Introduction to the Muscular System

You’ll remember that in the study of the skeletal system, each bone was viewed as an organ within the skeletal system. It is also the case that each of our approximately 600 muscles (accounting for 40% of our body weight) can be viewed as individual organs within the muscular system. Each of these organs will obviously have muscle fibers, but will also include nervous tissue and connective tissue. Muscle are important not only because they help us move, but also because they help us maintain proper posture and produce heat.

You’ll recall from earlier discussions that there exist three types of muscles: skeletal, cardiac, and smooth. All three types share common characteristics, such as: irritability, the property of being sensitive to incoming nerve impulses; contractility, which means the shortening of the muscle in response to incoming nerve stimuli; extensibility, the ability of a non-stimulated muscle to be stretched beyond its resting length by the action of an opposing muscle; and elasticity, the ability of an extended muscle to return to its original resting length.

Tendons and Associated Connective Tissue
Muscles are attached to bone by tendons and to other boney structures by ligaments. Remember from earlier discussions that our tendons are made by dense regular connective tissue that travels between the muscle and periosteum of bone. Some tendons of muscles are covered by a double-layered sheath, called a tendon sheath, that is filled with synovial fluid. This is the case in the tendon sheath that covers the long head of the biceps brachii. In other places, the tendons become long and flattened into a sheetlike structure as they come off the muscle and are referred to as an aponeurosis, an example being the galea aponeurotica between the frontalis muscle and the skull. In places such as the wrist and ankles, our tendon sheaths are themselves covered with a layer of connective tissue called a retinaculum that helps to keep them in place during muscle movement. An example is the flexor retinaculum that covers the tendons of the flexors of the forearm at the wrist.

To protect muscle fibers, to assist in efficiently transmitting the force of contraction through a muscle, and to provide pathways for nerves and blood vessels, muscle fibers are associated with each other through loose connective tissue. The classification of connective tissue within muscle is based upon the level of muscle tissue structure that the connective tissue encloses. For example, the individual muscle cell, called the muscle fiber, is enclosed within connective tissue referred to as endomysium. Multiple muscle fibers are bound together in bundles called fasiculi, and the connective tissue surrounding fasiculi is referred to as perimysium. Covering the whole muscle is a connective tissue referred to as epimysium.

Then, between the muscle and skin there often exists deep fascia that blends with the epimysium. Beyond this, there exists superficial fascia, which connects underlying muscle to overlaying skin.

Muscles at the Cellular Level
Muscle fibers are muscle cells. They are more elongated than most other cells. However, like other cells, they contain a cell membrane and cellular organelles, such as mitochondria. An important difference between muscle cells and other cells, however, is that muscle cells contain several nuclei and are striated. Some of the nomenclature of muscle cellular structure is unique to this type of cell. For example, the cell membrane of a muscle fiber is called a sarcolemma. The cytoplasm of a muscle cell we call sarcoplasm. Within this sarcoplasm exists a network of branched membranes called the sarcoplasmic reticulum. Then, there is also a system of T-tubules that runs perpendicular to the sarcoplasmic reticulum. The T-tubules and sarcoplasmic reticulum are important in the uptake and transport of calcium throughout the cell that is needed to carry out muscle contraction.

The framework of the sarcoplasmic reticulum and t-tubules surround many threadlike structures called myofibrils. These myofibrils are themselves composed of two kinds of myofilaments: actin and myosin. The actin are relatively thin filaments, while the myosin are relatively thick filaments. As the actin slides over and between the myosin, muscle contraction occurs.

I have summarized the terms of muscle structure below for you, from larger to smaller structure (top to bottom), for both macro and cellular levels of organization in muscle tissue:

Macro Level of Muscle Organization and Associated Connective Tissue:
Superficial fascia connecting muscle to skin
Deep fascia interwoven with Epimysium
Epimysium covering fasiculi (bundles of muscle fibers)
Fasiculi connected together by perimyisium
Each muscle fiber (muscle cell) covered in endomysium

Cellular Level of Muscle Organization
Sarcolemma, cell membrane
Sarcoplasm containing multiple myofilaments, as well as other organelles
Myofibrils composed of myofilaments
 

Two types of myofilaments: actin (thin) and myosin (thick)

Z line=a disc running transversely through an area composed only of actin; the distance between one Z line to the next Z line is called a sarcomere.

A band= the band stretching from one end of a myosin myofilament to the other; contains area of overlap between myosin and actin.

H zone=the subsection of an A band that contains only myosin

I band=the area between A bands that contains only actin and the Z lines.

Concepts of Gross Muscle Movement and Architecture
As a muscle contracts and tension is placed on the tendons, the bones attached to the muscle are moved. Depending on where the attachments of the muscle to the bone occur, one area of bone-muscle attachment will move more than the other bone-muscle attachment. Generally, the more moveable bone-muscle attachment is called the insertion and is moved toward the less moveable bone-muscle attachment called the origin. The thickened middle section of the muscle is called its belly (gaster). Even if you can’t remember which attachment is origin and which is insertion, as long as you remember the general area of attachments, this will be enough to help you understand the function of the muscle, and this understanding is really our objective.

