The Muscular System

Muscle System Functions

Provides voluntary movement of body Enables breathing,

blinking, and smiling Allows you to hop,

skip, jump, or do push-ups

Maintains posture Produces heat

Functions Continued Provides movement of

internal organs Moves food through

digestive tract Enables bladder control

Causes involuntary actions Reflex actions Adjusts opening of pupils Causes hair to stand on

end ( )

Properties of Muscles

Excitability: capacity of muscle to respond to a stimulus

Contractility: ability of a muscle to shorten and generate pulling force

Extensibility: muscle can be stretched back to its original length

Elasticity: ability of muscle to recoil to original resting length after stretched

Muscle Tissue Characteristics

Is made up of contractile fibers

Provides movement Controlled by the

nervous system Voluntary-

consciously controlled

Involuntary- not under conscious control

Cardiac

Skeletal

Smooth

Red vs white muscle fibers a.Red (slow) fibers- greater number of mitochondria

- contain high concentration of myoglobin

-react at a slow rate; do not undergo fatigue even with sustained contraction

- ex. Postural muscles which are opposed to gravity b. White (fast) fibers-contain little myoglobin/mitochon

- react rapidly and undergo anaerobic respiration

-generate force quickly but not for long durations

-ex. Fingers and eye movements ( darting motions) c. Intermediate fibers- postural muscles that are capable of

rapid contraction at times

- ex. Calf muscle-supports leg but also capable of running, walking, jumping

Red vs white muscle fiber

Nerve and Blood Vessel Supply

Motor neurons stimulate muscle fibers to contract Neuron axons branch so that each muscle fiber (muscle

cell) is innervated Form a neuromuscular junction (= myoneural junction)

Capillary beds surround muscle fibers Muscles require large amts of energy Extensive vascular network delivers necessary oxygen

and nutrients and carries away metabolic waste produced by muscle fibers

Nerve and Blood Vessel Supply

Types of Muscle Tissue

There are three main types of muscle tissue Skeletal

(striated) Cardiac

(heart) Smooth

(visceral)

Types of Muscle cont.Skeletal

Attached to bones Makes up 40% of body weight Responsible for locomotion, facial

expressions, posture, respiratory movements, other types of body movement

Voluntary in action; controlled by somatic motor neurons

Skeletal

Smooth Muscle

In the walls of hollow organs, blood vessels, eye, glands, uterus, skin

Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow,

In some locations, autorhythmic Controlled involuntarily by endocrine and autonomic nervous systems

Smooth

Heart: major source of movement of blood

Autorhythmic Controlled involuntarily by endocrine

and autonomic nervous systems

Cardiac Muscle

Cardiac

Comparison of Muscle Types

Muscle Type Cardiac

FunctionMovement of

bone

Walls of internal organs + in skinLocation

Attached to bone

Heart

SmoothSkeletal

Striated- light and dark bands

Many nuclei

StriatedOne or two

nuclei

CharacteristicsNon-striatedOne nucleus

(visceral)

Long + slender BranchingShape Spindle shape

Control Mode

Beating of heart

Involuntary Involuntary

Movement of internal organs

Voluntary

Sarcomere

Myofibril

Contain two types of protein filaments Actin and

Myosin Z disc- point of

anchor of actin Sarcomere-

functional unit of a myofibril, region between Z discs

Thin Filaments

ActinMolecule

Thick Filaments

Myosin Molecule

Z Disc

Sarcomere

Contractile Proteins: Actin and Myosin

Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs. Single filament contains roughly 300 myosin molecules Molecule consists of two heavy myosin molecules wound

together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

Myosin Filaments cont Myosin heads

1. Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)

2. Attached to the rod portion by a hinge region that can bend and straighten during contraction.

3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Actin (Thin) Myofilaments

1. F (fibrous) actin2. Tropomyosin3. Troponin

Two strands of fibrous (F) actin form a double helix elongating the myofilament; attached at either end at sarcomere. Composed of G actin

monomers each of which has a myosin-binding site

Actin site can bind myosin during muscle contraction.

• composed of 3 major proteins

Actin Filaments cont Tropomyosin: an elongated protein

winds along the groove of the F actin double helix.

Troponin is composed of three subunits: Tn-A : binds to actin Tn-T :binds to tropomyosin, Tn-C :binds to calcium ions.

