Neural Communications. Lecture 3

Transcription

1 Neural Communications Lecture 3

2 Neuronal Recording Techniques Electrode can be affine wire; or a salt filled capillary electrode. Electrode is a fine glass capillary of 0.1 micro-m diameter with salt solution. Extracellular Recordings Intracellular Recordings

3 Neuronal Recording Techniques

4 Neuronal Recording Techniques

5 Types of Electrical Signals 1. Localized potentials: They are passive and subject to attenuation 2. Action Potentials: They involve active processing and move fast as impulse and never attanuate

6 Localized Pot. 1. They are graded. The amplitude of the voltage changes with distance. Increase injected current increase the voltage. 2. Duration of potential varies with the duration of the current pulse, it rise and slowly due to capacitive effect of the membrane. (Recording from V1) 3. The changes are almost negligible at far distances, so localized potentials are useless for long-range signaling. (recording from V2)

7 Action Potentials A modest amount of electric current produce Localized Potentials. However, these localized potentials can be converted into action potentials once the membrane is depolarized to a certain level called Threshold. 1. It is all-or-nothing event. 2. Fixed amplitude and duration: Large current don t gives large action potentials and current of longer duration does not increase the duration of action potential. 3. Every action potentials is followed by a refractory period during which a second pulse cant be initiated. The frequency of repeated action potentials is limited by the refractory period. 4. The amplitude does not decline with distance.

8 Action Potentials 1. Once initiated: APs are independent on initiating event (larger current do not give larger action potentials, and currents of prolonged duration does not prolong the action potential) 2. Entire action potential sequence must be completed before starting a new one. The sequence also includes the refractory period. 3. Action potential don t decline with the distance.

9 Example

10 Neuron processes (armlike; extend from the cell body) Nerve fibers = axons Nerve impulse generators & transmitters One per neuron, although can branch into collaterals At terminal end branch a lot (e.g. 10,000/terminus) Receptive regions called dendrites Have receptors for neurotransmitters (chemicals released by other neurons)

11 Synapse Junctions between neurons Information is passed (usually chemically or electrically) Unidirectional Presynaptic (toward synapse) vs postsynaptic (away from synapse): most neurons function as both Synaptic cleft (tiny gap)

12 Synapse When the neuron is stimulated, the electrical signal (action potential) travels down the axon to the axon terminals. When the electrical signal reaches the end of the axon, it triggers a series of chemical changes in the axon terminal. Calcium ions (Ca++) flow into the axon terminal, which then initiates the release of neurotransmitters. A neurotransmitter is a molecule that is released from a neuron to relay information to another cell. Neurotransmitter molecules are stored in membranous sacs called vesicles in the axon terminal. Each vesicle contains thousands of molecules of a given neurotransmitter. For neurons to release their neurotransmitter, the vesicles fuse with the neuronal membrane and then release their contents, the neurotransmitter, via exocytosis. The neurotransmitter molecules are released into the synaptic space and diffuse across the synaptic space to the postsynaptic neuron. A neurotransmitter molecule can then bind to a special receptor on the membrane of the postsynaptic neuron. Receptors are membrane proteins that are able to bind a specific chemical substance, such as a neurotransmitter. For example, the dopamine receptor binds the neurotransmitter dopamine but does not bind other neurotransmitters such as serotonin. The chemical binding of neurotransmitter and receptor initiates changes in the postsynaptic neuron that may facilitate or inhibit an action potential in the postsynaptic neuron. If it does trigger an action potential, the communication process continues.

13 The story so far We have discussed the basic structure of neurons and the nature of the action potential. We have also discussed the firing of single neuron. It is time to describe the ways in which neurons communicate with each other. These communications make it possible for circuits of neurons to gather sensory information, make plans, and initiate behaviors.

14 Neurons convey information by transmitting messages to other neurons or other types of cells, such as muscles. How one neuron communicates with another neuron? The end of the axon branches off into several terminals. Each axon terminal is highly specialized to pass along action potentials to adjacent neurons, or target tissue, in the neural pathway. There are two ways of communication 1. Some cells communicate this information via electrical synapses. In such cases, the action potential simply travels from one cell to the next through specialized channels, called gap junctions, which connect the two cells.

