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. Neural networks consist of a series of interconnected neurons, and serve as the interface for neurons to communicate with each other.
A neural network or neural pathway is the interface through which neurons communicate with one another. These networks consist of a series of interconnected neurons whose activation sends a signal or impulse across the body. Neural networks : A neural network or neural pathway is the complex interface through which neurons communicate with one another.
The connections between neurons form a highly complex network. The basic kinds of connections between neurons are chemical synapses and electrical gap junctions, through which either chemical or electrical impulses are communicated between neurons. The method through which neurons interact with neighboring neurons usually consists of several axon terminals connecting through synapses to the dendrites on other neurons. If a stimulus creates a strong enough input signal in a nerve cell, the neuron sends an action potential and transmits this signal along its axon.
The axon of a nerve cell is responsible for transmitting information over a relatively long distance, and so most neural pathways are made up of axons. Some axons are encased in a lipid-coated myelin sheath, making them appear a bright white; others that lack myelin sheaths i. The process of synaptic transmission in neurons : Neurons interact with other neurons by sending a signal, or impulse, along their axon and across a synapse to the dendrites of a neighboring neuron.
Some neurons are responsible for conveying information over long distances. Some interneurons pass the signal on to motor neurons in your hand, which allows you to move your hand away.
Other interneurons send a signal to the pain center in your brain, and you experience pain. For instance, until recently, researchers believed that neuron creation occurred in adults in a region of the brain called the hippocampus. The hippocampus is involved in memory and learning. But a recent study is calling beliefs about hippocampal neurogenesis into question.
After analyzing hippocampus samples from 37 donors, researchers concluded that adults produce relatively few new hippocampal neurons. Though the results have yet to be confirmed, they come as a significant setback. Nervous system cells are called neurons. They have three distinct parts, including a cell body, axon, and dendrites. These parts help them to send and receive chemical and electrical signals.
While there are billions of neurons and thousands of varieties of neurons, they can be classified into three basic groups based on function: motor neurons, sensory neurons, and interneurons. The nervous system is very complex. Read these 11 fun facts and learn why it's so important. Synaptic pruning is a brain process that occurs between early childhood and adulthood. We'll tell you about research into how it affects certain….
The vagus nerve is the longest of the 12 cranial nerves. Here, learn about its anatomy, functions, and the kinds of health problems that can occur. Muscle twitching refers to small muscle contractions in the body. Learn more about the causes and treatment here. Parosmia is term used to describe health conditions that distort your sense of smell.
If you have parosmia, you may experience a loss of scent…. Health Conditions Discover Plan Connect. All these cells are packed into a three-pound organ about the size of both your fists stuck together.
You can think of your brain as a dense forest—the neuron forest—in which different kinds of trees grow near, around and on top of one another, their branches and roots intertwining. Just as all trees share a basic structure—roots, trunk, branches—but do not look exactly alike, all neurons are variations on a common structural theme.
The diversity of structures is extraordinary and scientists are still discovering brain cells that do not really look like any brain cell they have seen before. Different Types of Neurons click to enlarge.
Purkinje cell B. Granule cell C. Motor neuron D. Tripolar neuron E. Pyramidal Cell F. Chandelier cell G. Spindle neuron H. Stellate cell Credit: Ferris Jabr; based on reconstructions and drawings by Cajal. A model neuron. Before exploring the brain's cellular diversity, let's look at a model neuron. A typical neuron has three main structures: the cell body, the axon and the dendrites. The cell body contains the nucleus, which stores the cell's genes; the axon is a long slender cable that carries electrical signals known as action potentials away from the cell body toward other neurons; and the dendrites are shorter branching fibers that receive signals from other neurons.
Instead, an axon's branching tips communicate with the dendrites, axons and cell bodies of other neurons across tiny gaps called synapses.
Neurons classified by structure. Click to enlarge Credit: Ferris Jabr. Scientists have classified neurons into four main groups based on differences in shape. Multipolar neurons are the most common neuron in the vertebrate nervous system and their structure most closely matches that of the model neuron: a cell body from which emerges a single long axon as well as a crown of many shorter branching dendrites.
Unipolar neurons, the most common invertebrate neuron, feature a single primary projection that functions as both axon and dendrites. The unmyelinated regions between the myelin segments are called the nodes of Ranvier. In the peripheral nervous system , the myelin is produced by Schwann cells.
The cytoplasm , nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier.
This covering is the neurilemma , which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate.
Functionally, neurons are classified as afferent, efferent, or interneurons association neurons according to the direction in which they transmit impulses relative to the central nervous system. Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS.
They usually have long dendrites and relatively short axons. Efferent, or motor , neurons transmit impulses from the CNS to effector organs such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or association neurons, are located entirely within the CNS in which they form the connecting link between the afferent and efferent neurons.
0コメント