Motor Unit: The Nerve-Muscle Functional Unit

Mechanics of a Muscle Contraction

What stimulates a muscle to contract? Your nervous system

What cells are involved? Muscle cells and a motor neuron Motor neuron sends

impulse to muscle cells One neuron will form

synapses with many muscle cells

What is this called? A motor unit Let’s take a look under

the microscope.…A motor unit

A motor unit is a motor neuron and all the muscle fibers it supplies

The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500)

Muscles that control fine movements (fingers, eyes) have small motor units

Large weight-bearing muscles (thighs, hips) have large motor units

Neuromuscular Junction

Figure 9.7 (a-c)

Sarcoplasmic Reticulum (SR)

SR is an elaborate, smooth endoplasmic reticulum runs longitudinally and surrounds each myofibril Form chambers called terminal cisternae on either side

of the T-tubules A single T-tubule and the 2 terminal cisternae form

a triad SR stores Ca++ when muscle not is contracting

When stimulated, calcium released into sarcoplasm SR membrane has Ca++ pumps that function to pump

Ca++ out of the sarcoplasm back into the SR after contraction

Mechanics of a Muscle Contraction

Where does stimulation occur? Neuromuscular junction

How do motor neurons communicate with muscle cells? Neurotransmitters (typically

acetylcholine) carryimpulse signal across the gap

What happens when a muscle cell is stimulated? Calcium ions are released into the muscle cell

Myofibrils are surrounded by

calcium-containing

sarcoplasmic reticulum.

Neurotransmitters

Mechanics of a Muscle Contraction

What do calcium ions do? Cause interaction between actin and myosin

How do actin and myosin interact? Actin filaments slide over the myosin filaments.

What model explains this? Sliding Filament Model

What causes actin to slide over myosin? The head of myosin connects

to actin and pivots. What is this connection called?

cross-bridge The binding of the myosin heads

throughout the sarcomere occurs asynchronously… some myosin heads are

binding while other heads are releasing the actin filaments.

Role of Calcium Ions in Contraction When muscle is relaxed, attachment sites for

myosin heads are physically blocked by tropomyosin.

In order to contract a muscle, troponin must move tropomyosin.

Complex regulated calcium ion concentration.

Muscle fibers store Ca++ in sarcoplasmic reticulum.

Sliding Filament theory

Mechanics of a Muscle Contraction

What provides the energy to swivel the head of myosin? _____

How exactly does the sliding filament model work? In the sliding filament model of muscle contraction, the

(thin) actin filaments[red] (that are attachedto the Z-line) slide (areactually pulled) inward along the (thick)myosin filaments [blue], and the sarcomere (measuredfrom one Z line to thenext) is shortened.

ATP

Mechanics of a Muscle Contraction

When each sarcomere becomes shorter it causes each myofibril to become shorter.

When each myofibril becomesshorter it causes the muscle fibers to become shorter

When each muscle fiber shortens the overall muscle contracts.

Sarcomere

Myofibril in a state of contraction

Electrical changes during muscular contraction

• Depolarization- Na+ moves inside (+)

• Repolarization- (-) charge

• Absolute refractory period- depolarized membrane, a 2nd stimulus is ineffective ( no

response)

• Relative refractory period- during repolarization, stronger stimulus can cause a response

Mechanical changes- tension develops when filaments attempt to slide past each other

Isotonic contraction- ‘same tension’

- results in shortening of muscle but tension remains the same

- filaments are successful in sliding

- moved through a distance

-ex. Walking, lifting an object, bending knee, smiling

Types of Muscle Contraction

Isometric contraction- ‘same length’

- contraction without shortening

-increased tension due to exertion against an immovable object

-not successful in sliding

-Ex. Posture, holding an object, standing still, opposing gravity

The kymograph

Mechanical changes cont Muscle twitch- response after application of

threshold stimulus

-single, brief, jerky contraction Summation- stimuli applied in succession

-cells do not get chance to relax between stimuli

-can be summed up

Tetanus- application of stimuli in rapid succession

-no period of relation between them

-contraction bec sustained and prolonged

-may dev tension 4x as during a single twitch Treppe( staircase effect)- stimuli applied at

slower rate than tetanus

-increased fusion of twitches

-individual contraction gradually becomes stronger for a short time though stimulus strength is unchanged

