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Nervous System Guide - Part II
Terms in this set (68)
Define resting membrane potential.
is the voltage (potential difference) that exists across the plasma membrane during the resting state (a cell that is not conducting impulses) of an excitable cell.
How is resting membrane potential measured (i.e. what are the units)?
the amount of electric potential energy at one point in a circuit compared to the amount of electric potential energy at another point in the circuit
What is the numerical value of the resting membrane potential in a neuron?
virtually all functional activities of the nervous system are due to action potentials.
What three factors cause the unequal distribution of charges across the plasma membrane?
1) the differential permeability of the membrane to sodium and potassium ions.
2) the sodium-potassium pump.
3) the presence of nondiffusible, negatively charged molecules (proteins and phosphate groups) inside the cell.
Do all cells have a resting membrane potential?
all cells have a resting membrane potential
What ion is the membrane more permeable to?
Does sodium have a higher concentration inside or outside of the cell?
Outside of the cell
Does potassium have a higher concentration inside or outside of the cell?
Inside the cell
Sodium and potassium diffuse through _________channels.
Leakage channels are always _________. (open or closed).
Where are leakage channels located on the neuron?
Why is the inside of the plasma membrane more negative than the outside of the plasma membrane?
proteins, organic phosphates, and other organic anions that cannot leave the cell create a fixed negative charge on the inside of the membrane.
What structure in the membrane maintains the resting membrane potential?
How does the sodium/potassium pump maintain the resting membrane potential?
With each turn of the pump, two potassium ions are pumped into the cell and three
sodium ions are pumped out of the cell.
How many sodium ions does the pump move out of the cell?
three sodium ions are pumped out of the cell.
How many potassium ions does the pump move into the cell?
two potassium ions are pumped into the cell
How do neurons communicate?
neurons communicate by generating electrical signals in the form of changes in membrane potentials.
What change occurs as a result of the generation of an electrical signal?
What two factors result in changes in the membrane potential?
a. Anything that changes membrane permeability to any ion
b. Anything that alters ion concentration on either side of the membrane
What two types of signals (potentials) are produced by a neuron?
Graded potentials and Action Potentials
Which type of potential produces small signals that travel a short distance?
Where do graded potentials occur on the neuron?
occur on the dendrites and the cell body of neurons.
What triggers a graded potential?
triggered by the binding of a neurotransmitter to a receptor which causes chemically regulated gates to open.
Neurotransmitters bind to ____________gates?
Where are chemically regulated gates located on the neuron?
located on dendrites and cell body
If more neurotransmitter binds to chemically regulated gates, does that increase or decrease the voltage change across the membrane? Will the grated potential travel a shorter or longer distance?
The more neurotransmitter that binds (more
intense stimulus), the greater the voltage change across the membrane, and the
further the graded potential will travel.
the weakest stimulus capable of producing an action potential in irritable tissue. When the membrane is depolarized to ________, action potentials are generated and the process becomes self-generating along the axon. (-55 mV)
What is the numerical value for threshold in a neuron?
Name two types of postsynaptic potentials?
Excitatory Postsynaptic Potentials (EPSP)
Inhibitory Postsynaptic Potentials (IPSP)
Does an excitatory postsynaptic potential cause depolarization or hyperpolarization of the membrane?
How is an excitatory postsynaptic potential generated (i.e. how does it work)?
a. neurotransmitter binds to receptors on postsynaptic membrane which opens
chemically regulated gates that allow the simultaneous movement of both sodium and potassium ions. Since the electrochemical gradient for sodium is much steeper than for potassium, sodium influx is greater than potassium efflux and the membrane potential becomes more positive inside and is driven closer to threshold.
b. Current will flow on both sides of the membrane between the initial area of
the membrane that is depolarized and the area adjacent membrane area not yet
depolarized. Positive ions will move toward more negative areas and negative ions will move toward more positive areas. This results in depolarization in the adjacent area.
c. If the graded potential is strong enough to depolarize the membrane to
threshold when it reaches the axon hillock, voltage regulated gates will open
and an action potential will be generated that will travel along the axon.
Does an inhibitory postsynaptic potential cause depolarization or hyperpolarization of the membrane?
How is an inhibitory postsynaptic potential generated (i.e. how does it work)?
neurotransmitter binds to receptors on postsynaptic membrane which opens
chemically regulated gates which allow the movement of potassium out of the cell. This results in the membrane becoming more negative inside and is driven farther from threshold.
Which type of graded potential causes the resting membrane potential to become less negative?
Excitatory Postsynaptic Potentials (EPSP)
Which type of graded potential causes the resting membrane potential to become more negative?
Inhibitory Postsynaptic Potentials (IPSP)
Which type of potential produces large signals that travel over a long distance?
What type of gates are opened when a graded potential depolarizes the axon hillock to threshold?
Voltage regulated gates
Where are sodium voltage regulated gates located? What causes them to open?
located on axon hillock and axon; open in
response to changes in membrane potential (voltage). If threshold is reached at the axon hillock, then voltage regulated gates will open and an action potential will be generated.
Where are action potentials generated? Where do they occur on a neuron?
1) signals are conducted along the axon of a neuron or along the membrane of a muscle cell 2) occur in response to graded potentials that depolarize the membrane at the axon hillock to threshold (-55 mV).
