PSB 3002 EXAM 2 REVIEW QUESTIONS
Define (a) membrane potential (b) resting potential and (c) action potential. - Answers -
a.) membrane potential - the difference in charge across the membrane
(hyperpolarized: inside more negative than outside; depolarized: inside more positive)
b.) resting potential: charge of the neuron when at rest; -70 mV
c.) main electrical event of an axon, triggered by a threshold of excitation; rapid
depolarization followed by hyperpolarization
What two forces determine the distribution of ions across the cell membrane? -
Answers -1.) *diffusion:* molecules move from an area of higher concentration to an
area of lower concentration (DOWN the concentration gradient); molecules become
evenly distributed over time
2.) *electrostatic pressure:* opposite charges attract, like charges repel
Describe the concentration gradients across the cell membrane for (a) potassium (b)
chloride and (c) sodium. How are these gradients maintained while the cell is at rest? -
Answers -a.) *potassium (K+):* highly concentrated on the inside (diffuses OUT,
electrostatic pressure pushes in); little net movement
b.) *chloride (Ci-):* more concentrated outside than inside (diffuses INTO the cell,
electrostatic pressure pushes them out); no net movement
c.) *sodium (Na+):* more concentrated on the outside; diffusion AND electrostatic
pressure pushes sodium in; relatively impermeable to membrane except when
facilitated by a sodium-potassium pump
What is the function of the sodium-potassium pump? - Answers -located within the
membrane uses high levels of energy to pump 3 sodium out of the cell and 2 potassium
into the cell; helps keep sodium levels low inside the neuron
(a) Describe the ionic movements (and their relative timing) that occur during an action
potential. (b) What triggers an action potential? - Answers -*a.)*
1.) resting potential of neurons is -70mV. When it becomes more positive to reach the
threshold of excitation (about -55 mV), voltage-dependent sodium ion channels open.
2.) Na+ enters the cell, making the membrane potential even more positive
3.) once the membrane potential reaches around +40mV, Na+ channels close and
become refractory (can't open again until it goes back to resting potential)
, 4.) when Na+ channels become refractory, K+ (potassium) channels open and allow
potassium to leave the cell, making the membrane potential more negative
5.) K+ continues leaving the cell, making the inside more negative and eventually
causing membrane potential to return to normal levels (-70mV).
5.) once at normal levels, Na+ channels reset, and Na+ & K+ pumps help reestablish
membrane potential
*b.)*
an action potential is triggered when reaching the threshold of excitation (-55 mV),
which is triggered when neurotransmitters diffuse across the cleft to the postsynaptic
cell. once in the postsynaptic cell, it attaches to receptors, which opens ion channels +
depolarizes the postsynaptic cell.
How do the ions pass through the cell membrane? - Answers -ions are hydrophilic, so
they cannot easily cross the hydrophobic part of the membrane (tails). they must be
facilitated through the membrane with the help of transmembrane proteins which are
like pores that allow ions to pass through (such as ion channels).
What is the all-or-none law? - Answers -an action potential either occurs, or does not
occur; once initiated, is always transmitted down the axon at the same size
Explain how the presence of myelin on an axon speeds up conduction velocity. What is
this form of conduction called? - Answers -called *saltatory conduction*; myelin sheaths
create gaps (called nodes of ranvier). instead of having ion channels along the whole
axon, ion channels are ONLY at those nodes. that means that ion channels do not need
to wait for the adjacent ion channel to open and create an action potential in order to
trigger the opening of the next. instead, the conduction remains depolarized to the
threshold while traveling under myelin sheath (decremental conduction). once it reaches
the node of ranvier then, it can trigger the opening of ion channels + an action potential
at that node, rather than along the length of the whole axon
Describe the events that take place at the axon terminal from the time an action
potential arrives to the time neurotransmitter is released into the synapse. - Answers -
1.) vesicles containing NT dock against the presynaptic membrane (proteins on the
vesicle bind with proteins on the membrane)
2.) action potential comes along terminal buttons; if depolarized to a certain point,
calcium channels will open, and calcium will enter the cell.
3.) calcium binds with docking proteins on the presynaptic membrane, bringing about
the fusion of the vesicles to the membrane to release NT.
