Copyright © 2012 Pearson Education, Inc. Chapter 6 The Muscular System Betty McGuire Cornell...

Copyright © 2012 Pearson Education, Inc.

Chapter 6

The Muscular System

Betty McGuireCornell University

Lecture Presentation


The Muscular System

Function and characteristics of muscles Skeletal muscles working in pairs Contraction of muscles Voluntary movement Energy for muscle contraction Slow-twitch and fast-twitch muscle cells Building muscle


Function and Characteristics of Muscles

Three types of muscle Skeletal Cardiac Smooth



All muscles are Excitable (they respond to stimuli) Contractile (they can shorten) Extensible (they can stretch) Elastic (they can return to their original

length after being shortened or stretched)



Skeletal muscles are voluntary muscles responsible for Moving our body Maintaining posture Supporting internal organs Pushing against veins and lymphatic

vessels to move blood and lymph along Generating heat


Skeletal Muscles Working in Pairs

The body has more than 600 skeletal muscles

Synergistic muscles Muscles that must contract at the same

time to cause movement Antagonistic muscles

Movement is produced when one muscle of the pair contracts and the other relaxes

Example: the biceps muscle and triceps muscle of the upper arm



Tendon Band of connective tissue that attaches

a muscle to a bone Origin of a muscle

The end attached to the bone that remains relatively stationary during movement

Insertion of a muscle The end attached to the bone that

moves



Tendinitis Condition of having an inflamed tendon Caused by overuse, misuse, or age Healing is slow because tendons have a

poor blood supply Most effective treatment is rest



Muscle pull Also called a muscle strain or tear Caused by overstretching that damages

the muscle or tendon Treatment includes ice to reduce

swelling and keeping the muscle stretched


Contraction of Muscles

Fascicle A bundle of muscle cells

A skeletal muscle has many fascicles Each fascicle is surrounded by its own

connective tissue sheath The connective tissue sheaths of fascicles

merge at the ends of muscles to form tendons that attach the muscle to bone



A muscle cell = a muscle fiber When skeletal muscle cells are viewed

under a microscope, they have distinct bands called striations The striations are formed by the

arrangement of myofibrils within the cell Myofibrils are specialized bundles of

proteins



Each myofibril contains two types of myofilaments Myosin (thick) filaments Actin (thin) filaments

Actin filaments are more numerous



Each myofibril has tens of thousands of contractile units, called sarcomeres The ends of each sarcomere are marked by

dark protein bands called Z lines

Within each sarcomere the actin and myosin filaments are specifically arranged



One end of each actin filament is attached to a Z line

Myosin filaments lie in the middle of the sarcomere, and their ends partially overlap with surrounding actin filaments

The degree of overlap increases when the muscle contracts



Muscle contraction occurs at the molecular level

According to the sliding filament model, a muscle contracts when actin filaments slide past myosin filaments, shortening the sarcomere

Myosin molecules are shaped like two-headed golf clubs The club-shaped myosin heads are key

to moving actin filaments



The myosin head, also known as a cross-bridge, attaches to a nearby actin filament Then the head bends and swivels,

pulling the actin filament toward the midline of the sarcomere

The myosin head disengages from the actin filament

The movements of myosin require ATP The cycle begins again



Muscle contraction is controlled by the availability of calcium ions

Muscle cells contain the proteins troponin and tropomyosin

The troponin-tropomyosin complex and calcium ions regulate muscle contraction at the actin-myosin binding sites



When a muscle is relaxed, the troponin-tropomyosin complex covers the actin-myosin binding sites

Muscle contraction occurs when calcium ions bind to troponin, causing it to change shape

