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GOMEL STATE MEDICAL UNIVERSITY

Normal and Pathological Physiology Department

PHYSIOLOGY OF EXCITABLE TISSUES

Physiology of nerve tissue

Lecturer:

Victor Melnik

Professor,

Doctor of Biological Sciences

Lecture plan:

1. Membrane-ionic theory of the origin of the resting

membrane potential.

2. Membrane action potential (AP). Changes of

excitability during excitation. Laws of stimulation and

assessment of excitability. Lability.

3. Physiology of nerve fiber. Laws of excitement

conduction. Mechanisms of signal formation and

conduction in myelinated and unmyelinated fibers.

4. Parabiosis.

5. Physiology of synapses. Mechanisms of signal

transmission in chemical synapses. Principles and

features of excitation transmission in interneuronic

synapses.

6. Perception of external stimuli (reception).

Transformation of stimulus energy.

1. Membrane-ionic theory of the origin of the resting

membrane potential

All tissues are excitable, but conventionally they are divided

into excitable and non-excitable. Nervous, muscular, and

glandular tissues are excitable, as impulses which appear in

them go along the membrane. These impulses have an

important diagnostic value (for example, in

electrocardiography, electroencephalography,

electromyography, etc.).

The cell membrane is known to have an electric charge. Its

external surface is charged positively ―+‖ and the internal

one — negatively ―–―.

The difference between the charges of the external and

internal membrane sides is called the resting membrane

potential.

FIGURE — CHARGES DISTRIBUTION

BETWEEN INSIDE AND OUTSIDE OF THE CELL

IN ITS RESTING STATE

FIGURE — MEASUREMENT OF THE MEMBRANE POTENTIAL

OF THE NERVE FIBER USING A MICROELECTRODE

The formation of the resting membrane potential

(RMP) depends on the concentrations of К

, Nа

Са

, Сl

, as well as on the features of the cell

membrane. The cell membrane has 3 layers (Figure):

 External layer — mucopolysaccharides.

 Bimolecular lipid layer.

 Internal (protein) layer.

The membrane has channels which have the

properties of:

 Selectivity — the channels are divided into 4

groups: sodium, potassium, calcium, chloric.

Selectivity is not obligatory yet preferable.

 Electroexcitability.

FIGURE — STRUCTURE OF THE PLASMA

MEMBRANE

FIGURE — SODIUM AND POTASSIUM

CHANNELS

Many

channels can

be opened or

closed by gates

that are

regulated by

electrical

signals or

chemicals that

bind to the

channels. The

gating of protein

channels

provides a

means to

control their ion

permeability

(Figure).

Classification of ion channels:

By the amount of ions to which the channel

is permeable:

— Selective ion channels (permeable to one

type of ions).

— Non-selective ion channels (permeable to

several types of ions).

By the type of ions the selective channels are

divided into K

, Nа

, Са

, Сl

channels.

By the type of regulation (gating):

— Voltage-gated channels. They react to the

changes of the membrane potential. When the potential

reaches a certain value, the channel becomes activated

and ions pass through it down the concentration

gradient.

— Chemically-gated channels (ligand-gated

channels). In these channels the gates are opened by

the binding of a chemical substance (a ligand) with

receptors.

— Mechanically-gated channels. In these channels

the permeability is changed if there are some

mechanical actions on the membrane (these channels

are present in the membrane of the mechanoreceptors

of the blood vessels, skin, etc).

In cells at rest all sodium channels are closed.

There are leakage channels (non-specific), which are

permeable to all elements but are most permeable to

potassium. They are always open, and potassium ions

move through these channels down the concentration

and electrochemical gradients. According to the

membrane-ionic theory, the presence of the RMP is

caused by:

 Unequal ion concentration inside and outside the

cell.

 Different permeability of the channels to these ions.

There are many K

ions inside cells and few outside

them, opposite to Nа

. There are slightly more Сl

ions

outside cells than inside them. There are a great

number of organic anions inside cells.

The membrane of cells at rest is only

permeable to K

ions. At rest, potassium

ions constantly move outside cells, where

there is a high Nа

concentration.

Therefore, in cells at rest, the external

surface of the membrane is positively

charged. High-molecular organic

anions (proteins) are concentrated on

the internal surface of the membrane and

determine its negative charge. Due to

electrostatics they keep K

ions on the

other side of the membrane. The basic

role in the formation of the RMP

belongs to K

ions (Figure).

