Synaptic transmission

Synaptic transmission

Neurons transmit electrical signals via chemical and electrical synapses.

Biology

Keywords

signal transduction, stimulation, stimulus, signal, neuron, receptor, synapse, dendrite, axon, glial cell, telodendron, terminal bouton, channel protein, action potential, resting potential, basic nervous function, excitatory, inhibitory, neurotransmitter, ion channel, human, biology

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Scenes

Animation

  • dendrite - The short, branched projection of a neuron. It transmits signals towards the cell body.
  • axon - A large number of sodium ion channels are present along the entire length of the axon. These are responsible for the production and spread of the action potential. During neural activity, the receiving neuron is stimulated, which opens Na+ channels in its membrane. The flow of ions results in a local change in the membrane potential. The local changes are summed at the axon hillock. If the membrane potential reaches the threshold, an action potential is triggered, which then spreads along the axon reaching the terminal bouton where it initiates the stimulation of the next nerve cell.
  • cellular membrane - The neuron membrane contains special ion channels that produce the electrical signals of the cells. Stimulation generates local potentials in the membrane of the cell body and the dendrites. The local potentials are summed in the axon hillock and, if the sum reaches the threshold, an action potential is generated on the axon. The amplitude of the action potential is independent of the strength of the stimulus.
  • node of Ranvier - Axons are wrapped in glia cells; these cells collectively form the myelin sheath of the axon, which is an electrically insulating layer. Since the membrane is exposed to the intracellular space only at the gaps of the myelin sheath, known as nodes of Ranvier, the impulse can only form here. It propagates by 'jumping' from node to node. This method of transmission is much faster than in unmyelinated axons, where the impulse travels continuously down the axon like a wave.
  • glia cells - Axons are wrapped in these cells, which form the myelin sheath. This is an electrically insulating layer around the axon. The action potential is produced in the nodes of Ranvier and propagates by 'jumping' from node to node.
  • telodendron
  • terminal bouton - The enlarged terminal of the axon. It contains neurotransmitters. When an action potential arrives to the bouton, the neurotransmitters are released into the synaptic cleft and stimulate the receiving nerve cell.
  • excitatory nerve cell
  • inhibitory nerve cell
  • terminal bouton - The enlarged terminal of the axon. It contains neurotransmitters. When an action potential arrives, the neurotransmitters are released into the synaptic cleft and stimulate the receiving nerve cell.
  • mitochondrion - It provides energy (in the form of ATP) for the energy-consuming processes of the neurons. These processes include the release of ions through the membrane and the production and transport of neurotransmitters.
  • microtubules - They are important components of the cytoskeleton. They play a crucial role in transporting neurotransmitters to the terminal bouton.
  • presynaptic membrane - Synaptic vesicles fuse into this membrane when they release neurotransmitters into the synaptic cleft by exocytosis.
  • postsynaptic membrane - Neurotransmitters released from the terminal bouton diffuse through the synaptic cleft into this membrane. It contains a large number of ion channel receptors that open when the neurotransmitters bind to them.
  • ion channel receptor - When neurotransmitters bind to the receptors, these open up, permitting ions to enter the neuron. Excitatory neurotransmitters open cation channels and thereby depolarise the receiving neuron, while inhibitory neurotransmitters open anion channels and thereby hyperpolarise the receiving neuron.
  • synaptic vesicle - Membrane-enclosed vesicles that store neurotransmitters. When an action potential reaches the terminal bouton, the vesicles release neurotransmitters by exocytosis. Neurotransmitters then diffuse through the synaptic cleft to the membrane of the receiving neuron (postsynaptic membrane). There they bind to receptors and cause ion channels to open up. In excitatory synapses excitatory transmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory transmitters open anion channels and hyperpolarise the cell.
  • ion - They enter the neuron through the ion channels opened up by neurotransmitters. In excitatory synapses, excitatory neurotransmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory neurotransmitters open anion channels and thereby hyperpolarise the receiving neuron.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.
  • neurotransmitters - They are stored in synaptic vesicles in the terminal bouton of the axon. When an action potential reaches the bouton, the vesicles release neurotransmitters by exocytosis. In excitatory synapses, excitatory transmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory neurotransmitters open anion channels and hyperpolarise the cell.
  • postsynaptic membrane - Neurotransmitters released from the terminal bouton diffuse through the synaptic cleft to this membrane. It contains a large number of ion channel receptors that open when the neurotransmitters bind to them.
  • Na⁺ channel - It is opened by excitatory transmitters. This channel lets sodium ions to flow into the postsynaptic cell, which is thereby depolarised. If there is a sufficient amount of neutotransmitters, the depolarisation exceeds the threshold potential of the postsynaptic membrane, which produces an action potential.
  • K⁺ channel - When this channel is open, potassium ions flow out of the postsynaptic cell.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.
  • excitatory neurotransmitter - Most often it is acetylcholine or glutamic acid. Excitatory neurotransmitters open sodium and potassium ion channels in the membrane of the receiving neuron (that is, the postsynaptic membrane). This results in the depolarisation of the membrane. If there is a sufficient amount of neutotransmitters, the depolarisation exceeds the threshold potential of the postsynaptic membrane, which produces an action potential.
  • Excitatory synapse
  • postsynaptic membrane - Neurotransmitters released from the terminal bouton diffuse through the synaptic cleft to this membrane. It contains a large number of ion channel receptors that open when the neurotransmitters bind to them.
  • inhibitory neurotransmitter - Most often it is gamma-Aminobutyric acid (GABA) or glycine. Inhibitory neurotransmitters open chloride and potassium ion channels in the membrane of the receiving neuron (the postsynaptic membrane). This results in the hyperpolarisation of the membrane.
  • K⁺ channel - When this channel is open, potassium ions are released from the postsynaptic cell.
  • Cl⁻ channel - Chloride ions flow into the receiving neuron through this channel.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.
  • Inhibitory synapse
  • mV
  • threshold
  • -70
  • time
  • postsynaptic membrane
  • presynaptic membrane
  • connexon - Also known as a connexin hemichannel, it is a special channel that connects the membrane of the presynaptic and postsynaptic cells. It is mostly depolarising signals that spread via this channel.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.
  • Electrical synapse

