Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 48.4 Neurons Communicate With Other Cells at Synapses
Synapses are the critical junctions where neurons transmit signals to other cells. Depending on the type, signals may pass electrically or chemically. Chemical synapses dominate in complex nervous systems, relying on neurotransmitters to bridge the gap between cells.
1. Types of Synapses
- Electrical synapses: allow direct ion flow via gap junctions; rapid and synchronous (e.g., heart, escape reflexes)
- Chemical synapses: use neurotransmitters to transmit signals across the synaptic cleft; dominant in vertebrates
2. Chemical Synapse Function
- AP reaches the presynaptic terminal
- Depolarization opens voltage-gated Ca²⁺ channels
- Ca²⁺ triggers synaptic vesicle fusion and neurotransmitter release
- Neurotransmitters bind to postsynaptic receptors (often ligand-gated channels)
- Result: postsynaptic potential (EPSP or IPSP)
3. Postsynaptic Potentials
- EPSP (excitatory): depolarizes; Na⁺/K⁺ flow brings membrane closer to threshold
- IPSP (inhibitory): hyperpolarizes; K⁺ or Cl⁻ flow makes AP less likely
- Summed at the axon hillock to determine AP firing
4. Summation of Synaptic Input
- Temporal summation: multiple EPSPs from one synapse close in time
- Spatial summation: simultaneous EPSPs and IPSPs from different synapses
- Integration determines whether AP is triggered
5. Termination of Signaling
- Neurotransmitter clearance:
- Enzymatic breakdown (e.g. acetylcholinesterase)
- Reuptake into presynaptic terminal
- Uptake by glial cells
- Disruption (e.g. sarin gas) can cause fatal overstimulation
6. Metabotropic Receptors
- G protein-coupled receptors trigger signal transduction
- Slower but long-lasting (e.g., norepinephrine → cAMP)
- Enable modulation and support learning and memory
7. Diversity of Neurotransmitters
- Acetylcholine: excitatory at skeletal muscle; inhibitory at heart
- Amino acids:
- Glutamate: main excitatory NT in CNS
- GABA and Glycine: inhibitory via Cl⁻
- Biogenic amines: dopamine, serotonin, norepinephrine (mood, learning, targeted by drugs)
- Neuropeptides: e.g., substance P (pain), endorphins (analgesia)
- Gases: NO and CO act via second messengers; e.g., NO causes vasodilation
In a Nutshell
Neurons communicate through synapses, using either direct ion flow at electrical synapses or neurotransmitter release at chemical ones. Postsynaptic potentials integrate to determine neuronal output. A wide variety of neurotransmitters and receptor types allow the nervous system to produce fast, precise, or long-lasting responses essential for behavior, memory, and homeostasis.