Modules 4-6

What are the three major muscle types?

• cardiac muscles

• smooth muscles

• skeletal msucles

What are the 3 levels of skeletal muscle organization, from largest to smallest subunit? Describe their key points

1. Muscle → whole muscle is made up of individual muscle fibres which run the ENTIRE length of the muscle

2. Muscle fibre → run parallel to each other → surrounded by connective tissues → multinucleated, with a very large number of mitochondria

3. Myofibrils → the cell is divided along its length into discrete contractile units

what is another name for a single muscle cell?

a muscle fibre

describe the side view of a Myofibril

Pattern of light and dark bands (gives striated pattern)

describe the cross sectional view of a Myofibril

highly organized pattern of thick filaments and thin filaments

What are thin filaments made of?

actin

What are thick filaments made of?

myosin

is an I band of a myofibril light or dark?

light

is an A band of a myofibril light or dark?

dark

What is an A band of a myofibril?

a dark band made up of stacked thick and thin filaments

what is an I band of a myofibril?

portion of the myrofibril where the thin filaments don’t extend into the A band

what is the H zone of a myofibril?

slightly lighter portion of the A band, containing only protiens that hold the thick filaments (myosin) in a stack

What is the M line of a myofibril?

proteins that hold the thick filaments together in a stack

what is the Z line of the myofibril?

a verticle line in the middle of the I band

what does the distance from one Z line to the next represent?

a sarcomere - functional unit of skeletal muscle

How is muscle length increased in muscle growth?

new sarcomeres are added onto the ends

what kind of protein is myosin?

motor protein

what does myosin do with the use of ATP?

moves along actin filaments

what does each molecule of myosin consist of?

2 subunits forming a dimer

what proteins make up thin filaments?

actin, troponin and tropomyosin

what are actin filaments made up of?

individual spherical actin molecules that come together to forma double helix structure

what is tropomyosin? what’s its role in thin filaments?

• thin double helix protein

• lies end to end along the actin helix structure

• regulatory protein, prevents interactions of actin and myosin

what is troponin? what is its role in thin filaments?

• regulatory protein complex made of 3 polypeptides

• one binds to actin, one binds to tropomyosin and the other binds to Ca2+

what does the “power stroke” refer to?

the interaction between myosin and actin, leading to a shortening of the sarcomere

what are the steps of the cross-bridge cycle?

1. binding - myosin cross-bridge binds to actin molecule

2. power stroke - the myosin head bends, pulling the thin filament inward

3. detachment - cross-bridge detaches at the end of power stroke and returns to original conformation

4. binding - cross bridge binds to more dista; actin, and the cycle repeats

where does the body get the energy to shorten the sarcomere from?

excitation-contraction coupling

• conversion of electrical signals into actual contraction

what are the 2 membrane structures that help transmit signals to muscular fibres?

• sarcoplasmic reticulum

• T-tubules

what is a sarcoplasmic reticulum?

• membranous structure that runs parallel to muscle fibres

• storage site for Ca²+

how does the T tubule transmit a depolarization wave to the SR?

• The SR forms sacs adjacent to T-tubules

• dihydropyridine receptors lie on the surface of T-tubules

• touching the Dihy- receptors are the ryanodine receptors on the SR

• when the dihydropyridine receptors sense the depolarization wace, they influence the ryanodine receptors to undergo a conformational change. - Ca²+ enters the cytoplasm

what is a dihydropyridine receptor?

voltage sensor that senses the wave of excitation at it travels down T-tubules

what are ryanodine receptors?

receptors on the SR that are a form of Ca²+ channel

what ion serves as the primary trigger for muscles to contract?

Ca²+

why can’t contraction take place in a relaxed muscle?

because tropomyosin and troponin are blocking the myosin binding site on the actin molecules, preventing cross-bridge formation

what causes muscle relaxation?

decreased nerve activity at the neuromuscular junction

what is acetylcholinesterase?

enzyme that causes rapid hydrolysis of acetylcholine

what is happening in the SR when a muscle is relaxed?

