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The Part of Tens
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IN THIS PART ...
Discover the important brain circuits that are crucial
for thought and behavior.
See how new technologies are revolutionizing brain
science.
https://t.me/med1917
CHAPTER 19 Ten Most Important Neural Circuits 363
Chapter19
Ten Most Important
Neural Circuits
N
eurons are organized in ensembles called neural networks or circuits that
process information. In a circuit, a group of neurons are interconnected by
synapses to produce a specic function when activated. Information often
ows in from one place, through the circuit and out in another place. Circuits can
be simple and local, containing a small group of neurons in close proximity with
one input and one output. Or they can be more complex and long distance, involv-
ing integration of information from several areas with an output to dierent
regions. The diversity of neural circuit types has also been studied, identifying
diverging, converging, reverberating, and after-discharge circuits. This diversity
contributes to the broad scope of neural functions. Interconnected neural circuits
form large scale brain networks.
The term association cortex rst arose in the mid 20th century from the idea that
the neocortex was a nondierentiated device for learning stimulus-response con-
tingencies. In this early view, neural connectivity was initially random and non-
specialized. Research has now shown that connections between cortical areas and
subcortical areas are highly specic. Damage to particular areas of the brain
causes specic loss of function. After injury, plasticity mechanisms can often
reorganize synapses and circuits to overcome problems associated with the dam-
age, but sometimes not. This chapter describes ten particularly important circuits
in the brain and their functions.
IN THIS CHAPTER
»
Listing major neural circuits
»
Seeing what these ten brain
circuits do
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364 PART 4 The Part of Tens
The Reticular Formation
The reticular formation is an ancient brainstem circuit that extends throughout
much of the medulla, pons, and midbrain, and into the thalamus. It’s continuous
with the intermediate gray matter of the spinal cord and is organized in a similar
manner.
The word reticular is from the Latin for “net.” Refer to Chapter10 for more about
the reticular formation.
The reticular formation consists of more than 100 interconnected neural circuits
that control essential body functions and life processes such as sleep, wakefulness
and consciousness, heart rate, breathing, temperature, pain modulation, and
somatic motor control. Signicant damage to the reticular formation typically
results in death.
The reticular formation projects to the thalamus and neocortex to control states of
consciousness, awareness, and sleep-wake cycles via the reticular activating
system (RAS). The cerebral cortex requires input from the RAS to maintain aware-
ness and arousal. Minor or moderate damage to the RAS can result in loss of con-
sciousness and coma.
The reticular formation is also the origin of two descending motor pathways called
the reticulospinal tracts. These tracts control posture, balance, and tone, and are
also involved inlocomotion.
Somatic Reexes
Somatic reexes are the simplest neural circuits. The spinal cord and brainstem
reexes enable the body to adjust rapidly to sudden changes in the environment.
Reexes are rapid, involve sensory input and motor output, and don’t require
higher brain control. Chapter7 describes somatic reexes.
Here are a few examples of spinal reexes:
»
Withdrawal reex: A stimulus (for example, burning your hand) immediately
causes a movement (withdrawing your hand from the stove). The axon
terminals of pain receptor neurons synapse on and directly activate motor
neurons in the spinal cord. The pain axons simultaneously activate inter-
neurons to relax the opposing muscle.
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CHAPTER 19 Ten Most Important Neural Circuits 365
»
Knee jerk reex: Stimulation of the patellar tendon/ligament with a small
hammer stretches the spindles in the nearby quadriceps muscle, leading to
contraction of the quadriceps and kicking out the lower leg and foot. This
reex is involved in posture and balance and also contributes to locomotion.
For brainstem reexes the integrating unit and motor neurons are located in the
brainstem. Numerous brainstem reexes control eye movements. Other reexes
include the jaw jerk, gag, and cough reex. In the cough reex, stimulation of
receptors in the respiratory tract by foreign particles like dust or mucus sends
signals to the medulla. Activation of motor neurons sends signals to contract
muscles that produce the cough.
Some reexes can be modulated or suppressed. You can override a spinal reex if
you know in advance that the stimulus will occur. If you’re climbing a rope, the
pain in your hands may trigger a reex to let go, but your brain, knowing that let-
ting go would be even more dangerous, may override this reex. You can suppress
a cough for a short period, but not for very long if a strong irritant is present.
Central Pattern Generators
Spinal cord and brainstem circuits produce the basic rhythmic patterns for loco-
motion or movements called central pattern generators (CPGs). Once initiated, CPGs
depend on intrinsic properties of the CPG neurons and their connections, operat-
ing without motor commands from higher brain areas. Chapter9 describes CPGs.
CPGs are found in invertebrates and vertebrates in many regions of the nervous
system. Many CPGs normally produce rhythmic patterned outputs with sensory
feedback, suggesting that pattern generators operate in a stimulus-reex-
movement-stimulus cycle. However, CPGs are capable of generating locomotion
patterns without sensory feedback.
