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