The skeletal muscular system consists of skeletal muscles and accessory structures. This is the system that gives our bodies the ability to move. This muscle system is attached to the body’s skeleton at various points and through the contraction of its individual muscles, moves the skeleton in different directions. It is made up of around 640 different muscles.
Muscles have different functions and we can group them according to these functions
Let’s have a brief look at some of the major muscle groups and their functions. In grouping muscles according to function we come up with four major groups.
These are:
- PROTAGONISTS (Prime Movers).
They mainly produce a particular movement.
- ANTAGONISTS
These are muscles that oppose a particular movement.
- SYNERGISTS
These muscles help the prime movers or agonists. They also stabilize motion.
- FIXATORS
These are synergists that immobilize a bone. These include muscles for maintaining posture.
A muscle can perform more than one of these roles depending on what motion it is causing.
The individual muscles of the skeletal muscular system have different names. They can be named according to seven different criteria.
These criteria are:
- Muscle Location
For example the muscle called the temporalis in the head is located near the temporal bone.
- Muscle Shape.
These muscles are named after there shape. For example the trapezius muscle is roughly trapezoidal, the deltoids are triangular shaped (delta) and the rhomboids are diamond shaped.
- Muscle Size.
Some muscles include in their names words like maximus (largest), minimus (smallest), major (large), minor (small), longus (long) and brevis (short). As an example, the gluteus or glutes (buttocks muscles) include the gluteus maximus and the gluteus minimus.
- Direction Of Muscle Fiber.
The muscle fibers can run in the same direction as a particular body part or reference like the axis of a bone or midline of the body. The names of these muscles include words like: rectus (parallel), transverses (perpendicular) or oblique (at an angle).
- Number Of Muscle Origins.
The muscles are named after the number of origins they have. So for example biceps (bi) have two origins, triceps (tri) have three origins and quadriceps (quad) has four origins.
- Location Of Attachment To The Skeletal Structure.
These muscles are named after their point of origin and their point of insertion for example the sternocleidomastoid (sterno – cleido – mastoid) muscle along the side of the neck. This muscle originates from the sternum (sterno) and clavicle (cleido) and inserts on the mastoid process.
- Muscles By The Type Of Motion They Produce.
In this group we have flexors, extensors and abductor muscles.
Once you know these naming systems, you will be able to understand something about a muscle just by its name.
MUSCLE STRUCTURE
Muscle Cells
Muscle tissue is made up of muscle cells also known as muscle fiber. These fibers are bound together in groups known as myofibrils. These myofibrils are bound by a layer known as endomysium.
The myofibrils are bound together to form fascicles by a layer called the perimysium.
The fascicles are bound together to form the muscle. The muscle is bound by a layer known as the epimysium. These are all bound within an outer layer known as the deep fascia.
The deep fascia extends past the muscle it binds to form into the tendon.
Tendons
Tendons are the structures that bind the muscles to the bones of the skeleton structure or to other muscles. These tendons are made of dense fibrous connective tissue. These fibers are primarily collagen fibers.
Now the covering around the bone is known as the periosteum. So the layer covering the muscle known as the deep fascia extends past the muscle to form the tendon. This tendon then merges with the layer covering the bone called the periosteum to anchor or attach to the bone.
MUSCLE FIBER CONTRACTION
The neuromuscular junction is where the neuron of the nervous system joins the muscle. Each muscle fiber has a motor neuron nerve ending attached to it. The connection between a motor neuron and a muscle fiber is called a neuromuscular junction.
A muscle fiber has to be stimulated by a motor neuron nerve in order to contract. The nerve stimulates the muscle using chemical messengers or neurotransmitters called acetylcholine.
When exerting different levels of strength, like lifting a weight in the gym and lifting a piece of paper, different numbers of cell fibers are engaged. So the heavier the weight lifted, the more strength is needed and thus the more muscle fibers are engaged in the muscle contraction. Likewise the lighter the weight lifted, the less strength is needed and thus the less muscle fibers are needed and engaged in the contraction.
PARTS OF A MUSCLE
- The Origin.
As the name suggests, this is where the muscle originates. The origin is sometimes tricky to identify, but a general rule of thumb is that it is usually the more stationery of the two ends of a muscle which attach to bones. Take the bicep for example. It is attached to the bones of the shoulder area and bones of the forearm, just below the elbow.
So when the bicep is contracted the forearm moves. The shoulder does not move, and so the part of the bicep that attaches to the shoulder is the origin.
- The Belly.
The belly is the thick middle part of the muscle
- The Insertion.
The insertion is the opposite end of the origin. They usually cross at least one joint from the origin. This is with the exception of a few muscles. So the bicep for example has its origin at the shoulder area and so its insertion is at the other end of the bicep which is at the forearm/elbow area. The insertion actually crosses over the elbow joint to attach to the bone of the forearm.
Also when the muscle contracts, the insertion moves towards the origin of the muscle.
- The Head.
Some muscles have more than one belly. Where they originate is what is called the head. Sometimes the belly is referred to as the head. Not all muscles have heads. Examples of muscle heads are in the biceps brachii and triceps brachii. In biceps, bi means two and ceps mean heads, while in triceps tri means three and ceps means heads. Brachii means arm.
MUSCLE RELATIONSHIPS
Prime Movers / Protagonist
Prime movers are also known as the protagonists. These are the main muscles that bring about the intended movement.
Using the arm as an example, if you want to bend the elbow, you will contract the bicep. So when bending the arm the bicep becomes the prime mover or protagonist.
The Antagonist
An antagonist muscle is one which does the opposite of the protagonist muscle. So in the example of the arm, to bring the arm back straight again after bending because of bicep (protagonist) contraction, another muscle on the opposite side of the bicep will have to contract to do the opposite of what the bicep has done. The tricep does this by contracting and pulling the bent arm back straight. Thus the tricep is called the antagonist muscle.
The Synergist
Synergy is basically when things (in this case muscles) work together. Synergists are secondary muscles that help the prime mover muscles perform the desired movement.
One has to note that these above terms are relative, depending on the particular movement. So when bending an arm, the bicep is the protagonist and the tricep is the antagonist. When straightening the arm however, the triceps become the protagonist and the biceps become the antagonists.
So there we have it. A brief explanation about the skeletal muscular system. This was just an introduction to this system. There is soooo much more to cover about the muscular system. So have no fear, this is not the last of the muscular system. We shall cover much more in time.
