on 29-Mar-2020 (Sun)

Price is two traders making a buy and sell decision.

Price movement is a function of supply and demand.

Supply is the amount of a product which sellers want to sell at a particular price.

Demand is the amount of a product which buyers want to buy at a particular price.

Price will move with changes in supply and/or demand.

Price rises while demand is greater than supply, and while those buyers are willing to pay higher prices.

Price rises until we run out of buyers, or until supply increases sufficiently to absorb all the demand.

Price falls while supply is greater than demand, and while those sellers are willing to sell at lower prices.

Price falls until we run out of sellers, or until demand increases to the point it absorbs all the supply.

price movement is a result of supply/demand imbalance. And the supply/demand imbalance is created by trader’s sense of urgency to transact.

Bullish sentiment leads to bullish orderflow resulting in price rising.

Bearish sentiment leads to bearish orderflow resulting in price falling.

Neutral sentiment leads to narrow range sideways price action.

Price moves with changes in the forces of supply and demand.

Supply and demand change as the sentiment of the crowd changes.

The sentiment of the crowd changes with changes in the bullish or bearish sentiment of the market participants.

The net sum of all individual trader decisions and actions, form the Net Order Flow.

When Net Order Flow is bullish (demand greater than supply), price will rise.

When Net Order Flow is bearish (supply greater than demand), price will fall.

Price moves as a collective result of all traders’ bullish or bearish sentiment and their decisions to act in the market (buy or sell).

Bullish Pressure – sum of all demand in the market (forces attempting to push price up).

Bearish Pressure – sum of all supply in the market (forces attempting to push price down).

Net Order Flow – the resultant of bullish and bearish pressure. Net order flow (NOF) is bullish if price is going up, bearish if price is going down.

DIFFERENTIAL SENSITIVITY OF RECEPTORS

How do two types of sensory receptors detect different types of sensory stimuli? The answer is “by differential sensitivities.” That is, each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli

Modality of Sensation—The “Labeled Line” Principle

Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation. Yet, despite the fact that we experience these different modalities of sensation, nerve fibers transmit only impulses. Therefore, how do different nerve fibers transmit different modalities of sensation? The answer is that each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is deter- mined by the point in the nervous system to which the fiber leads

This specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle

LOCAL ELECTRICAL CURRENTS AT NERVE ENDINGS—RECEPTOR POTENTIALS

All sensory receptors have one feature in common. Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential

Mechanisms of Receptor Potentials.

Different receptors can be excited in one of several ways to cause recep- tor potentials: (1) by mechanical deformation of the receptor, which stretches the receptor membrane and opens ion channels; (2) by application of a chemical to the membrane, which also opens ion channels; (3) by change of the temperature of the membrane, which alters the permeability of the membrane; or (4) by the effects of electromagnetic radiation, such as light on a retinal visual receptor, which either directly or indirectly changes the receptor membrane characteristics and allows ions to flow through membrane channels

Maximum Receptor Potential Amplitude.

The maxi- mum amplitude of most sensory receptor potentials is about 100 millivolts, but this level occurs only at an extremely high intensity of sensory stimulus. This is about the same maximum voltage recorded in action potentials and is also the change in voltage when the membrane becomes maximally permeable to sodium ions

Relation of the Receptor Potential to Action Potentials.

When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials occur, as illustrated in Figure 47-2. Note also that the more the receptor potential rises above the threshold level, the greater becomes the action potential frequency

RECEPTOR POTENTIAL OF THE PACINIAN CORPUSCLE—AN EXAMPLE OF RECEPTOR FUNCTION

Note in Figure 47-1 that the Pacinian corpuscle has a central nerve fiber extending through its core. Surrounding this central nerve fiber are multiple concentric capsule layers, and thus compression anywhere on the outside of the corpuscle will elongate, indent, or otherwise deform the central fiber.

Figure 47-3 shows only the central fiber of the Pacin- ian corpuscle after all capsule layers but one have been removed. The tip of the central fiber inside the capsule is unmyelinated, but the fiber does become myelinated (the blue sheath shown in the figure) shortly before leaving the corpuscle to enter a peripheral sensory nerve

The receptor potential in turn induces a local circuit of current flow, shown by the arrows, that spreads along the nerve fiber. At the first node of Ranvier, which lies inside the capsule of the Pacinian corpuscle, the local current flow depolar- izes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system

Putting this principle together with the data in Figure 47-4, one can see that very intense stimulation of the receptor causes progressively less and less additional increase in numbers of action potentials. This exceedingly important principle is applicable to almost all sensory receptors. It allows the receptor to be sensitive to very weak sensory experience and yet not reach a maximum firing rate until the sensory experience is extreme.

ADAPTATION OF RECEPTORS

Another characteristic of all sensory receptors is that they adapt either partially or completely to any constant stimulus after a period of time. That is, when a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often to none at all.

Furthermore, some sensory receptors adapt to a far greater extent than do others. For example, the Pacinian corpuscles adapt to “extinction” within a few hundredths of a second, and the receptors at the bases of the hairs adapt to extinction within a second or more. It is probable that all other mechanoreceptors eventually adapt almost completely, but some require hours or days to do so, for which reason they are called “nonadapting” receptors

The longest measured time for almost com- plete adaptation of a mechanoreceptor is about 2 days, which is the adaptation time for many carotid and aortic barore ceptors; however, some physiologists believe that these specialized baroreceptors never fully adapt. Some of the nonmechanoreceptors—the chemoreceptors and pain receptors, for instance—probably never adapt completely

Mechanisms by Which Receptors Adapt

. The mecha- nism of receptor adaptation is different for each type of receptor, in much the same way that development of a receptor potential is an individual property. For instance, in the eye, the rods and cones adapt by changing the concentrations of their light-sensitive chemicals

