Structure of Muscle II: Microscopic Structure

Introduction to the Microstructure of Skeletal Muscle

• We'll start by looking at the bigger picture of muscle cells, like what you might see with your naked eye.

• Then, we'll dive deeper into the tiny, microscopic parts of skeletal muscles, which you'd need a microscope to see.

• This will help us understand how these tiny parts work together to make your muscles shorter (contract) and allow you to move.

Learning Objectives

• By the end of this explanation, you should be able to:

  • Point out and name the different parts of a sarcomere, which is the basic building block of muscle contraction.

  • Identify the three main parts of something called a "triad" and explain why it's so important for your skeletal muscles to work.

Sarcomere Structure

• A sarcomere is like a fundamental LEGO brick of a muscle; it's the smallest unit that can actually contract.

  • Think of a long muscle fiber (a single muscle cell) as being made up of many sarcomeres lined up one after another, like boxcars on a train.

  • Each sarcomere is surrounded by a special kind of internal "net" or "storage system" called the sarcoplasmic reticulum (SR), which is similar to a storage network found in other cells (the endoplasmic reticulum).

Sarcoplasmic Reticulum (SR)

• Key Functions:

  • It acts like a secret storage locker for calcium, which is super important because calcium is the signal that tells a muscle to contract.

  • It also helps to pass messages using special "tunnels" called transverse tubules (T Tubules).

Transverse Tubules (T Tubules)

• Definition: These are like deep tunnels or extensions of the muscle cell's outer membrane (called the sarcolemma) that reach deep inside the muscle cell.

• Function: They're like a communication highway, allowing messages from the outside surface of the muscle cell to quickly travel all the way to its inner parts.

Key Structural Components of the Sarcomere

• Z Disc: Imagine these as the "end lines" or boundaries of each individual sarcomere, marking where one muscle unit begins and another ends.

• I Band:

  • Definition: This is the lighter-looking area of the sarcomere, found right next to the Z Disc.

  • Composition: It's mainly made up of very thin protein strands called "thin filaments" and some helper proteins like titin.

  • Light Microscopy Appearance: Under a microscope, this section looks pale and bright (we call this "isotropic") because it doesn't block or scatter light much, due to having only one type of filament.

• A Band:

  • Definition: This is the darker-looking area of the sarcomere.

  • Composition: It contains both the thin filaments (from the I band) and thicker protein strands called "thick filaments," which overlap in this region.

  • Light Microscopy Appearance: Under a microscope, it appears dark (we call this "anisotropic") because the two different types of overlapping filaments scatter light significantly.

• H Zone:

  • Definition: This is the lighter, central part within the darker A band.

  • Composition: It's made up only of the central parts of the thick filaments. The "motor" sections (myosin heads) of the thick filaments aren't found here; they are further out.

  • Appearance: It looks a bit lighter than the rest of the A band because there are no thin filaments here.

• M Line:

  • Definition: This is the exact middle line that cuts through the center of the sarcomere, right in the middle of the H zone.

  • Composition: It contains the thick filaments and special proteins (myomesin) that help hold the thick filaments in place.

Contractile Mechanism of Sarcomeres

• When your muscles contract or shorten:

  • The sarcomeres themselves get shorter, pulling the Z Discs closer to the very center (the M line).

  • Key Point: It's super important to remember that the thin and thick filaments themselves do not shrink or change in length. Instead, they just slide past each other, increasing their overlap.

  • Mechanism: Think of it like a tug-of-war: tiny "heads" on the thick filaments (myosin heads) grab onto the thin filaments and pull them inwards, causing them to slide over the thick filaments. This is what we mean by "contraction."

  • This shortening only happens a tiny bit (at a micron level, $10^{-6}$ meters) for each sarcomere, but when you have millions of them linked together, it adds up to a noticeable shortening of your entire muscle by several centimeters.

• Visual Representation:

  • In a relaxed muscle, you'd see a relatively wide I band and a distinct H zone.

  • In a contracted muscle, the I band becomes much smaller, and the H zone can even disappear completely as the filaments slide past each other.

Regulation of Muscle Contraction

• Role of Sarcoplasmic Reticulum:

  • As we mentioned, it stores calcium.

  • When it releases these stored calcium ions, it's like sending a "go" signal that starts the muscle contraction.

• Components of the Triad:

  • This special structure is formed by three parts coming together:

    • Two enlarged sacs on either side of the SR, called "terminal cisternae."

    • One T Tubule, which runs between these two sacs (like a sandwich).

  • Function: The triad acts as a crucial communication hub, connecting signals from the outside of the muscle cell (via the T Tubules) to the internal calcium stores (in the SR), allowing for quick muscle activation.

Calcium Function in Muscle Contraction

• Calcium acts as a vital signaling molecule that essentially turns muscle contraction "on" or "off."

  • In a relaxed muscle, most of the calcium is kept neatly tucked away in the sarcoplasmic reticulum (SR).

• Voltage Sensors and Channels:

  • Dihydropyridine receptor (DHPR): This is a special protein in the T Tubules that acts like a "voltage sensor." It detects when an electrical message (an action potential) travels down the T Tubule.

  • Ryanodine receptor: This is a calcium release channel located in the SR membrane.

  • Interaction:

    • When an electrical signal (action potential) travels along the T Tubule, the DHPR (voltage sensor) gets activated.

    • This activation causes the nearby Ryanodine receptors (calcium gates) in the SR to open up.

    • When these gates open, calcium floods out of the SR and into the muscle cell's interior, kickstarting the process that leads to muscle contraction.

Summary of Key Points

• The fundamental unit responsible for muscle shortening is the sarcomere.

• Myofibrils, which are long strands within muscle cells, are basically many sarcomeres linked together end-to-end, from one Z Disc to the next Z Disc.

• The triad (made of the sarcoplasmic reticulum and T Tubules) is extremely important for sending signals that lead to muscle contraction by releasing calcium.