Nervous equipment throughout the pet kingdom differ in structure and also complexity, as portrayed by the selection of animals shown in figure 16.2. Some organisms, choose sea sponges, lack a true worried system. Others, like jellyfish, absence a true brain and instead have a system of separate but associated nerve cell (neurons) dubbed a “nerve net.” Echinoderms such together sea stars have actually nerve cell that room bundled into fibers referred to as nerves. Flatworms the the phylum Platyhelminthes have both a main nervous device (CNS), consisted of of a small “brain” and two nerve cords, and a peripheral nervous device (PNS) include a mechanism of nerves that expand throughout the body. The insect nervous mechanism is more facility but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can manage movements and behaviors there is no input indigenous the brain. Octopi may have the most complicated of invertebrate concerned systems—they have actually neurons the are organized in dedicated lobes and eyes that room structurally similar to vertebrate species.

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Figure 16.2. Nervous solution vary in structure and also complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells room bundled into fibers referred to as nerves. In animals exhibiting bilateral the opposite such together (c) planarians, neurons cluster right into an anterior brain that processes information. In enhancement to a brain, (d) arthropods have actually clusters that nerve cabinet bodies, referred to as peripheral ganglia, situated along the ventral nerve cord. Mollusks such as squid and also (e) octopi, which need to hunt come survive, have facility brains containing countless neurons. In (f) vertebrates, the mind and spinal cord comprise the central nervous system, when neurons expanding into the remainder of the body comprise the peripheral worried system. (credit e: change of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit transaction f: alteration of job-related by NIH)
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Figure 16.3. Neurons contain organelles typical to numerous other cells, such together a nucleus and also mitochondria. They likewise have more specialized structures, consisting of dendrites and also axons.

There room different types of neurons, and the functional duty of a offered neuron is intimately dependency on that is structure. Over there is an amazing diversity the neuron shapes and also sizes uncovered in various parts of the nervous mechanism (and throughout species), as illustrated by the neurons presented in figure 16.4.

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Figure 16.4. Over there is good diversity in the size and also shape that neurons transparent the nervous system. Examples include (a) a pyramidal cabinet from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and also (c) olfactory cell from the olfactory epithelium and also olfactory bulb.

if there space many identified neuron cabinet subtypes, neurons are broadly divided into four simple types: unipolar, bipolar, multipolar, and also pseudounipolar. Figure 16.5 illustrates this four basic neuron types. Unipolar neurons have only one framework that extends far from the soma. This neurons room not uncovered in vertebrates yet are uncovered in insects whereby they stimulate muscle or glands. A bipolar neuron has one axon and one dendrite expanding from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signal from photoreceptor cell that are sensitive come light and also transmits this signals to ganglion cell that bring the signal come the brain. Multipolar neurons space the most common form of neuron. Each multipolar neuron includes one axon and multiple dendrites. Multipolar neurons have the right to be uncovered in the main nervous mechanism (brain and also spinal cord). An example of a multipolar neuron is a Purkinje cell in the cerebellum, i m sorry has countless branching dendrites yet only one axon. Pseudounipolar cell share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single procedure that extends native the soma, like a unipolar cell, yet this process later branches into two distinctive structures, prefer a bipolar cell. Most sensory neurons space pseudounipolar and also have an axon the branches into two extensions: one connected to dendrites that obtain sensory information and also another the transmits this info to the spinal cord.

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Figure 16.5. Neurons are generally divided into 4 main types based on the number and placement the axons: (1) unipolar, (2) bipolar, (3) multipolar, and also (4) pseudounipolar.

Neurogenesis

At one time, scientists thought that civilization were born through all the neurons castle would ever have. Research performed throughout the last few decades shows that neurogenesis, the birth of new neurons, proceeds into adulthood. Neurogenesis was first discovered in songbirds that produce brand-new neurons while finding out songs. For mammals, new neurons likewise play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) every day. While most of the brand-new neurons will certainly die, researchers discovered that boost in the number of surviving brand-new neurons in the hippocampus associated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications likewise promote neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite restricted compared to renewal in various other tissues, research in this area might lead to new treatments for obstacle such together Alzheimer’s, stroke, and also epilepsy.

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how do scientists identify brand-new neurons? A researcher deserve to inject a compound called bromodeoxyuridine (BrdU) right into the mind of one animal. While every cells will be exposed come BrdU, BrdU will only be included into the DNA that newly generated cells that space in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher have the right to use fluorescent microscopy come visualize the visibility of BrdU, and thus new neurons, in mind tissue. Number 16.6 is a micrograph which mirrors fluorescently labeling neurons in the hippocampus that a rat.

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Figure 16.6. This micrograph shows fluorescently labeled brand-new neurons in a rat hippocampus. Cells that are actively dividing have bromodoxyuridine (BrdU) incorporated into their DNA and also are labeled in red. Cells the express glial fibrillary acidic protein (GFAP) space labeled in green. Astrocytes, however not neurons, express GFAP. Thus, cell that are labeled both red and green are actively dividing astrocytes, whereas cell labeled red just are actively dividing neurons. (credit: change of work-related by Dr. Maryam Faiz, et. Al., college of Barcelona; scale-bar data native Matt Russell)