Support & movement · Topic 6 of 18

Muscular System

Muscle Types & Contraction

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Anatomy · muscle tissue

Muscle Types & Contraction

Skeletal, cardiac and smooth muscle differ in control and striation; all contract as myosin pulls actin, triggered by ACh and calcium, powered by ATP.

01 · Muscle types3 types: skeletal, cardiac, smooth

02 · ContractionMyosin pulls actin → sarcomere shortens

03 · The triggerNerve releases ACh at the NMJ

What it is

Sarcolemma = membrane; sarcoplasm = cytoplasm; SR stores calcium

Parts & jobs

Three muscle types: skeletal, cardiac, smooth must know

Skeletal: striated, voluntary, multinucleated, moves bones must know

Cardiac: striated, involuntary, heart only, intercalated discs must know

Smooth: no striations, involuntary, walls of hollow organs and vessels must know

Sarcomere (Z-disc to Z-disc) = the contracting unit

Thick filament = myosin; thin filament = actin (+ troponin, tropomyosin)

Muscle → fascicles → fibers (epimysium, perimysium, endomysium)

Contraction, step by step

NMJ: motor neuron releases ACh → muscle fiber fires

Impulse runs down T-tubules → SR releases calcium

Calcium binds troponin → tropomyosin moves → myosin binds actin

ATP lets myosin release, re-cock and pull again

Relax: ACh broken down, calcium pumped back, actin covered again

The rules

Sliding filament: myosin pulls actin inward → sarcomere shortens must know

Skeletal or cardiac muscle?

Commonly confused

	Skeletal	Cardiac
Control	Voluntary	Involuntary
Striated?	Yes	Yes
Nuclei per cell	Many	Usually one
Cell shape	Long, cylindrical fibers	Short, branched cells joined by intercalated discs
Where	Attached to bones by tendons	Only in the heart wall

Mix-ups to avoid

Filaments keep their length; they slide, so the sarcomere shortens

More detail

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Gap junctions + pacemaker cells let the heart contract as a unit

Smooth muscle responds to autonomic nerves, hormones and stretch

Motor unit = one motor neuron + every fiber it controls

Muscle stores little ATP; running short causes fatigue

Pearl

Stripes mean sarcomeres: skeletal and cardiac muscle are striated, and smooth muscle is the one without stripes.

Sources · OpenStax Anatomy & Physiology 2e §10.1 Overview of Muscle Tissues · OpenStax Anatomy & Physiology 2e §10.2 Skeletal Muscle · OpenStax Anatomy & Physiology 2e §10.3 Muscle Fiber Contraction and Relaxation · OpenStax Anatomy & Physiology 2e §10.7 Cardiac Muscle Tissue · OpenStax Anatomy & Physiology 2e §10.8 Smooth Muscle · OpenStax Anatomy & Physiology 2e §10.4 Nervous System Control of Muscle Tension · Union Test Prep — HESI A&P study guide, page 3 (muscular system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Major Muscles & Their Actions

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Anatomy · major muscles

Major Muscles & Their Actions

Muscles move bones only by pulling, so they work in opposing pairs: a prime mover acts and its antagonist does the opposite.

01 · Muscle rolesAgonist acts; antagonist does the opposite

02 · Classic pairBiceps bend the elbow; triceps straighten it

03 · Thigh pairQuads straighten the knee; hamstrings bend it

What it is

Agonist = prime mover; antagonist = opposite action; synergist = helper must know

Biceps brachii flexes the elbow; triceps brachii extends it must know

Flexion bends, extension straightens; abduction away, adduction toward must know

Origin = fixed attachment; insertion = attachment on the moving bone

Parts & jobs

Quadriceps extend the knee; hamstrings flex the knee

Hamstrings: biceps femoris, semitendinosus, semimembranosus

Deltoid = rounds the shoulder; abducts (raises) the arm

Pectoralis major flexes + adducts the arm; latissimus dorsi extends + adducts it

Trapezius shrugs the shoulders and pulls the shoulder blades together

Sternocleidomastoid turns the head; both together flex it

Masseter = main chewing muscle (closes the jaw)

Diaphragm = dome muscle under the lungs; main breathing muscle

Rectus abdominis flexes the trunk: the "sit-up" muscle down the front of the abdomen

Gluteus maximus = largest buttock muscle; extends the thigh

Gastrocnemius + soleus → Achilles tendon; point the foot (tiptoe)

Tibialis anterior lifts the foot (dorsiflexion)

The rules

Muscles only pull, so they work in opposing pairs must know

Quadriceps or hamstrings?

Commonly confused

	Quadriceps femoris	Hamstrings
Where	Front (anterior) of the thigh	Back (posterior) of the thigh
Knee	Extend (straighten) it	Flex (bend) it
Hip	Help flex the thigh	Help extend the thigh
Muscles	Rectus femoris + 3 vastus muscles	Biceps femoris, semitendinosus, semimembranosus

More detail

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Names give location, size, shape, number of heads, direction or action

Inner-thigh adductors close the legs; gluteus medius abducts

Sartorius = longest muscle (hip to inner knee)

Rotator cuff = 4 tendons that steady the shoulder joint

Intercostals aid breathing; orbicularis oculi/oris close eye, move lips

Pearl

Every pull has an opposite: for each prime mover there is an antagonist that undoes its movement.

Sources · OpenStax Anatomy & Physiology 2e §11.1 Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems · OpenStax Anatomy & Physiology 2e §11.3 Axial Muscles of the Head, Neck, and Back · OpenStax Anatomy & Physiology 2e §11.4 Axial Muscles of the Abdominal Wall and Thorax · OpenStax Anatomy & Physiology 2e §11.5 Muscles of the Pectoral Girdle and Upper Limbs · OpenStax Anatomy & Physiology 2e §11.6 Appendicular Muscles of the Pelvic Girdle and Lower Limbs · OpenStax Anatomy & Physiology 2e §9.5 Types of Body Movements · OpenStax Anatomy & Physiology 2e §11.2 Naming Skeletal Muscles · LibreTexts Human Anatomy (Lange et al.) §9.6 Appendicular Muscles of the Pectoral Girdle and Upper Limbs · Union Test Prep — HESI A&P study guide, page 3 (muscular system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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