This change in shape moves tropomyosin, exposing the actin-myosin binding sites



Sarcoplasmic reticulum Form of smooth endoplasmic reticulum

found in muscle cells Stores calcium ions

Transverse tubules (T tubules) Pockets in the plasma membrane of a

muscle cell Carry signals from motor neurons deep

into the muscle cell to every sarcomere



Rigor mortis Muscle contraction will occur as long

ATP is present Without ATP, cross-bridges cannot

be broken Within 3 to 4 hours after death, the

muscles become stiff = rigor mortis Actin and myosin gradually break

down and muscles relax again after 2 to 3 days



Role of nerves in muscle contraction Neuromuscular junction

Junction between the tip of a motor neuron and a skeletal muscle cell

A nerve impulse travels down a motor neuron to the neuromuscular junction, where it causes the release of acetylcholine (a neurotransmitter) from the motor neuron

Acetylcholine diffuses across a small gap and binds to receptors on the plasma membrane of the muscle cell



Role of nerves in muscle contraction (cont.) The acetylcholine causes changes in

the permeability of the muscle cell, resulting in an electrochemical message similar to a nerve impulse

The message travels along the plasma membrane into the T tubules and then to the sarcoplasmic reticulum, releasing calcium ions for muscle contraction



Web Activity: Muscle Structure and Function



Muscular dystrophy (MD) If too many calcium ions enter a muscle cell, then

proteins may be destroyed, eventually causing the cell to die; on a large scale, muscles weaken

MD = a group of inherited conditions in which muscles weaken

Duchenne muscular dystrophy One of the most common forms The gene for production of the protein

dystrophin is defective Lack of dystrophin allows excess calcium ions to

enter muscle cells, eventually killing the cells


Voluntary Movement

Motor unit A motor neuron and all the muscle cells

it stimulates All the muscle cells in a given motor unit

contract together


Voluntary Movement

The number of muscle cells in a motor unit is highly variable Muscles responsible for precise movements

have fewer muscle cells in each motor unit than do muscles responsible for less precise movements

On average, there are 150 muscle cells in a motor unit


Voluntary Movement

Motor units and recruitment The strength of muscle contraction can

be increased by increasing the number of motor units that are stimulatedThis process, performed by the

nervous system, is called recruitment


Voluntary Movement

Muscle twitch Contraction of a muscle in response to a

single stimulus Twitches are very brief and typically not part

of normal movements


Voluntary Movement

If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first, due to summation


Voluntary Movement

Tetanus A sustained, powerful contraction

caused by very frequent stimuli Fatigue sets in when a muscle is unable to

contract even when stimulated Changing the frequency of stimulation is

another way to vary the contraction of muscles


Energy for Muscle Contraction

Muscle contraction requires an enormous amount of energy

ATP for muscle contraction comes from many sources, typically used in sequence ATP stored in muscle cells Creatine phosphate stored in muscle cells Anaerobic metabolic pathways Aerobic respiration


Slow-Twitch and Fast-Twitch Muscle Cells

Slow-twitch muscle cells Contract slowly, with great endurance Abundant mitochondria Packed with myoglobin (oxygen-binding

pigment) Dark, reddish appearance

Myoglobin Rich blood supply


Slow-Twitch and Fast-Twitch Muscle Cells

Fast-twitch muscle cells Contract rapidly and powerfully but with

much less endurance Can make and break cross-bridge

attachments more rapidly Have more actin and myosin Rely on anaerobic metabolic pathways to

generate ATP and therefore tire quickly


Building Muscle

Aerobic exercise Enough oxygen is delivered to the muscles

to keep them going for long periods Increases endurance and coordination Promotes development of new blood vessels Increases the number of mitochondria Typically does not increase size of muscles Examples: walking, jogging, swimming


Building Muscle

Resistance exercise Builds strength Muscles increase in size when they are

repeatedly made to exert more than 75% of their maximum force

Increases in muscle size reflect increases in the diameter of existing muscle cells

Example: weight lifting


Building Muscle

PLAY | Mitchell Report


Building Muscle

PLAY | Steroids

Copyright © 2012 Pearson Education, Inc. Chapter 6 The Muscular System Betty McGuire Cornell...

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