12 Central Nervous System and Senses

368

Central Processing of Acoustic

Information

Various qualities of sound must be coded for signal transmission in the acoustic pathway. These include the frequency, intensity and direction of sound waves as well as the distance of the sound source from the listener.

Frequency imaging. Tones of various frequencies are “imaged” along the cochlea, conducted in separate fibers of the auditory pathway and centrally identified. Assuming that a tone of 1000 Hz can just be distinguished from one of 1003 Hz (resembling true conditions), the frequency difference of 3 Hz corresponds to a relative frequency differential threshold of 0.003 (!p. 352). This fine differential capacity is mainly due to frequency imaging in the cochlea, amplification by its outer hair cells (!p. 366), and neuronal contrast along the auditory pathway (!p. 313 D). This fine tuning ensures that a certain frequency has a particularly low threshold at its “imaging” site. Adjacent fibers are not recruited until higher sound pressures are encountered.

Intensity. Higher intensity levels result in higher action potential frequencies in afferent nerve fibers and recruitment of neighboring nerve fibers (!A). The relative intensity differential threshold is 0.1 (!p. 352), which is very crude compared to the frequency differential threshold. Hence, differences in loudness of sound are not perceived by the human ear until the intensity level changes by a factor of over 1.1, that is, until the sound pressure changes by a factor of over !1,1 = 1,05.

Direction. Binaural hearing is needed to identify the direction of sound waves and is based on the following two effects. (1) Sound waves that strike the ear obliquely reach the averted ear later than the other, resulting in a lag time. The change in direction that a normal human subject can just barely detect (direction threshold) is roughly 3 degrees. This angle delays the arrival of the sound waves in the averted ear by about 3 · 10-5 s (!B, left). (2) Sound reaching the averted ear is also perceived as being quieter; differences as small as 1 dB can be distinguished. A lower sound pressure results in delayed firing of actions potentials, i.e., in increased latency (!B, right). Thus,

the impulses from the averted ear reach the CNS later (nucleus accessorius, !D5). Effects

(1) and (2) are additive effects (!B). The external ear helps to decide whether the sound is coming from front or back, above or below. Binaural hearing also helps to distinguish a certain voice against high background noise, e.g., at a party. Visibility of the speaker’s mouth also facilitates comprehension.

Distance to the sound source can be determined because high frequencies are attenuated more strongly than low frequencies during sound wave conduction. The longer the sound wave travels, the lower the proportion of high frequencies when it reaches the listener. This helps, for instance, to determine whether a thunderstorm is nearby or far away.

Auditory pathway (!D). The auditory nerve fibers with somata positioned in the spiral ganglion of the cochlea project from the cochlea (!D1) to the anterolateral ( !D2), posteroventral and dorsal cochlear nuclei (!D3). Afferents in these three nuclei exhibit tonotopicity, i.e., they are arranged according to tone frequency at different levels of complexity. In these areas, lateral inhibition (!p. 313 D) enhances contrast, i.e., suppresses noise. Binaural comparison of intensity and transit time of sound waves (direction of sound) takes place at the next-higher station of the auditory pathway, i.e. in the superior olive (!D4) and accessory nucleus (!D5). The next stations are in the nucleus of lateral lemniscus (!D6) and, after most fibers cross over to the opposite side, the inferior quadrigeminal bodies (!D7). They synapse with numerous afferents and serve as a reflex station (e.g., muscles of the middle ear; !p. 366). Here, sensory information from the cochlear nuclei is compared with spatial information from the superior olive. Via connections to the superior quadrigeminal bodies (!D8), they also ensure coordination of the auditory and visual space. By way of the thalamus (medial geniculate body, MGB; !D9), the afferents ultimately reach the primary auditory cortex (!D10) and the surrounding secondary auditory areas (!p. 311 E, areas 41 and 22). Analysis of complex sounds, short-term memory for comparison of tones, and tasks required for “eavesdropping” are some of their functions.

