Chapter 48: Neurons, Synapses, and Signaling

Overview

• Purpose: This chapter introduces neurons, synapses, and signaling as a precursor to understanding the nervous system in Chapter 49.
• Key Organism: The squid, due to its large axons, making it easier to study neuronal structures.

Neurons and Their Signals

• Neurons: Nerve cells that move info throughout the body using:
  • Electrical signals for long distances.
  • Chemical signals for short distances.
• Types:
  • Sensory neurons: Detect external stimuli/internal conditions.
  • Interneurons: Integrate sensory input and motor output.
  • Motor neurons: Trigger muscular/gland activity.

Nervous System Organization

• Central Nervous System (CNS): Integrates sensory input with motor output (brain and spinal cord).
• Peripheral Nervous System (PNS): Extends nerves throughout the body, carries info to/from CNS.
• Neuron Structure:
  • Cell Body: Contains organelles, nucleus.
  • Dendrites: Receive signals.
  • Axons: Transmit signals.
  • Myelin Sheaths: Improve conduction rates.

Supporting Cells

• Glial Cells: Support neurons (not involved in impulse conduction).
  • Astrocytes, Oligodendrocytes, Schwann cells.

Neural Circuits

• Simple Circuits: Presynaptic neuron stimulates a single postsynaptic neuron.
• Complex Circuits:
  • Diverging: One neuron, multiple postsynaptic cells.
  • Converging: Inputs from various sources into one output.
  • Reverberating: Signals sent linearly and back.
  • Parallel After-Discharge: Diverging and converging at different times.

Ion Channels and Membrane Potential

• Resting Potential: Typically -60 to -80 mV.
• Ion Concentrations: Potassium inside, sodium outside.
• Sodium-Potassium Pumps: Move ions using energy.
• Membrane Potential: Voltage across membrane.

Action Potentials

• Generation: Gated ion channels respond to stimuli, depolarization leads to action potentials.
• Phases:
  • Rising: Depolarization, increased sodium influx.
  • Falling: Sodium channels blocked, potassium exits.
  • Undershoot: Temporary hyperpolarization.
• Refractory Period: Prevents immediate reactivation of action potential.

Conduction and Synapses

• Speed: Increases with axon diameter, myelin sheaths.
• Synapses: Communication between neurons and other cells.
  • Electrical Synapses: Current transfers.
  • Chemical Synapses: Neurotransmitter crosses synaptic cleft.

Postsynaptic Potentials

• Excitatory (EPSPs): Move towards threshold.
• Inhibitory (IPSPs): Move away from threshold.
• Summation:
  • Spatial: Multiple inputs at once.
  • Temporal: Inputs in quick succession.

Neurotransmitters

• Groups:
  • Acetylcholine: Muscle stimulation, memory, learning.
  • Amino Acids: Glutamate, GABA, glycine.
  • Biogenic Amines: Dopamine, serotonin.
  • Neuropeptides: Substance P, endorphins.
  • Gases: Nitric oxide, carbon monoxide.