Synaptic Transmission and Graded Potentials: Study Notes for Anatomy & Physiology

Synapses

General Overview

Synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells, such as muscles or glands. They are essential for neural communication and integration in the nervous system.

• Electrical synapses: Direct cytoplasmic connections (gap junctions) allowing ions to flow rapidly between cells, enabling fast transmission.

• Chemical synapses: Use neurotransmitters to transmit signals across a synaptic cleft; slower but allow for complex modulation.

Key Terms: Presynaptic neuron, Postsynaptic neuron, Neurotransmitter, Synaptic cleft

Synaptic Mechanisms

Release and Recycling of Neurotransmitters

Neurotransmitters are released from the presynaptic axon terminal in response to an action potential. After release, they bind to receptors on the postsynaptic cell, initiating a response.

• Release: Triggered by calcium influx into the presynaptic terminal, causing vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft.

• Recycling: Neurotransmitters are removed from the synaptic cleft by reuptake into the presynaptic cell, enzymatic degradation, or diffusion away from the synapse.

Neurotransmitter Inactivation

To prevent continuous stimulation, neurotransmitters must be inactivated after their release.

• Enzymatic breakdown: Example: Acetylcholine is broken down by acetylcholinesterase.

• Reuptake: Neurotransmitters like serotonin and dopamine are taken back up into the presynaptic neuron.

• Diffusion: Some neurotransmitters simply diffuse away from the synaptic cleft.

Synaptic Transmission

Post-synaptic Steps

After neurotransmitter release, the postsynaptic cell responds via specific receptor mechanisms.

• Chemically gated ion channels (Ionotropic receptors): Neurotransmitter binding directly opens or closes ion channels, causing rapid changes in membrane potential.

• G protein-coupled receptors (Metabotropic receptors): Neurotransmitter binding activates intracellular signaling cascades, which can indirectly open or close ion channels and produce slower, longer-lasting effects.

Example: Glutamate acts on ionotropic receptors to excite neurons, while GABA acts on ionotropic receptors to inhibit neurons.

Classes of Postsynaptic Receptors

Receptor Type	Speed	Mechanism	Effect
Ionotropic	Fast (<100 ms)	Directly opens/closes ion channels	Rapid depolarization or hyperpolarization
Metabotropic	Slow (>100 ms)	Activates G-protein signaling; may open/close channels indirectly	Amplification, modulation, longer-lasting effects

Synaptic Potentials: Summation and Integration

Graded Potentials

Graded potentials are changes in membrane potential that vary in size and decay with distance from the site of origin. They are crucial for integrating synaptic inputs and determining whether a neuron will fire an action potential.

• Excitatory postsynaptic potential (EPSP): Depolarizes the membrane, increasing the likelihood of action potential generation.

• Inhibitory postsynaptic potential (IPSP): Hyperpolarizes the membrane, decreasing the likelihood of action potential generation.

Summation:

• Spatial summation: Multiple synapses activate simultaneously at different locations, combining their effects.

• Temporal summation: Rapid, repeated activation of a single synapse increases the overall effect.

Integration: The neuron integrates all EPSPs and IPSPs at the axon hillock (trigger zone) to determine if threshold is reached for action potential initiation.

Threshold and Action Potential Initiation

The threshold is the membrane potential at which an action potential is triggered. If the sum of graded potentials at the trigger zone reaches threshold, a runaway feedback loop leads to an action potential.

• Typical resting membrane potential: -70 mV

• Threshold potential: -55 mV

Equation (Nernst):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Equation (Goldman-Hodgkin-Katz):

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Termination of Synaptic Transmission

Mechanisms of Termination

Synaptic transmission is terminated by removing neurotransmitter from the synaptic cleft.

• Reuptake: Transporters pump neurotransmitter back into the presynaptic cell.

• Enzymatic degradation: Enzymes break down neurotransmitter molecules.

• Diffusion: Neurotransmitter diffuses away from the synapse.

Neurotransmitter Types and Functions

Main Neurotransmitters

Name	Main Location(s)	Main Function(s)
Glutamate (Glu)	CNS (everywhere)	Excites neuronal firing
GABA	CNS (everywhere)	Inhibits neuronal firing
Glycine	CNS (scattered)	Inhibition in many cases
Acetylcholine (ACh)	Nerve-muscle connections; Autonomic synapses; Some CNS synapses	Stimulates muscle contraction; slows heart rate; "Rest and digest"
Norepinephrine (NE)	Autonomic synapses; Some CNS synapses	Speeds heart rate; "Fight or flight"; emotion, arousal
Serotonin (5-HT)	Pons, Medulla, etc.	Broad effects on mood, sleep, and arousal
Dopamine (DA)	Basal ganglia; Frontal cortex; Limbic system	Motivation, reward, movement

Other Neurotransmitter Types

• Purines: AMP and ATP

• Gases: Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S)

• Peptides: Substance P, opioid peptides

• Lipid-derived: Eicosanoids, cannabinoids

Key Concepts

• There are two main types of postsynaptic receptors: ionotropic and metabotropic.

• The response of the postsynaptic cell depends on the receptor type, not the neurotransmitter itself.

• Drugs can mimic or inhibit neurotransmitter action by targeting receptors.

• Synaptic responses can be integrated through summation, affecting neuronal output.

Additional info: Some context and terminology were inferred and expanded for clarity and completeness, including definitions, equations, and examples.