MUSCLE TISSUEDr. Michael P. Gillespie

POSTURE / MOVEMENT

Stable posture results from a balance of competing forces.

Movement occurs when competing forces are unbalanced.

Force generated by muscles is the primary means for controlling the balance between posture and movement.

MUSCLE AS A SKELETAL STABILIZER Muscle generates force to stabilize the

skeletal system. Muscle tissue is coupled to the external

environment and internal control mechanisms provided by the nervous system allow it to respond to changes in the external environment.

Whole muscles consist of many individual muscle fibers.

Muscle adapts to the immediate (acute) and repeated long-term (chronic) external forces that can destabilize the body.

Fine control – surgery Large forces – dead-lift

TYPES OF MUSCLE TISSUE

Skeletal muscle tissue Cardiac muscle tissue

Autorhythmicity - pacemaker Smooth muscle tissue

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FUNCTIONS OF MUSCLE TISSUE Producing body movements Stabilizing body positions Storing and moving substances within

the body Sphincters – sustained contractions of ringlike

bands prevent outflow of the contents of a hollow organ

Cardiac muscle pumps nutrients and wastes through

Smooth muscle moves food, bile, gametes, and urine

Skeletal muscle contractions promote flow of lymph and return blood to the heart

Generating heat - thermogenesis 5

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PROPERTIES OF MUSCLE TISSUE

Electrical excitability Produces electrical signals – action potentials

Contractility Isometric contraction – tension without muscle

shortening Isotonic contraction – constant tension with

muscle shortening

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PROPERTIES OF MUSCLE TISSUE

Extensibility – ability of a muscle to stretch without being damaged

Elasticity Ability of a muscle to return to its original length

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CONNECTIVE TISSUE COMPONENTS

Fascia – a sheet of fibrous CT that supports or surrounds muscles and other organs Superficial fascia (subcutaneous layer) –

separates muscle from skin Deep fascia – holds muscles with similar

functions together Epimysium – outermost layer –

encircles whole muscles Perimysium

Surrounds groups of 10 – 100 individual muscle fibers separating them into bundles called fascicles

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CONNECTIVE TISSUE COMPONENTS

Endomysium Separates individual muscle fibers within the

fascicle

Tendon All 3 CT layers may extend beyond the muscle to

form a cord of dense regular CT that attaches muscle to the periosteum of bone

Aponeurosis A broad, flat layer of CT

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BASIC COMPONENTS OF MUSCLE

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MUSCLE SIZE

Whole muscles are made up of many individual muscle fibers.

These fibers range in thickness from 10 to 100 μm and in length from 1 to 50 cm.

Each muscle fiber is an individual muscle cell with many nuclei.

The individual muscle fibers contract, which will ultimately result in contraction of the entire muscle.

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NERVE AND BLOOD SUPPLY Skeletal muscles are well supplied with

nerves and blood vessels Neuromuscular junction – the structural point

of contact and the functional site of communication between a nerve and the muscle fiber

Capillaries are abundant – each muscle fiber comes into contact with 1 or more

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TWO TYPES OF PROTEINS IN MUSCLE

Contractile proteins Actin and myosin Shorten the muscle fiber and generate active

force Referred to as “active proteins”

Noncontractile proteins Titan and desmin

Titan provides tensile strength Desmin stabilizes adjacent sarcomeres

Make up the cytoskeleton within and between muscle fibers

Referred to as “structural” proteins 14

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SARCOLEMMA, T TUBULES, AND SARCOPLASM Sarcolemma – the plasma membrane of a

muscle cell T (transverse) tubules – Propogate action

potentials – extend to the outside of the muscle fiber

Sarcoplasm – cytoplasm of the muscle fiber Contains myoglobin – protein that binds with

oxygen

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MYOFIBRILS AND SARCOPLASMIC RETICULUM

Myofibril – the contractile elements of skeletal muscle

Sarcoplasmic reticulum (SR) – encircles each myofibril – stores CA2+ (its release triggers muscle contractions)

