CHAPTER 2 Introducing the brain CONTENTS Structure and function of the neuron 19 The gross organization of the brain 24 The cerebral cortex The subcortex 30 The midbrain and hindbrain Summary and key points of the chapter 33 Example essay questions 33 Recommended further reading 34 the phy that makes all ourmo at mak s,and to cons or even to create thoughts that no human has con book will scratch the surface of how this is all possible,but the purpose of this chapter is more mundane.It offers a basic guide to the structure of the brain,starting from a description of neurons and working up to a description of how these are organized into different neuroanatomical systems.The emphasis is on the human brain rather than the brain of other species. STRUCTURE AND FUNCTION OF THE NEURON All uro have basically thesame structure.They const of three ody (or son dendr an axon shown ir Although n have the same structure an note tha are son es be types o neurons in terms of the spatial arrangements of the dendrites and axon
It is hard to begin a chapter about the brain without waxing lyrical. The brain is the physical organ that makes all our mental life possible. It enables us to read these words, and to consider thoughts that we have never considered before—or even to create thoughts that no human has considered before. This book will scratch the surface of how this is all possible, but the purpose of this chapter is more mundane. It offers a basic guide to the structure of the brain, starting from a description of neurons and working up to a description of how these are organized into different neuroanatomical systems. The emphasis is on the human brain rather than the brain of other species. STRUCTURE AND FUNCTION OF THE NEURON All neurons have basically the same structure. They consist of three components: a cell body (or soma), dendrites and an axon, as shown in Figure 2.1. Although neurons have the same basic structure and function, it is important to note that there are some significant differences between different types of neurons in terms of the spatial arrangements of the dendrites and axon. CONTENTS Structure and function of the neuron 19 The gross organization of the brain 24 The cerebral cortex 28 The subcortex 30 The midbrain and hindbrain 32 Summary and key points of the chapter 33 Example essay questions 33 Recommended further reading 34 CHAPTER 2 Introducing the brain
20 THE STUDENTS GUIDE TO COGNITIVE NEUROSCIENCE FIGURE 2.1:Neurons consist fthree basic features:a A neuron 000 axons that send information. endrites In this diagram the axon KEY TERMS and supports.among other things.cognitive unction Cell body The cell body contains the nucleus and other organelles The nucleus Part of the neuron con contains the genetic code,and this is involved in protein synthesis.Proteins serve a wide variety of functions from providing scaffolding to chemical signaling (they can act as neurotransmitters and receptors in neurons). Dendrites Neurons receive information from other neurons and they make a"decision' Branching structures that about this information(by changing their own activity)that can then be passed on to other neurons.From the cell body,a number of branching structures called dendrites enable communication with other neurons.Dendrites receive Axon information from other neurons in close proximity.The number and structure Abranching structure of the dendritic branches can vary significantly depending on the type of tooherneuaian neuron (i.e.,where it is to be found in the brain).The axon,by contrast,sends transmits an action information to other neurons.Each neuron consists of many dendrites but potential. only a single axon(although the axon may be divided into several branches called collaterals). TEN INTERESTING FACTS ABOUT THE HUMAN BRAIN (1)There are 86 billion neurons in the human brain(Azevedo et al.,2009). (2)Each neuron connects with around 10,000 other neurons.As such,there are over 3,000 times one p uld eter(Nelson Bower,1990).This is the length of Manhattan Island.This leads to an important conclusion-namely,that neurons only connect with a small subset of other neurons.Neurons tend to communicate only with their neighbors in what has been termed a"small-world"archi- tecture (Sporns Zwi,2004).Long-range onnections are the exception rather than the rule (4)The idea that we only use 10 percent of the cells in our brain is generally considered a myth (Beyerstein,1999).It used to be thought that only around 10 percent of the cells in the brain were neurons(the rest being cells called glia),hence a plausible origin for the myth.This "fact"also turns out to be inaccurate.with the true ratio of neurons to glia being closer to 1:1 (Azevedo et al..2009).Glia serve a number of essential support functions;for example,they are involved in tissue repair and in the formation of myelin
20 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE The cell body contains the nucleus and other organelles. The nucleus contains the genetic code, and this is involved in protein synthesis. Proteins serve a wide variety of functions from providing scaffolding to chemical signaling (they can act as neurotransmitters and receptors in neurons). Neurons receive information from other neurons and they make a “decision” about this information (by changing their own activity) that can then be passed on to other neurons. From the cell body, a number of branching structures called dendrites enable communication with other neurons. Dendrites receive information from other neurons in close proximity. The number and structure of the dendritic branches can vary significantly depending on the type of neuron (i.e., where it is to be found in the brain). The axon, by contrast, sends information to other neurons. Each