Chapter 4 - Cell Anatomy and Physiology Flashcards
electricity
a natural phenomenon that is defined as the flow of electric charge
- charge is a property of matter that can be quantified and comes in 2 types, positive and negative
atoms
consist of protons (+), neutrons (0), and electrons (-)
electrical current
the flow of electrically charged particles from one point to another, measured in ampere (A)
electrical potential (voltage)
the difference in electrical charge between 2 points, and is measured in Volts with an instrument called a voltmeter
ions
electrically charged particles that can be either positive or negative
oscilloscope
a sensitive voltmeter that registers changes in voltage over time
microelectrode
an electrode small enough to be placed in the axon
speed of ions and electrons
ions have a max speed of 90 m/s, whereas electrons have a max speed of 270.000 km/s
cations
positively charged ions
anions
negatively charged ions
factors influencing the movement of anions and cations in and out of cells
- diffusion
- concentration gradient
- voltage gradient
diffusion
the movement of ions from a high concetration area to a low concentration area as a result of random motion
concentration gradient
the difference in concentration between 2 areas
voltage gradient
the difference in electrical charge between 2 areas
resting potential
the uneven distribution of electrical charge between the inside and outside of the cell membrane
resting potential when the axon is at rest
- intracellular side contains more K+ (potassium) and A- (large protein molecule)
- extracellular side contains more Na+ (sodium) and Cl- (chloride)
- overall inside is more negative than outside
channels, gates, and pumps
maintain the resting potential
- A- proteins always remain within the cell due to the membrane being impenetrable to large molecules
- K+ and Cl- channels allow k+ and Cl- ions to pass freely
- gates on Na+ channels keep positively charged Na+ ions out
- Na+/K+ pumps push out 3 Na+ ions from the intracellular fluid and inject 2 K+ (3:2)
- allows the membrane voltage to remain at its resting value of -70 mV
graded potentials
when the concentration of one of the ions at the unstimulated cell membrane changes, the voltage of the membrane changes
- these small fluctuations in the voltage across the cell membrane are called graded potentials
hyperpolarization
when the inside of the membrane is stimulated with a negative voltage, the membrane voltage becomes more negative
- increases the polarity of the membrane (charge difference between the inside and outside becomes larger)
depolarization
when a positive voltage is applied to the inside of the cell, the voltage becomes more positive
- polarity of the membrane is decreased (charge difference between inside and outside becomes smaller)
how the extracellular side becomes more positive
- increase in the outflow of K+ ions
- increase in influx of Cl- through the Cl- channels
action potential
a short but large reversal in the polarity of the membrane of an axon
- lasts about 1 ms and is an all-or-nothing potential
- triggered when the potential difference across the membrane exceeds a certain value (the firing threshold (-50 mV))
- intercellular side suddenly becomes relatively positive compared to the extracellular side, then quickly becomes more negative again (occurs when a high concentration of first Na+ and then K+ quickly cross the membrane)
- triggered by the axon hillock
voltage-activated ion channels
underlie action potentials
- are sensitive to the membrane voltage, and open or close depending on the voltage
phases of an action potential
- cell is at resting potential (-70 mV)
- when firing threshold is reached (-50 mV) the Na+ gate quickly opens, allowing the positive Na+ ions to flow inside the cell. This is the depolarization phase; voltage increases up to +30 mV. At this point the Na+ gates close
- soon after the K+ gate also opens. This is the repolarization phase; the inside of the cell becomes more negative again (-70 mV)
- K+ gates are slower than Na+ gates, so they remain open longer, thus the K+ efflux lasts longer and the inside of the cell becomes even more negative (hyperpolarization)
- K+ gates close, and membrane goes back to resting potential (-70 mV)
refractory period
a cell must wait until the action potential has ended before it can fire again
- result from the one-way gates of the voltage-sensitive Na+ and K+ channels that open and close
- due to refractory periods, neurons can only fire about 200 action potentials per second
absolute refractory period
when an axon is in the depolarization and repolarization phases, a new action potential absolutely cannot be fired
- during the absolutely refractory period, stimulation of the axon membrane will not lead to a new action potential
relative refractory period
when an axon is in the hyperpolarization phase, an increased electrical stimulation is required to produce another action potential
- during this phase, an action potential can be fired, but this requires a larger stimulation
nerve impulse
the propagation of an action potential along a neuronal axon
continuous conduction
the parts of an axon membrane that are close to where an action potential takes place are also brought to the firing threshold
- thus, their voltage-sensitive ion channels are activated, firing a new action potential
- spreads to adjacent parts of the membrane, which activates a new action potential
- a series of action potentials is propogated along the length of the axon
saltatory conduction
axons are surrounded by a myeling sheath, which prevents continuous conduction because there are no adjacent gates that can be activated
- however there are gaps in the insulation layer (nodes of Ranvier) and the action potential can jump from knot to knot
- ranvier nodes are close enough to each other that if an action potential is activated in one node, it can activate an action potential in an adjacent node
- saltatory conduction is faster and costs less energy than continuous conduction
excitatory postsynaptic potentials (EPSPs)
graded potentials that produce short depolarizations of a neuron membrane in response to stimulation
- because the voltage becomes less negative, this ensures that the neuron is more inclined to produce an action potential
- EPSPs are linked to an influx of Na+ ions
inhibitory postsynaptic potentials (IPSPs)
graded potentials that produce short hyperpolarizations of a neuronal membrane in response to stimulation
- because the membrane voltage becomes more negative, this ensures that a neuron is less inclined to produce an action potential
- IPSPs are linked to an efflux of K+ ions or an influx of Cl- ions
temporal summation
the relationship of 2 EPSPs or IPSPs that appear close to each other or equal
- adding up of graded potentials that occur in quick succession over time
spatial summation
when 2 EPSPs or IPSPs take place shortly after each other in time, they also take place shortly after each other on the membrane
- adding up of graded potentials that occur close in space
initial segment on the axon hillock
where the axon is attached to the cell body
- if the potential difference at the initial segment becomes -50 mV or even less negative, an action potential is generated, and the cell fires
back propagation
when an action potential moves from the initial segment to the dendrites