Sensory feedback does help coordinate ecient locomotion in the limbs. For
example, the brain controls the activity of spinal CPGs via descending axonal
projections that constitute high-level commands— such as switching of gaits—
while the spinal cord neural circuitry takes care of details— like adjusting gait for
stepping in holes or when running on slopes.
CPGs for locomotion are distributed throughout the lower thoracic and lumbar
spinal cord. These circuits produce motor neuron ring–patterns that synchro-
nize rhythmic contractions of exor-extensor muscles of the forelimbs and hind
limbs. As the hind limbs move in antiphase with each other, the forelimbs move
in the opposite antiphase, so your right arm goes forward at the same time as
your left leg.
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366 PART 4 The Part of Tens
Top-down control from the brain takes advantage of inputs from the visual and
vestibular systems. Your visual system helps to keep you from running into trees.
Your vestibular system compensates for stepping in holes and generates the
proper leaning posture if you’re running on a slope. CPGs are especially important
for controlling your balance in these cases.
CPGs use neural circuits in which activity in half of the circuit increases while the
other half decreases. This alternating phase activity is believed to be based on
circuits of neurons that are mutually inhibitory— one leg extends while the other
exes. These oscillators extend throughout the spinal cord and can receive both
sensory input and cortical modulation.
Thalamic Relay to the Cortex
The thalamus is at the center of a neural hub whose spokes go to and from the
neocortex and subcortical structures. The thalamus is not only the gateway to
the cerebral cortex, but it’s also an intrinsic part of nearly all cortical circuits.
The thalamus receives inputs from all the senses, as well as the basal ganglia
and cerebellum for motor control, and the reticular formation (refer to the
section “The Reticular Formation” earlier in this chapter). Parts2 and3 describe
other thalamic circuits.
The output of the thalamus makes a fast, ecient link among all four cerebral
lobes that parallels, and to some extent, can short-circuit pathways within the
neocortex between dierent areas. This means that the thalamus is controlling
the ow and processing of information throughout the brain by integrating the
output of cortical and subcortical processing. This makes the thalamus not just a
relay, but a true controller. Thalamic integration is essential for attention, aware-
ness, and consciousness.
Cerebellar Coordination
The cerebellum is an evolutionarily old part of the vertebrate brain. Unlike other
areas like the brainstem, the cerebellum increased in size and neuron number as
the neocortex expanded during evolution.
The cerebellum modulates and coordinates motor behavior. Its neural circuitry
detects errors between what is programmed for a particular movement and what
is executed. This depends on detection of variables like loads and uneven ground
while walking. It uses error computation to achieve correct sequences in motor
behavior based on practice. Chapter8 discusses cerebellar circuits and functions.
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CHAPTER 19 Ten Most Important Neural Circuits 367
The cerebellum achieves its capabilities by learning. When you rst learn to throw
a baseball, you have to think about what you’re doing and consciously adapt to
your mistakes. Eventually the process becomes automatic because the sequence
has become programmed by cerebellar circuits.
The cerebellum also plays an important role in cognition that involves spatial
relations and movement. If you plan to rearrange the furniture in your house, the
cerebellum is activated during this planning. Through connections with language
areas and the prefrontal cortex, the cerebellum also plays a role in language func-
tion and social cognition.
Hippocampal Circuitry and Activation
with the Cortex
The hippocampus is part of a brain circuit that encodes declarative and spatial
memories (refer to Chapter15). The hippocampus receives inputs from the entire
neocortex (through a region called the entorhinal cortex) and subcortical areas and
sends outputs back to the neocortex and other regions.
Within the hippocampal circuit, incoming activity to the dentate gyrus neurons
activates CA3 neurons, which then activate CA1 neurons. This leads to changes in
synaptic transmission in CA1 neurons, called synaptic plasticity, which is an under-
pinning of learning and memory. However, the hippocampus is only a temporary
memory store, and it must transfer information from CA1 neurons to neocortical
circuits for long-term storage.
Current models propose that during memory consolidation, the hippocampus
sends signals back to the neocortical areas that were originally activated by the
event itself. This signaling most likely occurs during rehearsal and sleep.
The result of the signaling is that the memories, which were stored temporarily
in the hippocampus, produce long-term changes in the neocortical areas and
circuits that were activated during the original stimulus.
The Amygdala Orbitofrontal Cortex Loop
Next to the hippocampus is the amygdala. The amygdala connects with the neo-
cortex, and like the hippocampus it’s implicated in encoding long-term memory.
The amygdala is particularly involved with processing stimuli and encoding
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368 PART 4 The Part of Tens
memories for experiences that have high emotional salience, such as danger and
fear. Chapters10 and15 describe the amygdala functions and circuitry.
