Membrane Potential

Voltage difference across plasma membrane

Resting potential

• -40 mV to -90 mV
• Negative relative to extracellular

Depolarized

• Less negative than resting potential

Hyperpolarized

• More negative than resting potential

Action Potential

Message that goes down the axon

A large depolarization of the axon

• "Firing" of the axon

Threshold

• Critical point

All or none

• Either threshold is reached or nothing happens
• Can not stop an action potential

Refractory period

Frequency coding

• Larger depolarizing stimulus, greater firing rate

Molecular Basis for Action Potentials

Cell membrane

• Porous skin of the neuron
• Intracellular fluid
• Extracellular fluid
• Ion channels
  • Potassium
  • Sodium
• Balance of charge
  • Intracellular more negative than extracellular
  • Resting potential (negative)

Action potential

• Initiation phase
  • Sodium channels open
• Depolarization phase
  • Sodium ions enter
    • Concentration gradient
    • Electrical gradient
• Repolarization phase
  • Sodium channels close
  • Potassium channels open
    • Potassium ions exit
• Sodium-potassium pump 

Central Pattern Generators

Rhythmic muscular contraction

• e.g., walking, swimming, peristaltic movement

Opposing muscle groups working in coordination

• Contract, relax, contract, relax, etc.

Required network of neurons

• Different neurons innervate different muscles

Simplest CPG

• Two neurons
• Reciprocal inhibition
• Postinhibitory rebound
  • Cell becomes more excitable after hyperpolarization (threshold is reduced)

Artificial Neural Networks (ANNs)

Computer programs

Must be trained

• Learning Based on neuronal structure
• Input units
  • Sensory neurons
• Hidden units
  • Interneurons
• Output units
  • Motor neurons

Are ANNs just like real nervous systems?

• No

What use are ANNs?

• Simulation
• Metaphor
• Hypothesis testing

Nervous System Organization

Central nervous system (CNS)

• Brain
• Spinal cord
• Non-regenerative

Peripheral nervous system (PNS)

• Nerves outside the CNS
• Regenerative

Classes of neurons

• Sensory neurons
  • Carry information from sensory organs to CNS
• Interneurons
  • Only in the CNS
  • Carry information from one neuron to another
  • Organize and integrate information
• Motor neurons
  • Carry information to muscles and glands

PNS Systems

Skeletal

• Muscles attached to bones
• Externally observable movements
• Neurons extend directly from CNS to muscle

Autonomic

• Visceral muscles and glands
• Indirect neuronal action from CNS to ganglion to target site
• Sympathetic division
  • Immediate responses to stressful stimuli
• Parasympathetic division
  • Regenerative, energy-conserving processes