Electrical and Synaptic Signaling 2

Neuron Signal Transmission

  • Signals are transmitted at synapses, allowing communication between neurons.
  • Incoming signals depolarize dendrites, leading to action potentials generated at the base of the axon.

Action Potentials

Nonmyelinated Nerve Cell
  1. Depolarization Initiation: Stimulation causes an inward rush of Na⁺ ions, reversing membrane polarity.
  2. Propagation: Action potential spreads as nearby regions become depolarized, triggering further Na⁺ influx.
  3. Repolarization: K⁺ channels open, allowing K⁺ to exit, restoring the resting state.
  4. Continuation: The sequence continues along the membrane, creating a propagated action potential.
Myelinated Nerve Cell
  1. Saltatory Propagation: Action potentials jump between nodes of Ranvier, allowing faster transmission compared to continuous propagation.
  2. Nodes of Ranvier: These are where voltage-sensitive Na⁺ channels are concentrated, enabling rapid depolarization.
  3. Myelin Sheath: Acts as an electrical insulator, enhancing signal speed and distance.

Synaptic Transmission

Types of Synapses
  1. Electrical Synapses: Direct communication via gap junctions; instantaneous signal transmission with no delay.
  2. Chemical Synapses: Separated by a synaptic cleft; neurotransmitters are released to transmit signals across the gap.
Neurotransmitters
  • Function: Relay signals across synapses; stored in presynaptic neurons and released upon action potential arrival.
  • Criteria:
    • Must elicit appropriate responses when in the synaptic cleft.
    • Naturally occurring in presynaptic neurons.
    • Released timely upon stimulation.
Types of Neurotransmitter Receptors
  1. Excitatory Receptors: Induce depolarization in postsynaptic neurons.
  2. Inhibitory Receptors: Hyperpolarize the postsynaptic neuron, reducing action potential likelihood.

Examples of Neurotransmitters

  1. Acetylcholine: Common in vertebrates; serves excitatory functions in cholinergic synapses.
  2. Catecholamines: Includes dopamine, norepinephrine, and epinephrine, playing roles in various synapses.
  3. Amino Acids: GABA, glutamate, glycine involved in inhibitory and excitatory functions.
  4. Neuropeptides: Short chains that can modulate pain perception, such as enkephalins.
  5. Endocannabinoids: Lipid derivatives that inhibit presynaptic neuron activity, e.g., THC in cannabis.

Neurotransmitter Secretion

  • Calcium Ion Role: Increased levels trigger neurotransmitter release from presynaptic terminals via exocytosis.
  • Vesicle Fusion: Neurotransmitters are secreted through the fusion of vesicles with the plasma membrane, facilitated by calcium influx.
Kiss-and-Run Exocytosis
  • A rapid release method where vesicles fuse transiently with the membrane to release some neurotransmitter and then reseal.

Neurotransmitter Detection

Types of Receptors
  1. Ligand-Gated Ion Channels: Quick reactions due to direct ion flow upon neurotransmitter binding.
  2. Metabotropic Receptors: Affect cellular processes indirectly through second-messenger systems.
Specific Receptor Examples
  1. Nicotinic Acetylcholine Receptor: Ligand-gated Na⁺ channel, causing depolarization.
  2. GABA Receptor: Ligand-gated Cl⁻ channel, resulting in hyperpolarization.
  3. NMDA Receptor: Ionotropic receptor for glutamate, allowing Ca²⁺ and other cations to pass through.

Antagonists and Agonists

  • Antagonists: Compete with neurotransmitters, preventing receptor activation (e.g., snake venom).
  • Agonists: Stimulate receptor activation, causing depolarization but cannot be quickly inactivated.

Inactivation of Neurotransmitters

  • Mechanisms:
  1. Reuptake: Transported back into presynaptic neurons or support cells.
  2. Degradation: Enzymatic breakdown (e.g., acetylcholine hydrolysis).
  3. Diffusion: Escape from the synaptic cleft.

Postsynaptic Potentials

  • Integration of Signals: Incremental changes result in excitatory or inhibitory postsynaptic potentials (EPSP/IPSP).
  • Summation Types:
  1. Temporal Summation: Rapid successive action potentials build up EPSPs.
  2. Spatial Summation: Many concurrent action potentials lead to simultaneous neurotransmitter release, increasing action potential probability.
Key Takeaways
  • Action potentials propagate faster in myelinated axons via nodes of Ranvier (saltatory conduction).
  • Synapses can be chemical or electrical, with distinct mechanisms for signal transmission.
  • Neurotransmitters are released due to elevated calcium levels, and their actions are mediated by different receptors which can be affected by various drugs.