Neurons and How They Work (p. 586)
28.1
28.2
28.3
Evolution of the Animal Nervous System (p. 586; Figs. 28.1, 28.2)
A. Animals sense changes in the environment using sensory neurons, and react to those changes
using motor neurons; these two types of neurons are linked by a nervous system.
B. Association neurons (or interneurons) are located in the brain and spinal cord and are part of
the central nervous system.
C. The peripheral nervous system consists of neurons leading to and from the central nervous
system.
D. Invertebrate Nervous Systems
1. The simplest nervous systems provide reflexes and are found in the Cnidaria.
2. More complex nervous systems, such as those in the flatworms, provide associative
activities.
3. The evolutionary path to vertebrates shows more complex sensory mechanisms,
differentiation into central and peripheral nervous systems, differentiation of sensory
and motor neurons, increased complexity of association, and elaboration of the brain.
Neurons Generate Nerve Impulses (p. 588; Fig. 28.3)
A. Neurons
1. Neurons are the basic structural units of the nervous system, and each is made up of
three parts.
a. Dendrites are branching cellular extensions that detect impulses and bring them into
the cell.
b. The cell body is the expanded, central portion of the cell that houses the nucleus.
c. Leading away from the cell body is a long, slender cellular extension called an axon
that conveys information away from the cell to the next neuron.
2. Neuroglia are the helper cells that protect and nourish neurons.
3. Two types of neuroglial cells, Schwann cells and oligodendrocytes, wrap axons in
insulating material called a myelin sheath.
4. Each Schwann cell or oligodendrocyte wraps a portion of a neuron, and there are gaps
between adjacent Schwann cells, called nodes of Ranvier.
5. The nervous impulse jumps rapidly from one node to the next as it travels down the
neuron.
B. The Nerve Impulse
1. When not transmitting an impulse, a neuron is said to be polarized and in its resting
state.
2. In this state, it has an excess of positively charged ions on the outside of the cell
membrane and an excess of negative ions on the inside.
3. When something interrupts the resting potential and there is enough pressure or other
stimulus to disturb the cell membrane of a neuron, voltage-gated channels open up,
allowing a rush of sodium to the inside of the cell; it is now depolarized.
4. Sodium channels close rapidly, and sodium is pumped back out of the cell.
5. If the stimulus is great enough, it will set up a chain reaction of sodium channels
opening down the length of the neuron and generating a nerve impulse.
6. This chain reaction is referred to as reaching action potential, and a nerve impulse
travels the length of the nerve.
The Synapse (p. 590; Figs. 28.4, 28.5, 28.6)
A. At the junctions between two neurons lies a small space called the synapse.
B. A nervous impulse is conveyed from the presynaptic neuron to the postsynaptic neuron.
C. Neurotransmitters
1. Since the nervous impulse cannot travel across a synapse, the presynaptic neuron
releases a chemical called a neurotransmitter into the synapse.
2. The postsynaptic neuron has specific receptors in its cell membrane that can detect the
neurotransmitter.
3. Once received, special ion channels in the postsynaptic membrane open up, causing a
change in electrical charge across the membrane.
D. Kinds of Synapses
1.
2.
28.4
Synapses are considered the control switches of the nervous system.
Some synapses are excitatory, meaning that the receipt of a given neurotransmitter
generates action potential in the postsynaptic neuron.
3. Other synapses are inhibitory, and receipt of a different neurotransmitter inhibits the
impulse from traveling further.
4. A process called integration results when excitatory and inhibitory synapses
counterbalance one another.
E. Neurotransmitters and Their Function
1. Acetylcholine is the neurotransmitter released at the neuromuscular junction.
a. It forms excitatory synapses with skeletal muscle but has the opposite effect on
cardiac muscle.
b. Glycine and GABA are inhibitory neurotransmitters.
c. Biogenic amines, including norepinephrine, dopamine, serotonin, and epinephrine,
have various effects on the body.
Addictive Drugs Act on Chemical Synapses (p. 592; Fig. 28.7)
A. Neuromodulators
1. At times, additional neurotransmitters are released that modify the transmission of an
impulse at a synapse, either prolonging or inhibiting it.
2. These additional chemicals are called neuromodulators, and are responsible for mood
and emotion in the brain.
B. Drug Addiction
1. Addictive drugs, such as cocaine, often act as neuromodulators.
2. The brain adjusts its synapses to accommodate such drugs, and the result is
physiological dependence, or addiction.
C. Is Addiction to Smoking Cigarettes Drug Addiction?
1. Cigarette smokers find it difficult to quit smoking because they have become addicted to
nicotine, a powerful neuromodulator.
2. Addiction to nicotine occurs because the brain compensates for the many changes
nicotine induces by making other changes; eventually the newly coordinated system
requires nicotine to achieve an appropriate balance of nerve pathway activities.
