BIOSIGNALS

BIOSIGNALS
generation
processing
analysis

BIOLOGICAL SYSTEMS
Biological system is an open dynamic system with continuous flow of matter,
energy and information.
Information transport in biological systems
The transport and processing of information in biological systems – human
organism – is facilitated by humoral or nervous mechanisms on a three levels.
The processes on the first lowest level are connected to the regulation of basic
biochemical reactions. The second level forms processes carried out in
autonomical systems utilizing humoral and nervous mechanisms, that control
functions of some organs (e.g. heart). The third, highest level covers the
information processing in central nervous system.

Energy in biological systems
Energy for all the vital functions of the body is obtained by either aerobic or
anaerobic metabolism:
- Aerobic metabolism releases energy from carbohydrates, proteins and fats by
the process of oxygenation. Aerobic metabolism is used by most cells, as it
produces large amounts of energy for prolonged periods of time, but requires
the presence of oxygen.
- Anaerobic metabolism, in contrast, does not need oxygen but is far less efficient
as it produces small amounts of energy and only functions for short periods
of time.
Therefore the body needs oxygen to produce the large amount of energy required
for most body functions, not only for moving, but also for breathing, eating,
digestion, osmotic function of kidneys, generation of action potentials and nerve
function. A part is consumed, transformed into heat energy.
This chemical-bound form of energy is stored in special molecules. The most
known is ATP (adenosine-5'-triphosphate).
The energy released by cleaving either a
phosphate (Pi) or pyrophosphate (PPi) unit
from ATP
ATP + H2O ADP + Pi G˚ = −30.5 kJ/mol (−7.3 kcal/mol)
ATP + H2O AMP + PPi G˚ = −45.6 kJ/mol (−10.9 kcal/mol)
at the standard conditions,
under typical cellular conditions, G is approximately −57 kJ/mol

Homeostasis and Adaptation
Homeostasis (from Greek: standing still similar) is the property of a system, either
open or closed, that regulates its internal environment and tends to maintain a
stable, constant condition.
All homeostatic control mechanisms have at least three interdependent
components for the variable being regulated: The receptor is the sensing component
that monitors and responds to changes in the environment. When the receptor senses
a stimulus, it sends information to a control center, the component that sets the
range at which a variable is maintained. The control center determines an appropriate
response to the stimulus. In most homeostatic mechanisms the control center is the
brain. The control center then sends signals to an effector, which can be muscles,
organs or other structures that receive signals from the control center. After receiving
the signal, a change occurs to correct the deviation by either enhancing it with
positive feedback (e.g. blood platelet accumulation) or depressing it with negative
feedback (regulation of blood pressure by vasoconstriction or vasodilatation).
Homeostasis

Homeostasis and Adaptation
Adaptation is the evolutionary process whereby a population becomes better suited
to its habitat.
May be structural, behavioral or physiological. Structural adaptations are
physical features of an organism (shape, body covering, defensive or offensive
armament); and also the internal organization). Behavioural adaptations are
composed of inherited behaviour chains and/or the ability to learn: behaviours may
be inherited in detail (instincts), or a tendency for learning may be inherited. E.g.:
searching for food, mating, vocalizations. Physiological adaptations permit the
organism to perform special functions (for instance making venom, secreting slime,
phototropism); but also more general functions such as growth and development,
temperature regulation, ionic balance and other aspects of homeostasis. Adaptation,
then, affects all aspects of the life of an organism.

BIOLOGICAL SIGNAL
is a summarizing term for all kinds of signals that can be (continually) measured
and monitored from biological beings. The term biosignal is often used to mean
bio-electrical signal but in fact, biosignal refers to both electrical and non-electrical
signals.
Electrical biosignals are usually taken to be (changes in) electric currents
produced by the sum of electrical potential differences across a specialized tissue,
organ or cell system like the nervous system.
Information in biosignals is often depreciated by a disturbance or noise. Therefore
the biosignals have to be properly processed – using a transformation or filtration
to extract required information.
There are a lots of methods and
algorithms for biosignal processing, 1-
or 2-dimensional, in time or in
frequency distribution.

