Chapter 2 – Nerve Cells And Nerve Impulses

Question 1

Cells that receive information and transmit it to other cells

Answer 1

Neurons

It is estimated that the adult human brain contains approximately 100 billion neurons but the exact number varies from person to person.

Question 2

Structure that separates the inside of the cell from the outside environment

Answer 2

Membrane

Composed of two layers of fat molecules that are free to flow round one another. Most chemicals cannot cross the membrane, but specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals

Question 3

Structure that contains the chromosomes

Answer 3

Nucleus

All animal cells have a nucleus except for mammalian red blood cells

Question 4

Structure that performs metabolic activities in a cell

Answer 4

Mitochondrion

Provides the energy that the cell requires for all other activities. Require fuel and oxygen to function

Question 5

Sites for cell synthesization of new protein molecules

Answer 5

Ribosomes

Question 6

Network of thin tubes that transport newly synthesized proteins to other locations

Answer 6

Endoplasmic reticulum

Question 7

What are the components of larger neurons?

Answer 7

Dendrites, a soma or cell body, an axon, and presynaptic terminals

Question 8

Neuron that receives excitation from other neurons and conducts impulses to a muscle

Answer 8

Motor neuron

Has its soma in the spinal cord. Receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle

Question 9

Neuron that is highly sensitive to a specific type of stimulation

Answer 9

Sensory neuron

Stimulation can be light, sound, or touch. Tiny branches lead directly from the receptors into the axon, and the cells soma is located on a little stalk off the main trunk

Question 10

Branching fibres from a neuron that receive information from other neurons

Answer 10

Dendrites

The branching fibres get narrower near their ends. Comes from the Greek root meaning tree. The surface is lined with specialized synaptic receptors at which the dendrite receives information from other neurons. The greater the surface area of a dendrite, the more information it can receive

Question 11

Short outgrowths that increase the surface area available for synapses

Answer 11

Dendritic spines

Question 12

Thin fibre of constant diameter; the neuron’s information centre

Answer 12

Axon

Conveys an impulse toward other neurons or an organ or muscle

Question 13

Structure containing the nucleus, ribosomes, and mitochondria

Answer 13

Cell body or soma

Most of the metabolic work of a neuron occurs here

Question 14

Insulating material that covers vertebrate axon

Answer 14

Myelin sheath

Question 15

Interruptions in the myelin sheath of vertebrate axons

Answer 15

Nodes of Ranvier

Question 16

point where an axon releases chemicals

Answer 16

Presynaptic terminal

End bulb or bouton

Question 17

Axon that brings information into a structure

Answer 17

Afferent axon

Question 18

Neuron that carries information away from a structure

Answer 18

Efferent axon

Every sensory neuron is an afferent to the rest of the nervous system, and every motor neuron is an efferent from the nervous system. Within the nervous system, a given neuron is an efferent from one structure and then afferent to another.

Question 19

Neuron whose axons and dendrites are all confined within a given structure

Answer 19

Interneuron or intrinsic neuron

For example, and intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus

Question 20

Describe variations among neurons

Answer 20

Neurons vary enormously in size, shape, and function. The shape of a given neuron determines its connections with other neurons and thereby determines its contribution to the nervous system. Neurons with wider branching connect with more targets.
The function of a neuron relates to its shape.

Question 21

Type of cell in the nervous system that, in contrast to neurons, does not conduct impulses over long distances

Answer 21

Glia

Derived from a Greek word meaning glue. Reflects early investigators ideas that glia were like glue that held the neurons together

Question 22

Star-shaped glia that synchronize the activity of the axons

Answer 22

Astrocytes

Wrap around the presynaptic terminals of a group of functionally related axons. By taking up ions released by axons and then releasing them back to axons, and astrocyte help synchronize the activity of the axons, enabling them to send messages in waves. Also remove waste material created when neurons die and control the amount of blood flow to each brain area. Dilate the blood vessels during periods of heightened activity in some brain areas to bring more nutrients into that area

Question 23

Cells that remove waste material and other micro organisms from the nervous system

Answer 23

Microglia

Very small cells, also remove waste material as well as viruses, fungi, and other microorganisms. Function like part of the immune system

