![]() ![]() While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. For mammals, new neurons also play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis: At one time, scientists believed that people were born with all the neurons they would ever have. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal cord. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. An example of a multipolar neuron is a Purkinje cell in the cerebellum, which has many branching dendrites but only one axon. Multipolar neurons can be found in the central nervous system (brain and spinal cord). Each multipolar neuron contains one axon and multiple dendrites. ![]() Multipolar neurons are the most common type of neuron. 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. A bipolar neuron has one axon and one dendrite extending from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. Unipolar neurons have only one structure that extends away from the soma. While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. These gaps are sites where the signal is “recharged” as it travels along the axon. Along the axon, there are periodic gaps in the myelin sheath. The myelin sheath is not actually part of the neuron. 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. Some axons are covered with myelin, which acts as an insulator to minimize the dissipation of the electrical signal as it travels down the axon, greatly increasing the speed of conduction. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Chemicals released at axon terminals allow signals to be communicated to these other cells. These terminals in turn synapse on other neurons, muscles, or target organs. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. Once a signal is received by the dendrite, it then travels passively to the cell body. There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. A neuron can be compared to an electrical wire-it transmits a signal from one place to another. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Regardless of the animal, the nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. Some organisms, like sea sponges, lack a true nervous system. Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 1. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |