Chapter 20: Skeletal Muscles and their Actions

Motor Unit: consists of one motor neuron and all the skeletal muscle fiber it controls

conists of one motor neuron and all the skeletal muscle fiber it controls

Motor Neuron:

A nerve cell that sends signals to muscle fibers

Axon:

the part of the neuron that carries the electrical signal

Neuromuscular Junction (NMJ):

point where the motor neuron communicates with a muscle fiber

Muscle Fibers:

individual muscle cells that contract to create movement

How a motor unit works:

1. Your brain/spinal cord sends a signal through a motor neuron.

2. The signal reaches the neuromuscular junction.

3. The muscle fibers controlled by that neuron are activated.

4. The fibers contract, producing force.

Small Motor Units:

control fewer muscle fibers and allow precise movements, such as moving your eyes.

Large Motor Units:

control many fibers and allow precise movements, such as when your leg muscles help you jump.

One motor neuron + all the muscle fibers it controls = ________

one motor unit

Muscles often work in ______ because muscles pull on bones rather than push them.

pairs

Agonist:

The prime mover

• the muscle primarily responsible for a particular movement

Antagonist:

The opposing muscle

• the muscle that produces the opposite movement and often relaxes or lengthens as the agonist contracts.

  • 
    

Example of Agonists and Antagonists:

Bending your elbow (flexion)

• agonist: biceps brachii

• antagonist: triceps brachii

Example of Agonists and Antagonists:

Straightening your knee (extension)

• agonist: quadriceps

• antagonist: hamstrings

Synergist:

Assists the agonist

Fixator:

stabilizes a bone or joint so the movement can occur efficiently

Origin:

the attachment usually associated with the less movable bone

Insertion:

attachment usually associated with the more moveable bone.

Tendon:

strong connective tissue that attaches muscle to bone or, in some cases, other structures.

When a muscle contracts it generally pulls its insertion toward its ______,

origin

Origin = generally stays put;

insertion = generally moves

Movement occurs at ______.

joints

Abduct:

away from midline

Adduct:

toward midline

Frontalis(surprised face):

raises eyebrows; wrinkles forehead

Masseter(chewing)

elevates mandible (closes jaw)

Temporalis(closes jaw):

elevates and rectracts mandible

Orbicular Oris (puckering):

closes and protrudes lips

Orbicularis Oculi(blinking):

closes eyelids

Zygomaticus(smiling):

raises corners of mouth.

Sternocleidomasteoid:

flexes the neck; rotates the head to the opposite side when one side contracts

Trapezius:

elevates, retracts, depresses, and upwardly rotates the scapula, depending on the fibers used

Pectoralis Major:

Flexex, adducts, and medially rotates the arm

External Abdominal Oblique:

compresses abdomen; flexes and rotates trunk

Rectus Abdominis:

flexes trunk; compresses abdomen

Transverse Abdominis

compresses abdominal contents and stabilizes the trunk

Serratus Posterior Superior:

assists with depression of the lower ribs during forced expiration

Serratus posterior inferior:

Assists with depression of the lower ribs during forced expiration

Latissimus dorsi:

Extends, adducts, and medially rotates the arm

Serratus anterior

Protracts the scapula and holds it against the rib cage

Rhomboids

Retract and stabilize the scapula

Platysma

Tenses skin of the neck; helps depress the lower lip and mandible