Muscle dalam badan manusia aladah salah satu elemen terpenting dalam kehidupan manusia.

7/30/2019 Muscle dalam badan manusia aladah salah satu elemen terpenting dalam kehidupan manusia.

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Muscle and Muscle Tissue

Make up about half of total body mass

Exerts force by converting chemical energy, ATP, to mechanical energy

Muscle tissue is classified based on

Shape

Number and position of nuclei

Presence of striations

Voluntary or involuntary control

Functional Characteristics of Muscle Tissue

Excitability

Ability to receive and respond to stimuli

Contractility

Ability to shorten after adequate stimulation

Extensibility

Ability to stretch

Elasticity

Ability to return to its original length after contraction

Functions of Muscle

Movement

Skeletal

Cardiac

Smooth

Maintain posture


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Stabilize joints

Generate heat

Types of Muscle Tissue

Skeletal muscle

Cells are elongated

Multinucleated

Nuclei are peripherally placed

Striated

Voluntary

They attach to and cover the bony skeleton

Cardiac

Cells branch

Single centrally placed nucleus

Striated

Involuntary

Located in the heart (Tab 9.3)

Smooth

Spindle shaped fibers

Centrally placed nucleus

Non-striated

Involuntary

Located in the walls of hollow visceral organs


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Types of Smooth Muscle

Smooth muscles in different organs differ in:

Fiber arrangement and organization

Responsiveness to stimuli

Innervation

Categorized into two main categories both of which are innervated by the ANS

and respond to hormonal controls

Single-unit smooth muscle

Multiunit smooth muscle

Single-Unit Smooth Muscle

Also called visceral muscle

Cell contract rhythmically and as a single unit

Cells are connected to each other via gap junctions and are arranged in opposing

sheets

Exhibit spontaneous action potentials (stress-relaxation response)

Multi-unit Smooth Muscle

Muscles fibers are independent of each other (Gap junctions rare)

Richly innervated and each nerve forms a motor unit with a number of muscle

fibers (spontaneous depolarizations are infrequent)

Responds to neural stimulation with graded contractions

Examples: Arrector pili muscles, eye muscles that adjust pupil size, muscles in

large airways and large arteries


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Gross Anatomy of Skeletal Muscle

Each muscle has a nerve and blood supply that allows neural control and ensures

adequate nutrient delivery and waste removal.

Connective tissue sheaths are found at various structural levels of each muscle:

endomysium surrounds each muscle fiber, perimysium surrounds groups of muscle

fibers, and epimysium surrounds whole muscles. (Fig 9.2)

Attachments span joints and cause movement to occur from the movable bone

(the muscles insertion) toward the less movable bone (the muscles origin)

Muscle attachments may be direct or indirect.

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the

sarcolemma.

Myofibrils account for roughly 80% of cellular volume, and contain the

contractile elements of the muscle cell. (Tab 9.1)

Striations are due to a repeating series of dark A bands and light I bands.

Myofilaments make up the myofibrils, and consist of thick and thin filaments.

(Fig 9.3)

Ultrastructure and Molecular Composition of the Myofilaments

There are two types of myofilaments in muscle cells: thick filaments composed of

bundles of myosin, and thin filaments composed of strands of actin.

Tropomyosin and troponin are regulatory proteins present in thin filaments. (Fig

9.4)


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The sarcoplasmic reticulum is a smooth endoplasmic reticulum surrounding each

myofibril.

T tubules are infoldings of the sarcolemma that conduct electrical impulses from

the surface of the cell to the terminal cisternae. (Fig 9.5)

Sliding Filament Theory

The sliding filament model of muscle contraction states that during contraction,

the thin filaments slide past the thick filaments. Overlap between the myofilaments

increases and the sarcomere shortens

Physiology of a Skeletal Muscle Fiber

The neuromuscular junction is a connection between an axon terminal and a

muscle fiber that is the route of electrical stimulation of the muscle cell.

A nerve impulse causes the release of acetylcholine to the synaptic cleft, which

binds to receptors on the motor end plate, triggering a series of electrical events on the

sarcolemma. (Fig 9.7)

Generation of an Action Potential Across the Sarcolemma

Like plasma and nerve cell membranes the sarcolemma is polarized

The potential difference between the extracellular space and the intracellular

space is called the Resting Potential

Resting Potential

Potential difference is the result of an unequal distribution of ions between the

inside and the outside of the cell


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Typical resting potential (Nerve) is -70mV meaning the inside of the cell is more

negative than the outside

Difference is due to selective ionic permeability of the cell membrane

It is maintained by Na+/K+ pump

K+ ion concentration is higher inside than outside

Negatively charged proteins are trapped inside the cell

Resting membrane potential is created because the membrane is selectively

permeable K+ ions

K+

ions diffuse down its concentration gradient

Na+ is not allowed to enter the cell thus the cell remains polarized

Transmission of action potentials lead to disruption of the ionic gradients which

must then be restored by the Na+/K+ pump that uses ATP to transport 3 Na+ out for every