Voice and Speech

here are as follows: [a:] as in glass; [i:] as in beat; [u:] as in food; [œ:] as in French peur; [!:] as in bought; [ø:] as in French peu or in German hören; [y:] as in French menu or in German trüb; [æ:] as in bad; [!:] as in head.

Consonants are described according to their site of articulation as labial (lips, teeth), e.g. P/B/W/F/M; dental (teeth, tongue), e.g. D/T/S/M; lingual (tongue, front of soft palate), e.g. L; guttural (back of tongue and soft palate), e.g. G/K. Consonants can be also defined according to their manner of articulation, e.g., plosives or stop consonants (P/B/T/D/K/G), fricatives

(F/V/W/S/Ch) and vibratives (R).

The frequency range of the voice, including formants, is roughly 40–2000 Hz. Sibilants like /s/ and /z/ have higher-frequency fractions. Individuals suffering from presbyacusis or other forms of sensorineural hearing loss are often unable to hear sibilants, making it impossible for them to distinguish between words like “bad” and “bass.” The tonal range (fundamental tone, !C) of the spoken voice is roughly one octave; that of the singing voice is roughly two octaves in untrained singers, and over three octaves in trained singers.

Language (see also p. 336). The main components of verbal communication are (a) auditory signal processing (!p. 368), (b) central speech production and (c) execution of motor speech function. The centers for speech comprehension are mainly located in the posterior part of area 22, i.e., Wernicke’s area (!p. 311 E). Lesions of it result in a loss of language comprehension capacity (sensory aphasia). The patient will speak fluently yet often incomprehensibly, but does not notice it because of his/her disturbed comprehension capacity. The patient is also unable to understand complicated sentences or written words. The centers for speech production are mainly located in areas 44 and 45, i.e., Broca’s area (!p. 311 E). It controlls the primary speech centers of the sensorimotor cortex.

Lesions of this and other cortical centers (e.g., gyrus angularis) result in disorders of speech production (motor aphasia). The typical patient is either completely unable to speak or can only express himself in telegraphic style. Another form of aphasia is characterized by the forgetfulness of words (anomic or amnestic aphasia). Lesions of executive motor centers (corticobulbar tracts, cerebellum) cause various speech disorders. Auditory feedback is extremely important for speech. When a person goes deaf, speech deteriorates to an appreciable extent. Children born deaf do not learn to speak.

Plate 12.31 Voice and Speech

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13 Appendix

Dimensions and Units

Physiology is the science of life processes and bodily functions. Since they are largely based on physical and chemical laws, the investigation, understanding, assessment, and manipulation of these functions is inseparably linked to the measurement of physical, chemical, and other parameters, such as blood pressure, hearing capacity, blood pH, and cardiac output. The units for measurement of these parameters are listed in this section. We have given preference to the international system of SI units (Système International d’Unités) for uniformity and ease of calculation. Non-SI units will be marked with an asterisk. Conversion factors for older units are also listed. Complicated or less common physiological units (e.g., wall tension, flow resistance, compliance) are generally explained in the book as they appear. However, some especially important terms that are often (not always correctly) used in physiology will be explained in the Appendix, e.g., concentration, activity, osmolality, osmotic pressure, oncotic pressure, and pH.

The seven base units of the SI system.

The base units are precisely defined autonomous units. All other units are derived by multiplying or dividing base units and are therefore referred to as derived units, e.g.:

—Area (length · length): m · m = m2

—Velocity (length/time): m/s = m · s–1.

If the new unit becomes too complicated, it is given a new name and a corresponding symbol, e.g., force = m · kg · s–2 = N (! Table 1).

Fractions and Multiples of Units

Prefixes are used to denote decimal multiples and fractions of a unit since it is both tedious and confusing to write large numbers. We generally write 10 kg (kilograms) and 10 µg (micrograms) instead of 10 000 g and 0.00001 g, for example. The prefixes, which are usually varied in 1000-unit increments, and the corresponding symbols and conversion factors are listed in Table 2. Prefixes are used with base units and the units derived from them (! Table 1), e.g., 103 Pa = 1 kPa. Decimal increments are used in some cases (e.g., da, h, d, and c; ! Table 2). Time is given in conventional nondecimal units, i.e., seconds (s), minutes (min), hours (h), and days (d).