neuron consists of many dendrites but only a single axon (although the axon may be divided into several branches called collaterals). FIGURE 2.1: Neurons consist of three basic features: a cell body, dendrites that receive information and axons that send information. In this diagram the axon is myelinated to speed the conduction time. (1) There are 86 billion neurons in the human brain (Azevedo et al., 2009). (2) Each neuron connects with around 10,000 other neurons. As such, there are over 3,000 times as many synapses in one person’s brain than there are stars in our whole galaxy. (3) If each neuron connected with every single other neuron, our brain would be 12.5 miles in diameter (Nelson & Bower, 1990). This is the length of Manhattan Island. This leads to an important conclusion—namely, that neurons only connect with a small subset of other neurons. Neurons tend to communicate only with their neighbors in what has been termed a “small-world” architecture (Sporns & Zwi, 2004). Long-range connections are the exception rather than the rule. (4) The idea that we only use 10 percent of the cells in our brain is generally considered a myth (Beyerstein, 1999). It used to be thought that only around 10 percent of the cells in the brain were neurons (the rest being cells called glia), hence a plausible origin for the myth. This “fact” also turns out to be inaccurate, with the true ratio of neurons to glia being closer to 1:1 (Azevedo et al., 2009). Glia serve a number of essential support functions; for example, they are involved in tissue repair and in the formation of myelin. TEN INTERESTING FACTS ABOUT THE HUMAN BRAIN KEY TERMS Neuron A type of cell that makes up the nervous system and supports, among other things, cognitive function. Cell body Part of the neuron containing the nucleus and other organelles. Dendrites Branching structures that carry information from other neurons. Axon A branching structure that carries information to other neurons and transmits an action potential
INTRODUCING THE BRAIN 21 (5)The brain makes up only 2 percent of body weight. (6)It is no erated.It was we ent of neurons and that new neurons are not generated.This idea is now untenable,at least in a region called the dentate gyrus (for a review,see Gross,2000). (7)On average.we lose a net amount of one cortical neuron per second.A study has shown that around 10 percent of our cortical neurons perish between the ages of 20 and 90 years- equivalent to 5,000 neurons per day (Pakkenberg&Gundersen,1997) (8)Identical twins do not have anatomically identical brains.A comparison of identical and nonidentical twins suggests that the three-dimensional cortical gyral pattern is determined primarily by non-genetic factors.although brain size is strongly heritable (Bartley et al..1997) (9)People with autism have larger brains in early life(Abell et al.,1999).They also have large ds to ac commodate the There is unlikely to be a si imple relationship between bra and intellect(most people with autism have low IQ).and brain efficiency may be unrelated to size. (10)Men have larger brains than women,but the female brain is more folded,implying an increase in surface area that may offset any size difference (Luders et al..2004).The total number of cortical neurons is related to gender,but not overall height or weight(Pakkenberg& Gundersen,1997) The terminal of anaon flattens out into a dise-shaped structure.It KEY TERMS is here that chemical signals enable communication between neurons via a small gap termed a synapse.The two neurons forming the synapse are Synapse the The small gap between neurons in whict When a presynaptic neuron is active,an electrical current (termed an action potential)is propagated down the length of the axon.When the action signaling between potential reaches the axon terminal,chemicals are released into the synaptic neurons cleft.These chemicals are termed neurotransmitters.(Note that a smal Action potential proportion of synapses,such as retinal gap junctions,signal electrically and A sudden change not chemically.)Neurotransmitters bind to receptors on the dendrites or cell (depolarization and body of the postsynaptic neuron and create a synaptic potential.The synaptic repola i)in the potential is conducted passively (i.e.,without creating an action potential) through the dendrites and soma of the postsynaptic neuron.These passive in axon.which fomms the currents form the basis of EEG.These different passive currents are summed oasis for how neu ons together and if their summed activity exceeds a certain threshold when they reach the beginning of the axon in the postsynaptic neuron,then an action the potential(an active electrical current)will be triggered in this neuron.In this and synchrony of action way different neurons can be said to be "communicating"with each other potentials). This is shown in Figure 2.2.It is important to note that each postsynaptic neuron sums together many synaptic potentials,which are generated at many different and distant dendritic sites (in contrast to a simple chain reaction released by one neuron between one neuron and the next).Passive conduction tends to be short range because the electrical signal is impeded by the resistance of the surrounding of other neurons matter.Active conduction enables long-range signaling between neurons by the propagation of action potentials
