Neuroscience Neuronal Communication Quiz

Unit 2: Part 3 - Synapses, Neuromuscular Junctions, and Neurotransmitters

• The nervous system relies on communication between neurons and effector cells, accomplished via action potentials.
• This communication facilitates the coordinated functioning of organ systems to maintain homeostasis.

The Big Picture

• Cells → Tissue → Organs → Organ systems
• The interaction between neurotransmitters (NT) and receptors determines cellular responses and action potentials.
• Communication between neurons and effector cells occurs through action potentials.

How the Message Continues Past an Individual Neuron

• Myelinated axons use saltatory conduction — action potentials jump between nodes of Ranvier.
• Myelin prevents current decay, allowing rapid signal transmission.

Synapses

• Synapses allow communication between neurons or between neurons and effector cells.
• There are axodendritic, axosomatic, and axoaxonic synapses.
• The presynaptic neuron releases neurotransmitters; the postsynaptic neuron receives them.

Types of Synapses

• Electrical synapses: Less common, signal transmission through gap junctions.
• Chemical synapses: More common, neurotransmitters relay signals across the synaptic cleft. Neurotransmitters being released from one neuron. opens/closes ligand-gate ion channels on neighboring neurons.

A Chemical Synapse

• Action potential arrives at the presynaptic axon terminal.
• Voltage-gated Ca2+ channels open, allowing calcium to enter.
• Calcium triggers synaptic vesicles to release neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic membrane, leading to a response in the postsynaptic neuron.

Events Occurring at a Chemical Synapse

• Step 1: Arrival of an action potential at the presynaptic terminal triggers neurotransmitter release (e.g., acetylcholine (ACh)).
• Step 2: Depolarization of the synaptic knob opens voltage-gated Ca2+ channels, causing calcium influx and neurotransmitter release.
• Step 3: Released neurotransmitters (ACh) bind to receptors on the postsynaptic membrane, causing depolarization.
• Step 4: The enzyme acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft, ending the signal.

Events Occurring at a Chemical Synapse (Steps 3 & 4)

• Step 3: The released neurotransmitter (e.g., ACh) diffuses across the cleft & binds to postsynaptic receptors. Excite or inhibit postsynaptic cells dependent on the receptor.
• Step 4: The effects are transient due to enzymes (e.g., AChE) breaking down the neurotransmitter.

Ion Passageways Across Membranes

• Four types of ion channels:
  • Leakage channels: Always open, specific ionic permeability.
  • Voltage-gated channels: Open or close in response to membrane potential changes.
  • Ligand-gated channels: Open or close in response to a specific chemical binding.
  • Mechanically gated channels: Open or close in response to physical forces.

Events Occurring at a Cholinergic Synapse

• Arrival of action potential → Calcium influx → ACh release → ACh binds receptors → Sodium influx → Depolarization → ACh breakdown by AChE → Return to resting state.

Chemical Synapses and Postsynaptic Potentials (PSPs)

• PSPs are changes in membrane potential in the postsynaptic cell.
• EPSPs (excitatory) increase the likelihood of an action potential.
• IPSPs (inhibitory) decrease the likelihood of an action potential.

Excitatory and Inhibitory Neurotransmitters

• Excitatory neurotransmitters increase the likelihood of an action potential (e.g., acetylcholine).
• Inhibitory neurotransmitters decrease the likelihood (e.g., GABA).

Intracellular and Extracellular Fluid Concentrations of Ions

• The concentrations of ions (like sodium, potassium, calcium, chloride) differ significantly between intracellular (inside the cell) and extracellular (outside the cell) fluids. This difference is crucial for generating and transmitting nerve impulses.

Neurotransmitters

• Various neurotransmitters like acetylcholine, norepinephrine, dopamine, serotonin, and GABA differ in their functions and actions.

Acetylcholine

• Acts in both the central and peripheral nervous systems (CNS & PNS)
• Binds to both muscarinic and nicotinic receptors.
• Can be excitatory or inhibitory, depending on the receptor type.

Norepinephrine

• Both CNS and PNS
• A "feel-good" neurotransmitter in the brain.
• Can be both excitatory or inhibitory
• A major neurotransmitter of the sympathetic nervous system
• Low levels can lead to depression.

Dopamine

• Primarily in the CNS
• Excitatory or inhibitory neurotransmitter
• A "feel good" neurotransmitter
• Cocaine and amphetamines affect dopamine re-uptake and release.
• Increased dopamine activity is linked to schizophrenia.

Serotonin

• Primarily in the CNS
• Primarily inhibitory neurotransmitter
• Involved in sleep, migraines, appetite, and mood regulation.

GABA

• Primary inhibitory neurotransmitter in the CNS
• GABA opening chloride channels causes hyperpolarization, making an action potential less likely.