how neurons generate signals the neuron at rest. stepping on a thumbtack reflexive withdrawal of...

How Neurons Generate SignalsThe Neuron at Rest

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.

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.

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.

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

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.

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

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.

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.