Length, Area, Volume

The meter (m) is the SI unit of length. Other units of length have also been used.

Examples:

1 ångström (Å) = 10-10 m = 0.1 nm 1 micron ( µ) = 10-6 m = 1 µm

1 millimicron (mµ) = 10-9 m = 1 nm

American and British units of length: 1 inch = 0.0254 m = 25.4 mm

1 foot = 0.3048 m

1 yard = 3 feet = 0.9144 m

1 (statute) mile = 1609.344 m !1.61 km

1 nautical mile = 1.853 km

The square meter (m2) is the derived SI unit of area, and the cubic meter (m3) is the corresponding unit of volume. When denoting the fractions or multiples of these units with prefixes (Table 2), please note that there are some peculiarities.

Examples:

1 m = 103 mm, but

1 mm2 = 106 mm2, and 1 m3 = 109 mm3

The liter (L or l)* is often used as a unit of volume for liquids and gases:

1 L = 10–3 m3 = 1 dm3 1 mL = 10–6 m3 = 1 cm3

1 µL = 10–9 m3 = 1 mm3.

Table 1 Derived units based on SI base units m, kg, s, cd, and A

1The solid angle of a sphere is defined as the angle subtended at the center of a sphere by an area (A) on its surface times the square of its radius (r2). A steradian (sr) is the solid angle for which r = 1 m and A = 1 m2, that is, 1 sr = 1 m2/m–2.

Table 2 Prefixes for fractions and multiples of units of measure

Dimensions and Units

13 Appendix

374

The relative molecular mass (Mr), or molecular “weight”, is the molecular mass of a substance divided by 1/12 the mass of a 12C atom. Since Mr is a ratio, it is a dimensionless unit.

The amount of substance, or mole (mol), is related to mass. One mole of substance contains as many elementary particles (atoms, molecules, ions) as 12 g of the nuclide of a 12C atom = 6.022 · 1023 particles. The conversion factor between moles and mass is therefore: 1 mole equals the mass of substance (in grams) corresponding to the relative molecular, ionic, or atomic mass of the substance. In other words, it expresses how much higher the mass of the atom, molecule, or ion is than 1/12 that of a 12C atom.

Examples:

—Relative molecular mass of H2O: 18

!1 mol H2O = 18 g H2O.

—Relative atomic mass of Na+: 23

!1 mol Na+ = 23 g Na+.

—Relative molecular mass of CaCl2:

=40 + (2 · 35.5) = 111

!1 mol CaCl2 = 111 g CaCl2.

(CaCl2 contains 2 mol Cl– and 1 mol Ca2+.)

The equivalent mass is calculated as moles divided by the valency of the ion in question and expressed in equivalents (Eq)*. The mole and equivalent values of monovalent ions are identical:

1 Eq Na+ = 1/1 mol Na+.

For bivalent ions, equivalent = 1/2 mole:

1 Eq Ca2+ = 1/2 mole Ca2+ or 1 mole Ca2+ = 2 Eq Ca2+.

The osmole (Osm) is also derived from the mole (see below).

Electrical Units

Electrical current is the flow of charged particles, e.g., of electrons through a wire or of ions through a cell membrane. The number of particles moving per unit time is measured in amperes (A). Electrical current cannot occur unless there is an electrical potential difference, in short also called potential, voltage, or tension. Batteries and generators are used to create such potentials. Most electrical potentials in the body are generated by ionic flow (! p. 32). The volt (V) is the SI unit of electrical potential (! Table 1).

How much electrical current flows at a given potential depends on the amount of electrical resistance, as is described in Ohm’s law

(voltage = current · resistance). The unit of electrical resistance is ohm (Ω) (! Table 1). Conductivity is the reciprocal of resistance (1/Ω) and is expressed in siemens (S), where S = Ω–1. In physiology, resistance is related to the membrane surface area (Ω · m2). The reciprocal of this defines the membrane conductance to a given ion: Ω–1 · m–2 = S · m–2 (! p. 32).

