SPPA 2050 Speech Anatomy and Physiology
Neuronal Structure and Function
Now, let us move on to the neuron. As will be the general pattern of behavior for SPPA 205, we
will begin discussion of the neuron with its basic structure, followed by its function. Although
there is a great deal of diversity in the look of neurons, the vast majority of neurons have four
structurally defined regions: the soma or cell body, the dendrites, the axon, and the presynaptic
terminal (also called an axon terminal arbor, or “Bouton”). The cell body is the metabolic center
of the neuron. Dendrites are often fine, tree-like extensions from the cell body. The axon also
projects from the cell body and is a tubular extension that can be quite long with a terminal
ending. Often the axon is wrapped in myelin, an insulating material (produced by
oligodendrocytes and Schwann cells), punctuated with nodes of Ranvier. The myelin and nodes
of Ranvier dramatically increase the speed at which neural signals can travel. Conduction
velocity (the speed of the signal) is also influenced by the diameter of the axon. The presynaptic
terminal of the axon is located very, very close to the dendrite or cell body of an adjacent neuron
(or a muscle cell – remember the muscle). The region where cell-to-cell communication can
occur is called the synapse and the region between the cells is called the synaptic cleft. Neurons
are polarized, meaning that information flows in a predictable and consistent direction. This flow
runs from the dendrites and cell body to the axon and finally to the axon terminal where it has the
potential of communicating with other cells. The means of communication for the neuron is
electrical and we will spend a little time discussing how this electrical charge is (1) generated at
the receiving end of the neuron, (2) propagated along the length of the neuron, and (3)
transmitted to the neighboring neurons.
The neuron has a separation of electrical charge across its cell membrane. A separation in charge
simply means an unequal number of positively and negatively charged elements (charged atoms
or ions, principally sodium, which has a positive charge, potassium, which also has a positive
charge and chlorine, which has a negative charge) inside and outside the cell. This is termed the
resting membrane potential. The resting membrane potential is negative indicating that the
inside of the cell is more negative than the outside of the cell. For a lot of neurons the resting
membrane potential is about –50 - -70 millivolts (1 millivolt = 1/1000 of a volt). The resting
membrane potential is established through active transport of ions across the cell membrane.
Active transport simple means that cellular energy (the active part) is needed to move the ions.
This is accomplished by a mechanism called the Sodium-Potassium pump (or Na-K pump).
The Na-K pump moves Na+ (sodium) ions out of the cell and moves K+ (potassium) ions into the
cell. Na+ and K+ have a single positive charge. The pump moves three Na+ ions out of the cell
for every two K+ that it lets in the cell. This causes an electrical gradient, where the charge inside
the cell is less than the charge outside the cell. Also, most of the Na + outside and K+ inside the
cell creates a chemical concentration gradient. This electrochemical gradient is the resting
membrane potential. If allowed, the Na+ ions would like to rush inside the cell to equalize both
their numbers relative to the outside of the cell and the equalize the negative charge across the
cell membrane. However, ions such as Na+ must pass through protein channels that are specially
designed for them. These channels may be open or closed depending upon the electrochemical
environment. At resting membrane potential, most of these channels are closed. Some ion
channels have special receptor sites on the outside of the cell. These receptor sites are much like
a lock in a door (which is the channel). It requires the correct key to open. This “key” takes the
form of a biochemical (often a neurotransmitter or NT). If there is a NT in the area it can attach
itself to the ion channel receptor site and change the permeability of that ion channel (e.g. unlock
and open the door). Opening or closing these receptor channels (Figure 2.7) will change the
permeability of the channels to ions and therefore can change the membrane potential. There is
another type of ion channel that you need to be aware of. These channels do not have receptor
sites and therefore do not operate in the lock and key manner. These voltage gated channels
SPPA 2050 Speech Anatomy and Physiology
(Figure 2.7) will change their permeability depending upon the membrane potential. If there is a
change in the membrane potential, these channels may open (or close). For example, a NT may
attach to a receptor site and open a Na+ channel. Given the electrochemical gradient that exists,
the Na+ will move into the cell making the membrane potential less negative. This change in
membrane potential could trigger a voltage gated Na+ channel to open causing more opportunity
for Na+ to enter the cell and further change membrane potential. There is a third type of channel
that occurs in specialized settings. Many sensory neurons (those that sense touch, pressure etc)
have ion channels that are opened/closed because of mechanical events in the environment. For
example, the hair cell in the ear has a small projection that is moved, indirectly, by sound
pressure. This movement of the ‘hair’ on the hair cell causes a mechanical change in the
permeability of ion channels, which change the membrane potential.
Neural signals are generated by changing this resting membrane potential (using the
aforementioned mechanisms). Increasing the resting potential (making it more negative) is called
hyperpolarization. Decreasing the resting potential (making it less negative) is called
depolarization. Hyperpolarization decreases the cell’s ability to generate a signal called an
action potential (discussed later) and is called inhibitory. Depolarization increases the cell’s
ability to generate an action potential and is therefore called excitatory.
As outlined before, signal transmission within the neuron involves receiving a signal from a
source, propagating the signal along the length of the neuron, and transmitting the signal to other
neurons. We’ll deal with each phase separately. Signal reception typically occurs at the dendrite
and cell body. These signals are called potentials and either depolarize (excite) or hyperpolarize
(inhibit) the cell (Figure 2.5). These potentials may come from sensory organs, such as a muscle
spindle (generator potentials) or from other neurons (postsynaptic potentials or PSP). If the
potential excites the cell, they are typically called excitatory postsynaptic potentials or EPSPs
for short. Those that inhibit the cell are called inhibitory postsynaptic potentials or IPSPs.
EPSPs and IPSPs are small and are often described as graded (can take on a variety of
magnitudes – large or small), local (potential only occurs in the region of the receiving end of the
neuron) and passive (potentials die out rather quickly). At any given point in time, there may be
a large number of these potentials (both excitatory and inhibitory) being generated in different
parts of the receiving end of the cell. There is a region called a trigger zone located near this
receiving area that is quite sensitive to these changes in the membrane potential. At the trigger
zone the various inhibitory and excitatory potentials will summate (add up). Summation can
occur because there are numerous PSPs occurring at the same time in different places along the
cell membrane (spatial summation) or a sequence of PSPs occur rapidly in time at the same
point on the cell membrane (temporal summation) (Figure 2.6). If the sum of the potentials is
excitatory and large enough to exceed some threshold value that is characteristic of that neuron,
then an action potential will be initiated (Figures 2.8-9). The action potential is a very large,
brief depolarization (up to 100 mV, about 1 ms in length). Unlike the receptor/synaptic
potentials, which are local and passive, the action potential is active (is assisted by the cell) and
will propagate the length of the axon without a reduction in its amplitude. While the changes in
the receptor/synaptic potential are “graded”, meaning they can be produced at a variety of levels
(positive, negative, bigger, smaller), the action potential is an all-or-none phenomenon. There
cannot be a “graded” or partial response. It either happens or it doesn’t. Once it does happen, the
action potential quickly moves down the length of the axon to the presynaptic terminal. The
myelin sheath speeds up the rate of travel as does the nodes of Ranvier. The action potentials
actually skip from one node to the next. Once the action potential reaches the presynaptic
terminal, the next phase of signal transmission begins – the transmission to neighboring cells.
This is described in the next paragraph. Recall that the action potential is a brief, large
depolarization. Following this depolarization, there is a rapid return back toward the resting
SPPA 2050 Speech Anatomy and Physiology
potential. This process takes a little time and actually results in a hyperpolarization of the cell for
a short period of time. What this means is that for a brief period of time, it is impossible to
generate another action potential and for a longer period of time it is more difficult to generate an
action potential. This brief period where another action potential is impossible is called the
refractory period of the neuron. It is important because it provides a limit on how fast a neuron
can fire (which can be as high as 1000/sec). In fact, the only way that a neuron can code
information is through how fast it fires (rate code) and when it fires (temporal code). Refer to
the figure below to get a visual image of this process. The curve above the trace of the membrane
potential simply implies that the required excitatory potential to elicit another action potential is
higher when closer in time to the previous action potential.
