Ch. 2 Flashcards
Chapter 2 of The Mind's Machine
Neurophysiology
Study of the specialized life processes that allow neurons to use chemical and electrical signals to process and transmit information
Polarized
There is a difference in electrical charge between the inside and the outside of the cell. Neurons are more negative on the inside than on the outside.
In a neuron at rest, majority of ions are
negatively charged or anions
Ions
electrically charged molecules
Anions
negatively charged ions
Cations
positively charged ions
Intracellular Fluid
(cytoplasm) The water solution found within cells
Extracellular Fluid
The fluid in the spaces between cells
Cell Membrane
The lipid bilayer that encloses a cell
Microelectrode
An especially small electrode used to record electrical potentials inside living cells
Is inside of a neuron or the extracellular fluid more negative?
the inside of a neuron
Resting Potential
The difference in electrical potential across the membrane of a nerve cell at rest
Millivolts (mV)
A thousandth of a volt
What is the resting potential of a neuron?
about -50 to -80 thousandths of a volt (negative sign indicates that the cell’s interior is more negative than the outside)
Ion Channel
A protein that acts as a tube to allow ions of a specific type to pass through the membrane. Other definition: A pore in the cell membrane that permits the passage of certain ions through the membrane when the channel is open.
Selective Permeability
The property of a membrane that allows some substances to pass through, but not others.
What ions is selectively allowed to cross the membrane? Which ion is not?
Potassium ions (K+) are allowed to enter or exit the cell fairly freely. However, sodium ions (Na+) cannot
Diffusion
the tendency for molecules of a substance to spread from regions of high concentration to regions of low concentration
Electrostatic Pressure
The propensity of charged molecules or ions to move toward areas with the opposite charge
How does electrostatic pressure relate to the ions in and out of the cell?
Positively charged cations (potassium) are attracted to the negatively charged interior of the cell. Anions are repelled by the cell interior and tend to exit to the extracellular fluid
Sodium-Potassium Pumps
The energetically expensive mechanism that pushes sodium ions out of a cell, and potassium ions in
Equilibrium Potential
The point at which the movement of ions across the membrane is balanced, as the electrostatic pressure pulling ions in one direction is offset by the diffusion force pushing them in opposite direction (the cell’s resting potential)
Action Potentials
Very brief but large changes in the resting membrane potential that arise in the initial segment of the axon, just after the axon hillock. Other definition: A rapid reversal of the membrane potential that momentarily makes the inside of a neuron positive with respect to the outside.
Hyperpolarization
an increase in membrane potential (the neuron becomes more negative on the inside, relative to the outside)
Depolarization
A decrease in membrane potential (brings membrane potential closer to zero- makes the inside of the neuron more like the outside)
Local Potentials
An electrical potential that is initiated by a stimulation at a specific site, is a graded response that spreads passively across the cell membrane, and decreases in strength with time and distance.
What happens to when the axon is depolarized to -40 mV or so?
This point has reached the threshold, so the action potential is provoked
Threshold
The stimulus intensity that is just adequate to trigger an action potential in an axon
All-or-None Property
The condition that the size (amplitude) of the action potential is independent of the size of the stimulus. Larger depolarizations do not produce larger action potentials. It either fires at its full amplitude, or it doesn’t fire at all.
Afterpotentials
Changes exhibited by the axons immediately following a spike due to the movement of ions in and out of the cell.
Voltage-Gated Na+ Channel
A Na+ selective channel that opens or closes in response to changes in the voltage of the local membrane potential. It mediates the action potential. If the axon is depolarized enough to reach threshold levels, the channel’s shape changes, opening the gate to allow Na+ ions through for a short while.
Steps of Action Potential
- Open K+ channels create the resting potential
- Any depolarizing force will bring the membrane potential closer to threshold
- At threshold, voltage-gated Na+ channels open, causing a rapid change of polarity– the action potential
- Na+ channels automatically close again; gated K+ channels open, repolarizing and even hyperpolarizing the cell (afterpotential)
- All gated channels close. The cell returns to its resting potential
Refractory
unresponsive
Absolute Refractory Phase
A brief period immediately following the production of an action potential, no amount of stimulation can induce another action potential because the voltage-gated Na+ channels can’t respond
Relative Refractory
Period of reduced sensitivity after the absolute refractory phase. Only strong stimulation can depolarize the axon to threshold to produce another action potential
Conduction Velocity
The speed of action potentials along the length of an axon
How fast is neural conduction?
over 300 miles per hour; this high rate of conduction ensures rapid sensory and motor processing
Saltatory Conduction
The form of conduction that is characteristic of myelinated axons, in which the action potential jumps from one node of Ranvier to the next
Multiple Sclerosis (MS)
A disease in which myelin is compromised, with highly variable effects on brain function
What happens when the action potential reaches the end of an axon?
The axon releases a chemical (neurotransmitter) into the synapse
Postsynaptic Potentials
The brief changes in the membrane potential of the postsynaptic cell in response to neurotransmitter
Excitatory Postsynaptic Potential (EPSP)
Postsynaptic membrane depolarization that pushes the postsynaptic cell a little closer to the threshold for an action potential.
