Nervous System Part 3: Synapses

Vocabulary flashcards covering key concepts from Nervous System Part 3: Synapses, including synaptic types, detailed chemical events, EPSPs, IPSPs, summation mechanisms, presynaptic modulation, and circuit pathways.

Synapse

A specialized junction between two neurons, or between a neuron and a target cell (such as a muscle or gland cell), that facilitates communication in the nervous system.

Presynaptic Neuron

The neuron that conducts electrical impulses toward a synapse and transmits chemical signals to another cell.

Postsynaptic Neuron

The neuron or effector cell (such as a muscle or gland cell in the PNS) that receives information and transmits electrical signals away from a synapse.

Axo-dendritic Synapse

A type of synapse formed between the axon terminal of a presynaptic neuron and the dendrites of a postsynaptic neuron.

Axo-somatic Synapse

A type of synapse formed between the axon terminal of a presynaptic neuron and the cell body (soma) of a postsynaptic neuron.

Axo-axonic Synapse

A type of synapse formed between the axon terminal of one neuron and the axon of another neuron.

Dendro-dendritic Synapse

A type of synapse formed directly between the dendrites of two neurons.

Sequence of Events at a Chemical Synapse

1. Action potential arrives at the axon terminal. 2. Voltage-gated $\text{Ca}^{2+}$ channels open and $\text{Ca}^{2+}$ enters. 3. $\text{Ca}^{2+}$ entry triggers synaptic vesicles to release neurotransmitters by exocytosis. 4. Neurotransmitters diffuse across the synaptic cleft and bind postsynaptic receptors. 5. Ion channels open, generating graded potentials. 6. Neurotransmitter effects terminate via reuptake, enzymatic degradation, or diffusion.

Excitatory Postsynaptic Potential (EPSP)

A local depolarization of the postsynaptic membrane caused by neurotransmitter binding opening chemically gated ion channels, allowing simultaneous $\text{Na}^+$ influx and K Efflux to pass simultaneously.

Inhibitory Postsynaptic Potential (IPSP)

A local hyperpolarization of the postsynaptic membrane caused by neurotransmitter binding opening channels permeable to $\text{K}^+$ (moving out) or $\text{Cl}^-$ (moving in), driving the membrane potential farther away from action potential threshold.

Temporal Summation

Postsynaptic integration occurring when one or more presynaptic neurons transmit impulses in rapid-fire succession, adding consecutive EPSPs on top of one another before previous signals can dissipate.

Spatial Summation

Postsynaptic integration occurring when a postsynaptic neuron is stimulated by a large number of terminals simultaneously across different locations, allowing many small EPSPs to add together.

Summation of EPSPs and IPSPs

The combined effect of simultaneous excitatory and inhibitory inputs on a postsynaptic neuron, where changes in membrane potential can cancel each other out unless excitatory signals predominate to reach threshold.

Presynaptic Inhibition

A regulatory mechanism where a modulatory neuron forms an axoaxonic synapse on a single axon collateral of a presynaptic neuron, blocking neurotransmitter release to selectively inhibit communication at one target synapse without affecting other targets.

Presynaptic Facilitation

A mechanism where an axoaxonic synapse increases calcium channel activation in a presynaptic axon terminal, leading to greater calcium entry, increased neurotransmitter release, and an enhanced postsynaptic effect.

Postsynaptic Inhibition

A regulatory mechanism where an inhibitory neuron directly modulates the postsynaptic cell body or dendrites, generating an IPSP that keeps the signal below threshold and inhibits response in all target cells equally.

Divergent Pathway

A neural circuit layout in which a single presynaptic neuron branches out to communicate with and influence a larger number of postsynaptic neurons.

Convergent Pathway

A neural circuit layout in which multiple presynaptic neurons provide input to converge upon and influence a smaller number of postsynaptic neurons.