Biology Unit 2: Synapses and Neurotransmitters

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

• The nervous system is a complex communication network
• Communication between cells is crucial for homeostasis and response
• Action potentials are essential for cellular communication, determined by the interaction between neurotransmitters (NT) and receptors.

Synapses

• Synapses allow communication between neurons and between neurons and effector cells (muscles or glands)
• Two types of synapses:
  • Electrical synapses: Less common, found in brain and heart, signal travels through gap junctions
  • Chemical synapses: More common, found throughout the body, neurotransmitters are released from one neuron and bind to receptors on a neighboring neuron

Chemical Synapse: Detailed Events

• Action potential arrives at the axon terminal
• Voltage-gated calcium channels open, allowing calcium ions (Ca²⁺) to enter
• Calcium trigger exocytosis of synaptic vesicles containing neurotransmitters
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
• Binding of neurotransmitters opens ion channels in the postsynaptic membrane. These channels are chemically-gated, or ligand-gated; they respond to specific ligands.
• This causes a change in the postsynaptic membrane potential, either excitatory (EPSP) or inhibitory (IPSP).
• Neurotransmitters are broken down or taken back into the presynaptic neuron to stop further signal transmission
• Example: Acetylcholine (ACh) is released from presynaptic axon terminals and binds to receptors on postsynaptic membranes, triggering ion channel opening
• Acetylcholinesterase (AChE) breaks down ACh

Types of Neurotransmitters

• Excitatory neurotransmitters: Increase the likelihood of an action potential. Example: Acetylcholine.
• Inhibitory neurotransmitters: Decrease the likelihood of an action potential. Example: GABA

Ion Channels

• Four types:
  • Leak channels (always open)
  • Voltage-gated channels (open/close in response to membrane potential changes)
  • Ligand-gated channels (open/close in response to a specific molecule binding)
  • Mechanically-gated channels (open/close in response to mechanical stimuli)
  • Understanding ion channels is essential in understanding how nerve signals are propagated and muscle contractions occur

Neurotransmitters: Specific Examples

• Acetylcholine (ACh): Both CNS and PNS functions. Binds to muscarinic and nicotinic receptors, with effects dependent on binding type. A major component of neuromuscular junctions.
• Norepinephrine: Both CNS and PNS functions. Can be excitatory or inhibitory; involved in the sympathetic nervous system, a "feel good" neurotransmitter.
• Dopamine: Primarily CNS function. Excitatory or inhibitory; its neurotransmission is affected by drugs like cocaine and amphetamines, implicated in schizophrenia.
• Serotonin: Primarily CNS function. Mainly inhibitory, regulating sleep, migraines, appetite, and mood.
• GABA: Primarily CNS function. A major inhibitory neurotransmitter. Binding to its receptor opens chloride channels (Cl⁻), increasing negative charge inside the neuron, making it less likely to generate an action potential.