Muscular System Human A & P

There are 3 types of muscle tissue: ◦ A. Skeletal

◦ B. Smooth

◦ C. Cardiac

◦ The essential function of a muscle is contraction, or shortening, and are responsible for essentially all of the body’s ability to move.

Skeletal Muscle Description

They attach to bones. They are huge, cigar-shaped,

multinucleated cells. The largest type (up to 30 cm). Striated (stripes in the fibers) Voluntary (conscious control—except

reflexes) Contract rapidly and with great force, but

also tire easily and require reset after short periods of activity.

Skeletal Muscle Structure

Thousands of skeletal muscle fibers are bundled together by connective tissue, which provides strength and structure to the muscle as a whole.

Each individual fiber is enclosed in an endomysium.

Several fibers are bundled together into a fascicle and covered with a coarse perimysium.

Several fasciles are bound together and covered by an epimysium, which covers the entire muscle.

Tendons and Aponeuroses

Skeletal muscle fibers blend or taper into tendons or aponeuroses. Aponeuroses attach indirectly to bones, cartilage, or connective tissue coverings.

Tendons anchor muscle to bone, but also provide durability and conserve space. They are tough collagenic fibers, so they can cross rough bony projections without tearing like muscles would.

Tendon pictures and diagrams

Apopneuroses

Smooth Muscle Description

Smooth muscle has no striations. It is involuntary (cannot consciously

control it). They are found mainly in the walls of

hollow visceral (internal) organs, such as the stomach, urinary bladder, and repertory passages.

They propel substances along a definite tract or pathway within the body, such as moving food through the digestive tract and emptying bowels and bladder.

Smooth Muscle Structure

Smooth muscle is spindle shaped with a single nucleus and are surrounded by a small endomysium.

They are arranged in layers, usually 2, one is circular, the other longitudinal. The 2 layers alternately contract and relax as they change the size and shape of the organ.

The movement is slow and sustained.

Smooth Muscle

Cardiac Muscle

Cardiac muscle is found only in the heart. It forms the bulk of the heart walls. It is striated and involuntary. The cardiac fibers are cushioned by small

amounts of soft connective tissue (endomysium) and arranged in spiral bundles (figure 8 shaped).

Cardiac fibers are branching cells joined by special junctions called intercalated discs.

Cardiac Muscle Fibers

How the heart muscle works

When the heart contracts, its internal chambers become smaller, forcing the blood into the large arteries leaving the heart. Cardiac muscle usually contracts at a fairly steady rate set by the heart’s “in-house” pacemaker.

Muscle Functions

1. Produce Movement—they enable us to respond quickly to changes in the external environment, as well as force fluid and other substances through internal channels.

2. Maintaining Posture-

3. Stabilizing Joints (muscle tendons)

4. Generating Heat—As ATP is used to power muscle contraction, nearly three fourths of its energy escapes as heat.

Organization of Skeletal Muscle

Cells = Fibers

Endomysium

Fascile

Perimysium

Epimysium

Connective tissue converges into either a tendon or aponeuroses.

Fascia

Muscles are organs—connective tissue (CT), blood vessels, nerves.

Muscles are arranged in different ways:

Attachment

Most are attached to two bones.

Origin—Fixed

Insertion—Moves

Skeletal Muscle Micro-anatomy

Skeletal Muscle cells have multiple nuclei. Many nuclei can be seen just below the plasma membrane (called a sarcolemma in muscle cells).

They have lots of mitochondria. The nuclei are pushed aside by long

ribbonlike organelles, the myofibrils, which nearly fill the cytoplasm. Myofibrils are bundles of fibers that fill the cell (1000+). They are composed of thick and thin microfilaments (thousands).

Myofibril

Muscle Fibers

Striations

Alternating bands, light (I) and dark (A) bands give the muscle cell its striped appearance.

The light I band has a midline interruption, a darker area called the Z line or disc.

The dark A band has a lighter central area called the H zone.

The M line in the center of the H zone contains protein rods that hold adjacent thick filaments together.

Striations

Alternating dark and light bands are due to the arrangement of thick and thin myofilaments.

A band, I band

Myofilaments

Thin filament—3 proteins ◦ Actin, troponin, tropomyosin ◦ Actin is anchored to Z disc. ◦ Do not extend, leaving the bare H zone.

Thick filament—1 protein ◦ Myosin ◦ Contain ATPase enzymes, which split ATP to

generate power for muscle contraction. ◦ Extend entire length of A band.

Z line to Z line is a sarcomere– the functional unit of muscle contraction.

Label the Sarcomere

The 5 golden rules of skeletal muscle activity

1. With a few exceptions, all skeletal muscles cross at least one joint.

2. Typically, the bulk of a skeletal muscle lies proximal to the joint crossed.

3. All skeletal muscles have at least two attachments: the origin and the insertion.

4. Skeletal muscles can only pull; they never push.

5. During contraction, a skeletal muscle insertion moves toward the origin.

Sliding Filament Theory of Muscle Contraction

In the sliding filament theory of muscle contraction, the actin fibers move inside myosin and cause the sarcomere to shorten and contract.

A motor unit is the motor neutron and the muscle fibers it controls

Sliding Filament Theory

When a muscle cell contracts, the thin filaments slide past the thick filaments, and the sarcomere shortens. This process comprised of several steps is called the Sliding Filament Theory. It is also called the Walk Along Theory or the Ratchet Theory.