Chapter 4: Neurons Use Electrical Signals to Transmit Information Flashcards
Electrographic Seizures
abnormal rhythmic neuronal discharges, may be recorded by an electroencephalogram
Electrical Stimulation
passage of an electrical current from the uninsulated tip of an electrode through tissue, resulting in changes in the electrical activity of the tissue
Voltmeter
device that measures the strength of electrical voltage by recording the difference in electrical potential between two points
Electroencephalogram (EEG)
graph of electrical activity from the brain, which is mainly composed of graded potentials from many neurons
Oscilloscope
specialized device that serves as a sensitive voltmeter, registering changes in voltage over time
Microelectrode
a microscopic insulated wire or a saltwater-filled glass tube whose uninsulated tip is used to stimulate or record from neurons
Diffusion
movement of ions from an area of higher concentration to an area of lower concentration through random motion
Concentration Gradient
difference in the relative abundance of a substance among regions of a container, allows the substance to diffuse from an area of higher concentration to an area of low concentration
Voltage Gradient
difference in charge between two regions that allows a flow of current if two regions are connected
What is the behavioral response to stimulation?
how do our nerve detect a stimulus (sensory) and inform the brain about it?
how does the brain decide what response should be made?
how does the brain command muscles to move to produce a behavioral response?
What is electricity?
a flow of electrons from a body that contains a higher charge (more electrons) to a body that contains a lower charge (fewer electrons)
What is electrical potential?
the ability to do work using stored electrical energy
What are volts?
difference of charge between the positive and negative poles
What is the relationship between electricity and biological tissue?
if it contains an electrical charge, this charge can be recorded
if it’s sensitive to an electrical charge, the tissue can be stimulated
What is the negative pole?
the source of electrons, higher charge
What is the positive pole?
location to which electrons flow, lower charge
Who was Galvani?
eighteenth century
electrical current applied to a dissected nerve induced a twitch in the muscle connected to the nerve, electricity flows along the nerve
What is electrical stimulation?
passing an electrical current from the tip of an electrode through brain tissue, resulting in changes in the electrical activity of the tissue
Who were Fritsch and Hitzig?
mid-nineteenth century
electrical stimulation of the neocortex causes movement (arms and legs)
What was Bartholow’s study from 1874?
first report of human brain stimulation
passed an insulated needle into the left posterior lobe so that the non-insulated portion rested entirely in the substance of the brain
the reference was placed in conduct with dura mater
when the circuit was closed, muscular contraction in the right upper and lower extremities ensued
What was Richard Caton’s study?
first attempt to measure electrical currents of the brain using a voltmeter and electrodes on the skull
noticed fluctuations in the voltmeter
early evidence that neurons send electrical messages
What is an electroencephalogram?
graph that records electrical activity through the skull or from the brain and represents graded potentials of many neurons
What was von Helmholtz’s experiment?
nineteenth century
stimulated brain nerve to calculate the information speed
flow of information in the nervous system is too slow to be a flow of electricity
What was Julius Berstein’s experiment?
1886
must be neuronal chemistry (based on ions) that produce electrical charges
How do tools measure neuron’s electrical activity?
when a single axon is stimulated, it produces a wave of excitation
if an electrode is connected to a voltmeter it can detect a change in electrical charge on that axon’s membrane as the wave passes
What is a volt?
a measure of a difference in electrical potential
What is a voltmeter?
a device that measures the difference in electrical potential between two bodies
Why was the giant axon of a squid studied?
much larger in a diameter than human axons
humans: 1 to 20 micrometers, squid: up to 1 millimeter
easier subject of experiments
used to discover the neuron’s ionically based activity
What are waves of information?
neurons can convey information as a wave, induced by stimulation on the cell body, that travels down the axon to its terminal
What is an oscilloscope?
a device that served as a sensitive voltmeter
used to record voltage changes on an axon
What are microelectrodes?
a set of electrodes small enough to place on or in and axon (1 mm in size)
can be used to measure a neuron’s electrical activity and deliver an electrical current to a single neuron (stimulation)
What are cations?
