Otis Singleton
Neurons are specialized cells that can transmit electrical signals through the nervous system. They are composed of three main parts: dendrites, the cell body, and the axon. Dendrites are thin, branched structures that receive signals from other neurons. The cell body carries out the neuron's basic functions, while the axon is a long, thin fiber that transmits nerve impulses, or action potentials, to other neurons. These action potentials are generated by the flow of positively charged ions across the neuronal membrane, creating a voltage difference that allows neurons to fire in rapid bursts. Myelin, a fatty membrane produced by glial cells, acts as an insulator around the axon to prevent the dissipation of electrical signals. Neurotransmitters, or chemical messengers, are then released from the axon terminals to excite or inhibit target neurons, completing the transmission of the electrical signal.
| Characteristics | Values |
|---|---|
| Where electrical signals occur in neurons | Along the cell membrane |
| How electrical signals are generated | Through the flow of positively charged ions across the neuronal membrane |
| How neurons maintain different concentrations of ions | By pumping out positively charged sodium ions and pumping in positively charged potassium ions |
| What is the role of the axon | To carry nerve impulses to other neurons |
| What is the role of dendrites | To receive signals from other neurons |
| What is an action potential | A brief electrical event that signals the neuron as "active" |
| How long does an action potential last | Approximately 1 millisecond |
| What is the role of myelin | To act as an insulator to prevent the dissipation of the electrical signal as it travels down the axon |
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What You'll Learn
Action potentials
Neurons are not naturally good conductors of electricity, but they have evolved mechanisms to generate electrical signals. These signals are based on the flow of ions across their plasma membranes. The neuronal action potential is a vital part of the propagation of impulses along nerve fibres, even at a distance. It is also crucial for communication between neurons through synapses.
The cycle of depolarization and repolarization is extremely rapid, taking about 2 milliseconds. This allows neurons to fire action potentials in rapid bursts, a common feature in neuronal communication. There are three stages in the generation of an action potential: depolarization, repolarization, and after-hyperpolarization.
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Neurotransmitters
There are more than 100 different neurotransmitters identified in humans, including acetylcholine, serotonin, dopamine, norepinephrine, glutamate, and endorphins. These neurotransmitters play various roles in the body, such as regulating appetite, sleep, memory, mood, muscle contraction, heart rate, and blood pressure. Imbalances in neurotransmitter levels have been linked to several diseases and mental disorders, including Alzheimer's disease, Parkinson's disease, depression, insomnia, and anxiety.
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Membrane potential
Neurons are not good conductors of electricity, but they have evolved mechanisms to generate electrical signals. These signals are based on the flow of ions across their plasma membranes. The electrical potential across the neuronal cell membrane is called the membrane potential.
The membrane potential is the result of the movement of several different ion species through various ion channels and transporters in the plasma membrane. These movements result in different electrostatic charges across the cell membrane. The neuronal membrane contains specialised proteins called channels, which form pores in the membrane that are selectively permeable to particular ions. The membrane potential is influenced by the opening and closing of these ion channels.
The resting membrane potential is the electrical potential difference across the plasma membrane when the cell is in a non-excited state. The resting potential for neurons ranges from -80 mV to -70 mV, with the interior of the cell having a negative baseline voltage. The opening and closing of ion channels can induce a departure from the resting potential. This is called depolarization if the interior voltage becomes less negative, and hyperpolarization if it becomes more negative.
The membrane potential has two basic functions. Firstly, it allows a cell to function as a battery, providing power to operate various "molecular devices" embedded in the membrane. Secondly, in electrically excitable cells such as neurons, it is used for transmitting signals between different parts of a cell. Signals are generated by the opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential. This change in the electric field can be quickly sensed by adjacent or more distant ion channels, which can then open or close as a result, reproducing the signal.
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Axons
The axon is the site of the action potential, a brief (~1 ms) electrical event that signals the neuron as 'active'. The action potential travels the length of the axon, causing the release of neurotransmitters into the synapse, which is the junction between the axon of one neuron and the dendrite of another. The neurotransmitters excite or inhibit the target neuron, allowing neurons to communicate with one another.
The structural integrity of axons requires the support of a surrounding protein called Perlecan. Without this protein, axonal segments can break apart during development, and the synaptic connections they form can die away.
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Dendrites
The dendrites receive synaptic inputs from axons, and the sum of these inputs determines whether the neuron will fire an action potential. The action potential is a brief electrical event that occurs when the neuron is 'active'. It travels the length of the axon and causes the release of neurotransmitters into the synapse.
The synaptic activity causes local changes in the electrical potential across the plasma membrane of the dendrite. This change in membrane potential will spread along the dendrite but becomes weaker with distance without an action potential. The dendrite's enlarged surface area allows it to receive signals from a large number of presynaptic neurons. Excitatory synapses terminate on dendritic spines, small protrusions with a high density of neurotransmitter receptors.
Most inhibitory synapses directly contact the dendritic shaft. In some types of neurons, signals are transmitted via their dendrites, rather than axons.
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Frequently asked questions
Electrical signals in neurons are called action potentials. They are rapid, temporary changes in membrane potential (electrical charge) that occur when positively charged sodium ions rush into a neuron and potassium ions rush out.
Neurons generate electrical signals through the flow of ions across their plasma membranes. They maintain different concentrations of certain ions across their cell membranes. Specifically, there is a high concentration of sodium ions outside the neuron and a high concentration of potassium ions inside.
Electrical signals occur in the axon, a long, thin fiber that carries nerve impulses to other neurons. The axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals.
