Chapter 4: The Nervous System

Question 1

Neurons are

Answer 1

responsible for the conduction of impulses.

Question 2

Neurons communicate using both

Answer 2

electrical and chemical forms of communication.

Question 3

Electrical communication occurs via

Answer 3

via ion exchange and the generation of membrane potentials down the length of the axon.

Question 4

Chemical communication occurs via

Answer 4

Chemical communication occurs via neurotransmitter release from the presynaptic cell and the binding of these neurotransmitters to the postsynaptic cell.

Question 5

Neurons consist of many different parts.

Answer 5

• Neurons consist of many different parts:
• Dendrites are appendages that receive signals from other cells.
• The cell body or soma is the location of the nucleus as well as organelles such as the endoplasmic reticulum and ribosomes.
• The axon hillock is where the cell body transitions to the axon, and where action potentials are initiated.
• The axon is a long appendage down which an action potential travels.
• The nerve terminal or synaptic bouton is the end of the axon from which neurotransmitters are released.
• Nodes of Ranvier are exposed areas of myelinated axons that permit salta-tory conduction.
• The synapse consists of the nerve terminal of the presynaptic neuron, the membrane of the postsynaptic cell, and the space between the two, called the synaptic cleft.

Question 6

Dendrites are

Answer 6

Dendrites are appendages that receive signals from other cells.

Question 7

The cell body or soma is the location of the

Answer 7

The cell body or soma is the location of the nucleus as well as organelles such as the endoplasmic reticulum and ribosomes.

Question 8

The axon hillock is where the

Answer 8

the cell body transitions to the axon, and where action potentials are initiated.

Question 9

The axon is a

Answer 9

The axon is a long appendage down which an action potential travels.

Question 10

The nerve terminal or synaptic bouton is the

Answer 10

The nerve terminal or synaptic bouton is the end of the axon from which neurotransmitters are released.

Question 11

Nodes of Ranvier are exposed areas of

Answer 11

Nodes of Ranvier are exposed areas of myelinated axons that permit salta-tory conduction.

Question 12

The synapse consists of the

Answer 12

The synapse consists of the nerve terminal of the presynaptic neuron, the membrane of the postsynaptic cell, and the space between the two, called the synaptic cleft.

Question 13

Many axons are coated in

Answer 13

Many axons are coated in myelin, an insulating substance that prevents signal loss.

Question 14

Myelin is created by

Answer 14

Myelin is created by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

Question 15

Myelin prevents

Answer 15

Myelin prevents dissipation of the neural impulse and crossing of neural impulses from adjacent neurons.

Question 16

Individual axons are

Answer 16

Individual axons are bundled into nerves or tracts.

Question 17

A single nerve may carry

Answer 17

A single nerve may carry multiple types of information, including sensory, motor, or both. Tracts contain only one type of information.

Question 18

Cell bodies of neurons of the same type within a nerve

Answer 18

Cell bodies of neurons of the same type within a nerve cluster together in ganglia in the peripheral nervous system.

Question 19

Cell bodies of the individual neurons within a

Answer 19

Cell bodies of the individual neurons within a tract cluster together in nuclei in the central nervous system.

Question 20

Neuroglia or glial cells are other cells

Answer 20

Neuroglia or glial cells are other cells within the nervous system in addition to neurons.

Question 21

Astrocytes does what?

Answer 21

Astrocytes nourish neurons and form the blood–brain barrier, which con-trols the transmission of solutes from the bloodstream into nervous tissue

Question 22

Ependymal cells line the

Answer 22

Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.

Question 23

Microglia are

Answer 23

Microglia are phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.

Question 24

Oligodendrocytes (CNS) and Schwann cells (PNS) produce

Answer 24

Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons.

Question 25

Transmission of Neural Impulses

Answer 25

STARTS

Question 26

All neurons exhibit a

Answer 26

All neurons exhibit a resting membrane potential of approximately −70 mV.

Question 27

Resting potential is maintained using

Answer 27

Resting potential is maintained using selective permeability of ions as well as the Na+/K+ ATPase.

Question 28

The Na+/K+ ATPase pumps

How many ions for each ?

Answer 28

The Na+/K+ ATPase pumps three sodium ions out of the cell for every two potassium ions pumped in.

Question 29

Incoming signals can be either

Answer 29

Incoming signals can be either excitatory or inhibitory.

Question 30

Excitatory signals cause

Answer 30

Excitatory signals cause depolarization of the neuron.

