The Mind's Machine Chapter 3: Neurophysiology

Neurophysiology

the study of the life processes of neurons

Ion

an atom or molecule that has acquired an electrical charge by gaining or losing one or more electrons

Anion

a negatively charged ion, such as a protein or chloride ion

Cation

a positively charged ion, such as a potassium or sodium ion

Intracellular Fluid

also called cytoplasm; the watery solution found within cells

Extracellular Fluid

the fluid in spaces between cells

Cell Membrane

the lipid bilayer that ensheathes a cell

Microelectrode

an especially small electrode used to record electrical potentials inside living cells

Resting Potential

the difference in electrical potential across the membrane of a nerve cell at rest

Millivolt (mV)

a thousandth of a volt

Ion Channel

a pore in the cell membrane that permits the passage of certain ions through the membrane when the channels are open

Potassium Ion (K+)

an atom that carries a positive charge

Sodium Ion (Na+)

an atom that carries a positive charge

Selective Permeability

the property of a membrane that allows some substances to pass through, but not others

Diffusion

the spontaneous spread of molecules from an area of high concentration to an area of low concentration until a uniform concentration is absorbed

Electrostatic Pressure

the propensity of charged molecules or ions to move toward areas with the opposite charge

Sodium-Potassium Pump

the energetically expensive mechanism that pushes sodium out of a cell, and potassium ions in

Equilibrium Potential

the point at which the movement of ions across the cell membrane is balanced, as the electrostatic pressure pulling ions in one direction is offset by the diffusion force pushing them in the opposite direction

Axon Hillock

the cone-shaped area on the cell body from which the axon originates

Hyperpolarization

an increase in membrane potential (the interior of the neuron becomes even more negative)

Depolarization

a decrease in membrane potential (the interior of the neuron becomes less negative)

Local Potential

an electrical potential that is initiated by stimulation at a specific site, which is a graded response that spreads passively across the cell membrane, decreasing in strength with time and distance

Threshold

the stimuli intensity that is just adequate to trigger an action potential in an axon

Action Potential

a rapid reversal of the membrane potential that momentarily makes the inside of the membrane positive with respect to the outside

All-or-None Property

referring to the fact that the size (amplitude) of the action potential is independent of the size of the stimulus

Afterpotential

the positive or negative change in membrane potential that may follow action potential

Voltage-Gated Na+ Channel

selective channel that opens or closes in response to changes in the voltage of the local membrane potential; mediates the action potential

Refractory

temporarily unresponsive or inactivated

Absolute Refractory Phase

a brief period of complete insensitivity to stimuli

Relative Refractory Phase

a period of reduced sensitivity during which only strong stimulation produces an action potential

Conduction Velocity

the speed of which an action potential is propagated along the length of an axon

Myelin

the fatty insulation around an axon, formed by glial cells; this sheath boosts the speed at which action potentials are conducted

Nodes of Ranvier

a gap between successive segments of the myelin sheath where the axon membrane is exposed

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)

literally "many scars"; a disorder characterized by the widespread degeneration of myelin

Neurotransmitter

the chemical released from the presynaptic axon terminal that serves as the basis of communications between neurons

Presynaptic

located on the "transmitting" side of a synapse

Postsynaptic

referring to the region of a synapse that receives and responds to a neurotransmitter

Postsynaptic Potential

a local potential that is initiated by stimulation at a synapse, which can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance

Excitatory Postsynaptic Potential (EPSP)

a depolarizing potential in the postsynaptic neuron that is normally caused by synaptic excitation; increases the probability that the postsynaptic neuron will fire an action potential

Inhibitory Postsynaptic Potential (IPSP)

a hyperpolarizing potential in the postsynaptic neuron; decreases the probability that the postsynaptic neuron will find an action potential

Chloride Ion (Cl-)

an atom that carries a negative charge

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 fired

Temporal Summation

the summation of postsynaptic potentials that reach the axon hillock at different times; the close in time the potentials occur, the more complete the summation is

