Muscular System

Function of skeletal muscles

1. Move the body

2. Maintain posture

3. Protect and Support

4. Regulate elimination of materiels

5. Produce Heat

Characteristics of Skeletal Muscles

1. Excitability - responsiveness

2. Conductivity

3. Contractility- (ATP)
   into mechanical energy

4. Extensibility - can be stretched

5. Elasticity - recoils

fascicle

Bundle of muscle fibers

epimysium

A layer of dense, irregular connective tissue surrounding a skeletal muscle.

perimysium

Fibrous sheath enveloping each of the fascicles of skeletal muscle fibers.

endomysium

Areolar connective tissue layer surrounding a muscle cell or fiber.

Tendon

Cord of dense regular connective tissue that connects muscle to bone.

aponeurosis

Flattened, sheet-like tendon

What is the largest to smallest organization of skeletal muscle structures?

Muscle → Fascicle → Muscle fiber → Myofibril → Myofilament → Sarcomere.

What is deep fascia and what does it do?

Dense irregular connective tissue external to epimysium; separates muscles, binds muscles with similar functions, contains nerves & blood vessels.

What are T-tubules and their function?

Deep invaginations of the sarcolemma that carry electrical impulses into the fiber to the sarcoplasmic reticulum.

What is the plasma membrane of a muscle fiber called?

Sarcolemma

What proteins make up thick filaments?

Myosin

What proteins make up thin filaments?

Actin, Tropomyosin, Troponin

What is the function of tropomyosin and troponin?

Tropomyosin blocks myosin-binding sites on actin; troponin binds Ca²⁺ to regulate contraction.

What is a sarcomere?

Functional contractile unit of a myofibril, from Z-disc to Z-disc.

What does the I band contain?

Only thin filaments; bisected by Z disc

What does the A band contain?

Entire thick filament; partially overlaps thin filaments; does not shorten during contraction.

What is the H zone?

Central part of A band with only thick filaments; shortens during contraction.

What is the M line?

Protein mesh in center of H zone; anchors thick filaments.

What are the three parts of the NMJ?

Synaptic knob, Motor end plate, Synaptic cleft.

What is the synaptic knob?

Expanded tip of axon; contains ACh vesicles, Ca²⁺ pumps, and voltage-gated Ca²⁺ channels.

What is the motor end plate?

Specialized sarcolemma region with junction folds and ACh receptors (chemically gated).

What is the synaptic cleft?

30 nm gap between synaptic knob and motor end plate; contains acetylcholinesterase.

What triggers the release of acetylcholine (ACh) from the synaptic knob?

Calcium (Ca²⁺) enters the synaptic knob through voltage-gated channels, binds to synaptic vesicles, causing exocytosis of ACh into the synaptic cleft.

What happens after ACh binds to receptors on the motor end plate?

The skeletal muscle fiber is excited, leading to local depolarization (end-plate potential).

How does acetylcholinesterase (AChE) function at the NMJ?

AChE breaks down ACh in the synaptic cleft, stopping stimulation of the muscle fiber and allowing relaxation.

How is an action potential propagated along the sarcolemma and T-tubules?

Sequential opening of voltage-gated Na⁺ channels causes depolarization; voltage-gated K⁺ channels then open for repolarization, restoring the resting membrane potential (−90 mV).

What are the four steps of crossbridge cycling?

1. Crossbridge formation – myosin binds actin
2. Power stroke – myosin pulls actin toward sarcomere center
3. Release of myosin head – ATP binds myosin, detaching it from actin
4. Resetting of myosin head – ATP is split into ADP + Pi, cocking myosin for another cycle

Do thin and thick filaments shorten during contraction?

No, thin filaments slide past thick filaments, shortening the sarcomere.

What sarcomere changes occur during contraction?

H zone disappears, I band narrows/disappears, Z discs move closer, A band stays the same.

Muscle cell are called…

Myocyte

Sarcoplasms are..

muscle fiber cytoplasm

Smooth muscle is…

Found in the walls of hollow visceral
organs
• Makes up the heart valves
• Function: force food and other substances
through internal body channels
• NOT striated
• Involuntary
• Has automaticity

Skeletal muscle anatomy is made from…

Muscle tissue
• Blood vessels
• Nerve fibers
• Connective tissue
• Requires nerve stimulation

Insertion Skeletal muscle attachment

attachment to movable bone

Origin Skeletal muscle attachment

attachment to immovable or less movable bone

Wht is the Sarcoplasmic reticulum

Network of smooth endoplasmic reticulum tubules surrounding each myofibril
• Most run longitudinally
• SR functions in regulation of intracellular Ca2+ levels
• Stores and releases Ca

Muscle fibers contain

myofibrils

During contraction of sarcomere filaments,

heads link thick and thin filaments together, forming crossbridges

Thin filaments are mostly composed of the protein

,actin, tropomyosin and troponin

wht does Tropomyosin do?

physically blocks myosin binding and cross bridge formation

Troponin’s 3 proteins

TnI, TnT & TnC

TnI binds to..

actin

TnT: binds to

tropomyosin

TnC: binds Ca

Ca²⁺

What happens when Ca²⁺ bind to TnC?

troponin changes, tropomyosin moves, allowing myosin to bind to actin and form cross bridges.

Elastic filament

composed of protein titin
• Holds thick filaments in place; helps recoil
after stretch

Nebulin, myomesin, C proteins bind…

filaments or sarcomeres together

T (transverse) tubules

Tube formed by protrusion of sarcolemma deep into cell interior
• Allow electrical nerve transmissions to reach deep into interior of each muscle fiber

Triad in the Sarcoplasmic reticulum is

an area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere

For Skeletal muscles to contract…

Be stimulated by a nerve ending
• Propagate AP (action potential) along its sarcolemma
• ↑intracellular Ca2+
• ATP

Events at the neuromuscular junction

1. AP arrives at axon terminal
2. Voltage-gated calcium channels open, calcium
enters motor neuron
3. Calcium entry causes release of ACh into
synaptic cleft
4. ACh binds ligand-gated Na+ channels on
sarcolemma…
5. …opening gates, and allowing Na+ to enter,
resulting in end plate potential ( muscle
contraction).
6. Acetylcholinesterase degrades ACh

Isotonic Contraction

a muscle action where the muscle changes length (shortens or lengthens) while maintaining relatively constant tension

Isometric Contraction

a type of strength training where a muscle generates tension without changing length or causing joint movement (pushing)

Auxotonic Contraction

Most contractions in daily life show some change in length and some change in tone

Concentric Contractions

Eccentric Contraction

Tonicity

Muscle twitch periods

• Latent period: excitation-contraction coupling;
  no muscle tension seen
  • Period of contraction: cross bridge formation;
  tension increases
  • Period of relaxation: Ca2+ reentry into SR;
  tension declines to zero

Smooth muscle has

• Spindle-shaped fibers
• Lack coarse connective tissue sheaths
• Longitudinal layer contraction: organ dilates and contracts