Nervous System 3 Flashcards
What makes a neuron an excitable cell?
The ability of a cell to respond to electrical/chemical stimuli by producing an action potential
Define action potentail
a brief change in electrical potential across the cell’s membrane involving the movement of ions (Na+ and K+) across the membrane – in response to stimulation
What is the resting membrane potential of a neuron ?
-70mV
What is the ionic basis of the resting membrane potential?
The difference in ion concentration between the inside and outside of the axon
Where is the greater concentration of K+ and Na+?
On the inside or outside of the cell?
Na+= greater con. on the outside
K+= greater con. on the inside
Which enzyme helps maintain the NA+/K+ concentrations in the cells?
Na+ /K+ ATPases
Ion pumps maintain this as well
What toxin can block Na+ /K+ ATPases ?
Ouabain
What is NA+ movement responsible for?
Action potential
What is K+ movement mainly responsible for?
Resting Membrane potential
as membrane is more permeable to K+
Describe the membrane potential graph that shows an action potential
x= time (msec)
y= membrane potential mV
Ek= lowest trough
E Na= almost peak
Describe an action potential in 8 steps
- Membrane at rest Pk>PNa+
- Membrane is passively depolarised
- Threshold for activation of voltage gated Na+ channels so Na+ depolaries membrane PNa> PK
- Positive Feedback cycle of depolarisation. Em overshoots zero.
- Em approaches ENa+. Driving force of the current decreases. Two time dependent events occur; Na+ channels inactivate, delayed rectifier K+ channels start to open ; PNa+ decreases and PK+ increases. membrane starts to depolarise
- Na+ channels inactivate and K+ current repolarizes the membrane and Em returns towards the resting value.
- Na+ channels are inactivated and K+ still opn so Pk>Pna and Em moves towards Ek. HYperpolarisation from A level
- Em return to resting value as K+ channel close
Describe the shape of the action potenial graph
Plateau then gradual increase then sharp peak then dip below plateau line before returning to the plateau
Name the two types of refractory periods
Absolute and relative
Define the relative refractory period
Na+ channels returning to their resting state BUT larger threshold for activation
Define Absolute refractory period.
Immediately after AP spike all the Na+ channels are inactivated and AP cannot be generated
Why are only a few ions needed to generate an Action Potential?
The extra and intracellular layers are conducting. The lipid membrane is non-conducting CAPACITOR ( stores electrical charge)
What properties of a neuron allow for rapid conduction of signals?
- Rapidly activated voltage-dependent channels that increase INWARD (Depolarising) current in response to DEPOLARISATION - positive feedback
- Propagation of voltage-change over membrane surface
- Myelination increases ratio of Rout/Rin resulting in increased conduction velocity of an axon. Faster AP
In the CNS and peripheral nerves what forms the Myelin?
CNS= oligodendrocytes Peripheral = Schwann cells
What does a myelinated axon allow?
Rapid Action Potential transmission Zero signal loss Small axon diameters Signal jumps from Node to Node (Node of Ranvier); signal can only occur at exposed axonal membrane Saltatory conduction (~100m/sec +) Lots of Na+ in meilein gaps
Where are voltage-gated channels clustered?
At the Nodes
Conduction Velocity in unmyelinated axons
Describe how this occurs
What is the typical velocity
Adjacent active and inactive regions exchange charge
This depolarizes the next section of the axon to threshold
The action potential does not travel in the reverse direction as the preceding section is in the refractory period
Typical velocity ~ 2 -5 m/sec (compare to 100m/sec for saltatory cond.)
Increase axon diameter increases velocity
Define synapse
Electrochemical communication between two or more neurons occur via synapses between axons and dendrites
Describe the first 2 steps of Synaptic transmission
1) The neurotransmitter must besynthesized and storedin vesicles so when an AP arrives at the nerve ending, the cell is ready to pass it along to the next neuron.
2) When an AP arrives at the terminal, the neurotransmitter must be quickly andefficiently released from the terminaland into the synaptic cleft.
Name 2 examples of neurotransmitters
Acetylcholine, Noradrenaline, GABA, glutamate
Describe the last 2 steps of Synaptic transmission
3) The neurotransmittermust thenbe recognized byselective receptors on the postsynaptic cell. This allows the signal to initiate another AP (‘propagation’ of signal). {Or, the receptors act to block the signals of other neurons also connecting to that postsynaptic neuron (‘inhibition’ of signal).}
4) Afterrecognition by the receptor, the neurotransmitter must be inactivated so that it does not continually occupy postsynaptic receptor sites.
State 2 reasons why neurotransmitter inactivation is beneficial
avoids constant stimulation of postsynaptic cell
frees up receptor sites to receive additional neurotransmitter molecules
Describe how the neurotransmitter is released at the synapse
AP arrived at the Synapse
Which triggers fusion of neurotransmitter containing vesicles to the plasma membrane and contents released into the synapse
This happens because the AP depolarizes the nerve terminal and opens voltage-gated Ca++ channels triggering fusion of neurotransmitter containing vesicles with the plasma membrane.
What happens to neurotransmitter release when extraceulluar Ca 2+ is removed and Na+ channels are closed?
No Ca2+ = no release
No Na+ channel = still release
What happens on the postsynaptic membrane?
The postsynaptic current depolarises the postsynaptic membrane. The postsynaptic potentials summate until the membrane reaches the threshold for action potential generation.
What are EPSPs?
Excitatory Postsynaptic Currents and Postsynaptic Potentials
Xesicular release (only 1-2 vesicles released)
EPSCs (excitatory post-synaptic currents) – current through ligand-gated channels / neurotransmitters
Membrane depolarised (Na+ moves into the cell)
Glutamate moves across the synapse
Many little response
What are IPSPs?
Inhibitory Postsynaptic Currents and Postsynaptic Potentials
current flow through inhibitory neurotransmitter activated channels
IPSPs can summate to reduce excitability, hyperpolarise cell (K+ moves out of the cell, Cl- ions moving into the cell)
Uses GABA to move across the synapse
leading to a reduced probability of AP generation
what properties of a neuron allow for rapid conduction of signals?
large diameter or myelination