positively charged ions
examples: sodium (Na+), potassium (K+)
What are anions?
negatively charged ions
examples: chloride (Cl-), protein molecules (A-)
What is diffusion?
movement of ions from an area of higher concentration to an area of lower concentration through random motion
results in dynamic equilibrium
What is a concentration gradient?
relative abundance of substance in space
differences in concentration of a substances allow the substance to diffuse from an area of higher concentration to an area of lower concentration
What is a voltage gradient?
difference in charge between two regions that allows a flow of current if the two regions are connected
opposite charges attract
similar charges repel
ions will move down a voltage gradient from an area of higher charge to an area of lower charge
What is equilibrium?
at equilibrium, the concentration gradient is equal to the voltage gradient
efflux (outflow) of chloride ions down the chloride concentration gradient is counteracted by the influx (inward flow) of chloride ions down the chloride voltage gradient
equilibrium occurs when the concentration gradient of chloride ions on the right side of the beaker is balanced by the voltage gradient of chloride ions on the left
Resting Potential
electrical charge across the insulating cell membrane in the absence of stimulation, a store of potential energy produced by a greater negative charge on the intracellular side relative to the extracellular side
Graded Potentials
small voltage fluctuation across the cell emebrane
Hyperpolarizations
increase in electrical charge across a membrane, usually due to inward flow of chloride or sodium ions or the outward flow of potassium ions
Depolarization
decrease in electrical charge across a membrane, usually due to the inward flow of sodium ions
Action Potential
large brief reversal in the polarity of an axon membrane
Threshold Potential
voltage on a neuronal membrane at which an action potential is triggered by the opening of sodium and potassium voltage-activated channels, about -50 mV relative to extracellular surround, also called the threshold limit
Voltage-Activated Channels
gated potential channel that opens or closes only at specific membrane voltages
Absolutely Refractory
the state of an axon in the repolarizing period, during which a new action potential cannot be elicited (with some exceptions) because gate 2 of sodium channels, which are not voltage activated, are closed
Relatively Refractory
the state of an axon in the later phase of an action potential, during which higher-intensity electrical current is required to produce another action potential, a phase during which potassium channels are still open
Nerve Impluse
propagation of an action potential on the membrane of an axon
Node of Ranvier
the part of an axon that is not covered by myelin
Saltatory Conduction
fast propagation of an action potential at successive nodes of Ranvier, saltatory means “leaping”
Multiple Sclerosis (MS)
nervous system disorder resulting from the loss of myelin around axons in the CNS
Autoimmune Disease
illness resulting from the loss of the immune system’s ability to discriminate between foreign pathogens in the body and the body itself, abnormal immune response by the body against substances and tissues normally present in the body
What is the resting potential?
electrical charge across a cell membrane in the absence of stimulation
a store of negative energy on the intracellular side relative to the extracellular side
the inside of the membrane at rest is -70 mV relative to the extracellular side
four charged particles take part in producing the resting potential
Na+ and Cl- in higher concentration outside the cell
K+ and A- in higher concentration inside the cell
How is the resting potential maintained?
- Because the membrane is relatively impermeable to large molecules the negatively charged proteins (A-) remain inside the cell
- Ungated potassium and chloride channels allow potassium and chloride ions to pass freely, but gates on sodium channels keep out positively charged sodium ions
- Na+ K+ pump forces out Na+ from the intracellular fluid and inject K+
What is the electrical makeup of the inside of the cell?
large proteins anions are manufactured inside cells
no membrane channels are large enough to allow these proteins to leave
cells accumulate positively charged potassium ions to balance out the negative protein anion charge
concentration gradient keeps potassium influx limited
residual potassium ions outside the membrane contribute to the charge across membrane
What is the electrical makeup of the outside of the cell?