Synaptic Transmission Sequence of Events

1. Action potential arrives at the presynaptic axon terminal
2. voltage-gated calcium channels in the membrane of the axon terminal open, allowing calcium ions (Ca++) to enter
3. Ca^2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft
4. Transmitter molecules bind to special receptor molecules in the postsynaptic membrane, leading - directly or indirectly - 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
5. The IPSPs and EPSPs in the postsynaptic cell spread toward the axon hillock (if the sum of all the EPSPs and IPSPs ultimately depolarizes the axon hillock enough to reach threshold, and action potential will arise.)
6. Synaptic transmission is rapidly stopped, so the message is brief and accurately reflects the activity of the presynaptic cell
7. synaptic transmitter may also activate presynaptic receptors, resulting in a decrease in transmitter release

synaptic vesicles

a small, spherical structure that contains molecules of neurotransmitter

synaptic cleft

the space between the presynaptic and postsynaptic neurons at a synapse; the gap measures about 20-40 nm

calcium ion (Ca2+)

a calcium atom that carries a double positive charge

synaptic delay

the brief delay between the arrival of an action potential at the axon terminal and the creation of a postsynaptic potential

ligand

a substance that binds to receptor molecules, such as a neurotransmitter or drug that binds postsynaptic receptors

Acetylcholine (ACh)

A neurotransmitter that is produced and released by parasympathetic postganglionic neurons, by motoneurons, and by neurons throughout the brain.

neurotransmitter receptor

Also called simply receptor. A specialized protein, often embedded in the cell membrane, that selectively senses and reacts to molecules of a corresponding neurotransmitter or hormone.

curare

a neurotoxin that causes paralysis by blocking acetylcholine receptors in muscle

bungarotoxin

A neurotoxin, isolated from the venom of the many-banded krait, that selectively blocks acetylcholine receptors.

Agonist

a substance that mimics or potentiates the actions of a transmitter or other signaling molecule

antagonist

a substance that blocks or attenuates the actions of a transmitter or signaling molecule

Cholinergic

Referring to cells that use acetylcholine as their synaptic transmitter.

degradation

the chemical breakdown of a neurotransmitter into inactive metabolites

acetlycholinesterase (AChE)

an enzyme that inactivates the transmitter acetylcholine

reuptake

the process by which released synaptic transmitter molecules are taken up and reused by the presynaptic neuron, thus stopping synaptic activity

transporter

a specialized membrane component that returns transmitter molecules to the presynaptic neuron for reuse

axo-dendritic synapse

a synapse at which a presynaptic axon terminal synapses onto a dendrite of the postsynaptic neuron, either via a dendritic spine or directly onto the dendrite itself

axo-somatic synapse

A synapse at which a presynaptic axon terminal synapses onto the cell body (soma) of the postsynaptic neuron.

axo-axonic synapse

a synapse at which a presynaptic axon terminal synapses onto the axon terminal of another neuron

dendro-dendritic synapse

a synapse at which a synaptic connection forms between the dendrites of two neurons

knee jerk reflex

a variant of the stretch reflex in which stretching of the tendon beneath the knee leads to an upward kick of the leg

Electroencephalogram (EEG)

a recording of gross electrical activity of the brain via large electrodes placed on the scalp

event-related potential (ERP)

Also called evoked potential. Averaged EEG recordings measuring brain responses to repeated presentations of a stimulus. Components of the ERP tend to be reliable because the background noise of the cortex has been averaged out.

Epilepsy

a brain disorder marked by major, sudden changes in the electrophysiological state of the brain that are referred to as seizures

seizure

a wave of abnormally synchronous electrical activity in the brain

tonic-clonic seizure

Also called grand mal seizure. A type of generalized epileptic seizure in which nerve cells fire in high-frequency bursts, usually accompanied by involuntary rhythmic contractions of the body

simple partial seizure

Also called absence attack. A seizure that is characterized by a spike-and-wave EEG and often involves a loss of awareness and inability to recall events surrounding the seizure.

