Module 1

1. Module 1

1.56. Page 5

Lesson 8

Module 1—The Nervous System

A Closer Look at Neurotransmitters

An illustration shows the connection between two neurons. The synapse is highlighted with a bright colour.

excitatory neurotransmitter: a neurotransmitter that promotes nerve impulse transmission in the postsynaptic membrane by opening sodium channels

inhibitory neurotransmitter: a neurotransmitter that hinders nerve impulse transmission in the postsynaptic neuron by hyperpolarizing it

Neurotransmitters are either excitatory or inhibitory at the synapse or at the neuromuscular junction. Sometimes the same neurotransmitter may be both inhibitory and excitatory, depending on the conditions in the body!

Read

dopamine: generally an excitatory neurotransmitter in the CNS

serotonin: an inhibitory neurotransmitter in the CNS

glutamate: an excitatory neurotransmitter in the CNS

GABA (gamma aminobutyric acid): an inhibitory neurotransmitter in the CNS

It is important to understand how neurotransmitters work. You should review pages 378 to 382 in your textbook. Begin reading at “Signal Transmission Across the Synapse.” Read up to the “Section Summary.”

By opening potassium channels or chloride channels in the postsynaptic membrane, inhibitory neurotransmitters make it harder to initiate depolarization in the second neuron because the neuron develops a much lower membrane potential—just like opening the windows on a winter day and letting the heat out makes it harder to warm up a room. Excitatory neurotransmitters open up sodium channels and make it easier to start depolarization because the membrane potential becomes less negative. These excitatory neurotransmitters act the same way as adding an electric heater to the room in order to offset the open window and make it easier to warm up the room. Acetylcholine and norepinephrine are just such excitatory neurotransmitters.

Acetylcholine is a common neurotransmitter in the somatic nervous system, and it is also found between the parasympathetic neurons of the autonomic nervous system. It is also found in some synapses of the brain. Norepinephrine (also called noradrenalin) is an important neurotransmitter in the sympathetic nervous system, and is also found in some synapses in the brain. Review pages 378 to 382 in your textbook and study the table below to learn more about some important neurotransmitters and their characteristics.

CHARACTERISTICS OF SELECTED NEUROTRANSMITTERS

Neurotransmitter	Secretion Location	Type	Effects of Abnormal Production
acetylcholine	CNS, PNS, neuromuscular junctions	mostly excitatory
norepinephrine	CNS, PNS, sympathetic neurons	excitatory in sympathetic neurons
dopamine	CNS	mostly excitatory
serotonin	CNS	generally inhibitory
glutamate	CNS	excitatory
GABA (gamma aminobutyric acid)	CNS	inhibitory
endorphins	CNS	inhibitory

Try This

Choose one of the following Try This activities.

TR 3. Using your textbook, the Internet, and any other sources that may be available to you, research what happens in the body when acetylcholine, norepinephrine, dopamine, serotonin, glutamate, GABA, or endorphins are not produced in appropriate amounts in the body. Add this new information to a table like the one above.

TR 4. You may choose to do questions 6, 7, and 8 on page 384 of the textbook. Check your answers with your teacher.

Discuss

The inability of the body to produce appropriate amounts of a neurotransmitter can result in serious consequences for the body. Michael J. Fox and Mohammed Ali are two famous people who have been afflicted with Parkinson’s disease. This disease seriously interrupts communication in the nervous system. Research the following questions about Parkinson’s disease. Post your research on the discussion board, or build a wiki with a group of students. If you are building a wiki, you can split up the questions for different group members to complete. After reviewing the shared research and based on the feedback from your peers, make any necessary modifications to your work. You may choose to further discuss your research with your teacher. Save your research in your course folder.

Define Parkinson’s disease.

List the symptoms that are associated with Parkinson’s disease, and explain these symptoms based on your understanding of this lesson.

Outline the causes of Parkinson’s disease.

Describe two possible treatments for someone with Parkinson’s disease.

Briefly explain what a stem cell is, and how it can be used to treat Parkinson’s disease.
Outline the societal and technological issues associated with using embryonic stem cell transplants to treat Parkinson’s disease.

Self-Check

Check your understanding of the concept of neurotransmitters by answering the following questions in full sentences. If at any point you require more help, consult with your teacher.

SC 7. Compare the effects of excitatory and inhibitory neurotransmitters on the postsynaptic membrane.

SC 8. Explain the relationship between acetylcholine and cholinesterase.

SC 9. Parkinson’s disease is associated with which neurotransmitter? Identify which area of the brain is affected by this disease and the evident symptom that results.

Check your work.

Self-Check Answers

SC 7. Both excitatory and inhibitory neurotransmitters affect the transmission of the nerve impulse across the synapse. (Similarity) Excitatory neurotransmitters generally make it easier to initiate depolarization in the dendrites of the postsynaptic neuron by opening sodium channels in the postsynaptic membrane and allowing sodium ions to diffuse into the postsynaptic neuron, thus initiating the action potential in the second neuron. Inhibitory neurotransmitters make it more difficult to initiate depolarization in the postsynaptic neuron by either opening potassium channels and allowing more potassium ions to leak out of the neuron (hyperpolarizing it), or by opening chloride channels and allowing negative chloride ions to leak into the neuron, therefore making the neuron more negative than its normal resting potential of – 60 or – 70 mV.

SC 8. After acetylcholine has opened sodium channels in the postsynaptic membrane and initiated depolarization in the postsynaptic neuron, it is decomposed by the enzyme cholinesterase, and the products are transported into the presynaptic neuron so that they can be resynthesized into acetylcholine. If cholinesterase is unable to catalyze the decomposition of acetylcholine, the acetylcholine remains in the synaptic cleft and in the receptors and stimulates the postsynaptic neuron continuously, which may cause muscle fatigue.

SC 9. Parkinson’s disease is associated with the neurotransmitter dopamine. Parkinson’s disease occurs when the nerve cells in the part of the brain that controls muscle movement are gradually destroyed and the neurons can no longer produce dopamine so that muscles can be co-ordinated. Consequently, some of the initial symptoms include muscle stiffness and difficulty bending the arms or legs; an unstable, stooped, or slumped-over posture; loss of balance; a slow, shuffling walk; varying degrees of shaking or muscle tremors; reduced ability to show facial expressions; slow speech; and loss of fine motor skills, such as the inability to write.