Chapter 3.2
How does a neurotransmitter "know" which receptor to bind to?
The neurotransmitter and the receptor have what is referred to as a lock-and-key relationship—specific neurotransmitters fit specific receptors similar to how a key fits a lock. The neurotransmitter binds to any receptor that it fits.
A(n) ________ facilitates or mimics the activity of a given neurotransmitter system. axon
agonist
drug that mimics or strengthens the effects of a neurotransmitter
agonist
phenomenon that incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation
all-or-none
Neurons
are the central building blocks of the nervous system, 100 billion strong at birth. Like all cells, neurons consist of several different parts, each serving a specialized function (Figure). A neuron's outer surface is made up of a semipermeable membrane. This membrane allows smaller molecules and molecules without an electrical charge to pass through it, while stopping larger or highly charged molecules.
Axons range in length from a fraction of an inch to several feet. In some axons, glial cells form a fatty substance known as the ..........., which coats the axon and acts as an insulator, increasing the speed at which the signal travels. The myelin sheath is crucial for the normal operation of the neurons within the nervous system: the loss of the insulation it provides can be detrimental to normal function. To understand how this works, let's consider an example. Multiple sclerosis (MS), an autoimmune disorder, involves a large-scale loss of the myelin sheath on axons throughout the nervous system. The resulting interference in the electrical signal prevents the quick transmittal of information by neurons and can lead to a number of symptoms, such as dizziness, fatigue, loss of motor control, and sexual dysfunction. While some treatments may help to modify the course of the disease and manage certain symptoms, there is currently no known cure for multiple sclerosis.
myelin sheath
Multiple sclerosis involves a breakdown of the ________.
myethith stealth
chemical messenger of the nervous system
neurotransmitter
The terminal buttons contain ........that house neurotransmitters, the chemical messengers of the nervous system.
synaptic vesicles
axon terminal containing synaptic vesicles
erminal button
Cocaine has two effects on synaptic transmission: it impairs reuptake of dopamine and it causes more dopamine to be released into the synapse. Would cocaine be classified as an agonist or antagonist? Why?
As a reuptake inhibitor, cocaine blocks the normal activity of dopamine at the receptor. The function causing more dopamine to be released into the synapse is agonist because it mimics and strengthens the effect of the neurotransmitter. Cocaine would be considered an agonist because by preventing the enzymatic degradation of the neurotransmitters, it increases the potential time that these neurotransmitters might be active in the synapse.
reuptake
As noted earlier, when the action potential arrives at the terminal button, the synaptic vesicles release their neurotransmitters into the synapse. The neurotransmitters travel across the synapse and bind to receptors on the dendrites of the adjacent neuron, and the process repeats itself in the new neuron (assuming the signal is sufficiently strong to trigger an action potential). Once the signal is delivered, excess neurotransmitters in the synapse drift away, are broken down into inactive fragments, or are reabsorbed in a process known as reuptake. Reuptake involves the neurotransmitter being pumped back into the neuron that released it, in order to clear the synapse (Figure). Clearing the synapse serves both to provide a clear "on" and "off" state between signals and to regulate the production of neurotransmitter (full synaptic vesicles provide signals that no additional neurotransmitters need to be produced). The synaptic space between two neurons is shown. Some neurotransmitters that have been released into the synapse are attaching to receptors while others undergo reuptake into the axon terminal. Reuptake involves moving a neurotransmitter from the synapse back into the axon terminal from which it was released.
summary of chapter 3.2
Glia and neurons are the two cell types that make up the nervous system. While glia generally play supporting roles, the communication between neurons is fundamental to all of the functions associated with the nervous system. Neuronal communication is made possible by the neuron's specialized structures. The soma contains the cell nucleus, and the dendrites extend from the soma in tree-like branches. The axon is another major extension of the cell body; axons are often covered by a myelin sheath, which increases the speed of transmission of neural impulses. At the end of the axon are terminal buttons that contain synaptic vesicles filled with neurotransmitters. Neuronal communication is an electrochemical event. The dendrites contain receptors for neurotransmitters released by nearby neurons. If the signals received from other neurons are sufficiently strong, an action potential will travel down the length of the axon to the terminal buttons, resulting in the release of neurotransmitters into the synapse. Action potentials operate on the all-or-none principle and involve the movement of Na+ and K+ across the neuronal membrane. Different neurotransmitters are associated with different functions. Often, psychological disorders involve imbalances in a given neurotransmitter system. Therefore, psychotropic drugs are prescribed in an attempt to bring the neurotransmitters back into balance. Drugs can act either as agonists or as antagonists for a given neurotransmitter system.
The action potential is an all-or-none phenomenon.
In simple terms, this means that an incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation. There is no in-between, and there is no turning off an action potential once it starts. Think of it like sending an email or a text message. You can think about sending it all you want, but the message is not sent until you hit the send button. Furthermore, once you send the message, there is no stopping it.
The neuron is a small information processor, and dendrites serve as input sites where signals are received from other neurons. These signals are transmitted electrically across the soma and down a major extension from the soma known as the ......, which ends at multiple terminal buttons.
axon
major extension of the soma
axon
branch-like extension of the soma that receives incoming signals from other neurons
dendrite
The ________ receive(s) incoming signals from other neurons. soma
dendrites
The soma has branching extensions known as
dendrites
nervous system cell that provides physical and metabolic support to neurons, including neuronal insulation and communication, and nutrient and waste transport
glial cell
An action potential involves Na+ moving ________ the cell and K+ moving ________ the cell.
inside; outside
protein on the cell surface where neurotransmitters attach
receptor
The synapse is a very small space between two neurons and is an important site where communication between neurons occurs. Once neurotransmitters are released into the synapse, they travel across the small space and bind with corresponding ............ on the dendrite of an adjacent neuron. Receptors, proteins on the cell surface where neurotransmitters attach, vary in shape, with different shapes "matching" different neurotransmitters.
receptors
neurotransmitter is pumped back into the neuron that released it
reuptake
The nucleus of the neuron is located in the....., or cell body.
soma
small gap between two neurons where communication occurs
synapse
storage site for neurotransmitters
synaptic vesicle
In healthy individuals, the neuronal signal moves rapidly down the axon to the terminal buttons, where ......... release neurotransmitters into the synapse (Figure). The synapse is a very small space between two neurons and is an important site where communication between neurons occurs.
synaptic vesicles
