ch 11 muscular tissue pt 1

All three types of muscle tissue, skeletal, smooth and cardiac, have these four

characteristics in common: they are excitable meaning they respond to stimuli, they

can contract, or shorten, they are extensible or stretchy and they are elastic meaning

that they can return to their original size or shape after stretching. These functions

are because of the structure of the myofiber, or muscle cell.

Myocytes

are much longer than a typical cell, as much as 30 cm long. But they’re very

narrow so you still can’t see it without a microscope. The image here is a microscopic

view of skeletal muscle tissue; notice how it appears to

be striped or striated, that’s

because of protein strands inside the cells are arranged in a regular pattern.

Myocytes are filled with these protein strands and that pushes the nucleus up against

the sides of the cell membrane.

Where do muscle cells come from? Remember the three layers of embryonic tissues

that form all specialized tissues in the body? The endoderm, mesoderm and

ectoderm. Well, the middle layer, the mesoderm forms muscle tissue. Some cells of

the mesoderm become myoblasts which are precursors to myocytes. Several

myoblasts fuse together to form a single muscle cell which is why myocytes have

multiple nuclei instead of just one nucleus like most other cells. Some myoblasts

remain as a stock of unspecialized cells just outside the cell membrane of a myocyte,

these are called satellite cells. They help the muscle cell grow and regenerate.

Here is a diagram of a myocyte. The plasma membrane is called the sarcolemma,

sacro is from the Latin word for flesh. The cytoplasm of a myocyte is called the

sarcoplasm. The sarcoplasm stores fuel for the muscle in the form of glycogen, which

is made fro glucose. It also stores another compound called myoglobin which is

similar to hemoglobin in red blood cells. Myoglobin’s job is to store oxygen for the

myocyte.

Notice that there are multiple nuclei in this myocyte and that they are pushed up

against that sarcolemma. That’s because most of the myocyte is filled with long cord

of protein called myofibrils, these are the

structures that contract, or shorten a

muscle.

You can also see the m

itochondria

packed

between the myofibrils.

If we zoom in on the myofibrils we see that they are covered by a network of tubes

called the sarcoplasmic reticulum, colored in blue in this diagram, that is essentially

smooth endoplasmic reticulum of muscle cells. The ends of the SR are swollen sacs

called terminal cisternae; terminal meaning end and cistern meaning something for

storage. So what do they store? Calcium. You’ll see why in a little bit. The next

structure I want you to look at are the transverse tubules shown in yellow between

the terminal cistern of one SR and the next. These T-tubules run vertically through

the myocyte from one side of the sarcolemma to the next. You need t know the

names of these structures in order to understand the events that happen during

muscle contraction. But before we get to that we need to examine the myofibrils.

Myofibrils is the name given to bundles of protein strands called myofilaments. They

come in two flavors: myosin which is thick and actin which is thin. To recap: bundles

of myofilaments make up myofibrils, bundles of myofibrils make up myofibers and

bundles of myofibers make up fascicles. Bundles of fascicles make up muscle organs.

A myosin molecule is shaped a bit like a golf club; myosin myofilaments are thick

because they are made of hundreds of the golf-club looking molecules bundled

together with all the tails aligned and the heads sticking outwards spiralling along the

length of the bundle.

Back to myofilaments. The action of the thin actin filament is regulated by two other

proteins, tropomyosin, shown as the white strands and troponin, the yellow bulbs.

Together they act like a switch to turn actin on and off; I like to think of troponin as a

door knob and tropomyosin as a door so access to actin can be opened or closed.

Back to myofilaments. The action of the thin actin filament is regulated by two other

proteins, tropomyosin, shown as the white strands and troponin, the yellow bulbs.

Together they act like a switch to turn actin on and off; I like to think of troponin as a

door knob and tropomyosin as a door so access to actin can be opened or closed.

Here we see a sarcomere, that is a contractile unit of a muscle cell. The image above

is a diagrammatic representation of a sarcomere and below is a micrograph. The

boundaries of a sarcomere are zig-zag lines called the z-disc or z line. Actin molecules

are anchored to the Z-disc by another protein called titin. Titin acts like a spring that

enables the muscle to recoil to its original position after contraction.. Myosin

filaments are anchored to the M-line in the idle of the sarcomere.

Actin and myosin are interspaced with each other; regions where they overlap appear

dark and are called A bands. Regions of actin filament only are light because there

filaments are thin , these are called

I bands.

You’ll notice that the middle region

where there are only myosin filaments, is not as dark as where myosin and actin

overlap, but not as light as where there are actin filaments only. This is called the H

band

. In the H

band myosin have no heads either and is called the bare zone

Recall that the sarcoplasmic reticulum stores calcium. When tat calcium is released

into the sarcoplasm it binds to troponin causing troponin to undergo a change in

shape. That change in shape in turn caused tropomyosin to move unblocking the

binding sites on actin. The myosin heads then bind to actin and pull on them so that

they slide over the myosin filaments. Click on the link to watch an animation of how

this happens