Ca²+ ATPase pumps the calcium back into the SR for when its needed again.

what are the 3 steps to the temporal relationship between the action potential and the mechanical action in muscles?

1. latent period

   • delay before contraction actually starts - when cross-bridge cycling is beginning

2. Contraction time

   • it takes time for the actin filaments to slide along the myosin

3. Relaxation time

   • ends when all Ca²+ is removed.

what is considered as the basic unit of muscle contraction?

a twitch - contraction of a single fibre

what are the two ways in which the body gets more muscle fibres to twitch?

• motor unit recruitment

• increasing frequency of stimulation

what is a motor unit?

a motor neuron branches out and innervates multiple muscle fibres - when that motor neuron is activated, all of its fibres contract

are the muscle fibres of a motor unit adjacent?

no

how does the body prevent fatigue during a sustained contraction?

the body can selectively rotate the activation of motor units

how does the body overcome the fact that skeletal muscle action potentials are relatively short, when it wants to sustain contraction?

the membrane potential quickly recovers to undergo another action potential

if a muscle fibre is restimulated after it has completely relaxed….

the second twitch has the same magnitude compared to the first

if a muscle fibre is restimulated before it has completely relaxed…

twitch summation occurs

if a muscle fibre is stimulated so rapidly that the twitches overlap…

tetanic contraction occurs

what are the two kinds of tetanic contraction? which is the stronger one?

• unfused

  • muscle fibres do not completely relax before the next stimulation

• fused (tetanus) (strongest)

  • no relaxation of the muscle fibres between stimuli

what determines the amount of tension that can be generated by a muscle at tetanus?

length of the muscle at the onset of contraction

explain the length-tension relationship for a less than optimal length muscle

• as the fibre length is shortened, thin filaments overlap → decreases efficiency of contraction, and tension

  • occurs at about 70% of resting muscle length

explain the length-tension relationship for an optimal length muscle

• maximal number of cross-bridge binding sites are available for cross-bridge binding

  • can achieve maximal contraction

explain the length-tension relationship for a greater than optimal length muscle

• less overlap of thick and thin filaments → less cross-bridges are available → less tension can develop

what are the common athletic muscular injuries?

1. Contusion - muscle is subject to sudden, heavy, extrinsic compressive force

2. strain - muscle fibres are exposed to an excessive force caused by intrinsic tension

3. laceration - deep cut or tear if the musclew

how are muscle contractions categorized at the level of the motor unit?

• isotonic

  • muscle tension remains constant as it changes length (bicep curl)

• isometric

  • muscle fibre tension increases as it remains at the same length (isometric holds)

how are muscle contractions categorized at the level of the whole muscle?

• concentric dynamic contractions

  • tension while the muscle shortens

• eccentric dynamic contractions

  • tension while the muscle lengthens

what are the 3 ways in which ATP is important in the contraction-relaxation process of skeletal muscle?

1. splitting of ATP to provide energy for power stroke

2. binding of new ATP to myosin head to release the cross-bridge

3. Active transport of Ca²+ back into SR

what are the 2 major types of fatigue?

• central fatigue

  • CNS decreases its activation of motor neurons

• muscle fatigue

  • reduces contractile activity before all ATP runs out → prevents rigor mortis

what are possible reasons for muscular fatigue to occur? how do they cause fatigue?

1. local accumulation of ADP and Pi from ATP hydrolysis

   • when ATP metabolite concentrations get too high, they interfere with cross-bridge cycling

2. accumulation of lactic acid

   • inhibits the enzymes of glycolysis, reducing ATP production

3. accumulation of extracellular K+

   • without ATP, the Na-K pump can’t function → membrane depolarization

4. glycogen depletion

   • muscle glycogen stores can become depleted

are all of the muscle fibres in one motor unit the same?

yes

what are the 2 classifications of muscle fibres?

• fast twitch muscle fibres (Type 2)

• Slow twitch muscle fibres (Type 1)

what are the slow twitch muscle fibres speed of contraction

slow rate

what are slow twitch fibres innervated by?

a2 motor neurons

how do a2 and a1 motor neurons compare to eachother?