Once the action potential reaches the presynaptic terminal, there has to be a way for information
to span the synaptic cleft to communicate with neighboring cells. This requires a change in the
mode of communication from electrical to biochemical. When the action potential reaches the
presynaptic terminal, it initiates something called a secretory potential, which mediates the
release of a chemical called a neurotransmitter into the synaptic cleft. The neurotransmitter
diffuses across the cleft to the neighboring cell where it interacts with the cell membrane of the
postsynaptic cell (if it is a neuron, it will be the dendrite or cell body) and produces a change in
the membrane potential of that cell (either excitatory or inhibitory, depending on what the
neurotransmitter is). Now this potential is unlikely to be big enough to elicit an action potential
all on its own. If this part is starting to sound familiar, that is because we already talked about it
earlier. This is the postsynaptic potential, but now it is happening in the next cell in the chain or
network. Everything we discussed earlier now applies to this new neuron. We’ve come full circle
in the physiology of the neuron.
Now a few points about neurotransmitters (NT). NT’s come in a variety of types. There are
dozens of NTs in the nervous system (e.g. glutamate, glycine, seretonin, dopamine,
acetylcholine). Some are located throughout the NS, while others are more localized. Some are
excitatory and some are inhibitory. Some we hear about often because too much or too little of
them are often linked to certain diseases such as Parkinson’s Disease (lack of the NT dopamine)
and that drugs (legal and otherwise) tend to influence the amount of NT or mimic its action.
Finally, for a little perspective, in the functioning human, a neuron will have numerous synapses
on its dendrite and cell body (the motor neuron can have 10,000) and can project its influence on
a very large number of cells. So let’s take a single neuron (we’ll call her Fred). Whether Fred
rises to action and speaks depends on the activities of the hundreds of Fred’s neighbors. If their
voices are loud enough, and Fred does rise to action she will in turn be a single voice in a chorus
of thousands. But if enough of them are saying the same thing, the results are behaviors (e.g.
speaking, thinking, sleeping) that we take for granted.
The Central Nervous System
The broad goal of this Chapter is to provide a general introduction to the central nervous system (CNS) and
its role in communication. It is not meant to be comprehensive and provides only a cursory overview of
many important CNS structures. See your text readings for more detailed treatment of the various sections.
Unit Organization
I. Anatomy of the Nervous System
I.
II.
III.
Anatomy of the Nervous System
Afferent and Efferent Pathways
Centers and Circuits for the Neural
Control of Speech
A. The Central Nervous System (CNS)
i. Basic organizational features of the CNS
SPPA 2050 Speech Anatomy and Physiology
a. “Slicing and Dicing” the CNS
The central nervous system can be
divided into the spinal cord and the brain. The
brain can be further divided into a number of
structures. The following is one way to organize
these structures.
Spinal cord
Brain
Hindbrain
Medulla
Pons
Cerebellum
Midbrain
Forebrain
Thalamus
Hypothalamus
Basal nuclei (ganglia)
Cerebrum
Further, the brain can be organized along
functional lines. For example, later on, we will
briefly discuss the limbic system, which is
distributed across a number of structures outlined
above.
You will often read about a structure called the
brainstem. This is a term that refers collectively
to the medulla, pons and midbrain.
b. Histologic organization of the CNS
If you inspect the gross appearance of
CNS tissue, you will quickly note that that some
tissue is white in color and other tissue takes on a
grayish color. This is an important distinction
since these different tissues contain different
kinds of material.
White matter contains
neuronal fibers that communicate between
different parts of the CNS. It is white in
appearance because it contains myelin, a fatty
substance that insulates the fibers.
It is
sometimes useful to think of white matter as
being analogous to the wiring in a house. Both
serve to primarily connect different regions of
their respective structures (house or CNS).
When you think white matter, think
connections between brain regions.
In
contrast, gray matter contains neuronal cell
bodies and synapses that allow communication
between neurons. As we learned way back in
Unit 1, the cell body and synapses are the
primary location of integration and modulation
of neural activity. Where the white matter is
analogous to the wiring in a house, the gray
matter is analogous to the switch panels in a
house. Gary matter and switch panels are points
where circuits converge and diverge. So, when
you think gray matter, think cell bodies and
connections between individual neurons. Gray
matter is often organized in focal regions of the
brain. A focal collection of nerve cells within
the CNS is called a nucleus. For example, the
cell bodies of the cranial nerves are located in
various nuclei within the brainstem. It is
important to recognize that damage to either
white matter or gray matter can disrupt normal
neural function.
