Neuronal Signaling and the Structure of the Nervous System
187
SECTION D SUMMARY
Central Nervous System: Brain
I. The brain is divided into six regions: cerebrum, diencephalon,
midbrain, pons, medulla oblongata, and cerebellum.
II. The cerebrum, made up of right and left cerebral hemispheres,
and the diencephalon together form the forebrain. The
cerebral cortex forms the outer shell of the cerebrum and is
divided into the parietal, frontal, occipital, and temporal lobes.
III. The diencephalon contains the thalamus and hypothalamus.
IV. The limbic system is a set of deep forebrain structures
associated with learning and emotion.
V. The cerebellum plays a role in posture, movement, and some
kinds of memory.
VI. The midbrain, pons, and medulla oblongata form the
brainstem, which contains the reticular formation.
Central Nervous System: Spinal Cord
I. The spinal cord is divided into two areas: central gray
matter, which contains nerve cell bodies and dendrites; and
white matter, which surrounds the gray matter and contains
myelinated axons organized into ascending or descending tracts.
II. The axons of the afferent and efferent neurons form the spinal
nerves.
Peripheral Nervous System
I. The peripheral nervous system consists of 43 paired nerves—
12 pairs of cranial nerves and 31 pairs of spinal nerves. Most
nerves contain the axons of both afferent and efferent neurons.
II. The efferent division of the peripheral nervous system is
divided into somatic and autonomic parts. The somatic fi bers
innervate skeletal muscle cells and release the neurotransmitter
acetylcholine.
Autonomic Nervous System
I. The autonomic nervous system innervates cardiac and smooth
muscle, glands, and gastrointestinal tract neurons. Each
autonomic pathway consists of a preganglionic neuron with its
cell body in the CNS and a postganglionic neuron with its cell
body in an autonomic ganglion outside the CNS.
II. The autonomic nervous system is divided into sympathetic
and parasympathetic components. The preganglionic neurons
in both the sympathetic and parasympathetic divisions
release acetylcholine; the postganglionic parasympathetic
neurons release mainly acetylcholine; and the postganglionic
sympathetics release mainly norepinephrine.
III. The adrenal medulla is a hormone-secreting part of the
sympathetic nervous system and secretes mainly epinephrine.
IV. Many effector organs that the autonomic nervous system
innervates receive dual innervation from the sympathetic and
parasympathetic division of the autonomic nervous system.
Blood Supply, Blood-Brain Barrier, and
Cerebrospinal Fluid
I. Inside the skull and vertebral column, the brain and spinal
cord are enclosed in and protected by the meninges.
II. Brain tissue depends on a continuous supply of glucose and
oxygen for metabolism.
Nicotine’s physiological effects are a complex result
of stimulation and desensitization of N-AChRs at these
diverse synapses. For example, at low doses nicotine
activates autonomic ganglia and stimulates the release of
catecholamines from the adrenal medulla. The sympathetic
components of these pathways dominate control of the
cardiovascular system under these conditions, and so
heart rate and blood pressure increase. Persistent high
blood pressure and increased work on the heart are part
of the reason that chronic nicotine use contributes to
cardiovascular disease. In the gastrointestinal system,
parasympathetic effects tend to dominate, leading to
activation of intestinal smooth muscle motor activity.
Brainstem control centers that regulate gastrointestinal
functions are also extremely sensitive to nicotine, and
vomiting or diarrhea can sometimes occur in individuals
who ingest high nicotine doses or in individuals who
have not been previously exposed to nicotine. At higher
doses of nicotine, the N-AChRs in these autonomic
pathways tend to desensitize and thus there is a depression
of all autonomic responses. At all doses of nicotine, the
neuromuscular junction receptors desensitize so rapidly that
the predominant effect on the musculature is relaxation.
Perhaps the most signifi cant effect of nicotine is its
stimulation of excitatory neurotransmitter release in the
central nervous system, particularly the release of dopamine
in the reward center pathways of the brain. These pathways
mediate pleasurable sensations associated with behaviors
that increase the survival of individuals and species, such
as feeding and sexual activity. Beginning just 7 seconds
after inhaling tobacco smoke, nicotine produces an overall
cognitive stimulation and euphoric sensation that strongly
reinforces the desire to smoke. Because of desensitization of
the N-AChRs, however, these effects wear off and the user
needs more nicotine to regain the pleasurable sensation. With
chronic use, a
tolerance
to nicotine develops such that it takes
progressively higher concentrations to achieve a given effect.
Overt
addiction
occurs when a smoker requires continuous
nicotine reinforcement just to feel “normal.” Nicotine carries
a higher risk of addiction than do most commonly used legal
and illegal drugs, such as alcohol, cocaine, and heroin. About
one-third of those who use nicotine become addicted, and a
variety of unpleasant withdrawal symptoms occur if nicotine
use is stopped, including: irritability, impatience, hostility,
anxiety, depressed mood, diffi culty concentrating, decreased
heart rate, increased appetite and weight gain. The addictive
power of nicotine most likely accounts for why more than 20
percent of adult Americans continue to smoke tobacco despite
the well-publicized fact that almost half a million people die
each year from its effects.
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