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CHAPTER 8 Controlling Movement through Brain Pathways 151
GABAergic (use the inhibitory neurotransmitter GABA), and they work to modulate
or gate functions on the nearby areas. They’re almost exclusively interneurons
that don’t project out of the thalamus.
The thalamic reticular areas receive aerent (inward) input from the brainstem
reticular formation and cortex and project to various thalamic nuclei within the
thalamus to regulate the ow of information through these to the cortex. The cor-
tical input tends to select ascending inputs according to what is most salient or
prominent within a sense, whereas the brainstem reticular input tends to switch
attention between senses according to threats and immediate contingencies.
Seeing the frontal eye elds
and superior colliculus
Anterior to the SMA is a cortical area called the frontal eye elds (FEFs). The FEFs
are involved in visual attention and initiating eye movements (voluntary
saccades), which are the rapid movements your eyes make when going from one
point to another, such as when you’re reading.
The FEF receive visual inputs in a pathway that starts with the retinal projection
to the superior colliculus, which projects to the medial dorsal nucleus of the thal-
amus, and then to the FEF cortex. The FEF contains a map that represents eye
movement saccade targets. When neurons in the FEF are electrically stimulated,
the eyes move to and look at areas of visual space that correspond to the stimula-
tion point.
The superior colliculus is a midbrain nucleus that receives projections from nearly
all retinal ganglion cells, except parvocellular ganglion cells. In nonmammalian
vertebrates, this structure is the main visual processing center, but in mammals it
appears to mostly code for eye movements. Electrical stimulation of the superior
colliculus, like the FEF, produces saccadic eye movements according to a well-
dened map across its surface.
Knowing, or Not Knowing,
Who’s In Control
This chapter explains that involuntary motor activity is controlled by the extrapy-
ramidal tracts that originate in the brainstem. These are, in turn, modulated by
the basal ganglia, cerebellum, and sensory and motor areas of the cerebral cortex.
These pathways are involved in reexes, locomotion, posture, and balance.
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152 PART 2 Controlling Muscles, Movement, and Survival
The chapter also describes how voluntary motor activity is controlled by neurons
in M1, which directly and indirectly activate alpha motor neurons. Neurons in M1
are modulated by the SMA or PMC.Both of these motor areas receive inputs from
the prefrontal cortex, sensory cortices and the thalamus, which relays informa-
tion from the basal ganglia and cerebellum about selection, sequence and correc-
tion of actions.
So, who’s in charge? Well, it depends on the circumstances. If you trip while walk-
ing, the extrapyramidal system may control your arms, legs, and trunk. If you’re
trying to hit a high backhand tennis shot that you rarely practice, you may depend
on your PMC.Turning the knob and opening the door to your house— which you
do every day— probably mostly activates your SMA.
But you may feel that you perform some movements entirely voluntarily, decided
by the real you somewhere in the brain, like deciding to get up and get some ice
cream from the freezer. Certainly you’re aware of this thought stirring you to
action. But maybe the thought existed before you were aware of it? Perhaps you
were slightly hungry, but you hadn’t clued in yet. Perhaps a sound you heard or
something you saw had an association with eating ice cream that you had forgot-
ten. Your blood sugar may be low, or some unconscious brain activity may have
re-created the fact that you bought a new ice cream avor yesterday but hadn’t
remembered until now.
Cold-blooded vertebrates that have basal ganglia but little or no neocortex have a
limited repertoire of complex behaviors that generally don’t depend much on
learning. Mammals in general, primates in particular, and humans specically
have a large number of complex behaviors, and they learn new ones throughout
their lifetimes. Nevertheless, the selection among these behaviors remains
embedded in old neural circuits, including the basal ganglia, but now within com-
plex memories and hierarchical goal systems.
The control system is hierarchical and distributed, but only one motor program
can control the muscles at a time. You can’t hit a forehand and a backhand at the
same time— you have to choose. You’re aware of the choice, certainly, but not
everything that may have gone into it.
