In this activity, you will view skeletal (a.k.a. striated or voluntary) muscle under the microscope.

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1 Week 08 Lab Muscle Tissue LEARNING OUTCOMES: Describe the major functions of muscle tissue. Compare and contrast the structural and functional characteristics of skeletal, cardiac and smooth muscle. Provide examples of the location of each type of muscle tissue in the body. Describe the anatomy of the neuromuscular junction. Explain the arrangement of smooth muscle layers and their ability to produce peristaltic contractions. INTRODUCTION: Muscle tissue is highly specialized to contract and produces most types of body movement. As you might expect, muscle cells tend to be elongated, providing a long axis for contraction. The three basic types of muscle tissue are explored in this lab. ACTIVITY 1: Skeletal Muscle In this activity, you will view skeletal (a.k.a. striated or voluntary) muscle under the microscope. Skeletal muscle, the "meat," or flesh, of the body, is attached to the skeleton. It is under voluntary control (consciously controlled), and its contraction moves the limbs and other external body parts. The cells of skeletal muscles are long, cylindrical, and multinucleate (several nuclei per cell), with the nuclei pushed to the periphery of the cells; they have obvious striations (stripes). 1. Obtain a slide of muscle tissue that contains skeletal muscle cut in longitudinal section and prepare to examine it microscopically. The best slides to view are the ones that are stained darkly. 2. In long section, one can appreciate the linear nature of skeletal muscle fibers. Note that the individual skeletal muscle fibers are linear, cylindrical cells with a diameter of approximately microns, depending on the specific muscle organ. Each cell runs parallel to the long axis of the organ, and each cell runs the entire length of the organ, from origin to insertion, without branching or making intracellular contacts with neighboring cells. Each muscle fiber is wrapped by a loose connective tissue called endomysium. Endomysium is a loose connective tissue sheath that consists of a network of fine 1

2 reticular fibers embedded in ground substance. Because it lacks significant amounts of elastic or collagen fibers, the endomysium is unstained in routine hematoxylin/eosin-stained preparations. The endomysium not only lends physical support to each muscle fiber, but also supports the small blood vessels and nerve arborizations that supply the tissue. 3. On the skeletal muscle slide, you need to: find the skeletal muscle fibers; find the nuclei, usually pushed up against the inside of the sarcolemma (muscle fiber plasma membrane); and identify the dark A bands and light I bands that make up the striations. ACTIVITY 2: The Neuromuscular Junction In this activity, you will observe both a model and a prepared microscope slide of a neuromuscular junction. Skeletal muscle cells are always stimulated by motor neurons via nerve impulses. The junction between a nerve fiber (axon) and a muscle cell is called a neuromuscular junction. Each motor axon breaks up into many branches called axon terminals as it nears the muscle, and each branch forms a neuromuscular junction with a single muscle cell. Thus a single neuron may stimulate many muscle fibers. Together, a neuron and all the muscle cells it stimulates make up the functional structure called the motor unit. The neuron and muscle fiber membranes, close as they are, do not actually touch. They are separated by a small fluid-filled gap called the synaptic cleft (see below). Within the axon terminals are mitochondria and vesicles containing a neurotransmitter chemical called acetylcholine. When a nerve impulse reaches the axon endings, some of the vesicles release their contents into the synaptic cleft. The acetylcholine rapidly diffuses across the junction and combines with the receptors on the sarcolemma. If sufficient acetylcholine is released, the permeability of the sarcolemma changes briefly, allowing more sodium ions to diffuse into the muscle fiber. If enough sodium enters the fiber, the result is generation of an action potential along its length. Week 08 Lab: Muscle Tissue 2

3 1. Examine a three-dimensional model of skeletal muscle cells that illustrates the neuromuscular junction. Identify the structures listed below: axon terminal synaptic vesicles synaptic cleft sarcolemma (note: the region of the sarcolemma just beneath the axon terminal is sometimes called the motor end plate) myofibrils A bands and I bands 2. Obtain a prepared slide of a neuromuscular junction. (Note: these slides may also be called motor end plates or axon terminals.) 3. On the neuromuscular junction slide, you need to: find the skeletal muscle fibers; find the axons that branch out across the muscle tissue, along with their boot-shaped axon terminals; and find the darkly-staining synaptic vesicles inside the axon terminals. Important note: Because these slides are whole mounts (as opposed to sections), you will not be able to see the synaptic clefts. They are squeezed shut when the coverslip of the slide is put on. Week 08 Lab: Muscle Tissue 3

4 ACTIVITY 3: Cardiac Muscle In this activity, you will view cardiac muscle tissue under the microscope. Cardiac muscle tissue occurs only in the walls of the heart. Cardiac muscle tissue is striated like skeletal muscle tissue. Each cardiac muscle fiber has a single nucleus (the cell is said to be uninucleated) and is branched. Cardiac muscle fibers are connected to one another by intercalated discs, special gap junctions that conduct contraction stimuli from one muscle fiber to the next. Unlike skeletal muscles, cardiac muscle is under involuntary control. For example, when you exercise, nerves of the autonomic nervous system cause an increase in your heart rate in order to deliver more blood to the active tissues. When you relax or sleep, the autonomic nervous system lowers your heart rate. 1. Place the slide of cardiac muscle tissue on the microscope stage and swing the low-power objective lens into position. Rotate the coarse adjustment knob to bring the image into focus. 2. Using the photomicrograph above as a reference, examine the heart muscle at low and medium magnifications. Try to find a region of the slide in which the cardiac muscle can be viewed longitudinally (rather than in cross-section).. How many nuclei do you see in each cardiac muscle fiber? How do cardiac muscle fibers compare in size with skeletal muscle fibers? 3. Increase the magnification to high and observe several muscle fibers. Do you see striations and branching? What structure connects adjacent fibers? Week 08 Lab: Muscle Tissue 4

5 ACTIVITY 4: Smooth Muscle In this activity, you will view smooth (a.k.a. involuntary) muscle under the microscope in a section of digestive tract. Smooth muscle, or visceral muscle, is found mainly in the walls of hollow organs (digestive and urinary tract organs, uterus, blood vessels). Typically it has two layers the circular and longitudinal layers that run at right angles to each other; consequently its contraction can constrict or dilate the lumen (cavity) of an organ and propel substances along predetermined pathways. This type of movement is called peristalsis and is similar to the way that earthworms crawl. Smooth muscle cells are quite different in appearance from those of skeletal or cardiac muscle. No striations are visible, and the uninucleate smooth muscle cells are spindle-shaped. longitudinal layer circular layer 1. Obtain a slide of serous/mucous membrane. (Note: you have already seen this slide, back in the lab on tissues.) This slide comes from some part of an animal s digestive tract (probably the small intestine). 2. Find the smooth muscle in the wall of the digestive organ. Use the photomicrograph above to help orient yourself. 3. On the serous/mucous membrane slide, you need to: find both layers of smooth muscle (they are toward the superficial side of the organ); describe the shape of the smooth muscle fibers in the circular layer (you can t do this in the longitudinal layer, because the fibers are coming out of the slide towards you); and recognize that smooth muscle fibers are unstriated. Week 08 Lab: Muscle Tissue 5