2. Nerve Cells, Neural Circuitry, and Behaviour

Question 1

The human brain contains a huge number of nerve cells, on the order of 10^11 neurons, that can be classified into at least a thousand different types. What matters more for behaviour than the variety of these neurons?

Answer 1

The complexity of human behaviour depends less on the variety of neurons than on their organisation into anatomical circuits with precise functions. One key organisational principle of the brain, therefore, is that nerve cells with similar properties can produce different actions because of the way they are interconnected.

Question 2

Because relatively few principles of organization give rise to considerable complexity, it is possible to learn a great deal about how the nervous system pro- duces behavior by focusing on five basic features of the nervous system.

Name each of these

Answer 2

1. The structural components of individual nerve cells;
2. The mechanisms by which neurons produce
   signals within and between nerve cells;
3. The patterns of connections between nerve cells and between nerve cells and their targets: muscles
   and gland effectors;
4. The relationship of different patterns of interconnection to different types of behaviour; and
5. How neurons and their connections are modified
   by experience

Question 3

There are two main classes of cells in the nervous system, what are they?

Answer 3

nerve cells, or neurons, and glial cells, or glia.

Question 4

A typical neuron has four morphologically defined regions. Name these

Answer 4

(1) The cell body (soma)
(2) Dendrites,
(3) Axon, and
(4) Presynaptic terminals

Question 5

What is generally contained in the soma?

Answer 5

The cell body or soma is the metabolic centre of the cell. It contains the nucleus, which contains the genes of the cell, and the endoplasmic reticulum, an extension of the nucleus where the cell’s proteins are synthesised.

Question 6

The cell body usually gives rise to two kinds of processes, what are these?

Answer 6

Several short dendrites and one long, tubular axon. Dendrites branch out in tree-like fashion and are the main apparatus for receiving incoming signals from other nerve cells. The axon typically extends some distance from the cell body and carries signals to other neurons.

Question 7

What distance range can an axon convey electrical signals?

Answer 7

ranging from 0.1 mm to 2 m

Question 8

What are these signals called and where are they initiated?

Answer 8

These electrical signals, called action potentials, are initiated at a specialised trigger region near the origin of the axon called the initial segment from which they propagate down the axon without failure or distortion at speeds of 1 to 100 m/s.

Question 9

To what extent does the amplitude of an action potential vary and what is this range?

Answer 9

The amplitude of an action potential traveling down the axon remains constant at 100 mV because the action potential is an all-or-none impulse that is regenerated at regular intervals along the axon.

Question 10

What is done to increase the speed at which action potentials are conducted?

Answer 10

To increase the speed by which action potentials are conducted, large axons are wrapped in an insulating sheath of a lipid substance, myelin. The sheath is interrupted at regular intervals by the nodes of Ranvier, uninsulated spots on the axon where the action potential is regenerated.

Question 11

What is located near the end of the axon?

Answer 11

Near its end the axon divides into fine branches that contact other neurons at specialised zones of communication known as synapses.

Question 12

What names are given to the cells 1. sending and 2. receiving this transmission?

Answer 12

The nerve cell transmitting a signal is called the presynaptic cell; the cell receiving the signal is the postsynaptic cell.

Question 13

From where do the presynaptic cells transmit signals from? (broadly)

Answer 13

The presynaptic cell transmits signals from specialised enlarged regions of its axon’s branches, called presynaptic terminals or nerve terminals.

Question 14

Where do the signals go from the presynaptic terminal?

Answer 14

The presynaptic and postsynaptic cells are separated by a very narrow space, the synaptic cleft. Most presynaptic terminals end on the postsynaptic neuron’s dendrites; but the terminals may also terminate on the cell body or, less often, at the beginning or end of the axon of the receiving cell

Question 15

The coherent structure of the neuron did not become clear until late in the 19th century (due to the prevailing belief that the cell theory did not apply to the brain, which they thought of as a continuous, web-like reticulum of very thin processes.) What allowed for this?

Answer 15

Ramón y Cajal began to use the silver-staining method introduced by Golgi, still used today. The stain reveals that there is no cytoplasmic continuity between neurons, even at synapses between two cells.

Question 16

What two advantages does this silver-staining method introduced by Golgi have?

Answer 16

First, in a random manner that is not understood, the silver solution stains only about 1% of the cells in any particular brain region, making it possible to examine a single neuron in isolation from its neighbours. Second, the neurons that do take up the stain are delineated in their entirety, including the cell body, axon, and full dendritic tree.

Question 17

How did Ramón y Cajal further utilise this method?