Question 24

Glia cells that build myelin sheaths. In the brain and spinal cord

Answer 24

Oligodendrocytes

Question 25

Glia cells that build myelin sheaths. In the periphery of the body

Answer 25

Schwann cells

Question 26

Cells that guide the migration of neurons and the growth of axons and dendrites during embryological development

Answer 26

Radial glia

When embryological development finishes, most radial glia differentiate into neurons, and a smaller number differentiate into astrocytes and oligodendrocytes

Question 27

Mechanism that excludes most chemicals from the brain

Answer 27

Blood-brain barrier

Question 28

A protein-mediated process that expends energy to enable a molecule to cross the membrane

Answer 28

Active transport

Question 29

A B1 vitamin necessary to use glucose

Answer 29

Thiamine

Question 30

Why do we need a blood-brain barrier?

Answer 30

When a virus invades a cell, mechanisms within the cell extrude virus particles through the membrane so that the immune system can find them, kill it, and the cell that contains it. Because the vertebrate brain does not replace damaged neurons, to minimize the risk of irreparable brain damage, the body builds a wall along the sides of the brain’s blood vessels which keeps out most viruses, bacteria, and harmful chemicals.

Question 31

How does the blood-brain barrier work?

Answer 31

Depends on the endothelial cells that form the walls of the capillaries. Outside the brain, such cells are separated by small gaps, but in the brain, they are joined so tightly that virtually nothing passes between them. This barrier keeps out useful chemicals as well as harmful ones. Useful chemicals include all fuels and amino acids, the building blocks for proteins. For the brain to function, it needs special mechanisms to get these chemicals across the blood-brain barrier.

Question 32

Identify one major advantage and one disadvantage of having a blood-brain barrier

Answer 32

The blood-brain barrier keeps out viruses, which is an advantage. It also keeps out most nutrients, a disadvantage.

Question 33

Which chemicals cross the blood-brain barrier passively

Answer 33

Small, uncharged molecules such as oxygen, carbon dioxide, and water cross the blood-brain barrier passively. So do chemicals that dissolves in the fats of the membrane such as vitamins A and D and all the drugs that affect the brain

Question 34

Which chemicals cross the blood-brain barrier by active transport?

Answer 34

Glucose the brains main fuel, amino acids the building blocks of proteins, purines, choline, a few vitamins, iron, and certain hormones

Question 35

A simple sugar. Vertebrate neurons depend almost entirely on it.

Answer 35

Glucose

Because the metabolic pathway that uses glucose requires oxygen, neurons need a steady supply of oxygen. The brain uses about 20% of all the oxygen consumed in the body.
Neurons depend so heavily on glucose because it is practically the only nutrient that crosses the blood-brain barrier after infancy, except for ketones.
To use glucose, the body needs vitamin B1, thiamine

Question 36

Which kind of glia cells wrap around the synaptic terminals of axons?

Answer 36

Astrocytes

Question 37

Difference in electrical charges between the inside and outside of the cell

Answer 37

Electrical gradient or polarization

Question 38

The difference in voltage in a resting neuron where the neuron inside the membrane has a slightly negative electrical potential with respect to the outside, mainly because of negatively charged proteins inside the cell

Answer 38

Resting potential

Question 39

Ability of some chemicals to pass more freely than others through a membrane

Answer 39

Selectively permeable

Question 40

Mechanism that actively transports sodium ions out of the cell while drawing in two potassium ions

Answer 40

Sodium-potassium pump

Question 41

Difference in distribution of ions across the neuron’s membrane

Answer 41

Concentration gradient

Question 42

Messages sent by axons

Answer 42

Action potentials

Question 43

Increased polarization across a membrane

Answer 43

Hyperpolarization

Example: using an electrode to apply a negative charge further increases the negative charge inside the neuron. When the stimulation ends, the charge returns to its original resting level

Question 44

To reduce polarization towards zero across a membrane

Answer 44

Depolarize or depolarization

Example: applying a small depolarizing current to a neuron reduces its polarization towards zero

Question 45

Minimum amount of membrane depolarization necessary to trigger an action potential

Answer 45

Threshold of excitation

When the potential reaches the threshold, the membrane opens it sodium channels and permits sodium ions to flow into the cell. The potential shoots up far beyond the strength of the stimulus.