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ATROPHY AND HYPERTROPHY

Muscular atrophy – wasting away of muscles Disuse Denervation

Muscular hypertrophy – an excessive increase in the diameter of muscle fibers

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FILAMENTS AND THE SARCOMERE

Filaments – structures within the myofibril Thin Thick

Sarcomere – basic functional unit of a myofibril

Z discs – separate one sarcomere from the next

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MYOFIBRIL ELECTRON MICROGRAPH

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FILAMENTS AND THE SARCOMERE

A band – predominantly thick filaments Zone of overlap at the ends of the A bands H zone – contains thick, but no thin filaments

I band – thin filaments M-line – middle of the sarcomere

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MUSCLE PROTEINS

Contractile proteins – generate force Myosin Actin

Regulatory proteins – switch contraction on and off

Structural proteins

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SLIDING FILAMENT MECHANISM

Muscle contraction occurs because myosin heads attach to the thin filaments at both ends of the sarcomere and pull them toward the M line.

The length of the filaments does not change; However, the sarcomeres shorten, thereby shortening the entire muscle.

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RELAXED & CONTRACTED MYOFIBRILS

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POWER STROKE OF CROSSBRIDGE CYCLING

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ROLE OF CA2+ IN CONTRACTION

An increase in calcium ion concentration in the cytosol initiates muscle contraction and a decrease in calcium ions stops it.

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MAJOR SEQUENCE OF EVENTS UNDERLYING MUSCLE FIBER ACTIVATION 1. Action potential is initiated and propagated down a motor

axon. 2. Acetylcholine is released from axon terminals at neuromuscular

junction. 3. Acetylcholine is bound to receptor sites on the motor endplate. 4. Sodium and potassium ions enter and depolarize the muscle

membrane. 5. Muscle action potential is propagated over membrane surface. 6. Transverse tubules are depolarized, leading to release of

calcium ions surrounding the myofibrils. 7. Calcium ions bind to troponin, which leads to the release of

inhibition of actin and myosin binding. The crossbridge between actin and myosin heads is created.

8. Actin combines with myosin adenosine triphosphate (ATP), an energy-providing molecule.

9. Energy is released to produce movement of myosin heads. 10. Myosin and actin slide relative to each other. 11. Actin and myosin bond (crossbridge) is broken and

reestablished if calcium concentration remains sufficiently high. 32

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RIGOR MORTIS

After death the cellular membranes become leaky.

Calcium ions are released and cause muscular contraction.

The muscles are in a state of rigidity called rigor mortis.

It begins 3-4 hours after death and lasts about 24 hours, until proteolytic enzymes break down (digest) the cross-bridges. 33

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NEUROMUSCULAR JUNCTION (NMJ)

Muscle action potentials arise at the NMJ. The NMJ is the site at which the motor neuron

contacts the skeletal muscle fiber. A synapse is the region where

communication occurs.

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NEUROMUSCULAR JUNTCION (NMJ)

The neuron cell communicates with the second by releasing a chemical called a neurotransmitter.

Synaptic vesicles containing the neurotransmitter acetylcholine (ach) are released at the NMJ.

The motor end plate is the muscular part of the NMJ. It contains acetylcholine receptors.

The enzyme acetlycholineesterase (AChE) breaks down ACh.

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PRODUCTION OF ATP

1. From creatine phosphate. When muscle fibers are relaxed they produce

more ATP than they need. This excess is used to synthesize creatine phosphate (an energy rich compound).

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PRODUCTION OF ATP

2. Anaerobic cellular respiration. Glucose undergoes glycolysis, yielding ATP and 2

molecules of pyruvic acid. Does not require oxygen.

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PRODUCTION OF ATP

3. Aerobic cellular respiration. The pyruvic acid enters the mitochondria where

it is broken down to form more ATP. Slower than anaerobic respiration, but yields

more ATP. Utilizes oxygen. 2 sources of oxygen.

Diffuses from bloodstream. Oxygen released from myoglobin.

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MUSCLE FATIGUE

Muscle fatigue is the inability of a muscle to contract forcefully after prolonged activity.

