Name: how neurons signal at synapses
Uploaded: 2023-04-30T11:35:22.000Z
Duration: 1 min 26 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Life Science

SC.912.L.14

SC.912.L.14.24

Chemical Synapses

Chemical synapses are specialized sites where electrical signals from a presynaptic neuron are relayed by chemical messengers, known as neurotransmitters, to the postsynaptic cell to propagate an electrical impulse.
They consist of an axon terminal of a presynaptic cell, here a neuron, containing numerous neurotransmitter-filled synaptic vesicles, a postsynaptic cell, in this case, another neuron, with neurotransmitter receptors, and the synaptic cleft, a fluid-filled space between the two.
The unidirectional information transfer at a chemical synapse begins with the arrival of an action potential at the axon terminal of the presynaptic neuron. The resulting membrane depolarization triggers voltage-gated calcium channels to open, allowing calcium ions to rush in. This calcium surge causes the synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
After diffusing through the synaptic cleft, the neurotransmitters bind to specific ion channels on the postsynaptic membrane. This binding opens specific ion channels, allowing appropriate ions to enter the postsynaptic cell and initiate excitatory or inhibitory responses.

chemical synapses

Chemical synapses are specialized sites between two neurons or between a neuron and a non-neuronal cell like a muscle, glandular or sensory cell.
Because chemical synapses depend on the release of neurotransmitter molecules from synaptic vesicles to pass on their signal, there is an approximately one millisecond delay between when the axon potential reaches the presynaptic terminal and when the neurotransmitter leads to opening of postsynaptic ion channels. Additionally, this signaling is unidirectional.
A synaptic cleft, usually 20-50 nm wide, separates the pre-and postsynaptic membranes.&#160; When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open. Calcium ions entering the cell initiate a signaling cascade that causes small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with the presynaptic membrane.
Fusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic cleft, the extracellular space between the presynaptic and postsynaptic membranes. The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane. The binding of a specific neurotransmitter causes particular ion channels, like ligand-gated channels, on the postsynaptic membrane to open. Neurotransmitters can either have excitatory or inhibitory effects on the postsynaptic membrane.
Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the postsynaptic membrane can &#34;reset&#34; and be ready to receive another signal.
Depending on the signal, a few or many neurotransmitter vesicles may be released, bringing about an excitatory or inhibitory postsynaptic response. This can result in an increased or decreased postsynaptic membrane potential depending on the bound neurotransmitter. Additionally, as the neurotransmitters&#39; availability in the synaptic cleft is regulated, this helps fine-tune the neuronal signal.
Clinical Relevance
Lambert-Eaton myasthenic syndrome is an autoimmune disorder wherein antibodies are targeted against the voltage-gated calcium channels that trigger the acetylcholine neurotransmitter release. Thus, the lowered levels of acetylcholine are insufficient to cause regular muscle contractions, resulting in muscle weakness.
In Myasthenia gravis, another autoimmune syndrome, in which autoantibodies block the acetylcholine from binding the postsynaptic membrane receptor at the neuromuscular junction, inhibiting contraction. This predominantly results in muscle weakness and diminished facial expression.
This text is adapted from <a href="https://openstax.org/books/biology-2e/pages/35-2-how-neurons-communicate">Openstax, Biology 2e, Section </a><a href="https://openstax.org/books/biology-2e/pages/35-2-how-neurons-communicate">35.2 How Neurons Communicate.</a>

Chemical synapses are specialized sites between two neurons or between a neuron and a non-neuronal cell like a muscle, glandular or sensory cell.
Because ...

Education

JoVE Core

Cell Biology

Unit 1

Channels and the Electrical Properties of Membranes

channels-and-the-electrical-properties-of-membranes

KEY TERMS AND CONCEPTS

how neurons signal at synapses

Chemical synapses let neurons pass signals to another neuron, a muscle cell, a gland cell, or a sensory cell. They use neurotransmitter molecules released from synaptic vesicles to carry the message. Because of that process, there is a short delay of about one millisecond between the arrival of the action potential and the opening of ion channels on the postsynaptic cell. Chemical synapses also send information in one direction only.

A synaptic cleft separates the presynaptic and postsynaptic membranes. This gap is usually 20 to 50 nm wide. When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell and make the presynaptic membrane even more depolarized.

That depolarization opens voltage-gated calcium channels. Calcium ions enter the terminal and start a signaling cascade. This cascade makes small membrane-bound synaptic vesicles fuse with the presynaptic membrane. The vesicles contain neurotransmitter molecules, so fusion releases neurotransmitter into the synaptic cleft.

The neurotransmitter then diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane. This binding opens specific ion channels, such as ligand-gated channels. Depending on the neurotransmitter and the signal, the postsynaptic response can be excitatory or inhibitory. The membrane potential may increase or decrease as a result.

After neurotransmission, the neurotransmitter must be cleared from the synaptic cleft. This reset step prepares the postsynaptic membrane to receive another signal. The amount of neurotransmitter released can also vary, since a few or many vesicles may fuse. That variation helps fine-tune the neuronal signal.

These synapses also matter in disease. In Lambert-Eaton myasthenic syndrome, antibodies target the voltage-gated calcium channels that trigger acetylcholine release. The lower acetylcholine level is not enough for normal muscle contraction, so muscle weakness results. In myasthenia gravis, autoantibodies block acetylcholine from binding to its receptor at the neuromuscular junction. This also leads to muscle weakness and a reduced facial expression.

How Neurons Signal at Synapses

aquaporins

Aquaporins and Membrane Transport

Aquaporins are membrane proteins that move water across cell membranes. Some aquaporins, called aquaglyceroporins, also transport glycerol. A few aquaporins may also help move volatile substances such as carbon dioxide and ammonia. These channels are found in many different tissues. Water-selective aquaporins are important in the kidneys, exocrine glands, and central nervous system. Aquaporins that may act as gas channels are often highly expressed in red blood cells, epithelial cells, and...

non-gated ion channels

Leak Channels in Resting Membranes

Leak channels are ion channels that let charged ions move across the plasma membrane without a gate. These proteins help maintain membrane potential, which is essential for cell survival. They can move more than a thousand ions in milliseconds during a cellular event. Unlike gated ion channels, non-gated channels have no opening trigger. They are also called leakage or passive channels. Instead, they switch between open and closed states at an intrinsic rate, so ions keep “leaking” through...

ligand-gated ion channels

How Ligand-Gated Ion Channels Work

Ligand-gated ion channels are membrane proteins that open when a chemical signal binds to them. They have a channel for ions to pass through and a ligand-binding site. When the ligand attaches, the channel opens and ions can move across the cell membrane. These channels are grouped into three subfamilies. The Cys-loop group includes nicotinic acetylcholine receptors, gamma-aminobutyric acid (GABA) receptors, glycine receptors, and 5-hydroxytryptamine receptors. The pore-loop group includes...

voltage-gated ion channels

How Voltage Gated Channels Control Signals

Voltage-gated ion channels control electrical signals in neurons, heart cells, and muscle cells. These transmembrane proteins open and close when the membrane potential changes. They sit in the cell membrane of electrically excitable cells and help ions move across it. Each channel includes a voltage-sensing domain that spans the lipid bilayer. Charged residues in this sensor shift when the membrane potential changes. That movement opens the channel and allows ions to pass through. Different...

mechanically-gated ion channels

Touch and Heat Sensors in Cell Membranes

Mechanically-gated ion channels are membrane proteins that open when cells experience physical stress. Tension, compression, swelling, and shear stress can change the shape of the protein. That change opens a transmembrane pore and lets ions pass to help transmit signals. In eukaryotes, these channels are found in several body regions, including neurons, lungs, skin, bladder, and heart. They support many normal physiological processes. They are also involved in disease when their function...

neuron structure

Neuron Signaling and Synapses

Neurons are the main cells of the nervous system that generate and pass along electrochemical signals. They communicate with one another mainly through neurotransmitters at special connections called synapses. Neurons can have different shapes, and those shapes often match their function. Most, however, share three main parts: a cell body, dendrites, and an axon. The cell body, or soma, contains the nucleus and the organelles needed for cell activity. Dendrites extend from the soma and usually...

resting membrane potential

How Neurons Keep a Negative Voltage

Neurons keep a negative voltage across their membrane when they are at rest. This membrane potential is the voltage difference between the inside and outside of a cell. In a resting neuron, it is usually around -70 millivolts, which means the inside is more negative than the outside and the cell is polarized. This negative charge depends on two main factors. The membrane is selectively permeable, so most ions and molecules cannot cross the lipid bilayer without help from ion channel proteins.

resting potential decay

Resting Membrane Potential Maintenance

The resting membrane potential stays near -70 mV in a neuron because the membrane allows ions to move in a selective way. At rest, the membrane is slightly permeable to sodium ions (Na+) and chloride ions (Cl-) and highly permeable to potassium ions (K+). Ion channels and pumps keep the concentration differences between the inside and outside of the cell in place. Potassium leak channels let K+ move out of the cell at rest. This outward movement makes the inside of the cell more negative. A...

action potential

How Nerve Signals Travel Down Axons

Nerve signals travel down axons as action potentials, which are electrochemical pulses that let neurons communicate. This signal moves to the axon terminals and triggers neurotransmitter release, passing information through the nervous system. An action potential is an all-or-none change in membrane potential that produces a fast spike in voltage. Neurons usually rest at about -70 millivolts. When they receive input from neurotransmitters or sensory stimuli, the membrane potential can become...

