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Chapter 9
motor unit will produce more force than activating a slow-
oxidative motor unit.
The process of increasing the number of motor units that
are active in a muscle at any given time is called
recruitment.
It is achieved by activating excitatory synaptic inputs to more
motor neurons. The greater the number of active motor neu-
rons, the more motor units recruited, and the greater the mus-
cle tension.
Motor neuron size plays an important role in the recruit-
ment of motor units. The size of a motor neuron refers to the
diameter of the nerve cell body, which usually correlates with
the diameter of its axon. Given the same number of sodium ions
entering a cell at a single excitatory synapse in a large and in a
small motor neuron, the small neuron will undergo a greater
depolarization because these ions will be distributed over a
smaller membrane surface area. Accordingly, given the same
level of synaptic input, the smallest neurons will be recruited
fi rst—that is, they will begin to generate action potentials fi rst.
The larger neurons will be recruited only as the level of synaptic
Table 9–3
Characteristics of the Three Types of Skeletal Muscle Fibers
Slow-Oxidative Fibers
(Type I)
Fast-Oxidative-Glycolytic Fibers
(Type IIa)
Fast-Glycolytic Fibers
(Type IIb)*
Primary source of ATP
production
Oxidative phosphorylation
Oxidative phosphorylation
Glycolysis
Mitochondria
Many
Many
Few
Capillaries
Many
Many
Few
Myoglobin content
High (red muscle)
High (red muscle)
Low (white muscle)
Glycolytic enzyme activity
Low
Intermediate
High
Glycogen content
Low
Intermediate
High
Rate of fatigue
Slow
Intermediate
Fast
Myosin-ATPase activity
Low
High
High
Contraction velocity
Slow
Fast
Fast
Fiber diameter
Small
Intermediate
Large
Motor unit size
Small
Intermediate
Large
Size of motor neuron
innervating fi ber
Small
Intermediate
Large
*Type IIb fi bers are sometimes designated as type IIx in the human muscle physiology literature.
Motor unit 2: fast-oxidative-glycolytic fibers
Motor unit 3: fast-glycolytic fibers
Motor unit 1: slow-oxidative fibers
Motor unit
1 recruited
Motor unit
2 recruited
Motor unit
3 recruited
Time
0
Whole-muscle tension
(b)
(a)
Figure 9–26
(a) Diagram of a cross section through a muscle composed of three
types of motor units. (b) Tetanic muscle tension resulting from
the successive recruitment of the three types of motor units. Note
that motor unit 3, composed of fast-glycolytic fi bers, produces the
greatest rise in tension because it is composed of the largest-diameter
fi bers and contains the largest number of fi bers per motor unit.
Table 9–4
Factors Determining Muscle Tension
1. Tension developed by each fi ber
a. Action potential frequency (frequency-tension relation)
b. Fiber length (length-tension relation)
c. Fiber diameter
d. Fatigue
2. Number of active fi bers
a. Number of fi bers per motor unit
b. Number of active motor units
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