Molly Ayala
Neurons are the cells of the nervous system that carry out neural impulses, which are electrochemical signals that allow for communication within the brain and between the brain and other parts of the body. Neurons communicate with each other via electrical events called 'action potentials' and chemical neurotransmitters. A neural impulse starts when a neuron receives a stimulus that activates a receptor and causes a change in voltage in the cell. This stimulus could be a change in temperature, pressure, or a neurotransmitter released from a neighbouring neuron. The neurotransmitter can either help (excite) or hinder (inhibit) the neuron from firing its own action potential.
| Characteristics | Values |
|---|---|
| How electrical impulses start in neurons | A neural impulse starts when a neuron receives a stimulus that activates a receptor and causes a change in voltage in the cell. |
| What is an action potential | An action potential is the electrical or neural signal generated by neurons to communicate with effector cells and tissues. |
| What is a nerve impulse | A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron. |
| What is a synapse | A synapse is the junction between two neurons. |
| What is a neurotransmitter | A neurotransmitter is a chemical released by a neuron following an action potential. |
| What is a resting potential | A resting potential is the state of a neuron when it is not actively transmitting a nerve impulse. |
| What is depolarization | Depolarization is the process by which a neuron becomes more positive due to an influx of positive ions. |
| What is hyperpolarization | Hyperpolarization is the process by which it becomes less likely that an action potential will be fired. |
| What is a sodium-potassium pump | A sodium-potassium pump is an ion pump that uses active transport and energy to move sodium ions out of the cell and potassium ions into the cell, maintaining the resting potential of a neuron. |
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What You'll Learn
Action potentials
Neurons are electrically excitable cells that transmit electrical impulses through action potentials. These action potentials are generated and propagated by changes in the cationic gradient (mainly sodium and potassium) across their plasma membranes. The lipid bilayer of the neuronal cell membrane acts as a capacitor, and the transmembrane channels act as resistors. This allows the neuron to maintain its resting membrane potential, which is normally negative on the inside compared to the outside.
When the resting membrane potential becomes less negative, it undergoes depolarization, which is the first stage of an action potential. This is caused by an influx of positively charged sodium ions into the neuron, which changes the electrochemical gradient. The sodium channels in the neuronal membrane open in response to a small depolarization of the membrane potential. This influx of sodium ions causes the inside of the neuron to become positively charged, opening the potassium channels and allowing potassium ions to leave the cell.
The second stage of an action potential is repolarization, which is a return to the membrane's resting potential caused by the outflow of potassium ions. The third stage is after-hyperpolarization, a recovery from a slight overshoot of the repolarization. The cycle of depolarization and repolarization is extremely rapid, taking about 2 milliseconds, allowing neurons to fire action potentials in rapid bursts.
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Sodium-potassium pump
Electrical impulses in neurons are known as action potentials. They are generated by the flow of positively charged ions across the neuronal membrane. This flow is made possible by the presence of specialised proteins called channels, which form pores in the membrane that are selectively permeable to positively charged ions.
The sodium-potassium pump is a crucial mechanism for maintaining the concentration gradients of sodium and potassium ions across the neuronal membrane. It moves 3 sodium ions out of the neuron and brings 2 potassium ions in, resulting in a net removal of one positive charge from the intracellular space. This pump is essential for preserving the membrane potential, which is the electrical potential across the neuronal membrane. The membrane potential arises due to the different distributions of positively and negatively charged ions inside and outside the neuron.
The sodium-potassium pump plays a vital role in the neuron's ability to generate and propagate action potentials. Under resting conditions, the potassium channels are more permeable to potassium ions than the sodium channels are to sodium ions. This results in a slow outward leak of potassium ions, larger than the inward leak of sodium ions. Consequently, the inside of the neuron becomes negatively charged relative to the outside.
When the sodium channels open, positively charged sodium ions rush into the neuron, causing a rapid shift in the membrane potential. This depolarization then opens the potassium channels, allowing potassium ions to flow out of the neuron. This transient switch in membrane potential is the action potential, which enables neurons to communicate with each other.
