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What is resting membrane potential?
A resting potential is the difference in voltage across a membrane. The greater the difference in charge across the membrane, the higher the potential. Resting membrane potential acts as a concentration gradient.
Is there a resting potential within a membrane?
The solutions inside or outside the membrane are electrically neutral. The resting potential only exist across a membrane.
What determines resting cell membrane potential?
1) differences of K+ inside and Na+ concentrations outside the cell.
2) differences in permeability of the plasma membrane to those ions.
Are their more channels for K+ or Na+ to cross the cell membrane?
There are more channels for K+ to leave the cell. The loss of K+ ions establishes a negative membrane potential.
Generally, Is there more K+ or Na+ within a cell membrane?
K+ concentration is higher inside the cell membrane.
Generally, Is there more K+ or Na+ outside a cell membrane?
Na+ concentration is higher outside the cell membrane.
What maintains cell membrane potential?
Na+, K+ ATPases, (Sodim Potassium Pumps), maintain the concentration gradient, resulting in the resting membrane potential of -70mV.
What does the Na+ K+ ATPase do?
Maintains potential of concentration gradient by pumping out Na+ and pumping in K+
What is a gated membrane channel?
Gated membrane channels are large proteins in the cell membrane that open and allow ions to pass when appropriate signals are received.
For an open channel, what factors determine in which direction ions will move through the channel?
The concentration gradient and the electrochemical gradient - together called the electrochemical gradient- determine the direction in which ions flow through an open membrane channel.
For which cation is there the greatest amount of leakage across the cell membrane?
There is more leakage of K+ than any other cation.
What are the three types of gated membrane channels that maintain resting membrane potential?
Mechanically gated, Chemically gated and Voltage gated.
IF more Na+ ions enter the cell what happens to the resting membrane potential?
As Na+ ions enter the cell the voltage increases. Moves from -70mV towards -55mV. This is Depolarization.
If more K+ ions leave the cell membrane what happens to the resting membrane potential?
As K+ ions leave the cell the voltage decreases. goes from -70mV towards balance at -90mV. This is Hyperpolarization.
What is a graded potential?
Graded potentials are short-lived, localized changes in membrane potential that can be either depolarization or hyperpolarization. Graded potentials are caused by chemically gated channels.
What is the relationship between magnitude and stimulus strength in graded potentials?
Magnitude varies directly with stimulus strength. The stronger the stimulus the more voltage changes and the further current flows.
What are chemically gated channels?
Chemically gated channels open or close in response to chemical (neurotransmitter) binding.
What are Mechanically gated channels?
Mechanically gated channels open or close in response to physical deformation.
What is an Action potential?
An Action potential is the principle method neurons send signals over long distances.
Are graded potentials caused by chemical gated channels or voltage gated channels?
Graded potentials are caused by chemically gated channels and are local in nature.
Are action potentials caused by chemical gated channels or voltage gated channels?
Action potentials are caused by voltage gated channels and are have extended effects.
What are the characteristics of local potentials.
1) Effective across short distance.
2) Increase and decrease membrane potential.(Depolarising and hyperpolarising)
3) Undergo summation.
Describe what happens before an action potential is fired?
The membrane potential is in flux as Local gates open in response to external chemical stimuli throughout the dendrites of the neuron. These gates depolarize or hyperpolarize the membrane potential. These changes are local, graded potentials.
As these local shifts happen, (summation) they are quickly dissipated through leakage channels. Also, The NA+ K+ pumps work to maintain the cell membrane potential. This cycle continues to happen as the normal resting state until enough stimulation is experienced, the threshold is reached, and the AP fires down the Axon relaying the stimulus.
What is threshold stimulus?
Stimulus (depolarization) that is large enough to open voltage gated channels.
What is threshold potential?
-55mV. Membrane potential at which channels open to initiate an action potential.
What are the 4 steps in the action potential cycle?
What happens during the Resting phase of an action potential?
