NEURAL CONTROL OF JAW MUSCLES
Motor Units
The smooth, graded contraction of a healthy muscle while
performing a task makes it appear that the constituent muscle
fibers are all contracting at the same time under continuous
control. However, nothing could be further from the truth: The
individual fibers are being activated many times per second, in
nearly random order. The summed forces generated by the fibers
produce the smooth contraction of the entire muscle.
Several muscle fibers are innervated by a single motoneuron;
these fire in concert whenever an action potential is conducted
along the neuron. This structure, the motoneuron and its connected
muscle fibers, is known as the motor unit; this is diagrammed in
Figure 1.
Smooth, graded muscle contractions are the result of summed
forces from single motor units (Mus). MUs fire in random order,
and usually discharge at rates from about 10-30/sec. The muscle
fibers in one motor unit are interdigitated with fibers from other
units, and may be spread over a fairly large area of the whole
muscle. In human muscles a single unit may extend laterally as far
as 20 mm (Buchthal et al., 1957). Many different units usually
occupy a single area of the muscle.
It is possible to estimate the number of muscle fibers in a
single motor unit from histological sections: The total number of
fibers in the muscle is counted in cross sections, and the total
number of axons is counted in sections of the motor nerve. In this
way the innervation ratios (axons:muscle fibers) shown in Table I
were obtained.
Table I. Innervation ratios of different human motor units
__________________________________________________________
Muscle
Ratio
Reference
__________________________________________________________
gastrocnemius
1:1934
Feinstein et al, 1955
anterior tibial
1:562
"
1st lumbrical
1:108
"
tensor tympani
1:30
stapedius
1:27
platysma
1:25
lateral rectus
1:9
laryngeal muscles
1:2-3
Ruëdi, 1959
temporalis
1:936
Carlsöö, 1958
masseter
1:640
Wersäll, 1958
"
Feinstein et al., 1955
"
"
__________________________________________________________
From studies such as these it is generally agreed that
muscles which control fine movements of the ear, eye, or speech
apparatus have the lowest number of muscle fibers per motoneuron,
i.e. the highest innervation ratio. Larger muscles such as those
of the leg, which produce less detailed movements, have lower innervation ratios (more muscle fibers per motoneuron). The masseter
and temporalis have relatively small innervation ratios,
indicating a limited amount of detailed movements in normal
functioning.
Graded Muscle Contractions
Muscle movements are determined by the activity of the
associated -motoneuron pools, and the coodination of different
muscles is brought about within the nervous system. The events
leading from excitation of an -motoneuron to muscle contraction
are the following:
Presynaptic action potential

