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CHAPTER 7 Moving Muscles with Motor Neurons 131
neuron is activated and sends signals to the alpha motor neuron to contract the
muscle that was stretched. It also signals, through an inhibitory interneuron, for
the antagonist muscle to relax. This spinal circuit works out the muscle contrac-
tion force necessary to keep you standing when you catch a box or when holding
your glass when lling it.
Golgi tendon organs
Sensory receptors within the muscle tendons are called Golgi tendon organs. They
provide feedback about muscle tension, the force of muscle contraction. This pro-
tects muscles from overcontraction for sustained muscle tension and in ne motor
control.
Golgi tendon organs are located at the junction between the muscle and the ten-
don that attaches to a bone. Golgi tendon organs are innervated by Type Ib sensory
neurons. This produces an opposite response to the stretch reex. Activation of
the Ib axon stimulates an inhibitory interneuron, which inhibits the alpha motor
neuron, and leads to relaxation of the attached muscle. The antagonist muscle is
contracted by the Ib axon through an excitatory interneuron.
Joint receptors for position
Somatosensory neurons that innervate muscles, tendons, and joints are called
proprioceptors. They’re types of mechanoreceptors activated by physical movement
of the sensory detector region of the neuron. Receptors in joints and tissues
around joints and nearby skin send information to the spinal cord about joint
position and movement. Free nerve endings report painful, extreme joint posi-
tions. They let you know when they just don’t bend that way!
Withdrawing a limb in the exor reex
The exor withdrawal reex is another example of a spinal reex. Touching
something sharp or painful can elicit this reex (see Figure7-4). The neural cir-
cuit for this reex exists at the level of the spinal cord and normally happens
without your conscious control.
The purpose of the exor withdrawal reex is to protect you from damaging stim-
uli so you don’t get hurt. It begins by activation of pain receptors (nociceptors) that
indicate tissue damage. These sensory receptors project to the spinal cord and
activate exor muscles that withdraw the limb.
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132 PART 2 Controlling Muscles, Movement, and Survival
Figure7-4 shows the components of a exor withdrawal reex. Touching a sharp
point activates pain sensory neurons that project to the spinal cord. Pain receptor
axons form a monosynaptic contact with alpha motor neurons on the same side to
activate the exor muscle (biceps) that moves the hand away from the sharp
point.
The axons also make a disynaptic connection that relaxes the extensor (triceps)
muscle on the same side. Polysynaptic connections produce arm extension on the
opposite side. The exion withdrawal response is also present in the lumbar spi-
nal cord for leg withdrawal when you stub your toe or step on a painful stimulus.
This fast reex happens at the spinal level before the pain signal even reaches
your brain!
Fine, steady control may require partial activation of both muscles simultane-
ously. Fine motor control happens by descending control from the brain acting on
both alpha motor neurons and spinal interneurons. Other interneurons may proj-
ect to other spinal cord segments, adjusting muscle contractions in your legs to
balance you when you move your arms.
Overriding a reex
Although local spinal cord neural circuits carry out the details of cortical brain
commands, sometimes cortical, high-level commands override the local reex
circuits. You can hold on to a hot cup of coee despite your reex to let go. You can
suppress a cough for a short period, but not for too long if a strong irritant is
present.
FIGURE7-4:
A spinal reex
neural circuit.
© John Wiley & Sons, Inc.
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CHAPTER 7 Moving Muscles with Motor Neurons 133
Exercise and Aging
Exercise builds muscle mass and strength. Muscles get bigger and stronger in
response to damage to the muscle bers from the exercise. Muscle cells can
increase in size, but new cells aren’t formed when muscles grow. Instead,during
growth, structural proteins including actin and myosin are added to muscle bers,
which increase the cell diameter. The ratio of the numbers of fast- and slow-
twitch bers in a muscle doesn’t change, but the ratio of the mass of muscle ber
types can be altered by dierent types of exercise.
Lack of activity causes the opposite to happen: Loss of skeletal muscle mass is
called muscle atrophy. This occurs when astronauts spend months in the low grav-
ity of space. Without the force of gravity to make muscles work, astronauts have
been known to lose nearly one-quarter of their muscle mass in a few months.
Muscle atrophy can occur following disuse or denervation through injury or dis-
ease and leads to muscle weakness and disability. The mechanism of muscle atro-
phy involves an imbalance between protein synthesis and protein degradation
leading to a loss of muscle proteins.Depending on the duration and extent of
atrophy, muscles may be restored with activity.
