Monocyte-Macrophage System and Synaptic Plasticity Mechanisms

The Monocyte-Macrophage System

Monocyte Development and Function

• Origin: Monocytes are formed in the bone marrow.
• Morphology: They possess a characteristic kidney-shaped nucleus.
• Circulation: They circulate in the blood for 72 hours before migrating through capillaries into tissue spaces.
• Maturation: Within 8 hours, they swell, increase in size, and develop a large number of lysosomes to become tissue macrophages.
• Systemic Role: Once they become tissue macrophages, they do not return to circulation. As both monocytes and tissue macrophages are phagocytic, they constitute the monocyte-macrophage system.

Mechanisms of Synaptic Plasticity

Presynaptic Mechanisms

1. Calcium Concentration: High-frequency stimulation of the presynaptic neuron leads to an accumulation of Ca²⁺. This increases neurotransmitter (NT) release, enhances the opening of ion channels in the postsynaptic neuron, and increases the amplitude of EPSPs and IPSPs, forming the basis of immediate memory.
2. Presynaptic Receptors: A third neuron terminating on a presynaptic knob influences the activity of the presynaptic neuron:
   • Presynaptic Facilitation: A third neuron releases an excitatory NT (e.g., serotonin), increasing NT release from the presynaptic neuron. This effect can last up to 3 weeks, serving as a basis for memory and learning.
   • Presynaptic Inhibition: A third neuron releases an inhibitory NT (e.g., GABA), decreasing NT release from the presynaptic neuron. This allows neighboring neurons to inhibit each other, accentuating the stimulation of specific postsynaptic neurons.
3. Autoreceptors: Located on the presynaptic neuron, these are activated by NTs released from the same neuron. Their stimulation results in the inhibition of further NT release, known as negative feedback inhibition.

Postsynaptic Mechanisms Affecting Synaptic Strength

• Up- and Down-regulation of Receptors: The number of receptors is dynamic; it increases with up-regulation and decreases with down-regulation, thereby increasing or decreasing the sensitivity of the synapse.
• Receptor Desensitization: In certain conditions, receptors respond once and then temporarily fail to respond, despite the continued presence of the NT.
• Effect of Co-transmitters and Neuromodulators: These may be secreted alongside NTs from the presynaptic neuron to modify the postsynaptic neuron's response to a specific NT.