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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 153
Chapter9
Focusing On the Spinal
Cord and Brain Stem
A
lthough the forebrain usually gets credit for being the smart one, the
spinal cord and brainstem are pretty intelligent too. The forebrain is
the architect, whereas the spine and brainstem are the contractors. The
forebrain, using its vision, memory, and goals, decides where you want to go or
what you want to say, like an architect decides the design of the building. But the
spinal cord and brainstem, the contractors, execute the plans, considering a ton of
variables, while also letting the forebrain know what’s going on outside and keep-
ing things going inside.
This chapter focuses on the two of the oldest parts of the central nervous system
(CNS) that evolved in vertebrates, the spinal cord and brain stem. They share
important functions:
»
Both contain pathways for signals traveling between the brain and the rest
of the body.
»
Both contain lower motor neurons that control voluntary movements and
autonomic functions.
»
Both relay sensory information from the body to the thalamus.
»
Both function as complex computers to control motor output and generate
important reexes.
IN THIS CHAPTER
»
Looking at spinal segments and
nerves
»
Motoring along with neural circuits
and reexes
»
Getting a sense of the peripheral
nervous system
»
Functioning with the brain stem
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154 PART 2 Controlling Muscles, Movement, and Survival
»
The spinal cord also produces the central pattern generator to control
locomotion, while the brainstem generates survival and homeostatic
functions.
Surveying the Spinal Cord
The brain and spinal cord comprise the CNS.The spinal cord begins at the bottom
of the brainstem, below the medulla. It begins with cervical segments and then
proceeds to thoracic, lumbar, sacral, and coccygeal segments. The next sections
focus on the spinal cord.
Protecting the spinal cord
The spinal cord is embedded in and protected by the vertebral column, composed
of vertebrae— a series of small bones that form the spine, shown in Figure9-1.
The backbone is a set of spinal vertebrae that run from the neck to the tail bone, a
remnant of humans’ pre-ape lineage.
FIGURE9-1:
The spinal cord
in relation to
the vertebrae
of the back.
© John Wiley & Sons, Inc.
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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 155
The spinal cord lies between the main vertebra bodies and the bones of the back
that you can feel through your skin. Between the vertebra bodies are spinal disks,
which are pads made of connective tissue that cushion and separate the vertebra
bodies. A herniated disk is a common back injury.
The spinal cord is also protected by the same three layers that cover the brain,
called the meninges:
»
Dura mater: The outer membrane containing connective tissue that protects
the spinal cord
»
Arachnoid mater: The layer under the dura mater consisting of web-like
membrane extensions that surround the subarachnoid space lled with
cerebrospinal uid (CSF)
»
Pia mater: The inner layer, a thin membrane that covers the entire
spinal cord
The meninges surround the groups of neurons, glia, and tracts that form the
spinal cord along its length.
Examining the gray and white matter
Looking at the spinal cord reveals the central gray matter area that contains neu-
rons, surrounded by white matter composed of axonal tracts. The gray matter is
shaped like a buttery and consists of neuronal cell bodies, dendrites, and syn-
apses. The white matter axon tracts transmit motor information down the spinal
cord to the segments and sensory information up to the brain.
The spinal cord gray matter is separated into functional regions called horns:
»
Dorsal horn: This horn contains sensory relay neurons. Both somatic and
visceral primary sensory neurons send sensory information to the
dorsal horn.
»
Ventral horn: This horn contains somatic lower motor neurons.
»
Lateral horn: This horn contains autonomic lower motor neurons.
All of the gray matter regions also contain spinal interneurons. Figure9-2 shows
a schematic section of the spinal cord.
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156 PART 2 Controlling Muscles, Movement, and Survival
In the center of the gray area is the central canal. It’s continuous with the fourth
ventricle in the medulla and contains cerebrospinal uid (CSF). CSF has several
functions (refer to Chapter 1) including nourishing and cushioning the spinal
cord. Because of the continuity between the spinal cord and brain CSF, anesthetics
can be injected between L3 and L5 into the spinal cord and will reach the brain.
Inputting and outputting
Inputs into and outputs from the spinal cord are anatomically separated. Both
somatic and visceral sensory axon inputs always enter into the spinal cord through
the dorsal side (toward the back of the human body) at the dorsal root. Both
somatic and autonomic motor axon outputs from the spinal cord always exit from
the ventral side (toward the front of the human body) via the ventral root.
