Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter: from the base of the spine to the toes.
The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along these types of axons, there are periodic gaps in the myelin sheath.
It is important to note that a single neuron does not act alone. Neuronal communication depends on the connections that neurons make with one another as well as with other cells, such as muscle cells. Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as , other neurons. There are different types of neurons; the functional role of a given neuron is intimately dependent on its structure.
There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system and across species. Neuron diversity : There is great diversity in the size and shape of neurons throughout the nervous system. Examples include a a pyramidal cell from the cerebral cortex, b a Purkinje cell from the cerebellar cortex, and c olfactory cells from the olfactory epithelium and olfactory bulb.
While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates, but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain.
Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system brain and spinal cord. The Purkinje cell, a multipolar neuron in the cerebellum, has many branching dendrites, but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single structure that extends from the soma like a unipolar cell , which later branches into two distinct structures like a bipolar cell.
Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receives sensory information and another that transmits this information to the spinal cord. Types of Neurons : Neurons are broadly divided into four main types based on the number and placement of axons: 1 unipolar, 2 bipolar, 3 multipolar, and 4 pseudounipolar. While glia or glial cells are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten.
Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be disastrous; most brain tumors are caused by mutations in glia.
There are several different types of glia with different functions. Astrocytes make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier: a structure that blocks entrance of toxic substances into the brain.
They have been shown, through calcium-imaging experiments, to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells, protecting the brain from invading microorganisms.
Oligodendrocytes form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes; one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as bridges for developing neurons as they migrate to their end destinations.
Cell processes, termed dendrites , extend superficially from the cell body into the molecular layer. A small axon arrow emerges from each cell body and runs through the granue cell layer. Note: Axons and dendrites of neurons are normally visible only in special stained sections such as this Golgi preparation. Schematic cartoon of a ventral horn Somatic Efferent SE multipolar neuron. SE neurons have their cell bodies in the CNS; their axons join peripheral nerves and innervate skeletal muscle.
As a multipoar neuron, a number of dendrites and one axon emanate from the cell body. The axon has few if any branches until its termination in a profusion of branches. The axon is specialized for conducting excitation from the input region to the output region of the neuron.
The cell body plus the dendrites of a mulltipolar neuron constitute a "dendritic zone" which receives synaptic input from other neurons. Along these types of axons, there are periodic gaps in the myelin sheath. It is important to note that a single neuron does not act alone.
Neuronal communication depends on the connections that neurons make with one another as well as with other cells, such as muscle cells. Dendrites from a single neuron may receive synaptic contact from many other neurons.
For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as , other neurons. There are different types of neurons; the functional role of a given neuron is intimately dependent on its structure.
There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system and across species. While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar.
Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates, but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain.
Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system brain and spinal cord. The Purkinje cell, a multipolar neuron in the cerebellum, has many branching dendrites, but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells.
A pseudounipolar cell has a single structure that extends from the soma like a unipolar cell , which later branches into two distinct structures like a bipolar cell.
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