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CHAPTER 10 Maintaining the Internal Environment 181
Because part of it lacks a blood-brain barrier, the hypothalamus is exposed to
circulating blood and responds to hormones including insulin, angiotensin, and
pituitary hormones. Nuclei within the hypothalamus also respond to blood osmo-
larity (solute concentration), plasma glucose concentration, and hormones
released by the GI tract to mediate responses to thirst and hunger.
For regulation of body temperature, thermoreceptors in the hypothalamus send
signals to autonomic motor neurons to stimulate heat production to raise the
temperature or for sweating and vasodilation to cool it. It also creates fevers in
response to infection.
The olfactory input— information about smells that come in through the nose—
to the hypothalamus can mediate aspects of sexual behavior. Consider the case of
pheromones, which are odorants animals produce that have unconscious behav-
ioral (often sexual) eects in others. (You generally aren’t even conscious of
detecting these pheromones.)
Linking the brain to the endocrine system
The endocrine system is the system of glands that secrete hormones directly into
the bloodstream to maintain homeostasis. It’s controlled by the autonomic ner-
vous system and the hypothalamus. Groups of glands that signal each other are
called an axis, such as the hypothalamic-pituitary-adrenal axis.
Examples of autonomic control of the endocrine system are the pancreas and
adrenal medulla. Sympathetic activation decreases insulin release from the pan-
creas, while parasympathetic activation increases it. Sympathetic activation
increases epinephrine and norepinephrine secretion from the adrenal medulla.
The hypothalamus controls the pituitary gland. The posterior pituitary is
essentially a group of axons from hypothalamic neurons that directly secrete
oxytocin and vasopressin into the blood. Vasopressin regulates the kidneys to help
reabsorb and conserve essential ions and proteins in the blood. It also constricts
the peripheral vasculature. Oxytocin is essential for female reproduction, trigger-
ing contraction of the uterus during childbirth, and maternal behaviors such as
pair bonding.
Other neurons in the hypothalamus send axons to the anterior pituitary where
they secrete neurohormones called hypothalamic-releasing hormones. These
stimulate the neurosecretory cells in the anterior pituitary gland to release spe-
cic pituitary hormones into the blood. The anterior pituitary releases seven hor-
mones involved in regulating growth during development, reproduction, control
of stress and thyroid hormone levels, and homeostasis.
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182 PART 2 Controlling Muscles, Movement, and Survival
Oscillating with brain waves
Neural oscillations, orbrainwaves, are rhythmic or repetitive patterns of neural
activity in theCNS that can be measured by EEG.EEG rhythms are categorized in
frequency bands:
»
Delta: Up to 4 Hertz (Hz), or cycles per second. These are the slowest waves
and are associated with the deepest levels of sleep.
»
Theta: 4 to 8Hz. These waves are associated with a highly relaxed state such
as while daydreaming during wakefulness and are present during lighter
stages of sleep.
»
Alpha: 8 to 13Hz. These waves are present during wakefulness, when in a
restful and relaxed state as when your eyes are closed. They also occur at
dierent stages of the wake-sleep cycle. Mu waves are a type of alpha wave
specic to the motor cortex.
»
Beta: 13 to 30Hz. These high frequency waves occur in the awake and alert
state during your daily activities.
»
Gamma: 30 to 100Hz. These are the highest frequency waves that are
present when you’re actively thinking, remembering and concentrating.
Neural tissue may generate brainwaves by activity within individual neurons,
through interactions among neurons, or both. Brainwaves are important for per-
ception, for the generation of rhythmic motor output, for information transfer
mechanisms, and for feature binding. Feature binding is the idea that dierent
features of a particular object, such as size, shape, and location, are represented in
neural ring at particular oscillation phases to create a person’s perception of the
object. Oscillatory brain activity is an important mechanism underlying
consciousness.
Sleeping and waking: Circadian rhythms
Another important nucleus in the hypothalamus is the suprachiasmatic nucleus (SCN).
This nucleus controls circadian rhythms, the sleep/wake cycle. In sleep, conscious-
ness is absent or severely reduced, but in a dierent way than during hibernation
or a coma. All animals sleep. The functions of sleep include restoration and repair
of cells and tissues; removal and replacement of damaged proteins; regulation of
metabolism, hormones and immune functions; and memory consolidation.
Synchronizing your intrinsic clocks with external light involves a special class of
retinal ganglion cells that project to the SCN.These cells have their own visually
activated G protein-coupled receptors that allows them to directly respond to
light.
