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CHAPTER 7 Moving Muscles with Motor Neurons 121
»
Cardiac muscle: This type produces heartbeats, contractions of the heart
muscle for pumping blood.
»
Skeletal muscle: This type is attached to bones through tendons to produce
movements of the body.
Body movements are controlled by the somatic motor system. Involuntary move-
ments include those for balance, posture, and coordination. Voluntary movements
are those you’re fully conscious of— and are typically controlled by higher-level
brain activity. Because muscles can only pull, and not push, many movements
require the coordination of an agonist and antagonistic (opposite) muscle pairs.
For example, most limb movements involve a exor muscle and an extensor
muscle pair.
A specialized type of striated muscle causes your heart to beat intrinsically with a
force like that of voluntary muscle. The heart beats on its own because of its own
pacemaker cells. The force and rate of your heartbeat isn’t usually under con-
scious control because the autonomic nervous system regulates it.
The following sections delve deeper into these two types of striated muscles.
Cardiac muscle
Cardiac muscle, striated muscle that is responsible for the involuntary contrac-
tions of the heart, is formed by cells called myocardiocytes or cardiomyocytes.
Cardiac muscle is very resistant to fatigue because it has a high density of
mitochondria. Like skeletal muscle, cardiac muscle relies on actin and myosin for
contraction, giving it a striated appearance from sarcomeres. Unlike skeletal
muscle, the heart doesn’t require innervation by motor neurons to contract.
The natural pacemaker of the heart is a cluster of cells called the sinoatrial node. It
generates electrical impulses that cause the heart’s chambers to contract, setting
the baseline heartbeat. The autonomic nervous system controls the heartbeat’s
force and rate, similar to how smooth muscle is controlled.
Skeletal muscle
Skeletal muscle consists of long, thin cells called muscle bers. Each muscle ber
typically extends the entire length of the muscle and is up to 30 centimeters long,
but less than 0.1 millimeter wide. Muscle bers are multinucleated cells that arise
from the fusion of immature myoblasts during development.
Skeletal muscle contraction is controlled by the activity of somatic alpha motor
neurons. Synaptic transmission occurs at the neuromuscular junction (NMJ)—
see Chapters5 and6. Acetylcholine is released by the motor neuron and binds to
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122 PART 2 Controlling Muscles, Movement, and Survival
receptors on the muscle cell, producing the end plate potential, which triggers a
muscle cell action potential. Each muscle action potential produces a contraction
of the muscle ber, called a twitch. Contraction is based on the movement of myo-
sin and actin myobrils. We describe the molecular events of muscle contraction
in the section “Pulling Your Weight: How Muscle Cells Contract” later in this
chapter.
The two major types of skeletal muscle bers are Type I (slow-twitch) bers and
Type II (fast-twitch) bers, both of which are present in most skeletal muscles,
but in varying ratios. The characteristics of both types are as follows:
»
Type I, slow-twitch bers: Muscles with a majority of slow-twitch bers are
dense with mitochondria and myoglobin, which give its characteristic dark red
color. Myoglobin is an oxygen-binding protein related to hemoglobin. Slow-
twitch bers can contract for long periods of time, but with less rate and force
than fast-twitch. They regenerate ATP in aerobic metabolism using oxygen in
mitochondria and don’t fatigue.
»
Type II, fast-twitch bers: Type II, fast-twitch bers contract rapidly and
powerfully. They’re the main ber type that contribute to the total force a
muscle can exert. The two subtypes include the following:
•
Type IIx bers: Known as the speed champions, these bers can contract
up to ten times faster than Type I bers. But type IIx bers fatigue rapidly
because they’re anaerobic (using glycolysis without oxygen to make ATP)
and are pale in color because they have less mitochondria and myoglobin.
•
Type IIa bers: In contrast, these bers are medium-speed bers that
contract at an intermediate speed, between that of Type I and Type IIx.
Type IIa use both anaerobic glycolysis and aerobic mitochondria to
produce ATP and are more resistant to fatigue.
Short intense bursts of activity using Type II bers result in delayed onset
muscle soreness. This was originally proposed to result from a buildup of
lactic acid. Recent research shows that pain results from small amounts of
damage to muscles and surrounding connective tissue, which increase muscle
growth through repair processes.
Type I and type IIa bers use aerobic metabolism, whereas type IIx bers use
mainly anaerobic metabolism. Performing endurance activities such as marathon
running depends mostly on aerobic slow-twitch bers. Short-duration power
movements, such as sprinting, use more anaerobic fast-twitch bers. This results
in an energy/oxygen debt that the body must pay back over time by consuming
foods that produce glucose (but a necessary and worthwhile one when escaping a
predator, for example).