Answer 17

Ramón y Cajal applied Golgi’s method to the embryonic nervous systems of many animals as well as humans. By examining the structure of neurons in almost every region of the nervous system, he could describe classes of nerve cells and map the precise connections between many of them.

Question 18

In this way Ramón y Cajal adduced, in addition to the neuron doctrine, two other principles of neural organisation that would prove particularly valuable in studying communication in the nervous system. Name and describe these two principles

Answer 18

The first of these has come to be known as the principle of dynamic polarisation. It states that electrical signals within a nerve cell flow only in one direction: from the receiving sites of the neuron, usually the dendrites and cell body, to the trigger region at the axon. From there the action potential is propagated along the entire length of the axon to its terminals.

The other principle advanced by Ramón y Cajal is that of connectional specificity, which states that nerve cells do not connect randomly with one another in the formation of networks. Rather each cell makes specific connections—at particular contact points—with certain postsynaptic target cells but not with others.

Question 19

Ramón y Cajal was also among the first to realise the feature that most distinguishes one type of neuron from another. What feature is this?

Answer 19

Form, specifically the number of the processes arising from the cell body. Neurons are thus classified into three large groups: unipolar, bipolar, and multipolar.

Question 20

Describe unipolar neurons

Answer 20

Unipolar neurons are the simplest because they have a single primary process, which usually gives rise to many branches. One branch serves as the axon; other branches function as receiving structures.

Question 21

Where are unipolar neurons often found?

Answer 21

These cells predominate in the nervous systems of invertebrates; in vertebrates they occur in the autonomic nervous system.

Question 22

Describe the typical structure of a Bipolar cell

Answer 22

Bipolar neurons have an oval soma that gives rise to two distinct processes: a dendritic structure that receives signals from the periphery of the body and an axon that carries information toward the central nervous system

Question 23

What types of neurons are often bipolar neurons?

Answer 23

Many sensory cells are bipo- lar, including those in the retina and in the olfactory epithelium of the nose.

Question 24

Describe the receptor neurons that convey touch, pressure, and pain signals to the spinal cord

Answer 24

The receptor neurons that con- vey touch, pressure, and pain signals to the spinal cord, are variants of bipolar cells called pseudo-unipolar cells. These cells develop initially as bipolar cells but the two cell processes fuse into a single continuous structure that emerges from a single point in the cell body.

The axon splits into two branches, one running to the periphery (to sensory receptors in the skin, joints, and muscle) and another to the spinal cord

Question 25

What cells predominate in the nervous system of vertebrates?

Answer 25

Multipolar neurons predominate in the nervous system of vertebrates.

Question 26

Describe traits of a multi-polar cell

Answer 26

They typically have a single axon and many dendritic structures emerging from various points around the cell body. Multipolar cells vary greatly in shape, especially in the length of their axons and in the extent, dimensions, and intricacy of their dendritic branching.

Question 27

What does the extent of branching typically correlate with?

Answer 27

Usually the extent of branching correlates with the number of synaptic contacts that other neurons make onto them. A spinal motor neuron with a relatively modest number of dendrites receives about 10,000 contacts—1,000 on the cell body and 9,000 on dendrites. The dendritic tree of a Purkinje cell in the cerebellum is much larger and bushier, receiving as many as a million contacts!

Diagrams of each of these nerves is found in docs

Question 28

How else can nerve cells be classified? Name these categories

Answer 28

Nerve cells are also classified into three major functional categories: sensory neurons, motor neurons, and interneurons.

Question 29

What function do each of these neurons carry out?

Answer 29

Sensory neurons carry information from the body’s peripheral sensors into the nervous system for the purpose of both perception and motor coordination.

Motor neurons carry commands from the brain or spinal cord to muscles and glands (efferent information).

Interneurons are the most numerous and carry signals between neurons

Question 30

What are meant by afferent neurons?

Answer 30

Some primary sensory neurons are called afferent neurons, and the two terms are used interchangeably. The term afferent (carried toward the central nervous system) applies to all information reaching the central nervous system from the periphery, whether or not this information leads to sensation. The term sensory should, strictly speaking, be applied only to afferent inputs that lead to perception.

Question 31

How are interneurons further subdivided?

Answer 31

Interneurons are the most numerous and are subdivided into two classes: relay and local. Relay or projection interneurons have long axons and convey signals over considerable distances, from one brain region to another. Local interneurons have short axons because they form connections with nearby neurons in local circuits.

Question 32

Describe how each functional classification can be subdivided further.