The peak of the action potential varies from one axon to another, but is consistent for a given axon

Question 46

Membrane channel whose permeability to sodium or some other ion depends on the voltage difference across the membrane

Answer 46

Voltage-gated channel

Question 47

Drugs that attach to the sodium channels of the membrane, stopping action potentials

Answer 47

Local anaesthetics

Question 48

Principle that the amplitude and velocity of an action potential are independent of the stimulus that initiated it

Answer 48

All-or-none law

Question 49

Time when the cell resists the production of further action potentials

Answer 49

Refractory period

Question 50

A time when the membrane is unable to produce an action potential

Answer 50

Absolute refractory period

Question 51

Time after the absolute refractory period that requires a stronger stimulus to initiate an action potential

Answer 51

Relative refractory period

Question 52

Describe sodium and potassium channels when the membrane is at rest

Answer 52

When the membrane is at rest, the sodium channels are closed, preventing almost all sodium flow. Potassium channels are nearly but not entirely closed, so potassium flows slowly

Question 53

Describe the concentration of sodium and potassium ions due to the sodium-potassium pump

Answer 53

The sodium-potassium pump is an active transport that requires energy. As a result of the sodium-potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are similarly more concentrated inside than outside.

The sodium-potassium pump is affective only because of the selective permeability of the membrane, which prevents the sodium ions that were pumped out of the neuron from leaking right back in again. When sodium ions are pumped out, they stay out. However, some of the potassium ion is pumped into the neuron slowly leak out, carrying a positive charge with them. That leakage increases the electrical gradient across the membrane.

Question 54

Describe the two forces that act on sodium when the neuron is at rest.

Answer 54

The electrical gradient and the concentration gradient which tend to push sodium into the cell

Electrical gradient: sodium is positively charged and the inside of the cell is negatively charged – opposite electrical charges attract, so the electrical gradient tends to pull sodium into the cell

Concentration gradient: the difference and distribution of ions across the membrane – sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell then to leave it. However, very few sodium ions across the membrane except by the sodium-potassium pump

Question 55

Describe how the electrical and concentration gradients act on potassium when the neuron is at rest.

Answer 55

Electrical gradient: potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in.

Concentration gradient: potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out. If the potassium channels were wide open, potassium would have a small net flow out of the cell. That is, the electrical gradient and concentration gradient for potassium are almost in balance, but not quite. The sodium-potassium pump has more potassium into the cell as fast as it flows out of the cell, so the two gradients cannot get completely in balance.

Question 56

When the membrane is at rest, are the sodium ions more concentrated inside the cell or outside? Where are the potassium ions more concentrated?

Answer 56

Sodium ions are more concentrated outside the cell; potassium is more concentrated inside

Question 57

When the membrane is at rest, what tends to drive the potassium ions out of the cell? What tends to draw them into the cell?

Answer 57

When the membrane is at rest, the concentration gradient tends to drive potassium ions out of the cell; the electrical gradient draws them into the cell. The sodium-potassium pump also draws them into the cell

Question 58

Why does the neuron use considerable energy to produce a resting potential?

Answer 58

The resting potential prepares the neuron to respond rapidly. Excitation of the neuron open channels that allow sodium to enter the cell rapidly. Because the membrane did it’s work in advance by maintaining the concentration gradient for sodium, the cell is prepared to respond vigourously to a stimulus. It is like an archer who pulls the bow in advance and then waits to fire at the appropriate moment

Question 59

What is the difference between a hyperpolarization and a depolarization?

Answer 59

A hyperpolarization is an exaggeration of the usual negative charge within a cell to a more negative level than usual. A depolarization is a decrease in the amount of negative charge within the cell

Question 60

What is the relationship between the threshold and an action potential?

Answer 60

A depolarization that passes the threshold produces an action potential. One that falls short of the threshold does not produce an action potential

Question 61

Three principles helpful for remembering the events behind the action potential:

Answer 61

1. At the start, sodium ions are mostly outside the neuron and potassium ions are mostly inside
2. When the membrane is depolarized, sodium and potassium channels in the membrane open
3. At the peak of the action potential, the sodium channels close

Question 62

Describe the events behind an action potential and what happens to sodium and potassium, which are regulated by voltage-gated channels

