B&C: Function and Structure of the Nervous System

Question 1

What are neurons?

Answer 1

Neurons are the basic signalling units that transmit information throughout the nervous system.

Question 2

What are the known functions of glial cells?

Answer 2

Glial cells provide structural support and electrical insulation to neurons, and modulating neuronal activity.

Question 3

What are dendrites?

Answer 3

branching extensions of the neuron that receive inputs from other neurons.

Question 4

What are spines?

Answer 4

Spines are little knobs attached by small necks to the surface of the dendrites and are specialized processes, where the dendrites receive inputs from other neurons

Question 5

Name some components of the soma

Answer 5

metabolic machinery that maintains the neuron: a nucleus, endoplasmic reticulum, a cytoskeleton, mitochondria, Golgi apparatus, and other common intracellular organelles

Question 6

Where does transmission occur in a neuron?

Answer 6

Synapse, a spe- cialized structure where two neurons come into close contact so that chemical or electrical signals can be passed from one cell to the next.

Question 7

What can happen if axons branch?

Answer 7

form axon collaterals that can transmit signals to more than one cell

Question 8

What are myelin and nodes of Ranvier?

Answer 8

Many axons are wrapped in layers of a fatty substance called myelin. Along the length of the axons, there are evenly spaced gaps in the myelin. These gaps are commonly referred to as the nodes of Ranvier

Question 9

When are neutrons presynaptic and when are they postsynaptic?

Answer 9

They are presynaptic when their axon makes a connection onto other neurons, and postsynaptic when other neurons make a connection onto their dendrites.

Question 10

How is the electrical potential defined?

Answer 10

the difference in the voltage across the neuronal membrane, or put simply, the voltage inside the neuron versus out- side the neuron.

Question 11

What do these voltages in the electrical potential depend on?

Answer 11

concentrations of potassium, sodium, and chloride ions as well as on charged protein molecules both inside and outside of the cell.

Question 12

What is the electrical potential of a neuron in its resting state?

Answer 12

the inside of a neuron is more negatively charged than the outside. The voltage difference across the neuronal membrane in the resting state is typically −70 millivolts (mV) inside

Question 13

What is this resting state known as?

Answer 13

resting potential or resting membrane potential.

Question 14

What does this resting potential mean?

Answer 14

This electrical potential difference means that the neuron has at its disposal a kind of battery; and like a battery, the stored energy can be used to do work— signaling work

Question 15

What does the neuronal membrane consist of?

Answer 15

The bulk of the neuronal membrane is a bilayer of fatty lipid molecules that separates the cytoplasm from the extracellular milieu.

Question 16

What is the effect of the membrane consisting of lipids?

Answer 16

it does not dissolve in the watery environments found inside and outside of the neuron. The lipid membrane blocks the flow of water- soluble substances between the inside and the outside of the neuron. It also prevents ions , proteins, and other water-soluble molecules from moving across it.

Question 17

What proteins are peppered along the membrane and what

Answer 17

transmembrane proteins that serve as conduits for ions to move across the neuronal membrane

Question 18

What are the two different types of these transmembrane proteins?

Answer 18

Ion channels are proteins with a pore through their centers, and they allow certain ions to flow down their concentration gradients. Ion pumps use energy to actively transport ions across the membrane against their concentration gradients, that is, from regions of low concentration to regions of higher concentration.

Question 19

What is meant by permeability?

Answer 19

The extent to which a particular ion can cross the membrane through a given ion channel

Question 20

What characteristic of the membrane’s permeability contributes to the membrane potential? What characteristic can change this potential?

Answer 20

The neuronal membrane is more permeable to K+ than to Na+ (or other) ions and neurons are excitable, meaning that they can change the permeability of their membranes.

Question 21

What are ion channels that are capable of changing their permeability for a particular ions called?

Answer 21

Gated ion channels

Question 22

Under normal conditions concentrations of which ions are greater inside and outside the cell?

Answer 22

Na+ and Cl− concentrations are greater outside of the cell, and K+ concentrations are greater inside the cell.

