fiber
Skeletal muscle fibers (skeletal muscle cells) run the entire length of the skeletal muscle (the organ) Each muscle fiber (cell) contains: - Myofibrils - Myoglobin - Inclusions - Mitochondria - Sarcoplasm - Sarcoplasmic Reticulum (SR) - Sarcolemma - Transverse Tubules (T Tubules)
Smooth muscle can be excited to contract by:
Pacemaker Cells Chemicals Autonomic Nerve Fibers (Autonomic nervous system)
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 1
Pacemaker activity (in a single-unit smooth muscle only), a chemical (hormone) or autonomic nerve fiber activation leads to an action potential being generated at the sarcolemma of the smooth muscle cell
Pacemaker Cells
Pacemaker cells are intrinsic to the smooth muscle They spontaneously depolarize generating action potentials to stimulate smooth muscle contraction Remember, this is only occurring in single-unit smooth muscle since multiunit smooth muscle does not contain pacemaker cells
Sarcolemma
Plasma membrane of muscle fiber
Sources of ATP to support skeletal muscle contraction: Aerobic catabolism of glucose
Produces more ATP via use of mitochondria that take the ATP and pyruvic acid made during glycolysis and use it, alongside oxygen, to make more ATP via the Krebs cycle and electron transport chain This system makes 38 ATP per glucose to be exact and is therefore very efficient Supports skeletal muscle activities for hours If you quickly convert anaerobic respiration to aerobic respiration you will not build up a lot of lactic acid and there will also be enough ATP produced to meet the demands of the skeletal muscles due to aerobic respiration being extremely efficient, thus muscle fatigue will be prevented
Why is each muscle type considered an organ?
Remember, one of the definitions of an organ is such that an organ must be composed of at least two types of tissues The other definition of an organ states it must be composed of at least two types of PRIMARY tissue Thus, each muscle type agrees with both definitions of an organ since each muscle tissue type has connective tissue wrappings, blood vessels, nerve fibers and therefore each muscle type is an organ
Myofibrils
Rod-like structures that run the entire length of the muscle fiber 80% of the volume of the muscle fiber is occupied by the myofibrils. Myofibrils contain 2 myofilaments: - Thick filaments - Thin filaments
Sarcomeres:
Sarcomeres are the Structural and Functional units of skeletal muscles because it is the smallest contractile unit that repeats throughout a skeletal muscle We have the skeletal muscle fibers (skeletal muscle cells) running the entire length of the skeletal muscle (the organ), then you have myofibrils running the entire length of the skeletal muscle fibers and then within the myofibrils you have the sarcomeres arranged end to end throughout the length of the myofibril A sarcomere is the distance between 2 successive Z discs in a myofibril.
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 5
The action potential spreads across the entire sarcolemma and into the T-tubules of the triads to cause calcium channels on the terminal cisternae of both sarcoplasmic reticulums (SR) to open
ORIGIN
The bone that does not move (the immovable bone) is the ORIGIN. Thus, when the skeletal muscle contracts, the insertion moves toward the origin since the insertion is the moveable bone!
INSERTION
The bone that moves (the movable bone) when the skeletal muscle contracts is known as the INSERTION
The Neuromuscular Junction:
The junction between the axonal terminal of a motor neuron and the motor end plate of the sarcolemma of a skeletal muscle fiber separated by a small space called the neuromuscular cleft (synaptic cleft) filled with extracellular fluid The highly folded region of the sarcolemma of the muscle fiber at the neuromuscular junction is called the Motor End Plate which expresses acetylcholine receptors Each skeletal muscle fiber (skeletal muscle cell), at its sarcolemma, will have ONE motor end plate, which means each skeletal muscle fiber can form only ONE neuromuscular junction with a singular axon terminal of ONE motor neuron to belong to that ONE SINGULAR motor unit
Components of a Sarcomere:
Thick filament is known as an A band Thin filaments alternating with "A" bands (thick filaments) The alternating pattern of the thick and thin filaments results in the characteristic striated appearance of skeletal muscle Z discs (Z lines) H zone M line I bands
Muscle Fatigue
This is a PHYSIOLOGICAL inability of a stimulated skeletal muscle to contract due to ATP DEFICIT which means the skeletal muscle demand for ATP is greater than the amount of ATP produced by the body, or in other words the body isn't making enough ATP to sustain skeletal muscle activity Thus, if a skeletal muscle undergoes contraction via the excitation-contraction coupling mechanism, then we know that cross bridges have formed within that skeletal muscle and those cross bridges have undergone the power stroke resulting in contraction, but if there is not enough ATP being produced by the body to lead to detachment of all the cross bridges, then that skeletal muscle cannot undergo another cross bridge cycle since the first cross bridge cycle has to complete before a new one can occur and thus the skeletal muscle will not contract even when its being stimulated
Why do sarcomeres of the optimum length generate the maximum force/tension of skeletal muscle contraction?