Coming in the back
Just outside the spinal cord on the dorsal side are the dorsal root ganglia (DRG). The
DRG contain the cell bodies of primary sensory neurons whose axon branches
project to the periphery and into the dorsal horn of the spinal cord. DRG contain
both primary somatosensory neurons that innervate skin, muscles and joints and
primary visceral sensory neurons that innervate internal organs.
As Figure9-2 shows, a single axon emerges from the sensory neuron body in the
DRG and then bifurcates (splits itself into two) within the ganglion. One axon
branch extends to the periphery to its target. DRG neurons don’t have dendrites or
form synapses at their sensory endings. Instead, sensory input (such as mechani-
cal stimulation) produces action potentials in the axon terminal at the target
(such as the skin).
FIGURE9-2:
Gray and white
matter inside the
spinal cord.
© John Wiley & Sons, Inc.
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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 157
These action potentials travel along the axon toward the DRG and then along the
other axon branch into the spinal cord dorsal horn. In the spinal cord, the sensory
axons form conventional synapses, releasing the neurotransmitter glutamate
onto sensory relay neurons and spinal interneurons.
DRG neurons can participate in monosynaptic and polysynaptic reexes (refer to
Chapter7). They also send both somatic and visceral sensory information about
the body to the brain along ascending white matter tracts in the spinal cord.
Going out the front
The output axons from motor neurons form the ventral root that exits from the
ventral side of the spinal cord. Motor neurons are located in two regions of the
spinal gray matter. Somatic motor output is from alpha and gamma lower motor
neurons whose cell bodies are located in the ventral gray area. Their axons project
to the motor end plate regions and spindles in skeletal muscles.
The cell bodies of autonomic motor neurons, called preganglionic neurons, are
located in the lateral horn. They extend axons to postganglionic neurons in auto-
nomic ganglia outside the spinal cord. Postganglionic neurons project axons to
internal organs and tissues to mediate autonomic responses. All sympathetic and a
few parasympathetic preganglionic neurons are located in the spinal cord (refer to
Chapter10). The majority of preganglionic parasympathetic neurons are located in
the brainstem (refer to the section “Noting essential nuclei” later in this chapter).
Joining together to form spinal nerves
Dorsal root and ventral root axons enter or leave the spinal cord in each segment
through several smaller bundles called rootlets that join together on the distal side
of the dorsal root ganglion. Spinal nerves contain both somatic sensory and
somatic motor axons. In addition, in some nerves, autonomic motor and visceral
sensory axons are present as well. Sensory axons traveling toward the spinal cord
are called aerents while the motor axons transmitting signals away from the spi-
nal cord are called eerents.
The interior of the spinal cord is part of the CNS, but the axons projecting outside
the spinal cord are part of the peripheral nervous system. All the spinal nerves
distal to the dorsal root ganglia consist of both sensory and motor axons, so
they’re called mixed nerves.
The human spinal cord consists of 31 segments with each giving rise to a spinal
nerve. The spinal nerves exit directly from the spinal cord in the upper segments
of the vertebral column. In the lower vertebrae, the nerves typically travel a seg-
ment or two down the column before exiting. Below the spinal cord are additional
fused vertebrae called spinal coccygeal segments, which aren’t associated with spi-
nal nerves.
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Figure9-3 shows where the segments are located along the spine as well as their
functions. Many nerves exiting the spinal cord can form a plexus, which is a net-
work of nerves. The two major nerve plexuses are thebrachial plexus, which
emerges from the cervical spine, and the lumbosacral plexus, which emerges from
the lumbosacral spine.
FIGURE9-3:
Spinal cord
segments and
some of their
functions.
© John Wiley & Sons, Inc.
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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 159
Injury to the spinal cord can result in partial paralysis or severe disability. The
extent of disability depends on the extent of the injury and the location of the seg-
ment of the spinal cord where damage occurs.
