Joints (Articulations)

Articulation – site where two or more bones meet

Functions of joints

Give the skeleton mobility

Hold the skeleton together

Classification of Joints: Structural

Structural classification focuses on the material binding bones together and whether or not a joint cavity is present

The three structural classifications are:

Fibrous

Cartilaginous

Synovial

Classification of Joints: Functional

Functional classification is based on the amount of movement allowed by the joint

The three functional classes of joints are:

Synarthroses – immovable

Amphiarthroses – slightly movable

Diarthroses – freely movable

Fibrous Structural Joints

The bones are joined by fibrous tissues

There is no joint cavity

Most are immovable

There are three types – sutures, syndesmoses, and gomphoses

Fibrous Structural Joints: Sutures

Occur between the bones of the skull

Comprised of interlocking junctions completely filled with connective tissue fibers

Bind bones tightly together, but allow for growth during youth

Fibrous Structural Joints: Syndesmoses

Bones are connected by a fibrous tissue ligament

Movement varies from immovable to slightly movable

Examples include the connection between the tibia and fibula, and the radius and ulna

Fibrous Structural Joints: Gomphoses

The peg-in-socket fibrous joint between a tooth and its alveolar socket

The fibrous connection is the periodontal ligament

Cartilaginous Joints

Articulating bones are united by cartilage

Lack a joint cavity

Two types – synchondroses and symphyses

Cartilaginous Joints: Synchondroses

A bar or plate of hyaline cartilage unites the bones

All synchondroses are synarthrotic

Examples include:

Epiphyseal plates of children

Joint between the costal cartilage of the first rib and the sternum

Cartilaginous Joints: Symphyses

Hyaline cartilage covers the articulating surface of the bone and is fused to an intervening pad of fibrocartilage

Amphiarthrotic joints designed for strength and flexibility

Examples include intervertebral joints and the pubic symphysis of the pelvis

Synovial Joints

Those joints in which the articulating bones are separated by a fluid-containing joint cavity

All are freely movable diarthroses

Examples – all limb joints, and most joints of the body

Synovial Joints: General Structure

Synovial joints all have the following

Articular cartilage

Synovial cavity

Articular capsule

Synovial fluid

Reinforcing ligaments

Synovial Joints: Friction-Reducing Structures

Bursae – flattened, fibrous sacs lined with synovial membranes and containing synovial fluid

Tendon sheath – elongated bursa that wraps completely around a tendon

Synovial Joints: Stability

Stability is determined by:

Articular surfaces – shape determines what movements are possible

Ligaments – unite bones and prevent excessive or undesirable motion

Muscle tone is accomplished by:

Muscle tendons across joints acting as stabilizing factors

Tendons that are kept tight at all times by muscle tone

Synovial Joints: Movement

The two muscle attachments across a joint are:

Origin – attachment to the immovable bone

Insertion – attachment to the movable bone

Described as movement along transverse, frontal, or sagittal planes

Synovial Joints: Range of Motion

Nonaxial – slipping movements only

Uniaxial – movement in one plane

Biaxial – movement in two planes

Multiaxial – movement in or around all three planes

Gliding Movements

One flat bone surface glides or slips over another similar surface

Examples – intercarpal and intertarsal joints, and between the flat articular processes of the vertebrae

Angular Movement

Flexion — bending movement that decreases the angle of the joint

Extension — reverse of flexion; joint angle is increased

Dorsiflexion and plantar flexion — up and down movement of the foot

Abduction — movement away from the midline

Adduction — movement toward the midline

Circumduction — movement describes a cone in space

Rotation

The turning of a bone around its own long axis

Examples

Between first two vertebrae

Hip and shoulder joints

Special Movements

Supination and pronation

Inversion and eversion

Protraction and retraction

Elevation and depression

Opposition

Types of Synovial Joints

Plane joints

Articular surfaces are essentially flat

Allow only slipping or gliding movements

Only examples of nonaxial joints

Hinge joints

Cylindrical projections of one bone fits into a trough-shaped surface on another

Motion is along a single plane

Uniaxial joints permit flexion and extension only

Examples: elbow and interphalangeal joints

Pivot Joints

Rounded end of one bone protrudes into a “sleeve,” or ring, composed of bone (and possibly ligaments) of another

Only uniaxial movement allowed

Examples: joint between the axis and the dens, and the proximal radioulnar joint

Condyloid, or Ellipsoidal, Joints

Oval articular surface of one bone fits into a complementary depression in another