This is because the optimal sarcomere length is such that you have overlap of the A band and the thin filaments while also still having an H zone (area of A band not overlapping with thin filaments) Thus, this length is optimal, since you have the maximum portion of thin filament over thick filament (A band) to allow maximum amounts of cross bridge cycle initiation to occur while also having the maximum opportunity for inward sliding of the thin filaments via the cross bridge cycle due to the presence of an H zone and hence maximum shortening of sarcomeres thereby resulting in maximum force/tension generated by the resulting contracting of the skeletal muscle Therefore, anything other than the optimal sarcomere length shows us a decrease in force/tension generated by the contracting skeletal muscle
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 6
This then causes the release of calcium ions into the sarcoplasm from the terminal cisternae of both sarcoplasmic reticulums (SR) of the triads This is therefore why it is critical for the T tubule to run through the terminal cisternae of two sarcoplasmic reticulums (SR) since we want to maximize calcium release into the sarcoplasm
Advantage of Direct Attachment of Skeletal muscles:
To provide physical protection to vital organs in the body cavities
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 8
With the tropomyosin blockade ended, activated myosin globular heads on the A bands (thick filaments) bind to the accessible myosin-binding sites on actin myofilaments (thin filaments) to form cross bridges (Step 1 in cross bridge cycle)
Attachment sites for each skeletal muscle:
Skeletal muscles span/cross over joints because when a skeletal muscle contracts it's going to move the joint thereby causing movement and thus skeletal muscles must have at least 2 attachment sites known as the ORIGIN and the INSERTION
Troponin I (TnI)
inhibitory subunit that binds to actin
1) Size of motor units activated
larger motor units generate more force/tension in a contracting skeletal muscle than smaller motor units This is because, if we remember, large size motor units consist of more skeletal muscle fibers (cells), hence there will be more sarcomeres to shorten in length (contract), hence there will be a stronger skeletal muscle contraction resulting in more force/tension being generated by the contracted skeletal muscle to get more work done than a smaller sized motor unit
M line
line that bisects the H zone and anchors the A bands
H zone
middle region of the A band not overlapping with the thin filaments
Z discs (Z lines)
anchor the thin filaments in myofibril
Troponin C (TnC)
binds calcium ions (Ca2+) when calcium is present
Troponin T (TnT)
binds to tropomyosin
Motor units come in different sizes based on the number of skeletal muscle fibers innervated by a motor neuron via its axon terminals:
small motor units medium-sized motor units large motor units
Sarcoplasmic Reticulum (SR)
Specialized smooth endoplasmic reticulum that stores/releases calcium into the sarcoplasm The expanded ends of SR are called TERMINAL CISTERNAE
The 3 major types of skeletal Muscle Fiber types are based on 2 criteria:
Speed of Contraction Major Pathway for ATP production
I bands
regions of the thin filaments not overlapping with the A band attached to Z disc
Major Pathway for ATP production: Aerobic Respiration
Oxidative muscle fiber types are muscle fiber types that quicky convert anaerobic respiration into aerobic respiration and thus mainly use AEROBIC respiration for ATP production Remember, aerobic respiration is very efficient
The Advantages of Indirect Attachment of Skeletal Muscles
1) A tendon occupies a smaller bone surface (efficient packaging) so the 650 skeletal muscles can attach to the 206 bones in the body 2) Several skeletal muscles binding to the same bones and thus having access to the same joint allows for interrelationships in the functions of skeletal muscles to stabilize the joint, such as synergistic muscles and antagonistic muscles 3) Protects skeletal muscles ("flesh") from direct contact with the rough bone surface which can tear the skeletal muscles 4) Allows for long bones to act as levers for movements when skeletal muscles that cross over joints contract.
