Action potentials and nervous system organization

• Gray text pages 183-185, 166-179

Brain organization

• Gray text pages 179-182, 191-199

Levels of Neural Organization

To understand the brain understand all levels

Neurons

Functions

• Signaling
• Information transfer
  • Within cells
  • Between cells

Types of Neurons

• Neuroglia
  • Astrocytes
  • Myelin
    • Oligodendrocytes
    • Schwann cells
• Neurons
  • Sensory
  • Interneuron
  • Motor

Structure

• Soma
• Axon
• Terminal
• Dendrites

Soma

Like any other cell

• Nucleus, mitochondria, plasma membrane

Axon

"Telephone wires" of the nervous system

• Communication
• Connect to
  • Dendrites
  • Soma

Myelin sheathe

• Oligodendrocytes (CNS)
• Schwann cells (PNS)

Axonal transport

• Microtubules ("axon escalators")
  • Cytoskeleton
• Neurotransmitters
• Proteins

Axon Terminal

The "transmitter"

Connects with

• Dendrites (common)
• Soma (common)
• Other axons (rare)

Communicates via

• Chemical transmitters
  • Neurotransmitters
• Electrical signals
• Excitatory or inhibitory signals

Dendrites

The "receivers"

Connects to axon terminals

Transfers signal received from another neuron to the soma

Axons and dendrites form synapses!

The Synapse

Where neurons communicate with one another

• Presynaptic cell
  • Transmitter
• Postsynaptic cell
  • Receiver
• Chemical and electrical synapses

Two hearts, no brain...

• Demonstrates chemical (neurotransmitter) synapse

Chemical Synapses

Asymmetric morphology

• Presynaptic
  • Axon terminal (contains neurotransmitter)
• Postsynaptic
  • Dendrite or soma

Synaptic cleft

• Space between pre- and postsynaptic elements

Transmission of signal

• Axon terminal releases neurotransmitter
• Neurotransmitters diffuse across synaptic cleft
• Postsynaptic receptors uptake neurotransmitter
• Signal travels to next neuron (unidirectional!)

Electrical Synapses

Symmetrical morphology

Pre- and postsynaptic elements physically indistinguishable

Bi-directional!

Gap junctions

• Cell-to-cell pores

Ion movement through gap junction

• Mediates signaling
• Low resistance pathways

Very fast

• Neurons are physically coupled
• No synaptic cleft to diffuse across

Dye coupling: Are their gap junctions here?

• Lucifer Yellow

Brain Drain...

Computers are often compared to human brains. In what ways are computers similar to brains? How are computers different from brains?

What neuronal tasks would benefit more from electrical synapses than chemical synapses?

What would go wrong if action potentials could travel both up and down axons?

The peripheral nervous system (PNS) can regenerate from damage whereas the central nervous system (CNS) can not. Could this possibly have some evolutionary advantage?