15 2. Most cells, however, communicate via chemical synapses. Such cells are separated by a space called a synaptic cleft and thus cannot transmit action potentials directly. Instead, chemicals called neurotransmitters are used to communicate the signal from one cell to the next. Some neurotransmitters are excitatory and depolarize the next cell, increasing the probability that an action potential will be fired. Others are inhibitory, causing the membrane of the next cell to hyperpolarize, thus decreasing the probability of that the next neuron will fire an action potential.

16 Acetylcholine Dopamine GABA (gammaaminobutyric acid) Glutamate Glycine Norepinephrine Serotonin Used by spinal cord motor neurons to cause muscle contraction and by many neurons in the brain to regulate memory. In most instances, acetylcholine is excitatory. Produces feelings of pleasure when released by the brain reward system. Dopamine has multiple functions depending on where in the brain it acts. It is usually inhibitory. The major inhibitory neurotransmitter in the brain. It is important in producing sleep, reducing anxiety, and forming memories. The most common excitatory neurotransmitter in the brain. It is important in learning and memory. Used mainly by neurons in the spinal cord. It probably always acts as an inhibitory neurotransmitter. Acts as a neurotransmitter and a hormone. In the peripheral nervous system, it is part of the fight-or-flight response. In the brain, it acts as a neurotransmitter regulating blood pressure and calmness. Norepinephrine is usually excitatory, but it is inhibitory in a few brain areas. Involved in many functions including mood, appetite, and sensory perception. In the spinal cord, serotonin is inhibitory in pain pathways.

17 Neurotransmitters (NT) Neurotransmitters exert their effects on cells by attaching to a particular region of a receptor molecule called the binding site. Binding Site The location on a receptor protein to which a ligand binds. A molecule of the chemical fits into the binding site the way a key fits into a lock; the shape of the binding site and the shape of the molecule of the neurotransmitter are complementary.

18 Neurotransmitters (NT) Neurotransmitters produce postsynaptic potentials brief depolarizations or hyperpolarizations that increase or decrease the rate of firing of the axon of the postsynaptic neuron. Postsynaptic Potential Alterations in the membrane potential of a postsynaptic neuron; produced by liberation of neurotransmitter at the synapse

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20 Neurotransmitters (NT) The presynaptic membrane, located at the end of the terminal button, faces the postsynaptic membrane, located on the neuron that receives the message (the postsynaptic neuron). Presynaptic Membrane The membrane of a terminal button that lies adjacent to the postsynaptic membrane and through which the neurotransmitter is released Postsynaptic Membrane The cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message

21 Release of NT These two membranes face each other across the synaptic cleft, a gap that varies in size from synapse to synapse but is usually around 20 nm wide. (A nanometer (nm) is one billionth of a meter.) Synaptic Cleft The space between the presynaptic membrane and the postsynaptic membrane. The synaptic cleft contains extracellular fluid, through which the neurotransmitter diffuses. A meshwork of filaments crosses the synaptic cleft and keeps the presynaptic and postsynaptic membranes in alignment.

22 Release of NT Synaptic vesicles are found in greatest numbers around the part of the presynaptic membrane that faces the synaptic cleft near the release zone, the region from which the neurotransmitter is released. When action potentials are conducted down an axon (and down all of its branches), something happens inside all of the terminal buttons: A number of small synaptic vesicles located just inside the presynaptic membrane fuse with the membrane and then break open, spilling their contents into the synaptic cleft. Release Zone A region of the interior of the presynaptic membrane of a synapse to which synaptic vesicles attach and release their neurotransmitter into the synaptic cleft

23 1. The process begins when a population of synaptic vesicles become docked against the presynaptic membrane, ready to release their neurotransmitter into the synaptic cleft. 4. Some of the calcium ions that enter the terminal button bind with the clusters of protein molecules. This event makes the segments of the clusters of protein molecules move apart, producing a fusion pore a hole through both membranes that enables them to fuse together. The process of fusion takes approximately 0.1 msec 2. Docking is accomplished when clusters of protein molecules attach to other protein molecules located in the presynaptic membrane 3. The release zone of the presynaptic membrane contains voltage-dependent calcium channels. When the membrane of the terminal button is depolarized by an arriving action potential, the calcium channels open. Thus, when the voltagedependent calcium channels open, Ca2+ flows into the cell.

