Module 4: Nerves

Question 1

Why are nerve cells and muscle cells considered “excitable”?

Answer 1

Because they can use the resting membrane potential to generate an electrochemical impulse called an action potential.

Question 2

How do nerve cells communicate with one another?

Answer 2

Through action potentials.

Question 3

What are dendrites? What is their function?

Answer 3

Thin, branching processes of the cell body. Function is to receive incoming signals and increase overall surface area of neuron so it can communicate with many other neurons. Number of dendrites on a nerve cell will vary depending on location in the body.

Question 4

What is the cell body (soma)? What is its function?

Answer 4

Control centre of the nerve cell, containing a nucleus and all necessary organelles for directing cellular activity.

Question 5

What is an axon? What is its function?

Answer 5

Projection of the cell body that carries outgoing signal to target cell in form of action potential. Axon may or may not be myelinated.

Question 6

What is the myelin sheath? What is its function?

Answer 6

Layered, phospholipid membrane sheath wrapped tightly around axon. Acts as an insulator for the axon, forcing the ionic changes that comprise an action potential to take place in only small exposed areas of the axon called nodes of Ranvier.

Question 7

What are the nodes of Ranvier? What is their function?

Answer 7

Small exposed areas of the axon that increase the speed of action potential because action potential jumps from node to node.

Question 8

What are collaterals? What is their function?

Answer 8

Branchings of the axon near its terminal end. Increase number of possible target cells with which the neuron can interact.

Question 9

What is the terminal bouton or axon terminal? What is its function?

Answer 9

Swelling at the end of an axon collateral. Swelling contains mitochondria and membrane bound vesicles containing various neurocrine molecules. Chemicals in axon terminal facilitate the transmission of signal across synapse to target cell.

Question 10

What is an action potential?

Answer 10

Rapid reversal of resting membrane. Phases include depolarization, repolarization, and hyperpolarization.

Question 11

What happens during depolarization?

Answer 11

Membrane potential rapidly changes from -70mV to +35mV.

Question 12

What happens during repolarization?

Answer 12

Membrane potential rapidly returns from +35mV to -70mV.

Question 13

What happens during hyperpolarization?

Answer 13

Membrane potential briefly becomes more negative, going from -70mV to -90mV. It then returns to -70mV, the resting membrane potential.

Question 14

What are the two special types of channels found in nerve and muscle cells? Where are they found?

Answer 14

Voltage-gated sodium and voltage-gated potassium channels. In the neuron, these are found on axon and are essential for generation of action potential.

Question 15

When do voltage-gated sodium channels open? Describe the events that occur.

Answer 15

Open during depolarization. When depolarization occurs, activation gate opens immediately, allowing Na+ to flow into cell. Then, inactivation gate, which takes longer to react to depolarization, closes and Na+ can no longer flow into cell. Channel then returns to resting configuration.

Question 16

Where are the two gates of the voltage-gated sodium channel located?

Answer 16

Both the activation and inactivation gate are located on intracellular side of cell.

Question 17

What is the resting configuration of the voltage-gated sodium channel?

Answer 17

Activation gate is closed and inactivation gate is open.

Question 18

What happens when the inactivation gate closes? What is this time period called?

Answer 18

When the gate closes, the channel will not open, regardless of strength of stimulation. Time period during this inactivation is called absolute refractory period.

Question 19

How many gates does a voltage-gated potassium channel have? How is/are the gate(s) different than those of a voltage-gated sodium channels?

Answer 19

This channel only has one gate, which opens during depolarization. These gates, however, do not open immediately like the voltage-gated sodium channel activation gate. They begin opening when the voltage-gated sodium channels start to become inactivated.

Question 20

Do voltage-gated potassium channels have an inactivation period?

Answer 20

No.

Question 21

Where is the voltage-gated potassium channel gate located?

Answer 21

On the intracellular side of the channel.

Question 22

When do the voltage-gated potassium channels begin to open?

Answer 22

While voltage-gated sodium channels are being inactivated.

Question 23

Where in the neuron does an action potential begin?

Answer 23

In a region of the neuron called the axon hillock. This area contains the largest number of voltage-gated channels.

Question 24

What is the absolute refractory period?

Answer 24

When the voltage-gated sodium channels are being inactivated, they will not open to fire another action potential, regardless of the strength of depolarization.

Question 25

What is the relative refractory period? What causes it?

Answer 25

When the membrane is hyperpolarized. This is caused by the voltage-gated potassium channels, which are not only slow to open but slow to close as well. This allows K+ ions to leave cell even after it has repolarized to -70mV.

Question 26

Is it possible to fire another action potential during the relative refractory period?

Answer 26

Yes, it is possible; however, it would require a stronger stimulus to reach threshold.

Question 27

Why does the relative refractory period reach -90mV?

Answer 27

Because that is the equilibrium potential of K+.

Question 28

Why do you need a strong depolarization for an action potential to occur? What happens if only two voltage-gated sodium channels open?

Answer 28

Small number of entering Na+ ions will cause small depolarization, but the cell will attempt to maintain its resting membrane potential at -70mV. Small buildup of positive charge affects other ions inside and outside cell. K+ will leave cell and Cl- will enter the cell. Movement of both of these ions will repolarize membrane potential back to normal. To generate action potential, initial depolarization must be strong enough to open all voltage-gated sodium channels to allow sufficient Na+ ions to enter.

