Neurotransmission
Encyclopedia
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s are released by a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
(the presynaptic neuron), and bind to and activate the receptors
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
of another neuron (the postsynaptic neuron). Neurotransmission usually takes place at a synapse
Chemical synapse
Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie...
, and occurs when an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
is initiated in the presynaptic neuron. The binding of neurotransmitters to receptors in the postsynaptic neuron can trigger either short term changes, like changes in the membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
called postsynaptic potential
Postsynaptic potential
Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials...
s, or longer term changes by the activation of signaling cascades
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
.
Nerve impulses are essential for the propagation of signals. These signals are sent to and from the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
via efferent
Efferent nerve
In the nervous system, efferent nerves, otherwise known as motor or effector neurons, carry nerve impulses away from the central nervous system to effectors such as muscles or glands...
and afferent neuron
Afferent nerve
In the nervous system, afferent neurons , carry nerve impulses from receptors or sense organs towards the central nervous system. This term can also be used to describe relative connections between structures. Afferent neurons communicate with specialized interneurons...
s in order to coordinate smooth
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
, skeletal
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
and cardiac muscle
Cardiac muscle
Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...
s, bodily secretions and organ functions critical for the long-term survival of multicellular vertebrate organisms such as mammals.
Neurons form networks through which nerve impulses travel. Each neuron receives as many as 15,000 connections from other neurons. Neurons do not touch each other; they have contact points called synapses. A neuron transports its information by way of a nerve impulse. When a nerve impulse arrives at the synapse, it releases neurotransmitters, which influence another cell, either in an inhibitory way or in an excitatory way. The next neuron may be connected to many more neurons, and if the total of excitatory influences is more than the inhibitory influences, it will also "fire", that is, it will create a new action potential at its axon hillock
Axon hillock
The axon hillock is a specialized part of the cell body of a neuron that connects to the axon. As a result, the axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon. For many years it was believed...
, in this way passing on the information to yet another next neuron, or resulting in an experience or an action.
Stages in neurotransmission at the synapse
- Synthesis of the neurotransmitter. This can take place in the cell body, in the axon, or in the axon terminalAxon terminalAxon terminals are distal terminations of the branches of an axon. An axon nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body, or soma, in order to transmit those impulses to other neurons.Neurons are...
. - Storage of the neurotransmitter in storage granules or vesicles in the axon terminal.
- Calcium enters the axon terminal during an action potential, causing releaseExocytosisExocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
of the neurotransmitter into the synaptic cleft. - After its release, the transmitter binds to and activates a receptor in the postsynaptic membrane.
- Deactivation of the neurotransmitter. The neurotransmitter is either destroyed enzymatically, or taken back into the terminal from which it came, where it can be reused, or degraded and removed.
Summation
Each neuron is connected with numerous other neurons, receiving numerous impulses from them.Summation is the adding together of these impulses at the axon hillock. If the neuron only gets excitatory impulses, it will also generate an action potential; but if the neuron gets as many inhibitory as excitatory impulses, the inhibition cancels out the excitation and the nerve impulse will stop there. Summation takes place at the axon hillock.
Spatial summation means several firings on different places of the neuron, that in themselves are not strong enough to cause a neuron to fire. However, if they fire simultaneously, their combined effects will cause an action potential.
Temporal summation means several firings at the same place, that won't cause an action potential if they have a pause in between, but when there are several firings in rapid succession, they will cause the neuron to reach the threshold for excitation.
Convergence and divergence
Neurotransmission implies both a convergence and a divergence of information. First one neuron is influenced by many others, resulting in a convergence of input.When the neuron fires, the signal is sent to many other neurons, resulting in a divergence of output. Many other neurons are influenced by this neuron.
Cotransmission
Cotransmission is the release of several types of neurotransmitters from a single nerve terminal. Cotransmission allows for more complex effects at postsynaptic receptors, and thus allows for more complex communication to occur between neurons.
In modern neuroscience, neurons are often classified by their cotransmitter, for example striatal GABAergic neurons utilize opioid peptide
Opioid peptide
Opioid peptides are short sequences of amino acids that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Opioid peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but they all resemble opiates...
s or substance P
Substance P
In the field of neuroscience, substance P is a neuropeptide: an undecapeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family. Substance P and its closely related neuropeptide neurokinin A are produced from a polyprotein precursor...
as their primary cotransmitter.
Some neurons can release at least two neurotransmitters at the same time, the other being a cotransmitter, in order to provide the stabilizing negative feedback required for meaningful encoding, in the absence of inhibitory interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s. Examples include:
- GABAGabâGabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
–glycineGlycineGlycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
co-release. - DopamineDopamineDopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...
–glutamate co-release. - AcetylcholineAcetylcholineThe chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
–glutamate co-release. - AcetylcholineAcetylcholineThe chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
(ACh)–vasoactive intestinal peptideVasoactive intestinal peptideVasoactive intestinal peptide also known as the vasoactive intestinal polypeptide or VIP is a peptide hormone containing 29 amino acid residues that is produced in many tissues of vertebrates including the gut, pancreas and suprachiasmatic nuclei of the hypothalamus in the brain...
(VIP) co-release. - AcetylcholineAcetylcholineThe chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
(ACh)–calcitonin gene-related peptideCalcitonin gene-related peptideCalcitonin gene related peptide is a member of the calcitonin family of peptides, which in humans exists in two forms, α-CGRP and β-CGRP. α-CGRP is a 37-amino acid peptide and is formed from the alternative splicing of the calcitonin/CGRP gene located on chromosome 11...
(CGRP) co-release. - Glutamate–dynorphinDynorphinDynorphins are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 , multiple active peptides are released: dynorphin A, dynorphin B, and α/β-neo-endorphin...
co-release (in hippocampusHippocampusThe hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
).
See also
- NeurotransmitterNeurotransmitterNeurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
- Serpentine receptor
- AutoreceptorAutoreceptorAn autoreceptor is a receptor located on presynaptic nerve cell membranes and serves as a part of a feedback loop in signal transduction. It is sensitive only to those neurotransmitters or hormones that are released by the neuron in whose membrane the autoreceptor sits.Canonically, a presynaptic...
- Neuromuscular transmission
- NeuropsychopharmacologyNeuropsychopharmacologyNeuropsychopharmacology is an interdisciplinary science related to psychopharmacology and fundamental neuroscience...