Answer 32

Sensory system interneurons can be classified according to the type of sensory stimuli to which they respond; these initial classifications can be broken down still further, into many subgroups according to location, density, and size. For example, the retinal ganglion cell interneurons, which respond to light, are classified into 13 types based on the size of the dendritic tree, the branching density, and the depth of its location in specific layers of the retina

Question 33

Are neurons or glia more numerous?

Answer 33

Glial cells greatly outnumber neurons—there are 2 to 10 times more glia than neurons in the vertebrate central nervous system.

Question 34

Name two ways in which glia differ from neurons

Answer 34

Glia differ from neurons morphologically; they do not form dendrites and axons. Glia also differ functionally. Although they arise from the same embryonic precursor cells, they do not have the same membrane properties as neurons; are not electrically excitable; and are not directly involved in electrical signalling, which is the function of nerve cells.

Question 35

The diversity in morphology of glial cells suggests that glia are probably as heterogeneous as neurons. Nonetheless, glia in the vertebrate nervous system can be divided into two major classes, name these.

Answer 35

microglia and macroglia

Question 36

What are microglia?

Answer 36

Microglia are immune system cells that are mobilised to present antigens and become phagocytes during injury, infection, or degenerative diseases.

Question 37

What are macroglia? (3)

Answer 37

There are three main types of macroglia: oligodendrocytes, Schwann cells, and astrocytes.

Question 38

Are micro or macro glia more numerous?

Answer 38

In the human brain about 80% of all the cells are macroglia.

Question 39

What are oligodendrocytes and schwann cells?

Answer 39

Oligodendrocytes and Schwann cells are small cells with relatively few processes. Both cells form the myelin sheath that insulates an axon by tightly winding their membranous processes around the axon in a spiral.

Question 40

What are the differences between oligodendrocytes and schwann cells?

Answer 40

Oligodendrocytes are found in the central nervous system; each cell envelops from one to 30 axonal segments (called internodes), depending on axon diameter. Schwann cells occur in the peripheral nervous system, where each envelops a single segment of one axon

These can be see in docs

Question 41

Upon myelination, how do oligodendrocytes and schwann cells influence axons?

Answer 41

Upon myelination, oligodendrocytes and Schwann cells influence axons by enhancing signal conduction and by segregating voltage-sensitive ion channels into distinct axonal domains (called node of Ranvier).

Question 42

Where do astrocytes get their name from?

Answer 42

Astrocytes, the third main class of glial cells, owe their name to their irregular, roughly star-shaped cell bodies and large numbers of processes

Question 43

What two main types of astrocytes are there? Describe where they are found and their rough structure

Answer 43

Protoplasmic astrocytes are found in the gray matter; their many processes end in sheet-like appendages.

Fibrous astrocytes are found in the white matter and have long, fine processes that contain large bundles of tightly packed intermediate filaments.

Question 44

What are meant by end-feet?

Answer 44

Both types of astrocytes have end-feet, dilatations that contact and surround capillaries and arterioles throughout the brain

Question 45

What differences are there between the end-feet of protoplasmic astrocytes and fibrous astrocytes?

Answer 45

The sheet-like processes of protoplasmic astrocytes envelop nerve cell bodies and synapses, whereas the end-feet of fibrous astrocytes contact axons at the nodes of Ranvier.

Question 46

What do we know about the functions of astrocytes? (4)

Answer 46

The functions of astrocytes are still mysterious. It is generally thought that astrocytes are not essential for information processing but support neurons in four ways:

1. Astrocytes separate cells, thereby insulating neuronal groups and synaptic connections from each other.
2. Because astrocytes are highly permeable to K+ , they help regulate the K+ concentration in the space between neurons. As we shall learn below, K+ flows out of neurons when they fire. Repetitive firing may create excess extracellular K+ that could interfere with signalling between cells in the vicinity. Astrocytes can take up the excess K+ and thus maintain the efficiency of signalling between neurons.
3. Astrocytes perform other important housekeeping chores that promote efficient signalling between neurons. For example, as we shall learn later, they take up neurotransmitters from synaptic zones after release.
4. Astrocytes help nourish surrounding neurons by releasing growth factors.

Question 47

Are astrocytes involved in signalling processes?

Answer 47

Although glial cells do not generate action potentials, they have recently been found to participate in neuron-glial signalling processes. The significance of this signalling is still poorly understood, but it may actively help regulate synapse development and function

Question 48

Describe the knee-jerk reflex broadly and the function it serves

Answer 48

The reflex is initiated when a transient imbalance of the body stretches the quadriceps extensor muscles of the leg. This stretching elicits sensory information that is conveyed to motor neurons, which in turn sends commands to the extensor muscles to contract so that balance is restored.

This reflex is useful for clinically, but the underlying mechanism is important because it continuously maintains normal tone in the quadriceps and prevents our knees from buckling when we stand or walk.

