Neurons and Electrical Signaling (CBNS 120)

Study Notes

Neurons have two main types of membrane: passive and active
Passive membranes have a constant electrical potential, while active membranes have a dynamic electrical potential that can change in response to stimuli
Examples of active membranes include "endogenous slow potentials" and "burster" neurons, which can generate repetitive action potentials

Information flows in a predictable and consistent direction within a nerve cell
Information flow begins at the receiving (input) sites on the dendrites and soma, and ends at the trigger zone at the axon hillock (initial segment)
At the axon hillock, the action potential is initiated and propagated unidirectionally along the axon to the presynaptic transmitter-release sites at the axon terminal

Based on morphology:
- Multipolar neurons (e.g. turtle spinal motor neurons)
- Unipolar neurons (e.g. crayfish swimmeret motor neurons)
Based on function and inputs/outputs:
- Motor neurons (efferents, effector cells)
  - Synapse onto muscle or gland tissue
  - In vertebrates, all motor neurons are excitatory
- Sensory neurons (afferents)
  - Transduce sensory information
  - Examples: heat, cold, light, mechanical pressure or stretch, chemical energy
- Neuroendocrine cells
  - Release neurohormones into the circulation
- Interneurons
  - Are both postsynaptic and presynaptic to other neurons
  - Can be either local- or projection-type interneurons

Local graded potentials:
- Graded changes in membrane potential that vary continuously in amplitude with stimulus strength
- Decay exponentially over distance
Action potentials:
- Transient, all-or-none reversal of the membrane potential produced by a regenerative inward current
- Do not decay over distance
Signal transduction in a typical vertebrate neuron:
- Local graded potentials can depolarize the neuron, leading to the generation of action potentials
- The more a local graded potential depolarizes the neuron, the higher the action potential frequency evoked

Membrane potential (EM): voltage or electrical potential (mV) across cell membranes
Depolarization: decrease in EM, or movement of EM in a positive direction
Hyperpolarization: increase in EM, or movement of EM in a negative direction
Synaptic potentials: 3 major types: chemical excitatory, chemical inhibitory, electrical

Extracellular recording:
- "Differential" extracellular recording from axons and whole nerves
- "Single-ended" extracellular recording from neuronal cell bodies
Intracellular recording:
- Sharp electrode and whole-cell patch
Optical recording:
- Voltage-sensitive dyes
- Calcium imaging

Golgi stain:
- Stains only an occasional neuron (about 5%) in a slice of tissue
- Enables visualization of entire nerve cells in very thick sections of brain tissue
Intracellular dye injection:
- Enables study of both electrophysiology and morphology of single neurons
- Examples: horseradish peroxidase (HRP), cobalt, biocytin, or Lucifer Yellow

Use of light to excite or inhibit cells, usually neurons, that have been genetically modified to express light-sensitive ion channels or pumps
Examples: channelrhodopsins (Ch1 and Ch2), halorhodopsin, and archaerhodopsin
Enables precise control of neural activity with high spatial and temporal resolution