Muscular System

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Muscle tissue

There are three  basic kinds of muscle tissue: Smooth, Skeletal and Cardiac. In the table below you can see some differences and similarities in the tissues.

type involuntary/voluntary striated vs smooth function location
Skeletal voluntary Striated Moves skeleton Skeleton
Smooth involuntary smooth propel substances hollow and tubular organs
cardiac involuntary striated pump blood heart

 

Now looking deeper into each tissue will give us a better understanding:

Skeletal muscle:

For each muscle there is-

1 nerve

1 artery

1-2 veins serving it.

These muscles have a rich blood supply due to its high demand of oxygen and glucose as well as its high waste products of carbon dioxide and lactic acid.

Organization of Skeletal muscle:

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As you can see at the microscopic level we have the muscle fiber which is wrapped by the endomysium, then groups of muscle fibers create fasicles which are then wrapped by perimysium and groups of fasicles are brought together to create muscle which is then wrapped by epimysium.

 

Now lets take a closer look at the muscle fiber. Muscle fiber is just a fancy way of saying a singular muscle cell. For some reason, muscles have all sorts of substitute names for things we find in most cells. Lets break those down now:

Muscle fiber = muscle cell

Sarco = Muscle

Sarcolemma = plasma membrane

Sarcoplasm = Cytoplasm

Sarcoplasmic reticulum = endoplasmic reticulum

Why change the names?

Recall the meaning of the prefix cyto to mean ‘cell’? Well, the prefix sarco means muscle.

So now lets discuss the skeletal muscle fiber.

Unlike a stereotypical cell, the skeletal muscle fiber has about 100 or more nuclei and instead of being centrally located, they’re located peripheral to the sarcolemma.

Now in the sarcoplasm which is the cytoplasm of the muscle fiber we have several components:

Glycosomes store glycogen for the fiber

Myoglobin stores the oxygen

Myofibrils contain the contractile elements of the fiber and there are several different types of myofibrils:

Thick myofilaments contain myosin which acts as a cross bridge to thin myofilaments

Thin myofilaments which contain actin which is the contractile protein that forms a helix

regulatory proteins are troponin and tropomyosin

elastic filaments are composed of titan

Sarcomere is the smallest functional unit of muscle it is what causes the striations we see in skeletal muscle.

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Sarcoplasmic reticulum aka endoplasmic reticulum stores the calcium of the fiber and releases calcium when the fiber receives the signal to contract. It also contains the terminal cisternae which acts as a reservoir.

Sarcolemma aka plasma membrane transmits the signal to contract and T-tubules which are the in folding’s of the sarcolemma that reach every myofilament.

Now lets discuss the bigger question: How on earth do all these components make our muscles move everyday?

The answer is simple but also complex. Lets break it down into a couple of charts:

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here we have the Neuromuscular junction. Sounds pretty complex but its actually pretty simple because all of the six steps are intertwined. If the action potential never arrived at the axon terminal, voltage gated calcium channels would have never opened and the calcium wouldn’t be able to flood into the axon terminal which would make it impossible for acetylcholine to be released. And that just brings us to the whole mess of activating the receptors on the sarcolemma, which would allow for sodium to get into the cell and potassium to get out which furthers the process of an action potential for the muscle fiber! Basically, what I’m trying to say is, the neuromuscular junction is important because it allows the fiber to initiate an action potential which is what causes contraction which brings me to…………….

The excitation contraction coupling which is how the action potential in the sarcolemma leads to the sliding of the filaments which occurs in the t tubules in the sarcolemma.

image

Okay so lets talk about what’s happening in these pictures:

Step 1: the action potential propagates along the sarcolemma and down T-tubules

Step 2: calcium ions are released from the t tubules

Step 3:  Calcium travels down the t tubule and binds to troponin ( this is the regulatory protein which blocks actin) this action removes the blocking action of tropomyosin on actin.

Step 4: Contraction is then able to begin as myosin binds to actin and forms a cross bridge.

Which then brings me to…. The cross bridge cycle:

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Step 1 is known as Cross Bridge Formation and what happens here is the myosin head attaches to actin myofilament thus forming the cross bridge.

image

Step 2 aka Power/ working stroke

3 things happen:

1. ADP and inorganic phosphate are released

2. myosin head pivots and bends

3.myosin head pulls the actin filaments toward the M-line (middle of the sarcomere)

image

Step 3 aka cross bridge detachment:

Myosin and actin link weakens and the myosin head detaches when ATP binds to myosin.

image

Step 4 aka Cocking of myosin head:

ATP is hydrolyzed to ADP and inorganic phosphate and the myosin head returns to high energy position.