FIGURE — IONIC MECHANISM OF THE FORMATION

OF THE RESTING MEMBRANE POTENTIAL

Despite the streams of ions coming through the

leakage channels, the ion concentrations are not

equivalent, i. e. they are always constant. This

does not happen because of the existence of Nа

-pumps in the membranes (Figure).

They continuously pump Nа

out of cells and

pump K

against the concentration gradient into

the cytoplasm. For 3 Nа

ions removed from a cell,

2 K

ions are introduced into it. The transmission of

ions against the concentration gradient is carried

out by active transport (with energy input).

FIGURE — STRUCTURE OF THE SODIUM-

POTASSIUM PUMP

Membrane potentials in different tissues

are characterized by different values: the

highest one is in muscular tissue —

80–90 mV, in nervous — 70 mV, in

connective — 35–40 mV, in epithelial

— 20 mV.

When the internal charge of the

membrane becomes less negative, it is

known as membrane depolarization. If

the internal charge of the membrane

becomes more negative, it is called

hyperpolarization.

2. Membrane action potential (AP)

Being imposed by a threshold stimulus, the

permeability of the membrane changes, and an

action potential (AP) or excitation occurs (Figure).

AP is rapid fluctuations of the membrane

potential during excitation.

The threshold stimulus is the minimal strength

which leads to the minimal response. To characterize

the threshold stimulus, the concept of rheobase (in

Greek, the root rhe translates to "current or flow", and

basi means "bottom or foundation") is used.

Apart from the threshold stimulus, there are

subthreshold stimuli which cannot generate

responses but induce a shift in cell metabolism.

Besides, there are superthreshold stimuli.

Having arisen, AP goes along the membrane

without changing its amplitude. It has the

following phases:

1. Slow depolarization (See figure «2»);

2. Fast depolarization (See figure «3»).

3. Fast repolarization (See figure «4»);

4. Slow repolarization or negative

afterpotential (See figure «5»).

5. Hyperpolarization or positive

afterpotential (See figure «6»).

FIGURE — MEMBRANE ACTION POTENTIAL

Phases of the

membrane action

potential:

(

2) Slow depolarization.

(

3) Fast depolarization.

(

4) Fast repolarization.

(

5) Slow repolarization.

(

6) Hyperpolarization.

Mechanism of the AP origin (Figure).

Under the effect of the threshold stimulus,

the cell membrane becomes permeable to

ions, which stream inside the cell at a

high speed (the flow of Na

ions into cells is

higher than the flow of K

ions outside cells).

The internal side of the membrane

becomes positive, and on its surface a

negative charge is formed. The changes of

the charges on the internal and external

surfaces of the membrane correspond to the

depolarization phase (See figure «1»).

Afterwards the sodium channels

close, and the potassium channels

which have been partially closed open.

ions go out of the cell. This AP

phase is called repolarization (See

figure «2»)

The action of the Nа

-K

-pump and

the RMP are restored (See figure «3»).

The basic role in the formation of

AP belongs to Na

ions.

FIGURE — SUCCESSIVE

OPENING AND CLOSURE OF CHANNELS

I. Sodium

channel

1. Opening

2. Closure

3. Opening

II. Potassium

channel

Changes of excitability during excitation

During the development of AP (excitation), the

excitability of cells changes (Figure).

The development of the slow depolarization phase

raises the excitability (hyperexcitability) creating

conditions for a response. Further, when the slow

depolarization phase is replaced by the fast one, the

excitability rapidly reduces and, when the

repolarization phase occurs, it starts to recover

again.

There are several periods of excitability:

1. Refractory period:

а) absolute;

b) relative.

2. Supernormal or exaltation period.

The refractory period is an interval of time during

which a cell cannot respond to the action of a stimulus.

The sodium channels are inactivated. During the

absolute refractory period, the cell does not respond to

the action of threshold or superthreshold stimuli.

The membrane repolarization leads to the reactivation

of the sodium channels. This is the relative refractory

period. During this period, a response may appear

under the action of the superthreshold stimulus.

During the supernormal period, excitability exceeds

the initial level. At this state the cell can respond to a

stimulus the strength of which is a bit lower than the

threshold one. The threshold of excitation is decreased

because the values of the membrane potential are close

to the critical level.

FIGURE — CHANGES OF THE MEMBRANE POTENTIAL

AND EXCITABILITY DURING EXCITATION

I — Changes of

the membrane

potential:

(1) Membrane

resting potential

Phases of

membrane action

potential:

(2) Slow

depolarization;

(3) Fast

depolarization.