Answer 62

At the resting potential, the sodium channels are close to permitting no sodium to cross and the potassium channels are almost closed allowing only a little flow of potassium.
As the membrane becomes depolarized, both the sodium and potassium channels begin to open, allowing for your flow. At first, opening the potassium channels makes little difference, because the concentration gradient and electrical gradient are almost in balance anyway. However, opening the sodium channels makes a big difference, because both the electrical gradient and the concentration gradient tend to drive sodium ions into the neuron. When the depolarization reaches the threshold of the membrane, the sodium channels open wide enough for sodium to flow freely. Driven by both the concentration gradient and the electrical gradient, the sodium ions into the cell rapidly, until the electrical potential across the membrane passes beyond zero to a reversed polarity.
Because of the persistent concentration gradient, sodium ions should still tend to diffuse into the cell. However, at the peak of the action potential, the sodium gates snapshot and resist re-opening for the next millisecond.
The depolarization of the membrane also opens potassium channels. At first, opening those channels made little difference. However, after so many sodium ions across the membrane, the inside of the cell has a slight positive charge instead of its usual negative charge. At this point both the concentration gradient and the electrical gradient drive potassium ions out of the cell. As a flow out of the axon, they carry with them a positive charge. Because the potassium channels remain open after the sodium channels clothes, enough potassium ions leave to drive the membrane beyond its usual resting level to a temporary hyperpolarization.
At the end of this process, the membrane has returned to its resting potential, but the inside of the neuron has slightly more sodium ions and slightly fewer potassium ions than before. Eventually, the sodium-potassium pump restores the original distribution of ions, but that process takes time.

Question 63

During the rise of the action potential, do sodium ions move into the cell or out of it? Why?

Answer 63

During the action potential, sodium ions move into the cell. The voltage-dependent sodium gates have opened, so sodium can move freely. Sodium is attracted to the inside of the cell by both an electrical and a concentration gradient

Question 64

As the membrane reaches the peak of the action potential, what brings the membrane down to the original resting potential?

Answer 64

After the peak of the action potential, potassium ions exit the cell, driving the membrane back to the resting potential. It is important to note that the sodium-potassium pump is not responsible for returning the membrane to its resting potential. The sodium-potassium pump is too slow for this purpose

Question 65

State the all-or-none law

Answer 65

According to the all-or-none law, the size and shape of the action potential are independent of the intensity of the stimulus that initiated it. That is, every depolarization beyond the threshold of excitation produces an action potential of about the same amplitude and velocity for a given axon.

Question 66

Does the all-or-none law apply to dendrites? Why or why not?

Answer 66

The all-or-none law does not apply to dendrites because they do not have action potentials

Question 67

Suppose researchers find that axon A can produce up to 1000 action potentials per second, but axon B can never produce more than 100 per second. What can we conclude about the refractory period of the two axons?

Answer 67

Axon a must have a shorter absolute refractory period, about 1 ms, whereas B has a longer absolute refractory period, about 10 ms

Question 68

A swelling where the axon exits the cell body

Answer 68

Axon hillock

Question 69

Transmission of an action potential down an axon

Answer 69

Propagation of the action potential

Question 70

An insulating material composed of fats and proteins

Answer 70

Myelin

Question 71

Axons covered with myelin sheaths

Answer 71

Myelinated axons

Question 72

The jumping of action potentials from node to node

Answer 72

Saltatory conduction

In addition to providing rapid conduction of impulses, saltatory conduction conserves energy: instead of admitting sodium ions at every point along the axon and then having to pump them out via the sodium-potassium pump, a myelinated axon admits sodium only at its nodes

Question 73

Neurons without an axon

Answer 73

Local neurons

Neurons without an axon exchange information with only their closest neighbors. Because they do not have an axon, they do not follow the all or none law. When a local neuron receives information from other neurons, it has a graded potential, a membrane potential that varies in magnitude in proportion to the intensity of the stimulus. The change in membrane potential is conducted two adjacent areas of the cell, in all directions, gradually decaying as it travels. Those various areas of the cell contact other neurons, which the excite or inhibit through synapses

Question 74

A membrane potential that varies in magnitude in proportion to the intensity of the stimulus

Answer 74

Graded potential

Question 75

In a myelinated axon, how would the action potential be affected if the nodes were much closer together? How might it be affected if the nodes were much farther apart?

Answer 75

If the nodes were closer, the action potential would travel more slowly. If they were much farther apart, the action potential would be faster if it could successfully jump from one node to the next. When the distance becomes too great, the current cannot diffuse from one node to the next and still remain above threshold, so the action potentials would stop.