Chapter 9
Muscles and Muscle Tissue
293
9
Except for the brief period following muscle cell excitation,
calcium ion concentrations in the cytosol are kept almost unde-
tectably low. Tere is a reason for this: Sustained high calcium
activates apoptosis, leading to cell death.
Homeostatic Imbalance
9.2
Rigor mortis
(death rigor) illustrates the fact that cross bridge
detachment is A±P driven. Most muscles begin to stiffen 3 to
4 hours aFer death. Peak rigidity occurs at 12 hours and then
gradually dissipates over the next 48 to 60 hours. Dying cells
are unable to exclude calcium (which is in higher concentration
in the extracellular fluid), and the calcium influx into muscle
cells promotes formation of myosin cross bridges. Shortly af-
ter breathing stops, A±P synthesis ceases, but A±P continues to
be consumed and cross bridge detachment is impossible. Actin
and myosin become irreversibly cross-linked, producing the
stiffness of rigor mortis, which gradually disappears as muscle
proteins break down aFer death.
Check Your Understanding
6.
What are the three structural components of a
neuromuscular junction?
7.
What is the final trigger for contraction? What is the initial
trigger?
8.
What prevents the filaments from sliding back to their
original position each time a myosin cross bridge detaches
from actin?
9.
What would happen if a muscle fiber suddenly ran out of
ATP when sarcomeres had only partially contracted?
For answers, see Appendix H.
Contraction of a Skeletal Muscle
Define motor unit and muscle twitch, and describe the events
occurring during the three phases of a muscle twitch.
Explain how smooth, graded contractions of a skeletal
muscle are produced.
Differentiate between isometric and isotonic contractions.
In its relaxed state, a muscle is soF and unimpressive, not what
you would expect of a prime mover of the body. However,
within a few milliseconds, it can contract to become a hard elas-
tic structure with dynamic characteristics that intrigue not only
biologists but engineers and physicists as well.
Before we consider muscle contraction on the organ level,
let’s note a few principles of muscle mechanics.
Te principles governing contraction of a single muscle fiber
and of a skeletal muscle consisting of a large number of fibers
are pretty much the same.
Te force exerted by a contracting muscle on an object is called
muscle tension
. Te opposing force exerted on the muscle by
the weight of the object to be moved is called the
load
.
A contracting muscle does not always shorten and move the
load. If muscle tension develops but the load is not moved,
the contraction is called
isometric
(“same measure”)—think
of trying to liF a 2000-lb car. If the muscle tension devel-
oped overcomes the load and muscle shortening occurs, the
contraction is
isotonic
(“same tension”), as when you liF a
5-lb sack of sugar. We will describe isometric and isotonic
contractions in detail, but for now the important thing to
remember when reading the accompanying graphs is this:
Increasing muscle tension
is measured for isometric contrac-
tions, whereas the
amount of muscle shortening
(distance in
millimeters) is measured for isotonic contractions.
A skeletal muscle contracts with varying force and for differ-
ent periods of time in response to stimuli of varying frequen-
cies and intensities. ±o understand how this occurs, we must
look at the nerve-muscle functional unit called a
motor unit
.
Tis is our next topic.
The Motor Unit
Each muscle is served by at least one
motor nerve
, and each mo-
tor nerve contains axons (fibrous extensions) of up to hundreds
of motor neurons. As an axon enters a muscle, it branches into
a number of terminals, each of which forms a neuromuscular
junction with a single muscle fiber. A
motor unit
consists of one
motor neuron and all the muscle fibers it innervates, or supplies
(Figure 9.13)
. When a motor neuron fires (transmits an action
potential), all the muscle fibers it innervates contract.
Te number of muscle fibers per motor unit may be as high as
several hundred or as few as four. Muscles that exert fine control
(such as those controlling the fingers and eyes) have small motor
units. By contrast, large, weight-bearing muscles, whose movements
are less precise (such as the hip muscles), have large motor units. Te
muscle fibers in a single motor unit are not clustered together but
are spread throughout the muscle. As a result, stimulation of a single
motor unit causes a weak contraction of the
entire
muscle.
The Muscle Twitch
Muscle contraction is easily investigated in the laboratory using
an isolated muscle. Te muscle is attached to an apparatus that
produces a
myogram
, a recording of contractile activity. Te
line recording the activity is called a
tracing
.
A
muscle twitch
is a motor unit’s response to a single ac-
tion potential of its motor neuron. Te muscle fibers contract
quickly and then relax. Every twitch myogram has three distinct
phases
(Figure 9.14a)
.
1.
Latent period.
Te
latent period
is the first few millisec-
onds following stimulation when excitation-contraction
coupling is occurring. During this period, cross bridges
begin to cycle but muscle tension is not yet measurable
and the myogram does not show a response.
2.
Period of contraction.
During the period of contraction,
cross bridges are active, from the onset to the peak of ten-
sion development, and the myogram tracing rises to a
peak. Tis period lasts 10–100 ms. If the tension (pull)
becomes great enough to overcome the resistance of the
load, the muscle shortens.
3.
Period of relaxation.
Tis final phase, lasting 10–100 ms, is
initiated by reentry of Ca
2
1
into the SR. Because contrac-
tile force is declining, muscle tension decreases to zero and
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