Signal Transmission at Synapses

Synapse

A region where communication occurs between two neurons or between a neuron and an effector cell

Synapse: Function

Allows information to be filtered and integrated that are essential for homeostasis

Synapse: Presynaptic Neuron

The neuron that carries a nerve impulse toward a synapse and sends the signa

Synapse: Postsynaptic Cell

A cell that receives the action potential, whether it be a postsynaptic neuron or an effector cell

Synapse: Postsynaptic Neuron

It receives a signal at the synapse and carries a nerve impulse away from the synapse.

Synapse: Effector Cell

It responds to the signal at the synapse

Axodendritic

Refers to synapses between neurons

Axosomatic

Refers to synapses between axon to dendrite

Axoaxonal

Refers to synapses between axons and cell body

Electrical Synapses

A region where action potentials conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions, most common in smooth muscle, cardiac muscle, and the developing embryo.

Electrical Synapses: Advantages

Faster Communication and Synchronization

Electrical Synapses: Advantages: Communication

Action potential passes directly from the presynaptic cell to the postsynaptic cell, unlike the delay communication at the chemical synapse

Electrical Synapses: Advantages: Synchronization

A large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions

Chemical Synapses

A region where neurons indirectly sends a message to another cell using chemical messengers called neurotransmitters, as they are separated by a synaptic cleft

Chemical Synapses: Synaptic Cleft

The small space (about 20–50 nm wide) separating the presynaptic and postsynaptic membranes at a chemical synapse that is filled with interstitial fluid

Chemical Synapse: Communication

Because the nerve impulse cannot cross the synaptic cleft, the presynaptic neuron converts the electrical signal into a chemical signal using a neurotransmitter across the synaptic cleft

Chemical Synapse: Movement

The neurotransmitter diffuses across the synaptic cleft through the interstitial fluid to eventually bind to receptors on the postsynaptic neuron.

Chemical Synapse: Postsynaptic Potential

A type of graded potential produced in the postsynaptic neuron in response to a neurotransmitter.

Chemical Synapse: Signal Sequence

Nerve impulse → neurotransmitter release → neurotransmitter crosses synaptic cleft → binds to postsynaptic receptors → postsynaptic potential.

Synaptic Delay

The short delay (about 0.5 msec) occurs while a neurotransmitter is released, crosses the synaptic cleft, and produces a response in the postsynaptic neuron.

Chemical Synapse: Example Pathway: Step 1

A nerve impulse arrives at the synaptic end bulb or varicosity of the presynaptic axon.

Chemical Synapse: Example Pathway: Step 2

The depolarizing phase of the nerve impulse opens voltage-gated Ca²⁺ channels in the synaptic end bulb to let Ca²⁺ flow into the presynaptic neuron due to the concentration gradient

Chemical Synapse: Example Pathway: Step 3

The increase in Ca²⁺ inside the presynaptic neuron triggers exocytosis of synaptic vesicles, which releases neurotransmitter molecules into the synaptic cleft.

Chemical Synapse: Example Pathway: Step 4

Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron.

Chemical Synapse: Example Pathway: Step 5

Neurotransmitter binding to ligand-gated channels opens the channels, allowing specific ions to flow across the postsynaptic membrane.

Chemical Synapse: Example Pathway: Step 6

Ion movement through the opened channels changes the voltage across the postsynaptic membrane, producing a postsynaptic potential.

Chemical Synapse: Step 7

When a depolarizing postsynaptic potential reaches threshold, it triggers a nerve impulse in the axon of the postsynaptic neuron.

Postsynaptic Potential: Threshold

A depolarizing postsynaptic potential must reach threshold to trigger a nerve impulse in the postsynaptic neuron.

Chemical Synapse: Overall Sequence

Nerve impulse → Ca²⁺ channels open → Ca²⁺ enters → synaptic vesicles release neurotransmitter → neurotransmitter binds receptors → ion channels open → postsynaptic potential forms → threshold reached → nerve impulse occurs.

Neurotransmitter: Types of Graded Potential

Either produces an excitatory or an inhibitory graded potential

Neurotransmitter: Excitatory Postsynaptic Potential

A temporary depolarization of a postsynaptic membrane caused by the influx of positively charged ions into the cell, but it does not initiate an action potential

Neurotransmitter: Inhibitory Postsynaptic Potential

A temporary hyperpolarization of a postsynaptic membrane caused by the efflux of positively charged ions out of the cell, making it less likely to fire an action potential

Neurotransmitter Receptors: Types

Based on whether the neurotransmitter binding site and the ion channel are components of the same protein or are components of different proteins: ionotropic receptors or metabotropic receptors

Ionotropic Receptors

A neurotransmitter receptor that contains the neurotransmitter binding site and the ion channel, which are components of the same protein, is also known as a ligand-gated channel

Ionotropic Receptor: No Neurotransmitter

When no neurotransmitter (ligand) is bound to an ionotropic receptor, its ion channel remains closed.