complex partial seizure

in epilepsy, a type of seizure that doesn't involve the entire brain and therefore can cause a wide variety of symptoms

aura

in epilepsy, the unusual sensations or premonition that may precede the beginning of a seizure

Electrical signals are the vocabulary of the nervous system

Neurons are more negative on the inside than on the outside, so we say they are polarized
- resting potential
- resting potential of the neuron reflects a balancing act between two opposing processes that drive K+ ions in and out of the neuron
- first is diffusion
- second is electrostatic pressure

A threshold amount of depolarization triggers an action potential
- two concepts are central to understanding how action potentials are triggered
- hyper polarization: increase in membrane potential
- depolarization: decrease in membrane potential
- action potential
- all or none property of action potential

Ionic mechanisms underlie the action potential
- action potential created by the sudden movement of Na+ ions into the axon
- applying pairs of stimuli that are spaced closer and closer together reveals a related phenomenon: beyond a certain point, only the first stimulus is able to elicit an action potential
- axonal membrane is said to be refractory (unresponsive) to the second stimulus

Action potentials are actively propagated along the axon
- the action potential is regenerated along the length of the axon
- axon potentials like flushing a toilet
- conduction velocity varies with the diameter of the axon
- fastest conduction velocities require more than just large axons
- myelin sheath greatly speeds conduction

Synapses cause local changes in the postsynaptic membrane potential
- when an axon releases neurotransmitter molecules into a synapse, they briefly alter the membrane potential of the other cell. because information is moving from the axon to the target cell on the other side of the synapse, we say that axon is from the presynaptic cell, and the target neuron on the other side of the synapse is the postsynaptic cell
- excitatory and inhibitory neurons get their names from their actions on postsynaptic neurons, not from their effects on behavior

Spatial summation and temporal summation integrate synaptic inputs
- neurons process information by integrating the postsynaptic potentials through both spatial summation (summing potentials from different locations) and temporal summations (summing potentials across time)

identify the two physical forces that make neurons more negatively charged inside than outside

Two opposing processes drive K+ ions in and out of the neuron: Diffusion and Electrostatic Pressure

Diffusion: tendency of molecules of a substance tot spread from regions of high concentration too regions of low concentration

Electrostatic Pressure: charged particles exert electrical forces one another: like charges repel, opposites attract
- K+ ions are attracted tot the negatively charged interior of the cell

understand the changes in a neuron's membrane that produces a large electrical signal called an action potential

Hyper-polarization:
- increasing the strength of hyper- polarizing stimuli leads to greater hyper-polarization of the neuron.
- farther from the stimulating electron, hyper polarization occurs simultaneously but is diminished

Depolarization:
- Increasing the strength of depolarizing stimuli leads to increasing depolarization of the neuron until the threshold is reached and an action potential is generated

+ size of action potential is independent of stimulus size
+ all or nothing property: fires full force or doesn't at all

explain the changes in channels and movement of ions that underlie the action potential

- the action potential is created by the sudden movement of Na+ ions into the axon

1. open K+ channels create the resting potential
2. any depolarizing force will bring the membrane potential closer to threshold
3. at threshold, voltage-gated Na+ channels open, causing a rapid change of polarity - the action potential
4. Na+ channels automatically close again; gated K+ channels open, depolarizing and even hyper polarizing the cell (after potential)
5. All gated channels close, the cell returns to its resting potential

understand how the action potential spreads along the length of an axon

* the action potential is regenerated along the length of the axon
- Na+ channels open, generating an action potential
- Myelin channels the depolarization down the axon interior
- Depolarization spreads pithing the axon very rapidly, like electricity through a wire
- the depolarized Na+ channels open, re-creating the action potential at the new node....
- ...and so on, hopping down the nodes in saltatory conduction

understand how each neuron uses electrical signals to integrate information from other neurons

Synapses cause local changes in the postsynaptic membrane potential
- when an axon releases neurotransmitter molecules into a synapse, they briefly alter the membrane potential of the other cell. because information is moving from the axon to the target cell on the other side of the synapse, we say that axon is from the presynaptic cell, and the target neuron on the other side of the synapse is the postsynaptic cell
- excitatory and inhibitory neurons get their names from their actions on postsynaptic neurons, not from their effects on behavior

