UMKC Anatomy 218 Unit 1 Exam 1.08

Dr. Allen

Skeletal Muscle Organization

From biggest → smallest:

Whole muscle
↓
Fascicle
↓
Muscle fiber
↓
Myofibril
↓
Myofilaments

• A fascicle = bundle of muscle fibers

• A muscle fiber/myofiber = an individual muscle cell

• A muscle fiber contains myofibrils

• Myofibrils contain myofilaments

The 3 connective-tissue coverings

Epimysium → covers entire muscle

Perimysium → covers fascicle

Endomysium → covers individual muscle fiber

EPI = exterior/entire muscle
PERI = around a bundle/fascicle
ENDO = innermost, individual fiber

• Endomysium = areolar CT with reticular fibers

• Perimysium + epimysium = dense irregular CT

Muscle Fiber Anatomy

Inside one muscle fiber.

Sarcolemma = plasma membrane of the muscle fiber
Sarcoplasm = cytoplasm of the muscle fiber
Sarcoplasmic reticulum = specialized version of the smooth ER

A skeletal muscle fiber is:

• Long

• Cylindrical

• Multinucleated = has multiple nuclei

• Nuclei are located toward the periphery/edges

• Packed with myofibrils

• Has lots of mitochondria because muscle is energetically demanding

Why multiple nuclei?

• During development, multiple myoblasts fuse together to form a large skeletal muscle fiber.

• Some myoblasts don't fuse and remain as satellite cells, which help with repair/regeneration.

Myofibrils, Sarcomeres & Myofilaments organization

Muscle fiber
↓
Myofibrils
↓
Sarcomeres
↓
Myofilaments

Sarcomere

A sarcomere is a repeating section of a myofibril.

The key boundaries:

Z disc | ← SARCOMERE → | Z disc

• extends from Z disc to Z disc

Myofilaments

There are two major contractile proteins:

Thin filament → ACTIN
Thick filament → MYOSIN

• Contractile: actin + myosin

• Regulatory: troponin + tropomyosin

• Structural: titin/connectin, nebulin, dystrophin

ACTIN = THIN
MYOSIN = THICK

Bands/lines

A band → contains thick filaments
I band → thin filaments
H zone → thick filaments
M line → middle
Z discs → boundaries of a sarcomere

Muscle Contraction Basics

Contraction starts at the neuromuscular junction (NMJ), where a nerve communicates with a muscle fiber.

The sequence begins:

1. Nerve impulse arrives
↓
2. Acetylcholine (ACh) is released into the synaptic cleft
↓
3. ACh binds receptors on the motor end plate
↓
4. An impulse travels along the sarcolemma and down the T-tubules
↓
5. Calcium (Ca²⁺) is released from the terminal cisternae into the sarcoplasm

ACh → neurotransmitter

Sarcolemma → carries the muscle impulse

T-tubules → carry the impulse deeper into the fiber

Ca²⁺ → released into sarcoplasm

Sliding filament changes

When a sarcomere contracts:

• Z discs move closer together

• I band gets narrower

• H zone gets smaller/disappears

• A band stays the SAME

• The actual thick and thin filaments do NOT get shorter — they slide past each other.

Calcium's next step

Ca²⁺ binds TROponin
→ moves tropomyosin
→ exposes binding sites on actin
→ myosin heads bind actin
→ myosin pulls the thin filaments inward
→ sarcomere shortens.

Calcium → TROponin → TROpomyosin moves

Motor units

A motor unit = one motor neuron + all the muscle fibers it controls.

Small motor unit → precise/fine control
Large motor unit → less precise, more fibers

Muscle fiber types

Type I / slow oxidative
→ small
→ aerobic
→ highly fatigue-resistant
→ good for sustained/postural activity

Type IIb / fast glycolytic
→ large
→ anaerobic
→ short bursts
→ fatigue quickly