A&P chapter 11

characteristics of muscle tissue

characteristics of muscle tissue

• excitability

• conductivity

• contractility

• extensibility

• elasticity

The plasma membrane is called the

sarcolemma

The cell’s cytoplasm is referred to as the

sarcoplasm

The sarcoplasm contains

normal organelles but also myofibrils, glycogen, myoglobin, and lots of mitochondria

Myofibrils are long cords made up of

protein

Myofibrils are this of the fiber

functional unit

Glycogen is a polysaccharide made up of

chains of glucose

Glycogen is used during intense exercise and is stored in these

glycosomes

Myoglobin is a protein that

stores oxygen

Myoglobin gives muscles their

red color

The role of the mitochondria in a muscle fiber is to

make ATP by combining sugar and oxygen

The smooth ER that forms a network around each myofibril is called

the sarcoplasmic reticulum

Dilated end-sacs of SR which cross the muscle fiber from one side to the other are called

terminal cisternae

The terminal cisternae stores

calcium

Tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side are called

T (transverse) tubules

A triad is made up of

a T tubule and two associated cisternae

Muscle fibers contain 100’s to 1000’s of

myofibrils

Myofibrils run

parallel to muscle length

Myofibrils make up this much of muscle fiber volume

80%

Myofibrils contain the

sarcomere

Myofibrils are made up of

myofilaments

Myofilaments are made of

contractile proteins (actin and myosin) + regulatory proteins (troponin and tropomyosin)

Thick filaments are made of

myosin

The two heads on myosin contain

actin-binding sites and an ATP-binding site

Thin filaments are mostly made of

actin

A single subunit of actin is called

G (globular) actin

A string of actin is called

F (filamentous) actin

G actin contain

the active sites for myosin heads

Two intertwined F actin filaments make up

the thin filaments

Tropomyosin

stabilizes the actin filaments & covers the binding site between actin and myosin in a relaxed muscle

Troponin

bonds the actin, tropomyosin, and calcium

Titin is a large, elastic, kind of “springy”

protein

Titin runs through the core of thick filaments and anchors it to

the Z disc and the M line

Titin helps stabilize and position

the thick filament

Titin prevents overstretching and

provides recoil to muscle

A sarcomere is everything between

one Z disc and another Z disc

These help link the myofibrils with the sarcolemma and to the bones

accessory proteins

Defects in this protein lead to muscular dystrophy

dystrophin

Muscular dystrophy results in

the destruction of muscle cells and the replacement with scar tissue

Someone diagnosed with muscular dystrophy has a life expectancy of about

20 years

The striations seen on muscle cells are due to

actin and myosin (thick and thin filaments)

A bands consist of

both thick and thin filaments

H bands are found

in the middle of the A band

H bands consist of

thick filaments only

M line is found

in the middle of the H band (where protein links thick filaments)

I bands consist of

thin filaments only

Z disc provides anchorage for

thin filaments and elastic filaments

Sliding filament theory states that muscles contract when

thick and thin filaments pull on one another, shortening the sarcomere

structural hierarchy

A synapse is any place where

a nerve fiber connects to another cell

A neuromuscular junction is a type of

synapse

One nerve fiber connects with the muscle fiber at

multiple positions, which increases the speed of the muscle contraction

A swollen end of a nerve fiber is called

a synaptic knob

The synaptic knob contains

synaptic vesicles with acetylcholine (ACh)

The gap between the synaptic knob and the sarcolemma is called

the synaptic cleft

The mat of collagen and glycoprotein that isolates the NMJ from surrounding tissue is called

the basal lamina

The basal lamina contains

acetylcholinesterase (AChE)