Release of acetylcholine

ACh binds to postsynaptic receptors

Opening of ionic channels in postsynaptic membrane

Depolarization and postsynaptic action potential

Release of Ca++ ions from T-tubules

Formation of actin-myosin cross bridges

Contraction
The unitary event produced by this process is the twitch, or
force which results from the excitation of a single muscle fiber.
This is shown in Figure 3, Part A. The bottom trace is the
presynaptic spike, and the top is the force generated by a single
muscle fiber. The delay between the action potential and the start
of the contraction is due to diffusion of the neurotransmitter
across the cleft and the excitation-contraction process in the
muscle fiber.
Part B shows the result of a second nerve action potential
arriving within a short time after the first. In this case the
forces from each spike can sum, to produce a larger force than
that of a single twitch. When the frequency of the motoneuron
spikes increases, the forces of individual fibers in a motor unit
are augmented in this way. In Part C, the steady force produced by
repetitive motoneuron activity is shown. This is known as a
tetanus, or tetanic contraction (not to be confused with the disease produced by Clostridium tetani, or tetany, the intermittent
muscle contractions resulting from hypocalcemia). At higher frequencies of motoneuron spikes, a fused tetanus is produced, where
no additional force is added by a further increase in frequency.
A second mechanism which is used to increase the force of
contraction of a muscle is recruitment of additional motor units.
As the overall level of "drive" to the -motoneuron pool is increased, the frequency of individual motoneurons increases and
additional motor units are activated. This is shown in Figure 4.
The top trace shows recordings obtained from thin wire electrodes
placed in the masseter muscle, and the bottom trace is the force
exerted on a transducer placed between the premolar teeth. As the
force is increased from zero and held at a roughly constant level,
a single motor unit begins to discharge. A further increase in
force level speeds up the first unit and causes a second, larger
one to start firing. A third increase in force speeds up both the
first and second units.
This observation contains two essential facts about the control of force in jaw muscles: First, the recruitment of additional
motor units occurs at higher levels of force, and second, units
with the smallest spikes are recruited before the ones with larger
spikes. This is known as the size principle, and was originally
described for the size of motoneuron spikes in cats (Henneman et
al., 1965); that is, the motoneurons with the smallest action
potentials were recruited first. Subsequently, it was found that
motor units with the smallest spikes were also recruited first in
peripheral muscles (Olson et al., 1968; Milner-Brown et al.,
1973b; Tanji and Kato, 1973). The size of the motor unit spikes is
related to the size of the component muscle fibers (Olson et al.,
1968; Goldberg and Derfler, 1977).
Pathology of Motor Units
Knowledge of the functions of single motor units makes possible an understanding of some of the major pathologies that affect movement in the periphery: For instance, anterior
poliomyelitis, which affected countless thousands of children
before a vaccine was developed, specifically attacked and killed
motoneurons in the anterior (ventral) spinal cord. Multiple
sclerosis has as one component a demyelination of the motor axons
in the ventral roots and somatic nerves. The condition of myasthenia gravis involves a blockage of the nicotinic acetylcholine
receptors on muscle fibers, leading to paralysis. And muscular
dystrophy includes muscle fiber degeneration and a direct
interference with the contractile ability of the muscle fibers.
Electromyography
The electromyogram (EMG) is a measure of the electrical activity
of a muscle. This signal is usually quite small (less than 1 mV),
so it must be recorded differentially, between two electrodes
placed close together, in or near the muscle. The signal in each
lead contains electrical noise, from lights or other equipment in
the room, but the difference between these two signals is mainly
due to muscle action potentials. The EMG may be recorded with flat
electrodes placed on the skin overlying a muscle, or single motor
unit signals may be recorded with fine-wire or needle
lectrodes
(Basmajian and De Luca, 1985).
In Figure 5, Part A, is shown a surface EMG recording from
the cheek overlying a human masseter muscle. The muscle is contracting against a bite-block placed between the teeth, at a level
about 40% of the maximum possible force. In this condition many
motor units are active, and the surface EMG is made up of the
interfering action currents of those muscle fibers which are
located near the recording electrodes. Part B shows the activity
of a single motor unit recorded by a surface electrode placed over
the masseter, when the only force exerted is to counteract the
force of gravity and keep the jaw closed. This unit is discharging
fairly regularly, which is typical, at a frequency of about 12
/sec. The signal in Part A is made up of one hundred or more
spikes such as those in Part B, occurring asynchronously.
The surface EMG is widely used to quantify the activity of a
muscle during the performance of some task. The average value of