(4) Fast

repolarization;

(5) Slow

repolarization

(6) Hyperpolar-

tion

II — Changes

of excitability:

(a) Normal

excitability

(b) Absolute

refractory period

(d) Supernormal

or exaltation

period

Laws of stimulation and assessment of excitability.

Lability.

The excitability of tissue depends on the threshold of its

irritability (rheobase). Rheobase is the minimal strength of a

stimulus that is able to cause excitation of tissue and induce

the minimal response. The lower the strength of the threshold

stimulus is, the higher the excitability of tissue is. However,

the response of tissue depends on the strength of the stimulus

to a certain extent.

The response of the cell also depends on the duration for

which the stimulus is applied. The threshold strength of the

stimulus is in the inverse relation with its duration.

The interrelations between the strength and duration of the

stimulus are demonstrated by the strength-duration curve. If

the strength of the current is «1», to induce a response from

tissue, the duration of the stimulus must be «а» (Figure).

Figure — The strength-duration

curve

Notes: 1 — rheobase; 2 — double rheobase;

a — useful time; b — chronaxie

The shortest duration for which a stimulus equal

to rheobase should react to induce a response is

called the useful time (Figure). If to double the

strength of the stimulus («2» — two rheobases), the

duration of the stimulus necessary to induce the

response decreases («b») (Figure).

The shortest duration for which a stimulus equal

to double rheobase should be applied on tissue to

cause a response is called chronaxie.

If the strength of a stimulus is equal to half of

rheobase (half of «1»), no response will arise

regardless of the duration of the stimulus. For

example, the reflex of withdrawing hands away from

a cold iron will not occur.

If tissue is exposed to a stimulus whose

strength is equal to triple rheobase, but whose

duration is too short (half of «b»), no response

will arise either. For example, if to touch a hot

iron very quickly, it is impossible to feel its

temperature (Figure).

Chronaxie characterizes the rate of excitation

generation. In different tissues it varies, which is

used for medical purposes, e.g. to determine the

damage of motor nerves.

Lability

To characterize the development of separate

APs, the concept of lability is used. Lability is the

rate of the development of the response to a

stimulus (separate APs). The higher lability is,

the more APs tissue can make per unit of time.

The measure of lability is the highest number of

stimuli to which tissue can respond by generating

APs per unit of time. The maximal rhythm of

excitation is limited by the duration of the absolute

refractory period. If the refractory period lasts for

0.5 msec, the maximal rhythm is 1,000 impulses

per second and more.

Nervous tissue possesses the highest

lability. It can generate up to 1,000

impulses per second. Muscular tissue can

conduct up to 500 impulses per second.

Synapses are least labile. However,

tissues cannot function at the maximum

rhythm for a long time. In natural

conditions tissue reacts to the excitation of

a lower rhythm which can be kept for a

long time. This rhythm is produced during

the supernormal period and is therefore

called optimal. In nerve fiber it is 500

impulses per second, in muscle fiber —

200 impulses per second.

During rhythmic excitation, lability can

increase or decrease. Decreased lability leads to

the development of the processes of inhibition

and its increase determines the properties of

tissues to adjust to a new higher rhythm of

impulses. The adjustment to the higher rhythm

is connected with the pumping of Nа

ions out

from the cytoplasm during excitation. Thus,

muscles are capable to adjust to a more frequent

rhythm of impulses coming to them from nerve

fibers. For example, if after a long flight you see

your parents at the airport, your tiredness

disappears for a while. This is connected with

the adjustment of your muscles to a higher

rhythm coming from the nerve centers.

FIGURE — LABILITY OF VARIOUS TISSUES

Synapses Muscles Nervous tissue

3. Physiology of nerve fiber

Nerves specialize on the conduction of

stimuli and connect the nerve centers

with executing organs. Nerves consist of

myelinated and unmyelinated fibers,

which are coated with the connective

tissue membrane (Tablе).

The surface of the axial cylinder of

nerve fiber is coated with the plasma

membrane, which performs the main

role in the generation and conduction of

excitement.

Table — Properties of different nerve fibers

Type

of fibers

Diameter,

mcm

Speed of

conduction,

m/sec

Functions

Aα

(myelinated)

13–22 70–120

Efferent

fibers conduct

excitation

skeletal muscles, afferent

fibers

conduct

excitation from

muscle

receptors

Aβ

(myelinated)

8–13 40–70

Afferent

fibers conduct

excitation

from

touch and

tendinous

receptors

Aγ

(myelinated)

4–8 15–40

Afferent

fibers conduct

excitation

from

touch and pressure

receptors,

efferent

fibers conduct excitation

skeletal

spindles

(myelinated)

1–3 3–14

Preganglionic

fibers of

the

vegetative

nervous system

(unmyelinate

0.5–1.0 0.5–1.0

Postganglionic

fibers of

the

vegetative

nervous

system,

afferent

fibers conduct

excitation

from

pain, temperature,

and

pressure

receptors

Myelinated fibers have an intercept sheath,

which is formed by myelin segments 1–2 mm long

(the myelin sheath). The gap between the two

segments is called the node of Ranvier (Figure).