Question 31

Inhibitory signals cause

Answer 31

Inhibitory signals cause hyperpolarization of the neuron.

Question 32

Temporal summation refers to the

Answer 32

Temporal summation refers to the integration of multiple signals near each other in time.

Question 33

Spatial summation refers to the

Answer 33

Spatial summation refers to the addition of multiple signals near each other in space.

Question 34

An action potential is used to

Answer 34

propagate signals down the axon.

Question 35

When enough excitatory stimulation occurs, the cell is

Answer 35

When enough excitatory stimulation occurs, the cell is depolarized to the threshold voltage and voltage-gated sodium channels open.

Question 36

Sodium flows into the neuron due to its strong

Answer 36

Sodium flows into the neuron due to its strong electrochemical gradient. This continues depolarizing the neuron.

Question 37

At the peak of the action potential (approximately__) what happens

Answer 37

At the peak of the action potential (approximately +35 mV), sodium chan-nels are inactivated and potassium channels open.

Question 38

Potassium flows out of the neuron due to its

Answer 38

Potassium flows out of the neuron due to its strong electrochemical gradient, repolarizing the cell. Potassium channels stay open long enough to over-shoot the action potential, resulting in a hyperpolarized neuron; then, the potassium channels close.

Question 39

The Na+/K+ ATPase brings the neuron

Answer 39

The Na+/K+ ATPase brings the neuron back to the resting potential and restores the sodium and potassium gradients.

Question 40

While the axon is hyperpolarized, it is in its

Answer 40

While the axon is hyperpolarized, it is in its refractory period. During the absolute refractory period, the cell is unable to fire another action potential. During the relative refractory period, the cell requires a larger than normal stimulus to fire an action potential.

Question 41

The impulse propagates down the length of the axon because

Answer 41

The impulse propagates down the length of the axon because the influx of sodium in one segment of the axon brings the subsequent segment of the axon to threshold. The fact that the preceding segment of the axon is in its refractory period means that the action potential can only travel in one direction.

Question 42

At the nerve terminal, neurotransmitters are

Answer 42

At the nerve terminal, neurotransmitters are released into the synapse.

Question 43

When the action potential arrives at the nerve terminal

Answer 43

When the action potential arrives at the nerve terminal, voltage-gated cal-cium channels open.

Question 44

The influx of calcium causes

Answer 44

The influx of calcium causes fusion of vesicles filled with neurotransmitters with the presynaptic membrane, resulting in exocytosis of neurotransmitters into the synaptic cleft.

Question 45

The neurotransmitters bind to receptors on the

Answer 45

The neurotransmitters bind to receptors on the postsynaptic cell, which may be ligand-gated ion channels or G protein-coupled receptors.

Question 46

Neurotransmitters must be cleared from the postsynaptic receptors to do what?

Answer 46

Neurotransmitters must be cleared from the postsynaptic receptors to stop the propagation of the signal. There are three ways this can happen:

• The neurotransmitter can be enzymatically broken down.
• The neurotransmitter can be absorbed back into the presynaptic cell by reuptake channels.

• The neurotransmitter can diffuse out of the synaptic cleft.

Question 47

Organization of the Human Nervous System

Answer 47

START

Question 48

There are three types of neurons in the

Answer 48

There are three types of neurons in the nervous system: motor (efferent) neu-rons, interneurons, and sensory (afferent) neurons.

Question 49

The nervous system is made up of the

Answer 49

The nervous system is made up of the central nervous system (CNS: brain and spinal cord) and peripheral nervous system (PNS: cranial and spinal nerves).

Question 50

In the CNS, white matter consists of

Answer 50

In the CNS, white matter consists of myelinated axons, and grey matter consists of unmyelinated cell bodies and dendrites. In the brain, white matter is deeper than grey matter. In the spinal cord, grey matter is deeper than white matter.

Question 51

The PNS is divided into the

Answer 51

The PNS is divided into the somatic (voluntary) and autonomic (automatic) nervous systems.

Question 52

The autonomic nervous system is further divided into the

Answer 52

The autonomic nervous system is further divided into the parasympathetic (rest-and-digest) and sympathetic (fight-or-flight) branches.

Question 53

Reflex arcs use the ability of interneurons in the spinal cord to do what?

Answer 53

Reflex arcs use the ability of interneurons in the spinal cord to relay informa-tion to the source of a stimulus while simultaneously routing it to the brain.

