Structure and Function of Axons and Dendrites - WikiLectures
Imaging ATP in axons reveals that they rely on glucose from the The neurons in the brain are made up of four distinct parts: the dendrites, Trevisiol et al. first uncovered a remarkable correlation between . Wellcome Trust. [edit part] The axon is the long projection of a nerve that can reach a length of tenths of centimeters, that conveys electrical impulses from the dendrites/soma of . Specifically, dendrites receive signals from other neurons, to the cell body The connection between axons that synapse on dendrites is called.
GABAergic neurons— gamma aminobutyric acid.
GABA is synthesized from glutamate neurotransmitters by the enzyme glutamate decarboxylase. Glutamate is one of two primary excitatory amino acid neurotransmitter, the other being Aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor often referred to as GPCR.
The function of NMDA receptors is dependent on Glycine receptor binding as a co-agonist within the channel pore.
NMDA receptors do not function without both ligands present. Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. Glutamate is synthesized from the amino acid glutamine by the enzyme glutamate synthase.
Dopamine is connected to mood and behavior and modulates both pre and post synaptic neurotransmission. Loss of dopamine neurons in the substantia nigra has been linked to Parkinson's disease. Dopamine is synthesized from the amino acid tyrosine.
Tyrosine is catalyzed into levadopa or L-DOPA by tyrosine hydroxlase, and levadopa is then converted into dopamine by amino acid decarboxylase. Serotonin 5-Hydroxytryptamine, 5-HT can act as excitatory or inhibitory. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase.
A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft. Synapse and Chemical synapse A signal propagating down an axon to the cell body and dendrites of the next cell Chemical synapse Neurons communicate with one another via synapseswhere either the axon terminal or an en passant bouton a type of terminal located along the length of the axon of one cell contacts another neuron's dendrite, soma or, less commonly, axon.2-Minute Neuroscience: The Neuron
Neurons such as Purkinje cells in the cerebellum can have over dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the supraoptic nucleushave only one or two dendrites, each of which receives thousands of synapses.
Synapses can be excitatory or inhibitory and either increase or decrease activity in the target neuron, respectively.
Some neurons also communicate via electrical synapses, which are direct, electrically conductive junctions between cells. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron. High cytosolic calcium in the axon terminal also triggers mitochondrial calcium uptake, which, in turn, activates mitochondrial energy metabolism to produce ATP to support continuous neurotransmission.
The human brain has a huge number of synapses. Each of the one hundred billion neurons has on average 7, synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about synapses 1 quadrillion. This number declines with age, stabilizing by adulthood.
Neuron - Wikipedia
Estimates vary for an adult, ranging from to 5 x synapses to trillion. Mechanisms for propagating action potentials[ edit ] InJohn Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties. By inserting electrodes into the giant squid axons, accurate measurements were made of the membrane potential.
The cell membrane of the axon and soma contain voltage-gated ion channels that allow the neuron to generate and propagate an electrical signal an action potential. Some neurons also generate subthreshold membrane potential oscillations. There are several stimuli that can activate a neuron leading to electrical activity, including pressurestretch, chemical transmitters, and changes of the electric potential across the cell membrane. Neurons must maintain the specific electrical properties that define their neuron type.
To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of myelin around their axons. The sheaths are formed by glial cells: The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy.
Multiple sclerosis is a neurological disorder that results from demyelination of axons in the central nervous system. Some neurons do not generate action potentials, but instead generate a graded electrical signalwhich in turn causes graded neurotransmitter release. Such non-spiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances. Neural coding[ edit ] Neural coding is concerned with how sensory and other information is represented in the brain by neurons.
The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or ensemble neuronal responses, and the relationships amongst the electrical activities of the neurons within the ensemble. In other words, if a neuron responds at all, then it must respond completely. Greater intensity of stimulation does not produce a stronger signal but can produce a higher frequency of firing.
There are different types of receptor responses to stimuli, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second.
This can be likened to an intrinsic property of light where to get greater intensity of a specific frequency color there have to be more photons, as the photons can't become "stronger" for a specific frequency.
There are a number of other receptor types that are called quickly adapting or phasic receptors, where firing decreases or stops with steady stimulus; examples include: The farther the origin of excitation from the soma cell bodythe greater the degree of the decrement until the current reaches the cell body. Measurements made in vivo suggest, that neuron's cable properties are not fixed quantities. It appears more efficient than the mathematical models indicate.
The most probable explanation is based on the presence of accumulations of the voltage-gated channels in various location of the dendritic membrane at the heads and necks of the dendritic spines, at the dendritic branching points, and even at the whole dendritic segments.
These accumulations hot-spots may recover the declining membrane potential and dramatically increase the effectiveness of conductance: Possible contribution to memory: Possible source of neuromodulators and tissue factors. Dendritic Spines Spiny dendrite of a striatal medium spiny neuron Dendritic spines are small outgrowth of the cell membrane of the dendrite.
This is where a single synapse with an axon typically takes place. There can be thousands of dendritic spines on a neuron.
A dendritic spine is a small membranous protrusion from a neuron's dendrite that typically receives input from a single synapse of an axon.
Dendritic spines behave as a storage site for synapses and are responsible for collecting post-synaptic potentials and transmitting them to the parent dendrite. Most spines have a bulbous head the spine headand a thin neck that connects the head of the spine to the shaft of the dendrite. The most notable classes of spine shape are: The variable spine shape and volume is thought to be correlated with the strength and maturity of each spine-synapse.
Electrical Properties The conduction is done by electrotonic conduction passive conduction of current. Due the spine's small size, the spine has a high input resistance. The spine's size is inversely proportional to the resistance. As like in any cable, the more space an electron has to travel through, the less resistance it will encounter in doing so. The synaptic potentials are relatively fast due to the relatively small capacitance of the spines, however the capacitance of the whole dendrite however becomes higher as the number of spines increases.