Classification of Biosignals
may be classified in many ways
according to:
• to their source or physical nature, where the classification respects the basic
physical characteristics of the considered process.
• to biomedical application. The biomedical signal is acquired and processed
with some diagnostic, monitoring, or other goal in mind. Classification may be
constructed according to the field of application, e.g., cardiology or neurology.
Such classification may be of interest when the goal is, for example, the study of
physiologic systems.
• to signal characteristics. From point of view of signal analysis, this is the
most relevant classification method. When the main goal is processing, it is not
relevant what is the source of the signal or to which biomedical system it belongs;
what matters are the signal characteristics.

We recognize two broad classes of signals:
- continuous signals
- discrete signals.
Continuous signals are described by a continuous function s (t) which provides
information about the signal at any given time. Discrete signals are described by a
sequence s (m) which provides information at a given discrete point on the time
axis.
Continuous
Discrete
value
value
time
time
Most of the biomedical signals are continuous. Since current technology
provides powerful tools for discrete signal processing, we most often transform a
continuous signal into a discrete one by a process known as sampling.

We divide signals into two main groups:
- deterministic
- stochastic signals.
Deterministic signals are signals that can be exactly described mathematically or
graphically. Real-world signals are never deterministic. There is always some
unknown and unpredictable noise added, some unpredictable change in the
parameters. It is very often convenient to approximate or model the signal by
means of a deterministic function(s).
An important family of deterministic signals is the periodic family. A periodic
signal is a deterministic signal that may be expressed by
s(t )= s(t + nT)
where n is an integer, and T is the period.
Under some conditions, the blood pressure
signal may be modeled by a complex periodic
signal, with the heart rate as its period and the
blood pressure wave shape as its basic wave
shape.

Most deterministic functions are nonperiodic. It is sometimes worthwhile to consider
an “almost periodic” type of signal. The ECG signal can sometimes be considered
“almost periodic.” The ECG’s RR interval is never constant; in addition, the PQRST
complex of one heartbeat is never exactly the same as that of another beat.
The most important class of signals is the stochastic class. Stochastic signals
cannot be expressed exactly; they can be described only in terms of probabilities.
Stationary stochastic processes are processes whose statistics do not
change in time. The expectation and the variance (as with any other statistical
mean) of a stationary process will be time-independent. Unfortunately, almost all
signals are nonstationary (e.g. sleep EEG signal)
the 3. and/or the 4.
stage of the nonREM
sleep phase

Types of biosignals
according to the origin
a) Bioelectric signals. They are generated by nerve
cells and muscle cells. Its source is the membrane potential,
which under certain conditions may be excited to generate an
action potential. In single cell measurements, where specific
microelectrodes are used as sensors, the action potential
itself is the biomedical signal. In more gross measurements,
where, for example, surface electrodes are used as sensors,
the electric field generated by the action of many cells,
distributed in the electrode’s vicinity, constitutes the bioelectric
signal. The electric field propagates through the biologic
medium, and thus the potential may be acquired at relatively
convenient locations on the surface, eliminating the need to
invade the system. The bioelectric signal requires a relatively
simple transducer for its acquisition. E.g.: ECG, EEG, EMG
and others.

) Bioimpedance signals. The impedance of the tissue contains important
information concerning its composition, blood volume, blood distribution, endocrine
activity, automatic nervous system activity, and more. The bioimpedance signal is
usually generated by injecting (or superficially) into the tissue under test sinusoidal
currents (frequency range of 50 kHz to 1 MHz, with low current densities of the
order of 20 A to 2 mA). The frequency range is chosen to minimize electrode
polarization problems, and the low current densities are chosen to avoid tissue
damage mainly due to heating effects. Bioimpedance measurements are usually
performed with 4 electrodes. Two source electrodes are connected to a current
source and are used to inject the current into the tissue. The two measurement
electrodes are placed on the tissue under investigation and are used to measure the
voltage drop generated by the current and the tissue impedance. E.g.: impedance
plethysmography or rheography.

c) Biomagnetic signals. Various organs, such as the brain, heart, and lungs,
produce extremely weak magnetic fields (10 −9 T to 10 −6 T). The measurements
of these fields provides information not included in other biosignals (such as
bioelectric signals). Due to the low level of the magnetic fields to be measured,
biomagnetic signals are usually of very low signal-to-noise ratio. Extreme caution
must be taken in designing the acquisition system of these signals.