Structural differences between Smooth muscle and Skeletal muscle
1) Smooth muscle lacks striations because the thick filaments and thin filaments are not arranged in an alternating pattern but rather, diagonally down the smooth muscle cell 2) Smooth muscle cells therefore lack sarcomeres because they lack Z lines since a sarcomere is the distance between two Z discs 3) Since smooth muscle cells lack Z lines, thin filaments are anchored by proteins called DENSE BODIES 4) Thin filaments in smooth muscle cells lack troponin which means the thin filament is composed of 2 types of proteins: actin and tropomyosin Tropomyosin does not block the myosin binding sites on actin even when smooth muscle is relaxed Since thin filaments in smooth muscle lack troponin (specifically troponin C), calcium ions bind to a regulatory protein called CALMODULIN 5) The sarcolemma of smooth muscle cell lacks the deep invaginations called transverse (T) tubules; thus the sarcolemma has shallow cavities called CAVEOLAE that contain extracellular fluid rich in calcium The sarcolemma also has calcium voltage-gated channels 6) The Sarcoplasmic reticulum (SR) in smooth muscle cell is poorly developed and the SR does not have the expanded ends called terminal cisternae In smooth muscle cell since both T-tubules and terminal cisternae are absent, Triads are also absent 7) Smooth muscle lacks neuromuscular junctions and instead contain diffuse junctions
Myoglobin
A red pigment that binds, stores and provides oxygen
Tropomyosin
A rod-shaped regulatory protein that spirals around the actin and blocks myosin binding sites on actin in a relaxed skeletal muscle
Chemicals
Chemicals such as hormones stimulate smooth muscle to contract Occurs in both single-unit and multiunit smooth muscle
Triad
Composed of a transverse tubule in between 2 terminal cisternae of two SR: Terminal cisterna-Ttubule-Terminal cisterna Function: - For the release of calcium ions into the sarcoplasm when the sarcolemma depolarizes during stimulus
2 Types of Smooth Muscle: Single-Unit Smooth Muscle
Composed of circular and longitudinal smooth muscle layers in the walls of organs Innervated by autonomic nerve fibers Electrically coupled by gap junctions allow all the smooth muscle cells of a single-unit smooth muscle (the organ) to depolarize in response to an action potential at the same time hence, in single-unit smooth muscle (the organ) all the smooth muscle cells contract at the same time which is known as FUNCTIONAL SYNCYTIUM These smooth muscles are either contracted or they aren't thanks to the electrically coupled gap junctions Stimulated to contract by chemicals **Exhibit pacemaker activity**
2 Types of Smooth Muscle: Multiunit Smooth Muscle
Composed of individual smooth muscle fibers Lacks gap junctions, thus smooth muscle cells contract independently and therefore not together as a unit Innervated by the autonomic nerve fibers Stimulated to contract by chemicals No pacemaker activity Examples: · Erector pili muscle · The muscles surrounding and controlling the pupil size
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 8
Cross bridge detachment occurs when the enzyme called myosin light chain phosphatase (MLCP) removes the inorganic phosphate (Pi) from the myosin globular heads (intracellular calcium levels fall during this time too)
Sarcoplasm
Cytoplasm of the muscle fiber
The Myofilaments
Each myofibril contains smaller structures called MYOFILAMENTS that are then separated into 2 main types: - Thick filaments - Thin filaments
ENDOMYSIUM
Each skeletal muscle fiber (skeletal muscle cell) is wrapped in a delicate connective tissue membrane called ENDOMYSIUM
True or False Question: Cardiac muscle cells lack triads because they lack both T tubules and terminal cisternae
False, cardiac muscle cells lack triad because although they have T tubules, their sarcoplasmic reticulum (SR) is poorly developed and thus lacks terminal cisternae
True or False Question: In smooth muscle cells, the power stroke occurs when both ADP and Pi dissociate from the myosin globular head