Excitatory signal

  • dendrite - The short, branched projection of a neuron. It transmits signals towards the cell body.
  • axon - A large number of sodium ion channels are present along the entire length of the axon. These are responsible for the production and spread of the action potential. During neural activity, the receiving neuron is stimulated, which opens Na+ channels in its membrane. The flow of ions results in a local change in the membrane potential. The local changes are summed at the axon hillock. If the membrane potential reaches the threshold, an action potential is triggered, which then spreads along the axon reaching the terminal bouton where it initiates the stimulation of the next nerve cell.
  • cellular membrane - The neuron membrane contains special ion channels that produce the electrical signals of the cells. Stimulation generates local potentials in the membrane of the cell body and the dendrites. The local potentials are summed in the axon hillock and, if the sum reaches the threshold, an action potential is generated on the axon. The amplitude of the action potential is independent of the strength of the stimulus.
  • node of Ranvier - Axons are wrapped in glia cells; these cells collectively form the myelin sheath of the axon, which is an electrically insulating layer. Since the membrane is exposed to the intracellular space only at the gaps of the myelin sheath, known as nodes of Ranvier, the impulse can only form here. It propagates by 'jumping' from node to node. This method of transmission is much faster than in unmyelinated axons, where the impulse travels continuously down the axon like a wave.
  • glia cells - Axons are wrapped in these cells, which form the myelin sheath. This is an electrically insulating layer around the axon. The action potential is produced in the nodes of Ranvier and propagates by 'jumping' from node to node.
  • telodendron
  • terminal bouton - The enlarged terminal of the axon. It contains neurotransmitters. When an action potential arrives to the bouton, the neurotransmitters are released into the synaptic cleft and stimulate the receiving nerve cell.
  • excitatory nerve cell

Electrical signals that form in neurons, that is, action potentials, spread along the axon towards the terminal bouton, where they are spread onto the neighbouring neurons via the synaptic clefts.