Central fatigue – a person may develop feelings of tiredness before actual muscle fatigue.

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OXYGEN DEBT OR RECOVERY OXYGEN UPTAKE

Added oxygen, over and above resting oxygen consumption, taken in after exercise.

Used to restore metabolic conditions. 1. To convert lactic acid back into glycogen

stores in the liver. 2. To resynthesize creatine phosphate and ATP in

muscle fibers. 3. To replace the oxygen removed from

hemoglobin.

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MOTOR UNITS

A motor unit consists of the somatic motor neuron and all the skeletal muscle fibers it stimulates.

A single motor neuron makes contact with an average of 150 muscle fibers.

All muscle fibers in one motor unit contract in unison.

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MOTOR UNIT

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TWITCH CONTRACTION

A twitch contraction is the brief contraction of all the muscle fibers in a motor unit in response to a single action potential.

A myogram is a record of a muscle contraction and illustrates the phases of contraction.

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REFRACTORY PERIOD

A period of lost excitability during which a muscle fiber cannot respond to stimulation.

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MOTOR UNIT RECRUITMENT

The process in which the number of active motor units increases.

The weakest motor units are recruited first, with progressively stronger units being added if the task requires more force.

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MUSCLE TONE

Even at rest a muscle exhibits a small amount of muscle tone – tension or tautness.

Flaccid – when motor units serving a muscle are damaged or cut.

Spastic – when motor units are over-stimulated.

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ISOTONIC AND ISOMETRIC CONTRACTIONS

Concentric isotonic activation (contraction) – a muscle shortens and pulls on another structure.

Eccentric isotonic activation – the length of a muscle increases during contraction.

Isometric activation – muscle tension is created; However, the muscle doesn’t shorten or lengthen.

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TYPES OF SKELETAL MUSCLE FIBERS

Slow oxidative (SO) fibers. Smallest of the fibers. Least powerful. Appear dark red – much myoglobin and many

capillaries. Resistant to fatigue.

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TYPES OF SKELETAL MUSCLE FIBERS

Fast oxidative-Glycolytic (FOG) fibers. Intermediate in diameter. Appear dark red – much myoglobin and many

capillaries. High level of intracellular glycogen. Resistant to fatigue.

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TYPES OF SKELETAL MUSCLE FIBERS

Fast Glycolitic (FG) fibers. Largest in diameter. Contain the most myofibrils, therefore more

powerful contractions. Appear white – low myoglobin and few

capillaries. Large amounts of glycogen – anaerobic

respiration. Fatigue quickly.

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TWITCH CLASSIFICATIONS “Slow twitch” - Muscle fibers innervated by small

motor neurons have twitch responses that are relatively long in duration and small in amplitude. Classified as S (slow) due to slower contractile

characteristics. Associated fibers are classified as SO fibers due to their

slow and oxidative histochemical profile. “Fast twitch” – muscle fibers associated with

larger motor neurons have twitch responses that are relatively brief in duration and higher in amplitude. Classified as FF (fast and easily fatigable). Associated fibers are classified as FG due to their fast

twitch, glycolytic profile. “Intermediate”

Classified as FR (fast fatigue-resistant). Associated fibers are classified as FOG due to utilization

of both oxidative and glycoltyic energy sources.

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MOTOR UNIT TYPES

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DISTRIBUTION AND RECRUITMENT OF DIFFERENT TYPES OF FIBERS

Most skeletal muscles are a mixture of all three types.

The continually active postural muscles have a high concentration of SO fibers.

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DISTRIBUTION AND RECRUITMENT OF DIFFERENT TYPES OF FIBERS

Muscles of the shoulders and arms are used briefly and for quick actions, therefore they have many FG fibers.

Muscle of the legs support the body and participate in quick activities, therefore they have many SO and FOG fibers.

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MUSCLE MORPHOLOGY

Muscle morphology describes the basic shape of the muscle.

The shape will influence the ultimate function of the muscle.

The two most common forms are fusiform and pennate.

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FUSIFORM & PENNATE MUSCLE FIBERS

Fusiform Fusiform muscles have fibers running parallel to

one another and the central tendon (i.e. biceps brachii).

Pennate (Latin – feather) Pennate muscles possess fibers that approach

their central tendon obliquely. Pennate muscles have a greater number of

muscle fibers and generate larger forces. Most muscles in the body are considered pennate. Subdivisions

Unipennate Bipennate Multipennate

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MUSCLE SHAPES

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FEATURES THAT AFFECT THE FORCE THROUGH A MUSCLE & ON THE TENDON

Physiological cross-sectional area The amount of active proteins available to

generate a contraction force With full activation, the maximal force potential

of a muscle is proportional to the sum of the cross-sectional area of all its fibers.

A thicker muscle generates greater force than a thinner muscle of similar morphology.

Pennation angle Pennation angle refers to the angle of orientation

between the muscle fibers and tendon.

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PENNATION ANGLE & DEGREE OF FORCE If muscle fibers attach parallel to the tendon

the angle is defined as 0 degrees (essentially all the force generated is transmitted across the joint).

If the pennation angle is greater than 0 degrees, then less of the force produced is transmitted through the tendon.

A muscle with a pennation angle of 0 degrees transmits 100% of its contractile force (theoretically).

A muscle with a pennation angle of 30 degrees transmits 86% of its contractile force.

Most human muscles have pennation angles that range from 0 to 30 degrees.

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PENNATION ANGLE & VECTOR OF FORCE

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PENNATE VS. FUSIFORM

In general, pennate muscles produce greater maximal force than fusiform muscles of similar volume.

Orienting muscle fibers obliquely to the central tendon allows for more total muscle fibers into a given length of muscle. This increases the physiological cross-sectional area and therefore the force.

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PASSIVE TENSION There are noncontractile elements of the muscle

and tendon. These noncontractile elements are referred to as

parallel and series elastic components of muscle. Series elastic components are tissues that lie

in series with active proteins. Parallel elastic components are tissues that

surround or lie in parallel with the active proteins. These are the extracellular connective tissues (epimysium, perimysium, and endomysium).

Stretching the whole muscle by extending the joint elongates both the parallel and series elastic components, generating a spring like resistance, or stiffness, within the muscle.

This resistance is referred to as passive tension. 65

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PARALLEL & SERIES ELASTIC COMPONENTS

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PASSIVE TENSION CONTINUED

The passive elements within a muscle begin generating passive tension after a critical length at which all of the relaxed (i.e. slack) tissue has been brought to an initial level of tension.

Tension progressively increases after this until the muscle reaches very high levels of stiffness.

Eventually, the tissue ruptures or fails. At very long lengths the muscle fibers begin to

lose their active force-generating capability because there is less overlap among the active proteins that generate force. The additional passive tension becomes very important.

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PURPOSE OF PASSIVE TENSION

Passive tension helps with movement and joint stabilization against the forces of gravity, physical contact, or other activated muscles.

Stretched muscle tissue stores potential energy which can be released to augment the overall force potential of a muscle.

The elasticity from the passive tension can serve as a damping mechanism that protects the structural components of the muscle and tendon.

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PASSIVE LENGTH-TENSION CURVE

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ACTIVE TENSION

Muscle tissue generates force actively in response to a stimulus from the nervous system.

The sarcomere is the fundamental active force generator within the muscle fiber.

The sliding filament hypothesis explains how the actin and myosin filaments can contract and exert their force.

Each myosin head attaches to an adjacent actin filament, forming a crossbridge.

The amount of force generated within each sarcomere depends on the number of simultaneously formed crossbridges. The greater the number of crossbridges, the greater the force generated within the sarcomere.

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ACTIVE LENGTH-TENSION CURVE The amount of active force depends upon the

instantaneous length of the muscle fiber. A change in fiber length- from either active

contraction or passive elongation- alters the amount of overlap between actin and myosin.

The ideal resting length of a muscle fiber (or individual sarcomere) is the length that allows the greatest number of crossbridges and therefore the greatest potential force.

As the sarcomere lengthens of shortens from its resting length, the number of potential crossbridges decreases so that lesser amounts of active force are generated.

