PSYC 370: Nerve Impulse

Basic Processes of Brain Function

neuronal transmission within a neuron, neurotransmission between neurons

Neuronal Modes

neurons can be at rest, excited, or inhibited

Cell Membrane

lipid bilayer, prevents transfer of most particles btw the inside and outside of the cell; protein channels/gates embedded

Lipid Bilayer

two layers of phospholipids

Cell Membrane Protein Channels/Gates

can be opened/closed; special doors allow transfer of particles btw inside and outside of cell

Ion Channels/Ion Gates

particles can only pass thru membrane thru ion channels or ion gates

Ions

small molecules with net electrical charge (pos or neg)

Proteins

too large to pass even through channels; negatively charged

Uneven Distribution of Particles Across Membrane

higher conc of negatively charged particles inside the neuron, higher conc of pos charged particles outside neuron

Membrane Potential

difference in electrical charge btw inside and outside of neuron; difference in distribution of pos and neg ions across the membrane

Resting Potential

membrane potential of neuron at rest; -70 mV on average; stored energy; store of neg energy on intracellular side relative to extracellular side; caused by excess neg proteins inside cell and by excess pos Na+ ions outside the cell

Sodium Potassium Pump

restores ions to its original place, Na+ outside neuron and K+ inside neuron; active mechanism; pumps 3 Na+ out and pulls in 2K+ ions; net exporter of positive charge (makes inside more neg)

4 Principles Keeping Ions in Place During Resting Potential

selective permeability, diffusion and conc gradients, electrostatic gradients, sodium/potassium pump

Selective Permeability

property of cell membranes that only allows certain molecules to enter or exit the cell; impermeable to proteins (too large), low permeability to Na (most sodium channels closed), low permeability to potassium (most gates are closed), high permeability to Cl (gates are open and Cl- could pass thru membrane)

Diffusion

force that moves particles from area of high conc to area of low conc

Equilibrium

end result of diffusion forces; particles are equally distributed in space

Electrostatic Forces

forces of attraction or repulsion between charged particles

Electrostatic Forces Gradient

Cl is neg and attracted to pos charges outside keeping Cl out; K is pos and attracted to neg inside so it stays in; Na is pos and attracted to negative inside but closed channels prevent crossing

Sodium Forces

conc gradient pushes it into cell; electrostatic forces attract into cell; selective permeability keeps it out of cell

Potassium Forces

conc gradient pushes it out of cell; electrostatic forces attract into the cell; selective permeability keeps it inside cell

Chloride Forces

conc gradient pushes Cl- inside cell, electrostatic forces push Cl- out of cell

Graded Potentials

small voltage fluctuation in cell membrane, can lead to depolarization or hyperpolarization

Depolarization

change in voltage in pos direction, inside becomes more positive; due to influx of Na+ thru Na+ channels

Hyperpolarization

change in voltage in neg direction; inside becomes more negative; due to efflux of K+ making extracellular side more pos or influx of Cl-

Action Potential Threshold

small depolarizations lead to voltage potential of -50 mV and all sodium channels open at same time, sodium rushes inside neuron and causes rapid depolarization

-50mV

threshold; voltage at which action potential is triggered

Action Potential

rapid and transient change in membrane potential; results in neuronal activity, neuron fires and sends signals to other neurons; only takes place in axon not in dendrites, originates in axon hillock and travels down axon

Repolarization

at peak of action potential Na+ channels close and K+ voltage gated channels open; K+ gets out, inside becomes less pos, and eventually negative again

Refractory Period

K+ channels stay open too long; K+ overshoot, state of hyperpolarization; period of hyperpolarization when it is difficult to generate another action potential

Propagation of Action Potential

axon potential begins at axon hillock; when neuron is depolarized Na+ ions enter cell and spread along axon in both directions; as Na+ spreads away from action potential it depolarizes that section of the axon and voltage gated channels open to propagate the action potential; each point along the membrane generates an action potential

What Prevents Action Potential From Traveling in Opposite Direction

refractory period

Myelin Sheath

speed ups propagation of action potential, need speed in mammalian brains; fatty glia cells; allows electrical conduction Pos

Myelin in Brain and Spinal Cord

oligodendrocytes

Myelin in Peripheral Nerves

schwann cells

Nodes of Ranvier

sections of axon not covered by myelin sheath, contain Na+ gates, sections where action potentials are generated

Postsynaptic Potentials

occur postsynaptically (in postsynaptic neuron)

EPSPs - Excitatory Postsynaptic Potentials

brief and small depolarization (graded depolarization), lead to some Na+ going into the cell

IPSPs - Inhibitory Postsynaptic Potentials

brief and small hyperpolarization of neuron membrane (grade hyperpolarization); lead to some K+ to leave the cell or Cl- to get into the cell

Individual EPSPs and IPSPs

subthreshold changes in voltage; cannot depolarize the membrane to reach threshold on their own

Mechanisms to Add EPSPs and IPSPs

temporal summation and spatial summation

Temporal Summation

same location different times; repeated potentials over time that occur at same location

Spatial Summation

graded potentials happen at same time on different dendrites

Axon Hillock

junction of cell body and axon, rich in voltage sensitive channels, where action potentials are initiated