Excitatory neurons increase the electrical activity of the neighbouring neurons and can generate an action potential in them.

Inhibitory signal

  • dendrite - The short, branched projection of a neuron. It transmits signals towards the cell body.
  • axon - A large number of sodium ion channels are present along the entire length of the axon. These are responsible for the production and spread of the action potential. During neural activity, the receiving neuron is stimulated, which opens Na+ channels in its membrane. The flow of ions results in a local change in the membrane potential. The local changes are summed at the axon hillock. If the membrane potential reaches the threshold, an action potential is triggered, which then spreads along the axon reaching the terminal bouton where it initiates the stimulation of the next nerve cell.
  • cellular membrane - The neuron membrane contains special ion channels that produce the electrical signals of the cells. Stimulation generates local potentials in the membrane of the cell body and the dendrites. The local potentials are summed in the axon hillock and, if the sum reaches the threshold, an action potential is generated on the axon. The amplitude of the action potential is independent of the strength of the stimulus.
  • node of Ranvier - Axons are wrapped in glia cells; these cells collectively form the myelin sheath of the axon, which is an electrically insulating layer. Since the membrane is exposed to the intracellular space only at the gaps of the myelin sheath, known as nodes of Ranvier, the impulse can only form here. It propagates by 'jumping' from node to node. This method of transmission is much faster than in unmyelinated axons, where the impulse travels continuously down the axon like a wave.
  • glia cells - Axons are wrapped in these cells, which form the myelin sheath. This is an electrically insulating layer around the axon. The action potential is produced in the nodes of Ranvier and propagates by 'jumping' from node to node.
  • telodendron
  • terminal bouton - The enlarged terminal of the axon. It contains neurotransmitters. When an action potential arrives to the bouton, the neurotransmitters are released into the synaptic cleft and stimulate the receiving nerve cell.
  • excitatory nerve cell
  • inhibitory nerve cell

Inhibitory neurons, however, reduce the electrical activity of the neighbouring neurons and thereby prevent the formation of action potential in them.

Chemical synapse

  • terminal bouton - The enlarged terminal of the axon. It contains neurotransmitters. When an action potential arrives, the neurotransmitters are released into the synaptic cleft and stimulate the receiving nerve cell.
  • mitochondrion - It provides energy (in the form of ATP) for the energy-consuming processes of the neurons. These processes include the release of ions through the membrane and the production and transport of neurotransmitters.
  • microtubules - They are important components of the cytoskeleton. They play a crucial role in transporting neurotransmitters to the terminal bouton.
  • presynaptic membrane - Synaptic vesicles fuse into this membrane when they release neurotransmitters into the synaptic cleft by exocytosis.
  • postsynaptic membrane - Neurotransmitters released from the terminal bouton diffuse through the synaptic cleft into this membrane. It contains a large number of ion channel receptors that open when the neurotransmitters bind to them.
  • ion channel receptor - When neurotransmitters bind to the receptors, these open up, permitting ions to enter the neuron. Excitatory neurotransmitters open cation channels and thereby depolarise the receiving neuron, while inhibitory neurotransmitters open anion channels and thereby hyperpolarise the receiving neuron.
  • synaptic vesicle - Membrane-enclosed vesicles that store neurotransmitters. When an action potential reaches the terminal bouton, the vesicles release neurotransmitters by exocytosis. Neurotransmitters then diffuse through the synaptic cleft to the membrane of the receiving neuron (postsynaptic membrane). There they bind to receptors and cause ion channels to open up. In excitatory synapses excitatory transmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory transmitters open anion channels and hyperpolarise the cell.
  • ion - They enter the neuron through the ion channels opened up by neurotransmitters. In excitatory synapses, excitatory neurotransmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory neurotransmitters open anion channels and thereby hyperpolarise the receiving neuron.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.
  • neurotransmitters - They are stored in synaptic vesicles in the terminal bouton of the axon. When an action potential reaches the bouton, the vesicles release neurotransmitters by exocytosis. In excitatory synapses, excitatory transmitters open cation channels and thereby depolarise the receiving neuron, while in inhibitory synapses inhibitory neurotransmitters open anion channels and hyperpolarise the cell.