sodium ions are kept out
the difference in concentrations of sodium contributes to the membrane’s resting potential
gates on the sodium ion channels are ordinarily closed, blocking the entry of most sodium ions
when sodium leaks into neuron (influx), sodium-potassium pumps regulate sodium and potassium flow, sodium ions are kept out
chloride ions move in and out of the cell through open channels
the equilibrium point is approx. the same as the membrane’s resting potential, and so chloride ions ordinarily contribute little to the resting potential
What are graded potentials?
if the concentration of any on the ions across the unstimulated cell membrane changes, the membrane voltage changes
these graded potentials are small voltage fluctuations across the cell membrane
What is hyperpolarization?
increase electrical charge across a membrane (more negative)
usually due to the inward flow of chloride ions or outward flow of potassium ions
What is depolarization?
decrease in electrical charge across a membrane (more positive)
usually due to an influx of sodium
How does blocking channels affect the body?
tetrodotoxin (TTX), which blocks sodium channels, also blocks depolarization
it can be lethal because it’s toxin impedes the electrical activity of neurons
What is an action potential?
large, brief reversal in polarity of an axon
lasts approximately 1 millisecond
voltage across membrane suddenly reverses
intracellular side more positive relative to extracellular side
How do action potentials occur?
voltage-activated ion channels: gated protein channels that open or close only a specific membrane voltages
sodium and potassium: closed at membrane’s resting potential, voltage-activated channels selectively open at -50 mV
combined flow of Na+ and K+ –> action potential
What is the threshold potential?
voltage at which an action potential is triggered
about -50 mV relative to extracellular surround
Na+ and K+ voltage-activated channels are attuned to the threshold voltage of -50mV
cell membrane changes to -50 mV = both types of channels open to allow ion flow across the membrane
after a process of depolarization, the membrane potential reverses and return to its resting potential
What is the role of voltage-activated ion channels in action potentials?
- When threshold voltage is met, voltage activated channels open briefly, K+ and Na+ channels open
- Sodium channels respond quicker than potassium, voltage change due to Na+ influx takes places before voltage change due to K+ efflux
- Sodium channels have two gates, once the membrane depolarizes to about +30 mV, one of the gates closes, Na+ influx begins quickly and ends quickly
- The potassium channels open more slowly than the sodium channels and they remain open longer, the efflux of K+ reverses the depolarization produced by Na+ influx and even hyperpolarizes the membrane
What is the absolute refractory period?
the state of an axon in the depolarization or repolarization period, during which a new action potential cannot (usually) be elicited
What is the relative refractory period?
during hyperpolarization, an action potential can only occur with a stronger electrical stimulus (more intense second stimulation)
What is a nerve impulse?
propagation of an action potential on the membrane of an axon
voltage change from one AP triggers neighboring voltage-activated channels
size and shape of action potential remain constant along the axon (all-or-none law)
What is the relationship between refractory periods and nerve action?
although an action potential can travel in either direction on an axon, refractory periods prevent it from reversing direction
refractory periods produce a single discrete impulse that travels away from the point of initial stimulation
when an action potential begins near the cell body, it usually travels down the axon to the terminals
What is myelin?
produced by oligodendroglia in the CNS and Schwann cells in the PNS
speeds up neural impulse
What are the nodes of Ranvier?
part of an axon that is covered by myelin
packed with voltage-activated channels
enables saltatory conduction: flow of energy, AP jumps from node to node
What is the relationship between myelin and action potential propagation?
propagation is energetically cheaper since action potentials regenerate at the nodes of ranvier
myelin improves conduction speed, action potential in unmyelinated axons have higher metabolic costs
nodes of ranvier are spaced ideally for salutatory conduction
What is multiple sclerosis?
myelin is damaged
disrupts the functioning of neurons: sensory loss, difficulty moving, fatigue
sudden symptomology
Excitatory Postsynaptic Potential (EPSP)
brief depolarization of a neuron membrane in response to stimulation, making the neuron more likely to produce an action potential
Inhibitory Postsynaptic Potential (IPSP)
brief hyperpolarization of a neuron membrane in response to stimulation, making the neuron less likely to produce an action potential
Initial Segment
area near where the axon meets the cell body that is rich in voltage-gated channels, which generate the action potential
Temporal Summation
addition of one graded potential to another that occur close in time
What is spatial summation?