Electrical work or energy is expressed in joules (J) or watt seconds (Ws), whereas electrical power is expressed in watts (W).

The electrical capacitance of a capacitor, e.g., a cell membrane, is the ratio of charge (C) to potential (V); it is expressed in farads (F) (! Table 1).

Direct current (DC) always flows in one direction, whereas the direction of flow of alternating current (AC) constantly changes. The frequency of one cycle of change per unit time is expressed in hertz (Hz). Mains current is generally 60 Hz in the USA and 50 Hz in Europe.

Temperature

Kelvin (K) is the SI unit of temperature. The lowest possible temperature is 0 K, or absolute zero. The Celsius or centigrade scale is derived from the Kelvin scale. The temperature in degrees Celsius (!C) can easily be converted into K:

!C = K - 273.15.

In the USA, temperatures are normally given in degrees Fahrenheit (!F). Conversions between Fahrenheit and Celsius are made as follows:

!F = (9/5 · !C) + 32

!C = (!F - 32) · 5/9.

Some important Kelvin, Celsius, and Fahrenheit temperature equivalents:

Dimensions and Units

375

Concentrations, Fractions, Activity

The word concentration is used to describe many different relationships in physiology and medicine. Concentration of a substance X is often abbreviated as [X]. Some concentrations are listed below:

—Mass concentration, or the mass of a substance per unit volume (e.g., g/L = kg/m3)

—Molar concentration, or the amount of a substance per unit volume (e.g., mol/L)

—Molal concentration, or the amount of substance per unit mass of solvent (e.g., mol/ kg H2O).

1 µg% = 10 µg/L

1 γ% = 10 µg/L.

Molarity is the molar concentration, which is expressed in mol/L (or mol/m3, mmol/L, etc.). Conversion factors are listed below:

1 M (molar) = 1 mol/L

1 N (normal) = (1/valency) · mol/L

1 mM (mmolar) = 1 mmol/L

1 Eq/L = (1/valency) · mol/L.

tive electrodes (e.g., for H+, Na+, K+, Cl -, or Ca2+). The activity and molality of a solution are identical when the total ionic strength ( µ) of the solution is very small, e.g., when the solution is an ideal solution. The ionic strength is dependent on the charge and concentration of all ions in the solution:

where zi is the valency and ci the molal concentration of a given ion “i”, and 1, 2, etc. represent the different types of ions in the solution. Owing to the high ionic strength of biological fluids, the solute particles influence each other. Consequently, the activity (a) of a solution is always significantly lower than its molar concentration (c). Activity is calculated as a = f · c, where f is the activity coefficient.

Example: At an ionic strength of 0.1 (as it is the case for a solution containing 100 mmol NaCl/ kg H2O), f = 0.76 for Na+. The activity important in biophysical processes is therefore roughly 25% lower than the molality of the solution.

In solutions that contain weak electrolytes which do not completely dissociate, the molality and activity of free ions also depend on the degree of electrolytic dissociation.

Fractions (“fractional concentrations”) are relative units:

—Mass ratio, i.e., mass fraction relative to total mass

—Molar ratio

—Volume ratio, i.e. volume fraction relative to total volume. The volume fraction (F) is commonly used in respiratory physiology.

Fractions are expressed in units of g/g, mol/ mol, and L/L respectively, i.e. in “units” of 1, 10–3, 10–6, etc. The unabbreviated unit (e.g., g/g) should be used whenever possible because it identifies the type of fraction in question. The fractions %, ‰, ppm (parts per million), and ppb (parts per billion) are used for all types of fractions.

Conversion: 1% = 0.01 1‰ = 1 · 10–3

1 vol% = 0.01 L/L

1 ppm = 1 · 10–6

1 ppb = 1 · 10–9

Osmolality, Osmotic / Oncotic Pressure

Osmolarity (Osm/L), a unit derived from molarity, is the concentration of all osmotically active particles in a solution, regardless of which compounds or mixtures are involved.