Muscles work as functional groups to accomplish movement of our bodies. Muscles that coordinate their contraction to move us in a particular direction are said to be synergistic (such as the Iliacus and Psoas Major that both flex the femur onto the pelvis). Those muscles that work in opposition to each other are termed antagonistic muscles (such as the biceps that flex the forearm versus the triceps that extend it). Skeletal muscle may contract in either an isotonic or isometric fashion. In an isotonic contraction, visible shortening of a muscle takes place as a fairly constant force of contraction is applied. In an isometric contraction, the force of contraction is not enough to cause movement, and visible muscle contraction does not occur.

Let’s consider briefly the motion of muscles as related to first, second and third class levers. In this discussion, bones are viewed as levers that move about joints that are described as fulcrums. In order to move the lever there must be an applied force (AF) coming from muscles that exceeds the resistance being encountered(R) as movement takes place over a fulcrum (F).

For example, the atlanto-occipital joint is a first class lever, which is like a seesaw. In this example, the effort from the contraction of the posterior neck muscles (AF) pulls across the fulcrum of the atlanto-occipital joint (F) and counteracts the resistance(R) from the weight of the head and face. The order of sequence of players, then, is AF,F,R.

A second-class lever is typified by a long crowbar placed underneath a rock, where a force is applied upward on the crowbar. In a second-class lever, the resistance is between the applied force and the fulcrum. An example of a second class lever in the body occurs as the calf muscles contract (AF) to move the weight of your body up(R) on your toes, where the metatarsal-phalangeal joints are the fulcrum (F). Our sequence of parts here is AF, R, F. Second-class levers provide a great deal of strength, though the lever doesn’t travel a very great distance or move very quickly.

An example of a third class lever is a person on the ground moving a ladder against a wall, where the base of the ladder moving on the ground is the fulcrum and the weight of the ladder is the resistance. In a third-class lever, movement can be quick, but you can’t move a great deal of weight with your lever. Elbow flexion is an example of this situation in the body. Here the elbow joint is the fulcrum (F), with the applied force for flexion being applied just distal to the joint where the biceps attaches at the radial tuberosity(E). The resistance comes from the weight of the forearm, most of which is distal to the site of our applied effort(R). The sequence of elements here is F, AF, R. Third-class levers are the most common in the body and provide the ability to move a lever large distances quickly, but move relatively little weight.

Fiber arrangements within muscles vary depending on the function of the muscle. For examples, muscles with fibers that run in parallel and contract over a distance provide good endurance, and are the most common type of skeletal muscle. An example of a parallel muscle is the rectus abdominis. Muscles with fibers that converge from a fan shape down to a single point of insertion on a tendon provide a good deal of power, such as the pectoralis major muscle, and are called convergent muscles. Muscle fibers arranged concentrically around an opening serve as sphincters when contracted, the external anal sphincter being an example. Pennate muscles are muscles in which many fibers are contained in a small area. Each of the muscle fibers attaching to the tendon attach at the same angle. While unipennate muscles exist, the more common theme is a bipennate structure, where a tendon receives attachments of muscle fibers on both sides. If the tendon receiving the muscle fibers has multiple branches, then the muscles is described as multipennate. Pennate muscles tire quickly but provide a great amount of strength when contracted.

The Neuromuscular Junction
The area of physical junction between muscles and nerves is termed the neuromuscular junction. Realize that both sensory and motor nerves serve muscles, but the following description is specific to the microanatomy of the motor nerve and muscle. Our motor nerve cells, originating from the CNS, send out lengthy processes distally called axons that transmit nerve impulses in the form of an ionic current. As the axons near the muscle fibers they branch into smaller units called axon terminals. The axon terminals reach the sarcolemma at the motor end plate. The passage of information from muscle to nerve initiates processes within the muscle fiber that results in actin sliding past and around myosin to produce contraction. A single motor neuron and the multiple muscle fibers it serves are called a motor unit. Large muscles that function in gross movement have many muscle fibers served by one motor neuron (about 500:1). Muscle recruited in fine motor skills may only have 10 muscle fibers served in a single motor unit. A motor neuron is an example of an efferent nerve cell, one that carries a message to the muscle; nerve cells carrying messages away from the muscle are called afferent nerve fibers

Naming Muscles
Muscles may or may not be named according to rules that make sense. Sometimes muscles are named according to their shape, ie the trapezoidal-shaped muscle called the ‘trapezoid’. At other times, the muscle will receive part of its name based on its size or length, such as ‘longus’, ‘minimus’, or ‘magnus’ (large). In other situations, the muscle may be named according to function and position, ie flexor digitorum superficialis, a superficial muscle that flexes the digits of the hand. In still other cases the orientation of muscle fibers is used, such as rectus abdominis. If a muscle name is helpful in remembering function, be grateful.

We can make two great divisions of muscles, those that are associated with the axial skeleton and those that are associated with the appendicular skeleton. We will begin our discussion of gross muscular anatomy with the muscles of the axial skeleton.

Muscles to Know