Download Nervous system summary

35–2 The Nervous System
The nervous system controls and coordinates body functions.
It responds to internal and external stimuli. Messages move
through the nervous system as electrical signals. The signals are
called impulses.
Neurons are nerve cells. Each neuron has a cell body, dendrites,
and an axon. The cell body is the largest part of a typical neuron.
It holds the nucleus. Short branches, called dendrites, carry
impulses toward the cell body. A long fiber, called the axon,
carries impulses away from the cell body. A myelin sheath
covers and insulates parts of some axons. A synapse is at the
end of an axon. One neuron transfers an impulse to another at
the synapse. Chemicals called neurotransmitters send impulses
across the synapse.
positive sodium ions flow into the neuron making the inside of the
cell temporarily more positive than the outside of the cell. After
the sodium ions enter, the potassium ions leave. This reversal of
charges across the membrane is the impulse, or action potential.
Once started, the action potential travels quickly down the
neuron’s axon. At the end of the axon, impulses can be
transmitted to the next cell.
There are three types of neurons: sensory neurons, motor
neurons, and interneurons. Sensory neurons carry impulses to
the spinal cord. Motor neurons carry impulses from the spinal
cord. Interneurons link sensory and motor neurons. A resting
neuron is one that is not carrying an impulse. When a neuron is
resting, the inside of the cell has a net negative charge and is
filled with potassium ions (K+). The outside of the cell has a net
positive charge and surrounded by sodium ions (Na+). This
charge difference across the cell membrane is called the resting
potential of the neuron. When a resting neuron is stimulated by
another neuron or by the environment, an impulse starts. The
Immediately after the ions change position, a protein called the
sodium/potassium pump returns the ions back to their original
positions.
The action potential travels down the axon until the impulse
reaches the axon terminal. The impulse must travel across the
space between the axon terminal and the dendrites of the next
neuron. This space between the two neurons is called the
synaptic cleft. The axon terminal releases neurotransmitters into
the space. The neurotransmitters diffuse across the space until
they attach to receptor proteins on the dendrites of the next
neuron. When the dendrites are stimulated by the
neurotransmitters, the begin the next impulse down the neuron.
35–3 Divisions of the Nervous System
The nervous system has two major divisions: the central nervous
system and the peripheral nervous system.
1) The central nervous system is made up of interneurons, which
make up the brain and spinal cord. It is the control center of the
body. It sends messages, processes information, and analyzes
information. The spinal cord connects the brain with the rest of
the body. Some reflexes are processed in the spinal cord. A
reflex is a quick, automatic response to a stimulus. The impulse
travels from the sensory neurons to the spinal cord, then an
impulse is sent directly to a motor neuron where a muscle causes
a movement. The movement from a reflex is not controlled by the
brain. The path of an impulse during a reflex is called the reflex
arc.
2) The peripheral nervous system has two divisions.
• The sensory division sends impulses from sensory neurons
to the central nervous system.
• The motor division sends impulses from the central
nervous system to muscles and glands. The most common
pathway an impulse travels through the nervous system is from
the sensory neurons, which receives the stimulus, to the
interneurons, which process the information, and finally to the
motor neurons, which reacts to the stimulus by moving muscles
or releasing hormones from glands.
1. What does the nervous system control?
2. What is an impulse?
3. What is the other name for a nerve cell?
4. What are the three main parts of a neuron?
5. Define each part does.
6. What insulates the axon?
7. What do neurotransmitters do?
8. Draw a neuron. Label the parts.
9. What do sensory neurons do?
10. What do motor neurons do?
11. What is a resting neuron?
12. What two ions are found around the axon?
13. Which ion is inside the axon and which is outside?
14. What is an action potential?
15. What is it called at the end of the axon?
16. What is a synaptic cleft?
17. What parts of a neuron are on both sides of the synaptic
cleft?
18. Where do the neurotransmitters come from, and where do
they go?
19. What makes up the central nervous system?
20. What does the central nervous system do?
21. What is a reflex?
22. What makes a reflex quick?
23. What are the two divisions of the peripheral nervous system?
24. What does each division do?
25. What is the normal pathway an impulse travels through the
nervous system?
