How Neurons Generate Signals

1. How Neurons Generate Signals The Neuron at Rest

2. Stepping on a Thumbtack • Reflexive withdrawal of the foot – a simple behavior controlled by a circuit with direct connections between: • Sensory neurons (responding to environment) • Motor neurons (controlling muscles) • Interneurons (to inhibit opposing muscles) • Projection neurons (carrying sensation to the brain) • Messages are carried via action potentials.

3. Biochemical Reactions • Ion – an electrically charged atom. • Polar covalent bonds – a molecule held together by sharing electrons (H2O). • Ionic bond – a molecule held together by the attraction of atoms with opposite charges (NaCl – table salt). • Cation – ion with a net positive charge. • Anion – ion with a net negative charge.

4. The Neuron at Rest • Neurons have potassium (K+) inside and sodium (Na+) outside in the extracellular fluid. • Ion channels in the cell wall (membrane) are selectively permeable to potassium, sodium or calcium. • Ion pumps maintain the cell’s inner environment.

5. How Ions Cross the Membrane • Diffusion – an ionic concentration gradient exists • Differences in electrical membrane potential and equilibrium potential • Ionic driving force • Ion pumps • Sodium/potassium, calcium

6. Electricity • Opposite charges attract, like charges repel. • Current (I) – movement of electrical charge, measured in amps. • Potential (voltage, V) – force exerted on a particle, difference between terminals, measured in volts. • Conductance (g) – relative ability of a charge to migrate, measured in Siemens.

7. Resting Potential • Membrane potential is voltage across the neuronal membrane. • Resting potential is the point at which all of the forces acting upon ions are in balance (equilibrium). • Diffusional and electrical forces are equal. • Vm = -65 mV

8. Two Important Equations • Nernst equation – calculates the equilibrium potential for a single ion, given knowledge of its concentration inside & outside the neuron. • Varies with body temperature. • Goldman equation – calculates the resting potential of a neuron, given knowledge of its permeability to various ions. • Varies with ion concentrations.

9. Regulation of Potassium • The neuron at rest is mostly permeable to potassium (K+) so the resting potential is close to EK. • Increasing potassium outside the cell leads to depolarization (an increase in the resting potential making it less negative). • To prevent this, potassium is tightly regulated by the body via glia & blood-brain barrier.