Chapter 9- Muscles and muscle tissue Flashcards
Muscle fiber
Individual muscle cell that makes up a single muscle
Tension
Force exerted by a muscle when it contracts
Load
Opposing force exerted on a muscle by the weight of an object
Mys-, myo-, and sarco-
Muscle
Muscle functions (4)
- Movement- involuntary or voluntary (skeletal muscle)
- Body posture and body position
- Joint stability
- Maintaining body temperature
How does muscle maintain body posture/body position?
Muscles work to hold us up against gravity. This is mainly the responsibility of skeletal muscle. The most important regions are the muscles in the neck and the trunk muscles (abdominals, obliques, back muscles).
How do muscles contribute to joint stability?
Mostly skeletal muscle. Muscles and tendons reinforce joints, tendons wrap around a joint to brace it
How do muscles maintain body temperature?
Mostly the responsibility of skeletal muscle. Skeletal muscle involuntarily contracts (shivering) when blood temperature drops- muscle contraction produces heat
Muscle characteristics (4)
- Excitability
- Contractility
- Extensibility
- Elasticity
Excitability
Membrane potential changes in response to stimulus. Action potential changes the membrane potential of a muscle cell
Contractility
Muscle cells shorten and begin to bulge
Extensibility
Muscle cells can lengthen/stretch. Allows the muscle to stretch without snapping and breaking
Elasticity
Healthy muscle cells return to their original shape (resting length)
Types of muscle tissue (3)
- Skeletal muscle
- Smooth muscle
- Cardiac muscle
Which muscle is the strongest in the body?
The masseter muscle (jaw) is probably the strongest- generates 200 pounds of force at the molars
How is skeletal muscle adaptable?
You can decide how much force you want to generate with a muscle
Skeletal muscle tissue
Voluntary muscle tissue, purpose is movement of body parts- attaches to and uses skeleton. Striated and multinucleate. Creates the most force but needs the most rest- can only use it for a certain period of time until it fatigues and you have to rest
Smooth muscle tissue
Involuntary muscle tissue, purpose is to move fluid and substances through the body. No striations (stripes), relatively consistent color, uninucleate.
Where is smooth muscle tissue found?
Found in urinary organs, reproductive organs, blood vessels, etc
Cardiac muscle tissue
Involuntary muscle tissue that moves blood through the body. Striated and uninucleate- one fiber has one nucleus. Only found in the heart
What determines the rate of contraction of cardiac muscle tissue?
Rate of contraction set by pacemaker cells. Pacemaker cells depolarize spontaneously- do not need an action potential, they will depolarize randomly
Innervation of skeletal muscle tissue
Each muscle receives 1 nerve (motor). Motor neurons stimulate muscle fibers to contract. Some motor neurons innervate less than 10 fibers, others can serve hundreds of fibers
Nerve branches several times to supply muscle fibers in one muscle. One nerve can serve about 150 muscle fibers
What neurotransmitter are we most concerned with for skeletal muscle tissue?
We are only concerned with acetylcholine, which has a stimulatory effect (the muscle contracts).
Vascularization of skeletal muscle tissue
Each muscle receives 1 artery and 1 vein. Function- brings in nutrients, removes waste (from ATP production, lactic acid). Muscles get a lot of blood- need lots of nutrients to produce ATP
Function of skeletal muscle connective tissue sheaths
Function- supports muscle, holds muscle together. Fibrous and tough in nature. Supports fibers to keep them from ripping if they contract too much
Layers of skeletal muscle connective tissue sheaths (3)
- Endomysium- inner
- Perimysium
- Epimysium- outer
All 3 layers are continuous with each other and with the tendon attaching the muscle to the bone. They are all made of the same kind of connective tissue.
Endomysium
Innermost connective tissue layer. Surrounds individual muscle fibers- myofibrils make up muscle fibers
Perimysium
Middle layer of connective tissue. Discrete bundles of muscle fibers grouped together, form fascicles
Epimysium
Outermost layer of connective tissue. Surrounds entire muscle, composed of bundles of fascicles
Direct skeletal muscle attachments to bone
Epimysium of muscle fuses directly to bone or cartilage (there are no tendons)
Indirect skeletal muscle attachments to bone
Tendon connects muscle to bone or another muscle
Origin
Where muscle attaches to a less movable bone- this is always proximal. Ex- the origin of the long head of the biceps brachii is the lip of the glenoid fossa, for the short head, it’s the coracoid process. When contracting the biceps, the shoulder (origin) does not move at all.