c. The CNS is symmetric
This is simply a reminder for you. As
with the rest of the body, the CNS is largely
symmetric. Sectioning the CNS along the
midsagittal plane will yield two anatomical
structures that are roughly mirror images of each
other. So, remember, when we discuss a
structure like the thalamus, it is important to
recall there is a left thalamus and a right
thalamus. We will learn in later discussions,
certain neural functions are often localized to left
or right parts of the CNS.
ii. The Cerebrum
The cerebrum is the largest part of the
human brain. It has a wrinkled surface, which
dramatically increases its surface area (works the
same way as crumpling a piece of paper you plan
to throw in the trash). The cerebral surface is
termed the cerebral cortex (bark). The cerebral
cortex is largely gray matter that is organized
into a series of histologically distinct layers.
Increasing the surface area of the cerebrum
dramatically increases the number of cell bodies
and synapses and hence increases the
opportunity for variations in neural connectivity.
This wrinkled cortex contains outfoldings and
infoldings. An outfolding is termed a gyrus, and
an infolding is termed a sulcus. Particularly
deep sulci are called fissures. There is sufficient
uniformity among brains to prompt anatomists to
name many of these gyri and sulci. There are
some key fissures that serve to separate the
cerebral cortex into smaller parts.
The
longitudinal fissure separates the cerebral
cortex into a left and right hemisphere. Within
each hemisphere, there are a number of
identifiable lobes. These include the frontal
lobe, the parietal lobe, the temporal lobe, the
occipital lobe and the limbic lobe. The central
sulcus/fissure (Rolandic fissure) separates the
frontal and parietal lobes and the lateral
SPPA 2050 Speech Anatomy and Physiology
sulcus/fissure (Sylvian fissure) separates the
temporal lobe from the fontal/parietal lobes.
Regions of the cerebral cortex are associated
with particular bodily functions. The frontal
lobe tends to be involved with motor and
executive function.
The temporal lobe is
principally involved in audition and olfaction.
The parietal lobe is involved in general sensation
and sensory integration. Finally the occipital
lobe in associated with visual function.
cerebral cortex. The thalamus is involved in
integrating peripheral sensory information of
different types with information from other parts
of the CNS. Therefore, damage to the thalamus
can affect basic sensory and more complex brain
functions (such as language formulation).
Similar to the basal ganglia, the thalamus
consists of a number of distinct nuclei.
As was noted earlier, white matter contains
axonal fibers that communicate between brain
areas. These bundles of fibers are often termed
fasciculi. White matter fiber tracts are organized
based on the structures between which they
communicate.
Association fibers connect
cortical areas in the same hemisphere.
Association fibers can be short (i.e. gyrus to
next gyrus) or long (i.e. connecting lobes).
Examples of long association fibers are the
superior longitudinal fasciculus, which
connects the frontal, parietal, occipital, and
temporal lobes and the arcuate fasciculus,
which connects the frontal, parietal and temporal
lobes.
Commissural
fibers
connect
hemispheres.
Examples include the corpus
callosum and the anterior commissure. Finally,
projection fibers connect cerebral structures
with structures in the brainstem and spinal cord.
• Mediodorsal nucleus: also involved in
• Anterior nucleus: involved in the brain’s
emotional circuits
iii. Basal nuclei/ganglia (BG)
There is a set of nuclei deep to the
cerebral cortex that are collectively termed the
basal nuclei or basal ganglia. The nuclei that
comprise the BG are (at least) the caudate (taillike) nucleus, the lenticular (lens-like) nucleus,
the subthalamic nucleus and the substantia
nigra. The lenticular nucleus can be divided into
the putamen and globus pallidus. The caudate
and putamen are often collectively termed the
striatum. The BG is most notably involved in
the control of movement. It has been suggested
that the BG helps stabilize the motor system.
Diseases related to BG function include
Parkinson’s Disease (reduced movement) and
Huntington’s Disease (uncontrollable “dancing”
like movement).
iv.