addition of one graded potential to another that occur close in space time
What is back propagation?
reverse movement of an action potential into the soma and dendritic field of a neuron, postulated to play a role in plastic changes that underlie learning
Optogenetics
transgenic technique that combines genetics and light to excite or inhibit targeted cells in living tissue
Stretch-Activated Channel
ion channel on a tactile sensory neuron that activates in response to stretching of the membrane, initiating a nerve impulse
End Plate
on a muscle, the receptor-ion complex that is activated by the release of the neurotransmitter acetylcholine from the terminal of a motor neuron
Transmitter-Activated Channel
receptor complex that has both a receptor site for a chemical and a pore through which ions can flow
How do neurons integrate information?
through dendritic spines, a neuron can establish more than 50,000 connections to other neurons
nerve impulses traveling from other neurons bombard the receiving neuron with excitatory and inhibitory inputs
the cell body, which lies between the dendritic tree and its axon, can receive inputs from many other neurons
how does rthe neuron integrate this enormous array of inputs into a nerve impulse?
What are excitatory postsynaptic potentials (EPSP)?
brief depolarization of a neuron membrane in response to stimulation
depolarized neuron is more likely to produce an action potential
are associated with the opening of sodium channels, allows influx of Na+
What are inhibitory postsynaptic potentials (IPSP)?
brief hyperpolarization of a neuron membrane in response to stimulation
hyperpolarized neuron is less likely to produce an action potential
associated with the opening of potassium channels (efflux of K+) or with the opening of chloride channels (influx of Cl-)
What is temporal summation?
pulses that occur at approximately the same time on a membrane are summed
What is spatial summation?
pulses that occur at approximately the same place on a membrane are summed
What is the summation of inputs?
a neuron sums all inputs that are close in time and space
it provides an indication of the summed influenced of multiple inputs
if the summed ionic inputs exceed the threshold (approximately -50mV) at the axon hillock, and action potential will be initiated
What is the role of ions in summation?
the influx of sodium ions accompanying one EPSP is added to the influx of sodium ions accompanying a second EPSP if the two occur close together in time and space
the same is true regarding effluxes of potassium ions
the neuron “analyzes” its inputs before deciding what to do
What is the axon hillock?
junction of cell body and axon
rich in voltage-activated channels
where EPSPs and IPSPs are integrated
where action potentials are initiated (usually)
What is back propagation?
reverse movement of an action potential from the axon hillock into the dendritic field
signals the dendritic field that the neuron is sending an action potential over its axon
may play a role in plastic changes in the neurons that underlie learning
How does sensory stimuli produce action potentials?
we receive information about the world in various ways
neurons related to these diverse receptors all have ion channels on their cell membranes
these ion channels initiate the chain of events that produces a nerve impulse
How does the sense of touch work?
each hair on our body allows us to detect the slightest displacement
dendrite of a touch neuron is wrapped around the base of each hair
hair displacement opens stretch-activated channels on the dendrite’s membrane
when channels open, they allow an influx of Na+ ions sufficient to depolarize the dendrite to its threshold level
How do nerve impulses produce movement?
spinal motor neurons send nerve impulses to synapses on muscle cells
axon of each motor neuron makes one or more synapses with target muscle
end plate: part of the muscle membrane that is contacted by the axon terminal
How does acetylcholine work?
chemical transmitter that the axon terminal releases at the muscle end plate
attaches to transmitter-activated channels
channels open, allowing Na+ and K+ ions across the muscle membrane to depolarize the muscle to the threshold
muscles then generate action potentials to contract
What are muscle contractions?
when a motor neuron’s axon collaterals contact a muscle fiber end plate
acetylcholine attaches to receptor sites on the end plate’s transmitter-activated channels, opening them
these large membrane channels allow simultaneous influx of Na+ and efflux of K+
triggers action potentials
causes the muscle to contract