Question 23

What combats potassium ions moving across the membrane to equalise concentration?

Answer 23

neurons use active trans- port proteins, known as ion pumps. neurons use a Na+/K+ pump that pumps Na+ ions out of the cell and K+ ions into the cell.

Question 24

How do pumps get the energy to push ions against concentration gradients?

Answer 24

Each pump is an enzyme that hydrolyzes adenosine triphosphate (ATP). For each molecule of ATP that is hydrolyzed, the resulting energy is used to move three Na+ ions out of the cell and two K+ ions into the cell

Question 25

Where does the concentration gradient want to push Na and K?

Answer 25

Na+ inside the neuron and K+ outside

Question 26

Why is there a difference in voltage between the outside and inside? What is the difference and what does this cause?

Answer 26

The force of the K+ concentration gradient pushes some K+ out of the cell, leaving the inside of the neuron slightly more negative than the outside. This creates another force, an electrical gradient, because each K+ ion carries one unit of positive charge out of the neuron as it moves across the membrane.

Question 27

What is the eventual result of the two gradients on K+

Answer 27

Eventually, the force of the concentration gradient pushing K+ out through the K+ channels is equal to the force of the electrical gradient driving K+ in, the opposing forces are said to reach electrochemical equilibrium. This results in the resting membrane potential, that −70 mV difference.

Question 28

What does the process of neuronal signalling begin with?

Answer 28

when excitatory postsynaptic potentials (EPSPs) at synaps- es on the neuron’s dendrites cause ionic currents to flow in the volume of the cell body.

Question 29

How is an epsp conducted and what is this called?

Answer 29

passively through the cytoplasm of the dendrite, cell body, and axon. Passive current conduction is called electrotonic conduction or decremental conduction

Question 30

What is the furthest distance a passive current will flow?

Answer 30

1mm

Question 31

Define the mechanism that regenerates and passes along the signal initiated in the synapse

Answer 31

An action potential is a rapid depolarization and repo- larization of a small region of the membrane caused by the opening and closing of ion channels.

Question 32

How far can action potentials travel and why?

Answer 32

Action potentials can travel for meters with no loss in signal strength, because they continuously regenerate the signal.

Question 33

What allows the signal to regenerate itself and where are these located?

Answer 33

voltage-gated ion channels located in the neuronal membrane. These are found at the spike-triggering zone in the axon hillock and along the axon.

Question 34

Where are these found in myelinated neurons?

Answer 34

the axon hill- ock and the nodes of Ranvier

Question 35

Where is the action potential initiated?

Answer 35

Spike triggering zone

Question 36

How does the spike- triggering zone initiate an action potential?

Answer 36

The passive electrical currents that are generated following EPSPs on multiple distant dendrites sum together at the axon hillock. This current flows across the neuronal membrane in the spike-triggering zone, depolarizing the membrane. If the depolarization is strong enough, meaning the membrane moves from its resting potential of about −70 mV to a less negative value of approximately −55 mV, an action potential is triggered.

Question 37

What is this depolarised membrane referred to as?

Answer 37

the threshold for initiating an action potential.

Question 38

Regarding ions, what happens when the threshold is reached?

Answer 38

voltage-gated Na+ channels open and Na+ flows rapidly into the neuron. This influx of positive ions further depo- larizes the neuron, opening additional voltage-gated Na+ channels

Question 39

What is this process called?

Answer 39

the Hodgkin–Huxley cycle.

Question 40

What proceeds the Hodgkin–Huxley cycle?

Answer 40

the voltage- gated K+ channels open, allowing K+ to flow out of the neuron down its concentration gradient, shifting the membrane potential back toward its resting potential

Question 41

The opening of the K+ channels outlasts the closing of the Na+ channels, what effect does this have?

Answer 41

a second repolarizing phase of the action poten- tial; and this drives the membrane potential toward the equilibrium potential of K+, which is even more negative
than the resting potential.

Question 42

What is meant by the equilibrium potential?

Answer 42

The equilibrium potential is the particular voltage at which there is no net flux of ions.