Spatial summation and temporal summation integrate synaptic inputs
- neurons process information by integrating the postsynaptic potentials through both spatial summation (summing potentials from different locations) and temporal summations (summing potentials across time)

Synaptic transmission requires a sequence of events

The steps that take place during chemical synaptic transmission are summarized
1. the action potential arrives at the presynaptic axon terminal
2. voltage-gated calcium channels in the membrane of the axon terminal open, allowing calcium ions to enter
3. Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft
4. transmitter molecules bind to special receptor molecules in the postsynaptic membrane, leading-directly or indirectly- 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
5. the IPSPs and EPSPs in the postsynaptic cell spread toward the axon hillock (if the sum of all the EPSPs and IPSPs ultimately depolarizes the axon hillock enough to reach threshold, an action potential will arise)
6. synaptic transmission is rapidly stopped, so the message is brief and accurately reflects the activity of the presynaptic cell
7. synaptic transmitter may also activate presynaptic receptors, resulting in a decrease in transmitter release

Action potentials cause the release of transmitter molecules into the synaptic cleft

Receptor molecules recognize transmitters
- synaptic transmission occurs when a chemical neurotransmitter diffuses across the synaptic cleft and binds to neurotransmitter receptor in the postsynaptic membrane

The action of synaptic transmitters is stopped rapidly
- 2 processes bring transmitter effects to a prompt halt:
1. degradation
2. reuptake

Neural circuits underlie reflexes

identify the sequence of steps that take place when one neuron releases a chemical signal to affect another

1. the action potential arrives at the presynaptic axon terminal
2. voltage-gated calcium channels in the membrane of the axon terminal open, allowing calcium ions to enter
3. Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft
4. transmitter molecules bind to special receptor molecules in the postsynaptic membrane, leading-directly or indirectly- 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
5. the IPSPs and EPSPs in the postsynaptic cell spread toward the axon hillock (if the sum of all the EPSPs and IPSPs ultimately depolarizes the axon hillock enough to reach threshold, an action potential will arise)
6. synaptic transmission is rapidly stopped, so the message is brief and accurately reflects the activity of the presynaptic cell
7. synaptic transmitter may also activate presynaptic receptors, resulting in a decrease in transmitter release

understand how a variety of chemical signals enables a diversity of neuronal responses to other neurons

the nature of the postsynaptic receptors at a synapse determines the action of the transmitter

ACh can function as either an inhibitory or an excitatory neurotransmitter, at different synapses. at excitatory synapses, binding of ACh to on retype of receptor opens channels for Na+ and K+ ions. at inhibitory synapses, ACh may act o another type of receptor to open channels that allow Cl- ions to ever, thereby hyperpolarizing the membrane

- the lock and key analogy is strengthened by the observation that various chemicals can fit onto receptor proteins and block the entrance of the key.

identify the interactions between neurons and muscles that underlie a simple reflex

- action potentials are triggered when he stretch receptor reaches threshold and speed along large sensory axons at about 100 m/s
- sensory neuron releases the neurotransmitter glutamate. about 0.5 ms later, excitatory postsynaptic potential (EPSP) appears in motor neuron
- EPSP spreads passively to axon hillock, where it triggers action potentials
- action potentials reach neuromuscular junctions. ACh is released as the neurotransmitter
- neuromuscular junction potential starts about 0.5 ms after arrival of presynaptic action potential. action potentials venerated in muscle fibers cause contractions that kick the leg out about 40 ms after the tap (referring to knee jerk reflex)

EEGs measure gross electrical activity of the human brain

electrical storms in the brain can cause seizures
- summing electrical activity over millions of nerve cells as detected by electrodes on the scalp, EEGs can reveal rapid changes in brain function - for example, in response to a brief, controlled stimulus that evokes an event-related potential (ERP). they can also reveal a seizure in someone with epilepsy.