First, the Pacinian corpuscle is a viscoelastic structure, so that when a distorting force is suddenly applied to one side of the corpuscle, this force is instantly transmit- ted by the viscous component of the corpuscle directly to the same side of the central nerve fiber, thus eliciting a receptor potential. However, within a few hundredths of a second, the fluid within the corpuscle redistributes and the receptor potential is no longer elicited. Thus, the receptor potential appears at the onset of compression but disappears within a small fraction of a second even though the compression continues

The second, much slower mechanism of adaptation of the Pacinian corpuscle results from a process called accommodation, which occurs in the nerve fiber itself. That is, even if by chance the central core fiber should continue to be distorted, the tip of the nerve fiber gradu- ally becomes “accommodated” to the stimulus. This prob- ably results from progressive “inactivation” of the sodium channels in the nerve fiber membrane, which means that sodium current flow through the channels causes them gradually to close, an effect that seems to occur for all or most cell membrane sodium channels, as was explained in Chapter 5

That is, part of the adaptation results from readjust- ments in the structure of the receptor, and part results from an electrical type of accommodation in the terminal nerve fibril

Slowly Adapting Receptors Detect Continuous Stimulus Strength—the “Tonic” Receptors.

Slowly adapt- ing receptors continue to transmit impulses to the brain as long as the stimulus is present (or at least for many minutes or hours). Therefore, they keep the brain con- stantly apprised of the status of the body and its relation to its surroundings. For instance, impulses from the muscle spindles and Golgi tendon apparatuses allow the nervous system to know the status of muscle contraction and load on the muscle tendon at each instant

Other slowly adapting receptors include (1) receptors of the macula in the vestibular apparatus, (2) pain receptors, (3) baroreceptors of the arterial tree, and (4) chemoreceptors of the carotid and aortic bodies. Because the slowly adapting receptors can continue to transmit information for many hours, or even days, they are called tonic receptors

Rapidly Adapting Receptors Detect Change in Stimulus Strength—the “Rate Receptors,” “Movement Receptors,” or “Phasic Receptors.”

Receptors that adapt rapidly cannot be used to transmit a continuous signal because they are stimulated only when the stimulus strength changes. Yet, they react strongly while a change is actually taking place. Therefore, these receptors are called rate receptors, movement receptors, or phasic receptors.

Thus, in the case of the Pacinian corpuscle, sudden pressure applied to the tissue excites this receptor for a few milliseconds, and then its excitation is over even though the pressure continues. Later, however, it trans- mits a signal again when the pressure is released. In other words, the Pacinian corpuscle is exceedingly important in apprising the nervous system of rapid tissue deforma- tions, but it is useless for transmitting information about constant conditions in the body

Predictive Function of the Rate Receptors.

If one knows the rate at which some change in bodily status is taking place, the state of the body a few seconds or even a few minutes later can be predicted.

For instance, the receptors of the semicircular canals in the vestibular apparatus of the ear detect the rate at which the head begins to turn when one runs around a curve. Using this information, a person can predict how much he or she will turn within the next 2 seconds and can adjust the motion of the legs ahead of time to keep from losing balance

Likewise, receptors located in or near the joints help detect the rates of movement of the different parts of the body. For instance, when one is running, information from the joint rate receptors allows the nervous system to predict where the feet will be during any precise fraction of the next second. Therefore, appro- priate motor signals can be transmitted to the muscles of the legs to make any necessary anticipatory correc- tions in position so that the person will not fall. Loss of this predictive function makes it impossible for the person to run

Nerve Fibers That Transmit Different Types of Signals and Their Physiological Classification

Some signals need to be transmitted to or from the central nervous system extremely rapidly; otherwise, the informa- tion would be useless. An example of this is the sensory signals that apprise the brain of the momentary positions of the legs at each fraction of a second during running

At the other extreme, some types of sensory information, such as that depicting prolonged, aching pain, do not need to be transmitted rapidly, and thus slowly conducting fibers will suffice.

General Classification of Nerve Fibers.

Shown in Figure 47-6 is a “general classification” and a “sensory nerve classification” of the different types of nerve fibers. In the general classification, the fibers are divided into types A and C, and the type A fibers are further subdivided into α, β, γ, and δ fibers

Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves. Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities. The C fibers constitute more than one half of the sensory fibers in most peripheral nerves, as well as all the postganglionic autonomic fibers

Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, covering a distance that is longer than a football field in 1 second. Conversely, the smallest fibers transmit impulses as slowly as 0.5 m/sec, requiring about 2 seconds to go from the big toe to the spinal cord

Alternative Classification Used by Sensory Physiologists.

Certain recording techniques have made it possible to separate the type Aα fibers into two subgroups, yet these same recording techniques cannot distinguish easily between Aβ and Aγ fibers.

Therefore, the following clas- sification is frequently used by sensory physiologists.

• Group Ia. Fibers from the annulospiral endings of muscle spindles (about 17 microns in diameter on aver- age; these fibers are α -type A fibers in the general classification).
• Group Ib. Fibers from the Golgi tendon organs (about 16 micrometers in diameter on average; these fibers also are α -type A fibers).
• Group II. Fibers from most discrete cutaneous tactile receptors and from the flower-spray endings of the muscle spindles (about 8 micrometers in diameter on average; these fibers are β - and γ-type A fibers in the general classification).
• Group III. Fibers carrying temperature, crude touch, and pricking pain sensations (about 3 micrometers in diameter on average; they are δ -type A fibers in the general classification).
• Group IV. Unmyelinated fibers carrying pain, itch, temperature, and crude touch sensations (0.5 to 2 micro- meters in diameter; they are type C fibers in the general classification)

TRANSMISSION OF SIGNALS OF DIFFERENT INTENSITY IN NERVE TRACTS—SPATIAL AND TEMPORAL SUMMATION

One of the characteristics of each signal that always must be conveyed is signal intensity—for instance, the intensity of pain. The different gradations of intensity can be trans- mitted either by using increasing numbers of parallel fibers or by sending more action potentials along a single fiber. These two mechanisms are called, respectively, spatial summation and temporal summation.

