Vertebrate Skeletal Muscle Structure
Human Anatomy & Physiology Lab #1 Today we will examine the muscular system of the rat, with particular emphasis on the muscles of the hind limb. The goal of this lab is to consider the relationship between fine and gross muscle structure and function. By focusing on rat hind limb muscle today, you will learn general principles of muscle structure and function. In addition, there are many similarities but also a few interesting differences between the anatomy of the leg in the rat and in humans. Muscle cells develop embryonically from mesenchyme cells, which form myoblasts that become myocytes, or muscle fibers. Based on fiber structure, there are 3 general types of muscle: 1) skeletal, 2) cardiac, and 3) smooth. Cardiac muscle is found in the walls of the heart. Smooth muscle is found in the walls of blood vessels, and in some visceral organs. Skeletal muscle fibers have a complex organization into distinct units or muscles. Skeletal muscles attach directly to the skeleton or indirectly attach via tendons (which are extensions of connective tissue in the muscle). The origin of a limb muscle is its proximal end; the insertion is its distal end. Usually skeletal muscles are arranged in groups of agonists/antagonists: one muscle (the agonist) pulls on a structure in one direction and the second muscle (the antagonist) pulls on it in the other direction (e.g., the triceps brachii is an antagonist of the biceps brachii). Even though we are focusing on the muscular system of the rat it is important to note that there are significant differences between some of the muscles of the rat (a quaruped) and a human (a biped), these differences are found in both the hindlimb and in the muscles of the back and shoulders. In the hindlimb the major differences between the rat and the human are the relative masses of some of the proximal hip musculature. The hamstring muscles make up the largest muscle group in the rat and include the biceps femoris, the semitendinosus and the semimembranosus. In the rat the hamstring muscles are twice as massive as the antagonistic group of muscles the quadriceps (vastus lateralis, vastus medialis, rectus femoris, vastus intermedius). In humans this trend is reversed and the quadricep muscle group weighs approximately twice as much as the hamstrings. Similarly the gluteus maximus in quadrupeds is a relatively small muscle and it accounts for only 6% of the mass of the muscles acting at the hip. In humans the gluteus maximus is the largest muscle that acts on the hip joint and it accounts for 18.3% of the muscle mass that is capable of acting at the hip and is thought to play an important role in human running (Lieberman et al., 2006). In addition humans are missing the caudifemoralis. Anatomical variation between the rat and the human can also be found in the muscles of the back and shoulders. In the rat there are three trapezius muscles the spinotrapezius, the acromiotrapezius and theclavotrapezius. In the human there is only one large trapezius muscle called the spinotrapezius. In the rat there is also three different deltoid muscles the spinodeltoid, the acromiodeltoid and the clavodeltoid. In the human there is only one deltoid muscle. OBJECTIVES 1) Remove the skin from the rat torso and limbs. 2) Dissect the rat to expose the muscles listed in Table 1. Dissect the muscles. 3) Find the muscles in Table 1 on the rat, and on human diagrams. Understand their basic anatomy (e.g., origin and insertion), function (e.g., flexor/extensor), and how they differ in rats and humans. 4) Demonstrate your mastery of the material by taking a quiz at the end of lab. Dissection Tips • Rely primarily on your fingers and the blunt probe and NOT the scalpel. You can be pretty rough as long as you tear tissue along its natural boundaries. • If you must make a cut, hold the scalpel like a pencil. Use your other fingers to control your cut. Don’t cut anything unless you can see below it and know what you are cutting into! A good technique is to insert the blunt probe deep to whatever you’re cutting and use it as a guide. You can cut along or beside it and know that you aren’t cutting anything deeper than you think. • Separate the muscles along their natural boundaries by using the sheets of connective tissue encasing the muscles (fascia), and muscle fiber orientation. Minimize the number of cuts you make through muscles, and only cut after you have determined the identity, origin and insertion of the muscle. • When it is necessary to remove a muscle, always cut muscles by going straight through the belly, as perpendicular as possible to the orientation of the fibers, and equidistant from the origin and insertion. With this technique, the location of the origin and insertion is preserved and the muscle can be ‘replaced’ by flopping the halves of the muscle back into place.
Instructions Begin by removing the skin from the rat then using your lab manual to identify the superficial muscle in table 1. Select which forelimb and hindlimb you are going to dissect and remove any additional, superficial fat and/or connective tissue from the surface of the pelvis, thigh and shank. Remove fat pads lateral to the tail base and in the depression behind the knee. Find the muscles listed in Table 1. You can make notes on and add further detail to the following diagrams. Refer to the human skeletons for muscle attachments and functions, and be sure to review the action of each muscle as you delineate it. You have at your disposal the drawings in the lab manual, as well as your TF. Use these to find the muscles in the rat. You may also want to read the text to help you if you get lost. When you are finding the muscles listed in Table 1 on the human, or just for a more detailed understanding, you can also refer to the copies of Netter’s guide to human anatomy. For the exam you are responsible for all the muscles on both the rat and human, the muscles origin and point of insertion.
Muscle group Muscle Key to finding
Gluteals Gluteus medius Locate the gluteus superficialisand look below it. Also move the femur
Gluteus superficialis (gluteus maximus in humans)
Quadriceps Rectus femoris Reflect the fascia latae. It is theonly quad that DOESN’T insert on the femur.
Vastus lateralis Reflect the fascia latae and palpate the femur, this muscle will be anterior to the femur.
Vastus medialis Reflect the sartorius and palpate the femur, this muscle will be anterior to the femur
Superficial leg flexors
Gracilis
Hamstrings Semitendonosus
Semimembranosus Caudalmost muscle of the thigh
Biceps femoris Palpate the femur from the lateral side, the fibers inserting posteriorly will be the Biceps femoris
Hip Adductors Adductor magnus Palpate the femur from medial side, these fibers will insert below the Vastus medialis
Foot Extensors Gastrocnemius Muscle with two heads (medial and lateral)
Soleus This is the only monoarticular muscle of the Triceps surae
Foot Flexor Tibialis anterior Located on the anterior surface of the tibia.
Brachium – Upper Arm
Triceps brachii Be sure to find all three heads of this muscle
Biceps brachii Located deep to the pectoralis
Lower Arm Pronator teres Muscle responsible for pronating the lower forelinb Palmaris longus Central muscle on the anterior forelimb
Dorsal Thorax & Abdomen
Spinodeltoid Spinodeltoid fibers travel at right angles to the fibers of the other two deltoid muscles.
Latissimus dorsi Large, sheet-like muscle that covers most of the dorsal surface of the thorax.
Rhomboids These muscles connect the skull and vertebral column to the scapula.
Supraspinatus Reflect the muscles covering the scapula and locate the spine of the scapula; the muscle located on the scapula anterior to the spine is the Supraspinatus
Infraspinatus Reflect the muscles covering the scapula and locate the spine of the scapula; the muscle located on the scapula posterior to the spine is the Infraspinatus
Teres major Connects the posterior border of the scapula to the humerus Thoracic trapezius This muscle inserts on the scapula and lays over top of the
latissimus dorsi.
Ventral Thorax Pectoralis superficialis This muscle has two portions an anterior and posterior portion. Part of this muscle is located deep to the pectoantebrachialis
Pectoralis profundus Located posterior to the pectoralis major.
Serratus ventralis Muscle has a serrated appearance and is located along the lateral side of the ribs.
Head & Neck Muscles
Masseter Large chewing muscle located in the cheek
Abdominal Muscles
Rectus abdominus These muscles travel parallel to each other on either side of the linea alba.
External oblique First layer of lateral abdominal muscles.
Internal oblique 2nd layer, should be perpendicular to external obliques Transversus abdominus May be difficult to find, will be just inside parietal
peritoneum. Common terms to describe the action of a muscle Extensor: straighten joints Flexor: bend joints Adductor: move appendages toward the median sagittal plane Abductor: move appendages away from the median sagittal plane Pronator: turn the dorsal surface of a limb anteriorly Supinator: turn the ventral surface of a limb anteriorly Levator: raise structures (Please note the spelling – “levator”, not “elevator”.) Depressor: lower structures References: Homberger, D.G. and W.F. Walker, Jr. 2004. Anatomy and Dissectionof the Rat. W.F. Freeman. 122 pages. Lieberman, D. E., Raichlen, D. A., Pontzer, H., Bramble, D. M. and Cutright‐Smith, E. (2006). The human gluteus maximus and its role in running. J Exp Biol 209, 2143‐55.
