Lecture 2: Synapses

Synapse

The anatomical structure where an impulse from one neuron is transmitted to another neuron or to an effector cell (muscle or gland).

Neural Transmission in CNS

Information is transmitted primarily via nerve action potentials (nerve impulses) in a chain of connected neurons.

Synaptic Functions of Neurons

Action potentials at synapses can be blocked, changed into prolonged/repetitive impulses, or integrated with other impulses.

Electrical Synapse

A type of synapse where two cells are directly connected via gap junctions for fast communication and synchronization.

Electrical Synapse Advantages

1. Fast communication (no synaptic delay) 2. Synchronization of groups of neurons or muscle fibers, increasing their sensitivity

Common Locations of Electrical Synapses

Found in cardiac and smooth muscle for synchronized contractions.

Directionality of Electrical Synapses

Bidirectional — impulses can pass in both directions between neurons.

Chemical Synapse

More common type of synapse; involves neurotransmitter release across a fluid-filled cleft (no gap junctions).

Presynaptic Neurotransmitter Release

The presynaptic neuron releases neurotransmitters into the synaptic cleft.

Effects of Neurotransmitter on Postsynaptic Neuron

Neurotransmitters can excite, inhibit, or modulate the sensitivity of the postsynaptic neuron.

Principle of One-Way Conduction

In chemical synapses, signals are transmitted only from the presynaptic to the postsynaptic neuron, ensuring directional control.

Presynaptic Vesicles

Contain neurotransmitters that are released into the synaptic cleft to influence the postsynaptic cell.

Presynaptic Mitochondria Function

Produce ATP to fuel vesicle formation, transport, and neurotransmitter synthesis.

Depolarization of Presynaptic Membrane

Activates voltage-gated calcium channels, leading to neurotransmitter release via exocytosis.

Neurotransmitter Release Amount

Directly proportional to the amount of calcium entering the presynaptic terminal.

Ionotropic Receptors

Ligand-gated ion channels on the postsynaptic membrane; produce fast, direct effects like EPSP or IPSP.

Metabotropic Receptors

Use second messenger systems for slower, prolonged effects such as those involved in memory.

Cation Channels at Synapse

Allow Na+ (and sometimes K+ or Ca2+) to enter, causing excitation (EPSP) by depolarizing the postsynaptic membrane.

Anion Channels at Synapse

Allow Cl⁻ to enter the postsynaptic cell, causing inhibition (IPSP) by hyperpolarizing the membrane.

Fate of Neurotransmitters in Synapse

1. Enzymatic degradation 2. Diffusion out of synaptic cleft 3. Reuptake into the presynaptic neuron for recycling

Excitatory Postsynaptic Potential (EPSP)

Caused by opening Na⁺ channels; depolarizes the membrane, increasing the likelihood of an action potential.

Inhibitory Postsynaptic Potential (IPSP)

Caused by opening Cl⁻ channels; hyperpolarizes the membrane, decreasing the likelihood of an action potential.

Synaptic Sensitivity and Memory

Repeated or prolonged activation can cause long-term potentiation via second messengers, supporting learning and memory.

Why Are Electrical Synapses Faster?

Because the two neurons are connected by gap junctions, allowing ions to flow directly between them in either direction.

Why Are Chemical Synapses One-Way?

Only the presynaptic cell releases neurotransmitters, and the postsynaptic cell has the receptors — preventing reverse transmission.

Importance of Mitochondria in Presynaptic Terminals

They supply energy for neurotransmitter synthesis and vesicle formation, movement, and exocytosis.

Role of Calcium in Neurotransmitter Release

Calcium influx during depolarization triggers vesicle fusion with the membrane, releasing neurotransmitters into the synaptic cleft.