Superconducting quantum interference devices (SQUID) are very sensitive
magnetometers used to measure extremely weak magnetic fields, based on
superconducting loops containing Josephson junctions. They are sensitive enough
to measure fields as low as 5×10 −18 T. The strength of the field at the Earth's
surface ranges from less than 3x10 -5 T in an area including most of South America
and South Africa to over 6x10 -5 T around the magnetic poles in northern Canada
and south of Australia, and in part of Siberia. ( A typical refrigerator magnet
produces 10 −2 T.

d) Bioacoustic signals. Many biomedical phenomena create acoustic noise. The
flow of blood in the heart, through the heart’s valves, or through blood vessels
generates typical acoustic noise. The flow of air through the upper and lower
airways and in the lungs creates acoustic sounds. These sounds, known as
coughs, snores, and chest and lung sounds, are used extensively in medicine.
Sounds are also generated in the digestive tract and in the joints. It also has been
observed that the contracting muscle produces an acoustic noise (muscle noise).
Since the acoustic energy propagates through the biologic medium, the
bioacoustic signal may be conveniently acquired on the surface, using acoustic
transducers (microphones or accelerometers).
e) Biochemical signals. Biochemical signals are the result of chemical
measurements from the living tissue or from samples analyzed in the clinical
laboratory. Measuring the concentration of various ions inside and in the vicinity of
a cell by means of specific ion electrodes is an example of such a signal. Partial
pressures of oxygen (pO 2
) and of carbon dioxide (pCO 2
) in the blood or respiratory
system are other examples or blood pH. Biochemical signals are most often very
low frequency signals. Most biochemical signals are actually dc signals.

f) Biomechanical signals. The term biomechanical signals includes all signals used
in the biomedicine fields that originate from some mechanical function of the biologic
system. These signals include motion and displacement signals, pressure and
tension and flow signals, and others. The measurement of biomechanical signals
requires a variety of transducers, not always simple and inexpensive.
The mechanical phenomenon does not propagate, as do the electric, magnetic, and
acoustic fields. The measurement therefore usually has to be performed at the exact
site. This very often complicates the measurement and forces it to be an invasive
one. E.g. blood pressure, non-directly – phonocardiography, Carotidography.
g) Biooptical signals. Biooptical signals are the result of optical functions of the
biologic system, occurring naturally or induced by the measurement. Blood
oxygenation may be estimated by measuring the transmitted and backscattered
light from a tissue (in vivo and in vitro) in several wavelengths (oximetry). Important
information about the fetus may be acquired by measuring fluorescence
characteristics of the amniotic fluid. Estimation of the heart output may be
performed by the dye dilution method, which requires the monitoring of the
appearance of recirculated dye in the bloodstream. The development of fiberoptic
technology has opened vast applications of biooptical signals.

h) Thermal biosignals – continuous or discrete carry information about the
temperature of the body core or temperature distribution on the surface. The
temperature measurement reflects physical and biochemical processes proceeded
in organism. The measurement is usually performed by a contact method using a
variety of thermometers. In special cases it is used 2D thermographic camera.
i) Radiological biosignals. These biosignals are formed by
interaction of ionizing interaction with biological structures.
They carry information about inner anatomical structures.
They play a significant role in diagnostics and therapy.
j) Ultrasonic biosignals. They are formed by interaction
with organism tissues. They carry information about
acoustic impedances of biological structures and their
anatomical changes. They are acquired by probes
containing piezoelectric transducer.

Electronic devices in Medicine
Block diagram of a diagnostic device
sensor
amplifier
A/D
converter
calibration
control
unit
signal
processing
data
storage
source
signal
display
data
transfer

Electronic devices in Medicine
Diagnostic systems
Electric signals are detected by sensors (mainly electrodes), while nonelectric
magnitudes are first converted by transducers into electric signals that can be
easily treated, transmitted, and stored. The output signals from electrodes or
sensors of the most monitored quantities are analog. For their processing, storage
and transport is necessary to do their amplification, filtration and digitalization
using A/D converter.
Different types of microprocessors are used to process biosignals (PC serves as a
control unit).
The control unit of modern electrocardiographs

Signal display usually respects user requirements. They are displays with
numerical or graphical information (light emitting diodes LED, LCD, monitors).
Data storage (e.g. Holter long-term (24- 48hrs) monitoring ECG, blood
pressure or pH of stomach).
Data transmission locally inside of the clinic or via hospital information system
(HIS), but also to other places by phone, internet or telemetrically.