Introducing the brain 21 The terminal of an axon flattens out into a disc-shaped structure. It is here that chemical signals enable communication between neurons via a small gap termed a synapse. The two neurons forming the synapse are referred to as presynaptic (before the synapse) and postsynaptic (after the synapse), reflecting the direction of information flow (from axon to dendrite). When a presynaptic neuron is active, an electrical current (termed an action potential) is propagated down the length of the axon. When the action potential reaches the axon terminal, chemicals are released into the synaptic cleft. These chemicals are termed neurotransmitters. (Note that a small proportion of synapses, such as retinal gap junctions, signal electrically and not chemically.) Neurotransmitters bind to receptors on the dendrites or cell body of the postsynaptic neuron and create a synaptic potential. The synaptic potential is conducted passively (i.e., without creating an action potential) through the dendrites and soma of the postsynaptic neuron. These passive currents form the basis of EEG. These different passive currents are summed together and if their summed activity exceeds a certain threshold when they reach the beginning of the axon in the postsynaptic neuron, then an action potential (an active electrical current) will be triggered in this neuron. In this way, different neurons can be said to be “communicating” with each other. This is shown in Figure 2.2. It is important to note that each postsynaptic neuron sums together many synaptic potentials, which are generated at many different and distant dendritic sites (in contrast to a simple chain reaction between one neuron and the next). Passive conduction tends to be short range because the electrical signal is impeded by the resistance of the surrounding matter. Active conduction enables long-range signaling between neurons by the propagation of action potentials. (5) The brain makes up only 2 percent of body weight. (6) It is no longer believed that neurons in the brain are incapable of being regenerated. It was once widely believed that we are born with our full complement of neurons and that new neurons are not generated. This idea is now untenable, at least in a region called the dentate gyrus (for a review, see Gross, 2000). (7) On average, we lose a net amount of one cortical neuron per second. A study has shown that around 10 percent of our cortical neurons perish between the ages of 20 and 90 years— equivalent to 85,000 neurons per day (Pakkenberg & Gundersen, 1997). (8) Identical twins do not have anatomically identical brains. A comparison of identical and nonidentical twins suggests that the three-dimensional cortical gyral pattern is determined primarily by non-genetic factors, although brain size is strongly heritable (Bartley et al., 1997). (9) People with autism have larger brains in early life (Abell et al., 1999). They also have large heads to accommodate them. There is unlikely to be a simple relationship between brain size and intellect (most people with autism have low IQ), and brain efficiency may be unrelated to size. (10) Men have larger brains than women, but the female brain is more folded, implying an increase in surface area that may offset any size difference (Luders et al., 2004). The total number of cortical neurons is related to gender, but not overall height or weight (Pakkenberg & Gundersen, 1997). KEY TERMS Synapse The small gap between neurons in which neurotransmitters are released, permitting signaling between neurons. Action potential A sudden change (depolarization and repolarization) in the electrical properties of the neuron membrane in an axon, which forms the basis for how neurons code information (in the form of the rate and synchrony of action potentials). Neurotransmitters Chemical signals that are released by one neuron and affect the properties of other neurons
22 THE STUDENT'S GUIDE TO COGNITIVE NEUROSCIENCE FIGURE 2.2:Electrical urrents are currents flow passively through dendrites and soma o neurons.but summed potential is strong enough at the start of the axon (called the hillock) 上 Pe) Potaptaadem Electrical signaling and the action potential Each neuron is surrounded by a cell membrane that acts as a barrier to the passage of certain chemicals.Within the membrane,certain protein molecules act as gatekeepers and allow particular chemicals in and out under certain conditions.These chemicals consist,among others,of charged sodium (Na*) and potassium(K)ions.The balance between these ions on the inside and outside of the memb ane is such that there is normally a resting potential of -70 mV across the membrane (the inside being negative elative to the outside) e-gated ion channels are of particular impo nce in the g of an action potential.They are found only in axons.which is why only the g action potentials The sequence of event igure 2.3) 之之ve current of sumcient no心 1. s across the axon membrane. 2 cho op cha opene a th poten sreduced (the e). 50m the ell me completely permea an the arge on the in de of the cell This sudden subs sequent repo larization in electrica ross the m The negative potential action potential 3 the cell is re red via the outward flow of K through voltage-gated K'channels and closing of the voltage-gated Na channels There is a brief period in which hyperpolarization occurs (the inside is more negative than at rest).This makes it more difficult for the axon to depolarize straight away and prevents the action potential from traveling backwards
