Notes
Skeletal Muscle Terms
Sections
Key Organizational Points:
Fascicles
Groups of muscle cells bound by connective tissue
- Fascicle alignment determines the strength and direction of force that muscles exert
Midline of the body
- Separates right and left halves
Parallel muscles
- Fascicles run in parallel
- Appear either strap-like or spindle-shaped
- The belly (aka, gaster) is the central portion of the muscle
- When parallel fascicles contract, the muscle length shortens
Circular muscle
- Fascicles are arranged around a circular opening
- When they contract they close off openings (such as the mouth)
Convergent muscle
- Fascicles converge at a tendon
- When they contract, the muscle can shorten along multiple axes
Pennate muscles
- Fascicles insert obliquely at a central tendon
- When they contract, the muscle shortens
Types of pennate muscles:
Bipennate muscle has fascicles that insert on both sides of the tendon
Unipennate muscle has fascicles that insert on only one side of the tendon
Multipennate muscle (such as the deltoid of the shoulder) comprises multiple tendons with oblique fascicle insertions
Movements:
Flexion of a joint
- Moves two bones closer together; muscles that do this are called flexors.
Extension of a joint
- Moves two bones further apart; muscles that do this are called extensors.
Abdudction
- Moves limb away from midline
Adduction
- Moves limb towards midline
Lateral rotation
- Turns limbs away from the midline of the body
Medial rotation
- Turns limbs towards the midline of the body
Skeletal muscle function
Agonist
- Produces the movement
Synergist
- Contributes to the movement
Antagonist
- Opposes the movement
Basis of Muscle Nomenclature:
- Orientational terminology (superior is on top and inferior is below; anterior is in front and posterior is behind)
- Depth (superficial means closer to the surface; deep means further from the surface)
- Anatomical Region (named for bony landmarks, etc.)
- Origin/Insertion (origin means that the bone that remains stable upon muscle contraction; insertion means that the bone moves upon muscle contraction).
- Fiber Direction (rectus = straight; brevis = short; longus = long; minor/minimus = small; quadratus = rectangular)
- Action (adductor, abductor, extensor, flexor)
Full-Length Text
- Here we will learn about skeletal muscle alignment, actions, and nomenclature, which will allows us to efficiently learn the origins, insertions, and functions of over 600 skeletal muscles found in the human body.
First we'll learn about muscle tissue alignment.
- Start a table.
- Denote that connective tissue groups muscle cells into fascicles.
- Fascicle alignment determines the strength and direction of force that muscles exert.
- Denote that we will draw a 4 muscle types of different fascicular alignment:
- Parallel.
- Circular.
- Convergent.
- Pennate.
- Indicate that we will draw them in both their relaxed and contracted states (always relaxed, first, then contracted).
Begin with a muscle with parallel oriented fascicles.
- Draw a muscle section.
- Show that the fascicles are aligned in parallel.
- Parallel muscles appear either strap-like or spindle-shaped (aka fusiform, which some authors consider to be a separate category).
- Label the belly (aka, gaster) in the central portion of the muscle.
- Next, indicate that when parallel fascicles contract, the muscle length shortens.
Now, let's draw a muscle with circular oriented fascicles.
- Draw a circular section of muscle.
- Show that the fascicles are arranged around a circular opening.
- Next, indicate that when they contract, these muscle types close off openings (such as the mouth).
- Next, let's draw a muscle with convergent oriented fascicles.
- Draw a fan-shaped muscle section.
- Show that its fascicles are aligned to converge at a tendon.
- Indicate that when convergent fascicles contract, the muscle can shorten along multiple axes.
Let's draw a muscle with pennate oriented fascicles.
- Draw a feather-shaped pennate muscle section.
- Show that it comprises fascicles that insert obliquely at a central tendon.
- Next, indicate that when pennate fascicles contract, the muscle shortens.
- Indicate that there are different types of pennate muscles:
- A bipennate muscle, which is shown here, has fascicles that insert on both sides of the tendon;
- A unipennate muscle has fascicles that insert on only one side of the tendon; and,
- A multipennate muscle (such as the deltoid of the shoulder) comprises multiple tendons with oblique fascicle insertions.
Now, let's learn key skeletal muscle actions.
- First, draw the skull and torso.
- Define the right and left sides.
- Next, draw the upper limbs with the elbows bent and the hands upright.
- Then, draw the lower limbs with both feet pointed to the left.
- Now, on the right upper limb, draw an arrow to show that movement of the forearm towards the upper arm creates flexion at the elbow; the muscles that produce this movement are called flexors.
- Flex your arm and feel your biceps contract.
- Then, use an arrow to show that movement of the forearm away from the upper arm creates extension at the elbow; the muscles that produce this movement are called extensors.
- Extend your arm and feel your triceps contract.
- Now, draw a vertical line through the midline of the body.
- Draw an arrow pointing from the torso to the right arm and indicate that movement of a limb away from the midline of the body is called abduction.
- Raise your upper arm to the side and feel deltoid abduct.
- Then, use an arrow pointing from the limb towards the torso and indicate that movement of a limb towards the midline of the body is called adduction.
- Bring your arm towards your chest and feel your pectoralis major contract.
- Next, draw an arrow to show that the left leg is rotated laterally, so that the knee and foot are turned away from the midline.
- Rotate your thigh outward and feel your lateral rotators of the deep hip contract.
- Then, draw an arrow to show that the right leg is rotated medially, so that the knee and foot are turned towards the midline.
- Rotate your thigh inward and feel your medial hamstrings contract.
- Skeletal muscles are also categorized as agonists, synergists, or antagonists, based on their function during joint movement.
- On the flexed left arm, draw an agonist anterior muscle and show it flex the forearm.
- On the posterior side, draw an antagonist muscle (so named because it opposes, or antagonizes, the movement of the agonist during flexion) and use a double-sided arrow to show that the extensor relaxes and lengthens during elbow flexion.
- Now, add a smaller flexor, called a synergist, which contributes to the work of the agonist.
- Recognize that when the elbow is extended rather than flexed, the roles are reversed: the extensor is the agonist, and the flexors are the antagonists.
- Draw another left upper limb in elbow extension.
- Show that the anterior muscle is now the antagonist; indicate (again with a double-sided arrow) that it lengthens during elbow extension.
- Then, show that the posterior muscle is now the agonist; indicate that it contracts to produce elbow extension.
Lastly, let's consider some key muscle nomenclature.
- Denote that we will address it either within our diagram or our table based on:
- Orientational terminology
- Depth
- Anatomical Region
- Origin/Insertion
- Fiber Direction
- Action
- Draw the following orientational planes: a vertical line to show that superior is on top and inferior is below; a muscle that is superior to another lies above it whereas a muscle that is inferior to another lies below it.
- Draw an oblique line in parallel to our diagram to show that anterior is in front and posterior is behind.
- Along the thigh show that superficial refers to the outer muscle and deep refers to the inner muscle.
- Then, consider that some muscle nomenclature is derived from the anatomical region.
- For instance, draw the right femur and show that one associated muscle is the rectus femoris.
- Now, denote that muscle origin means that the bone that remains stable upon muscle contraction.
- Insertion means that the bone moves upon muscle contraction.
- Now, denote that some muscles are named for the direction of their fibers.
- Denote that some examples of key words are:
- Rectus, which means straight,
- Brevis, which means short,
- Longus, which means long,
- Magnus and major, which mean large,
- Minor and minimus, which mean small, and,
- Quadratus, which means rectangular.
The next time you see a muscle name, try to dissect it.
- For example, the name "adductor magnus" tells you two things about the muscle: it is responsible for adduction, and it is a large muscle.