Mechanical Changes

Flow of events during muscular contraction

Sources of energy for muscular contraction

Ca2++ and myosin cause ATP breakdown Creatine phosphate- from excess ATP

-stored in muscles Glycogen breakdown through anaerobic

respiration

-only for short-term energy production

Tone- state of partial contraction which gives muscles a certain firmness

Plasticity- in smooth muscles only-ability to stretch w/o developing

lasting increase in tension - dev resistance to stretching at first-later tension decreases and muscle

adjusts to new length-in hollow visceral organs like urinary

bladder, stomach, small intestines

Control of a Muscle Contraction

How long does a muscle cell remain contracted? Until the release of acetylcholine

stops. How strongly does a muscle fiber

contract? To it’s fullest extent. All-or-none response

So what controls the strength of a contraction? Number of muscle cells recruited To get a stronger contraction, more

cells are stimulated A single cell can’t contract harder

Muscle Disorders

A strain is an injury to a muscle or tendon, and is often caused by overuse, force, or stretching.

Injured area experiences: pain and

soreness swelling warmth,

bruising, or redness

difficulty using or moving the injured area in a normal manner

Strain

Muscle Disorders

R.I.C.E. Rest: Stop all activities which

cause pain. Ice: Helps reduce swelling.

Never ice more than 10-15 min. at a time. Protect the skin.

Compression: Wrap the strained area to reduce swelling.

Elevation: Keep the strained area as close to the level of the heart as is conveniently possible to keep blood from pooling in the injured area.

Treatment for Muscle Injuries

Muscle Disorders

Muscle spasm- when A muscle (or even a few fibers of a muscle) involuntarily contract

Muscle cramp- involuntarily + forcibly contracted muscle that does not relax A forceful + sustained spasm

CrampsSpasms

Can last anywhere from a few seconds to a quarter of an hour

Caused by strain or injury

Muscle feels tied up in knots

Muscle Disorders

Tetanus is a preventable disease through vaccination Caused by bacteria that enters the body

through the skin Found in soil, dust and manure Toxin bacteria produces interferes with nerve

transmission to your muscles and causes them to seize up in painful spasms.

Tetanus typically starts in the jaw and muscles of the face, quickly spreading to the arms and legs.

“Lockjaw” Difficulty swallowing Intestines often seize up Bladder fails to empty Asphyxiation Cardiac arrest

Tetanus

Muscle Disorders

Produced naturally by the body to support such functions as fighting stress and promoting growth and development

Referred to as roids, juice, hype, weight trainers, gym candy, arnolds, stackers, or pumpers

People use steroid pills, gels, creams, or injections to improve their sports performance or the way they look.  

Anabolic steroids cause many different types of problems types of problems

premature balding or hair loss dizziness mood swings problems sleeping nausea and vomiting high blood pressure aching joints urinary problems shortening of final adult height increased risk of heart disease,

stroke, and some cancers

Anabolic Steroids

Muscle Disorders

People with cerebral palsy may have difficulty walking. They may also have trouble with tasks such as writing or using scissors.

Some people with cerebral palsy have other medical conditions, including seizure disorders or mental impairment.

Cerebral palsy happens when the areas of the brain that control movement and posture do not develop correctly or get damaged.

Cerebral Palsy

Muscle Disorders

A genetic condition that describes over 20 genetic and hereditary muscle diseases.

Characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.

In some cases, cardiac and smooth muscles are affected.

Progressive Muscular Wasting (weakness) Poor Balance and Frequent Falls Walking Difficulty + Waddling Gait Limited Range of Movement Scoliosis (curvature of the spine) Inability to Walk Muscle Atrophy and Drooping Eyelids

Muscular Dystrophy

Principal symptoms:

chronic autoimmune neuromuscular disease characterized by varying degrees of weakness of the skeletal muscles

Caused by a defect in the transmission of nerve impulses at the neuromuscular junction

Antibodies (produced by the body's own immune system) block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction which prevents the muscle contraction from occurring.

Myasthenia Gravis

Certain muscles such as those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing are often involved. The muscles that control breathing and neck and limb movements may also be affected.

Patients initially complain of drooping eye lids that get worse as the day goes on; they develop double vision, difficulty talking, and difficulty chewing.

Rigor mortis- stiffness of skeletal muscles after death

Myosin-actin crossbridges are still intact or in a state of midcontraction at the time of death

Crossbridges left attached due to depletion of ATP

Bonds not broken-rigid muscles Rigor mortis disappears as the body

decomposes