Insertion
Where muscle attaches to a movable bone, this is always distal. For the biceps brachii, it’s the radial tuberosity
Sarcolemma
Plasma membrane of muscle fibers
Sarcoplasm
C ytoplasm of muscle fibers
Glycosomes
Store large amounts of glycogen- cell can convert to glucose for ATP production
Myoglobin
Red pigment that stores oxygen. Similar to hemoglobin, but oxygen is stored within a muscle fiber. This is beneficial so we have raw materials for ATP production and muscles can produce it quickly.
Myofibrils
Make up bulk of individual muscle fiber. Gives skeletal muscle its striated appearance. Composed of alternating A bands and I bands- go along the length of the entire myofibril
A band
Composed of myofilaments, contains H zone at center- bisected by M line
M line
M line is where the myofilaments are anchored in place
I band
Composed of myofilaments. Contains Z disc at center, only have actin filaments, no myosin
Sarcomere
Formed by A band and I bands- region between two successive Z discs. Importance- the sarcomere is the smallest contractile unit of skeletal muscle tissue. It is the functional unit of muscle tissue- if you take away any other part, muscle contraction would not occur
Myofilaments
Actin and myosin containing portion of sarcomere. Function- actin and myosin interact to allow muscle contraction.
Types of contractile myofilaments (2)
- Thick filament (myosin)
2. Thin filament (actin)
Molecular composition of myosin
Composed of grouping of 6 chains: 4 light chains and 2 heavy chains that twist together to form a tail. Myosin head found at the end of each heavy chain
Importance of myosin
Myosin head uses ATP to link thick and thin filaments during contraction
Actin filaments
Twisting chains of G actin with myosin binding sites. The depression in actin is where myosin binds (active site).
2 regulatory proteins of actin
- Tropomyosin
2. Troponin
Tropomyosin
Arranged along length of thin filament. Importance- block myosin binding sites when the muscle is relaxed
Troponin
Globular protein associated with tropomyosin. Importance- binds tropomyosin to position it on the filament
Importance of troponin and tropomyosin
These proteins prevent unnecessary interactions between the myofilaments
What 2 intracellular structures regulate muscle contraction?
- Sarcoplasmic reticulum
2. T-tubules
Sarcoplasmic reticulum
Smooth endoplasmic reticulum of muscle. Extends over the surface of the myofibrils, thickens twice (terminal cisterns). Highly branched, extends length of myofibril
Terminal cisterns
The two thickened regions of the sarcoplasmic reticulum, formed at the A band and I band junction.
Sarcoplasmic reticulum function
Stores and releases intracellular calcium for muscle relaxation and contraction
T-tubules
Tube that protrudes deep into the cell and runs between terminal cistern- forming triad. Continuous with the sarcolemma
Importance of T-tubules
Increase surface area of muscle fiber- changes in membrane potential penetrate deep in muscle fiber. Nerve impulse runs along sarcolemma- T-tubules “pass” information to deep parts of muscle
What type of neurons stimulate muscle tissue?
Motor neurons
Which neurotransmitter stimulates muscle tissue?
Acetylcholine
Which types of ion channels are important for muscle contraction?
- Voltage gated- T-tubules- respond to change in membrane potential
- Chemically gated- sarcolemma- bind to ACh
Neuromuscular junction
The junction between a motor neuron and muscle fiber. The axon terminals of the neuron contain synaptic vesicles. The synaptic cleft here is filled with gel-like substance. Sarcolemma at cleft is folded to form junctional folds
Why is the sarcolemma folded at the neuromuscular junction?
Folds increase surface area for ACh binding
For stimulation of muscle fiber by motor neurons to occur, what steps must occur? (4)
- Transmission of action potential at neuromuscular junction
- Generation of action potential across sarcolemma
- Excitation- contraction coupling
- Cross-bridge formation and muscle contraction
End plate potential
Opening of chemically gated ion channels on sarcolemma creates end plate potential (EPP). An EPP is a specific graded potential, only in muscle cells that is always excitatory. EPP depolarizes sarcolemma. If strong enough, an action potential is generated.
When can cross bridge formation begin?
