Neuroscience: Chemical Messengers

Synapse

The junction where one neuron communicates with another neuron.

Presynaptic Terminal

The sending end of a neuron that releases neurotransmitters.

Postsynaptic Membrane

The receiving part of a neuron that contains receptors.

Synaptic Cleft

The tiny gap between two neurons at a synapse.

Chemical Synaptic Transmission

The process of sending signals across a synapse using neurotransmitters.

Why Neurotransmitters Are Needed

Electrical signals cannot directly cross the synaptic cleft.

Neurotransmitter

A chemical messenger released by neurons to communicate with other cells.

Synaptic Vesicle

A tiny membrane-bound sac that stores neurotransmitters.

Storage Vesicles

Vesicles that hold reserve neurotransmitters inside the axon terminal.

Releasable Vesicles

Vesicles positioned near the membrane and ready to release neurotransmitters.

Role of Calcium (Ca++) in Neurotransmitter Release

Calcium enters the presynaptic terminal and triggers neurotransmitter release.

Calcium Channels

Ion channels that open when an action potential reaches the axon terminal.

SNARE Proteins

Presynaptic proteins that help vesicles fuse with the cell membrane.

Vesicle Fusion

The process in which a vesicle merges with the membrane to release neurotransmitters.

Exocytosis

The release of neurotransmitters from a vesicle into the synaptic cleft.

Diffusion

The movement of neurotransmitters across the synaptic cleft.

Receptor

A specialized protein that detects neurotransmitters.

Speed of Synaptic Transmission

Neurotransmitters can cross the synaptic cleft in less than one millisecond.

Vesicle Recycling

The process of reforming and refilling synaptic vesicles after release.

Purpose of Vesicle Recycling

Allows neurotransmitters to be reused repeatedly.

Glial Cells at Synapses

Support neurotransmitter recycling and cleanup.

Transporters

Proteins that remove neurotransmitters from the synaptic cleft.

Function of Transporters

Clear neurotransmitters so new signals can occur.

Neurotransmitter Reuptake

The process of taking neurotransmitters back into cells after release.

Neurotransmitter Breakdown

The destruction of neurotransmitters by enzymes in the synaptic cleft.

Lock-and-Key Model

The idea that neurotransmitters fit specific receptors like keys fit locks.

Ionotropic Receptor

A receptor that directly opens an ion channel when activated.

Function of Ionotropic Receptors

Create rapid changes in membrane potential.

Ligand

A molecule that binds to a receptor.

Positive Ions

Ions such as sodium (Na+) and calcium (Ca++) that promote excitation.

Excitation

A process that makes a neuron more likely to fire an action potential.

Excitatory Post-Synaptic Potential (EPSP)

A small depolarization that makes a neuron more likely to fire.

EPSP Effect

Moves membrane potential closer to threshold.

Converging Synapses

The input of many synapses onto a single neuron.

Summation of EPSPs

The combining of multiple excitatory signals.

Threshold for Action Potential

The level of depolarization needed to trigger an action potential.

Glutamate

The main excitatory neurotransmitter in the brain.

Main Role of Glutamate

Increases the likelihood that neurons will fire.

Inhibition

A process that reduces the chance of a neuron firing.

Inhibitory Synapse

A synapse that decreases activity in the receiving neuron.

Inhibitory Post-Synaptic Potential (IPSP)

A change in membrane potential that makes firing less likely.

IPSP Effect

Moves membrane potential farther from threshold.

Purpose of Inhibition

Prevents excessive neuronal activity and balances excitation.

GABA

A major inhibitory neurotransmitter in the brain.

Function of GABA

Reduces neuronal activity.

Glycine

An inhibitory neurotransmitter commonly found in the spinal cord and brainstem.

Function of Glycine

Suppresses neuronal firing.

Balance of Excitation and Inhibition

Required for normal brain function and information processing.

Timing of Synaptic Transmission

Typically occurs in about one millisecond.

Importance of Neural Timing

Signals must arrive together to effectively influence a neuron.

Metabotropic Receptor

A receptor that works through chemical signaling pathways instead of ion channels.

Function of Metabotropic Receptors

Modify and regulate cellular processes inside neurons.

Neuromodulation

The adjustment of neuronal activity through metabotropic receptors.

Difference Between Ionotropic and Metabotropic Receptors

Ionotropic receptors act quickly through ion channels, while metabotropic receptors act slowly through intracellular pathways.

G-Protein

A molecule that carries signals from metabotropic receptors into the cell.

Second Messenger

An intracellular signaling molecule activated by metabotropic receptors.

Purpose of Second Messengers

Start chains of biochemical events inside neurons.

Metabolic Effects of Neuromodulation

Can alter ion channels, receptors, transporters, and gene expression.

Gene Expression in Neurons

The process by which genes are activated to produce proteins.

Speed of Neuromodulation

Slower than ionotropic signaling but longer-lasting.

Duration of Neuromodulation Effects

Can persist well beyond the original synaptic event.

Acetylcholine

A neurotransmitter that can act as both an ionotropic and metabotropic messenger.

Importance of Acetylcholine

The first neurotransmitter discovered.

Acetylcholine at Neuromuscular Junctions

Triggers muscle contraction.

Acetylcholine and Attention

Helps the brain focus on relevant information.

Dopamine

A neuromodulatory neurotransmitter associated with reward and motivation.

Function of Dopamine

Makes experiences feel rewarding and reinforces behavior.

Noradrenaline

A neuromodulatory neurotransmitter involved in stress and alertness.

Function of Noradrenaline

Helps coordinate responses to novelty and stress.

Neuromodulatory Neurons

Neurons that influence large regions of the brain through widespread projections.

Widespread Projections

Axons that extend throughout many parts of the brain.

Purpose of Neuromodulators

Fine-tune neural networks rather than transmit specific sensory information.

Reward System

Brain circuits influenced by dopamine that process positive experiences.

Stress Response

A coordinated brain response involving noradrenaline.

Locus Coeruleus (LC)

The brain region containing most noradrenaline-producing neurons.

Importance of the Locus Coeruleus

Distributes noradrenaline throughout the brain and spinal cord.

Neuromuscular Junction

A synapse between a motor neuron and a muscle fiber.

Striated Muscle

Skeletal muscle controlled by motor neurons.

Chemical Communication in the Brain

The primary method neurons use to communicate across synapses.

Most Important Excitatory Neurotransmitter

Glutamate.

Most Important Inhibitory Neurotransmitters

GABA and glycine.

Most Important Neuromodulators

Acetylcholine, dopamine, and noradrenaline.