Spatial Summation

Figure 47-7 shows the phenome- non of spatial summation, whereby increasing signal strength is transmitted by using progressively greater numbers of fibers. This figure shows a section of skin innervated by a large number of parallel pain fibers. Each of these fibers arborizes into hundreds of minute free nerve endings that serve as pain receptors.

The entire cluster of fibers from one pain fiber frequently covers an area of skin as large as 5 centimeters in diameter. This area is called the receptor field of that fiber. The number of endings is large in the center of the field but diminishes toward the periphery. One can also see from the figure that the arborizing fibrils overlap those from other pain fibers. Therefore, a pinprick of the skin usually stimulates endings from many different pain fibers simultaneously.

When the pinprick is in the center of the receptive field of a particular pain fiber, the degree of stimulation of that fiber is far greater than when it is in the periphery of the field because the number of free nerve endings in the middle of the field is much greater than at the periphery.

Thus, the lower part of Figure 47-7 shows three views of the cross section of the nerve bundle leading from the skin area. To the left is the effect of a weak stimulus, with only a single nerve fiber in the middle of the bundle stimulated strongly (represented by the red-colored fiber), whereas several adjacent fibers are stimulated weakly (half-red fibers). The other two views of the nerve cross section show the effect of a moderate stimulus and a strong stimulus, with progressively more fibers being stimulated. Thus, the stronger signals spread to more and more fibers. This process is the phenomenon of spatial summation

Temporal Summation.

A second means for transmit- ting signals of increasing strength is by increasing the frequency of nerve impulses in each fiber, which is called temporal summation. Figure 47-8 demonstrates this phenomenon, showing in the upper part a changing strength of signal and in the lower part the actual impulses transmitted by the nerve fiber.

fig 47 - 7

fig 47 - 8

fig 47 - 9

TRANSMISSION AND PROCESSING OF SIGNALS IN NEURONAL POOLS

The central nervous system is composed of thousands to millions of neuronal pools; some of these pools contain few neurons, whereas others have vast numbers. For instance, the entire cerebral cortex could be considered to be a single large neuronal pool. Other neuronal pools include the different basal ganglia and the specific nuclei in the thalamus, cerebellum, mesencephalon, pons, and medulla. Also, the entire dorsal gray matter of the spinal cord could be considered one long pool of neurons

Each neuronal pool has its own special organization that causes it to process signals in its own unique way, thus allowing the total consortium of pools to achieve the multitude of functions of the nervous system. Yet, despite their differences in function, the pools also have many similar principles of function, described in the following sections.

RELAYING OF SIGNALS THROUGH NEURONAL POOLS Organization of Neurons for Relaying Signals

. Figure 47-9 is a schematic diagram of several neurons in a neuronal pool, showing “input” fibers to the left and “output” fibers to the right. Each input fiber divides hun- dreds to thousands of times, providing a thousand or more terminal fibrils that spread into a large area in the pool to synapse with dendrites or cell bodies of the neurons in the pool. The dendrites usually also arborize and spread hundreds to thousands of micrometers in the pool

The neuronal area stimulated by each incoming nerve fiber is called its stimulatory field. Note that large numbers of the terminals from each input fiber lie on the nearest neuron in its “field,” but progressively fewer terminals lie on the neurons farther away

Threshold and Subthreshold Stimuli—Excitation or Facilitation.

As discussed in Chapter 46, discharge of a single excitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron. Instead, large numbers of input terminals must discharge on the same neuron either simultaneously or in rapid succession to cause excitation.

For instance, in Figure 47-9, let us assume that six terminals must discharge almost simulta- neously to excite any one of the neurons. Note that input fiber 1 has more than enough terminals to cause neuron a to discharge. The stimulus from input fiber 1 to this neuron is said to be an excitatory stimulus; it is also called a suprathreshold stimulus because it is above the thresh- old required for excitation. Input fiber 1 also contributes terminals to neurons b and c, but not enough to cause excitation. Nevertheless, discharge of these terminals makes both these neurons more likely to be excited by signals arriving through other incoming nerve fibers. Therefore, the stimuli to these neurons are said to be subthreshold, and the neurons are said to be facilitated. Similarly, for input fiber 2, the stimulus to neuron d is a suprathreshold stimulus, and the stimuli to neurons b and c are subthreshold, but facilitating, stimuli

Therefore, this is said to be the discharge zone of the incoming fiber, also called the excited zone or liminal zone. To each side, the neurons are facilitated but not excited, and these areas are called the facilitated zone, also called the subthreshold zone or subliminal zone

Inhibition of a Neuronal Pool.

Some incoming fibers inhibit neurons, rather than exciting them. This mecha- nism is the opposite of facilitation, and the entire field of the inhibitory branches is called the inhibitory zone. The degree of inhibition in the center of this zone is great because of large numbers of endings in the center and becomes progressively less toward its edges

Divergence of Signals Passing Through Neuronal Pools

Often it is important for weak signals entering a neuronal pool to excite far greater numbers of nerve fibers leaving the pool. This phenomenon is called divergence. Two major types of divergence occur and have entirely differ- ent purposes

An amplifying type of divergence is shown in Figure 47-11A. Amplifying divergence means simply that an input signal spreads to an increasing number of neurons as it passes through successive orders of neurons in its path. This type of divergence is characteristic of the cor- ticospinal pathway in its control of skeletal muscles, with a single large pyramidal cell in the motor cortex capable, under highly facilitated conditions, of exciting as many as 10,000 muscle fibers.