Thalamus
and
Hypothalamus
The thalamus is an anatomically and
functionally complex structure located deep to
the cerebral cortex near the BG. It is often
termed the “gateway to the cortex” because all
sensory information (exception olfaction) must
pass through thalamus before reaching the
•
•
•
•
emotion
Ventral anterior and ventral lateral nuclei:
receives input from basal nuclei and
cerebellum
Ventral posterior lateral (VPL) and ventral
posterior medial (VPM): relays information
on general body sensation (VPM: sensory
from head)
Medial geniculate body: processes auditory
information
Lateral geniculate body: processes visual
information
The hypothalamus is located inferior to the
thalamus (hence “hypo”) and is heavily involved
in the controls of autonomic nervous system
(ANS) function and endocrine system. The
hypothalamus is involved in the release of
hormones, controlling food and water intake,
regulating sexual behavior, sleep cycles, and
emotional responses. It has only an indirect role
in speech communication.
v. Cerebellum
The term cerebellum means “little
brain”. Even though it is called the little brain, it
contains roughly half the neurons in the entire
brain. The cerebellum is principally involved in
motor control. It aids in coordinating muscles to
produce smooth fluid motion, monitors ongoing
“state” of body and makes appropriate
adjustments in motor commands in response to
that “state”. This structure has also been
implicated in learning new motor tasks. Damage
to the cerebellum can result in ataxia, a
condition characterized by the decomposition of
movement, errors in timing and scaling of
movement, and tremor during movement
activities.
The cerebellum is located dorsal to the brainstem
and is attached to it by the superior, middle and
inferior cerebellar peduncles. The gray matter
SPPA 2050 Speech Anatomy and Physiology
of the cerebellum is located in a cortex (bark or
covering) & in structures referred to as the deep
cerebellar nuclei. The cerebellar cortex is the
primary information receiving area of the
cerebellum. Information (1) about body position
(from the brainstem and spinal cord), (2)
regarding vestibular status (balance) and (3)
from other motor control centers passes to the
cerebellar cortex through the inferior and middle
cerebellar peduncles. The deep cerebellar nuclei
are the primary information sending areas of the
cerebellum. Information is sent via the superior
cerebellar peduncle to structures such as the
thalamus, the brainstem and spinal cord, and the
cerebral cortex.
the body. Like the rest of the CNS, it contains
gray and white matter. The gray matter is a
butterfly-shaped (“H”-shaped) area in the middle
of the spinal cord. Each “wing” of the butterfly
is called a horn. There is a ventral horn which
contains the cell bodies of motor neurons, and a
dorsal horn, which receives sensory information.
The white matter that surrounds the gray matter
contain tracts that project (1) to the brain from
the body, (2) from the body to the brain and (3)
from one segmental level to another segmental
level. Tracts from white matter terminate and
arise from spinal cord gray matter. It is
important to remember that the stretch reflex is
mediated within the spinal cord.
vi. Limbic System
The limbic system is not a single
structure, but contains a number of CNS
structures including the limbic lobe, the fornix,
the hippocampus, the amygdala, the
mammilary bodies and the anterior nucleus of
thalamus. The limbic system is important for
regulating emotional and visceral responses. Its
specific role in speech and language is not
known, although damage in the area can cause
mutism (lack of any speech).
B. Peripheral Nervous System
vii. Brainstem
The brainstem includes the midbrain,
the pons and the medulla. It connects the spinal
cord with the forebrain. The brainstem performs
a variety of functions. It regulates some essential
life functions including the maintenance of body
temperature, control of respiration and heart rate,
swallowing and digestion. The brainstem also
contains many nuclei and fiber tracts. These
tracts contain fibers that project to/from higher
CNS structures from/to spinal cord/brainstem.
Nuclei include the cranial nerve nuclei, which
contain cell bodies of neurons that comprise the
cranial nerves. Damage to the brainstem can be
devastating. It can disrupt basic bodily function
causing death or an inability to survive without
artificial means. Damage to this area can also
cause severe motor and sensory impairment of
the body.
C. Nourishing the CNS
viii. Spinal cord
The spinal cord is that part of the CNS
that runs through the vertebral foramen. The
spinal cord is organized length-wise as a series
of segments that are quite similar in structure.
Each segment is associated with a pair of spinal
nerves. There is a tendency for a segment to be
associated with function at that particular level of
The peripheral nervous system, which includes
the cranial and spinal nerves was discussed in
Unit 1. However, you need to know the
following. You should know all 12 cranial
nerves by name and number. In addition, you
need to know the motor and sensory function
associated with CN V, VII, IX, X, XI, and XII.
