2.7: Lab Exercise 9- Microanatomy of the Muscular System

Lab Summary: In this lab, you will learn about the microscopic anatomy and physiology of skeletal muscle tissue. Possession of a solid understanding of the anatomy of skeletal muscle tissue will better enable you to comprehend the complex physiology of this tissue.

Your objectives for this lab are:

• In a model of the muscle fiber, describe the locations and functions of the following: o Endomysium
  • Sarcolemma
  • T-tubule
  • Mitochondria
  • Sarcoplasmic reticulum o Sarcomere
  • Myofibril
  • Myofilaments
  • Myosin
  • Actin
  • Zdisc
  • Iband
  • Hzone
  • Aband
  • Synaptic cleft
  • Synaptic vesicles
  • Acetylcholine
  • Motor neuron
  • Axon terminal
  • Neuromuscular junction
• In a slide of skeletal muscle (cross-section), describe the locations and functions of the following: o Epimysium
  • Perimysium
  • Fascicle
  • Endomysium
  • Muscle fiber
• In a slide of the neuromuscular junction, describe the structure and functions of the neuromuscular junction:
  • Axon
  • Axon terminals
  • Synaptic vesicles o NMJ

Background

The best-known function of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.

Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat.

Activity 9.1: Microanatomy of Muscle Tissue in Histology Slides

Connective Tissue Structures of Skeletal Muscle

Each skeletal muscle is an organ that consists of various integrated tissues. These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it and provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within the muscle (Figure \(\PageIndex{1}\)). Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently. Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle. Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue. Deep to the endomysium is the sarcolemma, the muscle’s cell membrane that helps contain the cellular contents and propagate the action potential throughout the muscle fiber.

Tendons, dense regular connective tissue structures, which connect muscles to bones, intertwine with the collagen of the epimysium. The other end of the tendon fuses with the periosteum surrounding the bone’s external surface. The tension created by contraction of the muscle fibers is then transferred through the mysia membranes, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places, the mysia membranes may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia (the connective tissue between skin and bones). Generally, an aponeurosis connects muscles to other muscles. The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an aponeurosis. As a reminder, ligaments connect bones to other bones.

Because skeletal muscles are active organs, each one is richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system.

Figure \(\PageIndex{1}\): The Three Connective Tissue Layers Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium.

The Neuromuscular Junction

Another specialization of the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ) (Figure \(\PageIndex{2}\)). This is where the muscle fiber first responds to signaling by the motor neuron. The response is a series of graded potentials (not full action potentials). Graded potentials are begun when acetylcholine is released from the axon terminal, diffuses across the synaptic cleft, and binds to ligand gated channels on the sarcolemma. The binding of acetylcholine to these channels allows Na+ ions to move into the muscle’s sarcoplasm, which increasingly moves the membrane potential to become more positive. Ultimately, the sum of many excitatory graded potentials will cause the muscle’s internal voltage to reach the depolarization stage and an action potential will flow through the muscle fiber. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the muscle fiber to contract.

Figure \(\PageIndex{2}\): Motor End Plate and Innervation At the NMJ, the axon terminal releases Acetylcholine (Ach). The motor end plate is the location of the ligand gated Ach receptors in the muscle fiber’s sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the ligand gated channels on the muscle’s sarcolemma.

Procedure for Activity 9.1: Microanatomy of Muscle Tissue in Histology Slides

Part 1: Cross-section of Skeletal Muscle

1. Obtain a slide of skeletal muscle (cross-section).
2. View the slide using the 10x or 40x objective, as directed by your instructor.
3. Use the information above to identify the structures listed in the objectives and describe their functions.
4. In the space below, draw a section of the skeletal muscle as seen in your slide. Label the epimysium, perimysium, fascicle, endomysium, and the muscle fiber.

Part 2: The Neuromuscular Junction

1. Obtain a slide of the neuromuscular junction.
2. View the slide using the 10x or 40x objective, as directed by your instructor.
3. Use the information above to identify the structures listed in the objectives and describe their functions.
4. In the space below, draw a section of the skeletal muscle with NMJ as seen in your slide. Label the axon, axon terminal containing synaptic vesicles, and a muscle fiber. Circle the NMJ.

Activity 9.2: Microanatomy of Muscle Tissue in Models

Skeletal Muscle Fibers

Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers. Skeletal muscle fibers can be quite large for human cells, with diameters up to 100 μm and lengths up to 30 cm (11.8 in) in the Sartorius of the upper leg. Skeletal muscles are multinucleated; multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction.

Some other terminology associated with muscle fibers is rooted in the Greek sarco, which means “flesh.” The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) is called the sarcoplasmic reticulum (SR) (Figure \(\PageIndex{3}\)). As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins, such as desmin, titin, and nebulin.

Figure \(\PageIndex{3}\): Muscle Fiber A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many myofilaments, which give the cell its striated appearance.

The Sarcomere

The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere, which is the functional unit of the muscle fiber. The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end. As myofibrils contract, the entire muscle cell contracts. Each sarcomere has a three-dimensional cylinder-like arrangement and is bordered by structures called Z-discs (also called Z-lines, because pictures are two-dimensional), to which the actin myofilaments are anchored (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): The Sarcomere The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber.

Excitation-Contraction Coupling

All living cells have membrane potentials, or electrical gradients across their membranes. Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances. For a skeletal muscle fiber to contract, its membrane must first be “excited”—in other words, it must be stimulated to fire an action potential. Electrical events preceded mechanical events! The muscle fiber and its motor neuron meet physically and functionally at the neuromuscular junction (NMJ), (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Motor End Plate and Innervation

Other Important Structures in Muscle Contraction

As you’ve read, getting a muscle to contract is a complex series of processes! There are several other structures involved in these processes (Figure \(\PageIndex{6}\)). The muscle fiber’s action potential sweeps along the sarcolemma and deep into the muscle’s fibers via the T-tubules, invaginations (inward foldings) of the sarcolemma. This excitation triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (the SR), where Ca2+ is stored. Multiple mitochondria serve as the site of aerobic cell respiration in which ATP is made to power the reactions that generate muscle contraction.

Figure \(\PageIndex{6}\): The T-tubule Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad—a “threesome” of membranes, with those of SR on two sides and the T- tubule sandwiched between them.

Procedure for Activity 9.2: Microanatomy of Muscle Tissue in Models

1. Obtain a model of a skeletal muscle fiber.
2. Use the information presented in Activities 1 and 2 to identify the following structures and describe their functions: Endomysium, Sarcolemma, T-tubule, Mitochondria, Sarcoplasmic reticulum (SR), Sarcomere, Myofibril, Myofilaments, Myosin, Actin, Z disc, I band, H zone, A band, Synaptic cleft, synaptic vesicles, acetylcholine, Motor neuron, Axon terminal, Neuromuscular junction

Additional Learning Resources (see electronic document in Canvas to access weblinks):

• Watch this video to learn about the microscopic anatomy and physiology of skeletal muscle:  https://youtu.be/qQpZnzuzAQI
• Watch this video to review the microscopic muscle fiber model:  https://www.youtube.com/watch?v=v-9VegCKF6U&feature=youtu.be
• Practice identifying muscles in models and identifying structure in a muscle fiber with this document:  Muscle Model Labeling Practice.pdf  (found in the Canvas Module containing the electronic copy of the SLM)
• More Practice identifying structures in muscle fibers models here:
  • http://academic.pgcc.edu/~aimholtz/AandP/PracPrac/2050_Lab22/musclehisto.html
  • https://pierceanatomy.weebly.com/nervous-212080-862029-823599.html