AChE breaks down

ACh, allowing fiber relaxation

A nerve impulse causes the release of neurotransmitters (ACh) via

exocytosis

Neurotransmitters diffuse across… and bind to receptors on…

the synaptic cleft… the surface of the sarcolemma

These increase the surface area, giving more space for ACh receptors

junctional folds

The binding of the neurotransmitter initializes… which causes…

an electrical signal on the sarcolemma… the contraction of the muscle fiber

Muscle and nerve cells are

electrically excitable

The membranes of muscles and nerves are

polarized

The polarization of muscle and nerve membranes is due to

a difference in the ionic concentrations of the ICF and the ECF

The process of moving toward equilibrium is known as

depolarization

The process of returning to resting membrane potential is known as

repolarization

The sodium-potassium pump resets

the action potential system

4 major steps of the behavior of skeletal muscle fibers

1. excitation

2. excitation - contraction coupling

3. contraction

4. relaxation

shortening of one leads to shortening of the other:

sarcomere → myofibril → muscle fiber → fascicle → muscle compartment → whole muscle

Each muscle cell is supplied by

one neuron

Each neuron supplies multiple

muscle cells

One nerve cell and all its associated muscle cells is

a motor unit

The average motor unit supplies

200 muscle fibers

3-6 muscle fibers make up… which specializes in fine motor control

a small motor unit

1000 muscle fibers make up… which specializes in strength

a large motor unit

A quick cycle of contraction and relaxation when stimulus is at threshold or higher is called

a twitch

Stimulating the nerve with higher voltages produces… because…

stronger contractions… higher voltages excite more nerve fibers which stimulate more motor units to contract

The process of using more motor units to strengthen contraction is known as

recruitment/multiple motor unit (MMU) summation

Higher frequency of stimulations increases… because higher frequency stimuli produce…

contraction strength… temporal (wave) summation

Warmer temperatures increase… because there is…

contraction strength… increased enzyme activity

Hydrated muscles increases… because there is…

contraction strength… proper spacing between thick and thin filaments

ATP supply depends on availability of

oxygen and organic energy source (e.g. glucose and fatty acids)

2 main pathways of ATP synthesis

1. anaerobic fermentation

2. aerobic respiration

Two enzyme systems control phosphate transfers in muscle cells

1. myokinase

2. creatine kinase

This enzyme transfers Phosphate from one ADP to another, converting the latter to ATP

myokinase

This enzyme obtains Phosphate from a phosphate-storage molecule creatine phosphate (CP) and gives it to ADP

creatine kinase

Excess postexercise oxygen consumption is also known as

oxygen debt

The loss of contractility due to prolonged use of muscle is known as

muscle fatigue

Muscle fatigue in high-intensity exercise results from

• potassium accumulation in the ECF which reduces excitability of the muscle fiber

• excess ADP and phosphate which slow cross-bridge movements (actin and myosin interaction)

Muscle fatigue in low-intensity exercise results from

• fuel depletion as glycogen and glucose levels decline

• electrolyte loss through sweat which can decrease muscle excitability

• central fatigue = when fewer motor signals are issued from the brain

Muscular strength depends on

• fascicle arrangement

• size of motor units

• multiple motor unit summation

• temporal summation

Muscle conditioning will

• increase muscle size (by increasing the number of myofibrils and forming more actin-myosin cross-bridges)

• increase muscle endurance

Types of muscle conditioning

• resistance training - builds strength only (ex: weightlifting)

• endurance training - builds endurance only (ex: aerobic exercise)

• cross-training - combines resistance and endurance training

2 major classes of muscle fibers

• slow-twitch → long-term motion

• fast-twitch → short-term motion

Differences between fiber types

Contributing factors to involuntary muscle spasms:

• too few electrolytes

• dehydration

• stress-induced muscle fatigue

• excessive caffeine or other stimulants

• lack of sleep/exhaustion

• poor muscle condition

• low blood sugar

Functional properties of cardiac muscle:

• contracts with regular rhythm

• works in sleep or wakefulness, without fail, and without conscious attention

• highly resistant to fatigue

• muscle cells of a given chamber must contract in unison

• contraction must last long enough to expel blood

Cardiac muscle is made of cells called

cardiocytes/myocariocytes

Cardiocytes are joined by

intercalated discs