Block diagram of a therapeutic device
source
life
functions
monitoring
signal
display
dose
monitoring
applicator
generator
control
unit
therapeutical
watch
Applicators are stimulation electrodes, tips, cavity resonators, ultrasound heads,
coils, optical fibers and others, that directly act on specified area of the tissue,
organ.

Signal digitalization
The A/D conversion ideally can be divided in two steps, the sampling process,
which converts the continuous signal in a discrete-time series and whose elements
are named samples, and a quantization procedure, which assigns the amplitude
value of each sample within a set of determined discrete values. Both processes
can modify the characteristics of the signal. A continuous time signal can be
completely recovered from its samples if, and only if, the sampling rate is greater
than twice the signal bandwidth (Shanon-Kotelnikov theorem).

Sensors of biosignals
different types of electrodes and sensors
Biomedical sensors
take signals representing biomedical variables and convert them into what is
usually an electrical signal. As such, the biomedical sensor serves as the interface
between a biologic and an electronic system.
It is possible to categorize all sensors as being either physical or chemical.
In the case of physical sensors, quantities such as geometric, mechanical, thermal,
and hydraulic variables are measured. In biomedical applications these can include
things such as muscle displacement, blood pressure, core body temperature, blood
flow, cerebrospinal fluid pressure, and bone growth.
The second major classification of sensing devices is chemical sensors. In this
case the sensors are concerned with measuring chemical quantities such as
identifying the presence of particular chemical compounds, detecting the
concentrations of various chemical species, and monitoring chemical activities
in the body for diagnostic and therapeutic applications.

Measurement of biological signals (physical quantities)
Some important biosignals do not have the character of the electrical potential or
voltage. The monitoring can be carried out only with the sensors transforming that
signal as a physical quantity to some form of the electrical signal.
Displacement sensors
Inductance sensors of a displacement use inductance changes of a coil at the
change of ferromagnetic slug position. The inductance is given by the formula:
where µ 0
is permeability of vacuum, µ r
is relative permeability of the ferromagnetic
slug, G is constant and characterizes the coil shape, N is the number of turns.

Capacitive Sensors for displacement monitoring use the changes of the capacitance
of the plate condenser based on the following formula:
S
where ε 0
is permittivity of vacuum, ε r
relative permittivity of dielectric (air), S is plate
area, d is plate distance.
If for example if one plate is fixed then the capacitance will vary
inversely with respect to the plate separation. This will result in a
hyperbolic capacitance-displacement characteristic. These
sensors can be used for monitoring of patient movement in bed,
but also can be applied for respiration measurements, gas or
liquid pressure.
Sensors of the blood velocities and flows
For the non-invasive measurement of the blood speeds or
flows there are used piezoelectric transducers in ultrasound
Dopller instruments. They are usually ceramic piezoelectric
transducers working on frequencies from 4 to 10 MHz in
continuous or pulse mode. The piezoelectric transducers can
be also used for blood pressure measurement or for
observation of heart valves movement in phonocardiography.
Sensor of the intraocular pressure

Sensors of the air flow
mesh
warming
Fleish’s pneumotachometer
measure flow rate by means of a transducer through which the patient breathes.
The air passes through a fine mesh which offers a small resistance to flow, with
the result that there will be a pressure drop across the mesh in proportion to the
flow rate. The instrument also calculates volume by integrating the flow signal.
The tube cone shaped ends provide laminar air flow around the pressure
transducer. The mesh warming facilitates water evaporation.

Sensors of temperature
There are many different sensors of temperature, but three find particularly wide
application to biomedical problems - metallic resistance thermometers,
thermistors, and thermocouples.
Metallic Resistance Thermometers
The electric resistance of a piece of metal or wire generally increases as the
temperature of that electric conductor increases. A linear approximation to this
relationship is given by
R = R 0
[ 1+α (T −T 0
)]
where R 0
is the resistance at temperature T 0
, is the temperature coefficient of
resistance, and T is the temperature at which the resistance is being measured.

Most metals have temperature coefficients of resistance of the order of 0.1– 0.4%/°C.
The noble metals are preferred for resistance thermometers, since they do not
corrode easily.
Metal resistance thermometers are often fabricated from fine-gauge insulated wire
that is wound into a small coil.