1) Resting is the first stage, All gated Na+ and K+ channels are closed.
What happens during the Depolarization phase of an action potential?
2) Depolarization is the second stage. Na+ channels open in response to local chemical stimulus and Na+ rushes into the cell. This rush depolarizes the cell locally and the local patch spreads opening more Na+ Channels. Once the threshold has been reached, -55mV depolarization becomes self generating. Many more gates open and allow more Na+ to enter the cell raising the membrane potential to +30mV.
What happens during the Repolarization phase of an action potential?
3) Repolarizing is the third stage. Na+ channels are inactivating and K+ channels open. The high membrane potential (+30mV) signals the change of gates. The AP spike stops rising and the incoming K+ ions reduce the membrane potential.
What happens during the Hyperpolarization phase of an action potential?
4) Hyperpolarizing is the fourth stage of of an action potential. The membrane potential has fallen lower than -70mV. Most K+ channels that opened during the repolarizing stage close. Some K+ channels remain open and Na+ channels reset.
During what phase of an action potential is Na+ entering in large amounts.
2) Depolarization. (Increase of charge)
During what phase of an action potential is K+ entering in large amounts.
3) Repolarization. (Decrease of charge)
What is the relationship of stimulus strength and action potential frequency.
The stronger the stimulus the more frequent the AP's are generated.
How can the nervous system distinguish the intensity of stimulus.
Quantity. More stimulus, more AP's.
What is the importance of refractory periods.
Refractory periods limit the rate that nerve impulses may be conducted.
Define absolute refractory period?
During the absolute refractory period the Na+ channels are resetting to their original resting state and the neuron cannot respond to another stimulus regardless of strength.
Why is the importance of an absolute refractory period?
Resets Na+ channels, ensures each AP is a separate all or none event and enforces one way transmission of the AP.
Define relative refractory period?
During the relative refractory period a stimulus that would have normally generated the AP is no longer sufficient, but an exceptionally strong stimulus can reopen the Na+ channels and send another AP.
Why is the importance of a relative refractory period?
Strong stimuli cause more frequent generation of AP's by intruding into the relative refractory period.
Describe the propagation of an action potential in a bare plasma membrane or dendrite:
Without voltage gated channels, current decays because current leaks across the membrane.
Describe the propagation of an action potential in an unmyelinated axon?
Voltage gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. This method is relatively slow.
Describe the propagation of an action potential in a myelinated axon?
Myelin keeps current in axons and voltage does not decay much. AP's are generated only in the nodes of ranvier and the current moves rapidly from node to node. This method is relatively fast.
How does myelin help conduct the current of a neuron?
Myelin acts as a insulator and keeps current from leaking or dissipating along the axon.
What is the relationship of Action Potentials and Graded Potentials?
Graded potentials stimulate Action potentials. Action potentials are larger than graded potentials and travel further.
Why dosen't an action potential potential get smaller as it propagates along an axon?
An action potential is regenerated anew at each membrane patch.
Why is conduction of action potentials faster in myelinated than in unmyelinated axons.
Conduction of axon potentials is faster in myelinated axons because myelin allows the axon membrane between nodes to change its voltage rapidly, and allows current to flow at only at the widely spaced nodes.
If an axon receives two stimuli close together in time, only one AP occurs. Why?
If a second stimulus occurs before the end of the absolute refractory period, no AP can occur because sodium channels are still inactivated.
Define saltatory conduction?
Saltatory conduction occurs in myelinated neurons. Saltatory conduction is faster than continuous conduction.
Junction that mediates information transfer from one neuron to another or neuron to effector cell.
Do synapses receive information from local stimulus?
No. Synapses connect the ends of Axons with cell bodies, dendrites, axons, or other effectors.
Describe Electrical synapses:
Electrical synapses contain protein channels that intimately connect the cytoplasm of adjacent neurons and allow ions and small molecules to flow directly from one neuron to the next. Very Fast, uni or bidirectional. Allow synchronizing of activity of all connected neurons.