False, the is what occurs in skeletal muscle contraction ONLY!!! In smooth muscle, the power stroke occurs when Pi alone, ADP was never bound to the myosin globular head, is still attached and the thin filaments slide DOWN the smooth muscle cell causing the shortening of the smooth muscle (i.e. smooth muscle contraction)
True or False Question: the thin filaments in smooth muscle cells are composed of actin, tropomyosin and troponin
False, thin filaments in smooth muscle cells are composed actin and tropomyosin
True or False Question: Cross bridge detachment in smooth muscle occurs when a new ATP binds to the myosin globular head of the A band
False, this is what occurs in skeletal muscle contraction ONLY!!! In smooth muscle, cross bridge detachment occurs when the enzyme called myosin light chain phosphatase (MLCP) removes the inorganic phosphate (Pi) from the myosin globular heads
True or False Question: Smooth muscle and cardiac muscle are both under involuntary control and both grow by hyperplasia and hypertrophy
False, yes both smooth muscle and cardiac muscle are under involuntary control, but only smooth muscle can grow by hyperplasia and hypertrophy whereas cardiac muscle only grows by hypertrophy
2 Main Categories of Skeletal Muscle Contraction:
Isometric Contraction Isotonic Contraction
Isometric Contraction
Force/tension generated by the skeletal muscle contraction is increasing while at a constant skeletal muscle length ("isometric" = same length, i.e. muscle length remains the same) Occurs when the weight of an object exceeds the force of contraction generated by the muscle An example would be trying to pick up a box that is too heavy This type of contraction is not explained by the excitation-contraction coupling mechanism (sliding filament mechanism) because as we remember that mechanism only describes the shortening length of skeletal muscle (contraction) which we do not see occurring here since the skeletal muscle length remains the same Instead, isometric contraction is better explained by recruitment since at some point you need to bring in the larger and larger motor units to cause stronger and stronger force of contraction so as to be able to move the weight, but as mentioned before if youre trying to lift something too heavy youll never generate enough force of contraction to overcome that weight, however recruitment is still occurring nonetheless
Comparison of the 3 types of Muscle: Calcium binds to tropomyosin C (TnC)
Found in both skeletal muscle and cardiac muscle only
Comparison of the 3 types of Muscle: Involuntary control of muscle contraction
Found in both smooth muscle and cardiac muscle
Comparison of the 3 types of Muscle: Voluntary control of muscle contraction
Found in skeletal muscle only
Comparison of the 3 types of Muscle: Functional syncytium
Found only in single-unit smooth muscle and cardiac muscle
Comparison of the 3 types of Muscle: Pacemaker activity
Found only in single-unit smooth muscle and cardiac muscle
Comparison of the 3 types of Muscle: Presence of gap junctions
Found only in single-unit smooth muscle and cardiac muscle
Comparison of the 3 types of Muscle: Presence of T-tubules
Found only in skeletal muscle and cardiac muscle
Comparison of the 3 types of Muscle: Sarcomere's present
Found only in skeletal muscle and cardiac muscle
Comparison of the 3 types of Muscle: Striation's present
Found only in skeletal muscle and cardiac muscle
Convergent Muscles
In a convergent muscle, the muscle fibers are spread over a broad area, but all the fibers converge at one common attachment site. Example: Pectoralis major
Parallel Muscles
In a parallel muscle, the fascicles are parallel to the long axis of the muscle. Most of the skeletal muscles in the body are parallel muscles. Example: Biceps brachii muscle
Pennate Muscles
In a pennate muscle, the fascicles form a common angle with the tendon. Example: Deltoid
Autonomic Nerve Fibers (Autonomic nervous system)