Both articular surfaces are oval

Biaxial joints permit all angular motions

Examples: radiocarpal (wrist) joints, and metacarpophalangeal (knuckle) joints

Saddle Joints

Similar to condyloid joints but allow greater movement

Each articular surface has both a concave and a convex surface

Example: carpometacarpal joint of the thumb

Ball-and-Socket Joints

A spherical or hemispherical head of one bone articulates with a cuplike socket of another

Multiaxial joints permit the most freely moving synovial joints

Examples: shoulder and hip joints

Sprains

The ligaments reinforcing a joint are stretched or torn

Partially torn ligaments slowly repair themselves

Completely torn ligaments require prompt surgical repair

Cartilage Injuries

The snap and pop of overstressed cartilage

Common aerobics injury

Repaired with arthroscopic surgery

Dislocations

Occur when bones are forced out of alignment

Usually accompanied by sprains, inflammation, and joint immobilization

Caused by serious falls and are common sports injuries

Subluxation – partial dislocation of a joint

Inflammatory and Degenerative Conditions

Bursitis

An inflammation of a bursa, usually caused by a blow or friction

Symptoms are pain and swelling

Treated with anti-inflammatory drugs; excessive fluid may be aspirated

Tendonitis

Inflammation of tendon sheaths typically caused by overuse

Symptoms and treatment are similar to bursitis

Arthritis

More than 100 different types of inflammatory or degenerative diseases that damage the joints

Most widespread crippling disease in the U.S.

Symptoms – pain, stiffness, and swelling of a joint

Acute forms are caused by bacteria and are treated with antibiotics

Chronic forms include osteoarthritis, rheumatoid arthritis, and gouty arthritis

Osteoarthritis (OA)

Most common chronic arthritis; often called “wear-and-tear” arthritis

Affects women more than men

85% of all Americans develop OA

More prevalent in the aged, and is probably related to the normal aging process

Osteoarthritis: Course

OA reflects the years of abrasion and compression causing increased production of metalloproteinase enzymes that break down cartilage

As one ages, cartilage is destroyed more quickly than it is replaced

The exposed bone ends thicken, enlarge, form bone spurs, and restrict movement

Joints most affected are the cervical and lumbar spine, fingers, knuckles, knees, and hips

Osteoarthritis: Treatments

OA is slow and irreversible

Treatments include:

Mild pain relievers, along with moderate activity

Magnetic therapy

Glucosamine sulfate decreases pain and inflammation

Rheumatoid Arthritis (RA)

Chronic, inflammatory, autoimmune disease of unknown cause, with an insidious onset

Usually arises between the ages of 40 to 50, but may occur at any age

Signs and symptoms include joint tenderness, anemia, osteoporosis, muscle atrophy, and cardiovascular problems

The course of RA is marked with exacerbations and remissions

Rheumatoid Arthritis: Course

RA begins with synovitis of the affected joint

Inflammatory chemicals are inappropriately released

Inflammatory blood cells migrate to the joint, causing swelling

Inflamed synovial membrane thickens into a pannus

Pannus erodes cartilage, scar tissue forms, articulating bone ends connect

The end result, ankylosis, produces bent, deformed fingers

Rheumatoid Arthritis: Treatment

Conservative therapy – aspirin, long-term use of antibiotics, and physical therapy

Progressive treatment – anti-inflammatory drugs or immunosuppressants

The drug Enbrel, a biological response modifier, neutralizes the harmful properties of inflammatory chemicals

Gouty Arthritis

Deposition of uric acid crystals in joints and soft tissues, followed by an inflammation response

Typically, gouty arthritis affects the joint at the base of the great toe

In untreated gouty arthritis, the bone ends fuse and immobilize the joint

Treatment – colchicine, nonsteroidal anti-inflammatory drugs, and glucocorticoids

Muscles and Muscle Tissue

Muscle Overview

The three types of muscle tissue are skeletal, cardiac, and smooth

These types differ in structure, location, function, and means of activation

Muscle Similarities

Skeletal and smooth muscle cells are elongated and are called muscle fibers

Muscle contraction depends on two kinds of myofilaments – actin and myosin

Muscle terminology is similar

Sarcolemma – muscle plasma membrane

Sarcoplasm – cytoplasm of a muscle cell

Prefixes – myo, mys, and sarco all refer to muscle

Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton

Has obvious stripes called striations

Is controlled voluntarily (i.e., by conscious control)

