Lecture 2 : Action potentials Flashcards
Resting state
-Na and K channels are closed
- leakage accounts for small movements of Na and K
- -70mv inside the cytoplasm
Cytoplasm is more negative that the plasma membrane
Depolarization
Na gates are opened, K gates are closed
Na rushes in the cytoplasm and the charge is reversed to +30mv
Cytoplasm is postie and plasma membrane becomes negative
* if voltage is at threshold, AP will travel down the axon
Repolarization
Na gates close, voltage sensitive K gates open
K exits the cell and the negative charge is restored.
-75mv
Hyperpolarization
K gates remain open and causes leakage of K
2 Functions of the Na/K pump
- keep the cell polarized at rest
- Return the ions to appropriate sides of membrane after AP
3 Na out of the cell
2K in
Role of the Na/K pump
Repolarization- restores the electrical conditions of the neuron but does not restore the ionic conditions
Threshold
change of about 15-20mV. in order to start AP change of 15-20mv has to occur
Absolute Refractory period
-time from opening of the Na activation gates until the closing of inactivation gates
Function of the Absolute Refractory period
- prevents the neuron from generation an AP
- ensures that each AP is separate
- enforces one way transmission of nerve impulses.
Relative refractory period
happens after absolute refractory period
- Na gates are closed
- K gates are open
- Repolarization and hyperpolarization occurs
Conduction Velocities of Axons
- rate of impulse propagation is determined by
Axon diameter and presence of myelin sheath
*** myelin increases the conduction velocities.
Saltatory conduction
- current passes through a myelinated axon only at the nodes of ranvier.
- Voltage gated Na channels are concentrated at these nodes.
Synapses
a junction that transfers information from one neuron :
- to another neuron
- to an effector cell
Chemical synapses
release and reception of neurotransmitters
- has axon terminal of the presynaptic neuron and receptor region of the post synaptic neuron.
Termination of Neurotransmitters
- are degraded by enzymes (AChE)
- are reabsorbed by astrocytes or the presynaptic terminals.
- diffuse from synaptic cleft
Postsynaptic potentials
two types:
EPSP- excitatory post synaptic potentials
IPSP- inhibitory post synaptic potentials
Summation
A single EPSP cannot include an action potential
EPSPs must summate temporally or spatially.
Temporal Summation-
occurs when a single presynaptic neuron fires many times in succession causing the postsynaptic neuron to reach its threshold and fire
Spatial summation
occurs when excitatory potentials from many different presynaptic neurons cause the postsynaptic neuron to reach its threshold and fire