When calcium enters the cytosol
Sliding filament model of muscle contraction
During contraction, thin filaments slide past thick filaments. Myosin heads of thick filament form cross bridges with actin on thin filament. Cross bridges form and break multiple times during contraction- myosin heads “slides” thin filament
What happens during the sliding filament model of muscle contraction? (4)
- I bands shorten
- Z discs come closer together
- H zone disappears
- A bands move closer together
Motor neurons in motor nerve branch to form
Neuromuscular junctions with a single muscle fiber. Every muscle fiber will get the exact same message
How many fibers one neuron innervates determines
How fine tuned the movement will be. Generally, the larger the number of fibers innervated by a neuron, the more coarse the movement will be. Less fibers are innervated by a neuron in the fingers- require small precise movements
Motor unit
A single motor neuron and all the muscle fibers it supplies. When the motor neuron fires, all fibers it innervates will contract. Fibers are not clumped together. Also, the number of muscle fibers a single motor neuron innervates influences movement.
Muscle twitch
Response of a muscle to a single stimulus where the muscle fibers quickly contract, then relax. Fast contraction, then a period of relaxation
How is muscle twitch measured?
By a myogram
3 phases of a myogram
- Latent period
- Period of contraction
- Period of relaxation
What occurs during the latent period of a myogram?
This is the first few milliseconds following stimulation. Excitation-contraction coupling occurs, but no tension observable in muscle. This is because tension comes from the sliding of the filaments
Tension in a muscle comes from
The sliding of muscle filaments
What occurs during the period of contraction in a myogram?
Active cross-bridge formation with building muscle tension. Maximum tension is generated by the end of this period.
What occurs during the period of relaxation in a myogram?
Cross bridge formation declines (filaments aren’t sliding), muscle tension declines to resting value
Graded muscle contractions
Smooth muscle contractions that vary in strength when different demands are placed on them. Ensures the muscle contraction is appropriate.
2 ways that graded muscle contractions can be graded
- Temporal summation
2. Recruitment/motor unit summation
Temporal summation
Increasing the frequency of stimulation of a muscle- increases the firing rate of a motor neuron so it can generate more force at the muscle
Recruitment/motor unit summation
Increasing the strength of stimulation of a muscle. Increasing the voltage of the stimulus causes more muscle fibers to contract- increases voltage contraction force
How does temporal summation happen?
Fires the stimulus in rapid succession- the second twitch hits the muscle before the first twitch has ended. Effect- muscle tension increases
Unfused (incomplete) tetanus
Rate of stimulation creates a sustained and quivering muscle contraction
Fused (complete) tetanus
Rate of stimulation creates smooth, sustained muscle contraction. No relaxation occurs- no time between action potentials- calcium doesn’t enter sarcoplasmic reticulum. Could end up damaging the muscle fiber
How does recruitment/motor unit summation happen?
Increase the number of motor units used for contraction, size principle of motor unit summation. Motor units are recruited asynchronously- some contracting, others relaxing
Size principle of motor unit summation
Motor units with smallest muscle fibers recruited first, motor units with largest muscle fibers recruited last- create most powerful contractions
Muscle tone
Only occurs in skeletal muscle tissue. This is when relaxed muscles are always slightly contracted due to spinal reflexes. The spinal cord is constantly sending impulses to the muscle- in this situation, the filaments aren’t sliding and the muscle isn’t doing any work. Does not produce movement
Why is muscle tone important?
Muscle tone keeps muscle tissue healthy and responsive, stabilizes joints, maintains posture
What is caused by a loss of muscle tone?
Loss of muscle tone leads to loss of responsiveness- muscle will not respond to stimuli. In individuals with hemiplegia, muscles on one side of the body have completely lost tone- can be caused by stroke.
Isotonic contraction
Muscle tension develops to overcome the load, muscle shortening occurs. Two subtypes,
2 subtypes of isotonic contraction
- Concentric contraction
2. Eccentric contraction
Concentric contraction
Muscle shortens and does work. Ex- upward motion of a bicep curl
Eccentric contraction
Muscle generates force as it lengthens- the sarcomere is lengthening, not shortening, cross bridges are still being formed. Ex- downward motion of a bicep curl. If this motion was not possible, you would have to drop the dumbbell straight down. This would damage the elbow, bones and ligaments
Isometric contraction
Tension develops in a muscle, but the length of the muscle does not change. Cross-bridge formation still occurs, but the thin filaments do not slide. Occurs when the load is not moved- when something is much too heavy.
