Biol 225 (A&P) - Muscular System Objectives #1 - #9 - Dr. Steele

Epimysium

This connective tissue covering surrounds and bundles all of the fascicles bound by the perimysium

Perimysium

This layer of connective tissue surrounds each fascicle (bundle of muscle fibers/cells) - it extends inward from the epimysium and separates the muscle tissue into small section (fascicles).

Endomysium

This layer of connective tissue surrounds each muscle cell/fiber within a fascicle - Consists of individual connective fibrous cells that cannot conduct an impulse

Periosteum

This is a continuous covering over bones that eventually "oozes" off and connects muscle to bone (tendons)

Aponeuroses

These connect muscle to muscle and are found in the connection between the muscles of the occipitofrontalis (epicranius) and the obliques

Tendons

These connect muscle to bone

Understand this diagram and the different levels of a muscle cell

Muscle fibers

Muscle cells are also known as these

Sarcolemma

This term is used to describe the cell membrane of a muscle cell

Sarcoplasm

This is the internal fluid of muscle cells

Multinucleate

The cells that form muscles keep fusing together and have many nuclei. They are formed by smaller cells (myoblasts) that join together - Myoblasts fuse to form myocytes

Myofibrils

These are what give muscle its stripes (seen where proteins do/do not overlap) - Actin and myosin - produce alternating light and dark striations

Sarcomeres

This is the area from one z-line to another - a repeating pattern of units creating striations - a lot of light passes through this and is referred to as the I-band

Striations

These patterns are created due to the filaments actin and myosin

Smaller

The size of filaments (actin and myosin), relative to a myofibril, is described as what?

Myosin

This filament is thicker and bulkier - considered a globular protein

Actin

This filament is longer and thinner, described as filamentous

The A-band

In a sarcomere, Which band's length is the exact same length as myosin?

I-band

This striation pattern (light band) in a sarcomere is composed of thin actin filaments held by direct attachment to z-lines, which appear in the center of these

A-band

This striation pattern (dark band) in a sarcomere is composed of thick myosin filaments overlapping thin actin filaments

Z-line

In a sarcomere, this appears in the center of I-bands and are shown by a zig-zag pattern - their job is to hold myosin filaments in place

H-zone

In a sarcomere, this is a slightly lighter central region of the A band consisting of only thick filaments

M-line

In a sarcomere, this is a thickening of the A band that consists of proteins that help hold the thick filaments in place

Tropomyosin and troponin

What are the two regulatory proteins that exert control over when actin and myosin can physically touch - Job is to control muscle contraction by blocking the binding of actin and myosin (prevent them from touching)?

Ca2+

What ion allows the binding of actin and myosin and overpowers the regulatory proteins?

Sarcoplasmic reticulum (SR)

This is a membranous cell organelle in a muscle cell that fills all available space around the myofibrils and is capable of the transfer of impulses around the entire myofibril

Transverse tubules

These connect the outside of a muscle cell to the SR and surround myofibril - this is continuous with the sarcolemma

Terminal cisternae

These are storage containers that surround the transverse tubules and contain Ca2+ due to active transport - contain voltage-gated ion channels, and when open, move Ca2+ with its concentration gradient (stores Ca2+ aside of a cell organelle)

Triad

This is formed when a t-tubule is combined with the surrounding cisternae

Motor unit

This is the entire system of a motor neuron and its connections to other cells

Neuromuscular junction

This consists of a motor neuron, a motor end plate, and a synaptic cleft - it is where the terminal axon/knob "attaches" to the motor end plate at the synapse

However many synaptic knobs are on the presynaptic neuron

How many muscle fibers can a neuromuscular junction connect to?