The second type of divergence, shown in Figure 46-11B, is divergence into multiple tracts. In this case, the signal is transmitted in two directions from the pool. For instance, information transmitted up the dorsal columns of the spinal cord takes two courses in the lower part of the brain: (1) into the cerebellum and (2) on through the lower regions of the brain to the thalamus and cerebral cortex. Likewise, in the thalamus, almost all sensory information is relayed both into still deeper structures of the thalamus and at the same time to discrete regions of the cerebral cortex

Convergence of Signals

Convergence means signals from multiple inputs uniting to excite a single neuron. Figure 47-12A shows con vergence from a single source—that is, multiple terminals from a single incoming fiber tract terminate on the same neuron. The importance of this type of convergence is that neurons are almost never excited by an action potential from a single input terminal. However, action potentials converging on the neuron from multiple terminals provide enough spatial summa- tion to bring the neuron to the threshold required for discharge

Convergence can also result from input signals (excit- atory or inhibitory) from multiple sources, as shown in Figure 47-12B. For instance, the interneurons of the spinal cord receive converging signals from (1) periph- eral nerve fibers entering the cord, (2) propriospinal fibers passing from one segment of the cord to another, (3) corticospinal fibers from the cerebral cortex, and (4) several other long pathways descending from the brain into the spinal cord. Then the signals from the interneu- rons converge on the anterior motor neurons to control muscle function

Such convergence allows summation of information from different sources, and the resulting response is a summated effect of all the different types of information. Convergence is one of the important means by which the central nervous system correlates, summates, and sorts different types of information

fig 47 -10

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fig 47 - 12

Neuronal Circuit With Both Excitatory and Inhibitory Output Signals

Sometimes an incoming signal to a neuronal pool causes an output excitatory signal going in one direction and at the same time an inhibitory signal going elsewhere. For instance, at the same time that an excitatory signal is transmitted by one set of neurons in the spinal cord to cause forward movement of a leg, an inhibitory signal is transmitted through a separate set of neurons to inhibit the muscles on the back of the leg so that they will not oppose the forward movement. This type of circuit is characteristic for controlling all antagonistic pairs of muscles, and it is called the reciprocal inhibition circuit

. Figure 47-13 shows the means by which the inhi- bition is achieved. The input fiber directly excites the excitatory output pathway, but it stimulates an inter- mediate inhibitory neuron (neuron 2), which secretes a different type of transmitter substance to inhibit the second output pathway from the pool. This type of circuit is also important in preventing overactivity in many parts of the brain

PROLONGATION OF A SIGNAL BY A NEURONAL POOL—“AFTERDISCHARGE”

Thus far, we have considered signals that are merely relayed through neuronal pools. However, in many instances, a signal entering a pool causes a prolonged output discharge, called afterdischarge, lasting a few mil- liseconds to as long as many minutes after the incoming signal is over. The most important mechanisms by which afterdischarge occurs are described in the following sections

Synaptic Afterdischarge.

When excitatory synapses discharge on the surfaces of dendrites or soma of a neuron, a postsynaptic electrical potential develops in the neuron and lasts for many milliseconds, especially when some of the long-acting synaptic transmitter sub- stances are involved. As long as this potential lasts, it can continue to excite the neuron, causing it to transmit a continuous train of output impulses, as was explained in Chapter 46. Thus, as a result of this synaptic “after- discharge” mechanism alone, it is possible for a single instantaneous input signal to cause a sustained signal output (a series of repetitive discharges) lasting for many milliseconds.

Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolongation.

One of the most important of all circuits in the entire nervous system is the reverberatory or oscillatory circuit. Such circuits are caused by positive feedback within the neuronal circuit that feeds back to re-excite the input of the same circuit. Consequently, once stimulated, the circuit may discharge repetitively for a long time.

Several possible varieties of reverberatory circuits are shown in Figure 46-14. The simplest, shown in Figure 47-14A, involves only a single neuron. In this case, the output neuron sends a collateral nerve fiber back to its own dendrites or soma to restimulate itself. Although the importance of this type of circuit is not clear, theoreti- cally, once the neuron discharges, the feedback stimuli could keep the neuron discharging for a protracted time thereafter.

. Figure 47-14B shows a few additional neurons in the feedback circuit, which causes a longer delay between initial discharge and the feedback signal. Figure 47-14C shows a more complex system in which both facilitatory and inhibitory fibers impinge on the reverberating circuit. A facilitatory signal enhances the intensity and frequency

Figure 46-14D shows that most reverberating path- ways are constituted of many parallel fibers. At each cell station, the terminal fibrils spread widely. In such a system, the total reverberating signal can be either weak or strong, depending on how many parallel nerve fibers are momentarily involved in the reverberation.

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Characteristics of Signal Prolongation From a Reverberatory Circuit.

Figure 47-15 shows output signals from a typical reverberatory circuit. The input stimulus may last only 1 millisecond or so, and yet the output can last for many milliseconds or even minutes. The figure demonstrates that the intensity of the output signal usually increases to a high value early in reverberation and then decreases to a critical point, at which it suddenly ceases entirely. The cause of this sudden cessation of reverberation is fatigue of synaptic junctions in the circuit. Fatigue beyond a certain critical level lowers the stimula- tion of the next neuron in the circuit below threshold level so that the circuit feedback is suddenly broken. The duration of the total signal before cessation can also be controlled by signals from other parts of the brain that inhibit or facilitate the circuit. Almost these exact patterns of output signals are recorded from the motor nerves exciting a muscle involved in a flexor reflex after pain stimulation of the foot (as shown later in Figure 47-18)

Continuous Signal Output from Some Neuronal Circuits Some neuronal circuits emit output signals continuously, even without excitatory input signals. At least two mech- anisms can cause this effect: (1) continuous intrinsic neu- ronal discharge and (2) continuous reverberatory signals

Continuous Discharge Caused by Intrinsic Neuronal Excitability. Neurons, like other excitable tissues, dis- charge repetitively if their level of excitatory membrane potential rises above a certain threshold level. The mem- brane potentials of many neurons even normally are high enough to cause them to emit impulses continually. This phenomenon occurs especially in many of the neurons of the cerebellum, as well as in most of the interneurons of the spinal cord. The rates at which these cells emit impulses can be increased by excitatory signals or decreased by inhibitory signals; inhibitory signals often can decrease the rate of firing to zero

Continuous Signals Emitted from Reverberating Circuits as a Means for Transmitting Information.

