Nervous System V Flashcards
What types of ion channels does conduction of AP require?
Voltage gated Na+ and K+ channels as well as leak channels
List the steps of the conduction of an AP:
- Resting membrane potential
- Depolarization stimulus
- Membrane depolarizes to threshold. Voltage gated Na+ and K+ channels begin to open
- Rapid Na+ entry depolarizes cell
- Na+ channels close and slower K+channels open
- K+ moves from cell to ECF
- K+ channels remain open and additional K+ leaves cell, hyperpolarizing it
- Voltage gated K+ channels close slowly, less K+ leaks out of cell
- Cell return to resting ion permeability and membrane potential
What channels are activated by depolarization?
Na+ and K+ channels, K+ open more slowly
What is the rising phase of an AP?
- depolarization
- depolarizing stimuli open voltage gated Na+ channels (-55 mV), allow Na+ travel down electrochemical gradient
-at +30 mV Na+ channels inactivate
What is falling phase of an AP?
- repolarization
- voltage gated K+ channels also open in response to depolarization, but slowly causing delayed efflux
What is the after-hyperpolarization phase of an AP?
- undershoot
- voltage gated K+ do not immediately close when reaching -70 mV causing membrane potential to dip below resting membrane potential
- leak channels bring back to -70 mV
How do voltage gated Na+ channels suddenly close at peak of AP?
They contain two gates: activation gate (quick) and inactivation gate (slower, on cytoplasmic side)
What does the double gating of Na+ create?
A refractory period
What is an absolute refractory period?
A second AP cannot be initiated 1-2 msec (inactivation gates locked)
What is a relative refractory period?
A second AP can be initiated but requires a larger than normal depolarizing stimulus (graded potential)
- 2-5 msec
- need to overcome more negative membrane potential (-80 to +30)
- need to overcome after depolarization phase
What is the purpose of a refractory period?
- ensures an AP travels in one direction
- limits the rate at which signals can be transmitted down a neuron
- creates space so brain can interpret stimulus
How is information encoded from AP’s?
The frequency of the APs
What is AP potential conduction (propagation)?
- travel over long distances without losing energy, size and amplitude is identical at trigger zone and axon terminal
Steps of AP conduction?
- Graded potentials enters trigger zone
- Voltage gated Na+ channels open, and Na+ enters axon
- Positive charges spread along adjacent sections of axon by local current flow
- Local current flow causes a new section of the membrane to depolarize
- Loss of K+ repolarizes membrane
- Refractory period prevents backward conduction
What is the first parameter determining velocity of APs?
The diameter of the axon: larger diameter axon will offer less internal resistance to current flow
- bringing adjacent regions of membrane to threshold faster
What is the second parameter determining velocity of APs?
The resistance of the axon membrane to ion leakage: current will spread to adjacent sections more rapidly if it is not lost via leak channels
Why is conduction velocity more rapid in myelinated axon?
No leakage in myelinated parts
- nodes of ranvier contain abundance of Na+ channels
- larger diameter axons (~120 m/sec)
- unmyelinated are smaller (~2 m/sec)
10um myelinated has same velocity of 500um unmyelinated
What is saltatory conduction?
Conduction from node to node
Demyelination?
- only nodes contain Na+ channels, AP cannot be maintained in unmyelinated region due to lack of Na+ channels
- current leaks out of unmyelinated region
Where do neurons communicate?
At synapses: presynaptic cell (neuron) to postsynaptic cell (neuron, muscle, target cell)
How many synapses can a postsynaptic neuron contain?
Up to 150,000
What are electrical synapses?
Some CNS neurons, cardiac muscle, smooth muscle
What are chemical synapses?
Majority of neurons in the nervous system use chemical signals to communicate
What is a neurocrine?
A chemical substance released from neurons used for cell-cell communication: neurotransmitters, neuromodulators, and neurohormones
What are neurotransmitters?
Chemical that is released, acts on a postsynaptic cell in close vicinity and causes rapid response in postsynaptic cell
- paracrine or autocrine
Paracrine vs autocrine?
Acts in local vicinity vs acts on themselves
What are neuromodulators?
Chemical that is released, acts on a postsynaptic cell in close vicinity and causes a slow response in postsynaptic cell
- paracrine or autocrine
What does different receptions at synapses cause?
Same neurocrine can act as a neurotransmitter at one and neuromodulator at another
What do neurohormones do?
Secreted into blood stream and act on targets throughout body