Define (a) membrane potential (b) resting potential and (c) action potential. - Answers -
a.) membrane potential - the difference in charge across the membrane
(hyperpolarized: inside more negative than outside; depolarized: inside more positive)
b.) resting potential: charge of the neuron when at rest; -70 mV
c.) main electrical event of an axon, triggered by a threshold of excitation; rapid
depolarization followed by hyperpolarization
What two forces determine the distribution of ions across the cell membrane? -
Answers -1.) *diffusion:* molecules move from an area of higher concentration to an
area of lower concentration (DOWN the concentration gradient); molecules become
evenly distributed over time
2.) *electrostatic pressure:* opposite charges attract, like charges repel
Describe the concentration gradients across the cell membrane for (a) potassium (b)
chloride and (c) sodium. How are these gradients maintained while the cell is at rest? -
Answers -a.) *potassium (K+):* highly concentrated on the inside (diffuses OUT,
electrostatic pressure pushes in); little net movement
b.) *chloride (Ci-):* more concentrated outside than inside (diffuses INTO the cell,
electrostatic pressure pushes them out); no net movement
c.) *sodium (Na+):* more concentrated on the outside; diffusion AND electrostatic
pressure pushes sodium in; relatively impermeable to membrane except when
facilitated by a sodium-potassium pump
What is the function of the sodium-potassium pump? - Answers -located within the
membrane uses high levels of energy to pump 3 sodium out of the cell and 2 potassium
into the cell; helps keep sodium levels low inside the neuron
(a) Describe the ionic movements (and their relative timing) that occur during an action
potential. (b) What triggers an action potential? - Answers -*a.)*
1.) resting potential of neurons is -70mV. When it becomes more positive to reach the
threshold of excitation (about -55 mV), voltage-dependent sodium ion channels open.
2.) Na+ enters the cell, making the membrane potential even more positive
3.) once the membrane potential reaches around +40mV, Na+ channels close and
become refractory (can't open again until it goes back to resting potential)
, 4.) when Na+ channels become refractory, K+ (potassium) channels open and allow
potassium to leave the cell, making the membrane potential more negative
5.) K+ continues leaving the cell, making the inside more negative and eventually
causing membrane potential to return to normal levels (-70mV).
5.) once at normal levels, Na+ channels reset, and Na+ & K+ pumps help reestablish
membrane potential
*b.)*
an action potential is triggered when reaching the threshold of excitation (-55 mV),
which is triggered when neurotransmitters diffuse across the cleft to the postsynaptic
cell. once in the postsynaptic cell, it attaches to receptors, which opens ion channels +
depolarizes the postsynaptic cell.
How do the ions pass through the cell membrane? - Answers -ions are hydrophilic, so
they cannot easily cross the hydrophobic part of the membrane (tails). they must be
facilitated through the membrane with the help of transmembrane proteins which are
like pores that allow ions to pass through (such as ion channels).
What is the all-or-none law? - Answers -an action potential either occurs, or does not
occur; once initiated, is always transmitted down the axon at the same size
Explain how the presence of myelin on an axon speeds up conduction velocity. What is
this form of conduction called? - Answers -called *saltatory conduction*; myelin sheaths
create gaps (called nodes of ranvier). instead of having ion channels along the whole
axon, ion channels are ONLY at those nodes. that means that ion channels do not need
to wait for the adjacent ion channel to open and create an action potential in order to
trigger the opening of the next. instead, the conduction remains depolarized to the
threshold while traveling under myelin sheath (decremental conduction). once it reaches
the node of ranvier then, it can trigger the opening of ion channels + an action potential
at that node, rather than along the length of the whole axon
Describe the events that take place at the axon terminal from the time an action
potential arrives to the time neurotransmitter is released into the synapse. - Answers -
1.) vesicles containing NT dock against the presynaptic membrane (proteins on the
vesicle bind with proteins on the membrane)
2.) action potential comes along terminal buttons; if depolarized to a certain point,
calcium channels will open, and calcium will enter the cell.
3.) calcium binds with docking proteins on the presynaptic membrane, bringing about
the fusion of the vesicles to the membrane to release NT.
