Neurophysiology: Synaptic Transmission Study Notes

Neurophysiology: Synaptic Transmission

Introduction

Synaptic transmission is the process by which neurons communicate with other neurons, muscle cells, or glandular cells via specialized junctions called synapses. This process is fundamental to the functioning of the nervous system and underlies all neural signaling, including sensation, movement, and cognition.

Chemical Synapse

Structure and Function

• Chemical synapse: A junction where a presynaptic neuron releases a chemical signal (neurotransmitter) across a synaptic cleft to a postsynaptic cell (neuron, muscle, or glandular cell).

• Neurotransmitters can be excitatory or inhibitory, depending on the type and location of the synapse.

• Types of synapses:

  • Axodendritic: Synapse between axon and dendrite.

  • Axosomatic: Synapse between axon and cell body.

  • Axoaxonal: Synapse between axon and another axon.

Example: Motor neurons synapsing onto muscle fibers to initiate contraction.

Mechanism of Synaptic Transmission

Steps in Chemical Synaptic Transmission

• An action potential arrives at the presynaptic terminal.

• Voltage-gated Ca2+ channels open, allowing Ca2+ influx.

• Ca2+ triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.

• Neurotransmitter binds to receptors on the postsynaptic membrane, opening ion channels and generating a graded potential.

Key Point: Binding of neurotransmitter opens ion channels, resulting in graded potentials (depolarization or hyperpolarization).

Summation

Temporal and Spatial Summation

• Temporal Summation: Graded potentials arrive at the postsynaptic neuron in rapid succession, allowing them to add together and potentially reach threshold for an action potential.

• Spatial Summation: Simultaneous graded potentials from multiple presynaptic neurons combine at the postsynaptic membrane, enhancing the overall effect.

Example: Multiple excitatory inputs arriving at the same time can trigger an action potential, even if each input alone is subthreshold.

Neural Integration of EPSPs and IPSPs

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

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

• The postsynaptic neuron integrates all incoming EPSPs and IPSPs at the trigger zone.

Facilitation and Inhibition

• If Excitatory > Inhibitory, but below threshold: Facilitation

• If Excitatory > Inhibitory, and reaches threshold: Action Potential (AP)

• If Inhibitory > Excitatory: Inhibition

Neurotransmitter Effects

Modification of Neurotransmitter Effects

• Synthesis of neurotransmitter can be stimulated or inhibited.

• Release can be blocked or enhanced.

• Removal from synaptic cleft can be stimulated or blocked.

• Receptor sites can be blocked or activated.

• Agonist: Substance that enhances a neurotransmitter's effects.

• Antagonist: Substance that blocks the action of a neurotransmitter.

Presynaptic Inhibition and Facilitation

Mechanisms

• Presynaptic inhibition: Release of neurotransmitter from the presynaptic neuron is reduced, often by inhibitory neurotransmitters (e.g., GABA), leading to decreased postsynaptic response.

• Presynaptic facilitation: Release of neurotransmitter is increased, often by excitatory neurotransmitters (e.g., serotonin), leading to enhanced postsynaptic response.

Example: GABA-mediated inhibition reduces Ca2+ entry and neurotransmitter release; serotonin-mediated facilitation increases Ca2+ entry and neurotransmitter release.

Synaptic Integration

Divergent and Convergent Circuits

• Divergence: One neuron sends signals to multiple target neurons ("one to many").

  • Example: A single neuron in the brain can activate hundreds of motor neurons in the spinal cord.

• Convergence: Multiple neurons send signals to a single target neuron ("many to one").

  • Example: Many photoreceptor cells in the retina converge on a single sensory neuron to aid in vision.

Circuit Type	Pattern	Example
Diverging	One input, many outputs	Motor neuron activating many muscle fibers
Converging	Many inputs, one output	Different sensory stimuli eliciting the same memory

Postsynaptic Cell Responses

Types of Receptors

• Channel-linked receptors: Directly open or close membrane ion channels, causing rapid synaptic transmission (depolarization or hyperpolarization).

• G Protein-linked receptors: Neurotransmitter binding activates intracellular second messengers (e.g., cAMP), leading to a variety of effects:

  • Open/close ion channels

  • Activate/inactivate enzymes

  • Turn on/off protein synthesis

Result: The effect depends on the type of neurotransmitter and the receptor type present on the postsynaptic cell. Not all cells have the same types of receptors.

Signal Termination

Mechanisms of Termination

• Uptake: Neurotransmitter is taken back into the presynaptic cell or adjacent neuron/glial cells for reuse or degradation.

• Enzymatic inactivation: Enzymes break down neurotransmitter in the synaptic cleft (e.g., acetylcholinesterase degrades acetylcholine).

• Diffusion: Neurotransmitter diffuses away from the synapse into surrounding tissue or blood.

Termination Method	Description	Example
Uptake	Reuptake into presynaptic neuron or glial cell	Serotonin reuptake
Enzymatic inactivation	Breakdown by enzymes in synaptic cleft	Acetylcholinesterase acting on acetylcholine
Diffusion	Neurotransmitter diffuses away from synapse	General neurotransmitter clearance

Additional info: The above notes expand on the original slides by providing definitions, examples, and context for each process, making the guide self-contained for exam preparation.