Thermistors
Unlike metals, semiconductor materials have an inverse relationship between
resistance and temperature. Their resistance as a function of temperature is given
by
where R 0
marks thermistor resistance at the reference temperature T 0
(298,15 K=25
°C), β is a constant determined by the materials that make up the thermistor, the
temperature constants for current materials are between 1500 and 7000 K.
Thermistors are usually made of a mixture of metal oxides such as FeO, NiO, MnO,
TiO, CoO.

Thermocouples
When different regions of an electric conductor or semiconductor are at different
temperatures, there is an electric potential between these regions that is directly
related to the temperature differences. This phenomenon, known as the Seebeck
effect, can be used to produce a temperature sensor known as a thermocouple
by taking a wire of metal or alloy A and another wire of metal or alloy B and
connecting them.
When these junctions are at different temperatures, a voltage proportional to the
temperature difference will be seen at the voltmeter
V = S AB
(Ts–Tr)
where S AB
is the Seebeck coefficient for the thermocouple made up of metals A
and B

Infrared thermometers
measure temperature by measuring infrared radiation emitted from objects. The
object's temperature can be determined if you know the amount of infrared energy
emitted by the object and its emissivity.
The Stefan–Boltzmann law, also known as Stefan's law, states that the total
energy radiated per unit surface area of a black body in unit time (known variously
as the black-body irradiance, energy flux density, radiant flux, or the emissive
power), H, is directly proportional to the fourth power of the black body's
thermodynamic temperature T:
H = ε . σ . T 4

Oximetry measures the degree of oxygen saturation in blood
The oxygen is under normal physiological conditions transported from lungs to
tissues in 2 different forms. Approximately 2 % of the total blood O 2
is dissolved in
plasma – it is linearly proportional to the p0 2
in blood. The rest (98 %) is carried on
erythrocytes, rather is reversible bound to hemoglobin producing oxyhemoglobin
HbO 2
. Based on this facts, there are 2 approaches to measure the blood
oxygenation: application of p0 2
sensor or measurement O 2
saturation (relative
content of HbO 2
in blood) with an oximeter.
Oximetry is based on the light absorption by blood (the color of fully oxygenated
blood is bright red)
A, B are coefficients dependent on specific
absorption of Hb and HbO 2 , H(λ1), H(λ2) are
optical densities of blood λ1 and λ2, resp.
Pulse oximeter facilitates non-invasive in vivo measurement of oxygen saturation in
blood. The sensor is formed by 2 light emitting diodes LED working on 2 different
wavelengths :
- λ 1
where is significant difference between light absorption of Hb and HbO 2
(red color
660 nm ),
- λ 2
where light absorption is independent on oxygen saturation in blood (the
absorbance of Hb is slightly lower than for HbO 2
(IR 960 nm ).

Biosignal processing
The most of the biosignals are acquired in the time domain. On the other hand for
their processing it is better to work with the frequency distribution (e.g. to
remove a noise by a frequency dependent filter).
The basic mathematical operation serves for biosignal transformation from the
time domain to the frequency domain is Fourier transformation.
For any periodic biosignal we can use Fourier series
and so describe the signals by means of the amplitudes a n
,b n
and phases of the
sine waves. ( = 2f is the angular frequency)

Electrical signals processing – 3 steps:
1. sensing
(electrical signals are collected as electrical voltages, with amplitude from 10 -6 V
(EEG) to 10 -2 V (EMG), their frequency is also in a wide range from 10 -1 to 10 3
Hz)
2. amplification
(2 types: direct current (voltage) amplifier and alternating amplifier, the both
have to be frequency independent)
3. recording

Biophysics of sensory perception
Classification and characteristics of receptors
Psychophysical laws

Our perception of the outside world depends on our five senses:
Cutaneous sensation
Gustation
Olfaction
Vision
Audition
Touch
Taste
Smell
Sight
Hearing
The elements of the peripheral nervous system that respond directly to stimuli are
called the sensory receptors.
A receptor is a transducer that produces electrical energy, impulse, from a form of
energy (mechanical, thermal, light).
The internal receptors (interoceptors, viscoreceptors and proprioceptors) monitor
position and function of the organs, muscles, joints, ligaments, etc.
The elements located at or near to the surface of the body - exteroceptors.

