Nervous System Part 2 Flashcards
Propagation of an Action Potential
Na+ influx causes local currents
Local currents cause depolarization of adjacent membrane areas
– Away from AP origin
– Toward axon’s terminals
Once initiated an AP is self-propagating
In non-myelinated axons>Continuous conduction
Absolute refractory period
Time from opening of Na+ channels until resetting of the channels (closed state)
Most Na+ channels are inactivated
Require repolarization to reset (closed state)
Enforces one-way transmission of nerve impulses
Relative Refractory Period
Follows absolute refractory period
Most Na+ channels have returned
to their resting state (closed state)
Some K+ channels still open
Repolarization is occurring
Threshold for AP generation is elevated
Inside of membrane more negative than resting state
Only exceptionally strong stimulus can stimulate an AP
Conduction Velocity
Speed of AP travelling down an axon
Conduction velocities of neurons vary widely (0.5 – 100 m/s)
Rate of AP propagation depends on:
1- Axon diameter
Larger diameter fibers = less resistance = faster impulse conduction
2- Degree of myelination
Myelin sheaths insulate and prevent leakage of charge
* Saltatory conduction is about 30 times faster than nonmyelinated axons
– Voltage-gated Na+ channels are located at nodes of Ranvier
– APs generated only at gaps
– Electrical signal appears to jump rapidly from gap to gap
Differences between graded potentials and axon potentials
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Synapses
a small gap between two neurons that allows them to communicate with each other
Electrical Synapses
Nerve impulse remains electrical
Neurons electrically coupled
– Joined by gap junctions
– Communication very rapid
– May be unidirectional or bidirectional
– Synchronize activity
More abundant in embryonic nervous tissue
Less common than chemical synapses
Chemical Synapses
Release and reception of chemical neurotransmitters
Electrical impulse changed to chemical across synapse,
then back into electrical
Typically composed of two parts:
- Axon terminal of presynaptic neuron
- Neurotransmitter receptor region on postsynaptic neuron’s membrane
a. Ionotropic (directly controls the flow of ions)
b. Metabotropic (indirectly influence ion channels through G protein
and/or a 2nd messenger )
Two parts separated by synaptic cleft (ECF)
Chemical synaptic signaling steps
1) An action potential arrives at the presynaptic axon terminal
2) calcium influx into the presynaptic terminal
3) synaptic vesicle release neurotransmitters into the synaptic cleft via exocytosis
4) neurotransmitters bind to receptors on the postsynaptic membrane
5) This opens ion channels, resulting in graded membrane potentials
6) neurotransmitter removal from the synaptic cleft through reuptake or enzymatic degradation.
Synaptic Delay
Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors
– 0.3–5.0 ms
Synaptic delay is rate-limiting step of neural transmission
* Summed across multiple synapses
* Reflex, involuntary (fewer synapses) vs. reaction times , voluntary (more synapse)
EPSP—excitatory postsynaptic potentials
Local depolarization of the postsynaptic membrane
Brings the neuron closer to AP threshold
An excitatory postsynaptic potential (EPSP) is a change in membrane voltage that makes a neuron more likely to fire an action potential. EPSPs are caused by neurotransmitters binding to receptors on the postsynaptic membrane.
IPSP—inhibitory postsynaptic potentials
Local hyperpolarization of the postsynaptic membrane.
Inner surface of membrane becomes more negative.
Drives the neuron away from AP threshold.
An inhibitory postsynaptic potential (IPSP) is a synaptic response that decreases the likelihood of a neuron firing an action potential. IPSPs occur when inhibitory neurotransmitters bind to receptors on the postsynaptic neuron.
Temporal summation
2 excitatory stimuli from one neuron close in time cause EPSPs that add together.
Be able to recognize this on a graph.
Spatial summation
2 simultaneous stimuli at different locations cause EPSPs that add together.