29. In the box below, draw a diagram that shows an impulse
moving from the sensory neurons, to the interneurons, to the
motor neurons. Label each type of neuron on your diagram.
26. Label the following parts in the diagram below
A.
B.
C.
D.
27. What order does an impulse travel down the neuron above?
30.How is a reflex ark different from the pathway you drew
above?
31
28. The diagram above shows an axon during an action
potential. What are the two ions in the circles above.
a.
b.
Sensory neuron
Motor neuron
How Do Drugs Affect Your Brain?
Drugs are chemicals. When someone puts these chemicals into their
body, either by smoking, injecting, inhaling, or eating them, they tap
into the brain’s communication system and tamper with the way nerve
cells normally send, receive, and process information. Different drugs—
because of their chemical structures—work differently. We know there
are at least two ways drugs work in the brain:


Imitating the brain’s natural chemical messengers
Overstimulating the “reward circuit” of the brain
Some drugs, like marijuana and heroin, have chemical structures that
mimic that of a neurotransmitter that naturally occurs in our bodies. In
fact, these drugs can “fool” our receptors, lock onto them, and activate
the nerve cells. However, they don't work the same way as a natural
neurotransmitter, and the neurons wind up sending abnormal messages
through the brain, which can cause problems both for our brains as well
as our bodies.
Other drugs, such as cocaine and methamphetamine, cause nerve cells to
release too much dopamine, which is a natural neurotransmitter, or
prevent the normal recycling of dopamine. This leads to exaggerated
messages in the brain, causing problems with communication channels.
It’s like the difference between someone whispering in your ear versus
someone shouting in a microphone.
The “High” From Drugs/Pleasure Effect
Most drugs of abuse—nicotine, cocaine, marijuana, and others—affect
the brain’s “reward” circuit, which is part of the limbic system.
Normally, the reward circuit responds to feelings of pleasure by
releasing the neurotransmitter dopamine. Dopamine creates feelings of
pleasure. Drugs take control of this system, causing large amounts of
dopamine to flood the system. This flood of dopamine is what causes
the “high” or intense excitement and happiness (sometimes called
euphoria) linked with drug use.
The Repeat Effect
Our brains are wired to make sure we will repeat healthy activities, like
eating, by connecting those activities with feeling good. Whenever this
reward circuit is kick-started, the brain notes that something important is
happening that needs to be remembered, and teaches us to do it again
and again, without thinking about it. Because drugs of abuse come in
and “hijack” the same circuit, people learn to use drugs in the same way.
After repeated drug use, the brain starts to adjust to the surges of
dopamine. Neurons may begin to reduce the number of dopamine
receptors or simply make less dopamine. The result is less dopamine
signaling in the brain—like turning down the volume on the dopamine
signal. Because some drugs are toxic, some neurons also may die.
As a result, the ability to feel any pleasure is reduced. The person feels
flat, lifeless, and depressed, and is unable to enjoy things that once
brought pleasure. Now the person needs drugs just to bring dopamine
levels up to normal, and more of the drug is needed to create a
dopamine flood, or “high”—an effect known as “tolerance.”
Long-Term Effects
Drug use can eventually lead to dramatic changes in neurons and brain
circuits. These changes can still be present even after the person has
stopped taking drugs. This is more likely to happen when a drug is taken
over and over.
Depressants, like alcohol, are drugs that slow down the normal function
of the central nervous system (CNS) and suppress the neural activities in
the brain. Depressants work by affecting the part of the brain that
controls a person’s bodily functions like breathing and heartbeat.
Depressants affect each person differently; response times to
depressants can range from immediate to several hours.
1. What are two ways drugs affect the brain?
2. What is different with the way Marijuana and methamphetamine
affects the brain?
3. What neurotransmitter do these drugs resemble?
4. What happens to the brain of people who repeatedly take drugs?
5. Why do people who repeatedly take drugs begin to suffer from
depression?
6. What do depressants do to the brain?