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CHAPTER 7 Moving Muscles with Motor Neurons 123
Twitching fast and slow:
Muscle composition
The ratio of fast- and slow-twitch skeletal muscle bers is varied in muscles in
humans. People with a smaller percentage of fast-twitch bers may be better
marathon runners; those with a larger percentage, better sprinters. One eect of
weight-bearing exercise is to increase the percentage of fast-twitch ber mass in
the muscle, which increases strength.
Pulling Your Weight: How Muscle
Cells Contract
Muscles can only pull, not push. Antagonistic muscle pairs control your limbs—
the exor exes (bends) the limb and the extensor extends (straightens) the limb.
Figure7-1 shows the antagonistic pair that moves the lower leg. The quadriceps is
the extensor that straightens the leg and supports you in an upright position. The
hamstring is the exor that bends the leg when you walk.
Muscle cells are excitable cells like neurons because they produce action poten-
tials. Skeletal muscles contract in a sequence of events. It starts with the motor
neuron releasing the neurotransmitter acetylcholine and ends with actin and
myosin laments sliding over each other in a contraction. We outline the events
in the next section.
FIGURE7-1:
An antagonistic
exor-extensor
muscle pair.
© John Wiley & Sons, Inc.
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124 PART 2 Controlling Muscles, Movement, and Survival
Somatic motor neurons control skeletal muscle contraction. Skeletal muscles are
innervated by alpha and gamma motor neurons (refer to the section “Under-
standing motor neurons” later in this chapter) whose cell bodies are located in the
spinal cord or brainstem, but whose axons are located in the spinal or cranial
nerves. Synaptic transmission at vertebrate muscles involves acetylcholine
released by alpha motor neurons. Read on to nd out how it all works.
Patterning muscle contractions
Skeletal muscles can be activated by reexes, nonconscious motor pathways,
rhythmic motor patterns, or consciously controlled voluntary movement
sequences, which we discuss in the following list. For typical skeletal muscles,
integration and output from the alpha motor neuron is the nal common pathway
for control of these movements.
»
Reexes: A reex involves the activation of a sensory receptor, such as a pain
receptor, that res action potentials in the limb when you touch something
sharp (refer to the section “Withdrawing a limb in the exor reex” later in this
chapter). The axon of this receptor projects to the spinal cord and excites, in a
reex arc, an alpha (lower) motor neuron that causes the body to withdraw
the limb. Reexes are also involved in proprioception.
»
Unconsciously controlled muscle movement: Many involuntary move-
ments involve control by neurons located in the brainstem that form the
extrapyramidal tracts. These tracts are involved in upright posture and
balance, control of head and eye movements, coordination, and rhythmic
motor patterns.
»
Rhythmic motor patterns: Circuits in the spinal cord that are spread along
the spine control rhythmic motor patterns. These circuits control sequences,
such as the alternating movement of the two legs for walking and the
alternating movement of the arms working in anti-phase with the legs.
»
Consciously controlled muscle movement sequences: The output from
upper motor neurons located primarily in the motor cortex control voluntary
movements. The motor cortex receives inputs from other areas in the frontal
lobe, sensory areas, thalamus, basal ganglia, and cerebellum. Upper motor
neurons project to the brainstem or spinal cord. They synapse on interneurons
and alpha motor neurons, which integrate many inputs for the control of
output to muscles. Chapter8 focuses on brain control of movement.
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CHAPTER 7 Moving Muscles with Motor Neurons 125
Understanding motor neurons
Alpha motor neurons are located in the spinal cord and brainstem. They receive
direct inputs from upper motor neurons, from nearby interneurons, and some
from sensory neurons. In the spinal cord their cell bodies lie in the ventral horn
gray area. Their axons project to muscles and are contained in peripheral nerves.
Thus, motor neurons are considered part of both the central and peripheral ner-
vous systems. In the brainstem, alpha motor neurons are located in cranial nerve
motor nuclei. Alpha motor neurons innervate extrafusal muscle, the major part of
the muscle that produces contraction and force.
The other somatic motor neuron type that leaves the spinal cord or brainstem is
the gamma motor neuron, which innervate intrafusal muscle bers called muscle
spindles, small regions inside the extrafusal muscle. Spindles are also innervated
by sensory neurons and provide sensory feedback from muscle for somatic reexes.
Eyeing the motor unit
A single alpha motor neuron, together with the many muscle bers it innervates,
is called a motor unit. Numerous motor units are typically involved in producing
the contraction of a single muscle. A motor pool is the group of alpha and gamma
motor neurons in the spinal cord that innervate and control a single muscle.
A given alpha motor neuron innervates only muscle bers of the same type (I, IIa,
or IIx). The alpha motor neuron action potential causes all the muscle bers to
which it projects to contract simultaneously. The force of the muscle contraction
is controlled by the motor neuron rate of ring and number of motor units that
are activated. The systematic recruitment of motor units is called the size principle,
which we discuss in the section “Recruiting motor units” later in this chapter.
