Muscular System

Muscle Tissue and Organization

Muscle Tissue

• Muscle tissue is one of the 4 primary tissue types

• Three types of muscle tissue– Skeletal – moves the body– Cardiac – heart muscle– Smooth – moves fluid and solids through the

digestive tract

Characteristics of Muscle Types

• SKELETAL– Voluntary– Striated– Multinucleate

• CARDIAC– Involuntary– Striated– Single nuclei– Intercalated discs

• SMOOTH– Involuntary– Not striated– Single nucleus

Features of Muscle Tissue

• Contractility – ability to shorten and pull

• Excitability – responds to stimuli

• Elasticity – muscle can rebound to its original shape after contraction

• Extensibility – ability to contract over a range of resting lengths

Functions of Skeletal Muscle

• Movement

• Posture

• Stabilize joints

• Support soft tissue

• Generation of heat

• Regulate entrances and exits (orifices)

Muscle Attachment

• TENDONS attach muscle to bone

• Dense regular CT

• Each muscle has an ORIGIN and INSERTION, and a specific ACTION

• The origin remains stationary while the insertion moves

Connective Tissue of Muscle

• Skeletal muscle has three layers of connective tissue

• 1. EPIMYSIUM – dense irregular CT that surrounds the entire muscle

• 2. PERIMYSIUM – divides muscle into compartments or bundles of muscle fibers called FASCICLES

• 3. ENDOMYSIUM – surrounds each muscle fiber (muscle cell)

Muscle Cell Terminology

• Muscle cells are very long; muscle fibers• SARCOLEMMA – cell membrane• SARCOPLASM – cytoplasm• Sarcoplasm is filled with thousands of

MYOFIBRILS that are responsible for contraction

• Myofibrils are composed of MYOFILAMENTS• Myofilaments are composed of the proteins

ACTIN and MYOSIN

Sarcomere

• Organization of thick(myosin) and thin filaments(actin) in the myofibrils

• Movements of these filaments causes muscle contraction

• Sliding filament theory– 1954 Sir Andrew Huxley and Rolf Niedeigerke– Myosin heads bind to the actin and pull or

“slide” the actin past the myosin to shorten the sarcomere

Sarcomere

• Z line = end of sarcomere• I band = thin (actin) filaments• H zone = thick (myosin) filaments• A band = zone of overlap, thick and thin

Thin Filaments

• Twisted strands of globular G actin molecules

• Each molecule of G actin has an active site that can bind to a myosin molecule

• Thin filaments also have two other proteins associated it– Tropomyosin – covers active sites on actin– Troponin - holds tropomyosin in place

Thick filaments

• Bundles of myosin molecules

• About 500 myosin molecules per bundle

• Myosin molecules have heads that can cross bridge to actin active sites

• The binding of myosin heads to actin result in muscle contraction

Figure 9.4b Sarcomere Structure

Muscle contraction-Sliding filament theory

• Contraction exerts a pull – tension• Interaction between actin and myosin

triggered by calcium ions and presence of ATP

• Sliding filament theory:– H band and I band get smaller– Zone of overlap gets larger– Z lines move closer together– Width of A band remains constant

Sliding Filament Theory

• Myosin heads cross bridge to the actin active sites

• Myosin attachment “pulls” the actin toward the center of the sarcomere

• Contraction begins with release of Ca2+

from the terminal cisternae of the sarcoplasmic reticulum

• The release of ions is the result of electrical stimulation of the muscle fiber

Terminal cisternae and T tubules

T-tubules

• The t-tubules distribute the electrical signal for contraction deep into the muscle fiber

• As the signal travels the terminal cisternae release calcium ions

• Release of calcium cause the troponin molecule to change shape

• Change in troponin causes a change in the position of tropomyosin, myosin can bind to action and contraction occurs!

Nervous System Control of Contraction

• Skeletal muscle fibers are controlled by a motor neuron

• Place where the nerve fiber and muscle meet is called the neuromuscular junction

• Synaptic terminal – end of axon

• Acetylcholine – neurotransmitter

• Motor unit – all the muscle fiber controlled by a single motor neuron

Neuromuscular Junction

Neuromuscular junction

Muscle contraction

Muscle relaxation

Sarcomere contraction

Sliding filaments

Sliding filaments

Sarcomere shortens, fiber contracts

Other components of the sarcomere

• Sarcoplasmic reticulum – stores Ca+² ions for muscle contraction

• Transverse tubules – carry impulse to stimulate and coordinate contraction

Sarcoplasmic reticulum

Types of Skeletal Muscle Fibers

• Fast fibers– Short duration, rapid fatigue– Anaerobic metabolism– Few mitochondria– Brief periods of intense exercise

• Slow fibers– Longer duration– Aerobic metabolism – Myoglobin present for oxygen binding– Marathon running

• Intermediate fibers– Greater resistance to fatigue– Similar to fast fibers but with more mitochondria

Organization of Muscle Fibers

• Parallel

• Convergent

• Unipennate

• Bipennate

• Multipennate

• Circular

Muscles and Leverage

• Muscles “work” by leverage, moving at a joint

• In the body the bone is the lever, the joint is the fulcrum

• Three types of levers in the body– First-class– Second-class– Third-class (most common)

Muscle Actions

• Prime movers– Muscle mainly responsible for producing a certain

action– Biceps brachii is prime mover for elbow flexion

• Synergist– Assists the prime mover

• Antagonists– Action opposite of prime mover– Triceps brachii is antagonist of biceps brachii