Electrical Synapses
The plasma membranes are set against each other crack with plasma of 3.5 nm with delays of 0.1 ms.
The action potential is taken directly from the cell pre-synaptic post-synaptic.
He met occasionally in the SN because it is not a typical structure of neurons.
The potential is transmitted through the connexons , much broader than the ion channels, gap junctions in .
not allow the integration of synaptic signals and bi-directional.
are used when needed and speed of transmission when synchronization is required in the activity of most cells.
- excitatory
In the visual system of vertebrates, the retina, in the heart.
Chemical synapses The synaptic cleft distances
neurons from 20 to 50 nm, the delay is 0.5-5ms.
The action potential is converted by the pre-synaptic neuron in neurotransmitter that depolarizes ( PPSE ) or hyperpolarized ( PPSI ) the post-synaptic neuron post-synaptic potential with a PPS. The NT never enters the post-synaptic cell, but only affects the slot.
These differ in the reaction of the post-synaptic neuron.
There is a classification by the morphology of synapses to identify at a glance whether a synapse is excitatory or inhibitory. The
Gray type I (30 nm synaptic cleft, asymmetric membrane specializations, active zones thick and wide, rounded synaptic vesicles, spines often present location and associated dendritic) correspond to excitatory. The Type II
Gray (20nm synaptic cleft, membrane specializations symmetrical active zones and thin not extensive, ovoid synaptic vesicles, spines normally absent and associated somatic location) corresponds to an injunction.
- direct or ionotropic
The NT goes to act on a dependent ion channel transmitter that allows the entry or exit of ions binding to the NT. It is fast enough, however, only one transmission is not sufficient for the development of the action potential.
- Neuro-muscular junction
not worth the classification of Gray because morphologically different from the synapses of the SN.
The post-synaptic membrane ( motor plate) is a muscle fiber in contact with a motor neuron and is much larger than a neural synapses. Membrane specializations are unique in both pre-synaptic neuron (thick filament as the active zone) and in the motor plate (junctional folds). The NT is
acetylcholine and was released only to excite, in amounts 200 times the normal synaptic communication. The channel is called the cholinergic nicotinic receptor.
-
The main excitatory NT glutamate. They are usually associated dendritic and serve hundreds or thousands of PPSE to get a post-synaptic action potential.
- The main inhibitors
NT are glycine and GABA. They are usually associated somatic or axo-axonic the fact that the PPSI is softer than the PPSE and thus the propagation delay is compensated PPSE.
There is a second mechanism called inhibition derivation which acts only when the neuron is excited. In fact, the ECL is close to the Vm, so opening up Cl-channels does not occur until no hyperpolarization Vm changes.
- or indirect or metabotropic G protein-coupled or second messenger
The NT goes to act on a membrane receptor, called membrane receptor associated G protein, which initiates a series of events inside the cell as the last step that have activation of the ion channel. It is much slower but it allows a greater reaction in duration and intensity. The G proteins are membrane proteins that are associated with NT-dependent receptors, which change within and split into three subunits.
Of course there is no single G protein, but of families. The G protein is not specific to neurons, but is typical of communication from other cell types.
The split occurs through the molecule of GTP (guanosintrifosfato) which replaces the GDP in the α subunit. Each subunit assumes
properties and different functions. The α subunit is looking for a target to trigger the reaction. In short
via the α subunit of G protein directly seeks an ion channel. In
Falls II messenger G protein α subunit activates an enzyme that produces a molecule called soluble II messenger that activates a second enzyme which then activates the ion channel. The second messenger functions as a sort of internal NT that can modify cellular metabolism and gene expression of the neuron, so the effect lasts much longer. The messenger molecule
The most common is the ' cAMP (Adenosinmonofosfato cyclic) derived from ATP is modified by the enzyme adenylate cyclase. The cAMP activates protein kinase which transfers a phosphate that ultimately activates the ion channel.
The first characteristic of synapses is the indirect ' amplification effect the NT. In fact, in one molecule of NT open ionotropic channels in metabotropic a molecule of thousands NT opens ion channels to effect the cascade of reactions inside.
The second characteristic of synapses is the indirect ' magnitude of the effect in the cell. The ionotropic effect is not very extensive, in the metabotropic effect extends throughout the neuron.
The third characteristic of synapses is the indirect effect life in the cell. The ionotropic effect is instantaneous, in the metabotropic effect lasts until all the internal reactions are not finished, but it is delayed.
indirect synapses are widely adaptable. In fact, the β subunit may in turn activate other II messenger. Some hormones affect other receptors that give rise to second-messenger cascades that can, for example, inhibit the synthesis of cAMP.
- excitatory
- Inhibitor
- Neuromodulatorie
are usually associated axons.
Integration post-synaptic potential
The electrical activity of neurons depends on the amount of excitatory and inhibitory synaptic signals received, the position where they are located synaptic contacts and constant space and the time constant of the various portions of the neuron. The spatial summation
is almost algebraically, PPS = PPSE - PPSI, to the threshold of action potential. In the spatial summation depends on the geometry of the synapse and space constant.
The efficacy of a synapse decreases with distance, but this varies from cell to cell and in different parts of the same cell. The degree of attenuation signal depends on the space constant, ie the distance at which the value of the PPS is the 37% of its initial value . The constant depends on the section of the dendrite and the resistance of its membrane.
The effect of PPS does not cease immediately after stimulation, in the post-synaptic neuron lasts for a variable period.
The calculation of the duration of the signal depends on the time constant and defines the time after which the PPS decreased by 37% compared to its initial value and varies between 1ms to 20ms.
The temporal summation takes place in dependence on the time constant, the more higher the more it is possible to trigger a potential action.
There are exceptions. Some neurons have a spontaneous firing rate produced by your metabolism. Some dendrites have very large need for riamplificare the PPS, but the transmission is normally elettrotonica.
0 comments:
Post a Comment