The amygdala projects to the orbitofrontal cortex (an area within the prefrontal
cortex), as well as to the hypothalamus and other brain areas associated with
emotional reactions to stimuli. The orbitofrontal cortex is involved in sensory
integration, modulation of visceral reactions, and learning for prediction and
decision-making for emotional behaviors.
Researchers have studied the encoding of fear memories in animal models of fear
conditioning, where a context that predicts some painful outcome becomes asso-
ciated with the outcome and itself induces fear. The amygdala also receives con-
nections back from the orbitofrontal cortex and medial prefrontal cortex, most
likely for the regulation of emotion.
Obsessive-compulsive disorder (OCD) and post-traumatic stress disorder (PTSD)
have been linked to changes in the amygdala. Lesions to the anterior temporal
lobe that include the amygdala can produce hypo-emotionality, loss of fear, and
hypersexuality. Imaging studies showed a signicant decrease in amygdala volume
in individuals with antisocial personality disorder.
The Conscious Triangle
The thalamus, frontal lobe, and parietal lobe are almost always activated during con-
sciousness. Moreover, their activity is also likely to be highly coupled, as coherent
EEG rhythms and synchronous neural ring show. Refer to Chapter17 for more
information on the current hypothesis about the basis of consciousness. So why
these three brain regions?
The prefrontal cortex is the main brain area responsible for working memory,
which is the mental representation of what’s most important about your current
situation. It’s also involved in attention and abstract thought. The orbitofrontal,
anterior cingulate, and insular cortices may support emotional aspects of
consciousness.
The thalamus links the neocortex to subcortical structures and the areas of neo-
cortex together. By controlling the activity of the neocortex, the thalamus appears
to control what emerges from the vast amount of subconscious neural activity as
conscious, accessible thought. One hypothesis is that conscious brain activity
requires a time and magnitude threshold. Enough neurons must be active over a
large enough region of the brain for at least half a second for conscious awareness
to occur and persist.
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CHAPTER 19 Ten Most Important Neural Circuits 369
The posterior parietal cortex is the seat of the convergence of somatosensory,
visual, and auditory sensory streams that allow you to locate yourself in the world
and be aware of this location. The posterior parietal cortex is connected to the
prefrontal cortex in the central executive network. Together these areas provide a
representation of the context of where and why you’re here. These areas are regu-
lated by the thalamus. Activity in this network may be what crosses the threshold
required for consciousness.
The Basal Ganglia Thalamus Loop
The basal ganglia are controllers of behavior, using the neocortex to generate
alternative motor plans and the cerebellum to help carry them out (refer to
Chapter8). The basal ganglia receive inputs to their outer nuclei, the caudate and
putamen (together called the striatum). The striatum projects to the inner nuclei,
mainly the globus pallidus. The output of the basal ganglia is chiey from its inner
nuclei to the motor nuclei of the thalamus that connects to the motor cortex.
Activity in the basal ganglia increases or decreases the thalamic output to motor
areas that generate commands for producing motor behavior. The other nuclei of
the basal ganglia, the substantia nigra and subthalamic nuclei, perform a crucial
modulatory role in this system.
The basal ganglia select current actions across an entire behavioral actionsequence
that is typically initiated in the prefrontal cortex. The basal ganglia receive inputs
about the state of the body with respect to the current goal being sought from
sensory cortices and the cerebellum, primarily through the thalamus. The basal
ganglia select among behavioral alternatives along the entire motor sequence by
projecting to the thalamus that then projects to appropriate regions of the motor
cortex to choose the next action in the sequence.
The Cingulate and Pulvinar Executives
The anterior cingulate cortex (ACC) lies on the medial surface of each frontal lobe
and plays key roles in cognitive, motor, and emotional processing (refer to
Chapter8 and16). The ACC comprises the anterior half of the cingulate gyrus. It’s
mesocortex, a phylogenetically earlier type of cortex that evolved in the earliest
mammals. The ACC receives inputs from the thalamus and the neocortex and
projects to the entorhinal cortex, the input region for the hippocampus.
The ACC controls neural processing throughout the neocortex, allocating this pro-
cessing according to task demands, making it a modulator of executive function.
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370 PART 4 The Part of Tens
The ACC is implicated in subjective experience associated with consciousness,
particularly consciousness of pain. It’s also one of the brain regions underlying
empathy. The ACC, together with the insula, is part of the salience network, a
network involved in detecting and determining the importance of external and
internal stimuli.
The posterior cingulate cortex also interacts with the prefrontal cortex, as part of the
default mode network. This network is active when you’re at wakeful rest when
daydreaming, thinking about yourself or others, or remembering the past.
The pulvinar (see Chapter 8), a nucleus in the most posterior region of the
thalamus, has widespread connections with all the neocortex. It’s an important
mediator of attention, particularly visual attention and eye movements associated
with it. The pulvinar integrates vision and goal pursuit to generate context-
specic motor responses to specic visual stimuli and ignoring irrelevant
visual stimuli.
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