channel rhodopsins

Light-Gated Ion Flow in Channel Rhodopsins

Channel rhodopsins are light-sensitive ion channels found in some organisms. They belong to the rhodopsin family, which also includes photoreceptors in bacteria, plants, and the vertebrate retina. These proteins use bound chromophores, such as retinal in rhodopsins and bilin in phytochromes, to respond to light. Rhodopsins are a group of cell surface proteins called G-protein coupled receptors. These proteins help move signals across biological membranes. Over evolution, the rhodopsin family...

patch clamp

Recording Ion Channel Currents with Patch Clamp

Patch clamp is a lab method used to record electrical currents across a cell membrane. It helps scientists study how ions move through channels in the membrane. These electrical signals are important for functions such as muscle contraction and nerve transmission. The method uses a glass micropipette filled with electrolyte solution. The pipette is sealed tightly to a small part of the cell membrane. This creates an electrically isolated patch, so ions moving through the channels flow into the...

electrical synapses

Gap Junctions in Electrical Signaling

Electrical synapses use gap junctions to pass signals directly between neurons and other cells. The presynaptic and postsynaptic membranes sit very close together, about 3.5 nm apart. Channel proteins connect the two cells and create a direct path for current. Unlike a chemical synapse, an electrical synapse does not rely on neurotransmitters crossing a synaptic cleft. The gap junction is built from two hexameric connexin hemichannels, also called connexons. Each adjacent cell contributes one...

Chemical synapses let neurons pass signals to another neuron, a muscle cell, a gland cell, or a sensory cell. They use neurotransmitter molecules released from synaptic vesicles to carry the message. Because of that process, there is a short delay of about one millisecond between the arrival of the action potential and the opening of ion channels on the postsynaptic cell. Chemical synapses also send information in one direction only. A synaptic cleft separates the presynaptic and postsynaptic...

excitatory and inhibitory effects of neurotransmitters

excitatory-and-inhibitory-effects-of-neurotransmitters

How Neurotransmitters Trigger or Block Signals

Neurotransmitters can either trigger or block signals at a chemical synapse. When an action potential reaches the presynaptic axon terminal, it causes the neuron to release neurotransmitters into the synaptic cleft. The released molecule then binds to specific receptors on the postsynaptic cell membrane. A molecule is usually identified as a neurotransmitter when it is present in the presynaptic neuron, released in response to strong presynaptic depolarization, and matched with receptors on...

muscle contraction

How Muscles Receive Signals to Contract

Muscle contraction begins when a nerve signal reaches a skeletal muscle fiber. At the motor endplate, a motor neuron releases acetylcholine, a neurotransmitter that carries the signal to the muscle cell. Acetylcholine binds to receptors on the sarcolemma, the muscle cell membrane, and lets sodium ions enter the cell. This starts an action potential in the muscle fiber and helps pass electrical signals from the brain to the muscle. The signal must also be controlled so the muscle does not stay...

the role of ion channels in neuronal computation

the-role-of-ion-channels-in-neuronal-computation

How Ion Channels Shape Neuron Firing

Ion channels help neurons decide when to fire an action potential. A postsynaptic neuron often receives signals from many presynaptic neurons, and the axon hillock adds those inputs together to judge whether firing is likely. In some cases, one strong EPSP, or excitatory postsynaptic potential, is enough to trigger a spike. In many cases, several EPSPs must arrive close together for the cell to reach threshold. This signal adding process is called summation. It can be temporal when one...

long-term potentiation

How Synapses Strengthen During Learning

Long-term potentiation, or LTP, is a way the brain strengthens synapses over time. A synapse is the chemical connection between neurons. LTP is one form of synaptic plasticity, which means changes in synapse strength. It works opposite to long-term depression, or LTD, which weakens synapses. Together, LTP and LTD are major mechanisms behind learning and memory. LTP can happen when a presynaptic neuron repeatedly fires and stimulates a postsynaptic neuron. This is called Hebbian LTP. It follows...

long-term depression

Synaptic Weakening in Memory Circuits

Long-term depression, or LTD, is a form of synaptic weakening in the brain. It is one way synaptic plasticity, or changes in the strength of chemical synapses, can occur over time between presynaptic and postsynaptic neurons. LTD works opposite long-term potentiation, or LTP, which strengthens synapses. Together, LTD and LTP help support learning and memory. LTD also helps keep the brain flexible. If synapses stayed maximally strengthened for too long, the brain would become less efficient and...

key terms and concepts

Trace how chemical synapses work, from calcium entry and vesicle fusion to neurotransmitter release, postsynaptic responses, and related muscle weakness disorders.

How Neurons Signal at Synapses | JoVE K12

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How Neurons Signal at Synapses

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