24 Recycling of the membrane Release of NT Kiss and run These synaptic vesicles release most or all of their neurotransmitter, the fusion pore closes, and the vesicles break away from the presynaptic membrane and get filled with neurotransmitter again. Merge and recycle Other vesicles merge and recycle and consequently lose their identity. The membranes of these vesicles merge with the presynaptic membrane. Little buds of membrane then pinch off into the cytoplasm and become synaptic vesicles. The appropriate proteins are inserted into the membrane of these vesicles, and the vesicles are filled with molecules of the neurotransmitter. Bulk endocytosis Large pieces of the membrane of the terminal button fold inward, break off, and enter the cytoplasm. New vesicles are formed from small buds that break off of these pieces of membrane.

25 Activation of NT Neurotransmitters then diffusing across the synaptic cleft and attaching to the binding sites of special protein molecules located in the postsynaptic membrane, called postsynaptic receptors. Once binding occurs, the postsynaptic receptors open neurotransmitterdependent ion channels, which permit the passage of specific ions into or out of the cell. Neurotransmitters open ion channels by at least two different methods: direct and indirect. We will discuss only direct method.

26 When a molecule of the appropriate neurotransmitter attaches to it, the ion channel opens. That receptor/ion channel is an ionotropic receptor.

27 Postsynaptic potentials can be either depolarizing (excitatory) or hyperpolarizing (inhibitory). What determines the nature of the postsynaptic potential at a particular synapse is determined by the characteristics of the postsynaptic receptors in particular, by the particular type of ion channel they open. Four major types of neurotransmitter-dependent ion channels are found in the postsynaptic membrane: sodium (Na+), potassium (K+), chloride (Cl ), and calcium (Ca2+)

28 Postsynaptic potentials The neurotransmitter-dependent sodium channel is the most important source of excitatory postsynaptic potentials. When sodium channels are opened, the result is a depolarization an excitatory postsynaptic potential (EPSP). Excitatory Postsynaptic Potential (EPSP) An excitatory depolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button

29 Postsynaptic potentials When potassium channels are open, they hyperpolarize the membrane, producing an inhibitory postsynaptic potential (IPSP). Inhibitory Postsynaptic Potential (IPSP) An inhibitory hyperpolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button

30 Postsynaptic potentials If the membrane is at the resting potential, nothing happens, because (as we saw earlier) the forces of diffusion and electrostatic pressure balance perfectly for the chloride ion. However, if the membrane potential has already been depolarized by the activity of excitatory synapses located nearby, then the opening of chloride channels will permit Cl to enter the cell. The influx of anions will bring the membrane potential back to its normal resting condition. Thus, the opening of chloride channels serves to neutralize EPSPs.

31 Postsynaptic potentials The fourth type of neurotransmitter-dependent ion channel is the calcium channel. Calcium ions (Ca2+), being positively charged and being located in highest concentration outside the cell, act like sodium ions; that is, the opening of calcium channels depolarizes the membrane, producing EPSPs. But calcium does even more. The entry of calcium into the terminal button triggers the migration of synaptic vesicles and the release of the neurotransmitter. In the dendrites of the postsynaptic cell, calcium binds with and activates special enzymes. These enzymes have a variety of effects, including the production of biochemical and structural changes in the postsynaptic neuron.

32 The more synapses a neuron has, the greater its information-processing capability. Each pyramidal cell in cerebral cortex has about 40,000 synaptic contacts with other neurons. The cerebral cortex alone (the main information-processing tissue of your brain) is estimated to have 100 trillion (10 14 ) synapses. So how a synaptic knob decide to fire?

33 Neural Integration Effects of Postsynaptic Potentials: Neural Integration Neurons are interconnected by means of synapses, and action potentials trigger the release of neurotransmitters, these chemicals initiate excitatory or inhibitory postsynaptic potentials. Excitatory postsynaptic potentials increase the likelihood that the postsynaptic neuron will fire; inhibitory postsynaptic potentials decrease this likelihood. (Remember, firing refers to the occurrence of an action potential.) Thus, the rate at which an axon fires is determined by the relative activity of the excitatory and inhibitory synapses on the soma and dendrites of that cell. If there are no active excitatory synapses, or if the activity of inhibitory synapses is particularly high, that rate could be close to zero.

