Muscles Test

11/20/25

What are the seven ways we name skeletal muscles?

1. Direction 

2. Location (near a bone or body region)

3. Relative Size

4. Number of Origins (Heads)

5. Shape

6. Location of Origin and/or Insertion

7. Action of Muscle (describes what muscle does)

Relative Size Examples (3)

• Maximus = Largest (Gluteus Maximus)

• Longus = Longest (Fibularis Longus)

• Brevis = Shortest (Extensor Pollicis Brevis)

Direction Examples (3)

• Rectus = Parallel (Rectus Abdominus)

• Transverse = Perpendicular (Transverse Abdominus)

• Oblique = @ 45 degree Angle (External Obliques)

Location Examples (2)

• Frontalis is on frontal bone

• Tibialis Anterior is next to Tibia

Number of Origins (Heads) Examples (2)

• Biceps = 2 Origins

• Triceps = 3 Origins

Shape Examples (2)

• Deltoid = Triangular

• Trapezius = Trapezoid

Location of Origin and Insertion Example

Sternocleidomastoid

> Origin: Sternum

> Insertion: Mastoid Process

Action of Muscle Example

Flexor Digitorum: Flexes Fingers

Prime Mover (Agonist)

Major responsibility for producing a specific movement

Antagonist

Muscle that oppose or reverse a particular movement

How are antagonists contracted?

They’re contracted slightly to provide some resistance to slow or stop movement

Synergists

Help prime mover by adding little extra force to movement and reduce unnecessary movement that occurs as a prime mover contracts

Fixators

Immobilize the bone

Adduction

Towards the midline or center

Abduction

Away from midline or center; spreading out

Extension

Straightens the joint

Flexion

Bends the joint

Circumduction

Circle or full rotation

Rotation

Head “yes” or “no” movements

Retraction

Pull head (chin) back

Protraction

Push head (chin) forward

Opposition

Thumb to finger

Supination

External rotation or flip hand over (palm up)

Pronation

Internal rotation or flip hand back (palm down)

Dorsiflexion

Bending foot up

Plantar Flexion

Bending foot down, point toes

Inversion

Foot move in towards body

Eversion

Foot turned out from body

Sarcolema

Cell membrane of a muscle fiber

Myofibrils

Contractile fiber in striated muscle cells

Myosin

Protein in a muscle fiber that forms the thick filaments and pulls on actin to help muscle fibers contract

Actin

Protein in a muscle fiber that forms the thin filaments that slide between thick filaments of the protein myosin, contracting muscle fibers

Sarcomere

Structural unit of a myofibril and the functional unit of muscle contraction

I Bands

The light, thin bands on the striation pattern of skeletal muscle fibers composed of thin filaments (actin) directly attached to Z Lines

Z Line

Forms a line (boundary) for the sarcomere

A Bands

The dark stripes in a muscle fiber where thick filaments (myosin) are found. They stay the same length during muscle contraction

H Zone

Center of the A band where only thick filaments (myosin) are present. It gets smaller when the muscle contracts.

M Line

The middle of the sarcomere within the A band. It holds the thick filaments (myosin) together and helps keep them aligned during contraction.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a specialized network of tubes in a muscle fiber that stores and releases calcium ions (Ca²⁺), which are essential for muscle contraction

Transverse Tubules (T-Tubules)

Tiny tubes in a muscle fiber that carry electrical signals (action potentials) deep into the muscle so all parts of the fiber can contract at the same time

What are the 5 parts of the Neuromuscular Junction?

1. Motor Neurons

2. Synapse

3. Neurotransmiter

4. Neuromuscular Junction

5. Synaptic Cleft

Motor Neuron

Neurons that control effectors (such as muscles)

Synapse

The functional connection between a neuron and another cell

Neurotransmitter

The chemical neurons released to communicated with the cells they control

Neuromuscular Junction

The synapse between a motor neuron and the muscle fiber that it controls

Synaptic Cleft

A small gap that separates the membrane of the neuron and the membrane of the muscle fiber

What are the two types of myofilaments?

1. Thick

2. Thin

Describe the thick filaments (type of myofilament)

Protein myosin (the most abundant)

Describe the thin filaments (type of myofilament)

Made of actin, plus regulatory proteins troponin and tropomyosin

Tropomyosin

A protein on the thin filament (actin) that blocks myosin from binding to actin when the muscle is relaxed

Troponin

Binds to tropomyosin on actin and calcium

What do troponin and tropomyosin control?

Interactions between myosin and actin

Sarcomere Units (2)

1. Contractile units of muscle

2. Structural units of myofibrils

What do sarcomeres form?