The classification according to function depends on the transduction
mechanism of the receptor.
Mechanoreceptors transduce mechanical factors such as touch, pressure,
vibration and strain into electrical activity.
Thermoreceptors respond to temperature
Photoreceptors to light
Chemoreceptors respond to chemicals in solution
Nociceptors are pain sensors, if the stimulus level is high enough
Some receptors are designed specifically to respond to the level of a stimulus,
whilst others are designed to respond to changes in the level. (e.g. some
mechanoreceptors respond directly to pressure and some respond to vibration)

Mechanoreceptors
associated with the sense of touch
structurally relatively simple
CUTANEOUS SENSATION
The categorization of the simple receptors depends on the geometry of the dendritic
endings
Encapsulated dendritic endings are contained in a protective capsule of connective
tissue. Some are designed to respond to light pressure, some to higher pressure,
some to strain and some to vibration.
-Pacinian corpuscles detect rapid vibrations, with large receptive fields
-Meissner’s corpuscle (glabrous skin only) detect slow vibrations
-Ruffini corpuscles (plus Merkel’s disks on glabrous skin) respond to
indentation, with slow adaptation
Free nerve endings around hairs detect movements of hairs
Nociceptors (pain receptors) and thermoreceptors belong into this section because
of their structural similarity

Mechanoreceptors respond to the stimulus of a mechanical load
are designed to respond to the intensity of the load (slowly adapting (SA)
receptors) and some are designed to respond to the rate of change of load (rapidly
adapting (RA) receptors)
The rate of generation of impulses is distinctly non-linear to the intensity of the
stimulus, and furthermore there is a fatigue effect so that the rate is not constant
even for constant stimulus.

Structural Categories of Sensory Receptors
Pain
temperature
Touch
Pressure

The Pacinian corpuscle
- the most abundant in the subcutaneous tissue under the skin, particularly in the
fingers, the soles of the feet and the external genitalia
- the largest of the receptors and the approximate shape of a rugby ball, typically
about 1 mm, and up to 2 mm, long and about half as wide
-resemble an onion in structure, with up to 60 layers of flattened cells surrounding a
central core, the whole being enclosed in a sheath of connective tissue. At the
centre is a single non-myelinated nerve fibre of up to 10 µm in diameter, which
becomes myelinated as it leaves the corpuscle.
- The Pacinian corpuscle serves most efficiently as a monitor of the rate of change
of load rather than to the intensity of the load itself: serves as a vibration
transducer.

The Pacinian corpuscle
Load applied slowly
Load applied rapidly
The presented model suggests that the mechanical stimulus reaching the core will
depend on the rate of loading.
Consider the application of a pinching load to two concentric ovoids filled with an
incompressible viscous fluid. If the load is applied very slowly then the fluid flows so
that the elastic membrane of the outer ovoid stretches along the axis to maintain
the enclosed volume of incompressible fluid.
If the load is applied rapidly, there will be a local high pressure under the point of
application of the load, and therefore some distortion of the circular section of the
membrane. This must be accommodated by a stretching along the axis.

Density of mechanoreceptors in human skin
Site
Tip of tongue
Tip of index finger
Lips
Edge of tongue
Palm
Forehead
Back of hand
Upper surface of foot
Neck
Back
Spatial discrimination
(cm)
0.2
0.5
0.75
1
1.2
2.5
3.2
4.0
5.5
6.8
Spatial density
(receptors per cm 2 )
25
4
2
1
0.7
0.16
0.1
0.06
0.03
0.02

Thermoreceptors
free nerve endings, respond to the stimulus of heat energy
there are few thermoreceptors in the human skin - of the order of 5 to 10 receptors
per cm 2 .
Some receptors are designed to monitor steady-state temperature, whilst others
respond efficiently to rapid changes of temperature.
Some of the steady-state receptors, the “hot detectors”, respond to high
temperatures, and some, the “cold detectors”, respond to low temperatures.
Warm receptors
sensitive to temperatures above 30 o C
unresponsive to temperature above 48 o C
Cold receptors
sensitive to temperature between
10 o C and 35 o C
Pain receptors
respond to temperatures below
10 o C
and above 45 o C
frequency AP/s
cold receptors
hot receptors
temperature °C

