Chapter 43

What are the two types of cells in the nervous system?

Neurons (nerve cells) and Glia

What do neurons do?

Generate and conduct electrical signals.

What are the two types of glia and their functions?

Macroglia - support and modulate neuronal function.
Microglia - small phagocytic cells, motile; function in major immune defense mechanism in the nervous system.

What are the two parts of the nervous system?

Central Nervous System (CNS) and Peripheral Nervous System (PNS).

What is the function of the Central Nervous System (CNS)?

It consists of brain and spinal cord and is the site of information processing and storage.

What is the function of the Peripheral Nervous System?

It provides communication between the central division and all of the rest of the body.

What are the main structures of a neuron?

Cell body, dendrites, axon, and axon terminal.

What is the significance of the cell body (a structure of a neuron)?

It contains neurons and organelles.

What is the significance of dendrites (a structure of a neuron)?

It brings info to the cell body (considered the input center).

What is the significance of axon (a structure of a neuron)?

It carries information away from the cell body.

What is the significance of the axon terminals (a structure of a neuron)?

It is located at the top of the axon and transmits messages to other cells.

How does the formation of a neuron affect neurons?

The form of a neuron determines its function.

What does the amount of dendrites reflect?

It reflects the number of sources of information coming to the neuron. Fewer dendrites results in fewer info processing.

How do neurons communicate with other cells?

They communicate via short distances and/or very long distances depending on the length of its axon.

How do all neurons process and communicate information?

They process and communicate info through changes in electric potential across their membranes.

What happens as a result of small changes in membrane electric potential?

These small changes generate large, rapidly reverse changes in membrane potential called action potentials (APs).

Draw out the movement of the action potential from the dendrites to the axon terminal.
What are possible targets for a neuron cell?

Possible targets are another neuron, muscle cells, and secretory cells (all of your endocrine cells).

What do axon terminals and the membrane of a target cell form when they are extremely close to one another?

Synapse

What are the different options that a target cell (postsynaptic cell) can be?

Another neuron, muscle cell, and a secretory cell.

When the AP (Action Potential) reaches the axon terminals, what are the two forms of a synapse that will transfer that information from the presynaptic cell to the postsynaptic cell.

Electrical and chemical synapse.

What do electrical synapses allow the action potential to do?

Adjacent cells are connected by an intercellular channel, gap junction, and electrical synapses allow the AP to pass directly between the two neurons.

What do chemical synapses allow the action potential to do?

Chemical synapses allow the AP to cause neurons to release a neurotransmitter that will excite or inhibit the neuron that detects the neurotransmitter.

What are most synapses in vertebrates, chemical or electrical?

Chemical

What does AP cause the terminal to release?

Neurotransmitter chemicals which diffuse to receptors on the postsynaptic cell.

What are the two possible results of a neurotransmitter binding to the postsynaptic cell?

The neurotransmitter can either excite or inhibit the postsynaptic cell.

How do neurons integrate information?

Neurons integrate information by summing excitatory and inhibitory inputs.

Glial cells may outnumber neurons in the human brain. There are two types: microglia and macroglia. What are the different types of macroglia?

Schwann cells, oligodendrocytes, ependymal cells, and astrocytes.

In the brain and spinal cord, what wraps around neuron axons?

Oligodendrocytes (a type of macroglia).

In the brain and spinal cord, oligodendrocytes wrap around neuron axons and form what?

When these cells wrap around the neuron axons, they form concentric layers of insulating cell membrane.

Which cells wrap the axons of other nerves (peripheral nerves)?

Schwann Cells

What do oligodendrocytes and schwann cells produce?

They both produce myelin, a type of cell membrane that covers axons and not all neurons are myelinated. (Myelin sheath)

What does myelin do?

It makes action potential more faster and provides electrical insulation to the neuron.

What do ependymal cells (a type of macroglia cell) do?

They line the central fluid filled chambers (ventricles) in the brain and produce cerebrospinal fluid.

What do astrocytes (a type of macroglia cell) do?

They contribute to the blood-brain barrier, which protects the brain from toxic substances in the blood.

What is the glymphatic system?

A waste clearance system in the CNS of vertebrates that utilizes perivascular channels, formed by astrocytes, to eliminate waste.

How does interstitial fluid enter the brain?

It enters through the perivascular spaces between arteries and astrocytes.

What are aquaporins?

Commonly referred to as water channels, they are integral membrane proteins that transport fluid across cell membranes.

What is the function of aquaporins in their cell membrane?

Aquaporins take up fluid and distribute it to the interstitial spaces in the brain.

How does fluid leave veins and what does the fluid take with it?

Fluid leaves through perivascular spaces of veins and takes metabolic waste products with it.

What other functions do astrocytes have?

• They can take up neurotransmitters from the synapse and thus control communication between pre-and postsynaptic cells.

• They store glycogen that can supply neurons with fuel.

• They release neurotransmitters that can alter the activities of neurons.

• They aid in repair and regeneration of neurons.

• They make contact with both blood vessels and neurons and signal changes in blood composition.

A projection of one astrocyte may make contact with roughly how many synapses?

100,000 synapses.

What is a tripartite synapse?

A synapse includes pre- and postsynaptic neurons as well as connections from astrocytes.

Compared with neuron B, neuron A would be more likely to?

a. communicate information over a short distance.
b. collect information from more cells.
c. depolarize after receiving an action potential.
d. process information more quickly from a single synapse.

b. collect information from more cells. (Neuron A has WAYYY more dendrites)

If you were seated on a postsynaptic cell waiting to receive an action potential from another neuron, which part(s) of the presynaptic cell would be closest?

a. axon
b. axon terminal
c. dendrites
d. cell body
e. axon hillock

b. axon terminal

Glial cells are estimated to be more numerous in the brain than are neurons, yet much less is known about them. What has made it harder to study the functions of glial cells than the functions of neurons?

a. their many different shapes make identification difficult.
b. they do not fire action potentials, which can be observed.
c. some serve as insulation for nerve cells, with very little activity.
d. they are the cause of most brain diseases.
e. they cannot be observed except when damaged, as in multiple sclerosis.

b. they do not fire action potentials, which can be observed.

What are ion transporters/pumps and ion channels responsible for?

They are responsible for the distribution of charges across the membrane that determine membrane potential.

What does a sodium-potassium pump do?

It moves Na+ to the outside and K+ to the inside, requires energy, and establishes concentration gradients.

What do ion channels in the membrane do?

They allow ions to pass through, but are selective— there are different channels for each type of ion.

Ions can move in either direction; what does net movement depend on?

It depends on concentration gradient and voltage difference (these two motive forces are called electrochemical gradient).

What two motive forces make up an electrochemical gradient?

A concentration gradient and voltage difference.

What is the inside or outside of a cell typically negative?

The inside of a cell is usually negative relative to the outside because “leak channels” allow some K+ ions to diffuse out.

Since potassium channels are open in the resting membrane and K+ ions diffuse out of the cell, what happens inside the cell as a result?

This leaves behind unbalanced negative charges inside the cell.

What happens as a result of the negative charge inside cells?

K+ ions diffuse back into the cell because of the negative electrical potential.

What is membrane potential?

Electrical charge difference across a cell membrane — due to a balance between the tendency of K+ ions to diffuse down their concentration gradient and the electrical potential that holds them back.

What is resting potential?

The steady state membrane potential of a neuron.

What are membrane potentials measured with?

Electrodes

What is the resting potential of an axon?

-60 → -70 millivolts (mV)

Is the inside of a cell at rest negative or positive?

Negative

What happens when a stimulus changes the permeability of the membrane?

It allows ions to move quickly, leading to a change in membrane potential (graded potential).

What is action potential?

A sudden, rapid reversal in the voltage across a portion of the cell membrane.

What happens when positively charged ions flow into the cell?

For 1 or 2 milliseconds, the inside of the cell becomes more positive than the outside.

Leak channels are always open, but some ion channels are “gated”. What are the three types of gated channels?

1. Voltage-gated channels

2. Chemically-gated channels

3. Mechanically-gated channels

What do voltage-gated channels respond to?

They respond to a change in voltage across the membrane.

What do chemically-gated channels depend on?

They depend on specific molecules that bind or alter the channel protein.

What do mechanically-gated channels respond to?

They respond to force applied to the membrane.

What alters the membrane potential?

Opening and closing of gated channels.

If Na+ channels open suddenly, what will happen?

Na+ diffuses in and the inside of the cell would become less negative.

When the inside of a neuron becomes less negative or more positive in comparison to its resting condition, what happens to the membrane?

The cell membrane becomes depolarized.

What happens if gated K+ channels open and K+ efflux increases over the normal leak rate?

The membrane potential becomes even more negative and the cell membrane becomes hyperpolarized.

What are action potentials?

They are sudden, transient, large changes in membrane potential, generated by the action of voltage-gated Na+ and K+ channels.

Are channels open or closed at resting potential?

They are closed. But, slight depolarization can cause them to open and the graded potential can spread by local current flow to the axon hillock.

What is the axon hillock?

It is the base of the axon; right where the axon starts and voltage-gated Na+ channels are concentrated in the hillock.

What happens when Na channels open and Na+ rushes into the axon?

The influx of positive ions causes more depolarization — a positive feedback effect.

What happens when the membrane is depolarized about 5 to 10 mV above resting potential?

A threshold is reached and a large number of sodium channels open and an action potential is generated. The axon then returns to resting potential as voltage-gated Na+ channels close and voltage-gated K+ channels open.

During what period do voltage-gated Na+ channels not open?

Refractory period.

Voltage-gated Na+ channels have two gates which are?

1. Activation gate — closed at rest but opens quickly at threshold.

2. Inactivation gate — open at rest and closes at threshold; reopens 1-2 milliseconds later than the activation gate closes.