These sections discuss how activity can aect muscles and release muscle hor-
mones and the dierences in muscles between males and females.
Use it or lose it: The eects of exercise
Exercise increases the mass of fast-twitch muscle. Endurance training, such as
long-distance running, can allow people to gain more of the slower Type IIa bers
versus the faster Type IIx bers.
For example, if a person is paralyzed, the ratio of fast-twitch Type II muscle bers
to slow-twitch Type I bers increases. Slow-twitch Type I bers may be lost
because of the lack of neural innervation that is necessary to maintain muscle.
Slowing down with age
Without some change in lifestyle, such as taking up weightlifting or strength
training, most people start losing muscle mass around age 25. By the age of 75,
individuals are reduced to about half the muscle mass they had before age 25.
During aging, changes also occur in the alpha motor neurons that control mus-
cles. These neurons re fewer action potentials per second in older versus younger
people. Declines in both the muscle and neural innervations result in weaker
muscles that act more slowly.
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134 PART 2 Controlling Muscles, Movement, and Survival
Comparing muscle mass in men
and women
Developing muscle mass is easier for males because of male hormones, the
androgens. Even in preschoolers, boys have signicantly more muscle mass than
girls of the same age.
The lower forces that females’ muscles generate may lead to increased joint insta-
bility in athletics, making injuries more likely. For example, damage to the ante-
rior cruciate ligament (ACL) in the knee is common in female athletes who play
soccer. Some dierences in skeletal composition also put females at a disadvan-
tage compared to males in athletics. Females’ wider pelvises put more stress on
their knees. Although androgen hormones help males add muscle mass, female
hormones such as estrogen have little eect on it. Females do have advantages in
some types of muscle endurance, however.
MYOKINES AND THE BRAIN
During exercise, muscle cells release protein factors into the blood called myokines.
Myokines in the circulation bind to and aect many dierent tissues, including the liver,
kidneys, fat tissue, and the brain. They’re proposed to increase metabolism and may
provide therapeutic eects of exercise in maintaining physical and mental health.
One myokine of interest to neurobiologists is the gene FNDC5 and its protein called
irisin. The levels of irisin increase following exercise. Though researchers haven’t identi-
ed the mechanism, irisin can cross the blood-brain barrier and has been proposed to
decrease neuroinammation and increase neuroplasticity. With these features, irisin is
an exciting candidate for the treatment of Alzheimer’s disease and depression.
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CHAPTER 8 Controlling Movement through Brain Pathways 135
Chapter8
Controlling Movement
through Brain Pathways
Y
our mother said it, and she was right: You’ve got to have goals. The fact is
that you do because animals, including humans, do things according to
goals. You may decide you want a better view of some squirrels outside. You
decide to walk to the window. You decide to move your right foot rst. (We didn’t
say they all needed to be big goals.) Actions start with a hierarchy of goals and
subgoals, translated into general and particular motor procedures. Where in your
brain does all this happen?
Goals and decisions arise from activity across many brain circuits. The important
brain areas for decisions about doing things include the frontal lobes, basal
ganglia, cerebellum, and thalamus, all of which we discuss in this chapter.
IN THIS CHAPTER
»
Moving through voluntary and
involuntary control
»
Integrating with the motor cortex
»
Being a boss like the prefrontal
cortex
»
Behaving dierent ways with the
basal ganglia
»
Coordinating with the cerebellum
»
Moving through the thalamus to the
neocortex
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136 PART 2 Controlling Muscles, Movement, and Survival
Overviewing Pathways for Controlling
Movement
Two systems evolved in mammals for brain control of movement, pyramidal
tracts and extrapyramidal tracts, which we examine in these sections. Both sys-
tems control skeletal muscle contraction. Of course, they need to work together,
and they do, to produce the variety of movements required for action and behavior
and also to stay balanced and be coordinated.
Focusing on pyramidal tracts
The pyramidal tracts control voluntary, conscious movement. The two main cate-
gories of pyramidal tracts are the following:
»
Corticospinal tract: The corticospinal tract travels from motor cortex through
the midbrain, pons, pyramids of the medulla, and beyond to synapse on
neurons in the ventral horn of the spinal cord to control voluntary movements
of the limbs and trunk.
