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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 171
nucleus (CN IV) is located in the inferior colliculus and also controls eye
movements.
Relaying and localizing sound
Auditory axons from the cochlear nucleus project to the superior olive. Neurons in
the superior olive project to the inferior colliculus (see Figure9-7). Neurons of the
inferior colliculus project to neurons in the auditory region of the thalamus.
Noting other functions
The red nucleus in the midbrain is the origin of the rubrospinal tract, a minor
control pathway for voluntary movement of upper limbs. The periaqueductal gray
has a role in pain processing, vocalizations, defensive behavior, and autonomic
functions.
The midbrain includes the substantia nigra and ventral tegmental area (VTA) that
contain dopaminergic neurons. The substantia nigra is heavily connected with the
basal ganglia in motor control. The VTA and other nuclei in the midbrain, such as
the locus coeruleus, which contains noradrenergic neurons, and raphe nuclei,
which contain serotonergic neurons, make extensive but diuse modulatory pro-
jections throughout the neocortex. These projections are involved in setting basic
brain states, such as what you’re attending to, alertness, drives, and goals.
FIGURE9-7:
Midbrain
superior and
inferior colliculi.
© John Wiley & Sons, Inc.
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172 PART 2 Controlling Muscles, Movement, and Survival
Recently, addiction researchers have focused considerable attention on the mid-
brain VTA. This nucleus sends dopaminergic axons to the nucleus accumbens,
which projects to the prefrontal cortex. This circuit has been shown to be impor-
tant in motivation, reward, and addiction.
Counting on the cranial nerves
The eyes, ears, nose, and tongue are special sense organs. The 12 cranial nerves
are involved in somatic, visceral, and special sensation and control of the somatic
and autonomic motor systems. Unlike the spinal nerves that are all mixed nerves,
some cranial nerves have dedicated sensory or motor functions and only four are
mixed nerves.
The cranial nerves are considered part of the PNS, except for the olfactory nerve
(I) and optic nerve (II). Cranial nerve III through XII originate in the brainstem
and travel outside the CNS.The olfactory and optic originate in and/or travel
through the CNS.The body has two of each cranial nerve, and the nerves are gen-
erally bilaterally symmetrical about the midline.
The 12 cranial nerves are as follows:
»
Olfactory nerve (I): Transmits the sense of smell from the nose. It terminates
in the olfactory bulb.
»
Optic nerve (II): Carries messages from retinal ganglion cells to the lateral
geniculate nucleus of the thalamus, the superior colliculus, and other retinal
recipient zones.
»
Oculomotor nerve (III): Motor nerve for controlling eye movements and
pupil dilation. Axons of this nerve originate in the oculomotor nucleus and
Edinger–Westphal nucleus.
»
Trochlear nerve (IV): A motor nerve that emanates from the dorsal midbrain
trochlear nucleus and controls the superior oblique eye muscles.
»
Trigeminal nerve (V): Sensory input comes from the face, and motor output
controls chewing. It originates in the trigeminal nucleus of the pons and the
spinal and mesencephalic trigeminal nucleus. The motor component comes
from the trigeminal motor nucleus.
»
Abducens nerve (VI): A motor nerve originating from the abducens nucleus.
It synapses on the lateral rectus eye muscle.
»
Facial nerve (VII): Has sensory and motor axons associated with the facial
nucleus, nucleus of the solitary tract, and superior salivary nuclei. Facial
expressions are controlled by motor innervation of this nerve. Taste
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CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 173
information from the front of the tongue also reaches the brain by this nerve.
Motor output includes control of the salivary and lacrimal glands.
»
Acoustic (vestibulocochlear) nerve (VIII): A sensory nerve that transmits
auditory and vestibular information to the brain from the spiral ganglion and
Scarpa’s ganglion. Its axons terminate in the cochlear and vestibular nuclei.
»
Glossopharyngeal nerve (IX): It transmits taste information from the back of
the tongue to the medulla. Axons in the nerve also come from the nucleus
ambiguous and inferior salivary nucleus. Motor output controls the secretion
of the parotid gland.
»
Vagus nerve (X): Originates in the posterolateral sulcus of the medulla,
nucleus ambiguus, dorsal vagal nucleus, and solitary nucleus. Motor output
controls laryngeal and pharyngeal muscles in the throat. It has a parasympa-
thetic output to control mucous membranes in the thorax and abdomen.
Taste sensory information from the epiglottis travels to the brain by the
vagus. The vagus is also crucial for controlling the vocal cords.
»
Accessory nerve (XI): Is a motor nerve originating mostly from the nucleus
ambiguus and spinal accessory nucleus. This nerve innervates the sternocleido-
mastoid and trapezius muscles that help to move the neck and turn the head.
»
Hypoglossal nerve (XII): Is primarily motor and originates in the medulla
hypoglossal nucleus. It controls the tongue and is important for speaking
and swallowing.
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CHAPTER 10 Maintaining the Internal Environment 175
Chapter10
Maintaining the Internal
Environment
T
he human brain evolved from primates, which evolved from mammals,
which, in turn, evolved from brains of lower vertebrates like amphibians.
