Objectives
- Identify the 3 types of muscle: smooth, skeletal, and cardiac
- Describe the morphological differences between the 3 types of muscle
- Distinguish muscle from types of connective tissue
Overview
Muscle functions through contraction of elongated, specialized cells composed of myofilaments found in the sarcoplasm (cytoplasm specific for muscle). Thin filaments are primarily composed of actin while thick filaments are primarily composed of myosin II.
Classification of muscle based on morphology:
- Striated muscle– exhibits cross-striations at light microscopic level
- Skeletal muscle– attached to bone and responsible for movement of axial and appendicular skeleton and eye movement
- Visceral striated– morphologically identical to skeletal muscle but specific for soft tissue such as the tongue, pharynx, portion of diaphragm, and upper esophagus
- Cardiac muscle– specifically found in the heart and base of large veins that empty into the heart
- Functional unit of striated muscle: sarcomere– z line to z line
- Smooth Muscle– no cross-striations; muscle cells, or fibers, that appear as elongated cells with tapered ends, lack highly arranged thick filaments, connected by gap junctions
Connective Tissue (CT) organization in skeletal muscle:
- Endomysium– layer of reticular fibers with some small blood vessels surrounding each muscle fiber made up of muscle fibrils (subunit of muscle fibers made up of myofilaments)
- Perimysium– thicker CT surrounding a bundle of muscle fibers, or fascicle.
- Epimysium– dense CT surrounding collection of fascicles (same as investing fascia in gross anatomy)
>Muscle Tissue Slides
There are 2 specimens of plastic-embedded skeletal muscle, one in longitudinal section and the other in cross-section. In the cross-section, focus on identifying the muscle fibers and arrangement of connective tissue. Examine the difference between nuclei of muscle fibers and nuclei of fibroblasts in the CT (endomysium and perimysium). Fibroblast nuclei can appear a bit wavy with a darker stain due to more heterochromatin while myocyte nuclei are fatter and lighter stained. In the longitudinal section, you can examine the sarcomeres, locating Z-lines, A-bands and I-bands.
The esophagus is an interesting example of muscle. In the muscularis externa layer of the esophagus, the upper portion (proximal) of the esophagus is contains skeletal muscle while the distal portion closest to the stomach contains smooth muscle and there is a mixture of both smooth and skeletal muscle in the middle. This cross-section is from the middle so it contains both smooth and skeletal muscle in the muscularis externa layer. There is dense irregular CT between the epithelium and muscularis externa, compare this to smooth muscle which looks similar but there are differences. You should see more nuclei per unit or volume in smooth muscle compared to CT. This is a great slide to review ALL of your basic tissues (except bone, cartilage, and cardiac muscle).
This specimen is from the heart. The lumen or inside of the chamber is located at the bottom of the slide. There is a very thin layer known as the endocardium which is made up of simple squamous epithelium (endothelium) and connective tissue. The rest of the specimen is the myocardium, containing muscle fibers and blood vessels. Use this slide to carefully look at cardiac muscle. Look for intercalated discs along with light and dark bands of the sarcomere. The other details of the sarcomere are more difficult to see here.
The uterus is a good example for observing disorganized smooth muscle. The muscle fibers go in every direction in order to create strong contractions. The epithelium in the uterus changes throughout the mensuration cycle and creates a thick layer compared to other organs. Use this slide to examine the smooth muscle and compare it to surrounding connective tissue.
This is a section through the forearm. It contains all the basic tissues (except bone, cartilage, and cardiac muscle) so it is a great slide to review previous topics. The center contains the forearm muscles surrounded by vessels, nerves, connective tissue, and skin.
>More Practice
At the gastroesophageal junction, there is a drastic change in the epithelium. Use this slide to review epithelium and identify muscle tissue.
Use this slide to identify muscle and connective tissue.
This specimen includes part of the heart, coronary vessels, and a heart valve. Use this slide to identify different types of tissue.
>More Practice Answers
Michigan 155. Gastroesophageal Junction
- Stratified squamous
- Simple columnar
- Smooth muscle (makes up the muscularis mucosae of the GI track).
- Black arrows- both are smooth muscle, the top is longitudinal section and the bottom is in cross-section; Yellow arrow- connective tissue
- Nerve (myenteric plexus)
- Adipocytes
GWU 09. Muscle-Tendon Junction
- Skeletal muscle
- Black arrows- muscle fibers; Green arrows: fibroblasts
- Dense regular CT; fibroblasts
UVA 36. Heart
- A) cardiac muscle; B) intercalated discs
- Simple squamous
- Smooth muscle
- A) Areolar tissue/adipose tissue (a type of loose CT); B) adipocytes
- Dense irregular CT