Question 43

What is the result of the hyperpolarisation (the membrane potential is even farther from the threshold required for triggering an action potential (e.g., around −80 mV))?

Answer 43

Hyperpolarization causes the K+ channels to close, resulting in the membrane potential gradually returning to its resting state

Question 44

What is meant by the absolute refractory period?

Answer 44

During this transient hyperpolarization state, the voltage-gated Na+ channels are unable to open, and another action potential cannot be generated.

Question 45

What period follows this?

Answer 45

he relative refractory period, during which the neuron can generate action potentials, but only with larger-than-normal depolarizing currents.

Question 46

What are the two consequences of the refractory period?

Answer 46

the neuron’s speed for generating action potentials is limited to about 200 action potentials per second. The other is that the passive current that flows from the action potential can- not reopen the ion-gated channels that generated it.

Question 47

Despite this what effect does the passive current have?

Answer 47

flow down the axon with enough strength to depolarize the membrane a bit farther on, opening voltage-gated channels in this next portion of the membrane. The result is that the action potential is propagated down the axon in one direction only—from the axon hillock toward the axon terminal.

Question 48

What is the effect of myelinated axons?

Answer 48

makes the axon super- resistant to voltage loss

Question 49

What effect does this have on action potentials?

Answer 49

allows passive currents generated by the action potential to be shunted farther down the axon. The result is that action potentials do not have to be generated as often, and they can be spread out along the axon at wider intervals. (the nodes of ranvier)

Question 50

What is this conduction process called?

Answer 50

saltatory conduction.

Question 51

How is the intensity of a stimulus conveyed in an action potential? What law does this demonstrate?

Answer 51

The intensity of a stimulus (e.g., a sensory signal) is communicated by the rate of firing of the action potentials; Since they all have the same amplitude there is an all or nothing law.

Question 52

What effect does the signal have when it reaches the axon terminal?

Answer 52

depolarization of the presynaptic membrane and to trig- ger neurotransmitter release.

Question 53

What is the name given to the gap between neurons at the synapse?

Answer 53

Synaptic cleft

Question 54

What two different types of synaptic transmission are there?

Answer 54

chemical and electrical transmission

Question 55

Describe the effect of an action potential in Chemical transmission

Answer 55

depolarization of the terminal membrane, causing voltage-gated Ca2+ channels to open. The opening of these channels triggers small vesicles containing neurotransmitter to fuse with the membrane at the synapse and release the transmitter into the synaptic cleft.

Question 56

What happens to these neurotransmitters?

Answer 56

diffuses across the cleft and, on reach- ing the postsynaptic membrane, binds with specific receptors embedded in the postsynaptic membrane (

Question 57

What effect do these neurotransmitters then have?

Answer 57

Neurotransmitter binding induces a change in the receptor, which opens specific ion channels and results in an influx of ions leading to either depolarization (excitation) or hyperpo- larization (inhibition) of the postsynaptic cell

Question 58

What is the effect of hyper polarisation in the post synaptic cleft?

Answer 58

Hyperpolarization of the postsynaptic neuron produces an inhibitory postsynaptic potential (IPSP).

Question 59

Name 4 factors required to make a molecule a neurotransmitter

Answer 59

■ It is synthesized by and localized within the presynaptic neuron, and stored in the presynaptic terminal before release.
■ It is released by the presynaptic neuron when action potentials depolarize the terminal
■ The postsynaptic neuron contains receptors specific for the neurotransmitter.
■ When artificially applied
to a postsynaptic cell, the
neurotransmitter elicits the same
response that stimulating the presynaptic neuron would.

Question 60

Name 4 categories of neurotransmitters and give examples for each

Answer 60

Amino acids (as- partate, gamma-aminobutyric acid (GABA), glutamate, and glycine.) Biogenic amines (serotonin, dopamine, and histamine.), Acetylcholine (ACh) in its own chemical class and a large group of neurotransmitters consists of slightly larger molecules, the neuropeptides

Question 61

What neurotransmitters are included in the catecholamines and what category do they belong to?

