chapter 4.4 Flashcards
Voltage-activated ion channels
ions channels that open or close in response to changes in the level of the membrane potential.
When the membrane potential of the axon is depolarized to the threshold of excitation by an EPSP
the voltage-activated sodium channels in the axon membrane open wide, and Na+ ions rush in, suddenly driving the membrane potential from about -70 to +50 mV.
The rapid change in the membrane potential associated with the influx of Na+ ions triggers
the opening of voltage-activated potassium channels. K+ ions near the membrane are driven out of the cell through these channels, first by their relatively high internal concentration and then, when the AP is near its peak, by the positive internal charge.
After about 1 millisecond, the sodium channels close,
which marks the end of the rising phase of the AP and the beginning of repolarization by the continued efflux f K+ ions.
Once repolarization has been achieved
the potassium channels gradually close. Although, too many K+ ions flow out of the neuron, and it is left hyperpolarized for a brief period of time.
AP only involves the ions right next to the membrane
A single AP has little effect on the relative concentrations of various ions inside and outside the neuron, and the resting ion concentrations next to the membrane are rapidly reestablished by the random movement of ions.
Sodium-potassium pumps
play only a minor role in the reestablishment of the resting potential.
Absolute refractory period
brief period of about 1 to 2 milliseconds after the initiation of an AP during which it is impossible to elicit a second one.
Relative refractory period
follows the absolute refractory period; period during which it is possible to fire the neuron again but only by applying higher-than-normal levels of stimulation. End of this period is the point at which the amount of stimulation necessary to fire a neuron returns to baseline.
(1) The relative refractory period is responsible for
responsible for the fact that action potentials normally travel along axons in only one direction. Portions of an axon over which an AP has just traveled are left momentarily refractory, making it impossible for an AP to reverse direction.
(2) The relative refractory period is responsible for
Responsible for the fact that the rate of neural firing is related to the intensity of the stimulation. High level of continual stimulation causes the neuron to fire again as its absolute refractory period is over. If stimulation is just sufficient to fire the neuron, then the neuron does not fire again until both the absolute and the relative refractory periods are over.
Conduction of APs differ from the conduction of EPSPs and IPSPs by:
(1) È The conduction of APs along an axon is nondecremental; APs do not grow weaker as they travel along the axonal membrane.
(2) APs are conducted more slowly than postsynaptic potentials.
The conduction of EPSPs and IPSPs is
passive, whereas the axonal conduction of APs is largely active.
The wave of excitation triggered by the generation of an AP near the axon hillock always
spreads passively back through the cell body and dendrites of the neuron.
Antidromic conduction
if electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an AP will be generated and will travel along the axon back to the cell body.
Orthodromic conduction
axonal conduction in the natural direction – from cell body to terminal buttons.
In myelinated axons
ions can pass through the axonal membrane only at the nodes of Ranvier. Axonal sodium channels are also concentrated at the Nodes of Ranvier.
When an AP is generated in a myelinated axon
the signal is conducted passively – instantly and decrementally – along the first segment of myelin to the next node of Ranvier. Still strong enough to open the voltage-activated sodium channels at the node and can generate another AP.
Saltatory conduction
transmission of APs in myelinated axons.
Conduction is faster
in large diameter axons and faster in those that are myelinated.
Mammalian motor neurons
synapse on skeletal muscles; large and myelinated and thus can conduct at speeds of 100 meters per second.
The maximum velocity of conduction in human motor neurons
is about 60 meters per second.
Conduction in mammalian interneurons
is typically passive and decremental.
Hodgkin-Huxley model
provided a simple effective introduction to what we now understand about the general ways in which neurons conduct signals. Based on the study of squid motor neurons.
Motor neurons
are simple, large, and readily accessible in the PNS.
Properties of cerebral neurons that are not shared by motor neurons:
(1) Many cerebral neurons fire continually even when they receive no input.
(2) Axons of some cerebral neurons can actively conduced both graded signals and APs.
(3) APs of different classes of cerebral neurons vary greatly in duration, amplitude, and frequency.
(4) Many cerebral neurons do not display APs.
(5) The dendrites of some cerebral neurons can actively conduct APs.
Postsynaptic potentials (PSPs)
are elicited on the cell body and dendrites. PSPs are conducted decrementally to the axon. When the summated PSPs exceed the threshold of excitation at the axon, an AP is triggered. The AP is conducted nondecrementally down the axon to the terminal button. Arrival of the AP at the terminal button triggers exocytosis.