Refer to the appropriate table. You should also
recall there are 31 spinal nerves.
i. Vascular Supply to CNS
Blood is the means by which the brain
is supplied oxygen and nutrients. Although the
brain is only 2 % of the total body mass, it uses
about 20 % of blood in the body. Glycogen is
the energy source for the brain. Unlike other
body parts, the brain can’t keep stores of
glycogen. Therefore, without blood (and its
oxygen and nutrients), the brain quickly becomes
“malnourished”.
Neural cell death occurs
rapidly (within a few minutes). Unfortunately,
neurons within the CNS do not routinely
regenerate so injuries become permanent.
Blood is supplied to the body from the heart via
arteries and blood is returned to the heart via
veins. In the CNS the term sinus refers to a
collection of veins. All arterial blood to the
brain come from the internal carotid and
vertebral arteries (remember symmetry –there
are two of each). The internal carotid artery
branches into a middle cerebral artery and an
anterior cerebral artery. The middle cerebral
artery supplies most of the brain’s lateral surface,
major portions of the frontal and temporal lobes
(including the insula), the basal ganglia and the
SPPA 2050 Speech Anatomy and Physiology
thalamus. The anterior cerebral artery supplies
the front and medial surface of the brain. The
left and right vertebral arteries combine at the
midline to form the basilar artery. The basilar
artery provides many small arterial branches that
supply the brainstem and cerebellum. The
basilar artery then splits to form the posterior
cerebral artery. The posterior cerebral artery
supplies the posterior temporal lobe and the
occipital lobe. The left and right anterior
cerebral arteries have a small “communicating”
artery that connects them. There are also
communicating arteries between the left (and
right) posterior cerebral artery and the left (and
right) internal carotid artery. This complex of
shared arterial circulation (or anastamosis) is
called the Circle of Willis. There is a real
advantage to this somewhat complex anatomical
setup (other than to provide good test questions
for instructors). A blockage of one or more of
the four arteries that bring blood into the brain
can occur and yet all areas of the brain can still
receive the precious blood they need to survive.
Mother nature can be quite clever.
responsible for headaches (you take the good
with the bad). The middle meningeal layer is the
arachnoid membrane. This is a web-like
structure that filled with some cerebrospinal
fluid. This fluid can serve to provide shock
absorption against blows to the head. The inner
meningeal layer is called the pia mater. It is
very closely associated with the brain surface
and follows it rough wrinkled appearance.
In the world of anatomy it is common to speak of
anatomical “spaces”. Often, these are not actual
empty spaces but potential spaces. Like a
collapsed balloon, these anatomical spaces are
typically empty, but have the potential to be
filled with stuff. Between each meningeal layer
is a designated space. The space between the
skull and dura mater is term the extradural
space. The space between dura and arachnoid
mater is called the subdural space and the space
between arachnoid and pia mater is called the
subarachnoid space. These terms are useful to
describe clinical conditions (see below).
Clinical note
Clinical note
A blockage, due to thrombosis (clot), embolism
(object floating through the bloodstream) or
bursting (hemorrhage) of blood vessels in the
brain is collectively termed a stroke or
cerebrovascular accident (CVA) which can
have devastating effects on communication
Protecting the CNS
Brain tissue is soft and gelatinous. Therefore, it
is prone to injury. There need to be ways to
protect this critical structure from wear and tear
of everyday life. One form of protection comes
from the meninges, which are a layered
wrapping around the brain and spinal cord.
Another form of protection comes from a
network of fluid filled cavities inside the brain
called the ventricles. The meninges are a
layered wrapping of the
ii. Protecting the CNS: The meninges
The meninges provide an external wrapping to
brain and spinal cord.
There are three
anatomically distinct layers or “mater”. The
most superficial layer is a thick tough, fibrous
layer called the dura mater. This provides a
good solid protection. However, this layer also
contains many of the sensory receptors that are
Many diseases and disorders associated with the
brain can actually be traced to the meninges.
Meningitis is an inflammation of the meninges
that can result in temporary and/or permanently
impaired neurologic function. A meningioma is
atumor of the meninges that can invade the brain
and cause serious impairment in function.
