Structure And Function Of Muscle Cells

Front 1

what are the 3 types of muscle?

Back 1

1) skeletal

2) smooth

3) cardiac

Front 2

briefly list some features of skeletal muscle...

Back 2

- fibres are large and cylindrical

- multinucleated

-appear striped or striated under the microscope

-many skeletal muscles we have voluntary control of but some are autonomic e.g the diaphragm is an example of automatic skeletal muscle.

Front 3

skeletal muscle...

Back 3

Front 4

briefly list some structures of cardiac muscle...

Back 4

-striated fibres but smaller

-branched fibres

-uninucleated

-cells are joined in series by junctions called intercalated discs (allows communication between cells so when one cardiac muscle fibre contracts, others contract)

Front 5

cardiac muscle...

Back 5

Front 6

list some features of smooth muscle...

Back 6

-small, spindle shaped fibres

-found in the walls of blood vessels and lower parts of the digestive system

-fibres lack striations (hence smooth)

-uninucleate- nucleus in the centre of the cell

Front 7

smooth muscle...

Back 7

Front 8

Studying skeletal muscle...

When you contract skeletal muscle, the muscle shortens and movement occurs. Why does movement occur?

Back 8

Movement occurs because each end of the muscle is attached to bone and between those two bone joints there will be a moveable joint.

Front 9

You have to have a nerve supply to each fibre (a nerve innovating each fibre).

Why is this important?

Back 9

It is important because not every fibre in skeletal muscle is used at the same time- they're used in groups.

The more muscle fibres you recruit, the shorter the time you can sustain activity for.

Front 10

To ensure each muscle fibre has a separate nerve supply you have connective tissue surrounding each one. This is known as...

Back 10

endomysium

The endomyseium allows you to electrically isolate one cell from the other.

Front 11

muscle fibres are organised into fascicles which are surrounded by the connective tissue layer called...

Back 11

perimysium

Front 12

The outer layer surrounding a number of fascicles is called...

Back 12

Epimyseium

Front 13

A diagram of this arrangement is...

Back 13

Front 14

Satellite cells are present in skeletal muscle. What is the role of satellite cells?

Back 14

Satellite cells are important when you want to repair or increase the size of a muscle.

Satellite cells can divide- one nuclei will add to the muscle and one will remain in the satellite cell ready to act as a stem cell should you require it- incorporate new cells into muscle.

Front 15

What is the basic contractile unit of a muscle?

Back 15

A sarcomere - one Z line to another Z line.

Front 16

Back 16

M line = myosin only

Z line = actin only

H band= actin and myosin

Front 17

Wrapping around the actin filament we have tropomyosin and nebulin. What is the role of nebulin?

Back 17

Nebulin is a template regulating the length of the actin filament.

(each nebulin molecule extends from the Z band along the length of one thin actin filament)

Front 18

Myosin is connected to titin. (from the Z disc to the M line).

What is the role of titin?

Back 18

Titin is important to the power you can get with your muscles.

Titin maintains the central position of the thick filaments in the sarcomere. During relaxation, titin generates passive tension through elastic extension when the sarcomere is stretched.

Front 19

Sarcomere with nebulin (associated with actin) and titin (associated with myosin) on it...

Back 19

Front 20

The sarcoplasmic reticulum is arranged as a repeating series of networks around myofibrils.

Where they meet is termed the terminal cisterna.

They act as reservoirs for Ca2+. Mitochondria are also present to provide the energy for muscle contraction.

Back 20

Front 21

The plasma membrane invaginates transversely forming a tubular system, T tubules, between the cisternae.

What is the role of T tubules?

Back 21

They contain VG sensor proteins which are activated when the membranes depolarizes inducing the sarcoplasmic reticulum to release calcium.

The T tubules and the adjacent cisternae form a triad.

Front 22

Describe the step by step process of skeletal muscle excitation...

Back 22

1) When the nerve signal arrives at the synaptic knob it stimulates the release of ACh into the cleft.

2) ACh released from the motor end plate binds to the nicotinic receptors, which incorporate a channel.

3) The channel opens, causing a small depolarisation which opens VG Na+ channels causing a larger depolarisation (action potential)

4) The depolarisation spreads over the plasma membrane and via the T tubules, these are physically linked to the calcium channels in the terminal cisternae of the sarcoplasmic reticulum.

5) The gates in the SR open and calcium diffuses into the cytosol.

6) Calcium ions are released into the sarcoplasm and binds to the troponin complex causing changes in the tropomyosin allowing the myosin head to attach and thus contraction is initiated.

Front 23

Ca2+ binds to troponin, what does this binding do?

Back 23

the binding of Ca2+ to troponin causes a conformational change which exposes areas on the actin so the myosin head can bind. (exposes the myosin binding site on actin)

Front 24

what is concentric contraction?

Back 24

concentric contraction causes movement of a joint, It is when you get a complete overlap of the actin and myosin filaments.

Front 25

what is eccentric contraction?

Back 25

when the muscle is stretched, the actin and myosin do not interact, the A band always remains the same.