Question 49

How can the knee-jerk reflex be tested clinically?

Answer 49

The tendon of the quadriceps femoris, an extensor muscle that moves the lower leg, is attached to the tibia through the tendon of the kneecap. Tapping this tendon just below the patella stretches the quadriceps femoris. This stretch initiates reflex contraction of the quadriceps muscle to produce the familiar knee jerk. By increasing the tension of a select group of muscles, the stretch reflex changes the position of the leg, suddenly extending it outward

Question 50

Describe the nerve cells involved in the knee jerk reflex

Answer 50

The cell bodies of the sensory neurons involved in the knee-jerk reflex are clustered near the spinal cord in the dorsal root ganglia. They are pseudo-unipolar cells; one branch of each cell’s axon runs to the quadriceps muscle at the periphery, whereas the other runs centrally into the spinal cord. The branch that innervates the quadriceps makes contact with stretch- sensitive receptors (muscle spindles) and is excited when the muscle is stretched. The branch reaching the spinal cord forms excitatory connections with the motor neurons that innervate the quadriceps and control its contraction. This branch also contacts local interneurons that inhibit the motor neurons controlling the opposing flexor muscles (In docs). These local interneurons are not involved in producing the stretch reflex itself, but by coordinating motor action they increase the stability of the reflex.

Question 51

Thus the electrical signals that produce the stretch reflex carry four kinds of information, list these

Answer 51

1. Sensory information is conveyed to the central nervous system (the spinal cord) from muscle.
2. Motor commands from the central nervous
   system are issued to the muscles that carry out the knee jerk.
3. Inhibitory commands are issued to motor neurons that innervate opposing muscles, providing coordination of muscle action.
4. Information about local neuronal activity related to the knee jerk is sent to higher centres of the central nervous system, permitting the brain to coordinate different behaviours either simultaneously or in series.

Question 52

What is meant by divergence and convergence?

Answer 52

The stretching of just one muscle, the quadriceps, activates several hundred sensory neurons, each of which makes direct contact with 45 to 50 motor neurons. This pattern of connection, in which one neuron activates many target cells, is called divergence

Conversely, a single motor cell in the knee jerk circuit receives 200 to 450 input con- tacts from approximately 130 sensory cells. This pattern of connection is called convergence

Question 53

When is divergence common and when is convergence common?

Answer 53

Divergence is especially common in the input stages of the nervous system; by distributing its signals to many target cells, a single neuron can exert wide and diverse influence.

Convergence is common at the output stages of the nervous system; a target motor cell that receives information from many sensory neurons is able to integrate information from many sources.

Question 54

What does convergence also ensure?

Answer 54

Convergence also ensures that a motor neuron is activated only if a sufficient number of sensory neurons become activated together.

Question 55

What is meant by feed-forward and feedbackinhibition?

Answer 55

Excitatory connections in the leg’s extensor muscles cause these muscles to contract, whereas connections with inhibitory interneurons prevent the antagonist flexor muscles from contracting. This feature of the circuit is an example of feed-forward inhibition

Neurons can also have connections that provide feedback inhibition. For example, a motor neuron may have excitatory connections with both a muscle and an inhibitory interneuron that in turn inhibits the motor neuron. The inhibitory interneuron is thus able to limit the ability of the motor neuron to excite the muscle.

Question 56

What other characteristic does the feedforward inhibition of the knee jerk flex have?

Answer 56

In the knee-jerk reflex, feed-forward inhibition is reciprocal, ensuring that the flexor and extensor pathways always inhibit each other so that only muscles appropriate for the movement and not those opposed to it are recruited.

Question 57

To produce a behavior, a stretch reflex for example, what must each participating sensory and motor nerve cell generate in sequence? (4)

Answer 57

To produce a behaviour, a stretch reflex for example, each participating sensory and motor nerve cell must generate four different signals in sequence, each at different site within the cell: an input signal, a trigger signal, a conducting signal, and an output signal.

Question 58

How can cells generate these four types of signals? (4)

Answer 58

Regardless of cell size and shape, transmitter biochemistry, or behavioural function, almost all neurons can be described by a model neuron that has four functional components that generate the four types of signals: a receptive component, a summing or integrative component, a long-range signalling component, and a secretory component (see in docs)

Question 59

What is meant by the resting membrane potential?

Answer 59

The different types of signals generated in a neuron are determined in part by the electrical properties of the cell membrane. Every cell, including a neuron, maintains a certain difference in the electrical potential on either side of the plasma membrane when the cell is at rest. This is called the resting membrane potential.