Hematoma
(bruising)
or
hemorrhage
(bleeding) can occur into meningeal spaces
(subdural, subarachnoid).
iii. Protecting the CNS: The ventricles and
cerebrospinal fluid
Within the brain, there are four interconnecting
fluid-filled spaces or ventricles. This fluid is
called cerebrospinal fluid (CSF) and it is
produced by the choroid plexus within the
ventricles. CSF is considered to be principally
protective,
providing
shock
absorption.
However, CSF may also have some nutritive
function. There are two lateral ventricles,
which are within each cerebral hemisphere. The
lateral ventricles communicate with the third
ventricle, which is in the region of the thalamus.
The third ventricle communicates with the
fourth ventricle via the cerebral aquaduct,
which is located ventral to the cerebellum and
dorsal to the pons.
The fourth ventricle
SPPA 2050 Speech Anatomy and Physiology
communicates with the central canal, which
runs down the center of the spinal cord.
(e.g. back). This is yet another example of the
relationship between structure and function.
Clinical note
In order for a sensory stimulus to reach the
somatosensory cortex (and thus be perceived),
the information must pass over at least three
neurons. The first order sensory neuron has its
cell body in dorsal root ganglion or cranial nerve
nuclei and its axon in a peripheral nerve. It
communicates the sensory stimulus to a second
order sensory neuron located in either the
dorsal gray matter of the spinal cord or a
brainstem nucleus. This neuron transmits the
sensory
information
from
the
spinal
cord/brainstem to the contralateral thalamus
where it synapses onto a third order sensory
neuron. This neuron carries information to the
somatosensory cortex via a large fiber bundle
called the corona radiata. Note that the
information crossed to the contralateral side of
the body when it was a second order neuron.
An excess CSF is called hydrocephalus
(literally, water on the brain), which can occur
for a number of reasons and impair normal
nervous system function
II. Afferent and Efferent Pathways
This section provides a very basic introduction to
the major afferent (sensory) and efferent (motor)
pathways in the CNS. These pathways are
critical for conscious and unconscious sensation,
voluntary movement, and sensorimotor
integration.
Before proceeding, it is important to point out
that at the level of the cerebral cortex (and most
subcortical structures), bodily representation is
“crossed”. That means that the sinistral side of
the body is represented in the dextral cortex and
vice versa. For this to occur, afferent and
efferent pathways must cross the midline
(decussate) of the CNS. The term contralateral
refers to the opposite side and the term ipsilateral
refers to the same side. For example, we might
say that the left cerebral cortex controls the
contralateral side of the body. We might also
say that the left vagus nerve provides the motor
supply to the ipsilateral intrinsic muscles of the
larynx.
i. Sensory Pathways
The primary cortical representation of
somatosensory (somato – body) function is
located in a long strip of cortex just posterior to
the central sulcus. This structure is called the
post-central gyrus, but is also referred to as the
sensory strip, the primary sensory cortex or
the somatosensory cortex. In terms of the
cortical map of Brodmann, this areas is assocated
with Brodmann’s Areas 1, 2, 3 and 5.
We say that the somatosensory cortex is
somatotopically organized. This means that the
bodily location of sensory information is
preserved within the cortex.
There is a
representation of the whole body along the
length of the somatorsensory strip. Further,
some body parts that we recognize to be highly
sensitive (e.g. lips, tongue, fingers) have larger
cortical representation than less sensitive parts
ii. Motor Pathways
The primary cortical representation of
movement is located in a long strip of cortex just
anterior to the central sulcus. This structure is
called the pre-central gyrus, but is also referred
to as the motor strip, the primary motor cortex
or the motor cortex. In terms of the cortical
map of Brodmann, this areas is assocated with
Brodmann’s Area 4.
Analogous to the somatosensory cortex, body
movements are highly localized and there is a
map of body parts in this strip of cortex. We
often say that the primary motor and sensory
cortices contains homunculi or “little men”.
Those structures that require more motor
precision (e.g. tongue and fingers) have larger
cortical representations than muscles systems
that produce more gross movements (e.g. trunk
muscles).
Communicating a motor command to the body
can involve as few as two neurons. The upper
motorneuron (UMN) has its cell body in the
motor cortex. Its axonal fibers descend from the
cortex through the internal capsule (white
matter that contains many fibers going to and
from the cortex) and then through the brainstem.