In most synapses, chemical substances known as neurotransmitters transmit signals between neurons. Therefore, these are called chemical synapses. In chemical synapses, when an action potential reaches the terminal bouton, synaptic vesicles release neurotransmitters into the synaptic cleft. The neurotransmitters diffuse through the synaptic cleft to the membrane of the receiving neuron, that is, to the postsynaptic membrane.

Neurotransmitters attach to the ion channel receptors in the postsynaptic membrane. Their attachment opens up the ion channels in the membrane, allowing ions to flow through. The more neurotransmitters are released, the more ions flow through the ion channels.

In excitatory chemical synapses, sodium ions flow through the postsynaptic membrane into the cell, while potassium ions flow out of the cell. The influx and efflux of ions increase the membrane potential compared to the resting potential; that is, they depolarise the membrane. If enough excitatory neurotransmitters are released to increase the membrane potential such that it reaches the threshold, an action potential is triggered.

Inhibitory neurotransmitters cause an influx of chloride ions and an efflux of potassium ions in the postsynaptic membrane. The influx and efflux of ions cause the decrease of the membrane potential; that is, they prevent the membrane potential from reaching the threshold and thereby inhibit the formation of an action potential. This is called hyperpolarisation.

Electrical synapse

  • postsynaptic membrane
  • presynaptic membrane
  • connexon - Also known as a connexin hemichannel, it is a special channel that connects the membrane of the presynaptic and postsynaptic cells. It is mostly depolarising signals that spread via this channel.
  • synaptic cleft - It is usually 20-30 nm wide (1 nm = 10⁻m). Neurotransmitters diffuse through the cleft from the presynaptic membrane to the postsynaptic membrane.

In some cases, electrical synapses are found between neurons. In this case, the synaptic cleft is only 2–3 nanometres wide. In electrical synapses, the membranes of the neighbouring neurons are connected by connexons. These are composed of connexin proteins and act as ion channels. Action potential can spread without any synaptic delay.

Narration

Electrical signals that form in neurons, that is, action potentials, spread along the axon towards the terminal bouton, where they are spread onto the neighbouring neurons via the synaptic clefts.

Excitatory neurons increase the electrical activity of the neighbouring neurons and can generate an action potential in them.

Inhibitory neurons, however, reduce the electrical activity of the neighbouring neurons and thereby prevent the formation of action potential in them.

In most synapses, chemical substances known as neurotransmitters transmit signals between neurons. Therefore, these are called chemical synapses. In chemical synapses, when an action potential reaches the terminal bouton, synaptic vesicles release neurotransmitters into the synaptic cleft. The neurotransmitters diffuse through the synaptic cleft to the membrane of the receiving neuron, that is, to the postsynaptic membrane.

Neurotransmitters attach to the ion channel receptors in the postsynaptic membrane. Their attachment opens up the ion channels in the membrane, allowing ions to flow through. The more neurotransmitters are released, the more ions flow through the ion channels.

In excitatory chemical synapses, sodium ions flow through the postsynaptic membrane into the cell, while potassium ions flow out of the cell. The influx and efflux of ions increase the membrane potential compared to the resting potential; that is, they depolarise the membrane. If enough excitatory neurotransmitters are released to increase the membrane potential such that it reaches the threshold, an action potential is triggered.

Inhibitory neurotransmitters cause an influx of chloride ions and an efflux of potassium ions in the postsynaptic membrane. The influx and efflux of ions cause the decrease of the membrane potential; that is, they prevent the membrane potential from reaching the threshold and thereby inhibit the formation of an action potential. This is called hyperpolarisation.

In some cases, electrical synapses are found between neurons. In this case, the synaptic cleft is only 2–3 nanometres wide. In electrical synapses, the membranes of the neighbouring neurons are connected by connexons. These are composed of connexin proteins and act as ion channels. Action potential can spread without any synaptic delay.

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