Synaptic Transmission and Graded Potentials in Neuronal Physiology

Synaptic Transmission

Overview of Synaptic Transmission

Synaptic transmission is the process by which neurons communicate with each other or with effector cells via synapses. This involves the release of neurotransmitters and the activation of receptors on the postsynaptic membrane.

• Neurotransmitter Release: The presence of Ca2+ inside the presynaptic cell causes synaptic vesicles to fuse with the membrane, releasing neurotransmitters into the synaptic cleft.

• Neurotransmitter Binding: Neurotransmitters bind to receptors on the postsynaptic neuron, which can trigger direct or indirect effects.

• Chemically Gated Ion Channels: These channels may remain open as long as the neurotransmitter is bound to the receptor. If the channels are not sensitive to changes in membrane potential, they will not respond to voltage changes.

Postsynaptic Cell Response

The response of the postsynaptic cell depends on several factors:

• Type of Neurotransmitter: Different neurotransmitters can have excitatory or inhibitory effects.

• Receptor Specificity: The specific receptor present on the postsynaptic cell determines the response.

• Multiple Receptors: There are multiple receptors for each neurotransmitter, each activating different ion channels and producing distinct effects.

Graded Potentials

Characteristics of Graded Potentials

Graded potentials (GPs) are changes in membrane potential that vary in size and decay with distance from their point of origin.

• Decay with Distance: GPs are largest at the synapse where they originate and decrease as they travel away from the synapse.

• Short-Distance Signals: Unlike action potentials (APs), GPs can only travel short distances because they decay as they move along the neuronal membrane. This applies to both excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

Summation of Graded Potentials

Temporal Summation

Temporal summation occurs when a rapidly firing presynaptic neuron causes EPSPs that are close together in time, leading to a greater depolarization.

• Temporal = Time: Multiple EPSPs from the same location, occurring in quick succession, can add together.

Spatial Summation

Spatial summation occurs when EPSPs are generated at different locations on the neuron at the same time, combining their effects.

• Spatial = Location: Multiple presynaptic neurons fire simultaneously at different synapses on the postsynaptic neuron.

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP)

An EPSP is a local depolarization of the postsynaptic membrane, making the neuron more likely to fire an action potential.

• Depolarization: EPSPs bring the neuron closer to the action potential threshold.

• "Fire" Signal: EPSPs encourage the neuron to generate an action potential.

Inhibitory Postsynaptic Potential (IPSP)

An IPSP is a hyperpolarization of the postsynaptic membrane, making the neuron less likely to fire an action potential.

• Hyperpolarization: IPSPs drive the neuron away from the action potential threshold.

• "Don't Fire" Signal: IPSPs inhibit the generation of an action potential.

Key Terms and Definitions

• Neurotransmitter: Chemical messenger released by neurons to transmit signals across a synapse.

• Synaptic Cleft: The small gap between the presynaptic and postsynaptic neurons.

• Postsynaptic Membrane: The membrane of the neuron or effector cell that receives the neurotransmitter signal.

• Action Potential (AP): A rapid, all-or-none electrical signal that travels along the axon of a neuron.

• Graded Potential (GP): A variable-strength change in membrane potential that decays with distance.

• Excitatory Postsynaptic Potential (EPSP): A depolarizing graded potential that increases the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): A hyperpolarizing graded potential that decreases the likelihood of an action potential.

Summary Table: Comparison of EPSPs and IPSPs

Feature	EPSP	IPSP
Effect on Membrane Potential	Depolarization	Hyperpolarization
Action Potential Likelihood	Increases (closer to threshold)	Decreases (farther from threshold)
Neurotransmitter Example	Glutamate	GABA
Ion Movement	Na+ influx	K+ efflux or Cl- influx

Relevant Equations

• Membrane Potential Change (Graded Potential):

$\Delta V = \frac{I_{syn}}{g_{syn}}$

• Summation of Potentials:

$V_{total} = V_{EPSP1} + V_{EPSP2} + \ldots + V_{IPSP1} + V_{IPSP2} + \ldots$

Additional info: Glutamate is a common excitatory neurotransmitter, while GABA is a common inhibitory neurotransmitter in the central nervous system.