The Central Nervous System (p. 594)
28.5
28.6
Evolution of the Vertebrate Brain (p. 594; Figs. 28.8, 28.9, 28.10)
A. The structure and function of the vertebrate brain have long been the subject of scientific
inquiry.
B. The Dominant Forebrain
C. The Expansion of the Cerebrum
How the Brain Works (p. 596; Figs. 28.11-28.14)
A. The Cerebrum Is the Control Center of the Brain
1. The cerebrum is the center for thought and association, and makes up about 85% of the
weight of the brain.
2. It is divided into right and left halves, called cerebral hemispheres.
3. Densely-packed neuron cell bodies lie in the thin cerebral cortex that covers the
cerebrum; it is here that much thinking occurs.
4. Deeper into the cerebrum are the myelinated nerve fibers that rapidly shuttle information
from one portion of the brain to another.
5. Bundles of neurons, called tracts, connect the two hemispheres.
6. The left side of the brain controls the right side of the body, and vice versa.
7. Different areas of the cerebral cortex control different body activities.
8. When brain blood vessels are clogged, it can lead to a stroke, and a portion of the brain
dies.
B. The Thalamus and Hypothalamus Process Information
1. Beneath the cerebrum are the thalamus and hypothalamus, which serve to process
information.
2.
28.7
The thalamus processes sensory information, including sound and balance, and sends
messages to the proper portion of the cerebrum.
3. The hypothalamus integrates the body's internal functions: it regulates heart rate, blood
pressure, the secretions of the pituitary, and many other organs.
4. An extensive network of neurons in the cerebrum and hypothalamus, called the limbic
system, controls hunger, thirst, pleasure, pain, and emotional drives.
C. The Cerebellum Coordinates Muscle Movements
1. The cerebellum, located at the back of the brain, is where the control of muscular
coordination, balance, and posture are located.
2. It is even better developed in birds because of the nature of their aerial acrobatics.
D. The Brain Stem Controls Vital Body Processes
1. The brain stem, including the midbrain, pons, and medulla oblongata, connects the brain
with the spinal cord. It controls breathing, heartbeat, and blood vessel diameter.
2. The reticular formation, a network of nerves, runs through the brain stem and connects
to other portions of the brain.
3. The reticular formation is responsible for shutting the brain down for sleep and for its
arousal when waking is needed.
E. Language and Other Higher Functions
1. The left hemisphere is the dominant one for language in 90% of right-handed people and
two-thirds of left-handers.
2. The dominant hemisphere is adept at sequential reasoning while the nondominant
hemisphere is adept at spatial reasoning, such as that which is needed for artwork and
music.
3. While the process of memory is not fully understood, there appear to be fundamental
differences between short-term and long-term memory.
4. Short-term memories are transient and are stored electrically in the form of a transient
neural excitation.
5. Long-term memories, by contrast, involve structural changes within certain neural
connections in the brain.
F. The Mechanism of Alzheimer’s Disease Is Still a Mystery
1. Two hypotheses have been proposed.
a. The nerve cells in the brain are killed from the outside in by abnormal production of
beta-amyloid protein (an external protein) causing plaque formation.
b. The nerve cells are killed from the inside out by an internal protein (tau) that causes
tangles inside of the nerve cell.
The Spinal Cord (p. 599; Figs. 28.15, 28.16)
A. The spinal cord begins at the brain stem and extends down through the vertebral column.
B. Sensory and motor nerve tracts run the length of the spinal cord, conveying messages at great
speed to the brain and back.
C. Spinal Cord Regeneration
1. In the past scientists have been unable to rejoin severed sections of the spinal cord.
2. With the discovery of fibroblast growth factor, neurobiologists have been at least partly
successful in gluing on nerves in rats with fibrin that had been mixed with fibroblast
growth factor.
3. This technique holds promise for future spinal cord repair.
The Peripheral Nervous System (p. 600)
28.8
Voluntary and Autonomic Nervous Systems (p. 600; Figs. 28.17, 28.18, 28.19)
A. Motor nerve pathways of the peripheral nervous system can be divided into the voluntary
nervous system and the autonomic nervous system.
B. Voluntary Nervous System
1. The voluntary nervous system controls skeletal muscles, either under conscious control
or as part of reflexes.
2. Reflexes are sudden, involuntary reactions that protect the body from danger.
3. These are usually wired through an interneuron housed within the spinal cord and do not
travel to the brain.
C. Autonomic Nervous System
1. The autonomic nervous system is involuntary and has two divisions.
2. The sympathetic nervous system prepares the body for danger and dominates in times of
stress.
a. Blood pressure and heart rate increase, and the body is ready for “fight-or-flight.”
3. The parasympathetic nervous system slows down reactions and conserves energy when
it is not needed.
4. Most glands, smooth muscles, and cardiac muscles are innervated with fibers from both
the sympathetic and parasympathetic nervous systems.