Question 54

In a monosynaptic reflex arc

Answer 54

In a monosynaptic reflex arc, the sensory (afferent, presynaptic) neuron fires directly onto the motor (efferent, postsynaptic) neuron.

Question 55

In a polysynaptic reflex arc, the

Answer 55

In a polysynaptic reflex arc, the sensory neuron may fire onto a motor neu-ron as well as interneurons that fire onto other motor neurons.

Question 56

Neurons are ?

Answer 56

Neurons are specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals.

Question 57

Axons ___; dendrites __ .

Answer 57

carry neural signals away from the soma carry signals toward the soma

Question 58

Myelin is produced by

Answer 58

1. Oligodendrocytes in the central nervous system 2. Schwann cells in the peripheral nervous system.

Question 59

at the end of the axon is the

Answer 59

nerve terminal or synaptic bouton (knob).

Question 60

Neurotransmitters released from

Answer 60

the axon terminal traverse the synaptic cleft and bind to receptors on the postsynaptic neuron.

Question 61

Multiple neurons may be bundled together to form a

Answer 61

nerve in the peripheral nervous system.

Question 62

In the central nervous system, axons may be bundled together to form

Answer 62

tracts Unlike nerves, tracts only carry one type of information. The cell bodies of neurons in the same tract are grouped into nuclei.

Question 63

REAL WORLD

Answer 63

Sometimes the body mounts an immune response against its own myelin, leading to the destruction of this insulating substance (demyelination) . Because myelin speeds the conduction of impulses along a neuron, the absence of myelin slows down information transfer . A common demyelinating disorder is multiple sclerosis (MS) . In MS, the myelin of the brain and spinal cord is selectively targeted . Because so many different kinds of neurons are demyelinated, MS patients experience a wide variety of symptoms including weakness, lack of balance, vision problems, and incontinence .

Question 64

Neurons are not the only cells in the nervous system. Neurons must be supported and myelinated by other cells. These cells are often called

Answer 64

glial cells, or neuroglia.

Question 65

Astrocytes

Answer 65

nourish neurons and form the blood–brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue.

Question 66

Ependymal cells

Answer 66

line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.

Question 67

Microglia

Answer 67

are phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.

Question 68

Oligodendrocytes (CNS) and Schwann cells (PNS) produce

Answer 68

produce myelin around axons.

Question 69

nourish neurons and form the blood–brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue.

Answer 69

Astrocytes

Question 70

line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.

Answer 70

Ependymal cells

Question 71

are phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.

Answer 71

Microglia

Question 72

___ and ___ produce myelin around axons.

Answer 72

Oligodendrocytes (CNS) and Schwann cells (PNS)

Question 73

action potentials

Answer 73

action potentials to relay electrical impulses down the axon to the synaptic bouton.

Question 74

A cell’s resting membrane potential is the

Answer 74

A cell’s resting membrane potential is the net electric potential difference that exists across the cell membrane, created by movement of charged molecules across that membrane.

**

For neurons, this potential is about −70 mV, with the inside of the neuron being negative relative to the outside. The two most important ions involved in gener-ating and maintaining the resting potential are potassium (K+) and sodium (Na+).

Question 75

The potassium concentration inside the cell averages about

Answer 75

140 mM, as compared to 4 mM outside of the cell.

This concentration difference makes it favorable for potassium to move to the outside of the cell. To facilitate the outward movement of potassium, the cell membrane has transmembrane potassium leak channels, which allow the slow leak of potassium out of the cell. As potassium continually leaks out of the cell, the cell loses a small amount of positive charge, leaving behind a small amount of negative charge and making the outside of the cell slightly positively charged.

Question 76

potassium leak channels

Answer 76

To facilitate the outward movement of potassium, the cell membrane has transmembrane potassium leak channels, which allow the slow leak of potassium out of the cell. As potassium continually leaks out of the cell, the cell loses a small amount of positive charge, leaving behind a small amount of negative charge and making the outside of the cell slightly positively charged.

Question 77

The potential difference that represents this potassium equilibrium is called the

Answer 77

The potential difference that represents this potassium equilibrium is called the equilibrium potential of potassium. Potassium’s equilibrium potential is around −90 mV. The negative sign is assigned due to con-vention, and because a positive ion (potassium) is leaving the cell.

Question 78

Sodium’s concen-tration gradient is the reverse of potassium’s, with a concentration of about ___ inside and ____ outside the cell.