Figure — Structure of myelinated fiber

The myelin sheath is deposited around the

axon by Schwann cells. The membrane of

Schwann cells first envelops the axon. Then

Schwann cells rotate around the axon many

times, laying down the multiple layers of the

Schwann cell membrane. Myelin is highly

resistant and besides it performs the isolating

function and takes part in the metabolism of

nerve fibers. A signal along myelinated fiber

goes only through the nodes of Ranvier, as

they have many sodium channels.

Unmyelinated fibers are of the similar structure

but have no myelin. Their surface is coated with

Schwann cells.

If to dissect nerve fiber, its peripheral end after a

while loses the ability to conduct signals and

degenerates. Myelin undergoes fatty degeneration

and transforms into fatty drops. The central end of

nerve fiber is able to regenerate. A growth bulb is

formed on it and grows towards the periphery (from

0.4 to 4.5 mm a day) and reaches the

corresponding organ or tissue. Therefore, their

innervations are recovered. Thus, the first signs of

the regeneration of muscle innervations can appear

after 5–6 weeks.

Laws of excitement conduction

• The anatomical and physiological integrity of

fibers is essential. Dissection or compression

affects the conductivity of nerves. If to cut a nerve

and separate both the ends of the cut at a distance

of 1 mm, excitation can skip from one end to the

other only through myelinated fibers.

• Signals may propagate along nerves in both

the directions. This law is typical only for fibers

isolated from the body, as inside the body signals

are transmitted through synapses which conduct

APs only in one direction.

• Isolated signal conduction, i.e. a signal from one

nerve fiber cannot skip to another one located in

parallel.

Mechanisms of signal formation and

conduction in myelinated and

unmyelinated fibers

The mechanism of signal conduction

in unmyelinated fibers. The action of the

threshold stimulus on the unmyelinated

fiber membrane changes its permeability to

Nа

ions, a great number of which flow

inside the fiber. In this area the charge of

the membrane changes (the internal

becomes positive, the external — negative).

It generates circular currents (movements

of charged particles) from «+» to «–» along

the whole fiber (Figure 3.9., «а»).

Features of signal conduction along

unmyelinated fibers:

 Signals go continuously and the whole fiber

is seized with excitation.

 Signals go at a low velocity.

Along unmyelinated fibers signals go to the

internal organs from the nerve centers. However,

the low velocity of the signals and their fading are

not always beneficiary to the human body. That is

why nature made an additional mechanism: the

conduction of signals along myelinated fibers.

Figure — The mechanism of signal

conduction in unmyelinated fibers

The mechanism of signal conduction

in myelinated fibers (Figure). The action

of the threshold stimulus on the membrane

of myelinated fibers at the Ranvier’s node

changes the permeability to Nа

ions,

which go inside the fiber. In this part the

charge of the membrane changes, which

also generates circular currents. These

currents go through the intercellular fluid to

the adjacent node, where the charge

changes again. Thus, the excitation

transmits from one part to another. The

reverse movement of the signal is

impossible, as the part through which it has

passed, is at the absolute refractory phase.

Figure — The mechanism of signal

conduction in myelinated fibers

Thus, in myelinated fibers action potentials

occur only at the nodes of Ranvier. The action

potentials are conducted from node to node, and

this is called saltatory conduction. Saltatory

conduction is important for two reasons. Firstly,

this mechanism increases the velocity of

transmission of nerve impulses. Secondly,

saltatory conduction conserves energy for the

axon because only the nodes depolarize,

therefore requiring less energy for re-establishing

the difference between the sodium and

potassium concentrations across the membrane

after a series of nerve impulses.

Features of signal conduction along

myelinated fibers:

 Signals go in intermittent motion

(saltatory conduction).

 Signals go at a high velocity.

In myelinated fibers signals are

transmitted from analyzers to the СNS,

skeletal muscles i.e. where a high speed of

responses is required.