 

In contrast lets discuss the components of a smooth muscle fiber:

As we saw in the chart, smooth muscle is found in the respiratory, digestive, urinary, reproductive tracts, arteries and veins.

It’s functions are to mix and propel contents of lumen of an organ which is commonly known as peristalsis. It also increases and decreases the diameter of airways and blood vessels which allows for blood and air flow.

Unlike Skeletal muscle, smooth muscle does not have any sarcomeres present which means its has no striations, hence the name smooth muscle.

Delving deeper into the smooth muscle down to the cellular level we find that smooth muscle fibers:

contain one central nucleus

have thick and thin filaments but unlike skeletal muscle, its disorganized and not arranged in sarcomeres.

On an organizational level the smooth muscle fiber is organized into layers:

Longitudinal layer– along the longest axis of the organ

circular layer– encircle the organ

oblique layer– not always present

 

Now how does it contract?

It contracts by way of sliding filaments, just like skeletal muscle.

 

Comparing and contrasting skeletal and smooth muscle:

Feature Skeletal Smooth
Location Skeleton – attached to bones Hollow and tubular organs
Voluntary vs. involuntary voluntary involuntary
Regulators Motor neurons (nervous system) Nervous system, hormones, intrinsic nervous system, stretching
neurotransmitters acetylcholine acetylcholine
norepinephrine
microscopic features > 100 nuclei (peripheral)
striated
cylindrical
1 nucleus (central)
smooth
spindle shaped
Sarcomeres? Yes No (scattered thick and thin filaments
Neuromuscular junction/ diffuse function Neuromuscular junction (highly structured) Diffuse junction (poorly structured)
Function Movement mix and propel contents of a body cavity
regulate airflow
changes diameter to regulate blood flow
connective tissues epimysium (muscle)
perimysium (fasicle)
endomysium (fiber)
endomysium
regulatory proteins troponin calmoduim
     
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Articulations

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What’s an articulation?

it’s a joint, which is a site where two or more bones meet.

Now, joints are classified in two ways:

1. By structure

2. By function

Structure Fibrous Cartilaginous Synovial
Function Synarthrosis Amphiarthrosis Diarthrosis

These categories interrelate in the sense that:

Fibrous joints are synarthrotic, meaning they are generally immovable joints

Cartilaginous joints are generally amphiarthrotic which is defined as a semi movable joint

Synovial joints are characterized as diarthrotic which means they are freely moveable.

Now lets get into detail:

Fibrous joints

Fibrous joints are made of dense fibrous connective tissue and there is no joint cavity present. There are three different types of fibrous joints:

1. Suture

2. Syndesmosis

3. Gomphosis

 

Moving onto Cartilaginous joints:

Much like the synovial joints, cartilaginous joints don’t have a joint cavity. Instead of dense fibrous connective tissue, cartilaginous joints are comprised of cartilage which connects articulating bones. These types of joints are not highly moveable and are usually classified as amphiarthrotic. There are two different types of cartilaginous joints:

1. Synchondrosis:

2. Symphysis:

Lastly, Synovial joints are defined as articulating bones separated by a fluid containing joint cavity. This is what sets synovial joints apart from cartilaginous joints and fibrous joints. Additionally, synovial joints are diarthrotic which means they are freely movable.

The components of a typical synovial joint include:

articular cartilage- lines the ends of bones in a joint

articular capsule-  encloses the joint cavity of a synovial joint.

synovial fluid- lubricates the joint surfaces and nourishes articular cartilages.

synovial cavity

reinforcing ligaments

nerve endings

Stabilizing factors:

1. Deep sockets

2. Close fit of articular surfaces

3. More ligaments

4. Muscle tone

Structures of the synovial joints:

Function Structure
Bursae act as cushion or buffer flattened fibrous sacs filled with synovial fluid
Tendon Sheaths reduce friction elongated tubelike bursae which wraps around tendons
Articular fat pads act as a cushion between fibrous capsule and synovial membrane or bone
Menisci Separate articular surfaces fibrocartilage pads

And there you have it. Joints and articulations in a nutshell, hope you enjoy 🙂