Muscle force also depends on the arrangement of muscle bers.
Typically, motor units, which innervate muscles that control large body parts—
like the legs— may synapse on and control up to 1,000 muscle bers, whereas
motor units that innervate small muscles may synapse on and control only ten
muscle bers. To achieve ne motor control, the body varies the activity of dier-
ent motor units to have a graded activation of muscle bers.
Polio is a viral infection that leads to loss of alpha motor neurons. Rehabilitation
from polio involves motor training so that the remaining alpha motor neurons can
innervate more muscle bers and restore some muscle force. As people age, they
lose some of the remaining alpha motor neurons. In post-polio syndrome, so few
alpha motor neurons are left that they can’t eectively innervate all the cells in
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126 PART 2 Controlling Muscles, Movement, and Survival
the muscle, and a person can regress to a pre-rehab state. Through vaccination,
polio isn’t a threat to more than 99 percent of the world’s population, though tiny
pockets of infection persist. New oral vaccines provide hope to eradicate polio.
Signaling at the neuromuscular junction
A single motor neuron axon branches and innervates many muscle cells of the
same type. Typically, one muscle cell receives only a single input from a motor
neuron axon terminal at the neuromuscular junction. During transmission, the
motor neuron releases acetylcholine into the synapse. Nicotinic acetylcholine
receptors are located at the muscle motor end plate. Binding of acetylcholine
opens the receptor channel and sodium ows into the muscle, producing the end
plate potential. This depolarization triggers an action potential in the muscle
membrane, by activation of voltage-gated sodium and potassium channels.
Coupling excitation contraction
The muscle cell action potential activates voltage-gated calcium channels in the
muscle (called L-type calcium channels). This causes the sarcoplasmic reticulum
within the muscle cell to release calcium. This increase in calcium causes actin
and myosin myolaments to slide over each other to shorten their overlap (see
Figure7-2) and the muscle cell contracts along its length. The process of muscle
contraction uses adenosine triphosphate (ATP) as the energy source.
Figure7-2 shows an alpha motor neuron axon branching into several terminals
on the left; on the right, you see a magnied view of the neuromuscular junction.
The curve shows the muscle end-plate potential.
FIGURE7-2:
The alpha motor
neuron terminal
at the end plate.
© John Wiley & Sons, Inc.
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CHAPTER 7 Moving Muscles with Motor Neurons 127
Sliding laments: Actin and myosin
The increased calcium in the muscle cell produces its eects by regulating a pro-
tein called troponin. In a relaxed muscle, calcium is low and troponin is bound to
the actin laments, preventing the actin from interacting with myosin. When the
sarcoplasmic reticulum releases calcium following the muscle action potential,
that calcium binds to troponin. This causes troponin to partially release from
actin, which allows the actin to bind with myosin.
Muscle cells are composed of multiple actin and myosin laments in series. Myo-
sin rst binds and then cleaves ATP, which induces the activated state of myosin.
After calcium is released and opens the actin-binding sites, activated myosin
binds to actin and function like a molecular ratchet. While bound to actin, the
myosin protein rotates, and as the myosin protein moves, it pulls the actin la-
ment along with it. The movement of multiple myosin proteins in its lament
drags the entire actin lament, shortening the sarcomere. This shortens the entire
muscle ber and together with simultaneously shortening bers, contracts the
muscle.
Figure7-3 shows how the muscle shortens, moving the Z lines closer together on
the left. On the right is a detail of how the myosin lament ratchets move the actin
brils along the myosin brils.
FIGURE7-3:
Myosin-actin
movement in the
sarcomere.
© John Wiley & Sons, Inc.
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128 PART 2 Controlling Muscles, Movement, and Survival
Controlling Muscle Contraction
Muscles can exert small or large forces. This enables a person to cradle an egg
with the same hand that can break a board in martial arts. Muscle cells are long
and thin and are organized lengthwise into muscle bers. The more muscle bers
contract, the shorter the muscle gets and the more force can is produced. Muscle
force is also dependent on how the bers are arranged in the muscle.
Muscle force is controlled in two ways:
»
By the alpha motor neuron ring rate: The number, type, and activity of the
alpha motors can precisely control muscle force to produce nely graded
contraction forces.
»
By the number and type of (slow or fast twitch) muscle cells activated:
This depends on the number and type of alpha motor neurons activated.
The following section explains these two mechanisms of muscle force control in
greater detail. We also describe the inputs to the alpha motor neurons and how
their axons are well insulated in nerves.
Modulating ring rate
One factor that determines muscle force is the frequency with which the muscle
bers are stimulated by their innervating alpha motor neurons, called the motor
unit ring rate. This varies from single action potentials that produce a single-
twitch contraction, to ring rates of hundreds of action potentials per second that
can produce maximal contraction, called a fused tetanic contraction. In general, the
ring of many alpha motor neurons increases gradually (despite the activation of
dierent muscle ber types— see the next section) as muscle force is increased.
