Electrical And Chemical Synapses Flashcards
What is the length between two cell membranes at a neuronal chemical synapse?
30nm
What is the gap length at a nerve muscle synapse?
50nm
Characteristics of electrical synapses
Bi directional Lack receptors Depolarization of pre synaptic membrane will produce despoliation of the post synaptic membrane Response is usually attenuated due to membrane resistance
Functional properties of electrical synapses
Reduced extra cellular space Cytoplasmic and continuity btw pre and post synaptic cells Mediating agent is ionic current Little synaptic delay Signal transmission is rapid
Explain Little synaptic delay
Transmission is limited only by the speed of electrotonic transmission across the short distance separating the pre synaptic and post synaptic elements at the synapse
What is the structure of gap junctions?
Hexameric complexes consist of subunits of connexonswhich are present in both pre and post synaptic membranes.
Classification of synapses
Electrical and chemical
Electrical pulse
Membrane of pre and post synaptic neurons come together to form a gap junction or low resistance bridge, direct jumping of electrical currents occur
Chemical synapses
Most common , gaps are approx 20nm wide
Types of synapses
Axo-axonic, axo- somatic , axo- dendritic
What time of synapse is this?
Axo-somatic: axon of pre-synaptic neuron communicates with the cell body of the post-synaptic neuron
what type of synaptic is this?
Axo-axonic: axon of pre-synaptic neuron communicates with the axon of the post-synaptic neuron
A reciprocal synaptic relationship between two dendrites
A single element is both pre- and postsynaptic to a second element.
Note the accumulations of synaptic vesicles on what is presumed to be the presynaptic side of the synapses
Diagram of chemical synapse
Synaptic cleft
Synaptic cleft - a gap between the two neurons across which the neurotransmitters migrate.
Post synaptic terminal
usually in the dendrites of receiving neurons. This contains receiving sites for the neurotransmitters.
Function of chemical synapse
- Increased extra-cellular space (typically 30 – 50 nm); no cytoplasmic continuity between pre- and postsynaptic elements
- Mediating agent is a chemical messenger (ACh, NA, peptides, etc.)
- Significant synaptic delay (at least 0.3 msec, sometimes 1 – 5 msec or longer)
- Unidirectional
Electrical and Chemical Synapses : how do this work?
Electrical and chemical synapses differ fundamentally in their transmission mechanisms. (A) At electrical synapses, gap junctions between pre- and postsynaptic membranes permit current to flow passively through intercellular channels (see blowup). This current flow changes the postsynaptic membrane potential, initiating (or in some instances inhibiting) the generation of postsynaptic action potentials.
(B) At chemical synapses, there is no intercellular continuity, and thus no direct flow of current from pre- to postsynaptic cell. Synaptic current flows across the postsynaptic membrane only in response to the secretion of neurotransmitters which open or close postsynaptic ion channels after binding to receptor molecules (see blowup).
What is a neuronal Transmitter?
Small molecules which carry the nerve impulse from the sending neuron to the receiving neuron.
Characteristic of transmitter
Occurs naturally in pre-synaptic terminals, and either the precursors and enzymes for its formation or an adequate, specific transport system for its uptake into the terminal must exist
Must be released from the terminals by nerve stimulation
A mechanism must exist for rapid inactivation of the released transmitter substance
A synaptic action must be identified for the substance, and local application must produce effects “exactly” like those of synaptically released transmitter substance
Drugs must produce similar effects upon actions of the substance and natural transmitter substance.
Neurotransmitter table
function of transmitters
two types of chemical synapses?
- Pre-synaptic transmission process
Determine the release of the chemical messenger
- Post-synaptic receptive processes-
determine the interaction between transmitter and receptor molecule in the postsynaptic cell
depolarising of the membrane signalifies EPSP OR IPSP
If depolarizing, it is an excitatory postsynaptic potential (EPSP); if hyperpolarizing, an inhibitory postsynaptic potential (IPSP).
Excitatory Post-synaptic Potentials (EPSPs)
The excitation of a post-synaptic neuron (transient depolarization) by axon endings that release a neurotransmitter that induces a local (sub-threshold) response (EPSPs)
Post-synaptic inhibition
The inhibition of a postsynaptic neuron by axon endings that release a neurotransmitter that induces hyperpolarization (inhibitory postsynaptic potentials)
Inhibitory Post-synaptic potential
A graded potential in the post-synaptic neuron which makes it more negative and, thus, less likely to fire.
2 mechanisms
K+ gates open up on the postsynaptic membrane and allow K+ ions to leave the neuron.
Chloride ion (Cl-) channels may open up and allow Cl- ions to enter the postsynaptic neuron.
Summation
Activity in one synaptic knob is small and occurs in several synaptic knobs. These responses summate to produce depolarization or hyperpolarization
Summation is divided into two
- Temporal
Repeated afferent stimuli cause new EPSPs before previous EPSPs have decayed in the same synaptic knob.
- Spatial
Activity occurs in more than one synaptic knob at the same time.
Activity on one knob facilitates activity in another to approach the firing level
Significance
Significance: excitation of a single pre-synaptic neuron never excites or inhibits the post-synaptic neuron as sufficient NT is not released
Facilitation
The process of lowering the threshold for propagation of the action potential by repeated use of a neural pathway or the summation of two or more sub-threshold impulses.
Each succeeding stimulus increased the duration of the AP in the pre-synaptic neuron
Voltage gated Ca2+ channels remain open
Subliminal stimulus primes the post-synaptic neuron so that another such stimulus will evoke discharge of the post-synaptic neuron
1st stimulus facilitates the effect due to prolonged exposure of post-synaptic neuron to the NT
Synaptic integration
Synaptic integration – the summation of both EPSP and IPSP at the post synaptic membrane
Determines whether synaptic transmission will occur or not
Signal Transduction Cascade
Figure 5.3 Signal-transduction cascade by which an extracellular ligand such as a peptide hormone can bind to its receptor to activate a G protein and, via the cascade, lead to the activation or inactivation of an ion channel, a protein kinase, or a phospholipase. The three shaded boxes to the right contain the major members of the categories to which the boxes are linked by the horizontal dashed lines.
Ionotropic and Meta-botropic Acetylcholine Receptors
A, This example illustrates a “nicotinic” acetylcholine receptor, which is a ligand-gated channel on the postsynaptic membrane. In a skeletal muscle, the end result is muscle contraction. B, This example illustrates a “muscarinic” acetylcholine receptor, which is coupled to a heterotrimeric G protein. In a cardiac muscle, the end result is decreased heart rate. Note that the presynaptic release of ACh is very similar here and in A. ACh, acetylcholine; GTP, guanosine triphosphate.