A reverberating circuit that does not fatigue enough to stop reverberation is a source of continuous impulses. Furthermore, excitatory impulses entering the reverber- ating pool can increase the output signal, whereas inhibi- tion can decrease or even extinguish the signal

(Fig 47 - 16)

Students who are familiar with radio transmitters will recognize this to be a carrier wave type of information transmission. That is, the excitatory and inhibitory control signals are not the cause of the output signal, but they do control its changing level of intensity. Note that this carrier wave system allows a decrease in signal intensity, as well as an increase, whereas up to this point, the types of information trans- mission we have discussed have been mainly positive information rather than negative information. This type of information transmission is used by the autonomic nervous system to control such functions as vascular tone, gut tone, degree of constriction of the iris in the eye, and heart rate. That is, the nerve excitatory signal to each of these areas can be either increased or decreased by accessory input signals into the reverberating neuronal pathway

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Rhythmical Signal Output

Many neuronal circuits emit rhythmical output signals— for instance, a rhythmical respiratory signal originates in the respiratory centers of the medulla and pons. This respiratory rhythmical signal continues throughout life.

Other rhythmical signals, such as those that cause scratching movements by the hind leg of a dog or the walking movements of any animal, require input stimuli into the respective circuits to initiate the rhythmical signals

Fig 47 - 17

Fig 47 - 18

All or almost all rhythmical signals that have been studied experimentally have been found to result from reverberating circuits or a succession of sequential reverberating circuits that feed excitatory or inhibitory signals in a circular pathway from one neuronal pool to the next. Excitatory or inhibitory signals can also increase or decrease the amplitude of the rhythmical signal output. Figure 47-17, for instance, shows changes in the respira- tory signal output in the phrenic nerve. When the carotid body is stimulated by arterial oxygen deficiency, both the frequency and the amplitude of the respiratory rhythmi- cal output signal increase progressively.

INSTABILITY AND STABILITY OF NEURONAL CIRCUITS

Almost every part of the brain connects either directly or indirectly with every other part, which creates a serious challenge. If the first part excites the second, the second the third, the third the fourth, and so on until finally the signal re-excites the first part, it is clear that an excitatory signal entering any part of the brain would set off a con- tinuous cycle of re-excitation of all parts. If this cycle should occur, the brain would be inundated by a mass of uncontrolled reverberating signals—signals that would be transmitting no information but, nevertheless, would be consuming the circuits of the brain so that none of the informational signals could be transmitted. Such an effect occurs in widespread areas of the brain during epileptic seizures. How does the central nervous system prevent this effect from happening all the time? The answer lies mainly in two basic mechanisms that function throughout the central nervous system: (1) inhibitory circuits and (2) fatigue of synapses

INHIBITORY CIRCUITS AS A MECHANISM FOR STABILIZING NERVOUS SYSTEM FUNCTION

Two types of inhibitory circuits in widespread areas of the brain help prevent excessive spread of signals: (1) inhibitory feedback circuits that return from the termini of pathways back to the initial excitatory neurons of the same pathways (these circuits occur in virtually all sensory nervous pathways and inhibit either the input neurons or the intermediate neurons in the sensory pathway when the termini become overly excited), and (2) some neuronal pools that exert gross inhibitory control over widespread areas of the brain (for instance, many of the basal ganglia exert inhibitory influences throughout the muscle control system)

SYNAPTIC FATIGUE AS A MEANS OF STABILIZING THE NERVOUS SYSTEM

Synaptic fatigue means simply that synaptic transmission becomes progressively weaker the more prolonged and more intense the period of excitation. Figure 47-18 shows three successive records of a flexor reflex elicited in an animal caused by inflicting pain in the footpad of the paw. Note in each record that the strength of contrac- tion progressively “decrements”—that is, its strength diminishes; much of this effect is caused by fatigue of synapses in the flexor reflex circuit. Furthermore, the shorter the interval between successive flexor reflexes, the less the intensity of the subsequent reflex response

Automatic Short-Term Adjustment of Pathway Sensitivity by the Fatigue Mechanism

Now let us apply this phenomenon of fatigue to other pathways in the brain. Those that are overused usually become fatigued, so their sensitivities decrease. Conversely, those that are underused become rested and their sensitivities increase.

Thus, fatigue and recovery from fatigue constitute an important short-term means of moderating the sensitivities of the different nervous system circuits. These functions help to keep the circuits operating in a range of sensitivity that allows effective function

Long-Term Changes in Synaptic Sensitivity Caused by Automatic Down-Regulation or Up-Regulation of Synaptic Receptors

The long-term sensitivities of synapses can be changed tremendously by up-regulating the number of receptor proteins at the synaptic sites when there is underactivity and down-regulating the receptors when there is overactivity

The mechanism for this pro- cess is the following: Receptor proteins are being formed constantly by the endoplasmic reticular–Golgi apparatus system and are constantly being inserted into the receptor neuron synaptic membrane. However, when the synapses are overused so that excesses of transmitter substance combine with the receptor proteins, many of these recep- tors are inactivated and removed from the synaptic membrane