22 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE Electrical signaling and the action potential Each neuron is surrounded by a cell membrane that acts as a barrier to the passage of certain chemicals. Within the membrane, certain protein molecules act as gatekeepers and allow particular chemicals in and out under certain conditions. These chemicals consist, among others, of charged sodium (Na+) and potassium (K+) ions. The balance between these ions on the inside and outside of the membrane is such that there is normally a resting potential of −70 mV across the membrane (the inside being negative relative to the outside). Voltage-gated ion channels are of particular importance in the generation of an action potential. They are found only in axons, which is why only the axon is capable of producing action potentials. The sequence of events is as follows (see also Figure 2.3): 1. If a passive current of sufficient strength flows across the axon membrane, this begins to open the voltage-gated Na+ channels. 2. When the channel is opened, then Na+ may enter the cell and the negative potential normally found on the inside is reduced (the cell is said to depolarize). At about −50 mV, the cell membrane becomes completely permeable and the charge on the inside of the cell momentarily reverses. This sudden depolarization and subsequent repolarization in electrical charge across the membrane is the action potential. 3. The negative potential of the cell is restored via the outward flow of K+ through voltage-gated K+ channels and closing of the voltage-gated Na+ channels. 4. There is a brief period in which hyperpolarization occurs (the inside is more negative than at rest). This makes it more difficult for the axon to depolarize straight away and prevents the action potential from traveling backwards. FIGURE 2.2: Electrical currents are actively transmitted through axons by an action potential. Electrical currents flow passively through dendrites and soma of neurons, but will initiate an action potential if their summed potential is strong enough at the start of the axon (called the hillock)
INTRODUCING THE BRAIN 23 FIGURE 23:The actio potential consists of number of phases. Time Na+channels close and open to pump Kout nnel -50m -70ml Depolarization dhaaneiomtinetaperate An action potential in one Na part of the opens adjacent voltage length of f the 11 xon ter the ac along the ay b e speeded up if the KEY TERM axon is myelinat Myeli atty a1 hat is depos cells (especially tho carry signals).It blocks the normal so the acti n potentia A fatty substance that jumps, via passive conduction,down the length of the axon at the points at is deposited around the which the myelin is absent (called nodes of Ranvier).Destruction of myelin is t speeds conduction found in a number of pathologies,notably multiple sclerosis. Chemical signaling and the postsynaptic neuron When the action potential reaches the axon terminal,the electrical signal initiates a sequence of events leading to the release of neurotransmitters into the synaptic cleft.Protein receptors in the membrane of the postsynaptic neurons bind to the neurotransmitters Many of the receptors are transmitter- gated ion channels(not to be confused with voltage-gated ion channels found in the axon).This sets up a localized flow of charged Na',K'or chloride(Cl),which creates the synaptic potential.Some neurotransmitters (e.g.,GABA)have an inhibitory effect on the postsynaptic neuron (i.e.,by making it less likely to fire).This can be achieved by making the inside of the neuron more negative than normal and hence harder to depolarize(e.g., by opening transmitter-gated Cchannels).Other neurotransmitters (e.g., glutamate)have excitatory effects on the postsynaptic neuron(i.e.,by making it more likely to fire).These synaptic potentials are then passively conducted as already described. Glutamate and GABA are the workhorse neurotransmitters of the brain in that nearly every neuron produces one or other of these.Note that it is not the chemicals themselves that make them excitatory and inhibitory.Rather it is the effect that they have on ion channels in the
Introducing the brain 23 An action potential in one part of the axon opens adjacent voltagesensitive Na+ channels, and so the action potential moves progressively down the length of the axon, starting from the cell body and ending at the axon terminal. The conduction of the action potential along the axon may be speeded up if the axon is myelinated. Myelin is a fatty substance that is deposited around the axon of some cells (especially those that carry motor signals). It blocks the normal Na+/K+ transfer and so the action potential jumps, via passive conduction, down the length of the axon at the points at which the myelin is absent (called nodes of Ranvier). Destruction of myelin is found in a number of pathologies, notably multiple sclerosis. Chemical signaling and the postsynaptic neuron When the action potential reaches the axon terminal, the electrical signal initiates a sequence of events leading to the release of neurotransmitters into the synaptic cleft. Protein receptors in the membrane of the postsynaptic neurons bind to the neurotransmitters. Many of the receptors are transmittergated ion channels (not to be confused with voltage-gated ion channels found in the axon). This sets up a localized flow of charged Na+, K+ or chloride (Cl– ), which creates the synaptic potential. Some neurotransmitters (e.g., GABA) have an inhibitory effect on the postsynaptic neuron (i.e., by making it less likely to fire). This can be achieved by making the inside of the neuron more negative than normal and hence harder to depolarize (e.g., by opening transmitter-gated Cl– channels). Other neurotransmitters (e.g., glutamate) have excitatory effects on the postsynaptic neuron (i.e., by making it more likely to fire). These synaptic potentials are then passively conducted as already described. Glutamate and GABA are the workhorse neurotransmitters of the brain in that nearly every neuron produces one or other of these. Note that it is not the chemicals themselves that make them excitatory and inhibitory. Rather it is the effect that they have on ion channels in the FIGURE 2.3: The action potential consists of a number of phases. KEY TERM Myelin A fatty substance that is deposited around the axon of some neurons that speeds conduction