Muscle Physiology- Gonsalves Exam #3

What are the general features of muscle fibers?

-Multinucleated cells

-Sarcolemma

-Very long compared to other cells (1-300mm)

-Not usually wide in diameter (10-100 microns)

- Sarcoplasm

-Myoglobin

What is a sarcolemma?

plasma membrane of a muscle cell

What is a sarcoplasm?

cytoplasm of a muscle cell; rich in glycogen and myoglobin

What is myoglobin?

Stores oxygen; similar to hemoglobin

What are the TWO special structures of a muscle fiber?

Myofibrils

Sarcoplasmic Reticulum

What is the ultrastructure of myofibrils?

- Muscle cell contains many parallel myofibrils

- Myofibrils have DARK bands (A bands) and LIGHT bands (I bands) that cause a "striated" appearance of muscle

- Thick filaments (myosin)

-Thin filaments (Actin)

-Sarcomere

-Z-line

-M-line

-H-zone

-A-band

-I- band

What are the A band and I band a result from?

An arrangement of overlapping and non-overlapping regions of two types of myofilaments.

What is a sarcomere?

Smallest contractile unit of muscle cell

What is a Z-line?

Connection of actin filaments; dividing line between two adjacent sarcomeres.

What is an M-line?

Connection of myosin filaments.

What is the H-zone?

Non-overlapping region of the myosin filaments around the M-line

What is the A-band?

Length of myosin filaments

What is the I-band?

Length of non-overlapping actin filaments

Each muscle cell (fiber) is composed of many what?

Myofibrils

Each myofibrils contains what?

Hundreds of accordion-like sarcomeres laid end-to-end.

When does muscle contraction occur?

When the sarcomeres contract by the sliding motion of actin and myosin filaments.

What is the molecular structure of myosin filaments?

-Thick filaments - 12-16 nm

-Composed of about 200 myosin proteins

-Has a golf club-like shape

-2 heads (cross-bridges)

-Tail- shaft of the thick filament

What are cross bridges?

When the myosin heads interact with thin filaments during a contraction; can bind to the actin filaments and use ATP

What is the molecular structure of actin filaments?

-Thin filaments- 5-7nm

-2 helical chains of F actin (G actin subunits)

-Tropomyosin

-Troponin

What can G actin bind with?

Myosin heads

What is tropomyosin?

Rod-like protein structure that helps to stiffen F actin structure

What is troponin?

Globular protein that can bind Ca2+ to regulate actin/myosin binding

What is the sarcoplasmic reticulum?

Smooth ER that house Ca2+

What does the sarcoplasmic reticulum surround?

Each myofibril

Where is sarcoplasmic reticulum fused to?

Each other at H-zones and A/I bands

What are terminal cisternae?

Enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules; around A/I bands

What are transverse tubules?

Passageways from extracellular space to the terminal cisternae of SR

What do the transverse tubules act as passages for?

Nerve message directly to SR

Glucose, Oxygen, Salts to fiber

What is the sliding filament model? Step-by-step

1. Ca2+ released from SR

2. Ca2+ binds to TnC region of Troponin

3. Troponin changes shape, moving Tropomyosin, exposing binding site on actin filaments.

4. Attachment

5. Power stroke

6. Detachment

7. Re-cocking the head

What is attachment in the sliding filament model?

Myosin head with ADP + Pi binds actin

What is a power stroke in the sliding filament model?

Myosin head bends, pulling along the actin filament, ADP + Pi are released

What is detachment in the sliding filament model?

ATP binds to the myosin head, causing detachment from Actin

What is re-cocking the head in the sliding filament model?

Hydrolysis of ATP --> ADP + Pi releases energy to re-cock the myosin

Some myosin heads are in contact with actin at all times, allowing _________ to occur?

Walking motion

1 cycle = ?

1% muscle contraction

Motion continues until what?

No more ATP is present or Ca2+ levels drop by re-uptake into SR

What is rigor motis?

Muscle stiffen because Myosin heads remain attached to the Actin filaments.

What is the neuromuscular junction?

Nerve/muscle intersection

What is the structure of the neuromuscular junction?

- 1 motor neuron/axon supplies several fibers

- 1 centrally located junction per fiber

- Synaptic vesicles

- Synaptic cleft

- Motor end plate

What are synaptic vesicles?

Sacs that contain acetylcholine (ACh- neurotransmitter)

What are synaptic clefts?

Space between the axon terminal and the sarcolemma of the muscle cell

What is the motor end plate?

Highly folded part of sarcolemma beneath the synaptic cleft; rich in ACh receptors

What occurs when a nerve signal causes release of ACh from the axon end?

1. Action potential along axon causes depolarization of axon terminal

2. Decreased membrane potential causes Voltage-Dependent Ca2+ channels on axon terminal to open

3. Ca2+ influx into axon terminal causes exocytosis of ACh containing synaptic vesicles

4. ACh diffuses across the synaptic cleft to bind to ACh receptors of the motor end plate.

What occurs during the electrical excitation of the sarcolemma?

1. ACh binds to ACh receptors which open ACh-Dependent Na+ channels

2. The Na+ channels allow Na+ to flow into the muscle cell, causing depolarization

3. Depolarization at the neuromuscular junctions spreads to adjacent sites

4. Voltage-dependent Na+ Channels at the adjacent sites open, allowing more Na+ in

5. A wave of depolarization, therefore, spreads across the entire cell

6. This cannot be stopped and is called an all-or-none response

7. Entire process occurs in about 1 millisecond (1/1000 second) 8. A refractory period occurs in which the muscle cell must repolarize its resting state.

The sarcolemma is what?

Polarized, meaning it has more negative charge inside than outside.