Question 60

What is the voltage of the inside of a typical resting neuron compared to outside of it?

Answer 60

In a typical resting neuron the voltage of the inside of the cell is about 65 mV more negative than the voltage outside the cell. Because the voltage outside the membrane is defined as zero, we say the resting membrane potential is –65 mV.

Question 61

To what extent does this resting potential vary in different cells and what is the range?

Answer 61

The resting potential in different nerve cells ranges from –40 to –80 mV; in muscle cells it is greater still, about –90 mV.

Question 62

What is this resting membrane potential resulting from?

Answer 62

The resting membrane potential results from two factors: the unequal distribution of electrically charged ions, in particular the positively charged Na+ and K+ ions, and the selective permeability of the membrane.

Question 63

The unequal distribution of positively charged ions on either side of the cell membrane is maintained by two main mechanisms. Describe these

Answer 63

Intracellular Na+ and K+ concentrations are largely controlled by a membrane protein that actively pumps Na+ out of the cell and K+ back into it. This Na+-K+ pump keeps the Na+ concentration in the cell low (about one-tenth the concentration outside the cell) and the K+ concentration high (about 20 times the concentration outside).

The cell membrane is selectively permeable to K+ because the otherwise impermeable membrane contains proteins that form pores called ion channels. The channels that are active when the cell is at rest are highly permeable to K+ but considerably less permeable to Na . The K ions tend to leak out of these open channels, down the ion’s concentration gradient. As K+ ions exit the cell, they leave behind a cloud of unneutralised negative charge on the inner surface of the membrane, so that the net charge inside the membrane is more negative than that outside.

Question 64

What major organ is involved in maintaining the extracellular concentrations of Na+ and K+?

Answer 64

The extracellular concentrations of Na+ and K+ are maintained by the kidneys.

Question 65

When is a cell, such as nerve and muscle, said to be excitable?

Answer 65

When its membrane potential can be quickly and significantly altered. This change serves as a signalling mechanism.

Question 66

What effect does reducing the membrane potential by 10 mV have in some neurons?

Answer 66

In some neurons reducing the membrane potential by 10 mV (from –65 to –55 mV) makes the membrane much more permeable to Na+ than to K . The resultant influx of positively charged Na+ neutralises the negative charge inside the cell and causes brief and explosive change in membrane potential to +40 mV. This action potential is conducted down the cell’s axon to the axon’s terminal, where it initiates an elaborate chemical communication with other neurons or muscle cells.

Question 67

How long does an action potential typically last and what happens in the axon following it?

Answer 67

An action potential typically lasts approximately 1 ms,
after which the membrane returns to its resting state,
with its normal separation of charges and higher permeability to K+ than to Na+ .

Question 68

Apart from longer distance action potentials what other signals do neurons produce?

Answer 68

In addition to the long distance signals represented by the action potential, nerve cells also produce local signals—receptor potentials and synaptic potentials—that are not actively propagated and that typically decay within just a few millimeters.

Question 69

Is the change in membrane potential that generates long-range and local signals a decrease or an increase from the resting potential?

Answer 69

The change in membrane potential that generates long-range and local signals can be either a decrease or an increase from the resting potential. The resting membrane potential therefore provides the baseline on which all signalling occurs.

Question 70

What is meant by depolarisation and hyperpolarisation and what effect do they have on the neuron?

Answer 70

A reduction in membrane potential is called depolarisation. Because depolarisation enhances a cell’s ability to generate an action potential, it is excitatory. In contrast, an increase in membrane potential is called hyperpolarisation. Hyperpolarisation makes a cell less likely to generate an action potential and is therefore inhibitory.

Question 71

What happens with the knee-jerk flex in regards to the ion channels?

Answer 71

In sensory neurons current flow is typically initiated by a physical stimulus, which activates specialised receptor proteins at the neuron’s receptive surface. In our example of the knee-jerk reflex, stretching of the muscle activates specific ion channels that open in response to stretch of the sensory neuron membrane. The opening of these channels when the cell is stretched permits the rapid influx of Na+ ions into the sensory cell.

Question 72

What is meant by a receptor potential?

Answer 72

This ionic current changes the membrane potential, producing a local signal called the receptor potential.

Question 73

What is the duration and amplitude of the receptor potential?

Answer 73

The amplitude and duration of a receptor potential depends on the intensity of the muscle stretch: The larger or longer-lasting the stretch, the larger or longer- lasting the resulting receptor potential. Thus, unlike the action potential, which is all or none, receptor potentials are graded.

Question 74

Are receptor potentials depolarising or hyperpolarising?

