65 terms

Lecture 11 - Muscle

What is skeletal muscle? Describe its activity.
Skeletal Muscle is muscle that can be voluntarily controlled (somatic). These generally stretch across joints (act across joints).

Skeletal muscles engage in strong, quick contractions that are discontinuous.

Skeletal muscle is striated.
What is cardiac muscle?
Cardiac muscle is muscle that cannot be voluntarily controlled (autonomic).

Cardiac muscles engage in strong, quick contractions that are continuous.

Cardiac muscle, like skeletal muscle, is striated.
What is smooth muscle?
Smooth muscle is muscle that cannot be voluntarily controlled (autonomic).

Smooth muscles engage in weak, slow contractions that are continuous.

Smooth muscle cells are not striated.
True or false: in a transverse section, cardiac muscle shows striations.

The striations in cardiac muscle are only visible in a longitudinal section.
Label the following image.
1. Myofibril
2. Muscle Fascicle
3. Muscle Fiber
4. Epimysium
5. Endomysium
6. Perimysium
List the fibers of a muscle cell from the smallest to the largest.
Myofibril, Muscle Fiber (Cell), Muscle Fascicle, Muscle Group, Dan-sized Muscle Group POWER!
What is endomysium?
Endomysium is the connective tissue surrounding one Muscle Fiber.
What is paramysium?
Paramysium is the connective tissue surrounding one Muscle Fascicle.
What is epimysium?
Epimysium is the connective tissue surrounding a group of Muscle Fascicles.
Label the following image.
ONE - Epimysium

TWO - Perimysium

THREE - Endomysium
True or false: skeletal muscle cells are multi-inucleated.
As seen above, skeletal muscle cells contain several nuclei.
What is the sarcoplasmic reticulum?
Sarcoplasmic reticulum is a special form of smooth ER.

Sarcoplasmic reticulum is a tubular network of smooth ER that stores and releases calcium.
Point out one skeletal muscle on the transverse cross-section of the following image.
Each of the blobs surrounded by an endomysium is a muscle fiber (muscle cell)
What is a sarcomere?
A sarcomere is the functional unit of the myofibril. A sarcomere is defined as the space between one z line and the next z line.

Sarcomeres are the repeating structural sub-units of the myofibril.
What are the components of a myofibril?
A myofibril has three components:

Actin - The thin component of the myofibril
Myosin II - The thick component of the myofibril
What are the three types of skeletal muscle?
Type 1 - Slow oxidative fibers (slow-twitch)
Type 2A - Fast oxdative/glycolytic fibers (medium size, fast twitch)
Type 2B - Fast glycolytic fibers (large size, fast-twitch)
What types of muscle fibers are fatigue-resistant?
Type 1 and Type 2A.

These are oxidative fibers - - they use oxygen as a source of energy.
Which muscle fibers have a high concentration of mitochondria?
Type 1 and Type 2A.

These skeletal muscle types are taking advantage of oxidation, which takes place in mitochondria.

Conversely, Type 2B is using glycolysis, which takes place in the cytoplasm.
Which muscle fibers have the lowest myoglobin content?
Type 2B.

Type 2B uses glyolysis, not requiring oxygen (which is what the purpose of myoglobin in the muscle cell).
What kind of fiber would be very prevalent in Ussain Bolt's body? How about a Boston Marathon champion?
Ussain Bolt almost certainly has tons of Type 2B muscle fibers.

A great marathon runner, on the other hand, would have lots of Type 1 muscle fibers.
Label the following image.
1. Type 1
2. Type 2B
What is this?
A cross-section of a muscle fiber. The striations are clearly visible.

The I band is the entire white portion.

The Z line is the thin line in the center of the I band.

