Chapter 3: Chemical Signaling by Neurotransmitters and Hormones

Question 1

What is a synapse?

Answer 1

A synapse is the pocket of space between two neurons (the presynaptic and postsynaptic cells). There are three types of synaptic connections: axodendritic, axosomatic, and axoaxonic.

Question 2

What is an axodendritic synapse?

Answer 2

Axodendritic synapses are where an axon terminal from a presynaptic cell communicates with a dendrite from a postsynaptic cell.

Question 3

What is an axosomatic synapse?

Answer 3

Axosomatic synapses are where an axon terminal from a presynaptic cell communicates with the nerve cell body from the postsynaptic cell.

Question 4

What is an axoaxonic synapse?

Answer 4

Axoaxonic synapses are when the axon terminal from one cell synapses to the axon terminal from another cell. In this case, the presynaptic cell alters neurotransmitter release from the postsynaptic cell directly at the terminal.

Question 5

What is a neurotransmitter?

Answer 5

Neurotransmitters are chemical substances that are released by neurons to communicate with other cells. While there are many criteria used to verify a chemical’s status as a neurotransmitter, not all are necessary. The most important criteria are that the chemical is synthesized in a group of nerve cells, it is released by those nerve cells, there are identified receptors for the chemical, and it mediates a response by the receiving cell.

Question 6

What is a classical neurotransmitter?

Answer 6

Classical neurotransmitters are the neurotransmitters that were discovered before the other categories of transmitters. Classical neurotransmitters (amino acids, monoamines, and acetylcholine) are mainly synthesized in the nerve terminal and then are transported into synaptic vesicles and released through exocytosis.

Question 7

How is the synaptic transmitter action of a released chemical terminated?

Answer 7

1. Reuptake by the presynaptic cell, postsynaptic cell, or astrocytes
2. Enzymatic breakdown

Question 8

What are the two major types of receptors?

Answer 8

1. Ionotropic receptors

2. Metabotropic receptors

Question 9

What are ionotropic receptors?

Answer 9

Ionotropic receptors are composed of multiple subunits and form an intrinsic ion channel that is permeable either to cations such as Na+ or to anions such as Cl–. These receptors respectively mediate fast excitatory or fast inhibitory transmission.

Question 10

What are metabotropic receptors?

Answer 10

Metabotropic receptors are coupled to G proteins in the cell membrane and mediate slower transmission involving ion channel opening or second-messenger synthesis or breakdown.

Question 11

Describe 5 of the 11 ways in which drugs can alter synaptic transmission.

Answer 11

1. Inhibits neurotransmitter synthesis by inhibiting a key enzyme needed for transmitter synthesis
2. Stimulates release of the neurotransmitter
3. Inhibits the release of the neurotransmitter
4. Stimulates autoreceptors, inhibiting release of the neurotransmitter
5. Blocks autoreceptors, increasing release of the neurotransmitter

Question 12

Describe the importance of the endocrine system.

Answer 12

1. drugs can adversely alter endocrine function
2. hormones may alter behavioral responses to drugs
3. hormones themselves sometimes have psychoactive properties
4. the endocrine system can be used as a window to the brain to help us determine the functioning of a specific neurotransmitter system by measuring changes in hormone secretion under appropriate conditions

Question 13

Why is nitric oxide considered to be an atypical neurotransmitter?

Answer 13

Nitric oxide is considered an atypical neurotransmitter because it has signalling functions outside the central nervous system. Nitric oxide has a role in regulating cardiovascular functioning.

Question 14

Describe neuropeptides?

Answer 14

Neuropeptides are a group of nonclassical neurotransmitters. They can only be synthesized in the cell body because their precursors are protein molecules which can only be made in the cell body. After proteins are made, they are packaged with enzymes in vesicles which break down the precursor into neuropeptides while they are transported to the axon terminal for release. As a result of how neuropeptides are synthesized, they are replenished more slowly than other types of neurotransmitters.

Question 15

Describe neuromodulators.

Answer 15

Neuromodulators indirectly affect the postsynaptic cell by enhancing, reducing or prolonging the effectiveness of neurotransmitters. They can also diffuse away from the site of release, allowing them to influence cells further away. However, the difference between neurotransmitters and neuromodulators is vague because some chemicals might act like a neurotransmitter in some circumstances and like a neuromodulator in others.

Question 16

What are the 7 steps of neurotransmitter synthesis, release, and inactivation?

Answer 16

Step 1: Neurotransmitters are synthesized by enzymatic reactions and stored in vesicles.

Step 2: An action potential reaches the presynaptic terminal.

Step 3: The terminal becomes depolarized and voltage-gated Ca2+ channels open.

Step 4: There is an influx of Ca2+ ions into the terminal through the channels.

Step 5: The Ca2+ ions cause the vesicles storing the neurotransmitters to fuse with the presynaptic membrane.

Step 6: Exocytosis.

Step 7: Termination of the synaptic signal.

Question 17

Explain exocytosis.

Answer 17

When fusing of the vesicle and axon terminal membranes exposes the inside of the vesicle to the outside of the cell, releasing the neurotransmitters into the synaptic cleft. Exocytosis is a Ca2+-dependent process.

Question 18

What are second messengers?

Answer 18

Second messagers activate protein kinases (a type of enzymes) which catalyze the addition of one or more phosphate groups (phosphorylate) to proteins in the cell. The added phosphate groups then alter the functioning of the protein.

Question 19

What are the 4 models that have been proposed to explain vesicle recycling?

Answer 19

1. Clathrin-mediated endocytosis
2. Ultrafast endocytosis
3. Kiss-and-run
4. Bulk endocytosis

Question 20

How are lipid and gaseous transmitters special?

Answer 20

1. Synthesized upon demand
2. Not stored in synaptic vesicles
3. Often function as retrograde messengers by signaling from the postsynaptic to the presynaptic cell

Question 21

What is neurotransmitter release controlled by?

Answer 21

1. Rate of cell firing
2. Release probability at a specific synapse
3. Inhibitory terminal and somatodendritic autoreceptors

Question 22

What purpose do neurotransmitter receptors serve?

Answer 22

Signaling information from the presynaptic to the postsynaptic cell. Unlike transporters, they do not carry neurotransmitter molecules across the cell membrane.

Question 23

What are allosteric modulators?

Answer 23

Allosteric modulators are molecules that either increase or decrease receptor activity by binding to sites on the receptor protein separate from the agonist binding site.

Question 24

Explain synaptic plasticity.

Answer 24

Synaptic plasticity refers to functional and structural changes in synaptic connectivity. Structurally, synaptic plasticity is manifested by changes in the size, shape, and/or number of dendritic spines.

Question 25

Describe the adrenal gland.

Answer 25

The adrenal gland is composed of the inner medulla and the outer cortex, both of which are activated by stress. The medulla secretes the hormones EPI and NE, whereas the cortex secretes glucocorticoid steroids such as cortisol and corticosterone.

Question 26

Explain the gonadal steroids.

Answer 26

Estrogens and progesterone are released from the ovaries in females, and androgens such as testosterone are released from the testes in males. These gonadal steroids are responsible for many of the secondary sex characteristics that appear after puberty.

Question 27

What hormone does the pineal gland synthesize?

Answer 27

The pineal gland synthesizes the hormone melatonin using serotonin as a precursor. Melatonin has been implicated in the regulation of various types of rhythmic activity, including sleep.