This provides a nearly continuous ability to increase force as the muscle load
demands.
Recruiting motor neurons
In stimulating muscles, motor neurons within a motor pool are activated in order
from smallest to largest to increase the force being exerted from weak to strong.
This is called the size principle. Alpha motor neurons that innervate Type I
slow-twitch muscle bers are activated rst. They’re smaller diameter motor
neurons and their target muscle bers are smaller in mass. When greater forces
are needed, the motor neurons that innervate Type II fast-twitch muscles are
activated next. They’re larger diameter motor neurons and their target muscle
bers have greater mass.
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CHAPTER 7 Moving Muscles with Motor Neurons 129
Receiving inputs
Alpha (lower) motor neurons receive inputs from upper motor neurons in motor
cortex, interneurons in the spinal cord or brain stem, and for some, from sensory
neurons that innervate muscles, tendons, and joints. Integration from all these
inputs controls the ring of alpha motor neurons. Gamma motor neurons are part
of the motor pool and control intrafusal muscle bers. Intrafusal bers form
muscle spindles involved in sensory feedback to the spinal cord for well-
controlled muscle activity.
Moving through action potentials
Alpha motor neuron axons are heavily myelinated by both oligodendrocytes and
Schwann cells (refer to Chapter 1). These features increase the speed of action
potential conduction, shortening the time between a central command or sensory
reex input, and action. Oligodendrocytes myelinate the short segment of the
alpha motor neuron axon within the spinal cord or brainstem. As alpha motor
neuron axons leave the spinal cord from the ventral root or the brainstem, they’re
wrapped by Schwann cells along the length of the spinal or cranial nerve.
Reexing without Reecting
Reexes are one of the most fundamental circuits in the central nervous system.
In a reex, sensory receptor activation produces a motor response through a very
short monosynaptic pathway. It doesn’t require any higher brain input, is very
rapid, and produces a stereotypical response. Here we describe the circuitry and
function of several types of reexes located in the spinal cord and brainstem.
Spinal reexes: Proprioception
and protection
To move your limbs, you need to know something about where they are when you
start, and then be able to keep tabs on where they are as the movement pro-
gresses. Sensory receptors in muscles, joints, and tendons work in reex circuits
that allow your brain to know where its parts are and how they are moving, in a
process called proprioception. In addition to providing important perception of
space and movements, proprioception also plays an essential role in balance and
posture.
When you encounter something harmful in the environment, activation of sen-
sory receptors causes your body to move quickly to avoid damage via the fast
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130 PART 2 Controlling Muscles, Movement, and Survival
reex circuits in your spinal cord that also adjust for dierent loads and
conditions.
What your brain does for limb position is like setting a thermostat to keep your
home at a comfortable temperature. The thermostat sensor turns the furnace on
if the temperature goes below a set point or o if the temperature goes above that
point. This works regardless of the outside temperature (load) or changes in the
furnace’s capacity (such as the air lter being dirty and clogged).
Muscle spindles and the stretch reex
Muscle spindles are innervated by sensory neurons. Type Ia sensory neurons
detect and transmit information about the degree of contraction or stretch that a
muscle is undergoing. When a muscle spindle is stretched, type Ia axons send
signals to the spinal cord where they activate alpha motor neurons that then cause
the muscles to contract. The sensitivity of the reex is regulated by gamma motor
neurons, which contract or relax the intrafusal muscle bers within the spindle.
The kneejerk reex is a classic example of a stretch reex. When a doctor taps the
rubber hammer on your patellar tendon-ligament, it activates a reex associated
with the knees buckling while standing, and the lower leg swings outward, which
keeps you standing, if you weren’t sitting. This reex contributes to balance, pos-
ture, and rhythmic pattern movement-walking.
Tapping on and deforming the patella tendon-ligament activates the Ia sensory
neurons in the quadriceps muscle spindle. The Ia sensory neuron sends action
potentials along its axon to the spinal cord where it branches. It forms one syn-
apse on the alpha motor neuron controlling the quadriceps, an extensor muscle,
causing it to contract and your lower leg swings forward. This is a monosynaptic
reex. Another synapse from the 1a sensory axon, using an inhibitory spinal inter-
neuron, leads to relaxation of the hamstring muscle.
Staying put with the loading reex
Although you often think of reexes as producing movement in response to a
stimulus, some reexes exist to keep you in the same place. If you were unloading
a truck and someone tossed you a box and you caught it, the new weight and load
on your legs would suddenly change. Another example: Pouring water into a glass
you’re holding increases the load on your arm, but you don’t spill the water.
These responses occur thanks to the loading reex, whose circuitry is similar to the
stretch reex. When a load occurs, the muscle spindle is stretched, the Ia sensory
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