Anatomy of a neuron (video) | Khan Academy
The axon arises from the soma (or sometimes from a dendrite) in a for the growth of axons and dendrites, which elongate by adding new material to their tips. An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Dendrites also dendrons, are branched protoplasmic extensions of a nerve cell that propagate Bipolar neurons have one axon and one dendritic tree at opposing ends of the cell body. Unipolar neurons have a stalk that extends from the cell.
Louis-Antoine Ranvier was the first to describe the gaps or nodes found on axons and for this contribution these axonal features are now commonly referred to as the Nodes of Ranvier.
These include modulation of sensory input, environmental pollutants, body temperature, and drug use.
What is a neuron? - Queensland Brain Institute - University of Queensland
One theory on the mechanism of dendritic arbor development is the Synaptotropic Hypothesis. The synaptotropic hypothesis proposes that input from a presynaptic to a postsynaptic cell and maturation of excitatory synaptic inputs eventually can change the course of synapse formation at dendritic and axonal arbors. A balance between metabolic costs of dendritic elaboration and the need to cover receptive field presumably determine the size and shape of dendrites.
A complex array of extracellular and intracellular cues modulates dendrite development including transcription factors, receptor-ligand interactions, various signaling pathways, local translational machinery, cytoskeletal elements, Golgi outposts and endosomes.
These contribute to the organization of the dendrites on individual cell bodies and the placement of these dendrites in the neuronal circuitry.
All these molecules interplay with each other in controlling dendritic morphogenesis including the acquisition of type specific dendritic arborization, the regulation of dendrite size and the organization of dendrites emanating from different neurons. This integration is both temporal, involving the summation of stimuli that arrive in rapid succession, as well as spatial, entailing the aggregation of excitatory and inhibitory inputs from separate branches.
This passive transmission means that voltage changes measured at the cell body are the result of activation of distal synapses propagating the electric signal towards the cell body without the aid of voltage-gated ion channels.
Passive cable theory describes how voltage changes at a particular location on a dendrite transmit this electrical signal through a system of converging dendrite segments of different diameters, lengths, and electrical properties.
Each ion species has its own corresponding protein channel located in the lipid bilayer of the cell membrane. Depending on the type of receptors that are activated, the effect on the target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way.AP1: NEURON: DENDRITES, SOMA, AXON
This entire sequence of events often takes place in less than a thousandth of a second. Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives.
The action potential is the final electrical step in the integration of synaptic messages at the scale of the neuron.
Difference Between Axon and Dendrite
Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals. While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as place cellsaxonal activity in both white and gray matter can also be recorded. Extracellular recordings of axon action potential propagation is distinct from somatic action potentials in three ways: The voltage change is triphasic.
Activity recorded on a tetrode is seen on only one of the four recording wires. In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including the alveus and the corpus callosum as well hippocampal gray matter. Although previous studies indicate an axonal origin of a single spike evoked by short-term pulses, physiological signals in vivo trigger the initiation of sequential spikes at the cell bodies of the neurons.
In terms of molecular mechanisms, voltage-gated sodium channels in the axons possess lower threshold and shorter refractory period in response to short-term pulses. If an axon that is not fully developed is cut, the polarity can change and other neurites can potentially become the axon. After the incision is made, the longest neurite will become the future axon and all the other neurites, including the original axon, will turn into dendrites.
Extracellular signaling[ edit ] The extracellular signals that propagate through the extracellular matrix surrounding neurons play a prominent role in axonal development. Netrin also known as UNC-6 a secreted protein, functions in axon formation. When the UNC-5 netrin receptor is mutated, several neurites are irregularly projected out of neurons and finally a single axon is extended anteriorly. PMGS asymmetrically distributes to the tip of the neurite that is destined to become the future axon.
Disrupting the activity of PI3K inhibits axonal development. The tips of the branched ends are formed by telodendria. The axon terminals are the swollen ends of the telodendria.
The axon terminals form the synaptic connection with a dendron of another neuron or with an effector organ. The membrane of the axon terminal is linked to the membrane of the target cell. Vesicles that contain neurotransmitters are present in the axon terminals to transmit the nerve impulses by means of chemical signals through the synaptic gap.
The axon hillock is the initial segment of an axon. It initiates the action potential. A cross-section of an axon is shown in figure 1.
Axon cross section 1 — Axon, 2 — Nucleus of the Schwann cell, 3 — Schwann cell, 4 — Myelin sheath The two types of axons are myelinated axons and non-myelinated axons. The myelin sheath forms an insulation on the axon to increase the speed of transmission of nerve impulses through the axon.
- Synapse structure
- Structure and Function of Axons and Dendrites
- What is a neuron?
This type of transmission of nerve impulses is called saltatory conduction. Schwann cells secrete myelin on the axons of the peripheral nervous system. Oligodendrocytes secrete myelin on the axons of the central nervous system. Myelinated axons are white in colour.
The gaps in the myelin sheath are called the nodes of Ranvier. The white matter of the brain and the spinal cord is composed of myelinated axons. What is a Dendrite A dendrite is a short-branched extension, which carries nerve impulses to the cell body from the synapses.
Many dendrites are extended from a single cell body of a nerve cell. Dendrites are highly branched structures.