Innervating autonomic nerve fibers are the bulbous swellings of the autonomic fibers called VARICOSITIES that form junctions (synaptic clefts) with smooth muscle called DIFFUSE JUNCTIONS and release neurotransmitters into the diffuse junctions. You will have SEVERAL varicosities forming SEVERAL diffuse junctions with ONE smooth muscle cell When the autonomic nerve fibers (varicosities) are activated, depending on the neurotransmitter released, it may stimulate a smooth muscle to contract OR a contracted smooth muscle to relax For example, if the neurotransmitter acetylcholine is released, single-unit smooth muscle in the wall of the bronchioles (tube-like structures that take air in/out of the lungs) contracts causing bronchoconstriction Another example is if the neurotransmitter norepinephrine is released, single-unit smooth muscle in the wall of the bronchioles relaxes causing bronchodilation
Transverse Tubules (T Tubules)
Involutions/invaginations of the sarcolemma into the sarcoplasm
Excitation-Contraction coupling of cardiac muscle:
Like single-unit smooth muscle Stimulated to contract by: 1) Pacemaker activity by sinoatrial (SA) node in the right atrium (Just like in single-unit smooth muscle) 2) Chemicals such as hormones can stimulate contraction (Just like in all smooth muscle) 3) Autonomic nervous system (Just like in all smooth muscle)
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 1
Motor neuron is activated in CNS voluntarily since as we remember, skeletal muscles are under voluntary control
How do the myosin globular heads become activated in the first place?
Myosin globular heads contain the enzyme ATPase Thus, when the ATP attached to the myosin globular head in the relaxed state is split into ADP + Pi via hydrolyzation by the ATPase enzyme, this results in the myosin head being energized/activated (cocked into the high-energy conformation) and ready to bind to accessible myosin-binding sites on actin myofilaments (Step 4 in the cross bridge cycle) ONLY activated myosin globular heads can bind to the accessible myosin binding sites on actin myofilaments to form cross bridges
Origin and Insertion Example: Brachioradialis
Origin (O): lateral supracondylar ridge at distal end of the humerus Insertion (I): base of styloid process of radius (the forearm) Thus, the brachioradialis is a muscle of the forearm that flexes the forearm at the elbow
Smooth Muscle Tissue Basic Information:
No striations Spindle-shaped cells Uninucleate cells It is an organ
Patterns of Arrangement of Fascicles in skeletal Muscles
Once again, muscle fibers in a skeletal muscle form bundles called fascicles. The muscle fibers in a single fascicle are parallel, but the organization/shape/arrangement of fascicles in the skeletal muscle can vary, as can the relationship between the fascicles and the associated tendon. Thus, one way of naming skeletal muscle fibers is to look at the organization/shape/arrangement of the fascicles The different patterns of fascicle organization/shape/arrangement form what are known as: - parallel muscles - convergent muscles - pennate muscles - circular muscles
RECRUITMENT
Recruitment refers the order in which motor units are activated in a skeletal muscle to cause that skeletal muscle to contract Smaller motor units are activated first followed by medium-sized motor units and then the large sized motor units The reason the motor units are activated from the smallest to the largest is because the small sized motor units tend to be fatigue resistant and as the motor units get larger, they become more fatigable Thus, if youre picking up a pen, you wouldn't be generating very much force/tension by the contracting skeletal muscles and thus only the small motor units may be activated If youre lifting a crate of books on the other hand you would probably be generating a pretty decent amount of force by the contracting skeletal muscles and thus potentially all the small and up to some of the large motor units could be activated
Smooth muscle as an organ consists of (The reason smooth muscle is an organ)
Smooth muscle tissue surrounded by endomysium, infiltrated with blood vessels and autonomic nerve fibers.
