Fear: it is something we have all experienced before. But have you ever
stopped to think how we know to be afraid of something, and for that matter,
how and why we respond to that fear? Research has shown that the major
parts of the brain that process what we call fear and anxiety include the
amygdala, the periaqueductal gray and the hippocampus. The following paper
discusses the anatomy, the functions, and the connections between the first
two of these structures, the amygdala and the PAG.
The amygdala is a large nuclear complex located in the dorsomedial portion
of the temporal lobe. It is divisible into a series of nuclei with two
major divisions: the corticomedial division and the basolateral division.
The corticomedial group is composed of the nucleus corticalis and the nucleus
medialis as well as a number of smaller cell masses. The basolateral group
is composed of the nucleus lateralis, the nucleus basalis, the nucleus
basalis accessorius, and the nucleus centralis. Other names for these subdivisions
are the lateral nucleus, the basal nucleus, the accessory basal nucleus
and the central nucleus respectively (Nieuwenhuys, Voogd, and Huijzen 191).
According to The Central Nervous System, the afferent projections of
the amygdala can be placed into one of the following four groups:
On the other hand, the efferent projections of the amygdala can be traced
to the following areas:
Although the anatomy of the amygdala is important, the function of this
structure and its subdivisions is also important. To determine the specific
functions or responses of the amygdaloid nuclei, tests can be performed
that specifically target the desired area. For example, certain nuclei
can be excited via electrical stimulation or destroyed via lesions. The
effects of these procedures can be studied physiologically. But first,
let us look at the amygdala as a whole.
Electrical stimulation of the amygdala, specifically the central nucleus,
produces a series of behavioral and anatomical changes nearly identical
to those produced in a state of fear. Targeting the central nucleus can
still give results for the entire amygdala because it receives input from
each of the other nuclei. Thus, electrical stimulation of the central nucleus
evokes increased heart rate and blood pressure, ulceration, urination and/or
defecation, increased respiration, increased vigilance, increased startle,
freezing facial expressions of fear and corticosteroid response depending
on the specific anatomical target of the amygdala. Thus, activation of
the amygdala elicits a set of behaviors resembling fear responses by projecting
to a number of target areas such as the hypothalamus, the central grey
and many of the cranial nerves (Davis 355-357).
Other studies specifically targeting the lateral and basolateral nuclei
have determined their individual functions as well. Most of these studies
deal with formation of memories of fear using classical conditioning where
an electrical shock is paired with a noise or light. In one such experiment,
to determine the amygdaloid nuclei's role in this type of memory formation,
researchers systematically destroyed, via lesions, specific areas of the
brain. If these lesions in any way impeded the fear conditioning, it was
assumed that this area is necessary in memory formation concerning fear
and anxiety.
In this experiment, the auditory cortex was destroyed and it had no
effect on the fear conditioning of the rats being used. Lesions were then
made in the auditory thalamus and midbrain. These resulted in complete
elimination of the rat's conditioning. These results suggested that once
the sound stimulus was processed in the auditory thalamus and midbrain
it traveled to another region other that the auditory cortex and this connection
is what results in physiological responses to the stimuli. Researchers
then traced fibers from the thalamus to subcortical regions and discovered
that only destruction of those leading to the amygdala had an effect on
fear conditioning.
Following this pattern, the experiment then involved discovering the
specific functions and connections between the amygdaloid nuclei. Previous
research had already revealed that lesions of the central nucleus had an
effect on heart rate, respiration, and vasodilation. This experiment also
showed that lesions of this nucleus prevented a rise in blood pressure
and freezing.
However, researchers also learned that the central nucleus is not directly
connected to the auditory thalamus. In fact, it is the lateral nucleus
that receives sensory information from the auditory thalamus which in turn
sends these signals to the basolateral, basomedial or accessory basal nucleus
which then communicates with the central nucleus. It is the central nucleus
that then connects to areas in the brain stem that elicit physiological
responses (LeDoux 50-57).
Another experiment produced similar results, this time using a visual
stimulus. Here, infusions of N-methyl-D-aspartate were used to create lesions
in the lateral and basolateral nuclei. Their experiments also revealed
that it is the lateral or basolateral nuclei ( or both ) that relays visual
information to the central nucleus. It was also discovered that lesions
of the most anterior portion of the basolateral nucleus blocked increased
startle levels normally produced by a series of footshocks. Thus, it is
this portion of the basolateral nucleus that processes shock information.
Overall, this study reveals that it is the lateral and basolateral amygdaloid
nuclei that are vital for the expression of fear-potentiated startle and
the anterior basolateral nuclei in increasing startle with a series of
shocks (Sananes and Davis 72-79).
The periaqueductal grey is another major structure involved in the processing
of fear. It is a large structure located in the midbrain, consisting of
small to medium neurons surrounding the aqueduct of Silvus, otherwise known
as the cerebral aqueduct. The periaqueductal grey runs from the posterior
commisure rostrally to the locus coeruleus caudally. Anatomically, there
are two ways to divide it: by cell type or on the basis of its anatomical
connections and functional representation of cardiovascular and behavioral
functions. In this later method, the periaqueductal grey is divided into
four major vertical columns situated dorsomedial, dorsolateral, lateral
and ventrolateral to the aqueduct.