Answer 74

Most receptor potentials are depolarising (excitatory). However, hyperpolarising (inhibitory) receptor potentials are found in the retina.

Question 75

How far does this graded potential typically travel?

Answer 75

The receptor potential is the first representation of stretch to be coded in the nervous system. This signal spreads passively, however, and therefore does not travel much farther than 1 to 2 mm. In fact, 1 mm down the axon the amplitude of the signal is only about one- third what it was at the site of generation.

Question 76

What is required for this graded potential signal to travel into the central nervous system then?

Answer 76

To be carried successfully to the central nervous system, the local signal must be amplified—it must generate an action potential. In the knee-jerk reflex the receptor potential in the sensory neuron must reach the first node of Ranvier in the axon. If it is large enough, the signal triggers an action potential that then propagates without failure to the axon terminals in the spinal cord

Question 77

An action potential is generated only if the receptor potential exceeds a certain voltage threshold. Does an increase in amplitude or duration of this potential result in any increase of amplitude or duration of the action potential?

Answer 77

Once this threshold is surpassed, any further increase in amplitude of the receptor potential can only increase the frequency with which the action potentials are generated, because action potentials have a constant amplitude.

The duration of the receptor potential determines the duration of the train of action potentials. Thus the graded amplitude and duration of the receptor potential is translated into a frequency code in the action potentials generated at the trigger zone.

(In docs)

Question 78

How does the action potential of the sensory neuron in the knee-jerk flex result in an input signal to the motor neuron?

Answer 78

In the knee-jerk reflex the action potential in the presynaptic terminal of the sensory neuron initiates the release of a chemical substance, or neurotransmitter, into the synaptic cleft. After diffusing across the cleft, the transmitter binds to receptor proteins in the postsynaptic membrane of the motor neuron, thereby directly or indirectly opening ion channels. The ensuing flow of current alters the membrane potential of the motor cell, a change called the synaptic potential.

Question 79

Is the synaptic potential graded or all-or-none?

Answer 79

Like the receptor potential, the synaptic potential is graded; its amplitude depends on how much transmitter is released. Synaptic potentials, like receptor potentials, spread passively and thus are local changes in potential unless the signal reaches beyond the axon’s initial segment and thus can give rise to an action potential.

Question 80

Is the synaptic potential depolarising or hyperpolarising?

Answer 80

In the same cell the synaptic potential can be either depolarising or hyperpolarising depending on the type of receptor molecule that is activated.

Question 81

What function does the trigger zone carry out in the neuron?

Answer 81

Sherrington first pointed out that the function of the nervous system is to weigh the consequences of different types of information and then decide on appropriate responses. This integrative function of the nervous system is clearly seen in the actions of the trigger zone of the neuron, the initial segment of the axon.

Question 82

Outline the role of Na+ throughout an action potential

Answer 82

Action potentials are generated by a sudden influx of Na+ through channels in the cell membrane that open and close in response to changes in membrane potential. When an input signal (a receptor potential or synaptic potential) depolarises an area of membrane, the local change in membrane potential opens local Na+ channels that allow Na+ to flow down its concentration gradient, from outside the cell where the Na+ concentration is high to inside where it is low.

Question 83

Why is the trigger zone sensitive to beginning an action potential?

Answer 83

Because the initial segment of the axon has the highest density of voltage-sensitive Na+ channels and therefore the lowest threshold for generating an action potential, an input signal spreading passively along the cell membrane is more likely to give rise to an action potential at the initial segment than at other sites in the cell. This part of the axon is therefore known as the trigger zone.

Question 84

To what extent do action potentials vary?

Answer 84

The remarkable feature of action potentials is that they are highly stereotyped, varying only subtly (but in some cases importantly) from one nerve cell to another. It has been found that all action potentials have a similar shape or wave-form. The action potentials carried into the nervous system by a sensory axon often are indistinguishable from those carried out of the nervous system to the muscles by a motor axon.

Question 85

What features of the conducting signal convey information? How so?

Answer 85

Only two features of the conducting signal convey information: the number of action potentials and the time intervals between them. “If they are crowded together the sensation is intense, if they are separated by long intervals the sensation is correspondingly feeble.”

Question 86

In addition to the frequency of action potentials how else can these two features convey important information?

Answer 86

In addition to the frequency of the action potentials, the pattern of action potentials also conveys important information. For example, some neurons are not silent in the absence of stimulation but are spontaneously active. Some spontaneously active nerve cells (beating neurons) fire action potentials regularly; other neurons (bursting neurons) fire in brief bursts of action potentials. These diverse cells respond differently to the same excitatory synaptic input. An excitatory synaptic potential may initiate one or more action potentials in a cell that does not have a spontaneous activity, but in spontaneously active cells that same input will modulate the rhythm by increasing the rate of firing of action potentials.