4. Parabiosis

The scientist N.E.Vvedensky proved that a

part of a nerve changes its lability under the

effect of an alterant (irritant). This happens

due to the fact that excitation lasts longer

within this part and, therefore, at a certain

stage of the alteration, the excitation is not

transmitted through the nerve.

The condition of low lability, i.e.

damage of the normal vital activity of the

nerve is called parabiosis. Parabiosis can be

observed under the action of narcotics, cold or

heat, under the influence of currents and other

stimuli.

The phenomenon of parabiosis was

studied on the example of a nerve-muscle

specimen which consisted of nervous

cells, nerve fibers, and muscles, which

reflected all the changes happening in

nerve fibers (Figure). During the

experiment, a stimulus (for example,

some narcotic on cotton wool) was

applied on some part of nerve fiber.

Through this part the stimulus

transferred and some changes could be

observed.

Figure — The scheme of the nerve-muscle specimen

of parabiosis

A,B,C — electrodes: A — experimental, B — control,

C — in the area of the alteration influence

As a result of the experiment, 3 phases (Figure) of

parabiosis were detected:

1. Provisional or equalizing phase. If to irritate

nerve fiber with stimuli of various strength (weak and

strong), the response of the muscle will always be

identical.

2. If the narcotic continues its action, there comes

the second phase — paradoxical. In this case strong

stimuli induce weak responses, and, on the contrary,

weak stimuli —strong ones.

3. If the effect of the narcotic is not stopped, neither

strong nor weak stimuli can induce a response. This

stage is called inhibitory.

Then, if to terminate the effect of the narcotic and to

wash the damaged part of the nerve, its properties are

recovered in the inverse sequence.

FIGURE — PHASES OF PARABIOSIS

NOTES: THE ARROWS ABOVE SHOW THE STRENGTH OF THE RESPONSE (THE

FORCE OF THE MUSCLE CONTRACTION). THE ARROWS BELOW SHOW THE

STRENGTH OF THE STIMULUS (CURRENT)

5. PHYSIOLOGY OF SYNAPSES

Synapses. Structure of synapses

Synapses are specialized structures which

provide the transmission of excitation from

one neuron to another or to target effector

cells.

The functional role of synapses:

1. They provide functional contacts between

nerves and organs.

2. They promote the regulatory activity of the

СNS.

3. They have plasticity (the amount of signal

which passes through a synapse can change, which

is of an important functional value).

4. They participate in the formation of memory.

The structure of chemical synapses

Nerve fibers approaching a cell form a thickening which

contacts with the cell. This part is the presynaptic

membrane. The opposite membrane is postsynaptic.

Between them there is a cleft filled with a plasma-like fluid.

In the presynaptic terminal, there are neuromediators,

which are capable to excite or inhibit the innervated cell

(Figure).

Myelinated nerve fibers approaching skeletal muscles

make fanlike branchings into end fibers (terminals). The

area of the synapse formation between the nerve

terminations and muscles is called the motor end plate.

The postsynaptic membrane of muscle fibers is thicker and

forms regular folds which increase the surface area of the

postsynaptic membrane. Therefore, a big amount of the

mediator may contact the postsynaptic membrane of

muscle fiber.

FIGURE — STRUCTURE OF THE SYNAPSE

NEUROMUSCULAR

SYNAPSE IS THE

SYNAPSE BETWEEN

MOTOR NEURON

AND

SKELETAL MUSCLE

CELL.

NEUROTRANSMITTER –

ACETYLCHOLINE

Figure — Neuromuscular

synapse

Classification of synapses

1. By the location:

a) peripheral: neuro-muscular, neuro-

secretory, receptor-neuronal;

b) central: axoaxonic, axosomatic,

axodendritic, dendrodendritic,

somatodendritic (Figure).

2. By the effect:

а) excitants;

b) inhibitors.

3. By the mechanisms of signal

conduction:

a) chemical;

b) electrical. They conduct excitation without

participation of the mediator at a high speed

and have bilateral signal conduction. The

structural basis of electrical synapses is the

nexus. These synapses are located in the

endocrine glands, epithelial tissue, СNS, and

heart.

c) mixed.

In some organs excitation can be transmitted

both through chemical and electrical synapses.

FIGURE — ELECTRICAL & CHEMICAL SYNAPSES

4. By the type of the secreted mediator, chemical

synapses are classified into:

а) adrenergic (the mediator is noradrenalin);

b) cholinergic (the mediator is acetylcholine);

c) serotoninergic;

d) glycinergic and others.