Front 26

what is isometric contraction?

Back 26

isometric contraction increase the tension but there is no change in length. (no movement of actin and myosin)

Front 27

The contraction cycle (skeletal muscle)

The shortening of a muscle involves rapid contraction cycles that move the thin filaments along the thick filament. There are 5 stages, list them:

Back 27

1) Attachment- the myosin head is tightly bound to the actin molecule of the thin filament (rigor state)

2) Release- ATP binds to the myosin head and induces the release from the actin therefore without ATP the muscle would remain in a state of rigor. The muscle is now relaxed.

3) Bending- the ATP causes further changes to the myosin head causing it to bend. The bending movement initiates the breakdown of ATP to ADP and inorganic phosphate both of which remain in the myosin head.

4) The myosin head binds to the new site and the inorganic phosphate is released. The effects are twofold, 1) it increases the binding affinity of the myosin for the actin. 2) the myosin head generates a force to straighten up and in doing so forces the thin filament along the thick filament creating the power stroke and shortening the sarcomere. During this stage, the ADP is lost from the myosin head,

5) the release of ADP results in the reattachment of the myosin head to the actin filament and the rigour state is re-established,

Front 28

The tension a muscle can generate is related to the number of cross bridges formed between the thick and thin filaments.

True or False?

Back 28

True

In an elongated fibre there are very few crossbridges and therefore little power can be generated.

As the sarcomere shortens, more crossbridges form and the force increases until the crossbridges reach the Z disc,and there are no new binding sites and therefore the tension decreases rapidly.

Front 29

In the resting state, there is a small amount of overlap between actin and myosin...

Back 29

as you begin to contract the muscle more of the myosin heads come into contact with actin---> increases tension.

Front 30

What is the first step in skeletal muscle contraction?

Back 30

The first step in skeletal muscle relaxation is the cessation of the nerve signal. Ach is recycled back by being resorbed into the synaptic knob.

Front 31

Active transport pumps in the sarcoplasmic reticulum begin to pump the calcium ions from the cytosol into where?

Back 31

the cisternae

Front 32

What does a decrease in calcium ions in the cytosol cause?

Back 32

Decreases in calcium ions causes calcium to unbind from the troponin allowing the tropomyosin to recover the binding sites and therefore myosin can no longer bind to the actin.

Tension is no longer produced or maintained as myosin can't bind to actin because tropomyosin is occupying the myosin binding site on actin. .

Front 33

STUDYING CARDIAC MUSCLE...

Cardiac muscle T tubule is called a 'Diad'

Back 33

Front 34

Therefore a difference between cardiac and skeletal muscle is the arrangement of T tubules.

Describe the arrangement of T tubules in cardiac and skeletal muscle...

Back 34

In cardiac muscle T tubules are arranged into Diads whereas in skeletal muscle T tubules are arranged into triads.

Front 35

In cardiac muscle the action potential originates spontaneously in the pacemaker cells and spreads through the gap junctions where?

Back 35

spreads through gap junctions in the intercalated discs to the contractile cells.

Front 36

What do desmosomes in the intercalated discs do?

Back 36

desmosomes in the intercalated discs link the adjacent cells forming a physical bond that resists mechanical stress and therefore resist stretching as the heart fills with blood and increases in tension as the muscle fibres contract.

(links intermediate filaments in adjacent cells)

Front 37

briefly describe contraction of cardiac muscle...

Back 37

the action potential enters from an adjacent cell opening the calcium gated voltage channels.

Calcium enters the cell which induces local release of calcium. The calcium ions binds to troponin to initiate contraction.

Front 38

When does relaxation of cardiac muscle occur?

Back 38

relaxation of cardiac muscle occurs when calcium unbinds from troponin and the calcium is pumped back into the sarcoplasmic reticulum to be stored ready for the next contraction.

Front 39

List some differences between cardiac and skeletal muscle...

Back 39

1) cardiac muscle cells are relatively small (10-20 micro meters in diameter)

2) A cardiac cell typically has a single centrally placed nucleus (skeletal cells are multinucleate)

3) the T tubules are short and broader (larger than those of skeletal muscle) and there are no triads.

4) The sarcoplasmic reticulum is not in the form of an expanded cisterna but organised as an anastomosing network.

5) Cardiac muscle is almost completely dependent on aerobic metabolism and therefore the cells contain large numbers of mitochondria. There are glycogen and lipid inclusions which are stores of energy and the myoglobin store oxygen.

6) each cardiac cell contact several others via intercalated discs.

Front 40

skeletal muscle contracts with neural stimulation whereas cardiac muscle contracts without neural stimulation...

Back 40

The pacemaker are specialised cardiac muscle cells which are more excitable than the others and therefore contract first.

Innervation of these cells by the nervous system can adjust the rate at which they contract.

Front 41

The intercalated discs contain desmosomes which transfer the force from cell to cell.

What do gap junctions allow the passage of?

Back 41

allow the passage of electrical signals.