Class I neurons are the smallest of the three types. They average 18
micrometers in length and 8.5 micrometers in width. After Nissel staining
the cells appear darkly stained with an axon projecting from one end of
the cell. The nucleus is rather large, oval in shape and stains slightly
lighter that the surrounding cytoplasm of the cell. The cell itself is
spindle shaped with the nucleus located in the center taking up the entire
width of the cell. Nissel substances are uniformly distributed and are
not found in clumps. Usually, class I neurons are found in the innermost
regions of the PAG, called the nucleus medialis. Overall this area has
a small population of neurons.
Class II neurons are slightly larger with the average diameter being
between 11 and 12.2 micrometers. They are fusiform or spherical in shape.
The nucleus in this cell type is usually irregularly shaped. The axon of
these cells is located in the central region, not at a pole as in class
I. Class II neurons generally have several dendrites. The Nissel substances
are arranged in clumps which are evenly distributed in the cytoplasm. Class
II neurons aggregate in the area just dorsal to the cerebral aqueduct named
the nucleus dorsalis along with a large number of glial cells.
Class III neurons are the largest of the tree types, with the average
length and width measuring 18.9 and 15 micrometers, respectively. These
cells are generally fusiform, spherical, or triangular in shape and have
a sparse amount of Nissel substance in their cytoplasm. This later fact
attributes to the fact that these cells stain very lightly. In most cases,
one axon and one dendrite can be identified. However, in the cases where
there is one axon and two dendrites, one of the dendrites is located at
the opposite pole as the axon and the other in the central region. The
nucleus is large in class III neurons, it is round, centrally located,
and darkly staining. These cells are found in the outer layer of the periaqueductal
grey known as the nucleus lateralis also along with a large number of glial
cells (Hamilton 1-7).
The periaqueductal grey can also be divided into four major longitudinal
columns. These columns are located dorsomedial, dorsolateral, lateral,
and ventrolateral to the cerebral aqueduct. The boundaries of these columns
are based on both anatomical and functional characteristics. These columns
are not of uniform width going caudally to rostrally. For example, the
dorsomedial and ventrolateral columns narrow as they travel caudally (Carrive
27-44).
The periaqueductal grey receives afferents from the following structures:
the amygdala, nucleus stria terminalis, dorsal hypothalamus, midline thalamus,
periventricular grey and the dorsolateral and ventrolateral midbrain tegmentum.
As for PAG efferents, the majority of the fibers leaving the PAG terminate
in the parabrachial nuclei, reticular formation, trigeminal motor nucleus
and nucleus ambiguus (Jurgens and Pratt 367).
Generally, the periaqueductal grey is known to be involved in some sort
of protection or defensive reactions. The research team of Jurgens and
Pratt investigated the role of the PAG in vocal expression of emotion.
Introducing dummy leopards to a group of squirrel monkeys was effective
in producing a fear elicited yapping calls. The team found that even the
smallest lesions of the PAG abolished the yapping call. Similarly, shrieking
calls were produced by gripping the squirrel monkeys forcibly. These calls
were also abolished, although the lesions of the PAG had to be larger in
this case (Jurgens and Pratt 367-377).
Another study investigated the role of the PAG in feline active defense
and quiet biting attack behavior. Affective defense behavior involves piloerection,
retraction of ears, baring of teeth, arching of back, pupillary dilation,
growling, hissing, unsheathing of claws, paw striking, urination, and sympathetic
cardiovascular reactions . Conversely, quiet biting attack behavior involves
the stalking of an anesthetized rat followed by a bite on the back of its
neck. In this type of behavior, there is no autonomic system activation
except for minimal pupillary dilation. In this study, affective defense
behavior was elicited via both electrical and chemical stimulation of the
dorsal half of the central grey. However, quiet biting attack behavior
was elicited from the ventral half of the PAG. Thus, this study discovered
two different pathways of fibers from the PAG that are associated with
affective defense behavior and quiet biting attack behavior separately,
in other words, a functional delineation (Shaikh, Barrett, and Siegel 9-23).
In another set of experiments, the function of particular columns of
the periaqueductal grey were ascertained. Specifically, the lateral and
ventrolateral columns were studied because they contain "topographically
distinct groups of neurons" which upon stimulation result in different
forms of defensive behaviors. Both play a vital role in the somatic and
autonomic components of defensive behavior. The results show that injection
of excitatory amino acids, which excite the cell body but not the axon,
into the lateral column increases somatic and autonomic reactions. These
reactions include elevated blood pressure, elevated heart rate, increased
vocalization, and increased hindlimb movement. These last two reactions
are characteristic of "fight" and "flight" respectively.