The A band is the entire dark portion.
Label the following image.
1. A band
2. H band
3. Z line
4. M line
5. Actin
What is alpha-actinin?
The purpose of the alpha-actinin is to hold actin in lattice at the Z disc.
What is Nebulin?
The purpose of Nebulin is to help anchor actin to the Z line.
What is myomesin?
The purpose of myomesin is to hold myosin at the M line.
What is C-protein?
C-protein has the same purpose as myomesin: holding myosin at the M line.
What is the purpose of Titin or Connectin?
Connects the thick filament (myosin II) to the Z disc.
What is the purpose of Desmin?
Desmin links myofibrils together.

Desmin is an intermediate filament.
What is Tropomodulin?
Tropomodulin is an actin capping protein.
What holds actin in place at the Z-line?
Both alpha-actinin and nebulin hold actin in place at the Z-line.
What holds myosin in place at the M-line?
Myomesin and C-protein holds myosin in place a the M- line.
What is the external lamina?
The external lamina is the equivalent of the basal lamina, but it is found in the endomysium.

The external lamina acts as an insulator for the muscle cell, preventing neighboring fiber cells from contracting because of contact with each other (rather than because of innervation).
What is dystrophin?
Dystrophin sits within the membrane, and binds to actin and dystroglycan. Dystroglycan passes through the sarcolemma (muscle cell's plasma membrane) and binds to laminin in the extracellular matrix.
Why is the dystophin complex important?
The dystrophin complex (actin connected to dystrophin, connected to dystroglycan, connected to the extracellular matrix) transfers the force of the actin/myosin contraction to the extracellular matrix.

This action is essential to harnessing muscle power.
What is Duchenne Muscular Dystrophy?
Duchenne Muscular Dystrophy is an X-linked genetic disease in which dystrophin is poorly produced.

Many of the individuals cells contain zero dystrophin (see image) making them unable to generate forceful muscle contractions.
In skeletal muscle, what is a triad? What does it do?
A triad consists of one t-tubule and two terminal cisternae of sarcoplasmic reticulum.

The purpose of a triad is to ensure that all of the fibers in a myofibril shorten at the same time.
What is a neuromuscular junction?
A neuromuscular junction is the space where skeletal muscle cells receive an action potential from the neuron.

Basically, the neuromuscular junction is the place where the axon of a skeletal muscle nerve reaches the skeletal muscle.
What is a motor unit?
A motor unit is a single motor neuron and all of the muscle fibers that it innervates.

Smaller motor units would give finer, more controlled contractions whereas large motor units would give larger, more powerful contractions.
What kinds of skeletal muscles would have a small motor unit? Large motor units?
Small motor units would be common in areas where fine muscle movement is important. Common examples would be fingers, eye muscles, etc.

Large motor units would be common in areas where large contractions are important. A Common example would be quadriceps.
What is a golgi tendon organ?
Mechano receptors that sense a physical change in the muscle environment and relay information to the CNS.

Basically, these receptors recognize changes in the length of muscle fibers and apprise the CNS.

Examples of these organs include: nuclear chain and bag fibers.
What is the purpose of a golgi tendon organ?
The point of golgi tendon organs is to allow the brain to control the strength of muscle reactions.
True or false: most cardiac cells have a single, centrally-oriented nucleus.

Unlike skeletal muscle cells, cardiac fibers are typically mono-nucleated.
What is an intercalated disc?
There are three regions in the intercalated disc. Two of them are important in cell-to-cell adhesion and the third is important in electrical coupling.

Basically, the intercalated disc helps to coordinate contractions among cardiac cells.
What are muscle diads? Where are muscle diads found?
A diad is a structure in cardiac myocytes that is located at the Z line or inbetween sarcomeres.

Diads are composed of a single t-tubule, paired with the terminal cisternae of a sarcoplasmic system.
Why are muscle diads and triads important?
Diads are important because they guide action potentials close to calcium sources so that muscle contractions follow closely behind action potentials.
What are purkinje fibers?
Purkinje fibers reach throughout the heart, propagating the current (action potential) to the cardio fibers.