Figure — Classification of synapses

Chemical synapses have some common properties:

 Excitation in synapses is transmitted only in one

direction. This is provided by the structure of synapses:

the mediator is released only from the presynaptic part

and it interacts with the receptors of the postsynaptic

membrane.

 The transmission of excitation in synapses is slower than

that in nerve fibers (synaptic delay).

 Excitation is transmitted with the help of special chemical

substances – mediators (neurotransmitters).

 In synapses the transformation of the excitation rhythm

occurs.

 Synapses have low lability.

 Synapses have rapid fatigability.

 Synapses have high sensitivity to chemical substances

(including pharmacological drugs — blockers and

others).

Mechanisms of signal transmission in chemical

synapses (on the example of nerve-muscular

synapses)

1. Release of the mediator into the synaptic cleft.

When APs reach the nerve termination (pre-synaptic

membrane), they generate its depolarization. As a

result, calcium ions go inside the terminal. The

increase of the calcium concentration in the nerve

termination promotes the release of acetylcholine into

the synaptic cleft.

2. Diffusion of the mediator to the postsynaptic

membrane and binding with receptors. The mediator

reaches the postsynaptic membrane and binds with

cholinoreceptors located on the postsynaptic

membrane.

FIGURE — STAGES OF SIGNAL TRANSMISSION IN SYNAPSE

Figure — Stages of signal transmission in the

synapse

3. The occurrence of excitation in muscle

fiber. As a result of the interaction of acetylcholine

with the receptors, sodium ions go through the

postsynaptic membrane into the cell and depolarize

the membrane (Figure).

If the initial level of the RMP is — 85 mV, it can

decrease to 10 mV, i.e. partial depolarization

occurs, the excitation does not go further, it stays

in the synapse. These mechanisms cause a synaptic

delay, which may last 0.2–1 ms. Partial

depolarization of the postsynaptic membrane is

called an excitatory postsynaptic potential

(EPP).

Influenced by the EPP in the next part of the

membrane of muscle fiber, there arises a

propagating AP, which produces a muscle

contraction.

In synapses the mediator depending on the

chemical structure can cause depolarization of the

postsynaptic membrane (the excitatory postsynaptic

potential is formed, which provides the exciting effect)

or hyperpolarization of the postsynaptic membrane

(the inhibitory postsynaptic potential is formed, which

provides the inhibitory effect).

4. The removal of acetylcholine from the

synaptic cleft. The enzyme acetylcholinesterase is

located on the external surface of the postsynaptic

membrane. This enzyme disintegrates acetylcholine

and inactivates it.

Some poisons and toxins like botulin can block the

conduction of signals through synapses. For example,

the poison curare contacts the receptors of the

postsynaptic membrane and interferes their interaction

with acetylcholine.

6. Perception of outside stimuli (reception)

Receptors are specific formations which transform

energy of a stimulus into an electrochemical potential

and then into the form of nervous excitation.

Classification of receptors.

By the character of sensations:

1) Visual.

2) Auditory.

3) Olfactory.

4) Gustatory.

5) Tactile.

By location:

1) Exteroreceptors — external (acoustic, visual).

2) Interoreceptors — internal (vestibular and

proprioreceptors).

By the character of stimuli:

1) Photoreceptors — (visual).

2) Mechanoreceptors — (touches and pressure).

3) Thermoreceptors — (cold and warmth).

4) Olfactory.

5) Gustatory.

6) Painreceptor.

By the location of stimuli:

1) Distant — (auditory, visual).

2) Contact — (gustatory, temperature, receptors of

pressure).

All the receptors have adaptation. Adaptation is

decreasing sensitivity to the long effect of a stimulus.

Transformation of stimulus energy

As a result of the interaction of a stimulus and

the receptor membrane, a receptor potential

(RP) appears. How does it happen?

During the contact of the stimulus with the

receptor membrane there is an increase of the

permeability of the membrane to sodium ions

and they get into the sensory terminal, which is

depolarized, and a RP is formed.

The initial conversion of the stimulus into the

RP is called transformation.

The RP excites the initial segment of the sensory

nerve generating a nervous impulse. The frequency

of nervous impulses depends on the RP amplitude.

There are primary-sensitive receptors, which

represent the endings of sensory nerves, and

secondary-sensitive receptors — separate cells

which receive stimulation. These cells are in contact

with the endings of sensory nerves. From these cells

the mediator is released and this results in the

formation of a nervous impulse. A set of receptors

that cause excitation of their own neurons is called

the receptive field; and the areas of the

concentration of receptors belonging to certain

sensory systems are called the reflexogenic zones.