In contrast, when the ventrolateral column of the PAG was injected with
an excitatory amino acid, autonomic and somatic activity decreased. This
included a drop in heart rate and blood pressure. Thus these two columns
seem to oppose each other functionally. In the lateral column alone, there
seems to be both a "fight" or threat display area and a "flight"
area. This was demonstrated again via excitatory amino acids. When the
"fight" area was stimulated, blood flow was constricted in the
hindlimb but was increased to the face. In contrast, when the "flight"
area was stimulated, blood flow to the hindlimb increased whereas blood
flow to the face was decreased. It is believed that the lateral column
of the PAG causes these "fight or flight" reactions in response
to cutaneous, superficial stimuli. Thus, a rise in blood pressure, heart
rate, and defensive posture is appropriate because it prepares the subject
physically to either fight or run. On the other hand, the ventrolateral
column is believed to elicit reactions caused by deeper visceral somatosensory
inputs. This could be described as an inescapable visceral pain or extreme
muscle fatigue. Therefore, a reaction that causes decreased movement and
muscle tone seems appropriate under these conditions (27-44).
So far, the anatomy and independent function of the amygdala and the
PAG have been discused. Therefore, the only topic that remains in this
overview of the anatomy of fear is the connections, both physical and anatomical,
between these two structures.
Different tracing methods have been used to determine the existence
of a connection between the amygdala and the periaqueductal grey. For example,
one study used tritium-labelled leucine (Jurgens and Pratt 374). This was
injected into the amygdala and inspection of the PAG revealed a direct
projection. Another experiment used c-fos immunoreactivity as a marker.
When the animals were exposed for fifteen minutes to an elevated plus maze,
an animal model of anxiety, c-fos immunoreactivity appeared in the piriform
cortex. This sends massive input to the amygdala, in several amygdaloid
nuclei in the periventricular system along the anteroposterior extension
of the hypothalamus and its projections to the dorsal PAG and other structures
(Graeff et. al. 123).
According to Micheal Davis, the central nucleus of the amygdala projects
to a region of the central grey and the effect of amygdaloid stimulation
is cessation of behavior in the form of freezing. In further research,
following infusion of a retrograde tracer into the dorsal PAG, the presence
of met-enkephalin and Fluoro-Gold within the central and lateral nucleus
of the amygdala were found (Shaikh, Lu, Seigel 116-117).
So, there is evidence demonstrating that the dorsal half of the PAG
elicits affective defense behavior. Yet stimulation of the amygdala somehow
suppresses this behavior via some sort of connection. Research led by Majid
B. Shaikh revealed exactly how the amygdala suppresses this affective defense
behavior at the level of the PAG. This research showed that the suppression
is mediated through mu and delta opioid peptides at the level of the midbrain
PAG. They believed that the source of the opioid afferents to the PAG is
the central nucleus of the amygdala. This belief was based on the following
information: the central nucleus suppresses this behavior when stimulated,
the central nucleus contains a large number of neurons that immunoreact
with enkephalins, and there is a direct projection from the central nucleus
to the PAG.
This research provided evidence that when an opioid receptor blockade
is set up in the PAG, the suppressive effects of the amygdala were totally
eliminated. The researchers first used the non-specific opioid antagonist
naxalone and this completely blocked the suppressive effect. Furthermore,
suppression was eliminated with B-FNA, a specific mu receptor antagonist.
However, when ICI 174,864, a specific receptor antagonist was used, the
suppressive effects of the amygdala was unchanged. Thus, the PAG must receive
afferents from another source besides the amygdala that operates via delta
receptors (Shaikh, Lu, Siegel 109-117).
The brain system that deals with fear and anxiety is thus a very complicated
one. The amygdala and the periaqueductal grey are two main constituents
in this system, representing the rostral and caudal ends respectively.
Both the amygdala and the PAG can be broken down, and each of these subdivisions
serves an important function in the processing of and reaction to stimuli
producing fear. Connections between these two structures also play an important
role in reaction to fear. Obviously, innumerable studies have been conducted
on fear and its anatomy, but like everything else, there is still a lot
yet to learn.
Works Cited
Carrive, Pascal. "The Periaqueductal Gray and Defensive Behavior:
Davis, Micheal. "The Role of the Amygdala in Fear and Anxiety."
Graeff, Frederico G., Maria Cristina L. Silveira, Regina L.
Hamilton, Betty L. "Cytoarchitectural Subdivisions of the
Jurgens, U. and R. Pratt. "Role of the Periaqueductal Grey in
Kim, Jeansok J., Richard A. Rison, and Micheal S. Fanselow.
LeDoux, Joseph E. "Emotion, Memory and the Brain." Scientific
Nieuwenhuys, Rudolf, Jan Voogd, and Christiaan van Huijzen. The
Sananes, Catherine B. and Micheal Davis. "N-Methyl-D-Aspartate
Shaikh, Majid B., Chang-Lin Lu, and Allan Siegel. "An
Shaikh, Majid B., Jeannette A. Barrett, and Allan Siegel. "The