3) Fast Glycolytic Muscle Fibers (also known as white fibers) Speed of contraction Activity of Myosin ATPase Pathway for ATP synthesis Myoglobin content Color Number of Mitochondria Number of Capillaries Fiber Diameter Recruitment order Glycogen content Rate of fatigue Activities suited for
Speed of contraction - Fast, since it contains the fast ATPase Activity of Myosin ATPase - Fast ATPase Pathway for ATP synthesis - Anaerobic respiration Myoglobin content - Low, since it mainly uses anaerobic respiration and therefore doesn't need as much oxygen Color - White (white fibers) Number of Mitochondria - Few, since anaerobic respiration is mainly taking place Number of Capillaries - Few, since you don't need constant supply of oxygen and glucose, since it stores enough glucose anyway Fiber Diameter - Large, since you don't need ample diffusion of oxygen and glucose, but they also contain more myofilaments, more sarcomeres and are thus very strong fibers Recruitment order - Third, and thus they belong to the large motor units Glycogen content - High, since you need a lot of stored glucose to supply the anaerobic respiration taking place Rate of fatigue - Fatigable, since you are mainly using anaerobic respiration and thus can have build up of lactic acid that will cause muscle fatigue Activities suited for - Intense but short lived (weightlifting or throwing a javelin)
2) Fast Oxidative Muscle Fibers (also known as intermediate fibers) Speed of contraction Activity of Myosin ATPase Pathway for ATP synthesis Myoglobin content Color Number of Mitochondria Number of Capillaries Fiber Diameter Recruitment order Glycogen content Rate of fatigue Activities suited for
Speed of contraction - Fast, since it contains the fast ATPase Activity of Myosin ATPase - Fast ATPase Pathway for ATP synthesis - Mainly Aerobic respiration Myoglobin content - Intermediate Color - Pinkish Red Number of Mitochondria - Many but not as many as slow oxidative fibers, since aerobic respiration is mainly taking place Number of Capillaries - Many but not as many as slow oxidative fibers, since you need to constantly supply oxygen and glucose to perform aerobic respiration Fiber Diameter - Intermediate, since you need an intermediate amount of oxygen and glucose to diffuse into the fiber, but they also contain an intermediate amount of myofilaments, an intermediate amount of sarcomeres and are thus intermediately strong fibers Recruitment order - Second, and thus belong to the medium motor units Glycogen content - Intermediate (store an intermediate amount of glucose) Rate of fatigue - Intermediate Activities suited for - Intermediate (jogging)
1) Slow Oxidative Muscle Fibers (also known as red fibers): Speed of contraction Activity of Myosin ATPase Pathway for ATP synthesis Myoglobin content Color Number of Mitochondria Number of Capillaries Fiber Diameter Recruitment order Glycogen content Rate of fatigue Activities suited for
Speed of contraction - Slow, since it contains the slow ATPase Activity of Myosin ATPase - Slow ATPase Pathway for ATP synthesis - Aerobic respiration Myoglobin content - High, since it is very red Color - Red (red fibers) due to having the highest content of myoglobin Number of Mitochondria - Many, since aerobic respiration is mainly taking place Number of Capillaries - Many, since you need to constantly supply oxygen and glucose to perform aerobic respiration Fiber Diameter - Small, since you need the most amount of oxygen and glucose to diffuse into the fiber, but they also contain the least amount of myofilaments, the least amount of sarcomeres and are thus the weakest fibers Recruitment order - First, and thus belong to the small motor units Glycogen content - Low, since it doesn't have a need to store glucose since it is always being delivered by the high number of capillaries Rate of fatigue - Fatigue Resistant, since it is mainly using aerobic respiration and so lactic acid build up that would lead to muscle fatigue will not occur Activities suited for - Endurance/Stamina type exercise (marathon)
2) Number of motor units activated
Strength of skeletal muscle contraction and thus force/tension generated by a contracting skeletal muscle increases as the number of motor units activated increases allowing increasing amount of work to get done Therefore, RECRUITMENT is taking place
3) Frequency of skeletal muscle activation