Nociceptors
free nerve endings networks
signal pain when the intensity of stimulation exceeds a particular threshold -
intense pressure, heat, acids, receptors that are sensitive to ATP level
there are rapid pain receptors, transmitting through myelinated fibers, that let
us know very quickly that something is wrong. The conduction velocity of these
nerve fibers is up to 30 m s -1 .
there are also persistent pain receptors, mostly non-myelinaled, that react
more slowly but maintain the sensation of pain - conduction velocities down to 2 m
s -1 and below.
Myelination
Most mammalian axons are myelinated.
The myelin sheath is provided by oligodendrocytes and Schwann cells.
Myelin is insulating, preventing passage of ions over the membrane.
Saltatory Conduction
Myelin
sheath
Node of
Ranvier

The chemical senses
gustation and olfaction, or taste and smell
They are dependent on the chemical stimulation of special cells called
chemoreceptors, which respond to chemicals in an aqueous solution.
They are regarded as the most primitive of the special senses.
Our perception of taste in particular is often a composite sense, in which we
supplement true taste information with cues from our sense of smell.

Gustation (taste)
The chemoreceptors responsible for the sense of taste are called taste buds.
We have about 10 000 of them, most of which are located on the tongue.
They are shed and replaced about every 7-10 days.
Different areas of the tongue react most effectively to particular tastes, suggesting
that the taste buds have some degree of specificity. Every taste bud appears to be
able to react to every taste, but the level of response varies. There is no obvious
structural difference between the taste buds that respond best to different tastes.
The task of transmission of the taste sensation to the brain is shared between two
of the cranial nerves — the seventh (facial) and the ninth (glossopharyngeal).
Epithelial cell receptors clustered
in taste buds.
Taste cells are not neurons, but
depolarize upon stimulation and
release chemical transmitters that
stimulate sensory neurons.

Classification of tastes
We normally classify tastes into four
categories.
In taste tests some compounds appear to fall
into different categories depending on
concentration. Even common salt, sodium
chloride, tastes sweet at very low
concentrations, close to the threshold of taste.
Classification of tastes
bitter
sour
sweet
salt
Location of taste sites
on the human tongue
Category
General
Specific
Sweet
Salt
Sour
Bitter
Many organic
compounds
Many inorganic
salts, metal ions
Acids, hydrogen
ion (H+)
Many alkaloids
Sugars, saccharin
Sodium chloride,
potassium chloride
Acetic acid (vinegar),
citric acid (lemon)
Caffeine, nicotine,
quinine, strychnine
Salt:
Na+ passes through channels
and activates specific receptor
cells, depolarizing the cells.
Sour:
Presence of H+.
Sweet and bitter:
Mediated by receptors coupled
to G-protein (gustducin).

Olfaction (smell)
The chemoreceptors responsible for the sense of smell, the olfactory receptors,
are located in the epithelial lining of the roof of the nasal cavity.
There are millions of these receptors. Man has a relatively poor sense of smell,
and part of the reason for this is the geometrical design of the olfactory system.
The olfactory receptors are situated in a side passage, off the main pathway of air
into the lungs.
Unlike the taste buds, which
consist of epithelial cells, the
olfactory receptors are actually
neurones.
They are unique in that they are
the only neurones in the body
that are replaced continually in
adult life. They typically last for
about 60 days.
The task of transmission of the
sensation of smell to the brain is
performed by the first cranial
nerve - the olfactory nerve.
Molecules bind to
receptors and act
through G-proteins
to increase cAMP.

Classification of smells
There are several theories of olfaction, and no doubt each contains an element of
the true. Many of the theories are underpinned by chemical models. One appealing
and simple model suggests that there are seven basic odours: camphoric, musky,
floral, pepperminty, ethereal, pungent and putrid. More complex models suggest 30
or more primary odours, and more recent work suggests that there are a thousand
or more separately identifiable odours.
Thresholds of smell
Our sense of smell is very sensitive - the threshold concentration for ethyl
mercaptan as 4 x 10 8 molecules per cm 3 . Given that there are about 5 x 10 19
molecules of nitrogen per cm 3 , this suggests that we can detect ethyl mercaptan in
air at a concentration of about one molecule in 10 11 . We might consider this to be
quite impressive, but the threshold of smell for a typical dog is about one thousand
times lower.