The brain and its parts have many complex functions, besides allowing you
to think deep thoughts. The brain integrates spinal cord control with sensory
information from the eyes and ears, allowing you to keep your balance. It creates
and retains long-term memories and produces the emotions and motivations
behind behaviors.
As if that weren’t enough, the brain is always working behind the scenes to con-
trol functions that are essential to life, like breathing and the regulation of heart
rate, blood pressure, and temperature. It also generates the rhythms of the body’s
internal clock, telling you when to sleep and when to wake. This chapter explores
all these functions and more.
IN THIS CHAPTER
»
Responding with the autonomic
nervous systems
»
Controlling sleeping and other body
functions
»
Feeling and thinking through the
limbic system
»
Interacting with older brain areas
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176 PART 2 Controlling Muscles, Movement, and Survival
Finding the Right Tone: The Autonomic
Nervous System
Your body does many things of which you’re not directly conscious. Breathing,
swallowing, and coughing are automatic reex responses that involve brain stem
control of skeletal muscles (see Chapter9). Regulation of blood ow, body tem-
perature, breathing rate, and circadian rhythms (your body’s internal clock)
involves your autonomic nervous system.
The body’s ability to regulate its internal physiological processes— such as body
temperature and rate of blood ow— and keep them stable and constant is called
homeostasis. These homeostatic functions are necessary for life. Homeostatic
functions of the nervous system evolved before all the brain’s higher cognitive
abilities. Even invertebrates, like insects, maintain internal homeostasis. These
sections discuss the divisions and functions of the autonomic nervous system, a
major regulator of homeostasis.
Subdividing the autonomic nervous system
The autonomic nervous system has two main subdivisions — sympathetic and
parasympathetic — which are, in a large sense, antagonistic to each other.
Both systems are constantly operating but their activities can be ramped up or
damped down:
»
The sympathetic system: This ght-or-ight system regulates your body for
fast and enduring physical action in focused or emergency situations. It
involves diverting blood supply from digestion to the muscles, mobilizing
glucose, dilating the pupil, and increasing heart rate and blood pressure.
»
The parasympathetic system: This rest and digest system does the opposite
of the sympathetic system. It’s the system that reduces blood supply to
skeletal muscle and enables digestion, immune responses, wound healing,
responses for reproduction, and other types of body maintenance.
See Figure10-1 for some of the functions of these two dierent branches of the
autonomic nervous system.
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CHAPTER 10 Maintaining the Internal Environment 177
Why does the autonomic nervous system have this dual, antagonistic structure?
Your ancestors once had to chase down their meals, often in dangerous situations,
before they could eat. The sympathetic (ght-or-ight) system increased their
heart rate and respiration for high activity, increasing blood sugar and diverting
blood ow away from the digestive system to skeletal muscles. After they cap-
tured their meal and avoided injury, their parasympathetic systems slowed down
their overall metabolism and directed blood ow to their digestive systems, allow-
ing the extraction of nutrients, and enhancing body repair and reproduction. All
vertebrate animals have this dual system.
The sympathetic and the parasympathetic systems involve a type of motor neuron
called a preganglionic neuron, with cell bodies located in spinal cord segments and
brainstem. Preganglionic neurons extend their axons to control postganglionic
neurons whose cell bodies are located in autonomic ganglia in the body or head.
Getting ready for action: The sympathetic system
The need to engage in strenuous, immediate, and enduring activity and deal with
emergency situations activates the sympathetic division. This system originates
with cholinergic preganglionic neurons located in the spinal cord thoracic and
lumbar segments (T1 to L2), also called the thoracolumbar division. In the spinal
cord, these motor neurons are located in the lateral horn and are called general
FIGURE10-1:
The autonomic
nervous system.
© John Wiley & Sons, Inc.
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178 PART 2 Controlling Muscles, Movement, and Survival
visceral eerents. These neurons synapse (releasing acetylcholine) with neurons in
sympathetic ganglia, clusters of neurons located outside the spinal cord often run-
ning roughly parallel to it.
Some sympathetic preganglionic neurons project their axons to chroman cells
in the adrenal medulla (part of the adrenal glands on the top of the kidneys). When
stimulated, the chroman cells release epinephrine and norepinephrine into the
bloodstream.
Postganglionic neurons in sympathetic ganglia extend their axons and synapse on
target organs such as the heart, lungs, blood vessels, and digestive tract. The
majority of postganglionic neurons use norepinephrine (noradrenaline) as their
neurotransmitter. The tissues express dierent types of adrenergic receptors that
can bind both norepinephrine and epinephrine. They’re metabotropic (G protein-
coupled) receptors that control ion channels and/or produce second messengers
that mediate their responses (refer to Chapter5).
Taking care of yourself: The parasympathetic
system
The parasympathetic system is active during rest, enhancing relaxation and
digestion and decreasing respiration and heart rate. Preganglionic parasympa-
thetic neurons are located in the brainstem (cranial nerves III, VII, IX, X) and the
sacral spinal cord (S2 through S4), also called the craniosacral division. These pre-
ganglionic neurons also release acetylcholine.
