The Muscular System: Structure and Physiology

ELAINE N. MARIEB

EIGHTH EDITION

6

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

PowerPoint® Lecture Slide Presentation by Jerry L. Cook, Sam Houston University

ESSENTIALSOF HUMANANATOMY

& PHYSIOLOGY

PART A

The Muscular System: Structure and

Physiology


Function of Muscles

Produce movement

Maintain posture

Stabilize joints

Generate heat


Characteristics of Muscles Muscle cells are elongated

muscle cell = muscle fiber

Contraction of muscles is due to the movement of microfilaments

All muscles share some terminology

Prefix myo refers to muscle

Prefix mys refers to muscle

Prefix sarco refers to flesh


Characteristics of Muscle Tissue Excitability

Muscle tissue (and nervous cells) receive and respond to stimuli by producing electrical signals

Contractability

Ability to shorten and thicken when stimulated

Extensibility

Ability to stretch without damaged

Elasticity

Ability to return to its original shape


Types of Muscle Tissue

Cardiac muscle

Skeletal muscle

Smooth muscle


Cardiac Muscle Has striations

Usually has a single nucleus

Joined to another muscle cell at an intercalated disc

Involuntary

Found only in the heart

Figure 6.2b


Smooth Muscle No striations

Spindle-shaped cells

Single nucleus

Involuntary – no conscious control

Found mainly in the walls of hollow organs


Skeletal Muscle Most are attached by tendons to bones

Cells are multinucleate

Striated – have visible banding

Voluntary – subject to conscious control

Cells are surrounded and bundled by connective tissue


Connective Tissue Wrappings of Skeletal Muscle

Tendon – cord-like structure, attaches muscle to bone

Endomysium – around single muscle fiber

Perimysium – around a fascicle (bundle) of fibers

Epimysium – covers the entire skeletal muscle

Figure 6.1


Connective Tissue Wrappings of Skeletal Muscle

Fascia – on the outside of the epimysium

Superficial: subcutaneous tissue; made of areolar connective tissue and adipose

Deep: holds muscles together and separates them into functional groups; made of dense irregular connective tissue

Figure 6.1


Skeletal Muscle Tissue

Each skeletal muscle is a separate organ composed of hundred to thousands of skeletal muscle cells called muscle fibers because of their elongated shapes. Connective tissue surround muscle fibers and whole muscles, blood vessels, and nerves penetrate muscle.


Microscopic Anatomy of Skeletal Muscle

Muscle fibers(cells) are arranged parallel to one another.

Sarcomeres are the basic functional unit of striated muscle fibers; occurs at the overlap of filaments.

Sarcolemma is the plasma membrane that covers each muscle fiber

Sarcoplasm is the muscle fiber’s cytoplasm.

Tranverse (T) tubules are tunnel-like extensions of the sarcolemma that pass through muscle fiber from side to side

Sarcoplasmic reticulum is a network of membrane –enclosed tubules that stores Ca2+ ions required for muscle contractions.

Myoglobin is found in the sarcoplasm; reddish pigment; stores oxygen until needed by mitochondria for ATP production.

Myofibrils extend along the entire length of the muscle fiber; are cylindrical; consists of two types of protein filaments: light & dark


Microscopic Anatomy of Skeletal Muscle


Organization of the Scarcomere Z-discs (lines) are zigzagging

zones that separates sarcomeres

A bands (dark bands) extend the entire length of thick filaments; at end thick & thin filaments overlap

I bands (light bands) are composed of thin filaments only

H zone is found at the center of each A band; contains only thick filaments


Organization of the Sarcomere Thick filaments are

composed of the protein myosin. Shaped like 2 golf clubs twisted together

Thin filaments are composed of the protein actin. Twisted into helix.

Thin filaments also contains the proteins tropomyosin & troponin


Sliding Filament Theory of Muscle Contraction

Overall summary of what happens in a sarcomere when a muscle contracts:

Myosin heads of thick filaments pull on thin filaments

Thin filaments slide toward center of sarcomere

I bands and H zone becomes narrower

I band and H zone disappears @ maximum contraction


Sliding Filament Theory of Muscle Contraction

Figure 6.7


Contraction Terminology Neuromuscular Junction (NMJ)- area of contact between

axon terminal & portion of sarcolemma

Axon terminal-branches of motor neuron that approaches, but not touch the sarcolemma

Acetylcholine (Ach)- neurotransmitter

Synaptic cleft- narrow gap that separates axon terminal of one neuron from muscle fiber

Motor end plate- part of sarcolemma that receives the neurotransmitter

Acetylcholinesterase (AChE)- enzyme that breaks down acetylcholine


Muscle Contraction Nerve impulse arrives at axon terminal of motor neuron

and triggers release of acetylcholine (ACh)…

ACh binds to its receptors and is activated, this causes Na/K ions to flow across membrane...

Inflow of Na ions generates a muscle action potential, which travels down sarcolemma & through T-tubules…

As the impulse moves down SR, Ca2+ is released from the SR to the thick and thin myofilaments…

Ca2+ binds to troponin molecules in thin filaments, causing the troponin-tropomyosin complex to change shape….


Muscle Contraction This change in shape of the troponin/tropomysosin complex

causes movement of the attached tropomyosin molecule….

Allowing the myosin head to contact/bind actin, causing the myosin head to swivel (this requires ATP!)

During the swivel, the myosin head is firmly attached to the actin, so when the head moves, it pulls the actin filament forward.

This is called the ”power stroke” of contraction

Many myosin heads are swiveling simultaneously and their collective efforts are enough to pull the entire actin filament past the myosin filament into the H zone and causes shortening (contraction) of the muscle fiber


Muscle Relaxation Neurotransmitter Ach (acetylcholine) is broken down by

AChE.

Muscle action potentials stop

Ca2+ release channels in the SR close

As levels of calcium in sarcoplasm falls, troponin releases calcium and slides back into original position where it covers the myosin binding sites

Thin filaments slips back into their relaxed positions.

The Muscular System: Structure and Physiology

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