SENSORY RECEPTORS

RECEPTORS

GATEWAY TO THE PERCEPTION

AND SENSATION

Registering of inputs, coding, integration

and adequate response

PROPERTIES OF THE SENSORY SYSTEM

According the type of the stimulus: According to function:

MECHANORECEPTORS Telereceptors

CHEMORECEPTORS Exteroreceptors

THERMORECEPTORS Proprioreceptors

PHOTORECEPTORS interoreceptors

NOCICEPTORS STIMULUS

Reception

Receptor – modified nerve or epithelial cell responsive to changes in external

or internal environment with the ability to code these changes as electrical potentials

Adequate stimulus – stimulus to which the receptor has lowest threshold – maximum

sensitivity

Transduction – transformation of the stimulus to membrane potential – to generator

potential– to action potential

Transmission – stimulus energies are transported to CNS in the form of action

potentials

Integration – sensory information is transported to CNS as frequency code (quantity

of the stimulus, quantity of environmental changes)

•Sensation is the awareness of changes in the internal and external environment

•Perception is the conscious interpretation of those stimuli

CLASSIFICATION OF

RECEPTORS - adaptation

TONIC – SLOWLY ADAPTING

With decrease of firing (AP

frequency) by constant stimulus

PHASIC– RAPIDLY ADAPTINGWith rapid decrease of firing (AP frequency)

by constant stimulus

ACCOMODATION – ADAPTATION

CHARACTERISTICS OF PHASIC

RECEPTORS

NONADAPTING RECEPTORS WITH

CONSTANT FIRING BY CONSTANT STIMULUS

NONADAPTING – PAIN

ALTERATIONS OF THE

MEMBRANE

POTENTIAL

ACTION POTENTIAL

TRANSMEMBRANE POTENTIAL

ION CONCENTRATIONS

OUTSIDE AND INSIDE

THE MEMBRANE AND

LIMITED PERMEABILITY

OF PARTICULAR IONS

CREATE

THE TRANSMEMBRANE

POTENTIAL

Sensory receptors accummulate changes in the environment or

in the body and transform them to electricity that is transmitted

to the brain via nerve fibres – amplitude coding

IRRITABILITY – the membrane can be excited by the stimulus, the increase of premeability

to a certain ion occurs, the response to the stimulus is limited and causes either depolarization or

hyperpolarization of the membrane, the response can be graded and is conducted with

decrement there is no refractory phase there is time and place summation

Temporal summation: repeated stimuli within a relatively short period of time can

have a cumulative effect

Spatial summation: stimuli occurring at different locations can have a cumulative effect.

Sir John Eccles (1903-

1997) showed temporal

summation in single cells.

Won the Nobel Prize in

1963 for his work on how

inhibitory and excitatory

processes occur at the

synapse.

http://dundeemedstudentnotes.files.wordpress.com/2012/04/untitled-picfewture6.png

Sensory organs Sensory receptors – they convert the energy from outer

environment to action potentials (electicity) to be sent to the central nervous system and brain cortex for perception, sensation and integration.

QUALITY OF THE STIMULUS (modality) depends on the receptor localisation and the fibers that connect the receptor with the projection centres (cortex)

Adequate stimulus1) produces receptor (generator, local) potential

– does not propagate, is only local 2) After reaching threshold level of depolarisation the action

potential arises – propagate to the brain centres (projection areas)

Example: Once we see the light, means, that the threshold was rerached, the action potential was created and propagated to the brain representation areas

QUANTITY OF THE STIMULUS (MODALITY) depends on the frequency of action potentials that arrive in defined time duration to the projection areas in the brain cortex

1. Stimulation of the membrane by subthreshold stimulus elicits local graded

excitation with decreasing of potential difference on the membrane

(depolarization) or with decreasing potential difference (hyperpolarization)

2. Stimulation with threshold stimulus initiates nerve impulse – action

potential (on axon hillock) and its conduction via the axon spikes -

transpolarization

Excitatory and inhibitory potential

EPSP is caused by opening of Na

channels in the postsynaptic membrane

EPSP is caused by the opening of Cl

channels in the postsynaptic membrane

Only a few types of cells can alter their membrane potential by varying the

membrane permeability to specific ions in response to stimulation

Ability to change the membrane potential have nervous and muscle cells

thanks to IRRITABILITY OR EXCITABILITY of their membranes

CONDUCTIVITY – the membrane

is excited by the stimulus and

when the axon membrane is

depolarized to a threshold level

the Na gates open and the

membrane becomes permeable

to Na (transpolarization)

valid for the axon - conduction

1) all or none law

2) refractory periods

3) intensity is coded by frequency

ALTERATIONS IN MEMBRANE POTENTIAL

receptor membrane is the real heart of the sensory system. It is a part of the plasma membrane of the sensory cell, which is in some way constructed so that a stimulus will cause a change in the membrane's permeability to some ion.

