Muscular System

Objectives

• Describe the three types of muscle tissue

• Describe how muscles contract

• Describe how muscles and bones interact to provide movement

Types of Muscle Tissue

• More than 40% of the mass of the average human body is muscle

• There are 3 types of muscle tissue:

– Skeletal Muscles

– Smooth Muscles

– Cardiac Muscles

Skeletal Muscle

• Skeletal muscle is responsible for voluntary movement

• Skeletal muscle is usually attached to bone

• Skeletal muscle is mostly controlled by the central nervous system

Skeletal Muscle

• Complete skeletal muscles consist of:

– Muscle fibers

– Connective tissues

– Blood vessels

– Nerves

Skeletal Muscle Structure

• Skeletal muscle tissue is made of elongated cells called muscle fibers– Each muscle fiber

contains many nuclei

– Each muscle fiber is crossed by light and dark stripes called striations

• Dense bundles of muscle fibers are called fascicles

Skeletal Muscle Structure

• Muscle fibers consist of bundles of threadlike structures called myofibrils

• Each myofibril is made up of two types of protein filaments– Thick filaments are made of the protein myosin– Thin filaments are made of the protein actin

• Actin is anchored at its end points to a structure called the Z line

• The region in between Z lines is called a sarcomere

Skeletal Muscle Structure OverviewMuscle

Fascicles

Muscle Fibers

Myofibrils

Filaments

Actin(Thin)

Myosin (Thick)

Smooth Muscles

• Smooth muscles are usually not under voluntary control

• A smooth muscle is spindle-shaped, has one nucleus, and is not striated

• Smooth muscles are found in the walls of hollow structures such as the stomach, blood vessels, and intestines

Smooth Muscles

• Most smooth muscle cells can function without nervous stimulation

– They are connected by gap junctions that allow electrical impulses to travel directly from one muscle cell to the neighboring muscle cell

Cardiac Muscles

• Cardiac Muscle is found only in the heart and control heart contractions

• Cardiac muscles shares features of skeletal and smooth muscle

• Like skeletal muscle:

– Has striations but is smaller

– Cells usually have one nucleus but may have two

Cardiac Muscles

• Like smooth muscle:

– Not under direct control of the central nervous system

– Cardiac cells are connected to their neighbors by gap junctions

Muscle Contraction

• The sarcomere is the functional unit of muscle contraction

• When a muscle contracts, actin and myosin interact to shorten the length of the sarcomere

• A muscle contracts when the thin filaments (actin) slide over the thick filaments (myosin)

• This process is powered by ATP

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Muscle Contraction

Control of Muscle Contraction• A skeletal muscle cell and a motor neuron connect at a point called the

neuromuscular junction

• Vesicles (pockets) in the axon terminals of the motor neuron release a neurotransmitter called acetylcholine

– This causes a nerve impulse

• The nerve impulse causes a release of calcium ions within the muscle fiber

– The calcium ions affect proteins that allow actin and myosin to interact

• The muscle cell remains contracted until the release of acetylcholine stops

Muscular Movement of Bones

• Skeletal muscles are generally attached to one end of a bone, stretch across a joint, and are attached to the end of another bone

• Muscles are attached to bone either directly or by a tough connective tissue called a tendon

Muscular Movement of Bones

• Origin – the point where muscle attaches to stationary bone

• Insertion – the point where muscle attaches to the moving bone

Muscular Movement of Bones

• Most skeletal muscles are arranged in opposing pairs– One muscle moves a limb in one

direction, and the other muscle moves it in the opposite direction

– Muscles move bones by pulling them

• Flexor – a muscle that bends a joint – example: biceps

• Extensor – a muscle that straightens a joint – example: triceps

• To produce movement, one muscle in a pair must relax while the opposite muscle contracts

Notes Review Questions

• Describe the three types of muscle tissue.– Skeletal

• Responsible for voluntary movement• Controlled by the central nervous system• Usually attached to bone• Have striations• Cells have many nuclei

– Smooth• Controls involuntary movement • No striations• Cells have one nucleus

– Cardiac• Controls contractions of the heart• Have striations• Cells usually have one nucleus

Notes Review Questions

• Describe the structure of muscles from the largest to the smallest structure.– Muscles are made of groups of fascicles. Fascicles are

made of bundles of muscle fibers. Muscle fibers are made of groups of myofibrils. Myofibrils are made of thick protein filaments called myosin and thin filaments called actin.

• Explain how muscles contract.– Muscles contract when the thin filaments (actin) in

the muscle fiber slide over the thick filaments (myosin).

Notes Review Questions

• Describe how muscles and bones interact to provide movement.– Skeletal muscles are attached to one end of a bone,

stretch across a joint, and are attached to the end of another bone. Muscles are either attached directly to the bone or by tendons. Most skeletal muscles are arranged in opposing pairs, where one muscle in a pair moves a limb in one direction and the other muscle in the pair moves the limb in the opposite direction. Muscles move bones by pulling them. To produce movement, one muscle in a pair must relax while the opposite muscle contracts. An example of this is the biceps and triceps.