COGNITIVE SCIENCE 17

The Electric Brain

Part 1 Jaime A. Pineda, Ph.D.

INPUT

INTEGRATION

OUTPUT

Law of Dynamic Polarization Dendrites generally receive synaptic input (i.e. are postsynaptic) and axons generally send synaptic output (i.e., are presynaptic)

Resting Membrane Potential (Vm) is ~ -70 mV in mammals

Vm is the voltage difference between the inside and outside of the cell

50 440

400 20

40 560

many few

Different concentrations of ions inside and outside the cell

Unequal distribution of ions across cellmembrane creates two major forces

osmotic pressure

electrostatic pressure

Other important factors

- Specific protein channels- Na+, K+, Cl-, Ca++

- Selective permeability of channels- leaky channels- K+ > Cl- > Na+

- Sodium-potassium pump

Na+ / K+ Pump

Restores equilibrium

Graded and Action Potentials•Graded

–Are small changes in membrane potential

–Are conducted passively

–Lose energy as they travel down dendrite/axon

•Action

–Are large changes in membrane potential

–Are conducted actively (regenerated)

–Do not lose energy as they travel down the axon

Graded Potentials

• Depolarization (EPSP)– Vm becomes more positive (Na+ flows in) and

the neuron is more likely to produce an action potential

• Hyperpolarization (IPSP)– Vm becomes more negative (K+ flows out) and

the neuron is less like to produce an action potential

Graded Potentials

• AM signals

• Degrade with distance (decremental conduction)

• Short distance

signaling V

Distance from source

Action Potentials

• FM signals

• Do not degrade with distance (non decremental conduction)

• Long distance signaling

Phases of the Action Potential

Firing threshold is the point at which the number ofactivated Na+ channels > inactivated Na+ channels

Absolute refractory period

Relative refractory period