the rectified signal, where the negative deflections are inverted
or eliminated, is known to increase with the force of contraction.
This type of EMG can be used to show the timing or coordination of
various muscles. The single-motor-unit EMG (Part B) is a direct
record of one motoneuron's activity in the central nervous system,
since one motor unit spike occurs for each motoneuron action
potential. This type of recording can be used to show the increase
in rate of a motor unit, and recruitment of additional motor units
with increased force, as shown in Figure 3.
In a clever use of the averaging technique, Milner-Brown et
al. (1973a) have used the single-unit EMG signal to trigger a
measurement of the total force being exerted by a muscle of the
hand. When many such records are averaged together, the extraneous
forces drop out, leaving only that due to the single motor unit.
It is possible to distinguish between slow-twitch and fast-twitch
fibers in this way, and to show that units which are recruited at
higher forces have higher twitch tensions and faster contractions,
in keeping with the size principle.
Muscle Spindles
Stretch-activated sensory receptors known as muscle spindles are
found throughout the body, especially in muscles associated with
postural activity (maintaining the skeleton upright against the
pull of gravity). These receptors contain at least two different
types of nerve endings: primary endings, which are sensitive to
dynamic and static stretch, and secondary endings which only
respond to static stretch.
In the jaw muscles, spindles are found only in the antigravity, or
closer muscles (masseter, temporal, medial pterygoid), and not in
the openers (Rowlerson, 1990).
Figure 8 shows the connections of the spindle afferents and
-motoneurons in the jaw closers. The intrafusal muscles (those
within the muscle spindles) are innervated by -efferent fibers
and act to stretch the spindle receptors. The extrafusal muscle
fibers (the contractile apparatus of the whole muscle) are innervated by the -motoneurons. Afferent activity resulting from
stretch of the spindle apparatus is conveyed in Ia fibers having
diameters from 13-20 m. This basic arrangement, known as the
monosynaptic reflex loop, also exists in the spinal segments of
the body. It is the most peripheral level of organization of motor
systems.
Afferent fibers in the jaw muscle spindles are unique in that
their cell bodies are located in the central nervous system, in
the trigeminal mesencephalic nucleus. (In the spinal system, they
are in the dorsal root ganglia.) Ia afferents end monosynaptically
on -motoneurons that excite the same muscle. Thus, stretch of a
spindle reflexly excites the muscle. This is accomplished with the
least possible delay in the nervous pathway; Ia afferents and motoneurons are the fastest nerve fibers in the body, conducting
at speeds up to 140 m/s.
The extra- and intrafusal muscle fibers are connected to the
spindles in a parallel arrangement. Thus, stretch of the entire
muscle increases the activity of the Ia afferent nerve fibers.
Contraction of the extrafusal muscle by -motoneuron excitation,
without concomitant excitation of -efferents, decreases the Ia
activity. And excitation of -efferents, either alone or in the
presence of -motoneuron firing, tends to increase the level of Ia
activity. The level of firing of -motoneurons is linked in turn
to the incoming activity of Ia afferent fibers.
This system embodies the principle of negative feedback for
control of muscle length: Stretching the muscle excites the
spindle, which makes the muscle contract to oppose the stretch.
The operation of this system in the leg muscles is presumably an
important mechanism by which we stand in an upright posture. In
the jaw muscles it also acts to oppose the force of gravity and
maintain the mandible in an elevated position. A comprehensive
view of the muscle spindle system in the jaw closers is shown in
Figure 9.
An input to the -motoneuron pool from the pyramidal motor
system excites the extrafusal muscle directly, causing shortening
and movement of the load. Inputs from extrapyramidal and other
sources via the -efferents have the effect of shortening the
intrafusal muscle fibers and stretching the muscle spindles. This
results in increased excitation of -motoneurons through the
monosynaptic reflex loop. Some possible functions of the  system
are to increase the muscle tone in preparation for rapid movement
or to compensate for changes in the weight being moved by a
muscle. If a harder contraction of the jaw-closers is needed to
bite through a resistant food bolus, for example, both the  and 
activity will be increased to provide the needed force. This constitutes a form of servo-assisted movement (Stein, 1980), where a
peripheral sensor detects the difference between the desired
movement and the actual movement and makes the needed corrections.
Golgi Tendon Organs
These are stretch-sensitive receptors located at muscletendon junctions in limb muscles, which inhibit the -motoneurons
of the muscles to which they are connected (Houk, 1979). Despite
some studies of sensory nerve fibers which resemble tendon organs
(Taylor and Davey, 1969; Smith, 1969), no convincing role for
these receptors has been found in the jaw muscle system (Beaudreau
and Jerge, 1968).
The roles of muscle spindles and  efferents in oral reflexes
will be discussed in the next chapter.