Motor neuron

This part of the neuromuscular junction controls effectors and causes the contraction of a skeletal muscle fiber when stimulated by its neuron

Motor end plate

This part of a neuromuscular junction is the area of the muscle fiber that is specialized to create a connection with the presynaptic neuron knobs - abundant in nuclei and mitochondria as well as having very folded sarcolemma

Synaptic cleft

This part of a neuromuscular junction is the small gap that separates the membrane of the neuron (knob) and the membrane of the muscle fiber (motor end plate) - this is where neurotransmitters are released into

1. There is a release of acetylcholine (ACh)

2. ACh binds to ligand-gated ion channels which allow the transfer of Na+ to stimulate the post synaptic cell due to the influx of positive charge, creating a second set of Na+ channels to open as an impulse/action potential - end plate potential and muscle action potential/impulse

3. Calcium is released form the SR and causes muscle contraction by freeing the binding sites of actin and myosin

How is a skeletal muscle fiber stimulated to contract?

Acetylcholine (ACh)

What is the only neurotransmitter used in skeletal muscle?

At the motor end plates

Where is the only place a neurotransmitter can bind?

1. Myofilaments do not shorten but slide over one another

2. Sarcomeres shorten, causing myofibrils to shorten

3. Areas of actin/myosin overlap increase

Describe the sliding filament model for skeletal muscle fiber contraction

Filaments do not change length, but their orientation to each other changes

In the sliding filament model for skeletal muscle fiber contraction, do filaments change in length? What changes?

Z-lines

As sarcomeres shorten, what becomes closer together?

The motor neuron releases acetylcholine as the neurotransmitter that binds to motor end plates and causes ligand-gated channels on the sarcolemma to open. This starts the transfer of an impulse as Na+ channels open following it

What is acetylcholine's role in excitation-contraction coupling?

Calcium has to be present for actin to bond to myosin and actually contract the muscle - if calcium is not present, contraction will not occur

What is Calcium's role in excitation-contraction coupling?

1. Motor neuron releases acetylcholine

2. Ligand-gated channels on the sarcolemma open due to presence of ACh

3. Na+ and K+ channels open (Na+ influx predominates)

4. End-plate potential results, threshold achieved, voltage-gated Na+ channels open

5. Sarcolemma and t-tubules depolarize

6. Voltage-gated channels on SR open - cisternae of SR store calcium - T-tubule and surrounding cistern = triad membranes

7. Ca2+ is released into the sarcoplasm by diffusion

8. Ca2+ binds to troponin, shifts regulatory proteins

9. Actin and myosin change position relative to one another

Explain excitation-contraction coupling

ACh

In excitation-contraction coupling, what opens ligand-gated ion channels on motor end plates?

Voltage-gated Na+ channels

In excitation-contraction coupling, if excitation is great enough and brings in enough Na+, what can be triggered just outside of the motor end plate to create an action potential?

Voltage-gated Na+ channels

These channels are located just outside of the motor end plate and can use the extra excitation from the neurotransmitter action at the motor end plate to stimulate them

The opening of Ca2+ channels

In excitation-contraction coupling, the impulse eventually travels along and depolarizes the sarcolemma and t-tubules, what does this lead to?

Comes from the sarcoplasmic reticulum (including cisternae) and the extracellular environment

When Ca2+ flows into the sarcoplasm by diffusion, where does it come from?

The I-band and H-zone

Z-line

In excitation-contraction coupling, when actin and myosin change position relative to one another, what two things become shorter? And, what is pulled towards the M-line?

ATP binds to myosin and hydrolyzes to provide energy to contract - ATP also binds to myosin after contraction to detach it from actin

What is the role of ATP in cross-bridge formation

1. ATP binds to myosin and hydrolyzes (ATP → ADP+P)

2. Activated myosin binds to actin in the presence of Ca2+

3. ADP+P is released at contraction , and myosin head pivots (power stroke) as it is slung forward

4. New ATP binds and myosin detaches from actin

5. Hydrolysis of new ATP returns myosin to its activated position

6. Cross-bridge cycling continues as long as Ca2+ and ATP are available

Explain cross-bridge formation

Calcium

Regulatory proteins prevent binding of actin and myosin in the absence of what ion?