»
Corticobulbar tract: The corticobulbar tract ends projects from motor cortex
to neurons in brainstem cranial nerve motor nuclei for the control of volun-
tary movements of the face, mouth, and tongue.
These tracts share common features. In both tracts, the majority of upper motor
neurons are located in the motor cortex. The upper motor neuron axons travel
together to form parts of the internal capsule and cerebral peduncles. They syn-
apse on and control both lower motor neurons and nearby interneurons.
Neurons in the motor cortex receive inputs from many brain areas, including the
prefrontal cortex, posterior parietal cortex, primary somatosensory cortex, and
thalamus. The thalamus relays motor information from the basal ganglia and
cerebellum.
Eyeing extrapyramidal tracts
The extrapyramidal tracts are primarily for involuntary, unconscious movement.
The upper motor neurons of the extrapyramidal system originate in the brainstem
and include the four extrapyramidal tracts. Three of the four tracts exert involun-
tary skeletal muscle control and are involved in posture, balance, muscle tone,
locomotion, and coordination of movements. The four tracts are as follows:
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CHAPTER 8 Controlling Movement through Brain Pathways 137
»
Reticulospinal: Origin is the reticular formation, a group of nuclei located in
the pons and medulla. The pontine and medullary reticulospinal tracts are
responsible primarily for postural control and locomotion.
»
Vestibulospinal: Origin is the vestibular nuclei in the pons and medulla.
Vestibular nuclei receive sensory input about head orientation from the
vestibular nerve. The vestibulospinal tract can aect muscle tone and
contributes to posture and maintaining the body’s and head’s balance.
»
Tectospinal: Origin is the superior colliculus (tectum) in the midbrain. The
tectospinal tract coordinates head and eye movements in response to visual
and auditory sensory stimuli.
»
Rubrospinal: Origin is the magnocellular red nucleus in midbrain. The
rubrospinal tract is involved in voluntary muscle movements and ne motor
control of the upper limbs.
The upper motor neurons of the extrapyramidal tracts receive their inputs from
sensory nuclei and cortex, cerebellum, basal ganglia, and motor cortex. We
describe the location and function of the extrapyramidal tracts in the spinal cord
in Chapter9.
Directing Voluntary Movement
with the Motor Cortex
The motor cortex is located at the posterior end of the frontal lobe in front of the
central sulcus and consists of three regions:
»
Primary motor cortex (M1): This region contains the majority of upper
motor neurons whose axons form the pyramidal tracts.
In M1, the upper motor neurons are large pyramidal neurons (a common type
of cortical neuron) called Betz cells and smaller non-Betz cells located in layer
V.These pyramidal neurons project their axons to the spinal cord forming the
corticospinal tract or to the brainstem forming the corticobulbar tract. M1
doesn’t contain granule cells that are typical in the input layer to most neocor-
tex (layer IV) and is called agranular.
»
Premotor cortex (PMC) and supplementary motor area (SMA): They’re
located anterior to M1. Together they’re often referred to as the secondary
motor cortex. Both the PMC and SMA provide major inputs to M1 and also
contain some upper motor neurons.
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138 PART 2 Controlling Muscles, Movement, and Survival
Many upper motor neurons synapse directly on alpha lower motor neurons (the
motor neurons that innervate muscle bers; refer to Chapter7) in direct control
pathways that allow ne, conscious control of muscles. Other upper motor neu-
rons synapse on spinal or brainstem interneurons. These interneurons provide
input to lower motor neurons and control their output via spinal and brainstem
circuits.
Mapping the movements: The homunculus
The motor map in M1 is a distorted map of the body’s muscles called a homunculus.
The motor map is similar to the somatosensory map on the other side of the
central sulcus in the parietal lobe. The corticobulbar tract originates in the
more lateral regions of M1 while the corticospinal tract originates in the medial
regions of M1.
The way this map functions suggests that it corresponds not to muscles as they’re
distributed, but to the movements mediated by those muscles. Single neurons in
M1 can activate several muscles related to one joint and coordinate muscle activity
associated with movement of the limb.
The distortion in the homunculus occurs because the size of the cortical represen-
tation of an area is proportional to the neness of the neural control, not the area
of the body. For example, the lips and face occupy a greater area of motor cortex
than the thighs. The control of nger movement involves more than one area.