Contracts rapidly but tires easily

Is responsible for overall body motility

Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

Cardiac Muscle Tissue

Occurs only in the heart

Is striated like skeletal muscle but is not voluntary

Contracts at a fairly steady rate set by the heart’s pacemaker

Neural controls allow the heart to respond to changes in bodily needs

Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages

Forces food and other substances through internal body channels

It is not striated and is involuntary

Functional Characteristics of Muscle Tissue

Excitability, or irritability – the ability to receive and respond to stimuli

Contractility – the ability to shorten forcibly

Extensibility – the ability to be stretched or extended

Elasticity – the ability to recoil and resume the original resting length

Muscle Function

Skeletal muscles are responsible for all locomotion

Cardiac muscle is responsible for coursing the blood through the body

Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs

Muscles also maintain posture, stabilize joints, and generate heat

Skeletal Muscle

Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

Muscle fibers are striated

Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins

Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins

Skeletal Muscle

The three connective tissue sheaths are:

Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles

Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle

Skeletal Muscle: Attachments

Most skeletal muscles span joints and are attached to bone in at least two places

When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

Muscles attach:

Directly – epimysium of the muscle is fused to the periosteum of a bone

Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber

Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma

Fibers are 10 to 100 mm in diameter, and up to several centimeters long

Each cell is a syncytium produced by fusion of embryonic cells

Sarcoplasm has numerous glycosomes (granules of stored glycogen) and a unique oxygen-binding protein called myoglobin

Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules

Myofibrils

Myofibrils are densely packed, rod like contractile elements

They make up most of the muscle volume

The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident

Sarcomeres

The smallest contractile unit of a muscle

The region of a myofibril between two successive Z discs

Composed of myofilaments made up of contractile proteins

Myofilaments are of two types – thick and thin

Myofilaments: Banding Pattern

Thick filaments – extend the entire length of an A band

Thin filaments – extend across the I band and partway into the A band

Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another

Thin filaments do not overlap thick filaments in the lighter H zone

M lines appear darker due to the presence of the protein desmin

Ultrastructure of Myofilaments: Thick Filaments

Thick filaments are composed of the protein myosin

Each myosin molecule has a rodlike tail and two globular heads

Tails – two interwoven, heavy polypeptide chains

Heads – two smaller, light polypeptide chains called cross bridges

Ultrastructure of Myofilaments: Thin Filaments

Thin filaments are chiefly composed of the protein actin

Each actin molecule is a helical polymer of globular subunits called G actin

The subunits contain the active sites to which myosin heads attach during contraction

Tropomyosin and troponin are regulatory subunits bound to actin

Sarcoplasmic Reticulum (SR)

SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril

Paired terminal cisternae form perpendicular cross channels

Functions in the regulation of intracellular calcium levels

Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction

T tubules associate with the paired terminal cisternae to form triads

T Tubules

T tubules are continuous with the sarcolemma

They conduct impulses to the deepest regions of the muscle

These impulses signal for the release of Ca²⁺ from adjacent terminal cisternae

Triad Relationships

T tubules and SR provide tightly linked signals for muscle contraction

A double zipper of integral membrane proteins protrudes into the intermembrane space

T tubule proteins act as voltage sensors

SR foot proteins are receptors that regulate Ca²⁺ release from the SR cisternae

Sliding Filament Model of Contraction

Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

In the relaxed state, thin and thick filaments overlap only slightly

Upon stimulation, myosin heads bind to actin and sliding begins

Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

As this event occurs throughout the sarcomeres, the muscle shortens

Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:

Be stimulated by a nerve ending

Propagate an electrical current, or action potential, along its sarcolemma

Have a rise in intracellular Ca²⁺ levels, the final trigger for contraction

Linking the electrical signal to the contraction is excitation-contraction coupling

Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons of the somatic nervous system

Axons of these neurons travel in nerves to muscle cells

Axons of motor neurons branch profusely as they enter muscles

Each axonal branch forms a neuromuscular junction with a single muscle fiber

Neuromuscular Junction

The neuromuscular junction is formed from:

Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction

Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

Electricity Definitions

Voltage (V) – measure of potential energy generated by separated charge

Potential difference – voltage measured between two points

Current (I) – the flow of electrical charge between two points

Resistance (R) – obstruction to charge flow

Insulator – substance with high electrical resistance

Conductor – substance with low electrical resistance

Electrical Current in the Body

Reflects the flow of ions rather than electrons

There is a potential on either side of membranes when:

The number of ions is different across the membrane

The membrane provides a resistance to ion flow

Role of Ion Channels

Types of plasma membrane ion channels:

Passive, or leakage, channels – always open

Chemically (ligand) gated channels – open with binding of a specific neurotransmitter

Voltage-gated channels – open and close in response to membrane potential

Mechanically gated channels – open and close in response to physical deformation of receptors

Operation of a Gated Channel

Example: Na⁺-K⁺ gated channel

Closed when a neurotransmitter is not bound to the extracellular receptor

Na⁺ cannot enter the cell and K⁺ cannot exit the cell

Open when a neurotransmitter is attached to the receptor

Na⁺ enters the cell and K⁺ exits the cell

Operation of a Voltage-Gated Channel

Example: Na⁺ channel

Closed when the intracellular environment is negative

Na⁺ cannot enter the cell

Open when the intracellular environment is positive

Na⁺ can enter the cell

Gated Channels

When gated channels are open:

Ions move quickly across the membrane

Movement is along their electrochemical gradients

An electrical current is created

Voltage changes across the membrane

Electrochemical Gradient

Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration

Ions flow along their electrical gradient when they move toward an area of opposite charge

Electrochemical gradient – the electrical and chemical gradients taken together

Neuromuscular Junction

When a nerve impulse reaches the end of an axon at the neuromuscular junction:

Voltage-regulated calcium channels open and allow Ca²⁺ to enter the axon

Ca²⁺ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane

This fusion releases ACh into the synaptic cleft via exocytosis

ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma

Binding of ACh to its receptors initiates an action potential in the muscle

Destruction of Acetylcholine

ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase

This destruction prevents continued muscle fiber contraction in the absence of additional stimuli

Action Potential

A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane

Role of Acetylcholine (ACh)

ACh binds its receptors at the motor end plate

Binding opens chemically (ligand) gated channels

Na⁺ and K⁺ diffuse out and the interior of the sarcolemma becomes less negative

This event is called depolarization

Depolarization

Initially, this is a local electrical event called end plate potential

Later, it ignites an action potential that spreads in all directions across the sarcolemma

Action Potential: Electrical Conditions of a Polarized Sarcolemma

The outside (extracellular) face is positive, while the inside face is negative

This difference in charge is the resting membrane potential

The predominant extracellular ion is Na⁺

The predominant intracellular ion is K⁺

The sarcolemma is relatively impermeable to both ions

Action Potential: Depolarization and Generation of the Action Potential

An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na⁺ (sodium channels open)

Na⁺ enters the cell, and the resting potential is decreased (depolarization occurs)

If the stimulus is strong enough, an action potential is initiated

Action Potential: Propagation of the Action Potential

Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch

Voltage-regulated Na⁺ channels now open in the adjacent patch causing it to depolarize

Thus, the action potential travels rapidly along the sarcolemma

Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

Action Potential: Repolarization

Immediately after the depolarization wave passes, the sarcolemma permeability changes

Na⁺ channels close and K⁺ channels open

K⁺ diffuses from the cell, restoring the electrical polarity of the sarcolemma

Action Potential: Repolarization

Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)

The ionic concentration of the resting state is restored by the
Na⁺-K⁺ pump

Excitation-Contraction Coupling

Once generated, the action potential:

Is propagated along the sarcolemma

Travels down the T tubules

Triggers Ca²⁺ release from terminal cisternae

Ca²⁺ binds to troponin and causes:

The blocking action of tropomyosin to cease

Actin active binding sites to be exposed

Myosin cross bridges alternately attach and detach

Thin filaments move toward the center of the sarcomere

Hydrolysis of ATP powers this cycling process

Ca²⁺ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes

Role of Ionic Calcium (Ca²⁺) in the Contraction Mechanism

At low intracellular Ca²⁺ concentration:

Tropomyosin blocks the binding sites on actin

Myosin cross bridges cannot attach to binding sites on actin

The relaxed state of the muscle is enforced

At higher intracellular Ca²⁺ concentrations:

Additional calcium binds to troponin (inactive troponin binds two Ca²⁺)

Calcium-activated troponin binds an additional two Ca²⁺ at a separate regulatory site

Calcium-activated troponin undergoes a conformational change

This change moves tropomyosin away from actin’s binding sites

Myosin head can now bind and cycle

This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin

Sequential Events of Contraction

Cross bridge formation – myosin cross bridge attaches to actin filament

Working (power) stroke – myosin head pivots and pulls actin filament toward M line

Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches

“Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state

Contraction of Skeletal Muscle Fibers

Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)

Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening

Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

Contraction of Skeletal Muscle (Organ Level)

Contraction of muscle fibers (cells) and muscles (organs) is similar

The two types of muscle contractions are:

Isometric contraction – increasing muscle tension (muscle does not shorten during contraction)

Isotonic contraction – decreasing muscle length (muscle shortens during contraction)

Motor Unit: The Nerve-Muscle Functional 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 four to several hundred

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

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

Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Muscle Twitch

A muscle twitch is the response of a muscle to a single, brief threshold stimulus

The three phases of a muscle twitch are:

Latent period –
first few milli-
seconds after
stimulation
when excitation-
contraction
coupling is
taking place

Period of contraction – cross bridges actively form and the muscle shortens

Period of relaxation –
Ca²⁺ is reabsorbed
into the SR, and
muscle tension
goes to zero

Graded Muscle Responses

Graded muscle responses are:

Variations in the degree of muscle contraction

Required for proper control of skeletal movement

Responses are graded by:

Changing the frequency of stimulation

Changing the strength of the stimulus

Muscle Response to Varying Stimuli

A single stimulus results in a single contractile response – a muscle twitch

Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation

More rapidly delivered stimuli result in incomplete tetanus

If stimuli are given quickly enough, complete tetanus results

Muscle Response: Stimulation Strength

Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs

Beyond threshold, muscle contracts more vigorously as stimulus strength is increased

Force of contraction is precisely controlled by multiple motor unit summation

This phenomenon, called recruitment, brings more and more muscle fibers into play

Treppe: The Staircase Effect

Staircase – increased contraction in response to multiple stimuli of the same strength

Contractions increase because:

There is increasing availability of Ca²⁺ in the sarcoplasm

Muscle enzyme systems become more efficient because heat is increased as muscle contracts

Muscle Tone

Muscle tone:

Is the constant, slightly contracted state of all muscles, which does not produce active movements

Keeps the muscles firm, healthy, and ready to respond to stimulus

Spinal reflexes account for muscle tone by:

Activating one motor unit and then another

Responding to activation of stretch receptors in muscles and tendons

Isotonic Contractions

In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and moves the load

The two types of isotonic contractions are concentric and eccentric

Concentric contractions – the muscle shortens and does work

Eccentric contractions – the muscle contracts as it lengthens

Isometric Contractions

Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens

Occurs if the load is greater than the tension the muscle is able to develop

Muscle Metabolism: Energy for Contraction

ATP is the only source used directly for contractile activity

As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:

The interaction of ADP with creatine phosphate (CP)

Anaerobic glycolysis

Aerobic respiration

Muscle Metabolism: Anaerobic Glycolysis

When muscle contractile activity reaches 70% of maximum:

Bulging muscles compress blood vessels

Oxygen delivery is impaired

Pyruvic acid is converted into lactic acid

The lactic acid:

Diffuses into the bloodstream

Is picked up and used as fuel by the liver, kidneys, and heart

Is converted back into pyruvic acid by the liver

Muscle Fatigue

Muscle fatigue – the muscle is in a state of physiological inability to contract

Muscle fatigue occurs when:

ATP production fails to keep pace with ATP use

There is a relative deficit of ATP, causing contractures

Lactic acid accumulates in the muscle

Ionic imbalances are present

Intense exercise produces rapid muscle fatigue (with rapid recovery)

Na⁺-K⁺ pumps cannot restore ionic balances quickly enough

Low-intensity exercise produces slow-developing fatigue

SR is damaged and Ca²⁺ regulation is disrupted

Oxygen Debt

Vigorous exercise causes dramatic changes in muscle chemistry

For a muscle to return to a resting state:

Oxygen reserves must be replenished

Lactic acid must be converted to pyruvic acid

Glycogen stores must be replaced

ATP and CP reserves must be resynthesized

Oxygen debt – the extra amount of O₂ needed for the above restorative processes

Heat Production During Muscle Activity

Only 40% of the energy released in muscle activity is useful as work

The remaining 60% is given off as heat

Dangerous heat levels are prevented by radiation of heat from the skin and sweating

Force of Muscle Contraction

The force of contraction is affected by:

The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction

The relative size of the muscle – the bulkier the muscle, the greater its strength

Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length

Muscle Fiber Type: Functional Characteristics

Speed of contraction – determined by speed in which ATPases split ATP

The two types of fibers are slow and fast

ATP-forming pathways

Oxidative fibers – use aerobic pathways

Glycolytic fibers – use anaerobic glycolysis

These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

Muscle Fiber Type: Speed of Contraction

Slow oxidative fibers contract slowly, have slow acting myosin ATPases, and are fatigue resistant

Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue

Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued

Smooth Muscle

Composed of spindle-shaped fibers with a diameter of 2-10 mm and lengths of several hundred mm

Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium

Organized into two layers (longitudinal and circular) of closely apposed fibers

Found in walls of hollow organs (except the heart)

Have essentially the same contractile mechanisms as skeletal muscle

Peristalsis

When the longitudinal layer contracts, the organ dilates and contracts

When the circular layer contracts, the organ elongates

Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs

Innervation of Smooth Muscle

Smooth muscle lacks neuromuscular junctions

Innervating nerves have bulbous swellings called varicosities

Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions

Microscopic Anatomy of Smooth Muscle

SR is less developed than in skeletal muscle and lacks a specific pattern

T tubules are absent

Plasma membranes have pouchlike infoldings called caveoli

Ca²⁺ is sequestered in the extracellular space near the caveoli, allowing rapid influx when channels are opened

There are no visible striations and no sarcomeres

Thin and thick filaments are present

Proportion and Organization of Myofilaments in Smooth Muscle

Ratio of thick to thin filaments is much lower than in skeletal muscle

Thick filaments have heads along their entire length

There is no troponin complex

Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner

Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals

Contraction of Smooth Muscle

Whole sheets of smooth muscle exhibit slow, synchronized contraction

They contract in unison, reflecting their electrical coupling with gap junctions

Action potentials are transmitted from cell to cell

Some smooth muscle cells:

Act as pacemakers and set the contractile pace for whole sheets of muscle

Are self-excitatory and depolarize without external stimuli

Contraction Mechanism

Actin and myosin interact according to the sliding filament mechanism

The final trigger for contractions is a rise in intracellular Ca²⁺

Ca²⁺ is released from the SR and from the extracellular space

Ca²⁺ interacts with calmodulin and myosin light chain kinase to activate myosin

Role of Calcium Ion

Ca²⁺ binds to calmodulin and activates it

Activated calmodulin activates the kinase enzyme

Activated kinase transfers phosphate from ATP to myosin cross bridges

Phosphorylated cross bridges interact with actin to produce shortening

Smooth muscle relaxes when intracellular Ca²⁺ levels drop

Special Features of Smooth Muscle Contraction

Unique characteristics of smooth muscle include:

Smooth muscle tone

Slow, prolonged contractile activity

Low energy requirements

Response to stretch

Hyperplasia

Certain smooth muscles can divide and increase their numbers by undergoing hyperplasia

This is shown by estrogen’s effect on the uterus

At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size

During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus

Response to Stretch

Smooth muscle exhibits a phenomenon called
stress-relaxation response in which:

Smooth muscle responds to stretch only briefly, and then adapts to its new length

The new length, however, retains its ability to contract

This enables organs such as the stomach and bladder to temporarily store contents

Types of Smooth Muscle: Single Unit

The cells of single-unit smooth muscle, commonly called visceral muscle:

Contract rhythmically as a unit

Are electrically coupled to one another via gap junctions

Often exhibit spontaneous action potentials

Are arranged in opposing sheets and exhibit stress-relaxation response

Types of Smooth Muscle: Multiunit

Multiunit smooth muscles are found:

In large airways to the lungs

In large arteries

In arrector pili muscles

Attached to hair follicles

In the internal eye muscles

Their characteristics include:

Rare gap junctions

Infrequent spontaneous depolarizations

Structurally independent muscle fibers

A rich nerve supply, which, with a number of muscle fibers, forms motor units

Graded contractions in response to neural stimuli

Microscopic Anatomy of Heart Muscle

Cardiac muscle is striated, short, fat, branched, and interconnected

The connective tissue endomysium acts as both tendon and insertion

Intercalated discs anchor cardiac cells together and allow free passage of ions

Heart muscle behaves as a functional syncytium

Muscular Dystrophy

Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy

Muscular Dystrophy

Duchenne muscular dystrophy (DMD)

Inherited, sex-linked disease carried by females and expressed in males (1/3500)

Diagnosed between the ages of 2-10

Victims become clumsy and fall frequently as their muscles fail

Muscular Dystrophy

Progresses from the extremities upward, and victims die of respiratory failure in their 20s

Caused by a lack of the cytoplasmic protein dystrophin

There is no cure, but myoblast transfer therapy shows promise