The sodium-potassium pump is also significant in various physiological and pathological processes. For example, in the heart, the inhibition of the sodium-potassium pump can increase intracellular sodium levels, leading to an increase in intracellular calcium via the sodium-calcium exchanger. This heightened calcium presence enhances the force of contraction, which is beneficial in treating heart failure and atrial fibrillation. Conversely, alcohol inhibits the sodium-potassium pump in the cerebellum, leading to ataxia and dystonia, demonstrating its role in body coordination.
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Neurons communicate via electrical events
Neurons are essentially electrical devices. They communicate with each other via electrical events called "action potentials" and chemical neurotransmitters. An action potential is a series of quick changes in voltage across a cell membrane. It is generated through the flow of positively charged ions across the neuronal membrane.
The neuronal membrane contains specialised proteins called channels, which form pores in the membrane that are selectively permeable to particular ions. These channels open and close in response to neurotransmitters or changes in the cell's membrane potential. When the channels open, they allow positively charged sodium ions to flow into the neuron, changing the electrochemical gradient and producing a further rise in the membrane potential. This causes the inside of the cell to become momentarily positively charged, a process known as depolarisation.
Depholarisation, in turn, opens the potassium channels, allowing potassium ions to leave the cell. This transient switch in membrane potential is the action potential. The cycle of depolarisation and repolarisation is extremely rapid, taking only about 2 milliseconds. This allows neurons to fire action potentials in rapid bursts, a common feature in neuronal communication.
At the junction between two neurons, or the synapse, an action potential causes neuron A to release a chemical neurotransmitter. The neurotransmitter can either excite or inhibit neuron B from firing its own action potential. Neurotransmitters are released from presynaptic terminals, which may branch to communicate with several postsynaptic neurons.
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Neurotransmitters
When a message or signal travels along a nerve cell, the electrical charge of the signal causes the vesicles of neurotransmitters to fuse with the nerve cell membrane. The neurotransmitters are then released from the axon terminal into the synaptic cleft, a small gap of less than 40 nanometers between the presynaptic axon terminal and the postsynaptic dendrite.
There are many different types of neurotransmitters, each with its own specific functions and roles in the body. For example, acetylcholine, the first neurotransmitter discovered, plays a role in muscle contractions, memory, heart rate, blood pressure, and gut motility. Imbalances in acetylcholine levels have been linked to Alzheimer's disease and seizures. Another example is serotonin, which is involved in regulating sleep, memory, appetite, mood, muscle contraction, and cardiovascular function. Low levels of serotonin have been observed in patients with depression.
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Synapses
A synapse is a link or connection between two neurons or a neuron and a gland or muscle cell. Synapses are the sites of transmission of electrical nerve impulses between neurons. They are also the sites of conversion of electrical signals to chemical signals and vice versa.
When an electrical signal travels down the axon of a neuron, it reaches the synapse, which is separated from the next neuron by a microscopic gap called the synaptic cleft. The arrival of the nerve impulse at the synapse causes the release of neurotransmitters, which are chemical messengers. These neurotransmitters then attach to the receptor molecules on the next neuron, thereby converting the electrical signal back into a chemical signal. The neurotransmitters can either excite or inhibit the next neuron from firing its own action potential.
The synapse, with its neurotransmitters, acts as a physiological valve, directing the conduction of nerve impulses in regular circuits and preventing random or chaotic stimulation of nerves. A single neuron can have tens of thousands of synapses, and the balance of excitatory and inhibitory inputs determines whether an action potential will result.
The creation of a synapse involves the axon tip of one neuron connecting to the main body or the dendrites of another neuron. The dendrites are the branch-like structures that receive synaptic inputs from axons. The sum total of dendritic inputs determines whether the neuron will fire an action potential.
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Frequently asked questions
Electrical impulses, also known as neural impulses, start when a neuron receives a stimulus that activates a receptor and causes a change in voltage in the cell. This change in voltage is called an action potential.
An action potential is a brief electrical event generated in the axon that signals the neuron as 'active'. It is a series of quick changes in voltage across a cell membrane.
An action potential occurs when the membrane potential of a specific cell rapidly rises and falls. This is caused by the influx of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential towards zero.
Neurons communicate with each other via electrical events called 'action potentials' and chemical neurotransmitters. At the junction between two neurons (synapse), an action potential causes neuron A to release a chemical neurotransmitter.
