2 Synapses and Neural Circuits
Learn how synapses transmit and integrate signals, how neural circuits coordinate activity, and how synaptic plasticity changes connections.
How synapses transmit signals
At a chemical , an action potential reaching the terminal opens voltage-gated calcium channels. Calcium enters the terminal and triggers synaptic vesicles to release into the . The diffuses across the gap and binds to matching receptors on the membrane.
The ’s effects stop when it is cleared, often through reuptake into cells, enzymatic breakdown, or diffusion away. Vesicles and their contents can be recycled for later release.
At electrical synapses, channels called gap junctions directly connect neighboring cells, allowing current to pass between them. This communication is very fast and can help synchronize activity. Chemical synapses are more common in the brain; they take slightly longer to transmit signals but allow more varied and adjustable effects.
Neurotransmitters and responses
Neurotransmitters are chemical messengers released by neurons. Examples include glutamate, the principal excitatory in the brain; GABA and glycine, which are commonly inhibitory; acetylcholine, used at the neuromuscular junction and in many brain circuits; and dopamine, serotonin, and norepinephrine, which often regulate or modulate activity across circuits.
A transmitter’s effect depends on the receptor it activates and on the receiving cell’s properties, so the same does not always have the same effect everywhere. Some receptors open ion channels directly and produce rapid responses. Other receptors start intracellular signaling cascades that can act more slowly and last longer.
A chemical can produce an , making the receiving neuron more likely to fire, or an , making firing less likely. These are graded changes, not full action potentials. If the resulting voltage change at the neuron’s trigger region reaches threshold, an action potential begins.
A neuron receives many inputs, and their combined effects depend on timing and location. Inputs arriving close together in time can summate temporally, while inputs from different synapses can summate spatially.
How neural circuits coordinate activity
A is a connected group of neurons whose activity helps perform a function. Signals travel through circuits as action potentials along axons and as synaptic effects from cell to cell. Connections can be feedforward, carrying activity onward; feedback, sending activity back to earlier parts of a pathway; or recurrent, allowing neurons to influence one another repeatedly.
When a sensory neuron responds to a touch, its activity can pass through several connected neurons. Some circuit cells relay the signal, others inhibit competing pathways, and others combine it with information from additional inputs. The resulting activity may contribute to a perception or guide a movement.
A neuron’s output reflects the pattern of activity across its network, not simply one incoming signal. Synapses and local circuits filter and integrate messages before they influence other parts of the nervous system.
How synaptic connections change
is a lasting change in how effectively one cell influences another. A can become stronger or weaker through changes in release, receptor number or sensitivity, or the structure of the connection. Some changes last briefly; others persist longer and may involve altered gene activity or structure.
is a well-studied form of strengthening, while is a lasting reduction in synaptic strength. In many brain circuits, patterns of activity involving glutamate receptors can trigger these changes.
Plasticity does not follow a simple rule that more activity always means a stronger connection. The direction and size of change depend on the , the activity pattern, and the cellular conditions. Changes across many synapses can reshape circuit responses and are important mechanisms underlying learning and memory.