Strength of skeletal muscle contraction and thus force/tension generated by the contracting skeletal muscle increases as the rate of skeletal muscle stimulation increases allowing increasing amount of work to get done The reason for this is because we know that with each activation of the skeletal muscle at the motor end plate, we are going to generate an action potential at the sarcolemma of the skeletal muscle fiber that will enter into the T tubules of the triads to cause calcium to be released into the sarcoplasm Now, if we increase the frequency of skeletal muscle activation then we are going to reach a point where you are not allowing the skeletal muscle to rest enough for all of the calcium in the sarcoplasm to be sequestered into the Sarcoplasmic reticulum (SR) before the next action potential enters the T tubules of the triads Thus, since you have more calcium in the sarcoplasm you are ending more tropomyosin blockades thereby exposing more myosin binding sites, which causes more myosin globular heads to bind, leading to more sliding of thin filaments occurring which of course translates into more shortening of the sarcomeres, more shortening of the myofibrils, more shortening of the skeletal muscle fibers (cells) and hence more shortening (contracting) of the skeletal muscle (the organ) (Thin Filament Sliding Mechanism of Muscle Contraction) and thus increased force/tension generated and more work getting done
Skeletal Muscle Tissue Basic Information:
Striations long, cylindrical cells called muscle fibers multinucleate cells It is an organ
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 5
The Calcium-Calmodulin complex activates an enzyme known as myosin light chain kinase (MLCK)
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 3
The entering calcium induces the release of more calcium from the poorly developed sarcoplasmic reticulum (SR) This process is known as the CALCIUM-INDUCED CALCIUM RELEASE MECHANISM. You need both sources of calcium to have enough intracellular calcium to start smooth muscle contraction Thus, calcium channel blockers can block calcium channels and prevent entry of extracellular calcium thereby inhibiting smooth muscle contraction
Circular Muscles
The fascicles are arranged in concentric rings They surround external body openings. Example: Orbicularis oris
Isotonic Contraction
The force/tension generated by the skeletal muscle contraction remains constant as the skeletal muscle length changes (isotonic = same force) Can be broken down into either a Concentric or Eccentric Contraction
Exam Question: What is the purpose of increase in intracellular calcium in skeletal muscle cells?
The purpose of intracellular calcium increase in skeletal muscle cells is to end the tropomyosin blockades of the myosin binding sites by binding to troponin C to cause a conformational change of the troponin thereby eliminating the tropomyosin blockades of the myosin binding sites
Exam Question: What is the purpose of increase in intracellular calcium in smooth muscle cells?
The purpose of intracellular calcium increase in smooth muscle cells is to bind to cadmodulin to form the calcium-cadmodulin complex that then activates MLCK which then functions to phosphorylate and subsequently activate the myosin globular heads of the A bands (thick filaments)
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 2
The sarcolemma of the smooth muscle cell contains voltage gated calcium channels which the action potential (signal) will cause to open thereby allowing calcium from the calcium rich extracellular fluid in the caveolae to enter into the smooth muscle cell
Speed of Contraction
The speed at which Myosin ATPase hydrolyzes ATP to activate the myosin globular heads There are 2 types: slow ATPase in slow fibers fast ATPase in fast fibers
Factors that affect the strength/force of skeletal muscle contraction:
The strength of contraction a skeletal muscle can undergo translates into how much force/tension can be generated by that contracting skeletal muscle and thus translates into how much work can be done by said skeletal muscle 1) Size of motor units activated 2) Number of motor units activated 3) Frequency of skeletal muscle activation 4) The length of the sarcomeres prior to skeletal muscle contraction
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 3
The vesicles fuse with the membrane of the axon terminal and via exocytosis release the neurotransmitter, ACETYLCHOLINE (Ach) into the neuromuscular cleft
Cardiac Muscle Cells Compared to Skeletal and Smooth Muscle Cells:
Uninucleate (similar to smooth muscle cells) Striated because presence of sarcomeres that therefore means the thick and thin filaments are arranged in an alternating fashion (like skeletal muscle) Transverse tubules (T tubules) present in the sarcolemma but triads are absent because cardiac muscle cells contain poorly developed Sarcoplasmic reticulum (SR) without terminal cisternae (like smooth muscle) Highly branched cells with membrane junctions called INTERCALATED DISCS which contain GAP JUNCTIONS and DESMOSOMES Cardiac cells are thus electrically coupled due to the presence of Gap junctions and the cardiac muscle contracts as a single unit, therefore it exhibits functional syncytium (like single-unit smooth muscle) Desmosomes are the anchoring junctions which will prevent cardiac muscle cells from separating, even after pressure, since you are going to generate a lot of pressure within the ventricle chambers Thin filament is composed of 3 types of proteins - actin, troponin, tropomyosin Tropomyosin blocks the myosin binding sites on action in a relaxed cardiac muscle (like skeletal muscle) During the excitation-contraction coupling of cardiac muscle, calcium binds to tropomyosin C (TnC), to end tropomyosin blockade on actin (just like in skeletal muscle) Cardiac muscle cells contain voltage gated calcium channels which will allow for extracellular calcium entry inducing calcium release from the poorly developed sarcoplasmic reticulum (SR) and thus perform the CALCIUM INDUCED CALCIUM RELEASE MECHANISM during the excitation-contraction coupling of cardiac muscle (just like in smooth muscle) This also means that calcium channel blockers can inhibit cardiac muscle contraction since they can prevent extracellular calcium entry which means you won't have enough calcium release from the SR and thus not enough intracellular calcium to start the mechanics of cardiac muscle contraction (just like in smooth muscle) Its an Organ
Sequence of Events in the Excitation-Contraction Coupling in Smooth Muscle: Step 7
Upon attachment, cross bridges form, and without the inorganic phosphate (Pi) being dissociated (it does not dissociate!!), the thin filaments will slide DOWN the smooth muscle cell (the power stroke) causing the shortening of the smooth muscle (i.e. smooth muscle contraction)
Comparison of the 3 types of Muscle: Calcium-induced calcium release mechanism
Used by both smooth muscle and cardiac muscle hence, calcium channel blockers inhibit contraction of smooth muscle and cardiac muscle. Calcium channel blockers do not inhibit skeletal muscle contraction
Sources of ATP to support skeletal muscle contraction: Stored ATP
Used first (supports 5 seconds of skeletal muscle activities)
Excitation-Contraction Coupling:
When MOTOR NEURONS that innervate skeletal muscles are activated, they stimulate the skeletal muscles to contract Motor neurons conduct impulses to skeletal muscles. Motor neuron makes contact with skeletal muscle fibers via its AXON TERMINALS Each axon terminal innervates one skeletal muscle fiber in the skeletal muscle to form the NEUROMUSCULAR JUNCTION A motor neuron and all the skeletal muscle fibers it innervates in a skeletal muscle via its axon terminals is called a MOTOR UNIT Therefore, if there were 10 motor neurons innervating a skeletal muscle, there will then be 10 different motor units within that skeletal muscle
Sequence of events in Excitation-Contraction Coupling Mechanism (sliding filament mechanism): Step 9
When the ADP and Pi dissociate (release) from the cross bridges, the attached myosin globular heads undergo a change of orientation termed the POWERSTROKE (Step 2 in the cross bridge cycle) This power stroke is when the myosin globular head changes from a right angle to a bent (oblique) position pulling the thin filament inward into the H zone and towards the M line resulting in sarcomere length shortening/decreasing (the distance between the Z discs shortens) which causes shortening of the myofibrils that then results in shortening of the skeletal muscle fibers (cells) and thus the length of the skeletal muscle (the organ) shortens (i.e. skeletal muscle contraction) (The Sliding Filament Mechanism of Muscle Contraction) The excitation-contraction coupling mechanism (sliding filament mechanism) also answers the previous question as to why/how the sliding of thin filaments occurs!
Indirect Attachment
Where the epimysium (outer connective tissue layer of the skeletal muscle) extends to form a TENDON which then anchors the skeletal muscle to the periosteum surrounding the bone Most skeletal muscles in the human body are attached indirectly to form the insertion site or origin site
Direct Attachment
Where the epimysium (outer connective tissue layer of the skeletal muscle) fuses directly to the periosteum surrounding the bone In other words, where the connective tissue of the skeletal muscles attach/fuses directly to the bone