• A2 are smaller than A1

• A2 have lower activation threshold than A1

• A2 have slower conduction speeds than A1

what are the metabolic properties of a slow twitch muscle fibre?

produce their ATP by aerobic process

what are fast twitch muscle fibres speed of contraction?

fast

what are fast twitch muscle fibres innervated by?

a1 motor neurons

what are the metabolic properties of fast twitch muscle fibres?

• Fast oxidative glycolysis fibres produce ATP by aerobic and anaerobic metabolism

• Fast glycolic fibres produce ATP by anaerobic means

what determines the colour of muscle fibres?

how fast they produce their energy

a muscle that is dense in mitochondria and myoglobin content is going to be what colour?

red

muscles that are less dense in mitochondria and myoglobin content are going to be what colour?

white

what colour is a fast glycolytic fibre?

white

what colour is a slow oxidative fibre?

red

what colour is a fast oxidative glycolytic fibre?

red

fill in the table

what are the 2 kinds of muscle receptors responsible for proprioception?

• muscle spindles

• golgi tendon organs

what do muscle spindles do?

they monitor changes in muscle length and play a key role in stretch reflexes

what do golgi tendon organs do?

primary purpose is to respond to changes in muscle tension

where are golgi tendon organs found?

at the junction of tendons and muscle fibres

how do golgi tendon organs respond to muscular stretch and contraction?

• muscle fibre contracts → tension pulls on tendons

• the stretch activates the afferent fibres in the tendons → stronger pull on the tendon = higher rate of firing in the golgi tendon organ afferents

what’s an intrafusal fibre?

a fibre found among extrafusal fibres, but only the ends of intrafusal fibres are contractile, not the middle

what’s an extrafusal fibre?

a regular muscle fibre

what are the 3 different levels of input involved in motor control?

• afferent neurons

  • spinal reflexes

  • maintain posture

• primary motor cortex

  • mediate fine voluntary movements of body parts

• brain stem

  • regulates overall body posture

what are the 3 kinds of muscle?

• skeletal

• smooth

• cardiac

where are smooth muscle cells found?

in the walls of hollow organs and tubes

where are cardiac muscle cells found?

the heart

do skeletal muscles have sarcomeres?

yes

do smooth muscle have sarcomeres?

no

what are the different filament types that participate in contraction of smooth muscle?

1. thick myosin filaments

   • longer than the skeletal ones

2. thin actin filaments

   • contain tropomyosin but not troponin

3. intermediate filaments

   • support the cytoskeletal framework

how are thick and thin filaments oriented in smooth muscle?

at angles, anchored by dense bodies

what does troponin do in skeletal muscle?

blocks cross-bridge formation until Ca²+ is present

what are the steps for myosin cross-bridge activation in smooth muscle?

1. during excitation, Ca²+ enters the smooth muscle cell and binds to calmodulin

2. Ca²+ -calmodulin complex binds to and activates myosin light chain kinase

3. once activated, the kinase phosphorylates myosin light chain, which allows the myosin cross-bridge to bind to actin

what is calmodulin?

calcium messenger protein

what is myosin light chain?

a protein associated with the myosin head. Aids in cross-bridge formation

do smooth muscle cells have T-tubules and Sarcoplasmic reticulum?

• no t tubules

• very little SR

what are the 2 ways in which smooth muscle cells receive calcium for muscle contraction?

• Ca2+ entry from the ECF

• Ca²+ release from the SR

how does smooth muscle receive Ca²+ from the ECF?

voltage gated dihydropyridine receptors function as Ca²+ channels

→ if the cell depolarizes enough, the channels open

how does smooth muscle receive calcium from the SR?

once Ca²+ enters, it can activate calmodulin OR it can stimulate the SR to release Ca²+ through CIRC

where is single unit smooth muscle predominantly found?

hollow organs

where is multiunit smooth muscle contraction predominantly found?

large blood vessels, small airways

is single unit smooth muscle contraction myogenic or neurogenic? what does that mean?

myogenic - self excitable and does not require nerve stimulation

is multiunit smooth muscle contraction neurogenic or myogenic? What does that mean?

neurogenic - innervated by nerves to contract