BY 30 – Fall 2001
14.1.
Graded Potentials
a. Because the resting potential of a neuron is not equal to the equilibrium potentials of either ______________________ or _____________________, any change in the ___________________________ to these ions will result in their movement.
b. In a typical neuron, the equilibrium potential for K+ is typically ____________________. Thus if membrane permeability to K+ increases beyond that at rest, K+ ions will move ___________________ the cell until the cell membrane potential is equal to ________________________. This movement of K+ is called a(n) _______________________ current, which will ______________________ the cell.
c. In a typical neuron, the equilibrium potential for Na+ is typically ____________________. Thus if membrane permeability to Na+ increases beyond that at rest, Na+ ions will move ___________________ the cell until the cell membrane potential is equal to __________________________. This movement of Na+ is called a(n) ________________________ current, which will ______________________ the cell.
d. By altering the membrane permeability to Na+ and K+, the cell can produce transient (that is, temporary) changes in membrane potential, which come in two basic types called _____________________ and ___________________ potentials. In general, ______________________ potentials are important as signals over short distances, while action potentials are important as signals over ____________________ distances.
e. A ____________________ potential is named as such because the size of the membrane potential change is variable depending on the size of the stimulus. These potentials can be either _________________ (involving an increase in K+ permeability) or ______________________ (involving an increase in Na+ permeability).
f. Graded potentials can ______________________ meaning that with repeated stimuli, the size of the graded potential will be greater (called ______________________ summation); or with 2 or more different simultaneous stimuli at the same site, the size of graded potential will be greater (called _______________________ summation).
g. Conduction of graded potentials is _____________________ meaning that the size of the potential becomes smaller as it spreads throughout the cell. In other words, the ______________________________ of the graded potential becomes smaller with increasing distance from the site of origin, primarily because of _________________________ of ions across the membrane.
14.2.
Action Potentials
a. An action potential involves an initial ___________________________ phase where the membrane potential changes from about _____________________ to _________________________. Then there is a ______________________ phase where the membrane potential returns to rest, but actually ________________________ first.
b. Unlike graded potentials, action potentials always involve _________________________ and the amplitude does not vary with stimulus. Also action potentials ______________________________ meaning that they move along a membrane without amplitude change. Action potentials only occur along membranes described as ___________________________ which includes the ____________________ and ___________________________of neurons and _______________________ cells.
c. Action potentials result from transient changes in membrane permeability, but unlike graded potentials involve _________________________ ion channels.
d. For an action potential to be generated, the cell membrane must depolarized to the _____________________ potential, usually about __________________, which is the point where ________________________________ just exceeds ________________________________ so that the net current (=movement of positive charge) is ________________________________.
e. The depolarization phase of the action potential involves opening _______________________ channels which begins a ___________________ feedback cycle in which ________________________________ opens more _____________ channels which leads to more ________________________, etc. As a result of this activity the cell membrane potential approaches ___________________ but only gets to about _________________________ because already Na+ channels are beginning to close.
f. The repolarization phase of the action potential involves opening ___________________________ channels which also open in response to cell _______________________ but open slower than Na+ channels. As these channels open, K+ flows ______________________ the cell, allowing the cell membrane potential to _________________________. Because they continue to stay open, the cell membrane potential actually ______________________. As these channels close, the cell membrane potential returns to rest.
g. Local anesthetics like Novocaine or lidocaine act by blocking ______________ channels which thus prevent _________________________ potentials. Without action potentials, sensory signals generated in response to injury cannot reach the __________________ giving rise to the sensation of pain.
h. ___________________ potentials are when depolarizing stimuli don’t reach threshold, and the cell membrane potential returns to rest. When the membrane potential does reach threshold, the action potential created will be of the same ________________________ and ______________________, a property that is called _________________________. Also actions potentials cannot _________________________ meaning that the size of the stimulus is indicated by the _____________________ of action potentials.
i. The __________________________ period of an action potential is the time period during which a second stimulus, no matter how strong, cannot generate a second action potential. The _______________________ period is the time period during which a second action potential can be generated, but only in response to a _________________________ stimulus.
j. Propagation of action potentials involves the ____________________ current bringing an adjacent region of membrane to threshold – thus propagation is where an action potential appears _______________________ down a length of membrane, each one of constant _____________________________ and __________________________. Propagation is one-way because at any one instance, the membrane behind is in the ________________________ period.
k. The velocity of propagation increases with ___________________________ of the axon and whether it is ________________________ or not. In the case of myelinated axons, conduction velocity is higher because the myelin increases ______________________ resistance which reduces charge leakage across the membrane. Conduction along myelinated axons is also called _________________________ because the action potential jumps from one _________________________ to the next.
l. Thus comparing graded and action potentials, the amplitude of a graded potential ______________________ with stimulus, while the amplitude of an action potential is _____________________.
m. Thus comparing graded and action potentials, graded potentials can ___________________________ while action potentials cannot.
n. Thus comparing graded and action potentials, action potentials have a _____________________ and a ______________________ period while graded potentials do not.
o. Thus comparing graded and action potentials, conduction is ______________________ in graded potentials but ____________________ in action potentials.
14.3.
Synapses
a. _______________________ are specialized junctions between two neurons in which the electrical activity in the first cell (called the ___________________ cell) affects the electrical activity of the second (called the _________________________ cell).
b. Synapses can be _______________________ (when two cells are joined by gap junctions); or _________________________ when the pre-synaptic cell releases ____________________________.
c. Functionally, synapses can be ______________________ (_____________) when the post-synaptic cell is depolarized; or ________________________ (________________) when it is hyperpolarized.
d. In a chemical synapse, the pre-synaptic cell forms a swelling called the _______________________ which contains a number of ________________, each of which contains neurotransmitter molecules. Neurotransmitters are usually made in the ___________________ and then transported into synaptic vesicles. Upon stimulation, the synaptic vesicles release their contents into the __________________. The membrane of the ____________________ cell has a thickening that represents neurotransmitter ______________________.
e. Synaptic transmission begins with ________________________ of the axon terminal which opens ______________________ channels. As a result of the __________________ influx, synaptic vesicles fuse with the pre-synaptic cell membrane releasing their contents. The neurotransmitters bind to ______________ on the post-synaptic cell, producing a(n)________________ or a(n) ___________________________.
f. The _____________________ delay refers to the 1 msec time period in which certain events such as Ca2+ entry into the cell and ___________________ are occurring.
g. One example of a neurotransmitter is acetylcholine (ACh), formed by the chemical reaction involving __________________________________ and ________________________________. The ACh is formed in the cytoplasm but stored in _______________________________ within the axon terminal, in this case known as a ______________________________ synapse. When released, ACh binds to _______________________ ACh receptors which act as cation channels allowing _______________________ ions to enter the cell. The result is a(n)__________________________________ in which the post-synaptic cell is depolarized. If there are enough EPSPs, the post-synaptic cell can generate a(n) ________________________ potential. The ACh left in the synaptic cleft is degraded by an enzyme called ________________________.