2 K+ it transports in

Action Potential

When a muscle cell receives excitatory impulse of sufficient strength,

depolarization occurs

The inside of the cell becomes progressively less negative and an action potential

is generated

Voltage changes on the membrane result in the opening of voltage-gated ion

channels (Fig 9.10)

An action potential begins when voltage-gated Na+ channels open in response to

depolarization

Na+ ions rush down its electrochemical gradient into the cell


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The segment of the cell where this occurs is depolarized

The Na+ channels then close

Voltage gated K+ ion channels then open

K+ ions rush out down its electrochemical gradient

The cell is repolarized (returns to a more negative potential)

Ionic concentrations of the resting state are restored by Na+/K+ ATPase

An action potential is therefore a transient reversal of the resting membrane

potential

The inside of the cell may become more negative than normal after repolarization

(hyperpolarization)

Immediately after an action potential, it may become very difficult or impossible

to initiate another action potential

This period is referred to as the refractory period (relative and absolute refractory

periods)

An action potential is propagated along the entire sarcolemma

All-or-none response

An action potential with consistent size and duration is produced only when a

threshold membrane potential is reached

Physiology of a Skeletal Muscle Fiber

Generation of an action potential across the sarcolemma occurs in response to

acetylcholine binding with receptors on the motor end plate. It involves the influx of

sodium ions, which makes the membrane potential slightly less negative.


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Excitation-contraction coupling is the sequence of events by which an action

potential on the sarcolemma results in the sliding of the myofilaments.

Ionic calcium in muscle contraction is kept at almost undetectable levels within

the cell through the regulatory action of intracellular proteins.

Muscle fiber contraction follows exposure of the myosin binding sites, and

follows a series of events (Fig 9.10, 9.11, and 9.12)

Contraction of a Skeletal Muscle

A motor unit consists of a motor neuron and all the muscle fibers it innervates. It

is smaller in muscles that exhibit fine control.

The muscle twitch is the response of a muscle to a single action potential on its

motor neuron. (Fig 9.13 & 9.14)

There are three kinds of graded muscle responses: wave summation, multiple

motor unit summation (recruitment), and treppe. (Fig 9.15 & 9.16)

Muscle Tone

A state of partial contraction exhibited by relaxed muscles

Results from spinal reflexes that activate a group of motor units in response to

stretch receptor activation in muscles and tendons

Does not produce movements

Keeps muscles healthy and firm so they can respond when stimulated

Helps stabilize joints and maintain posture

Isotonic Contraction


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Same tension

Muscle contracts and shortens to move a load

Concentric contractions occur when muscle contracts as it shortens (picking a

book)

Eccentric contractions (more forceful) occurs when the muscle contracts as it

lengthens (Fig 9.19)

Isometric Contractions

Same length

Occurs when a muscle tries to move a load that is greater than the force the

muscle is able to generate

Tension builds up in the muscle but it does not shorten

Muscle Metabolism

Muscles contain very little stored ATP, and consumed ATP is replenished rapidly

through phosphorylation by creatine phosphate, glycolysis and anaerobic respiration, and

aerobic respiration.

Muscles will function aerobically as long as there is adequate oxygen, but when

exercise demands exceed the ability of muscle metabolism to keep up with ATP demand,

metabolism converts to anaerobic glycolysis. (Fig 9.20)

Muscle fatigue is the physiological inability to contract due to the shortage of

available ATP.

Oxygen debt is the extra oxygen needed to replenish oxygen reserves, glycogen

stores, ATP and creatine phosphate reserves, as well as conversion of lactic acid to

pyruvic acid and then to glucose after vigorous muscle activity


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Heat production during muscle activity is considerable. It requires release of

excess heat through homeostatic mechanisms such as sweating and radiation from the

skin.

Effect of Exercise on Muscles

Aerobic, or endurance, exercise promotes an increase in capillary penetration, the

number of mitochondria, and increased synthesis of myoglobin, leading to more efficient

metabolism, but no hypertrophy.

Resistance exercise, such as weight lifting or isometric exercise, promotes an

increase in the number of mitochondria, myofilaments and myofibrils, and glycogen

storage, leading to hypertrophied cells.

Forces of Muscle Contraction

Number of muscle fibers stimulated

Size of the muscle fiber stimulated

Frequency of stimulation

Degree of Muscle stretch

Length-tension relationship

Velocity and Duration of Contraction

Velocity and duration of contraction is influenced by:

Fiber type

Fast and slow fibers exist

Difference in speed is dependent on the rate at which myosin ATPase splits ATP

Load


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Recruitment (Fig 9.21)

(See table 9.2 for comparison between fast and slow twitch muscle fibers)

Muscle dalam badan manusia aladah salah satu elemen terpenting dalam kehidupan manusia.

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