Patterns in myofibrils = Muscle striations

Zone of overlap in sarcomeres

The area where thick filaments (myosin) and thin filaments (actin) lie on top of each other

What causes muscle contraction?

Interactions of thick and thin filaments

What determines the interactions that cause muscle contractions?

Structures of protein muscles

Sliding Filament Theory

Muscles contract because thin filaments (actin) slide past thick filaments (myosin)

What happens to the zones and lines of the sarcomere in the Sliding Filament Theory?

• A band: same

• I band: smaller

• H band: smaller

• Zone of overlap: bigger

• Z lines: moves towards m line

• M line: stays put

What are the steps to induce a skeletal muscle to contract?

1. A motor neuron sends an electrical signal to the muscle.

2. The signal reaches the neuromuscular junction (where nerve meets muscle).

3. The nerve releases acetylcholine (ACh), a chemical messenger.

4. ACh makes the muscle fiber create its own electrical impulse.

5. This causes the muscle cell to release calcium.

6. Calcium triggers the sliding filament mechanism → the muscle contracts.

Step 1 of the Contraction Cycle

Cross-bridge formation

Myosin head attaches to actin (when calcium has moved tropomyosin out of the way).

Step 2 of the Contraction Cycle

Power stroke

The myosin head pulls the actin filament inward → this uses energy from ATP.

Step 3 of the Contraction Cycle

Cross-bridge detachment

A new ATP binds to myosin → myosin lets go of actin.

Step 4 of the Contraction Cycle

Myosin reactivation (re-cocking)

ATP is split (ATP → ADP + P), giving energy to reset the myosin head to its “ready” position.

Step 5 of the Contraction Cycle

Repeat as long as calcium + ATP are present

Myosin keeps grabbing, pulling, releasing, and resetting → muscle stays contracted

Skeletal Muscle Description

1. Long

2. Thin

3. Striated

4. Multi-Nucleated 

What proteins cause the striations of the skeletal muscle

1. Myosin

2. Actin

3. Troponin

4. Tropomyosin

Is Skeletal Muscle voluntary or involuntary?

voluntary or involuntary

Skeletal Muscle Location

Attached to bones by tendons for movement

Skeletal Muscle Contraction Type

Sustained and controlled

Skeletal Muscle stimulation of contraction

Released calcium binds troponin, which causes tropomyosine to shift to expose actin site that allows myosine head to attach. forms cross-bridge (with ATP available) to perform a power stroke

What is the smallest functional unit of contraction?

Sarcomere

Cardiac muscle description

• striated

• 1 nuclei

• branched

• intercalated disks

• located only in heart

Cardiac muscle voluntary or involuntary?

Involuntary

Cardiac muscle contraction type

Rythmic Contractions = All or Nothing

Smooth Muscle description

• lacks striations

• 1 nucleus

• thick/thin filaments arranged diagnolly

• intermediate filaments

What are the 2 types of smooth muscle?

Visceral and Multi-Unit

Where is visceral smooth muscle found?

Hollow Organs

Visceral Smooth Muscle contraction type

• Slow & Sustained

• muscle fibers work together AS A GROUP

Where is multi-unit smooth muscle found?

1. Iris of eyes

2. Blood vessels

Multi-Unit smooth muscle contraction type?

• Rapid

• muscle fibers work INDEPENDANTLY

Motor Unit Function

Motor neuron tellls muscle fibers to innervate (move)

Smooth muscle location example

Digestive system

smooth muscle example function

Peristalsis: Esophagus moves food down

smooth muscle contraction type

slow & sustained

What stimulates smooth muscle contractions

A lack of troponin, so calcium binds to the protein calmodulin to activate contracts

Smooth Muscle contraction neurotransmitters (2)

1. Norepinepherine

2. Acetycholine

How do bones connect to muscle

By tendons

Quadriceps (Front upper leg) muscles (4)

1. Rectus Femoris

2. Vastus Lateralis

3. Vastus Medialis

4. Vastus Intermedius

Hamstring (back upper leg) muscles (3)

1. Biceps Femoris

2. Semitendinosus

3. Semimembraneous

Antagonist example

Triceps brachii opposing the biceps during arm bending

Agonist example

Biceps Brachii during arm bending

What muscles are involuntary?

Smooth & Cardiac

What is the most abundant muscle protein?

Myosin

When does the main force of a contraction occur?

During the sliding of myosin along actin, which causes the muscle fibers to shorten

What IS a motor unit?

A group made up of a motor neuron and all the muscle fibers it controls

What are the I bands made of?

Actin filaments

What does the H zone contain?

Only myosin filaments

During contraction of sarcomere, what do calcium ions bond with?

Troponin

What is the MAIN neurotransmitter for muscle contraction?

ACh