Noting the spinal nerves
The top of the spinal cord starts where the medulla ends. The rst seven vertebral
segments are called cervical. From this region, eight cervical cord segments are
present, and eight pairs of cervical nerves (C1 to C8) exit to innervate the periphery.
The nerves leaving the various spinal segments are, in many cases, similar to
nerves leaving the brainstem, where they’re called cranial nerves. We discuss the
cranial nerves in the section “Moving and Surviving with the Brainstem” later in
this chapter.
The spinal cord has eight cervical segments with eight spinal nerves, but the spine
has only seven cervical vertebrae segments. Nerves C1 to C7 exit the spinal cord
from just above their corresponding vertebra. However, starting at the C7 to T1
(for thoracic 1) junction, the spinal nerve roots exit below their corresponding
vertebra. The spinal nerve emanating from between C7 and T1 is called C8. Here is
the lowdown on the dierent spinal nerves and their functions.
Cervical nerves
Cervical nerves C1 and C2 control head position via neck muscles. Nerves C3 and
C4 control the diaphragm for breathing. Nerve C5 controls upper arm muscles,
such as the biceps and deltoids. Nerve C6 controls the wrist and provides a partial
innervation of the biceps. Nerve C7 controls the triceps. Nerve C8 controls the
hands. Each cervical nerve also receives sensory information from the same area
it controls.
Thoracic nerves
For the thoracic nerves, T1 projects into the brachial plexus that innervates the
arms and upper back. T2 to T6 project to the intercostal muscles and receive sen-
sory information from the skin of the chest and upper back. T7 to T11 innervate
the skin and intercostal muscles of the abdomen and lower back. T12 projects with
the iliohypogastric nerve (a branch of L1) to innervate the gluteus muscles and skin
of the buttocks. All the thoracic segments contain sympathetic preganglionic neu-
rons that innervate the internal organs.
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Lumbar nerves
From the ve vertebrae in the lower back emerge ve pairs of lumbar spinal
nerves. These nerves innervate the lower abdomen, back, and buttocks. L1 through
L4 innervate the thigh muscles, such as the quadratus lumborum and iliopsoas for
exion. L2 through L5 mediate thigh adduction. Segments L1 through L3 give rise
to preganglionic sympathetic neurons.
Sacral and coccygeal nerves
The sacral nerves (S1 to S5) and coccygeal nerve emerge from the spinal cord
through openings called foramen in the fused sacral vertebrae. The nerves join
either the sacral or lumbosacral plexus. Sacral and coccygeal nerve functions
include innervation of the skin and muscles of the thighs, buttocks, legs, and feet.
S2 through S4 contain preganglionic parasympathetic neurons that innervate the
colon, rectum, bladder, and genital organs.
Moving around: Input and eectors
The spinal cord controls the trunk and limbs to act in unison with motor com-
mands from the brain arriving via the descending spinal cord tracts. Axons in
these tracts innervate alpha motor neurons and spinal interneurons to control the
motor output. Sensory input to the cord is provided by primary sensory neurons
and received by sensory relay neurons and spinal interneurons. Sensory input is
relayed to the brain along ascending tracts and also in spinal circuits. The spinal
cord neural circuitry integrates motor and sensory input to control motor output
through reex arcs. In addition, sensory and motor circuits coordinate multiple
segments of the spine.
Here are the inputs to and circuitry within the spinal cord:
»
Commands from the motor cortex: Upper motor neurons in the motor cortex
project axons to spinal cord alpha motor neurons and spinal interneurons via
the corticospinal tracts. They control voluntary limb and trunk movements.
»
Commands from the brainstem: Upper motor neurons in brainstem nuclei
project to alpha motor neurons and spinal interneurons via extrapyramidal
tracts: vestibulospinal, reticulospinal, and tectospinal tracts. They provide
involuntary muscle control for posture, balance, and head movements. The
fourth is the rubrospinal tract that provides minor voluntary control of upper
limb movement.
»
Monosynaptic and polysynaptic activation by sensory receptors: This
activation occurs in reex arcs involving primary sensory neurons that
innervate skin, muscles, and joints. Pain-withdrawal and kneejerk reexes
both involve synaptic connections from the sensory to the motor units.
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