The nernst potential is the potential inside the


The Nernst potential is the potential inside the
membrane.

The sign of the potential is positive (+) if the
ion diffusing from inside to outside is a
negative ion, and it is negative (-) if the ion is
positive.

Troponin and Its Role in Muscle
Contraction

Attached intermittently along the sides of the
tropomyosin molecules are still other protein
molecules called troponin.

These are actually complexes of three loosely
bound

protein subunits, each of which plays a
specific role in

controlling muscle contraction.


One of the subunits

(troponin I) has a strong affinity for actin,

Another (troponin T) for tropomyosin, and

a third (troponin C) for calcium ions.

This complex is believed to attach the
tropomyosin to the actin.The strong affinity of
the troponin for calcium ions is believed to
initiate the

contraction process.


Therefore, it is believed that the active sites
on

the normal actin filament of the relaxed
muscle are

inhibited or physically covered by the
troponin-tropomyosin complex.

Consequently, the sites cannot attach to the
heads of the myosin filaments to cause
contraction.

Before contraction can take place, the
inhibitory effect of the troponin-tropomyosin
complex must itself be inhibited.


This brings us to the role of the calcium ions.
In

the presence of large amounts of calcium
ions, the

inhibitory effect of the troponin-tropomyosin
on

the actin filaments is itself inhibited.


Smooth muscle is composed of far smaller
fibers—

usually 1 to 5 micrometers in diameter and
only 20

to 500 micrometers in length.

In contrast, skeletal muscle fibers are as much
as 30 times greater in diameter and hundreds
of times

as long.


Many of the same principles of contraction
apply to smooth muscle as to skeletal muscle.

Most important, essentially the same
attractive forces

between myosin and actin filaments cause
contraction in smooth muscle as in skeletal
muscle, but the internal physical arrangement
of smooth muscle fibers

is very different.


Smooth muscle can generally be

divided into two major types, which are shown
in Figure 8–1:

- multi-unit smooth muscle

- and unitary (or single-unit) smooth muscle.

Multi-Unit Smooth Muscle.

This type of smooth muscle is composed of discrete,

separate smooth muscle fibers. Each fiber operates
independently of the others and often is
innervated by a single nerve ending, as occurs for
skeletal muscle

fibers.

Further, the outer surfaces of these fibers , like those
of skeletal muscle fibers, are covered by a thin layer
of basement membrane–like substance, a mixture
of fine collagen and glycoprotein that helps
insulate the separate fibers from one another.


The most important characteristic of multi-
unit smooth muscle fibers is that each fiber
can contract independently of the others, and
their control is exerted mainly by nerve
signals.