Question 29

What is the threshold for generating an action potential?

Answer 29

-55mV

Question 30

How many of the available ions move through membrane during an action potential?

Answer 30

Only about one millionth of the ions. This means thousands of action potentials can be generated before concentration gradients for Na+ and K+ break down enough to prevent further action potentials.

Question 31

List the steps of an action potential.

Answer 31

1) Resting membrane potential
2) Na+ influx
3) Depolarization
4) Overshoot
5) K+ efflux
6) Repolarization
7) Hyperpolarization

Question 32

What is the movement of the action potential down the axon called?

Answer 32

Action propagation or conduction.

Question 33

List the steps of action propagation down an unmyelinated nerve.

Answer 33

1) Where action potential exists on axon, inside of membrane is positive (+35mV) with respect to outside because N+ have entered.
2) Positive charge is attracted to and moves toward area of membrane next to it at rest and has negative charge.
3) Because of buildup of positive charge, adjacent area of membrane now depolarizes.
4) Depolarization triggers voltage-gated sodium channels to open.
5) Na+ rushes into cell and depolarizes region to threshold, creating new action potential.
6) This process repeats and action potential is propagated along membrane.

Question 34

Why does action potential have a unidirectional nature (cannot go backwards along membrane)?

Answer 34

Because the voltage-gated sodium channels have been inactivated, which are in a state of absolute refractory. Therefore, this area cannot generate action potential. By the time channels are ready to open, action potential has travelled too far down axon to be able to affect them.

Question 35

What produces myelin?

Answer 35

Special cells called Schwann cells in peripheral nervous system (PNS) and oligodendrocytes in central nervous system (CNS).

Question 36

Where are the voltage-gated sodium and potassium channels in myelinated axons?

Answer 36

Only at the gaps between the myelin, called the nodes of Ranvier.

Question 37

List the steps of action propagation down a myelinated nerve.

Answer 37

1) Positive charge from existing action potential is attracted to and moves toward adjacent node of Ranvier that is negative.
2) Node of Ranvier now depolarizes.
3) Depolarization triggers voltage-gated sodium channels to open.
4) Na+ rushes into cell and depolarizes region to threshold and new action potential is generated.
5) By repetition of this process, action potential is propagated along membrane.

Question 38

What is propagation along myelinated nerve called?

Answer 38

Saltatory conduction because the action potential jumps from one node to the next.

Question 39

What is the all-or-nothing principle of action potentials?

Answer 39

You either have a full action potential after meeting threshold or none at all.

Question 40

What is multiple sclerosis?

Answer 40

Disease in which body’s natural immune system attacks and damages myelin surrounding axon of nerves. Damage can be so severe that it interrupts natural flow of action potentials to point where no transmission occurs. If damaged nerve is connected to muscle, muscle will not contract and person can suffer from paralysis.

Question 41

What is a chemical synapse?

Answer 41

Where neuron almost contacts nerve cell, muscle cell, or an organ (e.g. heart).

Question 42

What is the neuromuscular junction (NMJ)?

Answer 42

Synapse between neuron and muscle cell. Action potential from nerve cell triggers action potential in muscle cell, leading to muscle contraction.

Question 43

What is the neuron that contacts a muscle cell sometimes called?

Answer 43

Motor nerve fiber.

Question 44

What does the membrane of the presynaptic axon terminal contain?

Answer 44

Ca2+ voltage-gated channels. These channels open when cell membrane depolarizes.

Question 45

What does the axon terminal contain?

Answer 45

Synaptic vesicles that contain neurotransmitter acetylcholine (ACh).

Question 46

What does the basement membrane of the axon terminal contain?

Answer 46

Enzyme acetylcholinesterase (AChE).

Question 47

What is another name for the muscle cell membrane?

Answer 47

Sarcolemma

Question 48

Muscle cell membrane is thrown into folds. What is this region called? What does it contain?

Answer 48

Region is called the end plate and it contains receptors for acetylcholine, which are associated with ligand-gated ion channels.

Question 49

What is the gap between the motor fiber and muscle cell called?

Answer 49

Synaptic cleft.

Question 50

Describe the events at the neuromuscular junction.

Answer 50

1) Action potential on presynaptic motor nerve fiber triggers Ca2+ voltage-gated channels to open. Ca2+ enter cell.
2) Ca2+ trigger fusing of synaptic vesicles to membrane and release of ACh into synaptic cleft by exocytosis.
3) ACh diffuses across synaptic cleft and attaches to receptors on muscle cell/fiber membrane.
4) Ligand-gated ion channels open. Lots of Na+ enters cell and a few K+ leave. This triggers local depolarization called end plate potential (EPP). Note: There is no action potential yet!
5) Depolarization of EPP spreads to adjacent cell membrane where voltage-gated channels are located. These channels open and large amounts of Na+ flow into muscle cell and triggers action potential.
6) ACh is broken down to acetic acid and choline by enzyme AChE. Choline is taken back into axon terminal to be recycled.