Answer 78

Next, let’s consider in isolation the other important ion, sodium. Sodium’s concentration gradient is the reverse of potassium’s, with a concentration of about 12 mM inside and 145 mM outside of the cell, meaning there is a driving force pushing sodium into the cell. This movement is facilitated by sodium leak channels. The slow leak of sodium into the cell causes a build-up of electric potential. The equilibrium potential of sodium is around 60 mV, and is positive because sodium is moving into the cell.

Question 79

The resting potential is thus a tug-of-war:

Answer 79

Potassium’s movement pulls the cell potential toward −90 mV, while sodium’s movement pulls the cell potential the opposite way, toward +60 mV. But neither ion ever “wins” the tug of war. Instead, a balance of these two effects is reached at around −70 mV for the average nerve cell, as can be seen in Figure 4.3. This balance, this net effect of sodium and potassium’s equilibrium potentials, is the resting membrane potential. The resting potential is closer to potassium’s equilibrium potential because the cell is slightly more permeable to potassium. Neither ion is ever able to establish its own equilibrium, so both ions continue leaking across the cell membrane.

Question 80

MNEMONIC

Answer 80

To remember the direction of ion movement by Na/K ATPase, think pumpKin.

Question 81

Excitatory input causes

Answer 81

depolarization (raising the membrane potential, Vm, from its resting potential) and thus makes the neuron more likely to fire an action potential.

Question 82

Inhibitory input causes

Answer 82

hyperpolarization (lowering the membrane potential from its resting potential) and thus makes the neuron less likely to fire an action potential.

Question 83

If the axon hillock receives enough excitatory input to be depolarized to the threshold value (usually in the range of

Answer 83

−55 mV to −40 mV), an action potential will be triggered.

Question 84

Further, a postsynaptic neuron may receive information from several different presynaptic neurons, some of which are excitatory and some of which are inhibitory. The additive effect of multiple signals is known as

Answer 84

summation

Question 85

In temporal summation

Answer 85

In temporal summation, multiple signals are integrated during a relatively short period of time. A number of small excitatory signals firing at nearly the same moment could bring a postsynaptic cell to threshold, enabling an action potential.

Question 86

In spatial summation,

Answer 86

In spatial summation, the additive effects are based on the number and location of the incoming signals. A large num-ber of inhibitory signals firing directly on the soma will cause more profound hyper-polarization of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron.

Question 87

A graph of membrane potential vs. time during an action potential is shown in Figure 4.4.

Answer 87

If the cell is brought to threshold, voltage-gated sodium channels open in the mem-brane. As the name implies, these ion channels open in response to the change in potential of the membrane (depolarization) and permit the passage of sodium ions. There is a strong electrochemical gradient that promotes the migration of sodium into the cell. From an electrical standpoint, the interior of the cell is more negativethan the exterior of the cell, which favors the movement of positively charged sodium cations into the cell. From a chemical standpoint, there is a higher concentration of sodium outside the cell than inside, which also favors the movement of sodium into the cell. As sodium passes through these ion channels, the membrane potential becomes more positive; that is, the cell rapidly depolarizes. Sodium channels not only open in response to changes in membrane potential, but are also inactivated by them. When Vm approaches +35 mV, the sodium channels are inactivated and will have to be brought back near the resting potential to be deinactivated. Thus, these sodium channels can exist in three states: closed (before the cell reaches threshold, and after inactivation has been reversed), open (from threshold to approximately +35 mV), and inactive (from approximately +35 mV to the resting potential).

****The positive potential inside the cell not only triggers the voltage-gated sodium channels to inactivate, but also triggers the voltage-gated potassium channels to open. Once sodium has depolarized the cell, there is an electrochemical gradient favoring the efflux of potassium from the neuron. As positively charged potassium cations are driven out of the cell, there will be a restoration of the negative membrane potential called repolarization. The efflux of K+ causes an overshoot of the resting membrane potential, hyperpolarizing the neuron. This hyperpolarization serves an important function: it makes the neuron refractory to further action potentials. There are two types of refractory periods. During the absolute refractory period, no amount of stimulation can cause another action potential to occur. During the relative refractory period, there must be greater than normal stimulation to cause an action potential because the membrane is starting from a potential that is more negative than its resting value.
The Na+/K+ ATPase acts to restore not only the resting potential, but also the sodium and potassium gradients that have been partially dissipated by the action potential.