Neuro AM 03-21-25

Types of Synapses

1. Electrical Synapses:

   • Involve gap junctions, which are specialized channels that allow ions and small molecules to pass directly from one cell to another.

   • Very fast communication due to the direct passage of ions; this process minimizes delays in signaling.

   • Example: Sodium, potassium, and chloride ions flow freely between connected cells, allowing for rapid transmission of signals that can synchronize activity in networks of cells, such as in heart tissue.

2. Chemical Synapses:

   • More complex than electrical synapses and involve multiple steps for communication.

   • Release neurotransmitters at the presynaptic terminal that relay signals to the postsynaptic cell.

   • Steps for Communication:

     1. Action Potential Generation:

        • Triggered in neurons by excitatory stimuli, leading to the communication of information.

     2. Calcium Influx:

        • Action potentials increase calcium ion concentration in the presynaptic terminal, which is crucial for neurotransmitter release.

     3. Vesicle Movement:

        • Synaptic vesicles containing neurotransmitters migrate towards and fuse with the presynaptic membrane, a process influenced by calcium ions.

     4. Exocytosis:

        • The vesicles release their neurotransmitter contents into the synaptic cleft through exocytosis, a mechanism vital for communication.

     5. Neurotransmitter Diffusion:

        • Neurotransmitters rapidly diffuse across the synaptic cleft to bind to receptors on the postsynaptic membrane.

     6. Membrane Response:

        • The binding of neurotransmitters instigates a change in membrane potential (either slight depolarization or hyperpolarization, depending on the type of neurotransmitter and receptor).

     7. Reuptake:

        • After their action, neurotransmitters are often taken back into the presynaptic terminal, a process that helps modulate the strength of the signal and prevent continuous activation.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP):

  • Occurs when the membrane potential becomes more positive, increasing the likelihood of an action potential in the postsynaptic neuron, thus stimulating the cell.

• Inhibitory Postsynaptic Potential (IPSP):

  • Occurs when the membrane potential becomes more negative, decreasing the likelihood of an action potential, therefore inhibiting the cell’s activity.

Summation

• Refers to the integration of EPSPs and IPSPs that determines the overall change in membrane potential.

  • Types of Summation:

    • Spatial Summation:

      • Results from multiple synapses firing simultaneously on a postsynaptic neuron, enhancing the total excitatory input.

  • Example calculations:

    • EPSP from A (+3) and B (+2) results in a total of +5, promoting stimulation.

    • An IPSP from C (-3) combined with EPSP from B (+2) results in -1, indicating inhibition of the cell.

Synaptic Plasticity

• Refers to the ability of synapses to change their strength and efficacy based on activity or stimulation patterns, playing a crucial role in learning and memory.

• Involves several mechanisms:

  • Changes in the Amount of Neurotransmitter Released:

    • Synapses can adjust the quantity of neurotransmitters released, which can either enhance response (more neurotransmitters released) or diminish it (fewer neurotransmitters released).

  • Changes in the Number of Receptors:

    • The postsynaptic membrane can increase or decrease the number of receptors present, which affects the sensitivity of the neuron to neurotransmitters.

  • Changes in Presynaptic Structures:

    • This can include formation or loss of synapses, impacting the overall connectivity and functional network of neurons, essential for adaptive behavior.

Importance of Synaptic Plasticity

• Essential for learning and memory, as synaptic adjustments facilitate the encoding of new experiences and the processing of information in the brain.

• Directly relates to how the brain adapts to changing environmental conditions and reframes responses to stimuli.