This causes depolarization ofreceptor membrane –

RECEPTOR POTENTIALamplitude of the receptor potential depends of the strength of the stimulus

= AMPLITUDE CODE

SENSORY (RECEPTOR)

MEMBRANE

Occures on the border between receptor

Membrane and axon membrane

If the amplitude of the receptor potential in

this place reaches threshold level

ACTION POTENTIAL IS INITIATED

= FREQUENCY CODE

AP is caused by opening of Na channels

after the threshold stimulus

Action potential

Action potential is produced by

an increase in sodium diffusion

followed by an increase of

potassium diffusion

Both depolarization and repolarization

are produced by the diffusion of ions

down their concentration gradients

The Na/K pumps then rebuild the

concentration gradients of both ions

(sodium and potassium)

ACTION POTENTIAL, NERVE IMPULSE

treshold

Once a region of the axon membrane has been

depolarized to a threshold, the duration and the

amplitude of the AP is independent of the strenght

of the stimulus – ALL OR NONE LAW

ACTION POTENTIAL AND ITS REFRACTORY PERIODS

Three-neuronal afferent pathway from

sensory receptors to the brain cortex

I.order neuron

In the dorsal root ganglion

II. order neuron

In the spinal cord or in

the medulla

III. Order neuron

In the thalamus

The exception from

the three-neuronal rule is

the pathway of the smell

perception,

which transmits the sensory

signals directly from

olfactory area in the

nose to olfactory brain cortex

Somatic Pathways

First-order neurons – soma

reside in dorsal root or cranial

ganglia, and conduct

impulses from the skin to the

spinal cord or brain stem

Second-order neurons –

soma reside in the dorsal

horn of the spinal cord or

medullary nuclei and transmit

impulses to the thalamus or

cerebellum

Third-order neurons –

located in the thalamus and

conduct impulses to the

somatosensory cortex of the

cerebrum

http://www.austincc.edu/rfofi/NursingRvw/PhysText/PNSafferentpt1.html

SYNAPTIC CONNECTIONS

SYNAPSE – FUNCTIONAL CONNECTION OF NEURONS

1. Action potential reaches presynaptic button

2. Mediator (neurotransmitter) is released to synaptic cleft

3. Mediator contacts receptors inpostsynaptic membrane

4. Action potential in postsynaptic neuron is transmitted (or not) -depends onthe transmitter (excitatory/inhibitory)

Many synapses are activated on one neuron (up to 5000)

The voltage of each is about 1-2 mV (local, graded potentials)

The sum of local potentials which are either

EXCITATORY POSTSYNAPTIC POTENTIALS – EPSP or

INHIBITORY POSTSYNAPTIC POTENTIALS - IPSP

enables to reach threshold value for action potential on axon (depolarization) or

to get away from the threshold value for eliciting action potential (hyperpolarization).

SYNAPTIC INTEGRATION

PLACE AND TIME SUMMATION

(simultaneous (repeated stimulation

activation of the synapse causes

of high new PSP before the

number of former one is over)

synapses) one PSP lasts 15 ms

axodendritic, axosomatic, axoaxonal

SYNAPSE – FUNCTIONAL CONNECTION OF NEURONS

Each neuron get thousands of inputs

It integrates it to a single output – synaptic integrationThe output dependes on:

1. Strenght of presynaptic stimulation

2. Amount of released neurotransmitter

3. Amount of active PS receptors

EPSP – excitatory postsynaptic potentiál

IPSP – inhibitory postsynaptic potential

IPSP -Cl ions involved

EPSP - Na ions involved

Every synapse excites or depresses

the membrane of the neuron body or

neuron dendrites only LOCALY –

electricity is conducted to a nearby

place of the membrane and then

deceases

The sum of local potentials enables to

reach threshold value for action

potential on axon in case that overall

stimulation is higher than overall

depression

= depolarization of an axon

In case that overall depression prevail

(get away from the threshold value for

eliciting action potential)

= hyperpolarization of an axon

presynaptic

fibers

NEURON

NEUTRANSMITTERS

NEUROMEDIATORS

Neurotransmitter characteristics:

END TO END CONNECTION

1. Is produced by neurons, is released to synaptic cleft from the presynaptic

membrane after the arrival of action potentials.

