Lecture 23: Neuronal Physiology Part 1

What are the 4 distinct domains of a neuron?

• dendrites

• soma

• axon

• presynaptic terminals

What is true of an axon that goes to a distant brain area?

• example is a projection neuron

• functional implication: affects different brain area

• structure: dorsal root ganglion

What is true of an axon that stays in a local brain area?

• example is an interneuron

• functional implication: affects only nearby neurons

• structure: retinal bipolar cell

What is true of the pyramid-shaped spread of dendrites?

• example is a pyramidal cell

• functional implication: large area for receiving synaptic input; determines pattern of incoming axons that can interact with the cell

• structure: pyramidal cell

What is true of the radial-shaped spread of dendrites?

• example is a stellate cell

• functional implication: large area for receiving synaptic input; determines pattern of incoming axons that can interact with the cell

• stellate cell (contains dendritic spines)

What is true of a neuron that has one process that exits the cell body?

• example is a unipolar neuron (dorsal root ganglion cell)

• functional implication: small area for receiving synaptic input; highly specialized function

What is true of a neuron that has two processes that exit the cell body?

• example is a bipolar neuron (retinal bipolar cell)

• functional implication: small area for receiving synaptic input; highly specialized function

What is true of a neuron that has many processes that exit the cell body?

• example is a multipolar neuron (spinal motor neuron)

• functional implication: large area for receiving synaptic input; determines pattern of incoming axons that can interact with the cell

How do postsynaptic potentials look in the CNS?

• PSPs are graded, and decay with distance from site of synaptic input

• typically, a single EPSP is rarely strong enough to drive action potential firing on its own

What is spatial summation?

multiple sites of input at the same time

What is temporal summation?

multiple inputs over a short span of time (either from same presynaptic terminal or multiple terminals)

What does high internal resistance look like in a dendrite?

small diameter, not a lot of cytoplasm in dendrites in which ions can flow

What does low membrane resistance look like in a dendrite?

• no myelination

• channels permit leak of ionic current across membrane

What makes a dendrite leaky?

smaller length constant, low conductivity

Why might an EPSP of the same size at the synapse trigger an action potential in one neuron but not another?

EPSPs spread passively along dendrites and decay exponentially with distance. A dendrite with a long length constant (λ) loses less signal, so the EPSP can still reach threshold at the axon hillock. A dendrite with a short λ attenuates the EPSP below threshold, so no action potential fires.

How does dendrite length (relative to λ) affect passive PSP attenuation?

The longer the dendrite relative to λ, the more the PSP attenuates. An infinitely long dendrite decays to ~37% at one λ. A dendrite exactly 1λ long with a sealed end retains ~65% at its end. A very short dendrite (1/10 λ) shows almost no decay. Active membranes (with voltage-gated channels) show no attenuation at all.

What makes a dendrite active?

adding additional inward (depolarizing) current as signals propagate from distal dendrites to soma

What can be distributed along the length of the dendrite or clustered at certain sites?

voltage-gated channels

How can you boost synaptic signals?

signals can be boosted even if voltage-gated ion channel density is too low to generate APs

What is the difference between passive and active integration of postsynaptic potentials?

• Passive: EPSPs attenuate as they spread along the dendrite and simply sum at the soma, so many synaptic inputs are needed to reach threshold.

• Active: voltage-gated channels in the dendrite generate a dendritic spike that amplifies the input, so fewer synapses are needed to trigger an action potential.

What are the characteristics of dendritic spikes?

• slower, weaker than axonal APs

  • mediated by Ca²⁺ rather than Na⁺

• more depolarized threshold V_m for generation

• can provide retrograde signal that tells dendritic synapses that the neurons has generated an output

In Purkinje cells, what kinds of spikes occur in the dendrites vs. the soma, and why?

Dendrites have mostly Ca²⁺ channels, so they generate broad, slow Ca²⁺ spikes. The soma has mostly Na⁺ channels, so it generates fast, tall Na⁺ spikes (action potentials). Na⁺ spikes backpropagate into the dendrites but attenuate with distance.