Voltage-gated K+ channels contribute to the refractory period by remaining open. What happens when there is an efflux of K+ ions?

An efflux of K+ ions makes the membrane potential less negative than the resting potential for a brief period.

An action potential can travel over longer distances with no loss of signal. What are the two features of an action potential (AP)?

1. It is an all-or-nothing event — positive feedback to voltage-gated Na+ channels ensures the maximum action potential.

2. An action potential is self-regenerating — it spreads to adjacent membrane regions. It can’t reverse direction because of the refractory period of channels behind it.

Do action potentials travel faster in myelinated or nonmyelinated axons?

Myelinated axon

What are nodes of ranvier?

They are regularly spaced gaps in the myelin along an axon.

Where are action potentials generated?

They are generated at the nodes and the positive current flows down the inside of the axon. The current flows much faster through cytoplasm than ion channels can open and close.

What happens when the positive current reaches the next node?

The membrane becomes depolarized and another axon potential is generated.

What is saltatory conduction?

It is a form of propagation where an action potential appears to jump from node to node.

In a neuron at rest, the resting membrane potential is mainly due to:

a. the number of voltage-gated channels in the membrane.
b. the difference in the concentrations of Na+ inside and outside of the cell.
c. passive leak channels in the cell membrane.
d. the active transport of K+ ions out of the cell.
e. the electrical charge in the extracellular fluid.

c. passive leak channels in the cell membrane. (mainly potassium)

The conduction of an action potential is traveling from left to right in this myelinated axon. Which statement is true?

a. depolarization at node 2 is being caused by the opening of voltage-gated K+ channels.
b. the speed of the action potential in this axon depends mostly on its diameter.
c. Na+ channels have inactivated at node 1 and have entered a refractory state.

c. Na+ channels have inactivated at node 1 and have entered a refractory state.

Where do neurons communicate with other neurons or target cells?

At synapses.

What are the two types of synapses and their relevance on neurons communicating w/ other neurons or target cells?

1. Electrical synapse — action potential spreads directly to the postsynaptic cell (the other neuron or target cell).

2. Chemical synapse — neurotransmitters from a presynaptic cell induce changes in membrane potential of a postsynaptic cell.

What are neuromuscular junctions?

Chemical synapses between motor neurons and skeletal muscle cells.

What is the neurotransmitter in the case of a motor neuron and skeletal muscle cell?

Acetylcholine (ACh) which diffuses across the synaptic cleft to the motor end plate on the muscle cell.

What is the relationship between actional potential and the neurotransmitter ACh?

An action potential causes the release of the neurotransmitter ACh when voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. And vesicles release ACh into the synaptic cleft by exocytosis.

What happens after ACh diffuses across the synaptic cleft?

It binds to receptors (AChRs) on the motor end plate.

What are AChRs?

They are receptors for ACh and are gated channels that allow Na+ and K+ to flow through.

Extra info: An increase in Na+ depolarizes the membrane and the amount of depolarization depends on the number of ACh receptors (AChRs).

What must happen to stop the action of a neurotransmitter?

The neurotransmitter must be cleared from the synaptic cleft after release in order to stop its action. ACh is broken down by AChE (an enzyme).

What happens if the enzyme to break down ACh, AChE, is inhibited?

ACh will stay in the synaptic cleft and cause spastic muscle paralysis and eventually death.

Extra info: some nerve gases and insecticides inhibit AChE.

Synapses between motor neurons and muscle cells are excitatory. What does ACh always cause?

ACh causes depolarization! Other synapses can be inhibitory if the postsynaptic response is hyperpolarization (the cell becomes more negative).

Neurons have multiple synapses. True or False?

True. Neurons have many synapses and can receive different chemical messages.

What are the 3 main neurotransmitters in the brain (they are amino acids)?

1. Glutamate — it’s excitatory (causes depolarization)

2. Glycine — it’s inhibitory

3. Aminobutyric acid (GABA) — it’s inhibitory

What are other neurotransmitters in the brain?

1. Monoamines — examples include dopamine, norepinephrine, and serotonin

2. Peptides — examples include endorphins and enkephalins— modulate pain

3. Nitric oxide

In the vertebrate neuromuscular junction, acetylcholine is transmitted from the presynaptic neuron to the postsynaptic motor end plate through:

a. endocytosis of acetylcholine
b. diffusion
c. formation of vesicles in the motor end plate
d. acetylcholine membrane pumps
e. closing of voltage-gated Ca 2+ channels

b. diffusion

Which event at this neuromuscular junction is paired correctly with its location of where it occurs?

a. 7: Neurotransmitter is released.
b. 4: Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
c. 3: Voltage-gated calcium channels open.

b. 4: Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

Extra Notes:
- At 7, the breakdown of ACh occurs via acetylcholinesterase (AChE)
- At 3, vesicles bind to the membrane and release ACh into the cleft