One hydrolyzed ATP (ATP → ADP+P)

How many ATP/ADP+P are responsible for stimulating one myosin filament to bind to one actin filament

Once the Ca2+ supply is returned back into the SR or extracellular matrix so that the regulatory proteins can block binding sites of actin and myosin - Ca2+ pumps re-establish the gradient

How does cross-bridge cycling end?

1. Ca2+ is pumped back into the SR and extracellular fluid because voltage-gated ion channels are closed, so the pump re-establishes concentrations

2. Binding sites are blocked when Ca2+ is removed and muscles cannot contract

3. Sarcomere returns to its original length

Explain muscle relaxation

Rigor mortis

This is the contraction of dead muscle cells

- Muscle cells run out of ATP and the muscles contract due to the presence of Ca2+ (bc pumps fail and cannot reset contraction)

- Ca2+ present frees binding sites allowing energized myosin to sling forward

- Once myosin and actin sling forward in contraction, the energy is used and the cells cannot relax because there is no ATP to free the binding site

What is the cause of rigor mortis?

They are long, skinny, and covered by connective fibrous tissue

Describe muscle cells

Myofibrils

These are long and thin structures that are smaller than a cell and are located inside one muscle cell

Skeletal muscle cells are individualized (covered by dense fibrous connective tissue) and independent of one another

Describe the connectivity of skeletal muscle cells

Impulses transfer cell-to-cell in cardiac muscle so that the muscle can contract every single time and have a complete reaction

How do impulses travel in cardiac and smooth muscle muscle?

Because skeletal muscle cells do not communicate with one another since they are independent, allowing us to only perform a certain extent of an action compared to a complete action of a cardiac or smooth muscle

Why can skeletal muscle do partial contractions?

Skeletal muscle cells are independent from one another

Are skeletal muscle cells interdependent or independent of one another?

Smooth and cardiac muscle cells are interdependent on one another

In cardiac and smooth muscle, are cells interdependent or independent?

Bands

When looking at sarcomeres and striations, what do we use as reference points?

Transfer of electrical signals (impulses)

The membrane that compartmentalizes myofibrils and is connected to the outside of the cell has what function?

No, the impulse stays in the skeletal muscle fiber because those fibers are separated from neighboring cells

Does an impulse travel from one skeletal muscle fiber to another?

A nerve

Skeletal muscle requires what to tell them what to do?

Damage cannot be at the cell body because separation damages beyond repair, partially because of high nucleus concentration

The only way you can recover from nerve damage is if it is not at this part of the neuron

Pumps - the calcium there can then diffuse with its gradient

Where does all of the calcium in the synaptic cleft come from?

Highly folded

What is a characteristic of the motor end plate?

Overshoot

What allows stimulation and opening of adjacent ion channels to the motor end plate?

yes

In the sliding filament model, do myofibrils change length?

Closer together as each sarcomere becomes shorter

In the sliding filament model, in which direction do Z-lines travel?

The sarcoplasmic reticulum

What stores calcium which creates a regulatory protein shift?

I-band

This band is on either side of a z-line where there is only actin, this decreases in size during contraction

Z-lines

Which lines are pulled closer together when myosin binds to actin?

It decreases

What happens to the H-band during contraction?

A-band increases overlap but stay the same length

What happens to the A-band during contraction?

The impulse is what lets the calcium out of storage to free binding sites to start contraction

What is a reason why skeletal muscle requires an impulse?

Excitation

What is the impulse that leads to a contraction

Contraction

This occurs when filaments slide and the muscle gets shorter

Nerve impulse

You need this to cause contraction

T-tubules/cisternae

Excitation stimulates all of the __________ and __________ that are continuous with the cell membrane

During the power stroke

In cross-bridge cycling, when is energy released (ADP+P is spent)?