2-4 Synaptic Transmission

Synaptic Transmission Overview

• Synaptic transmission is the process by which a neuron communicates with its target cell.

• It involves the release of neurotransmitters from the presynaptic neuron which bind to receptors on the postsynaptic neuron.

• Key ions involved: Na+ and K+ play roles in action potentials and synaptic transmission.

• Depolarization and hyperpolarization of the postsynaptic neuron contribute to the signal processing and transmission.

Components of Synaptic Transmission

Pre Synaptic Neuron

• Action Potential: A rapid rise and fall in membrane potential that allows communication.

• Voltage-gated Na+ channel: Opens up during the action potential to allow Na+ influx, leading to depolarization.

• Ligand-gated Na+ channel: Opens in response to the binding of neurotransmitters, allowing Na+ entry.

Post Synaptic Neuron

• Receives action potential from the presynaptic neuron through chemical signals.

• Action potential in post synaptic neuron can be triggered by neurotransmitter binding.

Types of Synapses

Synapse Definition

• A synapse is a junction where communication occurs between neurons or between a neuron and another cell (muscle or gland).

• Two main types: Electrical and Chemical.

Electrical Synapses

• Found in smooth muscle, cardiac muscle, and certain neurons.

• Cells are connected via gap junctions, allowing direct electrical communication.

• Allow rapid signaling through direct cell contact.

Chemical Synapses

• Most common type involving neurotransmitters.

• Neurotransmitter released from the presynaptic terminal diffuses across the synaptic cleft.

• Allows communication between cells not in direct contact.

Neurotransmitter Release

• The process of neurotransmitter release involves:

  1. Arrival of Action Potential: Triggers voltage-gated Ca2+ channels to open.

  2. Calcium Influx: Calcium ions (Ca2+) enter the presynaptic neuron, triggering neurotransmitter release from vesicles by exocytosis.

  3. Neurotransmitter Action: Neurotransmitters bind to specific receptors on the postsynaptic neuron, causing graded potentials.

Types of Neurotransmitters

Acetylcholine (ACh)

• ACh opens ligand-gated channels directly when bound to receptors.

• Excitatory in skeletal muscle; effects vary in autonomic nervous system.

• Cleared away by Acetylcholinesterase (AChE), which hydrolyzes ACh into acetate and choline.

Monoamines

• Neuromodulators derived from single amino acids.

• Include serotonin, histamine, catecholamines (dopamine, norepinephrine, epinephrine).

• Process of action involves release via exocytosis and reuptake by presynaptic neuron.

• Monoamine oxidase (MAO): Enzyme responsible for degrading monoamines, targeted by certain medications.

Summary of Synaptic Communication

Summary of Synaptic Communication

• Very fast signaling process with local action.

• Requires close proximity between communicating neurons.

• Composition of signaling is dictated by the types of neurotransmitters and receptors involved in the synapse.