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CHAPTER 10 Maintaining the Internal Environment 183
Sleep cycles through several characteristic stages, each of which has unique prop-
erties. If you observe someone sleeping, you can see periods in which their eyes
are moving nearly continuously, whereas during other periods the eyes move.
This distinguishes rapid eye movement (REM) sleep from non-REM (NREM)
sleep.
The predominant brain waves that can be detected with electroencephalography
(EEG) also dier between sleep and wakefulness, and between dierent sleep
phases. Figure 10-2 shows a typical night’s sleep phase plot, which is called a
hypnogram.
Dreaming with REM sleep
REM sleep is the most like being awake, and most dreams occur in REM sleep.
These high frequencies are similar to those seen in waking EEGs and may repre-
sent perceptual and motor experiences associated with dreaming. Infants not only
sleep more than adults, but they also have a larger proportion of REM sleep. No
one is sure why people dream. One theory is that synapses that are activated dur-
ing waking activities are pared back (in a process known as normalization) so that
the brain is ready for renewed synapse growth during memory encoding. REM
sleep has shown to be important for formation of long-term declarative memories
(see Chapter15).
REM sleep is characterized by high frequencies in the EEG.These high frequencies
are associated with motor activities that occur in dreams, such as running, that
are associated with frontal-lobe activity. However, the motor commands from the
frontal lobe are blocked in the spinal cord so that actual movement doesn’t occur.
FIGURE10-2:
A hypnogram.
© John Wiley & Sons, Inc.
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184 PART 2 Controlling Muscles, Movement, and Survival
Going through NREM sleep
When you rst fall asleep, you transition through the NREM stages to deep sleep.
You then move up through those stages to REM sleep. Often after you descend
through the phases in order, N1 to N4, the sleep then oscillates irregularly between
phases after that. Late in the sleep period the phase sequences tend to be shal-
lower, sometimes not reaching N3 or N4, with some brief transitions from REM to
momentary wakefulness. Slow-wave sleep consists of N3 of non-REM sleep.
Slow-wave sleep has also been shown to be important for formation of long-term
declarative memory (refer to Chapter15).
Sleep phases are characterized by distinct changes in EEG patterns, examples of
which you can see in Figure10-3. Wakefulness is associated with beta rhythms in
the EEG.Mental relaxation and meditation shift the EEG rhythms from beta to
alpha. In the earliest stage of sleep, characteristic slow eye movements occur,
technically called slow rolling eye movements (SREMs). About this time, alpha waves
nearly disappear from the EEG, and theta waves begin to predominate. Stage N2
of sleep is also dominated by theta waves, but they change to delta waves in N3
and N4. N4 is the deepest stage of sleep, from which it’s the most dicult to
awaken.
FIGURE10- 3:
EEG sleep
patterns during
dierent stages
of sleep.
© John Wiley & Sons, Inc.
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CHAPTER 10 Maintaining the Internal Environment 185
Staying Alive with the Reticular Formation
The reticular formation is an ancient brainstem area that courses throughout much
of the subcortex. The word reticular is derived from the Latin word for net. It com-
prises more than 100 neural circuits that control essential body functions and
extends up from spinal projections to the medulla, through the midbrain and
pons, to reticular area of the thalamus. The reticular formation interacts with
numerous brain areas to control life processes, such as regulation of breathing
and heart rate, temperature, and wakefulness. Damage to the reticular formation
typically results in coma or death. Here we examine reticular formation in greater
detail.
Separating the reticular formation
Some motor neurons in the primary motor cortex send axon branches to nuclei in
the reticular formation in the medulla. These reticular nuclei are the origin of the
reticulospinal tracts that send axons to the spinal cord motor areas to maintain bal-
ance during body movements. Reticular nuclei also receive visual and auditory
inputs relayed from the cerebellum for motor coordination.
The reticular formation in the medulla can be divided into groups of nuclei that
form three major columns. These nuclei send axonal projections through the
medulla and into the midbrain. The columns are as follows:
»
Median column: Also known as the Raphe nuclei, it sends axons diusely
through the neocortex that release serotonin.
»
Medial column: Also known as the magnocellular column, it sends projections
to the medulla and midbrain and may be involved in sleep/wakefulness
control.
»
Lateral column: Also known as the parvocellular column, it projects to the
medulla and midbrain. Its functions aren’t well characterized.
