Neural Propagation

DPT 5035

Graded/Local Potentials

Small local voltage changes, of varying (“graded”) amplitudes (can be of multiple sizes– less vs. more channels)

Graded potentials are seen at three general locations. Where are they?

1. Sensory Receptors: always excitatory

2. Neuromuscular Junction: always excitatory

3. Synaptic potential: either excitatory (move toward zero) or inhibitory (take away from zero)

EPSP (synapse specific)

Excitatory post-synaptic potential

IPSP (synapse specific)

Inhibitory post-synaptic potential

Sodium channels cause what type of post-synaptic potentials?

Excitatory

Potassium channels cause what type of post-synaptic potentials?

Inhibitory

Synaptic Potentials

• Produced by opening of different ion channels in the neuron membrane

• EPSPs and IPSPs occur only in nerve cells, at synapses

• Not at sensory receptors or muscle end plates

• Exist only locally on small area of neuron membrane, spreading only very short distances.

• Are not actively propagated along long axons

EPSP

• A small graded electrical potential (depolarizing), that increases the excitability of the post-synaptic neuron

• Depolarizes the inside of the neuron, bringing the resting potential closer to zero.

• Increases the excitability of the neuron, bringing the potential closer to threshold for an action potential

• Triggered by binding of a chemical neurotransmitter at a synapse site (dendrites or soma)

• Ion channels open (only briefly) that allow more neuron permeability to Na+, resulting in increased positive charge entering the neuron

IPSP

• A small graded potential (hyperpolarizing) that decreases the excitability of the neuron, making it less likely to have an Action Potential

• Ion channels open, allowing for more neuron permeability to K+ or Cl-

• Increased positive charge (K+) leaving or negative charge (Cl-) entering

• Resting potential declines from -65 mV toward the Cl- and K+ equilibrium potentials of -90mV

• Channels are open only briefly, so only few ions flow

• Membrane Potential goes from -65 to -75mV, making it less likely to generate an Action Potential

Passive Propagation

• For spread of graded potentials (EPSPs & IPSPs)

• Depends only on diffusion of ions (ions diffuse from the synapse site on the soma to the axon hillock)

• Passive because it does not require cell energy (ATP) or opening of voltage-gated ion channels

• Works only for short distances

• Called electrotonic propagation

• Can occur bidirectionally

Passive Propagation Picture

• Leakage of ions occurs

• Graded potential decays as it spreads over time and distance

Temporal Summation

• At one site, two potentials occur sequentially (close together in time)

• The two potentials can add together, creating a larger graded potential and move the membrane potential farther from resting (A+A+A…)

Spatial Summation

• Potentials come in from two different synapses in different locations onto one cell

• The graded potentials passively spread along until they occupy the same space of membrane

Temporal Summation and Spatial Summation Picture

Slide 11