Purkinje fibers are specialized to rapidly conduct impulses. They do this by having calcium channels that open very quickly and by limiting the number of myofibrils in their cells.
How are purkinje fibers in cardiac muscle different from other cardiac fibers?
Purkinje fibers are bi-nucleated, whereas typical cardiac fibers are mono-nucleated.

Additionally, Purkinje fibers have unique channels for releasing calcium and carry relatively few myofibrils.
Where is smooth muscle found?
Smooth muscle is found in blood vessels, hollow organs, the muscles of the eye, and in skin.
What is an eosinophilic cytoplasm? Which muscle type contains an eosinophilic cytoplasm?
Eosinophilic cytoplasm tends to stain very pink.

Smooth muscle cells contain this type of cytoplasm because they contain very few negatively charged bodies in the cytoplasm.

Hemotoxylin stains darker than eosin.
True or false: smooth muscle cells are organized into myofibrils.

Smooth muscle cells are not organized like cardiac and skeletal muscle. This is why cross-striations are not visible in smooth muscle.
Describe the surroundings of the smooth muscle.
Smooth muscle cells are attached to each other by both adhesions and gap junctions.

Smooth muscle cells are surrounded by an external lamina.
Smooth muscle cell nuclei appear similar to what kind of tissue? How can we tell them apart?
Smooth muscle cell nuclei appear similar to the nuclei of dense regular and irregular tissue.

We can distinguish smooth muscle nuclei because they tend to have a "corkscrew" morphology.

In other words, smooth muscle nuclei look like "squiggly" fibroblasts.
In smooth muscle, what is a "dense body."
Dense bodies anchor thin and intermediate filaments from smooth muscle to the plasma membrane.

Dense bodies are rich in alpha-actinin. They serve as the anchors upon which thin filaments exert force.

Dense bodies are homologous to the Z line in skeletal muscle.
How does smooth muscle contract?
The contractile proteins in smooth muscle are actin and myosin II.

The thick filaments are aligned between the thin filaments. Repositioning of the myosin heads move adjacent thin filaments in opposite directions.

Thin filaments are inserted into dense bodies that are attached to the cell membrane, the force generated by their movement alters the shape of the smooth muscle cell.
What is myasthenia gravis?
Myasthenia gravis is an auto-immune disease where the body produces antibodies that bind to acetylcholine receptors on the motor end plate and inhibit the excitatory effects of acetycholine. This results in general weakness of the muscles.
True or False. Cardiac muscle cells are long, simple, tubular structures.
False. Cardiac muscle cells are short and branched. In addition, their nucleus is located in the center of the cell as opposed to skeletal muscle where the nuclei are on the periphery.
What is the only connection formed between cardiac cells?
The only connection between cardiac muscle cells are intercalated discs.
What are the two components of an intercalated disc?
1. A desmosome (macula adherens and fascia adherens)
2. Gap junctions
The gap junctions in these cells are specialized to propagate current.
Intercalated discs have two parts, a transverse and longitudinal part between cells
Transverse has a desmosome (macula adherens) and fascia adherens
Longitudinal have the gap junctions
Intercalated discs have a transverse component and a longitudinal (lateral) component. Which parts of the intercalated disc do you find in each component?
The transverse component has macula adherens and fascia adherens.

The longitudinal component has a specialized gap junction.
What is it important that the electrical impulse of a heartbeat begins at the apex of the heart and works it way up? What makes sure this happens?
Blood will gather in the right ventricle, located and the bottom-right part of the heart. The electrical current needs to start there and work its way up so that the cardiac muscle will squeeze the blood out of the ventricle, into the pulmonary arteries, and then into the left atrium. Purkinje fibers do this.
Why do smooth muscle cells contract all at once?
Smooth muscle cells are connected by gap junctions. Also, some smooth muscle cells will contract in response to being stretched.
How is it that skeletal muscle cells/fibers can contract simultaneously?
How is it that cardiac muscle cells/fibers can contract simultaneously?
Diads, Purkinje fibers, and intercalated discs
How is it that smooth muscle cells/fibers can contract simultaneously?
Gap junctions