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CROSSBRIDGE

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ACTIVE LENGTH-TENSION CURVE

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LENGTH-FORCE (LENGTH-TENSION)

The ideal resting length of the muscle fiber allows for the optimum length-force relationship.

While the phrase length-force is more appropriate, the term length-tension is used instead due to its wide acceptance in the physiology literature.

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TOTAL LENGTH-TENSION CURVE OF MUSCLE

The active length-tension curve, when combined with the passive length-tension curve, yields the total length-tension curve of muscle.

The combination of active force and passive tension allows for a large range of muscle forces over a wide range of muscle length.

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TOTAL LENGTH-TENSION CURVE

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ISOMETRIC MUSCLE FORCE

Isometric activation of a muscle produces force without significant change in its length.

This occurs when the joint over which an activated muscle crosses is constrained from movement.

Constraint can occur from a force produced by an antagonistic muscle or from an external source.

Isometrically produced forces provide stability to the joints and the body as a whole.

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MAXIMAL ISOMETRIC FORCE AS AN INDICATOR OF A MUSCLE’S PEAK STRENGTH Maximal isometric force of a muscle is often used

as a general indicator of a muscle’s peak strength and can indicate neuromuscular recovery after injury.

A muscle’s internal torque generation can be measured isometrically at several joint angles.

The magnitude of isometric torque differs considerably based on the angle of the joint at the time of activation, even with maximal effort.

The internal torque produced isometrically by a muscle group can be determined by asking an individual to produce a maximal effort contraction against a known external torque.

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DYNANOMETER

A dynamometer is an instrument used to measure force, moment of force (torque), or power.

In the fields of rehabilitation, therapy, kinesiology, and ergonomics, force dynanometers are used to measure back, grip, arm, or leg strength in order to evaluate physical status, performance, or task demands.

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HAND DYNANOMETER

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RESISTING A FORCE

The nervous system stimulates a muscle to resist a force by concentric, eccentric, or isometric activation.

During concentric activation, the muscle shortens (contracts). The internal (muscle) torque exceeds the external (load) torque.

During eccentric activation, the external torque exceeds the internal torque. The muscle is driven by the nervous system to contract but it is elongated in response to a more dominating force (an external force or an antagonistic muscle).

During isometric activation, the length of the muscle remains nearly constant, as the internal and external torques are equally matched.

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MODULATING FORCE THROUGH CONCENTRIC AND ECCENTRIC ACTIVATION During concentric and eccentric activations, a very

specific relationship exists between a muscle’s maximum force output and its velocity of contraction (or elongation).

Concentric activation A muscle contracts at a maximum velocity when the

load is negligible. As the load increases, the maximum contraction

velocity of the muscle decreases. Eventually, a very large load results in a contraction

velocity of zero (i.e. isometric state). Eccentric activation

A load that barely exceeds the isometric force level causes the muscle to lengthen slowly.

Speed of lengthening increases as a greater load is applied.

There is a maximal load level the muscle cannot resist, beyond which the muscle uncontrollably lengthens.

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RELATIONSHIP BETWEEN LOAD AND MAXIMAL SHORTENING VELOCITY

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FORCE-VELOCITY CURVE

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POWER & WORK Power, or the rate of work, can be expressed as a product of

force and contraction velocity. A constant power output of a muscle can be sustained by

increasing the load (resistance) while proportionately decreasing the contraction velocity, or vise versa. [switching gears on a bike]

Positive work A muscle undergoing a concentric contraction against a load is doing

positive work on a load. Negative work

A muscle undergoing eccentric activation against an overbearing load is doing negative work.

A muscle can act as either an active accelerator of movement against a load while the muscle is contracting (i.e. concentric activation) or as a “brake” or decelerator when a load is applied and the activated muscle is lengthening (i.e. eccentric activation).

The quadriceps muscles act concentrically when one ascends the stairs and lifts the weight of the body (positive work). The quadriceps perform eccentrically as they lower the body down the stairs in a controlled fashion (negative work).

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