At this point most of these fiber cross over
(decussate) to the contralateral side of the body.
The UMN then synapses onto a lower
motorneuron (LMN), whose cell body is
located either in cranial nerve nuclei in the
brainstem (head and neck muscles) or in the
SPPA 2050 Speech Anatomy and Physiology
ventral horn of the spinal cord (below the head).
The LMN leaves the CNS and synapses onto the
muscle fiber. Because there are no intervening
neurons between the LMN and the muscle, the
LMN is sometimes called the final common
pathway. Any instruction for a muscle to move
must pass through this neuron. This part of the
motor system is sometimes called the direct
motor system because it contains so few
synapses. It is also called the pyramidal motor
system because the white matter tracts through
which the UMN run are pyramidal shaped when
they pass through the medulla.
Clinical Note
Damage to upper motorneuron and lower
motorneuron result in quite different clinical
syndromes. UMN damage can result in spastic
paralysis, increased muscle tone and
exaggerated reflexes. In contrast, damage to
LMN can result in flaccid paralysis, absent or
reduced reflexes, decreased muscle tone and
atrophy of muscle.
Extrapyramidal (Indirect) Motor System
The pyramidal motor system is associated with
voluntary (willful) movement.
It includes
primary motor cortex (60 %) and other cortical
structures including the premotor (anterior to
motor strip) and sensory cortices (40%).
However, this is not the only neural system that
controls movement. As mentioned earlier there
are other neural structures (e.g. the cerebellum
and the basal nuclei) that a involved in regulating
movement. It is common in neurology to use the
term extrapyramidal or indirect motor system
to refer to a system that influences and regulates
the motor instructions that are sent to the
periphery.
The basal nuclei/ganglia is the
principle structure in this system.
While
voluntary movement is initiated by the direct
system, this indirect system is crucial to the
production of stabile, skillfully executed
movements. As we learned earlier, damage to
this system can seriously disrupt voluntary
movement control.
Sensorimotor Regulation
Although we discuss sensory and motor function
separately, it is important to recognize these
systems are inextricably linked. For example,
when we move our bodies, sensations are
generated (recall the muscle spindle). We make
use of this sensory information to evaluate
whether our movements are reaching their
intended targets and make adjustments if
necessary.
Speech production stimulates a
number of sensory modalities including touch,
kinesthesia/proprioception,
and
audition.
Although short term disruptions to sensory
systems do tend not to interfere with speech
production, longer term sensory loss (i.e. hearing
loss) can result in deterioration of speech motor
skills.
III. Centers and Circuits for the Neural
Control of Speech
It is often suggested that high level neural
functions are “localized” to a region of the brain.
For example, language is predominantly
represented in the dominant (i.e. left)
hemisphere. This argument could be extended to
suggest that there are very specific anatomical
locations for the range of language and cognitive
functions. However, this does not seem to be the
case. Although specific brain structures appear
critical for speech and language, no single
structure “houses” all the machinery needed. For
example, stimulating some portion of the brain
can result in events necessary for speech (e.g.
vocalization), but not “full-blown” speech. It
appears that speech and language functions are
distributed across many structures that include,
but are not limited to
Broca’s area
Broca’s Area is located anterior to the
primary motor area (the face and mouth area) on
or near the inferior frontal gyrus of the dominant
hemisphere (usually left). This area is associated
with planning speech production and damage in
this region can result in an expressive or Broca’s
aphasia (loss of language) that compromises an
individuals ability to express themselves using
language, while preserving an ability to
understand others.
Supplementary motor area
The supplementary motor area (SMA)
appears to be involved in planning motor
sequences including preparation of movement,
performing “internally” generated motor plans
and is implicated in the planning of propositional
speech.
Wernicke’s area
Wernicke’s area is important for the
understanding and formulation of language.
Damage in this area often results in an aphasia
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with problems understanding or repeating
language (hence the term receptive or
Wernicke’s aphasia). With deficits of this type,
patients exhibit spoken language that while
fluent, makes little sense
Other important areas
Primary motor cortex (“speech” muscles)
Basal nuclei
Cerebellum
Thalamus
Somatosensory cortex
Primary & association areas of auditory cortex