chapter 12 pt 3

Neurotransmission and Neuromodulation

Acetylcholine and Synaptic Transmission

  • Some acetylcholine molecules cross the synaptic cleft to interact with acetylcholine receptors on target cells.

  • Other mechanisms for neurotransmitter removal include:

    • Reuptake into the synaptic knob.

    • Diffusion away from the synaptic cleft.

  • Certain prescription drugs are designed to influence the quantity of neurotransmitter available in the synaptic cleft.

Neuromodulation (Pages 476-477)

  1. Definition of Neuromodulation:

    • The release of chemicals known as neuromodulators from cells that locally regulate or alter neuronal responses to neurotransmitters.

  2. Effects of Neuromodulation:

    • Results in either facilitation or inhibition.

Facilitation vs. Inhibition

  • Facilitation:

    • Occurs when there is an increased response from a postsynaptic neuron.

    • May result from:

    • An increased amount of neurotransmitter.

    • An increased number of receptors on the postsynaptic neuron.

  • Inhibition:

    • Occurs when there is a decreased response from a postsynaptic neuron.

    • May result from:

    • A decreased amount of neurotransmitter.

    • A decreased number of receptors on postsynaptic neurons.

Nitric Oxide as a Neuromodulator

  • Unique Characteristics:

    • Nitric oxide is considered an unusual neurotransmitter and is classified by some as a neuromodulator.

  • Synthesis:

    • Produced from the amino acid arginine, synthesized on an 'as-needed' basis.

  • Functions:

    • Enters presynaptic neurons allowing for retrograde communication.

    • Plays a role in memory development by stimulating presynaptic neurons to increase neurotransmitter release.

    • Released from motor neurons that innervate blood vessels, which causes vasodilation.

Endocannabinoids

  • Definition:

    • Molecules that bind to the same receptors as the active ingredient tetrahydrocannabinol (THC).

  • Similarities to Nitric Oxide:

    • Small, nonpolar molecules produced and released on-demand from postsynaptic neurons.

  • Effects:

    • Decrease neurotransmitter release from presynaptic neurons.

    • Alter learning and memory processes.

    • Affect appetite regulation and suppress nausea.

Neural Integration and Neuronal Pools in the CNS (Page 478)

  • The nervous system facilitates coordination and integration of neuronal activity through neuronal pools, consisting of billions of interneurons organized in complex patterns.

  • Types of Neuronal Circuits:

    1. Converging Circuit:

    • Input from multiple presynaptic neurons converges on a single postsynaptic neuron.

    1. Diverging Circuit:

    • Information spreads from one presynaptic neuron to several postsynaptic neurons or from one pool to multiple pools.

    1. Reverberating Circuit:

    • Utilizes feedback to create repeated, cyclical stimulation. It continues until interrupted by inhibitory stimuli or synaptic fatigue, where neurotransmitter production in a presynaptic cell is exhausted.

    1. Parallel-After-Discharge Circuit:

    • Input is transmitted simultaneously along multiple neuron pathways to a common postsynaptic cell.