34 The release of the neurotransmitter produces depolarizing EPSPs in the dendrites of the neuron. These EPSPs (represented in red) are then transmitted down the dendrites, across the soma, to the axon hillock located at the base of the axon. If the depolarization is still strong enough when it reaches this point, the axon will fire. Now let s consider what would happen if, at the same time, inhibitory synapses also become active. Inhibitory postsynaptic potentials are hyperpolarizing they bring the membrane potential away from the threshold of excitation. Thus, they tend to cancel the effects of excitatory postsynaptic potentials.

35 Neural Integration The rate at which a neuron fires is controlled by the relative activity of the excitatory and inhibitory synapses on its dendrites and soma. If the activity of excitatory synapses goes up, the rate of firing will go up. If the activity of inhibitory synapses goes up, the rate of firing will go down. Note that neural inhibition (that is, an inhibitory postsynaptic potential) does not always produce behavioral inhibition. For example, suppose a group of neurons inhibits a particular movement. If these neurons are inhibited, they will no longer suppress the behavior.

36 Thus, inhibition of the inhibitory neurons makes the behavior more likely to occur. Of course, the same is true for neural excitation. Excitation of neurons that inhibit a behavior suppresses that behavior. For example, when we are dreaming, a particular set of inhibitory neurons in the brain becomes active and prevents us from getting up and acting out our dreams. Neurons are elements in complex circuits; without knowing the details of these circuits, one cannot predict the effects of the excitation or inhibition of one set of neurons on an organism s behavior.

37 Electrical communication Some synapses are electrical; the membranes meet and almost touch, forming a gap junction. Gap Junction is a special junction between cells that permits direct communication by means of electrical coupling. The membranes on both sides of a gap junction contain channels that permit ions to diffuse from one cell to another. Thus, changes in the membrane potential of one neuron induce changes in the membrane of the other.

38 A single action potential in a synaptic knob does not produce enough activity to make a postsynaptic cell fire. An EPSP may be produced, but it fades before reaching threshold. A typical EPSP is a voltage change of only 0.5 mv and lasts only 15 to 20 ms. If a neuron has an MP of 70 mv and a threshold of 55 mv, it needs at least 30 EPSPs to reach threshold and fire. There are two ways in which EPSPs can add up to do this, and both may occur simultaneously.

39 Spatial summation. This occurs when EPSPs from several synapses add up to threshold at the axon hillock. Any one synapse may admit only a moderate amount of Na+ into the cell, but several synapses acting together admit enough Na+ to reach threshold. The presynaptic neurons cooperate to induce the postsynaptic neuron to fire. Temporal summation. This occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades. This allows the EPSPs to add up over time to a threshold voltage that triggers an action potential. Temporal summation can occur if even one presynaptic neuron stimulates the postsynaptic neuron intensely enough.

40 Neural coding The nervous system must interpret and pass along both quantitative and qualitative information about its environment whether a light is dim or bright, red or green; whether a taste is mild or intense, salty or sour; whether a sound is loud or soft, high-pitched or low. Considering the complexity of information to be communicated about conditions in and around the body, it seems a marvel that it can be done in the form of something as simple as action potentials particularly since all the action potentials of a given neuron are identical. The way in which the nervous system converts information to a meaningful pattern of action potentials is called neural coding (or sensory coding when it occurs in the sense organs).

41 Neural coding Qualitative Information coding: The most important mechanism for transmitting qualitative information is the labeled line code. This code is based on the fact that each nerve fiber to the brain leads from a receptor that specifically recognizes a particular stimulus type. Nerve fibers in the optic nerve, for example, carry signals only from light receptors in the eye; these fibers will never carry information about taste or sound. The brain therefore interprets any signals in those fibers in terms of light even if the signals result from artificial stimulation of the nerve.

42 Neural coding Quantitative information information about the intensity of a stimulus strength depends on the fact that the more strongly a neuron is stimulated, the more frequently it fires. A weak stimulus may cause a neuron to generate 6 action potentials per second, and a strong stimulus, 600 per second. Thus, the central nervous system can judge stimulus strength from the firing frequency of afferent neurons This figure is based on recordings made from a sensory fiber of the frog sciatic nerve as the gastrocnemius muscle was stretched by suspending weights from it. As the stimulus strength (weight) and stretch increase, the firing frequency of the neuron increases. Firing frequency is a coded message that informs the CNS of stimulus intensity.