Controlling functions
The reticular formation controls the cardiovascular system via its cardiac and vas-
omotor centers in the medulla. It’s also involved in pain modulation. Some pain
signals from the body are relayed by medulla nuclei that project to the cortex. In
addition, descending axons from cortical pain perception areas provide input to
medulla nuclei. These nuclei extend axons to the spinal cord that can inhibit
transmission of some pain signals to the brain.
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186 PART 2 Controlling Muscles, Movement, and Survival
Ascending projections from the reticular formation to the thalamus and cortex
control or gate the sensory signals reaching these structures. The reticular forma-
tion is a gateway for essential functions of states of consciousness and awareness
versus sleep. Other motor nuclei in the reticular formation control the ability of
the eyes to track objects and central pattern generators that coordinate muscles
for breathing and swallowing.
Modulating activity: Reticular
activating system
Reticular formation nuclei that modulate activity of the cerebral cortex are part of
the reticular activating system (RAS). The RAS is a neural circuit by which the brain-
stem interacts with the cortex through relays in thalamic nuclei. The RAS also
includes the dorsal hypothalamus. The RAS midbrain reticular formation controls
transitions from relaxed wakefulness to focused attention.
The main neurotransmitters used by the RAS are acetylcholine and noradrenaline;
they operate in an antagonistic manner to regulate states of awareness and con-
sciousness by controlling thalamo-cortical activity. Projections that release ace-
tylcholine from the RAS to the thalamus, basal forebrain, substantia nigra, and
cerebellum are increased during wakefulness and REM sleep.
Most of the noradrenaline output of the RAS is from the locus coeruleus in the
pons. These neurons project to many of the same areas to which the cholinergic
RAS system projects, but with opposite eects: They’re active during waking and
slow-wave sleep, but not during REM sleep. The locus coeruleus also projects to
the cerebral cortex and the spinal cord.
Continuing through the thalamus
and cortex
Extensions of the RAS project to the thalamus in the thalamic reticular nucleus
and peri-geniculate nucleus. Thalamic reticular nuclei receive input from dorsal
thalamic nuclei and the cerebral cortex, in addition to inputs from brainstem
reticular nuclei.
The function of the thalamic reticular areas appears to be to modulate the infor-
mation projected from other nuclei in the thalamus to the cortex. For example,
although you may be concentrating on reading a sign in front of you, rapid move-
ment toward you in your peripheral vision may pull your attention toward the
movement. This would also be true if you heard a loud noise behind you. The
reticular formation controls not only whether you’re awake or asleep, but also
important aspects of what you pay attention to while awake.
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CHAPTER 10 Maintaining the Internal Environment 187
Controlling Your Motives:
The Limbic System
The word limbic derives from the Latin limbus or “edge.” It describesanatomical
structures that border on the edges of the thalamus. The term limbic system refers
to a group of nuclei and tracts above the brainstem and below the neocortex (sub-
cortical), originally postulated to control drives and instincts. It’s phylogeneti-
cally very old and exists in nonmammalian vertebrates that perform complex
behaviors with little or no neocortex. They use the limbic system and basal ganglia
for neural control of behavior.
As originally conceived, the limbic system consists of the cingulate cortex, hip-
pocampal complex, amygdala, septum, anterior thalamic nuclei, limbic cortex,
and hypothalamus. Most regions of the limbic system have extensive connections
beyond the limbic system and functions beyond emotional control of behavior.
Also, the limbic system doesn’t have exclusive rights to emotion. Emotions are
contributed by and controlled by other regions of the brain.
The use of the term limbic system is controversial among neurobiologists. Lumping
these brain structures together as a limbic system probably isn’t valid. Still, the
idea of an evolutionarily older, subcortical processing system whose output is an
emotional state that aects behavior still has some validity, and this term is still
used today.
Evolution tried two other types of cortex before settling on building the majority
of the mammalian brain from neocortex. These are allocortex and mesocortex,
shown in Figure10-4. These evolutionarily older types of cortex are associated
with the limbic system. The limbic system is a collection of structures from the
midbrain, diencephalon, allocortex, and mesocortex.
Going from allocortex to mesocortex
to neocortex
Allocortex and mesocortex predate the neocortex in evolution. But these older
cortical areas are crucial for brain function even in animals like humans who have
a very large neocortex. Allocortex is the oldest cortex, having three to ve layers.
It’s the type of brain structure in the hippocampal complex (called archicortex) and
primary olfactory cortex (called paleocortex). Mesocortex is a six-layered brain
structure in what’s called the paralimbic region between neocortex and allocortex.