It is indeed fortunate that up-regulation and down- regulation of receptors, as well as other control mecha- nisms for adjusting synaptic sensitivity, continually adjust the sensitivity in each circuit to almost the exact level required for proper function. Think for a moment how serious it would be if the sensitivities of only a few of these circuits were abnormally high; one might then expect almost continual muscle cramps, seizures, psy- chotic disturbances, hallucinations, mental tension, or other nervous disorders. Fortunately, the automatic con- trols normally readjust the sensitivities of the circuits back to controllable ranges of reactivity any time the cir- cuits begin to be too active or too depressed

The somatic senses are the nervous mechanisms that collect sensory information from all over the body. These senses are in contradistinction to the special senses, which mean specifically vision, hearing, smell, taste, and equilibrium

CLASSIFICATION OF SOMATIC SENSES

The somatic senses can be classified into three physiological types: (1) the mechanoreceptive somatic senses, which include both tactile and position sensations that are stim- ulated by mechanical displacement of some tissue of the body; (2) the thermoreceptive senses, which detect heat and cold; and (3) the pain sense, which is activated by factors that damage the tissues

The tactile senses include touch, pressure, vibration, and tickle senses, and the posi- tion senses include static position and rate of movement senses.

Other Classifications of Somatic Sensations

Somatic sensations are also often grouped together in other classes, as follows: Exteroreceptive sensations are those from the surface of the body. Proprioceptive sensations are those relating to the physical state of the body, including position sen- sations, tendon and muscle sensations, pressure sensa- tions from the bottom of the feet, and even the sensation of equilibrium (which is often considered a “special” sen- sation rather than a somatic sensation). Visceral sensations are those from the viscera of the body; in using this term, one usually refers specifically to sensations from the internal organs. Deep sensations are those that come from deep tissues, such as from fasciae, muscles, and bone. These sensations include mainly “deep” pressure, pain, and vibration

Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration

Although touch, pressure, and vibration are frequently classified as sepa- rate sensations, they are all detected by the same types of receptors. There are three principal differences among them: (1) touch sensation generally results from stimula- tion of tactile receptors in the skin or in tissues immedi- ately beneath the skin; (2) pressure sensation generally results from deformation of deeper tissues; and (3) vibra- tion sensation results from rapidly repetitive sensory signals, but some of the same types of receptors as those for touch and pressure are used.

Tactile Receptors

There are at least six entirely different types of tactile receptors, but many more similar to these also exist.

First, some free nerve endings, which are found every- where in the skin and in many other tissues, can detect touch and pressure. For instance, even light contact with the cornea of the eye, which contains no other type of nerve ending besides free nerve endings, can nevertheless elicit touch and pressure sensations.

Second, a touch receptor with great sensitivity is the Meissner’s corpuscle (illustrated in Figure 47-1), an elon- gated encapsulated nerve ending of a large (type Aβ) myelinated sensory nerve fiber. Inside the capsulation are many branching terminal nerve filaments. These corpuscles are present in the nonhairy parts of the skin and are particularly abundant in the fingertips, lips, and other areas of the skin where one’s ability to discern spatial locations of touch sensations is highly developed. Meissner corpuscles adapt in a fraction of a second after they are stimulated, which means that they are particu- larly sensitive to movement of objects over the surface of the skin, as well as to low-frequency vibration

Third, the fingertips and other areas that contain large numbers of Meissner’s corpuscles usually also contain large numbers of expanded tip tactile receptors, one type of which is Merkel’s discs, shown in Figure 48-1. The hairy parts of the skin also contain moderate numbers of expanded tip receptors, even though they have almost no Meissner’s corpuscles. These receptors differ from Meissner’s corpuscles in that they transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Therefore, they are responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin

Merkel discs are often grouped together in a receptor organ called the Iggo dome receptor, which projects upward against the underside of the epithelium of the skin, as is also shown in Figure 48-1. This upward pro- jection causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor. Also note that the entire group of Merkel’s discs is innervated by a single large myelinated nerve fiber (type Aβ). These receptors, along with the Meissner’s corpuscles discussed earlier, play extremely important roles in localizing touch sensations to specific surface areas of the body and in determining the texture of what is felt

Fourth, slight movement of any hair on the body stim- ulates a nerve fiber entwining its base. Thus, each hair and its basal nerve fiber, called the hair end-organ, are also touch receptors. A receptor adapts readily and, like Meissner’s corpuscles, detects mainly (a) movement of objects on the surface of the body or (b) initial contact with the body

Fifth, located in the deeper layers of the skin and also in still deeper internal tissues are many Ruffini’s endings, which are multibranched, encapsulated endings, as shown in Figure 47-1. These endings adapt very slowly and, therefore, are important for signaling continuous states of deformation of the tissues, such as heavy prolonged touch and pressure signals. They are also found in joint capsules and help to signal the degree of joint rotation

Sixth, Pacinian corpuscles, which were discussed in detail in Chapter 47, lie both immediately beneath the skin and deep in the fascial tissues of the body. They are stimulated only by rapid local compression of the tissues because they adapt in a few hundredths of a second

Therefore, they are particularly important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues

Fig 48 - 1

Transmission of Tactile Signals in Peripheral Nerve Fibers

Almost all specialized sensory receptors, such as Meissner’s corpuscles, Iggo dome receptors, hair recep- tors, Pacinian corpuscles, and Ruffini’s endings, transmit their signals in type Aβ nerve fibers that have transmis- sion velocities ranging from 30 to 70 m/sec. Conversely, free nerve ending tactile receptors transmit signals mainly by way of the small type Aδ myelinated fibers that conduct at velocities of only 5 to 30 m/sec.