43 Neural Circuits

44 Neural Circuits

45 Neural Circuits Reverberating (oscillatory) circuit: Some circuits are constructed so that once the presynaptic cell is stimulated, it will cause the postsynaptic cell to transmit a series of nerve impulses. One such circuit is called a reverberating (oscillatory) circuit. In this pattern, the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones, however, sending impulses back through the circuit again and again. The output signal may last from a few seconds to many hours, depending on the number of synapses and arrangement of neurons in the circuit. Inhibitory neurons may turn off a reverberating circuit after a period of time. Among the body responses thought to be the result of output signals from reverberating circuits are breathing (A reverberating circuit sends repetitious signals to your diaphragm and intercostal muscles, for example, to make you inhale. When the circuit stops firing, you exhale; the next time it fires, you inhale again. ), coordinated muscular activities etc.

46 Neural Circuits A fourth type of circuit is the parallel after-discharge circuit. In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. It is thought that parallel afterdischarge circuits may be employed for precise activities such as mathematical calculations.

47 The story so far. The excitation as a result of integration could result in neuronal firing (spikes) Spikes are important since other neurons receive them (as a signals) Neurons communicate with spikes Information is coded by spikes So if we can manage to measure the spiking time, we decipher how the brain works.

48 Example: Neural integration Neural integration of bilateral olfactory inputs enhances signal-to-noise ratio. (a) Schematic diagram of the bilateral olfactory input pathways and a hypothetical central neuron (grey circle) receiving those inputs. Information is transmitted as spiking activity. Typically, in the absence of any olfactory stimulus, the receptor neurons tend to show a baseline spiking response that contributes to the 'noise' in the system. (b) Neural integration can reduce uncorrelated noise. The plots on the left represent the firing rate of two receptor neurons over time. The baseline fluctuations observed in the two independent channels (left) are reduced after integrating them (right), thus improving signal-tonoise ratio. This improvement may be the chief contribution of dual olfactory inputs to chemotaxis. The green box indicates the release of a puff of odor.

49 Neural integration Single spiking time is meaningless and to extract useful information, we have to average 1. a group of neurons in a local circuit where neuro codes the same information 2. over a time window to obtain the firing rate r r = = Local circuit = 6 Hz Time window = 1 sec

50 Hence we have firing rate of a group of neurons Neural integration r 1 So we can have a network of these local groups w 1: synaptic strength R = f w r j ) ( j w n r n

51 r i is the firing rate of input local circuit The neurons at output local circuits receives signals in the form N i= 1 w i r i The output firing rate of the output local circuit is then given by R R = f ( N i= 1 w i r i where f is the activation function, generally a Sigmoidal function of some sort ) w i weight, (synaptic strength) measuring the strength of the interaction between neurons.

52 EEG and multi cell activity

53 Learning in the Brain Brains learn Altering strength between neurons Creating/deleting connections Hebb s Postulate (Hebbian Learning) When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased. Long Term Potentiation (LTP) Cellular basis for learning and memory LTP is the long-lasting strengthening of the connection between two nerve cells in response to stimulation Discovered in many regions of the cortex

54 The EEG measures not action potentials but summation of graded Post Synaptic Potentials (PSPs) What is summation of graded Post Synaptic Potentials (PSPs)? Let s revisit

55 Types of Synapses CNS Synapses Axodendritic: Axon to dendrite Axosomatic: Axon to cell body Axoaxonic: Axon to axon Dendrodendritic: Dendrite to dendrite

56 Principles of Synaptic Integration

57 Principles of Synaptic Integration EPSP Summation Allows for neurons to perform sophisticated computations Integration: EPSPs added together to produce significant postsynaptic depolarization Spatial summation : adding together of EPSPs generated simultaneously at different synapses Temporal summation : adding together of EPSPs generated at the same synapse in rapid succession (within 1-15 msec of one another)

58 The Contribution of Dendritic Properties to Synaptic Integration Dendrite as a straight cable : EPSPs have to travel down to spike-initiation zone to generate action potential Membrane depolarization falls off exponentially with increasing distance V x = V o /e x/ Vo : depolarization at the origin : Dendritic length constant Distance where the depolarization is 37% of origin (V = 0.37 V o ) In reality, dendrites have branches, changing diameter..