Answer 61

In the category biogenic amines, includes dopamine, norepinephrine, and epinephrine

Question 62

What five groups are neuropeptides divided into? Include an examples for 4 of them

Answer 62

1.Tachykinins; (brain-gut peptides). This group includes substance P, which affects vasoconstriction and is a spinal neurotransmitter involved in pain.
2.Neurohypophyseal hormones. Oxytocin and vasopressin are in this group.
3. Hypothalamic releasing hormones. This group includes corticotropin-releasing hormone, involved in the stress response, and somatostatin, an inhibitor
of growth hormone.
4. Opioid peptides. permits the neuropeptide to bind to opiate receptors. It includes the endorphins and enkephalins.
5. Other neuropeptides. This group includes peptides that do not fit neatly into another category.

Question 63

What purpose do oxytocin and vasopressin serve?

Answer 63

The former is involved in
mammary functions and has been tagged the “love hormone” for its role in pair bonding and maternal behaviors; the latter is an antidiuretic hormone.

Question 64

Is a neuron tied to one neurotransmitter?

Answer 64

No a neuron may have multiple neurotransmitters however the neurotransmitters may be released together or separately, depending on the conditions of stimulation. (depending on rate of stimulation of action potential)

Question 65

What does the effect of a neurotransmitter depend on? (2)

Answer 65

The receptor rather than the transmitter; might cause one to increase firing and the other to decrease firing. Also on the connections of neurons that use the transmitter.

Question 66

Name 6 types of neurotransmitters that usually have an excretory effect

Answer 66

Catecholamines, Serotonin, Glutamate, ACh, histamine and some of the neuropeptides

Question 67

Name 3 types of neurotransmitters that usually have a inhibitory effect

Answer 67

Glycamine, GABA and some of the peptides

Question 68

What is meant by conditional neurotransmitters?

Answer 68

Some neurotransmitters act only in concert with other factors; their action is conditioned on the presence of another transmitter in the synaptic cleft or activity in the neuronal circuit

Question 69

What three ways can the removal of a transmitter be carried out to prevent further signal transduction?

Answer 69

(a) by active reuptake of the substance back into the presynaptic ter- minal, (b) by enzymatic breakdown of the transmitter in the synaptic cleft, or (c) merely by diffusion of the neurotransmitter away from the region of the synapse or site of action

Question 70

Name a group of neurotransmitters that are removed from the synaptic cleft by reuptake mechanisms

Answer 70

biogenic amines (dopamine, norepinephrine, epinephrine, histamine, and serotonin).

Question 71

What is the reuptake mechanism mediated by?

Answer 71

active transporters, which are transmembrane proteins that pump the neurotransmitter back across the presynaptic membrane.

Question 72

Give an example of a neurotransmitter that is eliminated from the synaptic cleft by enzymatic action and the enzyme that breaks it down

Answer 72

The enzyme acetylcholinesterase (AChE), which is located in the synaptic cleft, breaks down ACh

Question 73

In electrical synapses what replaces synaptic clefts?

Answer 73

neuronal membranes are touching at specializations called “gap junctions”, and the cytoplasms of the two neurons are essentially continuous.

Question 74

What allows the electrical transmissions to occur?

Answer 74

gap junction channels create pores connecting the cytoplasms of the two neurons. As a result, the two neurons are isopotential (i.e., have the same electrical potential), meaning that electrical changes in one are reflected instantaneously in the other.

Question 75

Can signals travel both directions?

Answer 75

Yes, under most circumstances, the communication is bidirectional; however, so-called rectifying synapses limit current flow in one direction, as is typical in chemical synapses.

Question 76

When are electrical synapses useful? (2)

Answer 76

Electrical synapses are useful when information must be conducted rapidly, such as in the escape reflex of some invertebrates or when groups of neurons should operate synchronously, as with some hypothalamic neurosecretory neurons.

Question 77

What are the limitations of electrical synapses

Answer 77

They are much less plastic than chemical synapses, and they cannot amplify a signal (whereas an action potential that triggers a chemi- cal synapse could cause a large release of neurotransmit- ter, thus amplifying the signal).