Some tactile free nerve endings transmit by way of type C unmyelinated fibers at velocities from a fraction of a meter up to 2 m/sec; these nerve endings send signals into the spinal cord and lower brain stem, probably sub- serving mainly the sensation of tickle

Thus, the more critical types of sensory signals—those that help to determine precise localization on the skin, minute gradations of intensity, or rapid changes in sensory signal intensity—are all transmitted in more rapidly con- ducting types of sensory nerve fibers. Conversely, the cruder types of signals, such as pressure, poorly localized touch, and especially tickle, are transmitted by way of much slower, very small nerve fibers that require much less space in the nerve bundle than the fast fibers

Detection of Vibration

All tactile receptors are in- volved in detection of vibration, although different recep- tors detect different frequencies of vibration. Pacinian corpuscles can detect signal vibrations from 30 to 800 cycles/sec because they respond extremely rapidly to minute and rapid deformations of the tissues. They also transmit their signals over type Aβ nerve fibers, which can transmit as many as 1000 impulses per second. Low- frequency vibrations from 2 up to 80 cycles per second, in contrast, stimulate other tactile receptors, especially Meissner’s corpuscles, which adapt less rapidly than do Pacinian corpuscles

Detection of Tickle and Itch by Mechanoreceptive Free Nerve Endings

Neurophysiological studies have demonstrated the existence of very sensitive, rapidly adapting mechanoreceptive free nerve endings that elicit only the tickle and itch sensations. Furthermore, these endings are found almost exclusively in superficial layers of the skin, which is also the only tissue from which the tickle and itch sensations usually can be elicited. These sensations are transmitted by very small type C, unmy- elinated fibers similar to those that transmit the aching, slow type of pain

The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite, and the elicited signals then activate the scratch reflex or other maneuvers that rid the host of the irritant. Itch can be relieved by scratching if this action removes the irritant or if the scratch is strong enough to elicit pain. The pain signals are believed to suppress the itch signals in the cord by lateral inhibition, as described in Chapter 49

SENSORY PATHWAYS FOR TRANSMITTING SOMATIC SIGNALS INTO THE CENTRAL NERVOUS SYSTEM

Almost all sensory information from the somatic seg- ments of the body enters the spinal cord through the dorsal roots of the spinal nerves. However, from the entry point into the cord and then to the brain, the sensory signals are carried through one of two alternative sensory pathways: (1) the dorsal column–medial lemniscal system or (2) the anterolateral system. These two systems come back together partially at the level of the thalamus

The dorsal column–medial lemniscal system, as its name implies, carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord. Then, after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain stem to the thalamus by way of the medial lemniscus. Conversely, signals in the anterolateral system, imme- diately after entering the spinal cord from the dorsal spinal nerve roots, synapse in the dorsal horns of the spinal gray matter, then cross to the opposite side of the cord and ascend through the anterior and lateral white columns of the cord. They terminate at all levels of the lower brain stem and in the thalamus

The dorsal column–medial lemniscal system is com- posed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec, whereas the anterolateral system is composed of smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second up to 40 m/sec

Another difference between the two systems is that the dorsal column–medial lemniscal system has a high degree of spatial orientation of the nerve fibers with respect to their origin, whereas the anterolateral system has much less spatial orientation. These differences immediately characterize the types of sensory information that can be transmitted by the two systems. That is, sensory informa- tion that must be transmitted rapidly with temporal and spatial fidelity is transmitted mainly in the dorsal column– medial lemniscal system; that which does not need to be transmitted rapidly or with great spatial fidelity is trans- mitted mainly in the anterolateral system

The anterolateral system has a special capability that the dorsal system does not have—that is, the ability to transmit a broad spectrum of sensory modalities, such as pain, warmth, cold, and crude tactile sensations. Most of these sensory modalities are discussed in detail in Chapter 49. The dorsal system is limited to discrete types of mech- anoreceptive sensations

Dorsal Column–Medial Lemniscal System

1. Touch sensations requiring a high degree of localization of the stimulus
2. Touch sensations requiring transmission of fine gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints 6. Pressure sensations related to fine degrees of judgment of pressure intensity

Anterolateral System

1. Pain
2. Thermal sensations, including both warmth and cold sensations
3. Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body
4. Tickle and itch sensations
5. Sexual sensations

ANATOMY OF THE DORSAL COLUMN–MEDIAL LEMNISCAL SYSTEM

Upon entering the spinal cord through the spinal nerve dorsal roots, the large myelinated fibers from the special- ized mechanoreceptors divide almost immediately to form a medial branch and a lateral branch, shown by the right-hand fiber entering through the spinal root in Figure 48-2. The medial branch turns medially first and then upward in the dorsal column, proceeding by way of the dorsal column pathway all the way to the brain.

The lateral branch enters the dorsal horn of the cord gray matter, then divides many times to provide terminals that synapse with local neurons in the intermediate and anterior portions of the cord gray matter. These local neurons in turn serve three functions:

1. A major share of them give off fibers that enter the dorsal columns of the cord and then travel upward to the brain.

2. Many of the fibers are very short and terminate locally in the spinal cord gray matter to elicit local spinal cord reflexes, which are discussed in Chapter 55.

3. Others give rise to the spinocerebellar tracts, which we discuss in Chapter 57 in relation to the function of the cerebellum

Fig 48 - 2

Dorsal Column–Medial Lemniscal Pathway.

Note in Figure 48-3 that nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile nuclei). From there, second-order neurons decussate imme- diately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus. In this pathway through the brain stem, each medial lemnis- cus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory functions for the head that the dorsal column fibers sub- serve for the body. In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex. From the ventrobasal complex, third-order nerve fibers project, as shown in Figure 48-4, mainly to the post- central gyrus of the cerebral cortex, which is called somatic sensory area I (as shown in Figure 48-6, these fibers also project to a smaller area in the lateral parietal cortex called somatic sensory area II).