Front 42

Cardiac muscle cells are much smaller than skeletal muscle, contains branched fibres and you do have striations on cardiac muscle cells..

Back 42

Front 43

Studying SMOOTH muscle...

List locations in the body where smooth muscle is found...

Back 43

in the walls of:

1) blood vessels

2) Gastrointestinal tract and gallbladder

3) ureter and urinary bladder

4) uterus

5) respiratory system

6) eye

Front 44

what is the length of a typical smooth muscle cell?

Back 44

from 20 to 500 micrometers

they are longitudional, spindle shaped cells

Front 45

smooth muscle contraction can either be phasic or tonic.

In the oesophagus, is contraction phasic or tonic?

Back 45

In the oesophagus it is phasic, contracting only when food enters whilst in the intestine there is phasic contraction and relaxation to push the contents along the gut.

Front 46

where is tonic contraction found?

Back 46

Tonic contraction is found in sphincters which only relax to open allowing material to pass through.

Front 47

list some features of contraction of smooth muscle cells...

Back 47

1) smooth muscle cells have cytoplasm filled with filaments of actin, and lesser amount of myosin.

2) no striations

3) electrical excitation spreads from cell to cell via gap junctions

4) contraction is controlled by hormones or balance between sympathetic and parasympathetic divisions of the autonomic nervous system.

Front 48

describe the steps in smooth muscle contraction...

Back 48

1) calcium enters the cell and induces release of calcium ions from the sarcoplasmic reticulum.

2) calcium binds to calmodulin (CaM)

3) calmodulin activates myosin light chain kinase (MLCK) resulting in phosphorylation which leads to the activation of the myosin head to attach to actin.

4) in the presence of ATP, the myosin head bends producing contraction.

Front 49

the regulatory proteins in skeletal and cardiac muscle are tropomyosin and troponin.

What are the regulatory proteins in smooth muscle?

Back 49

calmodulin and light chain myokinase

Front 50

what are the calcium sources for the 3 types of muscle?

Back 50

1) skeletal muscle ---> sarcoplasmic reticulum

2) cardiac muscle ---> sarcoplasmic reticulum and extracellular fluid

3) smooth muscle---> mainly extracellular fluid.

Front 51

In skeletal and cardiac muscle the calcium receptors are troponin of the actin.

What are the calcium receptors in smooth muscle?

Back 51

calmodulin of myosin.

Front 52

In smooth muscle, myosin light chain kinase phosphorylates myosin light-chain, triggering cross-bridge cycling.

true or false?

Back 52

true

Front 53

in cardiac and skeletal muscle calcium-induced calcium release from the sarcoplasmic reticulum is the main mechanism of contraction.

what is the main mechanism in smooth muscle?

Back 53

In smooth muscle it is voltage induced calcium release.

(no T tubules in smooth muscle also)

Front 54

In smooth muscle, calcium ions enters the cytosol from the extracellular space through voltage-dependent channels in the sarcolemma.

True or false?

Back 54

true

Front 55

summary points

In skeletal muscle, what is the diameter of a muscle fibre?

Back 55

about 100-120 micrometers

Front 56

what is the epimyseium?

Back 56

A sheath of dense connective tissue surrounding a collection of fascicles that constitutes the muscle

Front 57

Where are the nuclei positioned in skeletal muscle?

Back 57

At the periphery

Front 58

why are skeletal muscle fibres multinucleated?

Back 58

because they are formed from the fusion of hundreds of precursor cells

Front 59

what is the diameter of a myofibril?

Back 59

about 1 mircometer

Front 60

what is the length of a sarcomere?

Back 60

About 2.5 mircometers

Front 61

what shape are smooth muscle cells?

Back 61

spindle shaped

Front 62

what is the position of smooth muscle cells nucleus?

Back 62

central

Front 63

is there any connective tissue separating the individual cells in smooth muscle cells?

Back 63

Yes- but very little- therefore it is difficult to resolve the boundaries between cells

Front 64

would you expect to find rough endoplasmic reticulum in smooth muscle cells?

Back 64

Yes- smooth muscle cells make the extracellular matrix components but also need rough endoplasmic reticulum for production of their own membrane proteins and organelles.

Front 65

what happens to the nucleus of contracted cells? (smooth muscle)

Back 65

the cells twist like a corkscrew when it contracts due to the diagonal arrangement of the actin filaments

Front 66

In cardiac muscle, striations are visible, true or false?

Back 66

True

Front 67

where are the nuclei situated in cardiac muscle?

Back 67

centrally (as in in the smooth muscle but not skeletal, skeletal nuclei are at the periphery)

Front 68

In cardiac muscle, are the fibres straight or branching?

Back 68

Branching

Front 69

in cardiac muscle are many blood vessels present?

Back 69

yes

Front 70

what is the name of the dark line seen on electromicrographs?

Back 70

intercalated discs

(gap junctions and adheren junctions form parts of the intercalated disc)

Front 71

what is the function of intercalated discs?

Back 71

to allow contractile waves to pass from cell to cell