Recent precision functional imaging studies show that M1 is more complex than
originally reported in the brain surgery studies. Imaging studies identied several
areas that don’t appear to control specic muscles. The areas were named the
somatocognitive action network and may function in goal-oriented action, arousal,
pain, and physiological control.
Population coding
Neurons in M1 that control the arms are most active when the arms are reaching
in a specic direction, but they respond for other directions near the preferred
direction. The average of thousands of these units produces a much tighter direc-
tionality than could be specied by a single neuron via what’s called a population
code. Individual neurons can be regarded as vectors with a direction where their
ring is at a maximum, and they re proportionately less for movements in other
directions. The average ring of all neurons then controls muscles to move a limb
in the direction we want to move.
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CHAPTER 8 Controlling Movement through Brain Pathways 139
Coordinating Muscle Groups
Almost every movement you make involves a sequence of at least dozens of dif-
ferent muscles. Try extending your arm out to the side. This movement is simple
to do, but it still requires many muscles: Your trunk counterbalances the move-
ment, and your legs muscles stabilize your body.
Although the spinal cord is pretty good at coordinating ordinary movement, evo-
lution hasn’t yet programmed your body to dance the foxtrot or type on the com-
puter. Voluntary movements, especially new ones, require learning organized
sequences of muscle commands, which involve the SMA and PMC that we discuss
here, as well as the basal ganglia and cerebellum, which we describe in the sec-
tions “Eyeing the Basal Ganglia and Its Nuclei” and “Correcting Errors: The Cer-
ebellum” later in this chapter.
Guiding with internal goals and the SMA
One way to look at the frontal lobe is that is has an axis of abstraction. The most
abstract representations of your goals exist in the prefrontal cortex at the most
anterior part of the frontal lobe. The neurons that control many of your muscles
are in M1, at the most posterior part of the frontal lobe. In between are the SMA
and PMC, as well as the frontal eye elds and Broca’s area (refer to the sections
“Seeing the frontal eye elds and superior colliculus” and “Producing speech:
Broca’s area” later in this chapter).
The SMA is located just anterior and medial to M1. The PMC is anterior and lateral
to M1. Both the SMA and PMC are posterior to the prefrontal cortex. Figure8-1
shows a diagram of the cerebral cortex (right) with M1 stippled.
FIGURE8-1:
SMA (internal)
and PMC
(external) motor
control circuits.
© John Wiley & Sons, Inc.
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140 PART 2 Controlling Muscles, Movement, and Survival
You can imagine that the prefrontal cortex has the most abstract representation of
some goal, like getting someone’s attention across the room. You know that you
could do this in many ways. You could call out, wave, walk over, send someone
else over, or send a text. Whatever option you choose must be translated into an
appropriate sequence of commands for the specic muscles.
The prefrontal cortex is where the abstract plans originate. If you’re going to do
something you know how to do, you activate the SMA.Both the prefrontal cortex
and SMA communicate with the basal ganglia to choose what you’re going to do.
The SMA projects to M1 to control exactly which muscles you’re going to use.
Additional functions attributed to the SMA are postural stabilization and coordi-
nation of both sides of the body.
Functional imaging studies show the SMA is activated even if you just imagine
doing something like moving your arms and legs, without actually doing it.
Externally monitored performance
and the PMC
When you’re learning to do something new, you need sensory feedback from the
posterior parietal cortex, which integrates somatosensory, visual, and auditory
inputs. You also need feedback from the cerebellum, which monitors errors and
learns timing sequences. When doing unfamiliar tasks, the input to M1 comes
more from the PMC, which receives information from the posterior parietal cortex
and cerebellum.
Besides projecting to M1, the PMC also projects directly to the spinal cord and has
reciprocal connections with the SMA. The PMC has a distinctive histological
appearance— it has a granular layer IV that’s distinct from the agranular M1.
Setting Goals with the Prefrontal Cortex
The frontal lobes are the center of goal pursuit. The SMA and PMC areas organize
and sequence groups of muscles to accomplish movement goals, such as running
while bringing your tennis racquet back in readiness to stop and hit a forehand.
Anterior to SMA and PMC are two regions called the frontal eye elds, which are
involved in visual attention, and Broca’s area, which is involved in language.
Everything anterior to these areas is called the prefrontal cortex. The frontal
lobes constitute about one-third of the neocortex, and more than half of the
frontal lobes consist of the prefrontal cortex. The following sections explore what
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