The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals. Dendrites extend from the neuron cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites are covered with synapses formed by the ends of axons from other neurons.
Illustration by Lydia V. The brain is what it is because of the structural and functional properties of interconnected neurons. The mammalian brain contains between million and billion neurons, depending on the species. Each mammalian neuron consists of a cell body , dendrites , and an axon.
When neurons receive or send messages, they transmit electrical impulses along their axons, which can range in length from a tiny fraction of an inch or centimeter to three feet about one meter or more. Many axons are covered with a layered myelin sheath, which accelerates the transmission of electrical signals along the axon.
This sheath is made by specialized cells called glia. In the brain, the glia that make the sheath are called oligodendrocytes, and in the peripheral nervous system, they are known as Schwann cells. Damage to the myelin sheath from disease can cause severe impairment of nerve-cell function. In addition, some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves.
The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all. The frequency of action potentials is correlated with the intensity of a stimulus.
This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus. Reuptake refers to the reabsorption of a neurotransmitter by a presynaptic sending neuron after it has performed its function of transmitting a neural impulse. Reuptake is necessary for normal synaptic physiology because it allows for the recycling of neurotransmitters and regulates the neurotransmitter level in the synapse, thereby controlling how long a signal resulting from neurotransmitter release lasts.
The synapse is the site at which a chemical or electrical exchange occurs between the presynaptic and postsynaptic cells. The synapse is the junction where neurons trade information. It is not a physical component of a cell but rather a name for the gap between two cells: the presynaptic cell giving the signal and the postsynaptic cell receiving the signal. There are two types of possible reactions at the synapse—chemical or electrical. During a chemical reaction, a chemical called a neurotransmitter is released from one cell into another.
In an electrical reaction, the electrical charge of one cell is influenced by the charge an adjacent cell. The electrical response of a neuron to multiple synaptic inputs : Synaptic responses summate in order to bring the postsynaptic neuron to the threshold of excitation, so it can fire an action potential represented by the peak on the chart.
The process of a chemical reaction at the synapse has some important differences from an electrical reaction. Chemical synapses are much more complex than electrical synapses, which makes them slower, but also allows them to generate different results. Like electrical reactions, chemical reactions involve electrical modifications at the postsynaptic membrane, but chemical reactions also require chemical messengers, such as neurotransmitters, to operate. Neurotransmitters are chemicals that transmit signals from a neuron across a synapse to a target cell.
Neurotransmitters are chemicals that transmit signals from a neuron to a target cell across a synapse. When called upon to deliver messages, they are released from their synaptic vesicles on the presynaptic giving side of the synapse, diffuse across the synaptic cleft, and bind to receptors in the membrane on the postsynaptic receiving side. An action potential is necessary for neurotransmitters to be released, which means that neurons must reach a certain threshold of electric stimulation in order to complete the reaction.
A neuron has a negative charge inside the cell membrane relative to the outside of the cell membrane; when stimulation occurs and the neuron reaches the threshold of excitement this polarity is reversed. This allows the signal to pass through the neuron. When the chemical message reaches the axon terminal, channels in the postsynaptic cell membrane open up to receive neurotransmitters from vesicles in the presynaptic cell.
Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell that is, decreasing the voltage gradient of the cell, thus bringing it further away from an action potential , while excitatory neurotransmitters cause depolarization bringing it closer to an action potential.
Neurotransmitters match up with receptors like a key in a lock. A neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.
There are several systems of neurotransmitters found at various synapses in the nervous system. The following groups refer to the specific chemicals, and within the groups are specific systems, some of which block other chemicals from entering the cell and some of which permit the entrance of chemicals that were blocked before.
The cholinergic system is a neurotransmitter system of its own, and is based on the neurotransmitter acetylcholine ACh. This system is found in the autonomic nervous system, as well as distributed throughout the brain. The cholinergic system has two types of receptors: the nicotinic receptor and the acetylcholine receptor, which is known as the muscarinic receptor.
Both of these receptors are named for chemicals that interact with the receptor in addition to the neurotransmitter acetylcholine. Nicotine, the chemical in tobacco, binds to the nicotinic receptor and activates it similarly to acetylcholine. Muscarine, a chemical product of certain mushrooms, binds to the muscarinic receptor. Another group of neurotransmitters are amino acids, including glutamate Glu , GABA gamma-aminobutyric acid, a derivative of glutamate , and glycine Gly.
These amino acids have an amino group and a carboxyl group in their chemical structures. Glutamate is one of the 20 amino acids used to make proteins.
Each amino acid neurotransmitter is its own system, namely the glutamatergic, GABAergic, and glycinergic systems. They each have their own receptors and do not interact with each other.
Amino acid neurotransmitters are eliminated from the synapse by reuptake. A pump in the cell membrane of the presynaptic element, or sometimes a neighboring glial cell, clears the amino acid from the synaptic cleft so that it can be recycled, repackaged in vesicles, and released again.
The reuptake process : This illustration shows the process of reuptake, in which leftover neurotransmitters are returned to vesicles in the presynaptic cell. Another class of neurotransmitter is the biogenic amine, a group of neurotransmitters made enzymatically from amino acids.
They have amino groups in them, but do not have carboxyl groups and are therefore no longer classified as amino acids. A neuropeptide is a neurotransmitter molecule made up of chains of amino acids connected by peptide bonds, similar to proteins. However, proteins are long molecules while some neuropeptides are quite short. Neuropeptides are often released at synapses in combination with another neurotransmitter. Dopamine is the best-known neurotransmitter of the catecholamine group.
The brain includes several distinct dopamine systems, one of which plays a major role in reward-motivated behavior. Most types of reward increase the level of dopamine in the brain, and a variety of addictive drugs increase dopamine neuronal activity.
Other brain dopamine systems are involved in motor control and in controlling the release of several other important hormones. The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein.
If there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing more likely to reach an action potential or hyperpolarizing less likely to reach an action potential effect is also dependent on the receptor.
When acetylcholine binds to the nicotinic receptor, the postsynaptic cell is depolarized. However, when acetylcholine binds to the muscarinic receptor, it might cause depolarization or hyperpolarization of the target cell.
The amino acid neurotransmitters glutamate, glycine, and GABA are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid because Glu receptors in the adult cause depolarization of the postsynaptic cell.
Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization, making the receiving cell less likely to reach an action potential. On the other hand, when an excess of the neurotransmitter dopamine blocks glutamate receptors, disorders like schizophrenia can occur.
Neurons communicate through synapses - contact points between the axon terminals on one side and dendrites or cell bodies on the other. Here, in a nanometre-wide gap, electrical signals coming via the axon are converted into chemical signals through the release of neurotransmitters, and then promptly converted back into electricity as information moves from neuron to neuron.
Myelin acts as a form of insulation for axons, helping to send their signals over long distances. For this reason, myelin is mostly found in neurons that connect different brain regions, rather than in the neurons whose axons remain in the local region. Neurons cannot properly communicate if axons are damaged or broken. Scientists at QBI are working to better understand the underlying processes and genetics involved. Since axons are much longer than the rest of the cell, they need to be maintained by transporting essential molecules and organelles through them.
QBI scientists have discovered that the gene mec is involved in stabilising the internal neuronal structure to support proper transport within the axon and its maintenance. A mutation of this gene, and others with similar functions, can disrupt this process, leading to damaged axons and eventual disease.
QBI researchers have also discovered two proteins involved in axon degeneration in the roundworm C.
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