What energy source does skeletal muscle use for contractile activity?
ATP is the only energy source used directly for contractile activity
How does skeletal muscle store ATP?
Skeletal muscle stores very little ATP. It’s stored in glycogen- can be converted to glucose and then ATP. ATP is regenerated as fast as it is used.
3 pathways used to regenerate ATP
- Direct phosphorylation
- Anaerobic pathways
- Aerobic pathways
Direct phosphorylation
Creates ATP from ADP and Pi using creatine phosphate (CP). This reaction is reversible, creatine phosphate is restored during rest. Does not require oxygen, supplies about 15 seconds worth of ATP.
What enzyme catalyzes direct phosphorylation?
Creatine kinase
What are the products of glycolysis?
Glucose broken down to form 2 ATP and lactic acid. Lactic acid is not used by muscle- diffuses into blood vessels
Glycolysis
Anaerobic pathway to form ATP from glucose. Does not require oxygen. This is a fast method to create ATP- faster than aerobic pathways.
Drawbacks of glycolysis (2)
- A lot of glucose used for low ATP yield
2. Lactic acid build up results in DOMS
DOMS
Delayed onset muscle soreness, results from lactic acid buildup in muscles from glycolysis.
Cellular respiration
Creates 95% of ATP used by muscle during rest and light to moderate long term exercise. Uses pyruvic acid that is produced in an anaerobic pathway, also requires oxygen and mitochondria. Produces 32 ATP, H2O, and CO2
Drawbacks of cellular respiration (3)
- Slow process- used for long term/endurance activities
- Requires constant oxygen and a lot of glucose- fast heart rate helps circulate blood to transport oxygen
- Sometimes the heart can’t beat fast enough, so the muscle cell will revert back to glycolysis
Muscle fatigue
Occurs when the muscle is physiologically incapable of contracting- it’s starting to run out of ATP. The muscle cells will die if you have absolutely no ATP- ATP is needed not just for exercise, but for normal metabolic functions
Function of muscle fatigue
Muscle fatigue prevents complete depletion of ATP in muscle
Rate and duration of muscle fatigue depends on
The activity. For example, low intensity exercise of prolonged duration. With marathon running, it takes longer for the muscles to fatigue, but also takes longer for the muscles to recover.
Excess post exercise oxygen consumption (EPOC)
EPOC is used to completely restore muscle function. It is “paying back” an oxygen debt- the amount of oxygen required post exercise to return muscle to pre-exercise state
EPOC involves (4)
- Restoring oxygen reserves
- Accumulated lactic acid is converted to pyruvic acid- lactic acid on its own is useless for the cell
- Glycogen stores replenished
- ATP and CP stores replenished
Force of muscle contraction is determined by
The number of cross bridges formed between myosin and actin filaments
Factors that influence force of muscle contraction (4)
- Frequency of stimulation- temporal summation
- Number of muscle fibers recruited- motor unit summation
- Size of muscle fiber
- Degree of muscle stretch
How does bulkier muscle influence force of contraction?
Bulkier muscle generates more tension and creates more force- has larger motor units
Hypertrophy
Increased size of muscle fibers in muscle to increase force generated, occurs through weight lifting and resistance training.
What is the rate of hypertrophy dependent on (3)?
- Genetics- some people are able to put on muscle very easily due to genetics
- Sex- individuals with more testosterone are more prone to hypertrophy
- Nutrition- proteins and carbohydrates are macronutrients that matter most when building muscle
How does degree of muscle influence force of muscle contraction?
Force a muscle creates varies with how much it is stretched. Length-tension relationship
Muscle fiber type can influence (3)
- Speed of contraction
- Load and recruitment
- Pathway of ATP production- fibers vary in how they prefer to produce ATP
Speed of muscle contraction is dependent on (2)
- How fast ATP is split- how fast cross bridges can form and break
- Electrical activity of motor neurons, fast neurons= fast contraction
How does the size of the load affect the speed of muscle contraction?
Small loads allow faster contraction
How does the amount of motor unit recruitment influence the speed of contraction?
More motor unit recruitment= faster contraction
What type of pathways do oxidative fibers use?
Oxidative fibers use aerobic pathways
What type of pathways do glycolytic fibers use?
Glycolytic fibers use anaerobic pathways
What type of organs have smooth muscle tissue?
