Synapse Structure and Function Quiz

Synapse Structure and Function

• A synapse is a specialized junction where one neuron communicates with another neuron or another cell type, such as a muscle or gland. Information typically flows from a neuron to its target.
• The presynaptic neuron is the first part of the connection to communicate, while the target is considered postsynaptic.
• The synaptic knob, a small round or oval shaped structure, contains vesicles and mitochondria.
• The postsynaptic region contains receptors for neurotransmitters.
• The synaptic cleft is a tiny space between the two neurons. (200-300 angstroms)
• Neurotransmitters are molecules released from the presynaptic neuron into the cleft; then they bind to receptors on the postsynaptic membrane.

Synaptic Transmission

• Neurotransmitter release occurs in three steps:

  • Release of neurotransmitters into the synaptic cleft.
  • Action of the neurotransmitters on the postsynaptic membrane.
  • Termination of synaptic transmission.

• Neurotransmitter release is triggered by an action potential arriving at the synaptic knob. This triggers an influx of calcium ions

• This causes vesicles containing neurotransmitters to fuse with the membrane and release neurotransmitters via exocytosis into the synaptic cleft.

• Neurotransmitters are removed from the cleft via uptake and degrading enzymes.

Postsynaptic Potentials

• Postsynaptic potentials (PSPs) are two major types of signals in a neuron:
  • Excitatory Postsynaptic Potentials (EPSPs)
    • These cause partial depolarization of the postsynaptic membrane, bringing it closer to threshold.
  • Inhibitory Postsynaptic Potentials (IPSPs)
    • These cause partial hyperpolarization of the postsynaptic membrane, moving it further away from threshold.
• EPSPs are triggered by excitatory neurotransmitters binding to receptors, usually allowing sodium ions to enter the cell.
• IPSPs are triggered by inhibitory neurotransmitters binding to receptors, usually allowing chloride ions to enter the cell or potassium ions to leave the cell.
• Summation of EPSPs and IPSPs determines whether or not an action potential occurs in the postsynaptic neuron.

Termination of Synaptic Transmission

• Synaptic transmission ends when the neurotransmitter is removed from the synaptic cleft. This happens through various mechanisms:
  • Active reuptake of the neurotransmitter into the presynaptic terminal.
  • Enzymatic degradation (breaking down the neurotransmitter).
  • Diffusion of the neurotransmitter out of the synaptic cleft.

General Properties of Synaptic Transmission

• One-way conduction: impulses move in one direction (from presynaptic to postsynaptic). This is because the neurotransmitter is located only in the presynaptic neuron.
• Synaptic delay (approximately 0.5 milliseconds): the time it takes for the neurotransmitter to be released, diffuse across the synapse, and bind to receptors. This delay is a result of several steps in the process.
• Synaptic fatigue: the synapse's ability to transmit signals progressively diminishes if stimulated continuously for a long period. A depletion of neurotransmitters could be the cause of the progressive decline.

Effects of Environmental Factors

• Hypoxia: Reduced oxygen supply to neurons leads to loss of excitability and cessation of synaptic transmission, which can result in coma quickly (within less than 7 seconds).
• pH: Changes in pH (acidosis or alkalosis) alter neuronal excitability; alkalosis generally increases excitability, while acidosis typically reduces it.

Effects of Drugs

• Some drugs affect synaptic transmission by altering neuronal excitability.
  • Caffeine and theophylline decrease the threshold for excitation, thereby increasing neuronal excitability.
  • Anesthetics and hypnotics increase the threshold, thus decreasing neuronal excitability and synaptic transmission.

Test your knowledge on synapse structure and function, including the roles of presynaptic and postsynaptic neurons, synaptic transmission, and neurotransmitter action. This quiz will cover key concepts and terminology related to how neurons communicate with each other and other cells.