Question 87

What effect can inhibitory inputs have in an active cell?

Answer 87

Inhibitory inputs have little information value in a silent cell. By contrast, in spontaneously active cells inhibition can have a powerful sculpting role. By establishing periods of silence in otherwise ongoing activity, inhibition can produce a complex pattern of alternating firing and silence where none existed. These subtle differences in firing patterns may have important functional consequences for the information transfer between neurons.

Question 88

What have these observations regarding the excitatory and inhibitory effects on firing rhythm led researchers to attempt?

Answer 88

This has led mathematical modellers of neuronal networks to attempt to delineate neural codes in which information is also carried by the fine-grained pattern of firing— the exact timing of action potentials

Question 89

Describe the action potentials generated for cortical pyramidal cells after intracellular injection of a depolarising current pulse (can be seen in docs)

Answer 89

Intracellular injection of a depolarising current pulse in a cortical pyramidal cell results in a train of action potentials that decline in frequency. This pattern of activity is known as regular firing. Some cortical cells generated bursts of three or more action potentials, even when depolarised only for a short period.

Question 90

Describe the action potentials generated for cerebellar Purkinje cells after intracellular injection of a depolarising current pulse (can be seen in docs)

Answer 90

Cerebellar Purkinje cells generate high-frequency trains of action potentials in their cell bodies that are disrupted by the
generation of Ca2+ action potentials in their dendrites. These cells can also generate plateau potentials from the persistent activation of Na+ conductances.

Question 91

Describe the action potentials generated for thalamic relay cells after intracellular injection of a depolarising current pulse (can be seen in docs)

Answer 91

Thalamic relay cells may generate action potentials either as bursts or tonic trains of action potentials owing to the presence of a large low-threshold Ca2+ current.

Question 92

Describe the action potentials generated for Medial habenular cells after intracellular injection of a depolarising current pulse (can be seen in docs)

Answer 92

Medial habenular cells generate action potentials at a steady and slow rate, in a pacemaker fashion.

Question 93

When an action potential reaches a neuron’s terminal it stimulates the release of chemical substances from the cell. What are these chemical substances?

Answer 93

These substances, called neurotransmitters, can be small organic molecules, such as l-glutamate and acetylcholine, or peptides like substance P or LHRH (luteinizing hormone releasing hormone).

Question 94

Describe the process of this neurotransmitter release at the synapse with the appropriate vocabulary

Answer 94

Neurotransmitter molecules are held in sub-cellular organelles called synaptic vesicles, which accumulate at specialised release sites in the terminals of the axon called active zones. To eject their transmitter substance into the synaptic cleft, the vesicles move up to and fuse with the neuron’s plasma membrane, then burst open, a process known as exocytosis.

Question 95

How is the amount of neurotransmitter released determined?

Answer 95

Once released, the neurotransmitter is the neuron’s output signal. Like the input signal, it is graded. The amount of transmitter released is determined by the number and frequency of the action potentials that reach the presynaptic terminals

Question 96

What occurs after release of the neurotransmitter?

Answer 96

After release the transmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron. This binding causes the postsynaptic cell to generate a synaptic potential.

Question 97

What determines whether the signal is inhibitory or excitatory?

Answer 97

Whether the synaptic potential has an excitatory or inhibitory effect depends on the type of receptor in the postsynaptic cell, not on the particular chemical neurotransmitter. The same transmitter substance can have different effects at different receptors.

Question 98

The model of neuronal signalling we have outlined is a simplification that applies to most neurons but there are some important variations. Give two examples of this at the cell level

Answer 98

For example, some neurons do not generate action potentials. These are typically local interneurons without a conductive component; they have no axon or such a short one that regeneration of the signal is not required. In these neurons the input signals are summed and spread passively to the pre- synaptic terminal region nearby where transmitter is released.

Neurons that are spontaneously active do not require sensory or synaptic inputs to fire action potentials because they have a special class of ion channels that permit Na+ current flow even in the absence of excitatory synaptic input.

Question 99

Where do nerve cells differ the most?

Answer 99

At the molecular level

Question 100

Even cells that are similar morphologically can differ importantly in molecular details. Give an example of this

Answer 100

For example, they can have different combinations of ion channels. Different ion channels provide neurons with various thresholds, excitability properties, and firing patterns. Neurons with different ion channels can therefore encode synaptic potentials into different firing patterns and thereby convey different information.

Question 101

How else may neurons differ from each other at the molecular level?