Fig 48 - 3

Spatial Orientation of the Nerve Fibers in the Dorsal Column–Medial Lemniscal System

One of the distinguishing features of the dorsal column– medial lemniscal system is a distinct spatial orientation of nerve fibers from the individual parts of the body that is maintained throughout. For instance, in the dorsal columns of the spinal cord, the fibers from the lower parts of the body lie toward the center of the cord, whereas those that enter the cord at progressively higher segmen- tal levels form successive layers laterally the right side of the thalamus, and the right side of the body is represented in the left side of the thalamus

In the thalamus, distinct spatial orientation is still maintained, with the tail end of the body represented by the most lateral portions of the ventrobasal complex and the head and face represented by the medial areas of the complex. Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented in

SOMATOSENSORY CORTEX

Figure 48-5 is a map of the human cerebral cortex, showing that it is divided into about 50 distinct areas called Brodmann’s areas based on histological structural differences. This map is important because virtually all neurophysiologists and neurologists use it to refer by number to many of the different functional areas of the human cortex.

Fig 48 - 5

Fig 48 - 4

Fig 48 - 6

Note in Figure 48-5 the large central fissure (also called central sulcus) that extends horizontally across the brain. In general, sensory signals from all modalities of sensation terminate in the cerebral cortex immediately posterior to the central fissure. Generally, the anterior half of the parietal lobe is concerned almost entirely with reception and interpretation of somatosensory signals, but the posterior half of the parietal lobe provides still higher levels of interpretation. Visual signals terminate in the occipital lobe, and audi- tory signals terminate in the temporal lobe.

Conversely, the portion of the cerebral cortex anterior to the central fissure and constituting the posterior half of the frontal lobe is called the motor cortex and is devoted almost entirely to control of muscle contractions and body movements. A major share of this motor control is in response to somatosensory signals received from the sensory portions of the cortex, which keep the motor cortex informed at each instant about the positions and motions of the different body parts

Somatosensory Areas I and II

Figure 48-6 shows two separate sensory areas in the anterior parietal lobe called somatosensory area I and somatosensory area II. The reason for this division into two areas is that a distinct and separate spatial orientation of the different parts of the body is found in each of these two areas. However, somatosensory area I is so much more extensive and so much more important than somatosensory area II that in popular usage, the term “somatosensory cortex” almost always means area I

Somatosensory area I has a high degree of localization of the different parts of the body, as shown by the names of virtually all parts of the body in Figure 48-6. By contrast, localization is poor in somatosensory area II, although roughly, the face is represented anteriorly, the arms centrally, and the legs posteriorly

Much less is known about the function of somatosen- sory area II. It is known that signals enter this area from the brain stem, transmitted upward from both sides of the body. In addition, many signals come secondarily from somatosensory area I, as well as from other sensory areas of the brain, even from the visual and auditory areas. Projections from somatosensory area I are required for function of somatosensory area II. However, removal of parts of somatosensory area II has no apparent effect on the response of neurons in somatosensory area I. Thus, much of what we know about somatic sensation appears to be explained by the functions of somatosensory area I

Spatial Orientation of Signals from Different Parts of the Body in Somatosensory Area I

Somatosensory area I lies immediately behind the central fissure, located in the postcentral gyrus of the human cerebral cortex (in Brodmann’s areas 3, 1, and 2)

Figure 48-7 shows a cross section through the brain at the level of the postcentral gyrus, demonstrating repre- sentations of the different parts of the body in separate regions of somatosensory area I. Note, however, that each lateral side of the cortex receives sensory information almost exclusively from the opposite side of the body.

Some areas of the body are represented by large areas in the somatic cortex—the lips the greatest of all, followed by the face and thumb—whereas the trunk and lower part of the body are represented by relatively small areas. The sizes of these areas are directly proportional to the number of specialized sensory receptors in each respective peripheral area of the body. For instance, great number of specialized nerve endings are found in the lips and thumb, whereas only a few are present in the skin of the body trunk.

Note also that the head is represented in the most lateral portion of somatosensory area I, and the lower part of the body is represented medially

Layers of the Somatosensory Cortex and Their Function

The cerebral cortex contains six layers of neurons, beginning with layer I next to the brain surface and extending progressively deeper to layer VI, shown in Figure 48-8.

Fig 48 - 7

Fig 48 - 8

As would be expected, the neurons in each layer perform functions different from those in other layers. Some of these functions are:

1. The incoming sensory signal excites neuronal layer IV first; the signal then spreads toward the surface of the cortex and also toward deeper layers.

2. Layers I and II receive diffuse, nonspecific input signals from lower brain centers that facilitate specific regions of the cortex; this system is described in Chapter 58. This input mainly controls the overall level of excitability of the respective regions stimulated

3. The neurons in layers II and III send axons to related portions of the cerebral cortex on the opposite side of the brain through the corpus callosum.

4. The neurons in layers V and VI send axons to the deeper parts of the nervous system. Those in layer V are generally larger and project to more distant areas, such as to the basal ganglia, brain stem, and spinal cord, where they control signal transmission. From layer VI, especially large numbers of axons extend to the thalamus, providing signals from the cerebral cortex that interact with and help to control the excitatory levels of incoming sensory signals entering the thalamus

The Sensory Cortex Is Organized in Vertical Columns of Neurons; Each Column Detects a Different Sensory Spot on the Body with a Specific Sensory Modality

Functionally, the neurons of the somatosensory cortex are arranged in vertical columns extending all the way through the six layers of the cortex, with each column having a diameter of 0.3 to 0.5 millimeter and containing perhaps 10,000 neuronal cell bodies. Each of these columns serves a single specific sensory modality; some columns respond to stretch receptors around joints, some to stimulation of tactile hairs, others to discrete localized pressure points on the skin, and so forth. At layer IV, where the input sensory signals first enter the cortex, the columns of neurons function almost entirely separately from one another. At other levels of the columns, inter- actions occur that initiate analysis of the meanings of the sensory signals