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Action Potential and Synapses

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Action Potential and Synapses 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 NONADAPTING RECEPTORS WITH CONSTANT FIRING BY CONSTANT STIMULUS NONADAPTING – PAIN TONIC – SLOWLY ADAPTING With decrease of firing (AP frequency) by constant stimulus PHASIC– RAPIDLY ADAPTING With rapid decrease of firing (AP frequency) by constant stimulus ACCOMODATION – ADAPTATION CHARACTERISTICS OF PHASIC RECEPTORS 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 stimulus 1) 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 EPSP is caused by the opening of Cl channels in the postsynaptic membrane channels in the postsynaptic membrane ALTERATIONS IN MEMBRANE POTENTIAL 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 SENSORY (RECEPTOR) MEMBRANE 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 of receptor membrane – Occures on the border between receptor RECEPTOR POTENTIAL Membrane and axon membrane amplitude of the receptor potential If the amplitude of the receptor potential in depends of the strength of the this place reaches threshold level stimulus ACTION POTENTIAL IS INITIATED = AMPLITUDE CODE = FREQUENCY CODE AP is caused by opening of Na channels after the threshold stimulus Action potential ACTION POTENTIAL, NERVE IMPULSE treshold 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 Once a region of the axon membrane has been down their concentration gradients depolarized to a threshold, the duration and the The Na/K pumps then rebuild the amplitude of the AP is independent of the strenght concentration gradients of both ions of the stimulus – ALL OR NONE LAW (sodium and potassium) ACTION POTENTIAL AND ITS REFRACTORY PERIODS Three-neuronal afferent pathway from sensory receptors to the brain cortex The exception from the three-neuronal rule is III. Order neuron the pathway of the smell In the thalamus perception, which transmits the sensory II. order neuron signals directly from In the spinal cord or in olfactory area in the the medulla nose to olfactory brain cortex I.order neuron In the dorsal root ganglion 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 in postsynaptic membrane 4. Action potential in postsynaptic neuron is transmitted (or not) - depends on the transmitter (excitatory/inhibitory) SYNAPSE – FUNCTIONAL CONNECTION OF NEURONS 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 Each neuron get thousands of inputs It integrates it to a single output – synaptic integration The 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 EPSP - Na ions involved IPSP -Cl ions involved NEURON 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) presynaptic = hyperpolarization of an axon fibers 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
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