59 Postsynaptic Potentials Signaling in Dendrites & Soma V m changes dendrites & soma Excitatory: + Inhibitory: -

60 Postsynaptic Potentials - PSPs Chemically-gated ion channels Graded Summation Duration milliseconds Decremental Weaken over time & space

61 Postsynaptic Potentials Soma & Dendrites Chemically-gated channels Passive current Graded Summation EPSP Excitatory Depolarization Na+ influx AP more likely IPSP Inhibitory Hyperpolarization K+ efflux AP less likely

62 EPSPs Excitatory Postsynaptic Potential Depolarization (+) V m becomes more positive Na+ influx

63 EPSP Record here + + Depolarization more likely to Action Potential V m -55 mv - 65 mv AT REST - Time

64 Temporal Summation + V m -55 mv + 2 pulses same synapse ~ - 65 mv AT REST - Time

65 Temporal Summation + + Repeated stimulation V m -55 mv - 65 mv AT REST - Time

66 Spatial Summation V m -55 mv Multiple synapses - 65 mv AT REST - Time

67 IPSPs Inhibitory Postsynaptic Potential similar to EPSPs but opposite hyperpolarization (-) V m becomes more negative K+ efflux

68 IPSP - + V m -55 mv - 65 mv Hyperpolarization less likely to fire also summate (max) AT REST - Time

69 EPSPs & IPSPs summate CANCEL EACH OTHER Net stimulation EPSPs + IPSPs = net effects Or we Integrate over time

70 + EPSP - IPSP - 65 mv

71 Dipole

72 Summation of post synaptic potentials If an excitatory neurotransmitter is released at the apical dendrites of a cortical pyramidal cell, current will flow from the extracellular space into the cell, yielding a net negativity on the outside of the cell in the region of the apical dendrites. To complete the circuit, current will also flow out of the cell body and basal dendrites, yielding a net positivity in this area. Together, the negativity at the apical dendrites and the positivity at the cell body create a tiny dipole (a dipole is simply a pair of positive and negative electrical charges separated by a small distance)

73 Aligned neurons and synchronous activity Neurons which are radially symmetric, randomly oriented or asynchronously activated do not produce externally observable electric or magnetic fields. Neurons which are non radially symmetric, spatially aligned and synchronously activated add up to produce externally observable electric or magnetic fields

74 The summation of the individual dipoles is complicated by the fact that the cortex is not flat, but instead has many folds. But the summation of many dipoles is essentially equivalent to a single dipole formed by averaging the orientations of the individual dipoles. This averaged dipole is called an equivalent current dipole (ECD). It is important to note, however, that whenever the individual dipoles are more than 90 degrees from each other, they will cancel each other to some extent, with complete cancellation at 180 degrees. Why its hard to record cerebellar activity from the scalp For example, the Purkinje cells in the cerebellar cortex are beautifully aligned with each other and oriented perpendicular to the cortical surface, but the cortical surface is so highly folded that the dipoles in one small patch of cerebellar cortex will almost always be cancelled by dipoles in a nearby but oppositely oriented patch, making it difficult or impossible to record cerebellar activity from the scalp.

75 Volume conduction Alignment of the dipoles is primarily responsible for over the skull recordings. But it cause a problem as well. When a dipole is present in a conductive medium such as the brain, current is conducted throughout that medium until it reaches the surface. This is called volume conduction. The voltage that will be present at any given point on the surface of the scalp will depend on the position and orientation of the generator dipole. Electricity does not just run directly between the two poles of a dipole in a conductive medium, but instead spreads out through the conductor. Consequently, potentials spread out as they travel through the brain. In addition, because electricity tends to follow the path of least resistance, potentials tend to spread laterally when they encounter the high resistance of the skull. Volume conduction eliminating algorithms will briefly be covered later.

76 Volume conduction The voltage that will be present at any given point on the surface of the scalp will depend on the position and orientation of the generator dipole. Consider a radial oriented dipole. The resultant electric potential on the scalp is as in figure.

77 Magnetic Field of Brain Blurring of the electrical field can be addressed by an other way. An electrical dipole is always surrounded by a magnetic field, and these fields summate in the same manner as voltages. Thus, whenever potential is generated, a magnetic field is also generated, running around the dipole. This is called magnetoencephalogram (MEG). However, magnetic recordings are very expensive because supercooling is expensive and because an expensive magnetically shielded recording chamber is necessary to attenuate the Earth s relatively large magnetic field.