However, measurements with osmometers as well as the biophysical application of osmotic concentration refer to the number of osmoles per unit volume of solvent as opposed to the total volume of the solution. This and the fact that volume is temperature-dependent are the reasons why osmolality (Osm/kg H2O) is generally more suitable.

Ideal osmolality is derived from the molality of the substances in question. If, for example, 1 mmol (180 mg) of glucose is dissolved in 1 kg

changes when electrolytes that dissociate are used, e.g., NaCl Na+ + Cl–. Both of these ions are osmotically active. When a substance that dissociates is dissolved in 1 kg of water, the ideal osmolality equals the molality times the number of dissociation products, e.g., 1 mmol NaCl/kg H2O = 2 mOsm/kg H2O.

Electrolytes weaker than NaCl do not dissociate completely. Therefore, their degree of electrolytic dissociation must be considered.

These rules apply only to ideal solutions, i.e., those that are extremely dilute. As mentioned above, bodily fluids are nonideal (or real) solutions because their real osmolality is lower than the ideal osmolality. The real osmolality is calculated by multiplying the ideal osmolality by the osmotic coefficient (g). The osmotic coefficient is concentration-dependent and amounts to, for example, approximately 0.926 for NaCl with an (ideal) osmolality of 300 mOsm/kg H2O. The real osmolality of this NaCl solution thus amounts to 0.926 · 300 = 278 mOsm/kg H2O.

Solutions with a real osmolality equal to that of plasma (!290 mOsm/kg H2 O) are said to be isosmolal. Those whose osmolality is higher or lower than that of plasma are hyperosmolal or hyposmolal.

Osmolality and Tonicity

Each osmotically active particle in solution (cf. real osmolality) exerts an osmotic pressure (π) as described by van’t Hoff’s equation:

where R is the universal gas constant (8.314 J · K–1 · Osm–1), T is the absolute temperature in K, and cosm is the real osmolality in Osm · (m3

H2O)–1 = mOsm · (L H2O)–1. If two solutions of different osmolality ( cosm) are separated by a water-permeable selective membrane , cosm will exert an osmotic pressure difference (Δπ) across the membrane in steady state if the membrane is less permeable to the solutes than to water. In this case, the selectivity of the membrane, or its relative impermeability to the solutes, is described by the reflection coefficient (σ), which is assigned a value between 1 (impermeable) and 0 (as permeable as water). The reflection coefficient of a semipermeable membrane is σ = 1. By combining van’t Hoff’s and Staverman’s equations, the osmotic pressure difference (Δπ) can be calculated as follows:

Equation 13.3 shows that a solution with the same osmolality as plasma will exert the same osmotic pressure on a membrane in steady state (i.e., that the solution and plasma will be isotonic) only if σ = 1. In other words, the membrane must be strictly semipermeable.

Isotonicity, or equality of osmotic pressure, exists between plasma and the cytosol of red blood cells (and other cells of the body) in steady state. When the red cells are mixed in a urea solution with an osmolality of 290 mOsm/kg H2O, isotonicity does not prevail after urea (σ " 1) starts to diffuse into the red cells. The interior of the red blood cells therefore becomes hypertonic, and water is drawn inside the cell due to osmosis (! p. 24). As a result, the erythrocytes continuously swell until they burst.

An osmotic gradient resulting in the subsequent flow of water therefore occurs in all parts of the body in which dissolved particles pass through water-permeable cell membranes or cell layers. This occurs, for example, when Na+ and Cl– pass through the epithelium of the small intestine or proximal renal tubule. The extent of this water flow or volume flow Jv (m3 · s–1) is dependent on the hydraulic conductivity k (m · s–1 · Pa–1) of the membrane (i.e., its permeability to water), the area A of passage (m2), and the pressure difference, which, in this case, is equivalent to the osmotic pressure difference Δπ (Pa):

Dimensions and Units

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