Title: Three Dimensional Sarcomere Analysis

Introduction: The sarcomere is the basic functional unit of striated muscular contraction and is bordered by Z lines. I bands are areas of only thin filaments. A bands correspond to the length of the thick filament and the H Zone is the measurement in the center of the A band that contains only thick filaments. The detailed images of transverse and longitudinal cross sections of specimens produced by the transmission electron microscope (TEM) allow for a technical analysis of sarcomere morphology and the sliding filament model of muscular contraction.

Objectives: You will learn to analyze various micrographs and recognize the limitations; to conceptualize two dimensional longitudinal and transverse cross sections of a sarcomere; to construct a working, 3-D, sliding filament model using basic materials and information obtained from the micrographs; and to relate muscle morphology and physiology.

Materials: (Per group) Prepared light microscope slides and TEM micrographs of cross and transverse sections of striated muscle (see diagrams below); 24 heavy duty pipe cleaners of six different colors; 2 strands costume beads; silly putty; glass beads; 24"x18" corkboard; freezer wrap; 2 yards of thin wire; marking pens; scissors; T pins; 2 dimensional analysis of sarcomere.

Procedure/Results: 1) Use light microscope to view prepared slides of striated muscles on low and high power. Sketch and label components of sarcomere on result sheet. 2) Observe various TEM micrographs of transverse and longitudinal cross sections. Sketch and label images. 3) Make your own 3-D, functional (working) model of a sarcomere. Limit model to one thick filament (composed of staggered arrays of myosin molecules) with appropriate surrounding thin filaments, two Z lines and each component in the fourth step. 4) Cover cork board with freezer wrap and attach 3-D model to board with pins and label: A) sarcomere, B) thin filament, C) thick filament, D) actin, E) myosin, F) tropomyosin, G) troponin, H) calcium binding site, I) calcium, J) Z line, K) I band, L) A band, M) M line, N) H zone, 0) cross-bridge, P) site of ATP® ADP + Pi + energy, 5) Demonstrate working sarcomere.

Conclusion: 1) Explain the necessity of the following for muscular contractions: A) motor neurons, B) sarcoplasmic reticulum, C) mitochondria. 2) A) What misconceptions of structure could you obtain from the analysis of only longitudinal TEM cross sections? B) Explain. 3) How do longitudinal and transverse TEM micrograph cross sections allow for accurate 3-D structural analysis? 4) Explain the advantages of TEM micrographs over light micrographs. Predict and label diagrams of TEM micrographs for the transverse and longitudinal cross sections of an axon. 6) Discuss the relationship between sarcomere morphology and the sliding filament model of muscular contraction.

Postlab: Group discussion and analysis of the various proposed models. Include a brief summary in the lab report.

Why is the above my favorite activity, and where does it fit into the curriculum?

Sarcomere models are not available and sarcomeres are very difficult to visualize from two dimensional text book format. I designed and authored this activity years ago to help students conceptualize a real three dimensional sarcomere. We study sarcomeres during our exploration of striated muscle in animal form and function unit. It is my favorite activity because students enjoy creating and manipulating their own working model of a sarcomere, and they profit from their analysis of the various unique models created by their classmates and not multiple machine-made replicas.

IMAGES FOR SACROMERE ACTIVITY

Sarcomeres are very important in the study of muscles.  The morphology helps to indicate the function.  The following are  micrographs of sarcomeres.

Cross Section Electron Micrograph of a Sarcomere

Diagram of Cross Section of Electron Micrograph of a Sarcomere

Transverse Electron Micrograph of a Sarcomere

Light Micrograph of Sarcomere Shows Striations

Students contruct a model of a sarcomere.