ACh triggers an electrical excitation of the sarcolemma by opening what?

Chemically gated Na+ channels, allow positive charges to rush into the cell. The muscle becomes less negative or "depolarized."

When causes the refractory period to occur?

-When Voltage-Dependent Na+ Channels close, Voltage-Dependent.

What happens to K+ channels during the refractory period?

They open, and the Sodium/Potassium Pump rebalances the ion concentrations.

Re-polarization generally takes how long?

Very little time (3 milliseconds)

How long can a contraction last?

Up to 100 milliseconds (1/10 sec). Limits how fast the cell can "re-fire" and contract.

After binding to ACh receptors on sarcolemma, ACh is quickly broken down by an enzyme known as what?

Acetylcholinesterase (AChE)

What is myasthenia gravis?

Autoimmune disease where immune system attacks ACh receptors

What are ACh antagonists?

curare (snake venom + other venoms) and atropine; chemicals that block an ACh receptor

What is the latent period?

Time between excitation and contraction

Action potential passes down _______ from the sarcolemma surface

T Tubules

T Tubule depolarization causes the release of _______ from the sarcoplasmic reticulum

Ca2+

Ca2+ increases causes uncoupling of _________ and sliding of filaments previously mentioned

Troponin

What pumps Ca2+ back into the sarcoplasmic reticulum?

ATP-Dependent Ca2+ Pumps

Low Ca2+ levels allows ___________ blockade of actin and muscle relaxes

Troponin/Tropomyosin

Calcium sequesters do what?

Bind Ca2+ in the cell so it will not form Calcium Phosphate crystals ----> Calmodulin + Calsequestrin

What is a motor unit?

A single motor neuron and all of the muscle cells stimulated by it

# of muscle cells per motor neuron?

4-400

Muscles of fine control?

(fingers, eyes and face): fewer muscle cells per neuron

Muscles of posture and gross movement?

(gluteus maximus): more muscle cells per neuron

What is distributed on muscle fibers throughout the muscle (not one region)?

Axon terminals

Stimulation of one motor unit causes what?

Weak contraction throughout the whole muscle

What is a muscle twitch?

The response of a muscle to a single short electrical stimulus

What is a strong twitch?

Many motor units activated

What is a weak twitch?

Few motor units are activated

What is a latent period? (3 ms)

Time after stimulation for coupling to occur and contraction to start.

What is a contraction period? (10-100 ms)

From beginning of contraction to maximum force (tension)

What is a relaxation period? (10-100ms)

Time from maximum force to original relaxed state

What is a Frequency of Stimulation ( Wave Summation)?

A motor unit may be stimulated over and over again so no relaxation period is possible.

What are the THREE (3) factors of frequency of stimulation?

1. Cannot be greater than 1 every 3 ms (REFRACTORY PERIOD)

2. Motor neurons generally deliver action potentials in volleys with varying frequency

3. Tetanus

What is tetanus?

Smooth muscle contraction that occurs when summation is so great that the relaxation period disappears.

What is Summation of Multiple Motor Units?

As strength of stimulus is increased, more and more motor units are activated in the muscle shelf.

What is the threshold stimulus?

Levels of stimulus at which first motor units are activated.

What is the maximal stimulus?

Level of stimulus at which all motor units of a muscle are activated.

What is an example of a body part showing summation of motor units well?

Muscles of the hand

When weak force and delicate motion is needed, ______ motor units are activated

Few

What is Asynchronous Motor Unit Stimulation?

Motor units activated in different cycles average out to produce a smooth muscle contraction

What is Treppe: The Staircase Effect?

When a muscle is first used, it will show a gradual increase in force with a maximal stimulus until it is "warmed up."

What is muscle tone?

Slightly contracted state of muscle that is maintained by reflexes originating in the spinal cord. Maintains posture and readiness for active contraction

What is muscle tension?

Force generated by a muscle

What is load?

Force-resisting movement of a muscle. Muscle tension must be greater than the load to move it

What is isometric contraction?

Muscle doesn't change length (trying to list a box that is too heavy

What is isotonic contraction?

Muscle that moves the load (doing bicep curls with weights)

What is force of contraction?

Determined by several factors

What determines the force of contraction?

Number of motor units activated

Size of muscle (in cross section)

Size in muscle is increased by increasing what?

Size of individual muscle cells not the # of cells

What are series elastic elements?

Sheath around the muscle and the connective tissue tendons that attach muscle to bone

"Stretching" of non-contractile parts allows time for muscle to produce what?

Tetanic contraction

What is the degree of muscle stretch (actin-myosin overlap)?

Optimal force can be generated when muscle is between 80-120% of resting length

What is the effect of load on a muscle?

Smaller the load ---> faster the contraction

Larger the load ----> Slower the contraction/less duration

What are Red slow-twitch fibers (small, red)?

-Slow twitch; slow acting myosin ATPases

-Lots of myoglobin (red) to store oxygen

-Many mitochondria, active enzymes

-Use fat as primary fuel source

-Very aerobic, long duration contraction

What is White Fast-Twitch Fibers (large, pale)?

-Fast twitch; fast acting myosin ATPases

-Few mitochondria, primarily anaerobic

-Glycogen stores used for anaerobic respiration

-Lactic acid produced, fatigues quickly

-Rapid, intense, short duration

What is Intermediate Fast-Twitch Fibers (medium, pink)?

-Fast twitch; fast acting myosin ATPases

-Aerobic with myoglobin present

-Somewhat resistant to fatigue

Most muscle have combinations of what?

All 3 types of fibers

What can determine muscle combinations in fiber types?

Genetics

What is aerobic exercise?

That requiring steady oxygen