3) generated at the axon hillock region
Name and describe the three sequential changes in membrane permeability that occur during an action potential
1) Increase in Sodium Permeability and Reversal of the Membrane Potential
2) Decrease in Sodium Permeability
3) Increase in Potassium Permeability of Repolarization
What causes the upward spike of an action potential?
As more sodium ions enter, the voltage changes further which opens more voltage-gated sodium channels. As a result, the membrane potential becomes less and less negative and then overshoots to about +30mV as sodium ions rush inward along their electrochemical gradient. This rapid depolarization and polarity reversal produces the sharply upward spike of the action potential.
What causes the sodium voltage regulated gates to close?
As the membrane potential becomes increasingly more positive, the positive intracellular charge resists further sodium entry.The sodium gates close after a few milliseconds of depolarization and the membrane becomes increasing impermeable to sodium and sodium influx declines and finally stops completely.
Where are the potassium voltage regulated gates located on the neuron?
located on axon hillock and axon
reestablishment of the resting membrane potential after depolarization has occurred.
What causes the downward spike of an action potential?
The abrupt decline in sodium permeability and the increased permeability to potassium contribute to the repolarization process.
Describe the propagation of an action potential along an unmyelinated axon?
The sodium ions that moved into the axon then move laterally from the area of polarity reversal toward the neuron area that is still polarized. As a result, local current flows are established. These current flows depolarize adjacent membrane areas in the forward direction, which opens voltage gated channels and triggers an AP there. An AP is a self propagating event that continues along the axon (once initiated).
Describe the propagation of an action potential along a myelinated axon? This type of conduction is
also known as _________conduction.
A myelinated sheath dramatically increases the rate of impulse propagation. This is due to the fact that myelin acts as an insulator to prevent almost all leakage of charge. Between the Schwann cells are Nodes of Ranvier (i.e. the axon is bare at the nodes). Almost all the sodium channels are concentrated at the nodes. Therefore, when an AP is generated in a myelinated fiber, it is forced to move to the next node (approximately 1mm), where it triggers another AP. This type of conduction is Saltatory conduction, because the electrical signal appears to jump from node to node along the axon.
In which type of conduction does the action potential jump from node to node.
What are the names of the gaps between the nodes?
Nodes of Ranvier
Is impulse conduction faster in myelinated or unmyelinated fibers?
Does myelin sheaths increase or decrease the rate of impulse conduction? Why?
A myelinated sheath dramatically increases the rate of impulse propagation. This is due to the fact that myelin acts as an insulator to prevent almost all leakage of charge.
Explain the all or none phenomenon.
action potentials follow the all or none principle - it either happens completely or it doesn't happen at all. If threshold is reached, then the action potential will be generated to the full extent. A greater stimulus does not lead to a greater action potential.
Explain absolute refractory period.
the period of time following stimulation in which no new action potentials can be generated by a second stimulus regardless of the strength of the stimulus. During the time, the sodium voltage gates are open and no new action potential can be generated. This ensures that the action potential is an all or none event and that the impulse travels in one direction.
Explain relative refractory period.
this is the period of time when the sodium gates are closed and the potassium gates are open. During this time, the axon's threshold for impulse generation is substantially elevated. Only an exceptionally strong (suprathreshold) stimulus can reopen the sodium gates and allow another impulse to be generated.
the functional junction between a neuron and another cell.
Between what two cells does a synapse occur in the CNS?
In the CNS the junction is between two neurons (Neuro-Neuro Junction)
Name three locations of a synapse in the PNS.
a. the junction is either between two neurons (neuro-neuro junction)
b. between a neuron and a muscle cell (neuromuscular junction)
c. or between a neuron and an effector cell within a gland (neuro-effector cell junction)
Name two types of synapses. Which type involves a neurotransmitter?
Describe an electrical synapse. Where do they occur?
exist between neurons in the brain. Also found in cardiac muscle and smooth muscle. At these synapses, the plasma membrane of adjacent cells are linked together by gap junctions. When an electrical signal is generated in one cell, it is directly transferred to the adjacent cell by means of ions flowing through the gap junctions.
Describe a chemical synapse. Where do they occur?
these types of synapses are specialized for the release and reception of chemical neurotransmitters. Neurotransmitters function to open or close ion channels that influence membrane permeability, and consequently membrane potential. A chemical synapse is made up of two parts: the axonal terminal of the presynaptic neuron and the receptor region located on the membrane of a dendrite or the cell body of the postsynaptic neuron.
What are the two parts of a chemical synapse?
Describe an excitatory chemical synapse.
one that brings the membrane potential of the postsynaptic membrane neuron closer to the threshold for generating an action potential. This depolarization is called an excitatory postsynaptic potential.
Describe an inhibitory chemical synapse.
one that brings the membrane potential of the postsynaptic membrane neuron further from the threshold point needed to generate an action potential or stabilizes the membrane potential at the resting value.
Name three factors that affect conduction rate. Explain how they affect conduction rate.
1. Diameter of Nerve Fiber:
3. Myelin sheath
The membrane is 75 times more permeable to potassium ions than to sodium ions.
The membrane is only slightly permeable to sodium ions.
This type of conduction along myelinated axons is
also known as _________conduction
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