Inhibitory Postsynaptic Potential (IPSP)
The postsynaptic membrane potential becomes even more negative, or hyperpolarized. THis moves the cell membrane potential away from threshold- it decreases the probability that the neuron will fire an action potential
What determines whether a synapse excites or inhibits the postsynaptic cell?
- Whether the neurotransmitter released by the presynaptic cell generate and EPSP or generate an IPSE.
- Also depends on what sort of receptor the postsynaptic cell possesses.
Spatial Summation
The summation of postsynaptic potentials that reach the axon hillock from different locations across the cell body. If this summation reaches threshold, an action potential is triggered
Temporal Summation
The summation of postsynaptic potentials that reach the axon hillock at different times. The closer in time the potentials occur, the greater the summation
How is information arriving from various parts of the neuron weighted?
In terms of the distance to the axon hillock and the path resistance along the way
What are the steps that take place during chemical synaptic transmission?
- The action potential arrives at the presynaptic axon terminal
- Voltage-gated calcium channels in the membrane of the axon terminal open, allowing calcium ions (Ca2+) to enter
- Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft
- Transmitter molecules bind to special receptor molecules in the postsynaptic membrane, leading to the opening of ion channels in the postsynaptic membrane. The resulting flow of ions creates a local EPSP or IPSP in the postsynaptic neuron
- The IPSPs and EPSPs in the postsynaptic cell spread toward the axon hillock. (If sum of all the EPSPs and IPSPs depolarizes the axon hillock enough to reach threshold, and action potential will arise)
- Synaptic transmission is rapidly stopped, so the message is brief and accurately reflects the activity of the presynaptic cell
- Synaptic transmitter may also activate presynaptic receptors, as a way of monitoring the extent of transmitter release.
Synaptic Vesicles
A small, spherical structure that contains molecules of neurotransmitter
What happens when an action potential reaches a presynaptic terminal?
It causes hundreds of synaptic vesicles near the presynaptic membrane to fuse with the membrane and discharge molecules of neurotransmitter into the synaptic cleft
Synaptic Delay
The time needed for Ca2+ to enter the terminal, for the vesicles to fuse with the membrane, for the transmitter to diffuse across the synaptic cleft, and for transmitter molecules to interact with their receptors before the postsynaptic cell responds
Ligand
A substance that binds to receptor molecules, such as a neurotransmitter or drug that binds to postsynaptic receptors. Molecule has to be correct shape in order to fit into a receptor protein.
Acetylcholine (ACh)
A neurotransmitter that is produced and released by many neurons in the brain
Where does ACh go in the postsynaptic neuron?
Fits into areas called ligand-binding sites in neurotransmitter receptor molecules embedded in the postsynaptic membrane
Curare and Bungarotoxin
Potent poisons that block ACh receptors
Agonists
A molecule that mimics the actions of a neurotransmitter
Antagonists
Molecules that interfere with or prevent the action of a neurotransmitter
Because the activation of the synapse is brief, how many signals can the synapse send?
The synapse can signal over a thousand times per second, potentially sending a lot of information
Degradation
The chemical breakdown of a neurotransmitter by special enzymes (makes it inactive)
Reuptake
Transmitter molecules are swiftly cleared from the synaptic cleft by being absorbed back into the axon terminal that released them
Transporters
A specialized membrane component that returns transmitter molecules to the presynaptic neuron for reuse
After reuptake what can happen to the transmitter molecules?
The may be repackaged into newly formed synaptic vesicles to await re-release, conserving the resources for making new transmitter molecules
Axo-Dendritic Synapses
A synapse at which a presynaptic axon terminal synapses onto a dendrite of the postsynaptic neuron. From axon to dendrite
Axo-Somatic Synapses
A synapse at which a presynaptic axon terminal synapses onto the cell body (soma) of the postsynaptic neuron. From axon to cell body
Axo-Axonic Synapses
A synapse at which a presynaptic axon terminal synapses onto the axon terminal of another neuron
Dendro-Dendritic Synapses
A synapse at which a synaptic connection forms between the dendrites of two neurons
Why does the knee-jerk reflex happen so quickly?
Several factors:
1. both the sensory and the motor axons involved are myelinated and of large diameter, so they conduct action potentials rapidly
2. the sensory cells synapse directly on the motor neurons
3. Both the central synapse and the neuromuscular junction are fast synapses
Electroencephalograms (EEGs)
A recording of gross electrical activity of the brain via large electrodes placed on the scalp
Event-Related Potentials (ERPs)
Averaged EEG recordings measuring brain responses to repeated presentations of a stimulus
Epilepsy
A brain disorder marked by major, sudden changes in the electrophysiological state of the brain that are referred to as seizures
Seizures
A wave of abnormally synchronous electrical activity in the brain
Tonic-Clonic Seizures
The most severe seizure disorder the includes loss of consciousness and rhythmic convulsions. Accompanied by synchronized EEG activity all over the brain.
Simple Partial Seizures
More subtle seizures that have spike-and-wave EEG activity for 5-15 seconds at a time. Includes a loss of awareness and inability to recall events surrounding the seizure
Complex Partial Seizures
Seizures that do not involve the entire brain and thus can produce a wide variety of symptoms, often preceded by an unusual sensation, or aura