Answer 101

Neurons also differ in the chemical substances they use as transmitters and in the receptors that receive transmitter substances from other neurons.

Question 102

How are these physiological differences in neurons relevant to disease?

Answer 102

Because of physiological differences among neurons, a disease may affect one class of neurons but not others. Certain diseases strike only motor neurons (amyotrophic lateral sclerosis and poliomyelitis), whereas others affect primarily sensory neurons (tabes dorsalis, a late stage of syphilis). Parkinson disease, a disorder of voluntary movement, damages a small population of interneurons that use dopamine as a neurotransmitter.

Question 103

Is the nervous system more or less susceptible to disease than other organs? Why?

Answer 103

Because the nervous system has so many cell types and variations at the molecular level, it is susceptible to more diseases (psychiatric as well as neurological) than any other organ system of the body.

Question 104

What is meant by a command cell?

Answer 104

The stretch reflex illustrates how interactions between just a few types of nerve cells can constitute a functional circuit that produces a simple behaviour, even though the number of neurons involved is large. In invertebrate animals, and in some lower-order vertebrates, a single cell (a so-called command cell) can initiate a complex behavioural sequence.

Question 105

What (relatively) complex human behaviours have been found to be activated by a command cell?

Answer 105

As far as we know no complex human behaviour is initiated by a single neuron. Rather, each behaviour is generated by the actions of many cells.

Question 106

Broadly speaking, what are the three components of neural control and behaviour?

Answer 106

Sensory input, intermediate processing, and motor output. In vertebrates each component is likely to be mediated by a single group or several distinct groups of neurons.

Question 107

What is meant by parallel processing?

Answer 107

Each component may have multiple neural pathways that simultaneously provide the same or similar information. The deployment of several neuronal groups or pathways to convey similar information is called parallel processing. Parallel processing also occurs in a single pathway when different neurons in the pathway perform similar computations simultaneously.

Question 108

What could be the reasoning behind parallel processing in regards to evolution?

Answer 108

Parallel processing makes enormous sense as an evolutionary strategy for building a more powerful brain, for it increases both the speed and reliability of function within the central nervous system.

Question 109

How has this concept of parallel processing been adapted outside the realm of neuroscience?

Answer 109

The branch of computer science known as artificial intelligence originally used serial processing to simulate the brain’s cognitive processes—pattern recognition, learning, memory, and motor performance. These serial models performed many tasks rather well, including playing chess. However, they performed poorly with other computations that the brain does almost instantaneously, such as recognising faces or comprehending speech.

Question 110

Many theoretical neurobiologists have turned to different types of models that include parallel processing. What are these called?

Answer 110

Neural networks

Question 111

Describe the process of neural networks. What determines the outcome?

Answer 111

In these models, elements of the system process information simultaneously using both feed-forward and feedback connections. Interestingly, in systems with feedback circuits it is the dynamic activity of the system that determines the outcome of computation, not inputs or initial conditions.

Question 112

What do neural network models capture well about actual neural circuits that traditional deterministic models have difficulty?

Answer 112

The highly recurrent architecture of most actual neural circuits and also the ability of the brain to function in the absence of specific sensory input from outside the body such as during thinking, sleep, and the generation of endogenous rhythms.

Question 113

What could neural network models further suggest about how we analyse the brain?

Answer 113

Neural network models also show that analysis of individual elements of a system may not be enough to decode the action potential code. According to this neural network view, what makes the brain a remarkable information processing organ is not the complexity of its neurons but the fact that it has many elements interconnected in a variety of complex ways.

Question 114

How is behaviour modified if the nervous system is wired so precisely? How can changes in the neural control of behaviour occur when connections between the signalling units, the neurons, are set during early development?

Answer 114

Several solutions for this dilemma have been pro- posed. The proposal that has proven most farsighted is the plasticity hypothesis which suggests that the application of a stimulus leads to changes of a twofold kind in the nervous system:

The first property, by virtue of which the nerve cells react to the incoming impulse, we call excitability, and changes arising because of this property we shall call changes due to excitability.

The second property, by virtue of which certain permanent functional transformations arise in particular systems of neurons as a result of appropriate stimuli or their combination, we shall call plasticity and the corresponding changes plastic changes.

Question 115

To what extent is there evidence behind this theory of plasticity?

Answer 115

There is now considerable evidence for functional plasticity at chemical synapses. These synapses often have a remarkable capacity for short-term physiological changes (lasting seconds to hours) that increase or decrease synaptic effectiveness. Long-term changes (lasting days) can give rise to further physiological changes that lead to anatomical alterations, including pruning of preexisting synapses and even growth of new ones.