Ionotropic Receptor: Neurotransmitter Binding

When the correct neurotransmitter binds to an ionotropic receptor, its ion channel opens, producing either an EPSP or an IPSP

Ionotropic Receptor: Excitatory Postsynaptic Potential

Many excitatory neurotransmitters bind to ionotropic receptors containing cation channels that allow Na⁺, K⁺, and Ca²⁺ to cross the postsynaptic membrane

Ionotropic Receptor: Excitatory Postsynaptic Potential: Effect

An EPSP depolarizes the postsynaptic cell, making it more likely to reach threshold and generate a nerve impulse.

Ionotropic Receptor: Inhibitory Postsynaptic Potential

Many inhibitory neurotransmitters bind to ionotropic receptors containing Cl⁻ channels that allow Cl⁻ ions to diffuse into the postsynaptic cell

Ionotropic Receptor: Inhibitory Postsynaptic Potential: Effect

An IPSP hyperpolarizes the postsynaptic cell, making it less likely to reach threshold and generate a nerve impulse.

EPSP vs. IPSP: Main Difference

EPSP = depolarization → more likely to fire. IPSP = hyperpolarization → less likely to fire.

Metabotropic Receptors

A ligand-gated channel that contains the neurotransmitter binding site but lacks an ion channel, yet coupled by

Metabotropic Receptor

A neurotransmitter receptor that has a neurotransmitter-binding site but does not contain an ion channel as part of its structure, but is connected to a separate ion channel through a membrane protein called a G protein.

Metabotropic Receptor: G Protein

A G protein is a membrane protein that connects a metabotropic receptor to a separate ion channel.

Metabotropic Receptor: G Protein: Function

When a neurotransmitter binds to the receptor, the G protein can either directly open or close the ion channel, or indirectly activate a “second messenger” in the cytosol, which then opens or closes the ion channel.

Metabotropic Receptor: Direct Pathway

Neurotransmitter binds → G protein activates → G protein directly opens/closes ion channel.

Metabotropic Receptor: Indirect Pathway

Neurotransmitter binds → G protein activates → second messenger activates → ion channel opens/closes.

Metabotropic Receptor: Inhibitory Function

Some inhibitory neurotransmitters bind to metabotropic receptors that are connected to K⁺ channels.

Metabotropic Receptor: K⁺ Channels

When an inhibitory neurotransmitter activates a metabotropic receptor, the receptor can cause its linked K⁺ channels to open and allow K⁺ diffuses out of the postsynaptic cell, making the cell more negative (hyperpolarized)

Metabotropic Receptor: IPSP

Opening K⁺ channels through an inhibitory metabotropic receptor produces an IPSP by causing K⁺ to leave the postsynaptic cell and making the inside more negative.

IPSP: K⁺ Mechanism

K⁺ exits the postsynaptic cell → inside becomes more negative → hyperpolarization → IPSP → neuron is less likely to fire.

Neurotransmitter: Different Effects

The same neurotransmitter can be excitatory at one synapse and inhibitory at another, depending on the type of receptor it binds to.

Neurotransmitter Removal

Neurotransmitters must be removed from the synaptic cleft so their effects on the postsynaptic cell do not continue indefinitely to prepare the synapse for another signal.

Neurotransmitter Removal: Methods

Diffusion, Enzymatic Degradation, and Uptake by Cells.

Neurotransmitter Removal: Diffusion

Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect.

Neurotransmitter Removal: Enzymatic Degradation

Some enzymes could break down certain neurotransmitters

Neurotransmitter Removal: Uptake

Neurotransmitters can be removed from the synaptic cleft by being actively transported back into the neuron that released them (reuptake) or into neighboring neuroglia (uptake) by neurotransmitter transporters.

Synaptic Integration

The process by which a postsynaptic neuron combines (sums) the postsynaptic potentials it receives from many synapses.

Synaptic Input: Number of Synapses

A typical neuron in the CNS receives input from about 1,000–10,000 synapses.

Synaptic Integration: Summation

Summation is the process in which graded potentials, including EPSPs and IPSPs, add together to increase the likelihood of meeting the treshold.

Summation: Types

Spatial and Temporal Summation

Summation: Spatial

Process in which several presynaptic end bulbs release neurotransmitters simultaneously, producing postsynaptic potentials at different locations at the same time.

Summation: Temporal

Process in which one presynaptic neuron repeatedly releases neurotransmitter, producing postsynaptic potentials at same locations at different time.

Synaptic Integration: Overall Effect

The postsynaptic neuron's response depends on the sum of all excitatory and inhibitory effects acting on it at that time.

Synaptic Integration: EPSP

If excitatory effects are greater than inhibitory effects but still below threshold, an EPSP occurs without triggering a nerve impulse because the neuron is partially depolarized; subsequent stimuli can more easily reach threshold through summation.

Synaptic Integration: Nerve Impulse

If total excitatory effects are greater than inhibitory effects and reach threshold, one or more nerve impulses are triggered and continue as long as the EPSP remains at or above threshold.

Synaptic Integration: IPSP

If total inhibitory effects are greater than excitatory effects, the postsynaptic membrane hyperpolarizes (IPSP), making the neuron less likely or unable to reach threshold and generate a nerve impulse.