Hollow organs. The heart is the exception
2 layers of smooth muscle tissue
- Longitudinal layer
2. Circular layer
Longitudinal layer of smooth muscle
Muscle fibers run the length of the organ. This is the more superficial layer. When contracted, the organ shortens
Circular layer of smooth muscle
Muscle fibers run the circumference of the organ. This is deep to the longitudinal layer. When contracted, the organ lengthens
How do the layers of smooth muscle contract?
The layers contract asynchronously- one contracts, then the other. They don’t contract at the same time. Like an earthworm- one part of the body contracts and forms a sort of wave motion- part of the organ becomes thinner, the other gets larger
Microanatomy of smooth muscle
Muscle fibers are short, spindle shaped. Covered only by endomysium. Muscle fibers have gap junctions- depolarization spreads from cell to cell. No striations or sarcomeres- there are still thick and thin filaments, but not like skeletal muscle
Varicosities
Bulb like swellings of autonomic fibers scattered over smooth muscle tissue surface. Creates diffuse junctions- wide synaptic clefts that release neurotransmitter to multiple muscle fibers. Smooth muscle doesn’t have neuromuscular junctions like skeletal muscle.
Smooth muscle sarcoplasmic reticulum
Smooth muscle fibers have no T-tubules and less sarcoplasmic reticulum. Sarcoplasmic reticulum releases some calcium and the caveolae of the sarcolemma has calcium channels
Caveolae
Caveolae are depressions in smooth muscle fibers
Where does most calcium for smooth muscle contraction come from?
The extracellular space
How are smooth muscle’s thick and thin filaments different from the ones found in skeletal muscle?
There are less thick filaments than thin filaments. Also, myosin heads are found along entire length of thick filament
In skeletal muscle, myosin heads are only found at one end.
Calmodulin
Protein that acts as a calcium binding site in smooth muscle. Smooth muscle doesn’t have troponin
How are thick and thin filaments arranged in smooth muscle?
Thick and thin filaments arranged diagonally, the filaments spiral down axis of muscle fiber. Effect- when a muscle fiber contracts, it twists- like wringing out a towel
Intermediate filaments- smooth muscle
Criss cross across muscle fiber. Fibers are non-contractile- do not shorten when the rest of the muscle does. Attach to the cell at dense bodies
Intermediate filaments function
Forms a cytoskeleton- transmits pulling force to surrounding muscle tissue. Contributes to hollow organs acting as a single unit- if one cell starts to pull, the others will pull with it.
Smooth muscle contraction steps (5)
- Calcium ions enter the cytosol from the ECF via voltage dependent or voltage independent calcium channels, or from the scant SR
- Calcium binds to and activates calmodulin
- Activated calmodulin activates the myosin light chain kinase enzymes
- The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases
- Activated myosin forms cross bridges with actin of the thin filaments. Shortening begins.
Unitary smooth muscle
Most common- found in all hollow organs except for the heart. The characteristics described in the notes are all characteristics of unitary smooth muscle.
Multi unit smooth muscle
Have no gap junctions or spontaneous depolarization- muscle fibers are structurally independent of the other fibers. They require a direct message from the autonomic nervous system. Forms motor units- can vary contraction and graded contractions with recruitment. Has things in common with both smooth muscle and skeletal muscle
Where is multi unit smooth muscle typically found?
Found in arrector pili, smooth muscle of airways, internal eye muscles
Smooth muscle’s response to a neurotransmitter is dependent on
Receptor molecules on the sarcolemma. Ex- acetylcholine binding in bronchioles is stimulatory- contract norepinephrine binding in bronchioles is inhibitory- relax. Neurotransmitters can be either inhibitory or excitatory.
Influence of hormones/chemical factors on smooth muscle
Some smooth muscle has no innervation and responds only to local chemicals. Others spontaneously depolarize. These hormones/chemicals can enhance or inhibit calcium entry into sarcoplasm
How does smooth muscle respond to stretch?
Smooth muscle responds to stretch by contracting. Importance- increases ability to push substances through organ. For example, the stomach will contract when stretched out by food
Stress-relaxation response
If an organ is filled slowly, it will not contract strongly. If too much food is pushed into the small intestine, the small intestine would rip
Can smooth muscle generate more tension when stretched?
Yes- smooth muscle can stretch more and can generate more tension while stretched. It can still contract when stretched 150% its length