It includes the cingulate gyrus, insula, some of the orbitofrontal cortex, and the
parahippocampal gyrus.
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188 PART 2 Controlling Muscles, Movement, and Survival
FIGURE10-4:
Neocortex,
mesocortex, and
allocortex.
© John Wiley & Sons, Inc.
EMOTIONAL PROCESSING
Although science ction often portrays emotionless characters such as Mr. Spock as
superior beings, would we as humans really be better o without emotions? Why would
we have evolved emotions if they only get us into trouble? The answer is that emotions
allow us to avoid trouble far more often than they cause it.
Remember: Emotions existed before reason. Feelings— the content of emotions—
are the brain’s way of telling us if something is good or bad, and whether it’s important,
without having to go through a long and perhaps faulty chain of reasoning about the
results of some action (or inaction). Emotions are the means by which the part of your
brain that processes incoming sensory information communicates likely outcomes to
the part of your brain that executes actions. This communication depends on a set of
sub-neocortical structures called the limbic system that include brain regions that are
cortex, but not neocortex.
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CHAPTER 10 Maintaining the Internal Environment 189
Organizing thoughts and activities
with the cingulate gyrus
One of the original limbic areas is the cingulate gyrus, a mesocortical area that
lies mostly between the corpus callosum ber tract and the neocortex (see
Figure10-5).
The cingulate gyrus receives inputs from the thalamus and the neocortex, and
projects to the entorhinal cortex around the hippocampus via the cingulum. One
function proposed for the cingulate gyrus is to allocate processing among other
cortical areas, making it a mediator of executive function.
The cingulate gyrus is typically divided into anterior and posterior regions, with
the dividing line above the anterior thalamus:
»
Anterior cingulate cortex: This area appears to be involved in motivation,
the reward anticipation, and the perception of pain or even the expectation of
pain. It’s also involved in attention allocation, decision making, and impulse
control. Imaging studies show that it’s activated when you make mistakes and
when you receive conicting information.
»
Posterior cingulate cortex: Less is known about this area, but it appears to
be involved in awareness and the allocation of working memory along with
the dorsolateral prefrontal cortex. It’s a central node of the default mode
network that’s active when you daydream. It has been implicated in pain
and episodic memory retrieval.
FIGURE10-5:
The limbic
system.
© John Wiley & Sons, Inc.
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190 PART 2 Controlling Muscles, Movement, and Survival
Creating memories with the hippocampus
You can remember the security code you’ve been texted for only about as long as
it takes to type it in, but you never forget your Social Security number (unless you
develop a form of dementia like Alzheimer’s disease). The hippocampus controls
the formation of long-term memories like facts and words, as well as what are
called episodic memories of events.
Short-term memory is used in what’s called working memory, mediated by the
lateral prefrontal cortex. Information can pass from short-term to long-term
memory if you’ve rehearsed it, you’re repeatedly exposed to, or it’s an important
fact or event. The hippocampus is required for the encoding of long-term declara-
tive memories for facts and events. Other types of memory such as procedural
memory for skills and habits are encoded by dierent regions of the brain.
Chapter15 describes the role of the hippocampus and mechanisms of memory.
The synapses between specic neurons in the hippocampus undergo synaptic
plasticity during memory encoding. To produce a long-term memory, the hippo-
campus signals back to the neurons in the neocortex that provided the informa-
tion to be associated and remembered. Neurons in the neocortex form their own
memory circuit that can be activated by future cues.
The output of the hippocampus is through a ber tract called the mbria. The
axons from the hippocampi on each side of the brain merge at the midline where
the tract is called the fornix (refer to Figure10-5). The fornix divides at the ante-
rior commissure. The anterior axon tract is called the precommissural fornix. It
projects to the nucleus accumbens, which is involved in reward and implicated in
addiction (see the section “Motivating with the basal ganglia” later in this chap-
ter) and to septal nuclei. Some fornix axons form the post-commissural fornix
and project through the hypothalamus to the mammillary bodies. These in turn
project to the anterior nuclei of the thalamus, which projects to the cingulate
cortex.
Feeling emotional with the
amygdala and beyond
The amygdala is a central structure for detection and integration of salient emo-
tional stimuli and contributes to emotion stimulated responses. It’s involved in
modifying responses in sensory cortex to favor relevant items. It recruits top-
down attention in the frontal-parietal cortex and contributes to hippocampal
dependent memory. It also drives autonomic responses by inputs to the hypothal-
amus and brainstem.
https://t.me/med1917
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