What are the three types of neuron to neuron synapses?
What is a neuromuscular junction?
Synapse junction between neuron and muscle sending the signal to contract the muscle.
Function of Chemical synapses?
Specialized for release and reception of chemical neurotransmitters. They are the mechanism of action and effecting muscles or glands.
Form of Chemical synapse?
1) Knoblike axon terminal of presynaptic neuron.
2) Neurotransmitter receptor region n the membrane of dendrite or cell body of postsynaptic neuron.
What is the synaptic cleft?
Space between presynaptic neuron and postsynaptic neuron. Fluid filled approximately 30 to 50 nm.
What are the six steps of transfer of a chemical synapse? Fig 11.17
1) Action potential arrives at axon terminal.
2) Voltage gated Ca++ channels open and Ca++ enter the axon terminal.
3) CA++ entry causes neurotransmitter containing vesicles to release their contents by exocytis.
4) Neutransmitters diffuse across the synaptic cleft and binds to specific receptors on postsynaptic membrane.
5) Binding of neurotransmitter opens ion channels, resulting in graded potentials.
6) Neurotransmitter effects are terminated.
What is the first step of information transfer across a synapse?
1) Action potential arrives at axon terminal and depolarizes the membrane.
What are the effects of depolarizing the membrane of a synapse knob?
Gives the signal to open Na+ and CA++ gates. Ca++ floods down its electrochemical gradient into the terminal from the extracellular fluid.
What does Ca++ flooding into the terminal of the axon propagate?
The flood of Ca++ acts as an intracellular messenger. Through a chain of events, vesicles fuse with with the axon membrane and empty their contents by exocytosis.
How do neurotransmitters effect the postsynaptic neuron?
When a neurotransmitter binds to the receptor protein on the postsynaptic membrane, the protein changes shape. The new shape opens or closes ion channels. (excited or inhibited)
What channels do excitatory neurotransmitters (EPSP) open?
Na+ and K+ fluxes where Na+ and K+ diffuse simultaneously through the membrane in opposite directions. This rapidly depolarizes the membrane sending the membrane towards threshold. If the membrane reaches threshold it sends another AP.
What channels do inhibitory neurotransmitters (IPSP) open?
Opens K+or Cl- channels. K+ movesout of teh cell and Cl- moves in Hyperpolarizing the membrane moving away from another AP.
What are the effects of Cl- entering the membrane?
Cl- has a strong negative charge reducing the membrane potential and inhibiting further transmission of stimulus.
Can a single EPSP induce an AP in the postsynaptic neuron?
No. But if thousands of excitatory terminals are firing on the same postsynaptic neuron, or if there are many delivering rapid impulses, the probability of reaching threshold depolarization increases greatly.
Describe summation of EPSP's
Many excitatory impulses combining together to reach threshold depolarization.
What are the two types of summation in a postsynaptic membrane?
Spatial Summation and Temporal summation.
Describe spatial summation:
Large volumes of excitatory transmitters sending impulses at the same time.
Describe the effects of a cell receiving EPSP's and IPSP's:
EPSP's strongly depolarize the membrane potential but IPSP's strongly hyperpolarize membrane potential. If a cell is receiving both types of signals, the cell has a dramatic change of Na+, K+ and CL- ions until the signals cease or threshold potential is reached and an AP is propagated.
describe "facilitation" of EPSP's:
Process by which neurons are made more excitable by subthreshold stimuli.
Describe neuron convergence:
Input from more than a single neuron. Two or more neurons converge on a single neuron. Allows spatial summation.
What are different types of chemical messengers?
Neurotransmitters, Hormones, Neurohormones, paracrines.
Protein or glycoprotein in the plasma membrane or in the interior of a target cell that binds a specific chemical messenger. (It is the receptors that accept the neurotransmitters of a postsynaptic neuron and allow Na+, K+ and Cl- to pass through the membrane.)
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