They extend their axons over long distances to reach postganglionic neurons in
ganglia located near their target organs. Postganglionic parasympathetic neurons
release acetylcholine. Their target organs express muscarinic receptors, a metabo-
tropic (G protein-coupled) receptor type for acetylcholine (refer to Chapter5).
Inputting to the autonomic nervous system
The autonomic nervous system continuously senses conditions within the body
using visceral sensory neurons that innervate blood vessels and internal organs.
They detect levels of chemicals and metabolites, gases, and other conditions
important for body function. Here are some mechanoreceptors:
»
Chemoreceptors sense levels of oxygen, carbon dioxide, and sugar.
»
Baroreceptors in blood vessels detect arterial pressure.
»
Thermoreceptors detect temperature.
»
Other mechanoreceptors detect the stretch of organs.
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CHAPTER 10 Maintaining the Internal Environment 179
Visceral pain receptors sense signals that indicate damage to tissue or organs
(refer to the next section for more information).
The cell bodies of visceral sensory neurons are located in dorsal root ganglia and
sensory ganglia of cranial nerves. Visceral sensory information is transmitted to
the thalamus, brainstem, or hypothalamus. Conscious perception of visceral states
is relayed from the thalamus to the insula where the brain interprets feelings of
hunger, fullness, internal pain, or nausea. Sensory input to the brainstem or
hypothalamus is integrated to control autonomic motor output. The output from
the autonomic system is an unconscious response. However you can be conscious
of some autonomic eects such as your heart beating faster.
Responding with visceral reexes
Visceral reexes include simple spinal reexes in which sensory neurons activate
spinal interneurons, which in turn activate preganglionic autonomic neurons. An
example is the micturition (urination) reex in which the bladder’s smooth muscle
contracts when it’s lled with urine.
Visceral reexes can also involve cranial nerves that transmit to brainstem nuclei.
An example is the baroreceptor reex. When blood pressure is elevated, barore-
ceptors in the carotid sinus, where the internal and external carotid arteries meet,
are activated. They transmit visceral sensory information along the glossopha-
ryngeal nerve to the brainstem. This input is integrated in the cardiac center in
the medulla and stimulates the vagus nerve output to the heart. This parasympa-
thetic response decreases heart rate, which decreases blood pressure.
The visceral sensory region of the brainstem is called the nucleus of the solitary
tract, also called the nucleus tractus solitarius (NTS)— refer to Chapter9. It receives
all visceral aerents. Through projections with cardiorespiratory and gastrointes-
tinal regulatory centers, it’s an essential part of the regulatory centers for internal
homeostasis. It’s also part of several brainstem reexes.
Controlling the Autonomic
Nervous System
Control of the autonomic nervous system is provided by the brainstem, spinal
cord, and hypothalamus. We discuss this system in greater detail here.
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180 PART 2 Controlling Muscles, Movement, and Survival
Regulating autonomic output with
the brainstem and spinal cord
In the brainstem, the reticular formation has the most direct inuence over auto-
nomic functions. The reticular formation is a phylogenetically ancient area with
nearly a hundred interconnected nuclei. The medulla contains cardiac centers that
regulate heart rate, the vasomotor center that regulates blood vessel diameter,
and the nuclei that control digestion. We discuss other key functions of the retic-
ular formation in the section “Staying Alive with Reticular Formation” later in
this chapter. In the midbrain, the periaqueductal gray controls autonomic
responses, including the sympathetic fear response to threat.
Control of autonomic functions by interneurons in the spinal cord involves vis-
ceral spinal reexes described in the section “Responding with visceral reexes”
earlier in this chapter. Although the urination reex involves a reex arc within
the spinal cord, it can be consciously inhibited by the brain, which voluntarily
controls the urinary output.
Integrating autonomic control
by the hypothalamus
The hypothalamus is located below the left and right thalami and above the mid-
brain. It’s composed of 11 nuclei and integrates inputs from numerous brain
regions to mediate essential homeostatic functions. It’s a central controller of the
autonomic nervous system, controlling heartbeat, blood pressure, and digestion.
It senses body temperature, hunger, and thirst and regulates behavioral responses
to these. It connects the brain to the endocrine system by regulating output from
the pituitary gland. And it’s involved in circadian rhythms, sleep, and arousal.
The hypothalamus receives inputs from the bloodstream and receives neuronal
input from the brainstem reticular formation, NTS, ventrolateral medulla, limbic
system, septal nucleus, hippocampus, olfactory bulb, and cerebral cortex.
Hypothalamic nuclei in posterior and lateral regions control sympathetic
responses. Nuclei in anterior and medial regions control parasympathetic actions.
Neurons in these nuclei send axons to preganglionic neurons in the spinal cord
and brainstem, providing some direct control of autonomic output. Other nuclei
provide input to the reticular formation and periaqueductal gray to control auto-
nomic output indirectly.
The hypothalamus receives inputs from the basal forebrain and limbic system,
including the hippocampus and amygdala. The amygdala is a key limbic area for
emotions, including fear. The amygdala provides direct input to the hypothalamus
and also to the periaqueductal gray for fear-related behaviors.
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