Bio 1106- Lesson 9

Communication Via Neurons

• Course: BIOL 1106

• Institution: Virginia Tech, Department of Biological Sciences

Lesson Learning Objectives

• Explain the ionic basis of:

  • Resting membrane potential

  • Action potential

  • Synaptic potentials

• Describe how gated ion channels produce these signals.

• Discuss the role of myelination in nervous system function.

• Distinguish between electrical and chemical synapses.

• Explain how information in an action potential is transmitted across a chemical synapse and integrated in the postsynaptic neuron.

• Describe the mode of action of excitatory and inhibitory neurotransmitters.

Organization of the Nervous System

• Nervous System Divisions:

  • CNS (Central Nervous System)

  • PNS (Peripheral Nervous System)

• Key Components:

  • Sensory Neurons: transmit sensory information

  • Interneurons: process information in CNS

  • Motor Neurons: command muscle movements

Nerve Impulse Transmission

• Key Concepts:

  • Electrical potential: Inside is more negative than outside.

  • Stimulation results in electrical changes via:

    • Membrane transport proteins

    • Ion channels:

      • Leakage channels

      • Gated channels

  • Sodium-potassium pump operates to maintain resting potential.

The Sodium-Potassium Pump

• Function:

  • Pumps Na+ out and K+ into the cell.

• Steps of Operation:

  1. Intracellular sodium binds to the carrier.

  2. ATP phosphorylates protein; Na+ is released outside.

  3. K+ binds to the protein, causing dephosphorylation.

  4. Potassium is released inside the cell, and process repeats.

Establishment of the Resting Membrane Potential

• Measured Values:

  • Intracellular environment: -70 mV

  • Extracellular environment: 0 mV

• Electrochemical gradients maintained by sodium-potassium pump and ion channels.

Disruptions to Resting Potential

• Key Types of Changes:

  • Graded Potentials:

    • Depolarization: Making inside less negative.

    • Hyperpolarization: Making inside more negative.

• Gated channels can lead to action potentials or graded potentials.

Membrane Potentials

• Graded Potentials:

  • Small stimuli cause small depolarizations.

• Threshold: Requires a significant stimulus to trigger an action potential.

• Hyperpolarization: Certain stimuli can make the membrane potential more negative.

Action Potential Phases

1. Resting Phase:

   • Equilibrium state at -70 mV.

2. Rising Phase:

   • Sodium channels open, causing rapid depolarization.

3. Top of Curve:

   • Maximum voltage is reached; sodium channels inactivate.

4. Falling Phase:

   • Potassium channels open, resulting in repolarization.

5. Undershoot:

   • Brief hyperpolarization occurs before returning to rest state.

Propagation of Action Potential

• In Unmyelinated Axons:

  • Action potentials are continuous along the axon due to sequential opening and closing of channels.

• In Myelinated Axons (Saltatory Conduction):

  • Action potentials jump between nodes (nodes of Ranvier), increasing speed.

Effect of Diameter on Impulse Conduction

• Conduction Velocity Table:

  Axon Diameter (μm)	Myelin	Velocity (m/s)
  500	No	25
  20	Yes	120
  10	Yes	50
  5	Yes	20
  1	No	2

Synapses

• Types of Synapses:

  • Electrical Synapses: Via gap junctions.

  • Chemical Synapses: Use neurotransmitters.

• Synaptic Cleft: Space between presynaptic and postsynaptic neurons, where neurotransmitter diffusion occurs.

Neurotransmitters

• Types and Functions:

  • Acetylcholine (ACh): Excitatory at the neuromuscular junction.

  • Amino Acids: Glutamate (excitatory), Glycine (inhibitory), GABA (inhibitory).

  • Biogenic Amines:

    • Epinephrine: Hormonal response.

    • Norepinephrine: Sympathetic nervous response.

    • Dopamine: Movement control in brain.

    • Serotonin: Sleep regulation and emotional state.

Synaptic Integration

• EPSP (Excitatory Postsynaptic Potential): Can lead to action potential if sufficient.

• IPSP (Inhibitory Postsynaptic Potential): Makes action potential less likely.

• Modes of Integration:

  • Spatial Summation: EPSPs from multiple presynaptic neurons.

  • Temporal Summation: Repeated EPSPs from one neuron.'