2. It must have an effect on postsynaptic neuron

2. After trensmitting the signal it must be quickly degraded - deactivated

4. It has to have the same effect on postsynaptic neuron during experimental use as

in vivo

NEUROMODULATORS

DIFUSE MODULATORY SYSTEMS

CENTRES ARE SMALL SUBCORTICAL NUCLEI

Localised in brain stemm

One neuron releases its modulator

to the ECF and could influence

Up to 100 000 neurons in the CNS

Characteristics of the neuromodulators:

1. They do not transmitt the neuronal impulses

2. They influence synthesis, degradation a reabsorption of the

neurotransmitters

3. They have regulatory effects upon synaptic transmission adnd

moreover on the extrasynaptic neuronal receptors

NEUROTRANSMITTERS- NEUROMODULATORSMore than 50 chemical substances

1. Small molecules with rapid effects

Stored in axonal vesicules

Effect on postsynaptic membrane approx. 1 ms, - opening of ion channels,

Brief inactivation, recycled, fromed in the body of neurons

Class I. ACH

Class II. AmInes : NA, A, Dopamín, serotonín, histamín

Class III. Aminoacids: GABA, Glycín, Glutamate, Aspartate

Class IV. NO

2. NEUROPEPTIDS, prolonged effects, are integral part of protein molecules

In neuronal bodies, are fromed in the bodies and compose the vesicules inside of them,

then they are brought to the axonal terminals with longlasting effect (hod. až dni)

pôsobí na iónové kanály, metabolizmus bunky, moduluje expresiu génov.

A. Hypothalamic releasing hormones

B. Pituitary peptides: beta-endorfín, MSH, Prolaktin, GH, vazopresin, oxytocin,

ACTH, LH, TSH

C. Peptids operating in GIT and brain: Leucin enkefalin, methionín enkefalin,

Substancia P, gastrin, cholecystokinin, VIP, Neurotensin, insulín, glukagon

D. Z iných tkanív: angiotensín II, Bradykinín, Karnosín, calcitonín, sleep peptides

THE CYCLE OF

NEUROTRANSMITTER

• THE RELASE (METABOLISM) OF NEUROTRANSMITTER

must be quick so as the new

signal could follow

• Mechanisma/ Reuptake to presynaptic

neuron or to glial cell

b/ Degradation by specific enzymes

c/ Combination of both

CONDUCTION OF

ACTION POTENTIALS

ALL OR NONE LAW

CONSTATNT REGENERATION OF DEPOLARIZATION OF THE MEMBRANE

CONDUCTION OF ACTION POTENTIALS WITHOUT DECREMENT

CONDUCTION OF THE NERVE IMPULSES – ACTION POTENTIALS

osciloscop

CONDUCTION OF THE NERVE IMPULSES – ACTION POTENTIALS

Conduction on unmyelinated fibers

= without myelin sheath around the axon

Action potential is regenerated on the adjacent

region of the excitable membrane of an axon

Conduction on myelinated fibers

= with myelin sheath wrapped around the axon

made of Schwann cells

Action potential is propagated by

SALTATORY CONDUCTION

(“jumps” from one Ranvier node to another)

CONDUCTION OF THE NERVE IMPULSES

ON UNMYELINATED FIBERS

Each AP injects positive charges (sodium

ions)

Into the axon

These are conducted by the cable

properties

of the axon to an adjacent region that still

has

a membrane potential of –65 mV.

When this adjacent region of the

membrane

reaches threshold level of depolarization

It too produces an AP as its voltage

regulated

gates open

STRENGTH DURATION CURVE

RHEOBASE – MINIMUM STIMULUS INTENSITY

When the stimulus strength is below the rheobase,

stimulation is ineffective even when stimulus

duration is very long. CHRONAXY – THE STIMULUS DURATION

CORRESPONDING TO TWICE THE RHEOBASE

Significance of the Chronaxie?

Given that two nerves have the same Rheobase,

Chronaxy the can give an indication of their

relative excitabilities. nerve B is the more excitable.

The curve for the slower fibres

would be shifted to the right, longer

stimulus duration would be

needed to bring the

slower fibres to threshold.

DIAGRAM OF TIME DURATION

NEEDED FOR ELICITING

THE ACTION POTENTIAL

DEPENDING ON STIMULUS

INTENSITY IN THE SAME

NERVE

FREQUENCY CODING OF

THE STIMULUS INTENSITY

THE STONGER THE INTENSITY

OF THE STIMULUS, THE MORE

ACTION POTENTIALS ARE

TRANSMITTED VIA AXON TO

CNS IN CERTAIN PERIOD

OF TIME

= HIGHER FREQUENCY