Neurons (nerve cells)
Neurons, or nerve cells, are specialized eukaryotic cells that form the foundation of the nervous system, which comprises the brain, spinal cord, sensory organs, and peripheral nerves. These cells play a crucial role in transmitting information throughout the body, facilitating communication between different body parts. Neurons carry messages using an electrochemical process, where electrical impulses are transformed into chemical signals by neurotransmitters at synapses, the junctions between neurons.
Structurally, neurons consist of a soma (cell body), dendrites, and an axon. The soma houses the nucleus and cytoplasm, while dendrites extend from the soma to receive incoming signals. The axon, which can be quite long—sometimes reaching nearly three feet—transmits messages to other cells. A protective myelin sheath covers the axons of long neurons, enhancing signal transmission speed.
Neurons can be classified based on their functions into three main types: sensory neurons, which convey information to the nervous system; motor neurons, which relay commands from the nervous system to muscles and glands; and interneurons, which serve as connectors between sensory and motor neurons. Additionally, neurons are categorized by their structural characteristics, with types including unipolar, bipolar, and multipolar neurons, reflecting the number of extensions they have. Understanding neurons is vital to comprehending how the nervous system operates and influences bodily functions.
Subject Terms
Neurons (nerve cells)
A cell is a small building block of life, and groups of cells compose living organisms. Neurons are specialized cells that make up the nervous system, which includes the brain, spinal cord, sensory organs, and nerves in the body. Nerves connect the brain, spinal cord, and sensory organs to the rest of the body. The nervous system controls the body and allows it to communicate with its parts.
Overview of a Cell
Cells make up every living organism. They hold all the biological information necessary for a living thing to stay alive and function. Each cell performs a specific function. Cells are very tiny, only about the diameter of a single strand of human hair. They can be seen only with the aid of a microscope. Not all cells look the same. Many have adapted for functional purposes and survival in different environments. For example, muscle cells have elasticity to allow them to contract and relax.
Organisms may be made of a single cell or many cells. A prokaryote is a single-celled organism without a nucleus. The nucleus is the central part of a cell that controls it and contains genetic material. A eukaryote may be single-celled or multicellular. Unlike a prokaryotic cell, a eukaryotic cell has a nucleus. A bacterium is an example of a prokaryotic cell, and a neuron is an example of a eukaryotic cell.
A cell membrane surrounds each cell. It acts like a barrier to protect the inside of a cell from the environment outside the cell. Cells contain a thick liquid known as cytoplasm, which is about 70 percent water. Molecules such as nucleic acids, proteins, carbohydrates, and lipids make up the other 30 percent.
Nucleic acids play a part in a cell's genetic code. Two types exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA work together to build and maintain a cell. Most DNA is found in the nucleus of a cell, while RNA is located in the cytoplasm.
Proteins are molecules made from amino acids, or small chains of molecules. Proteins have many purposes. Proteins known as enzymes convert other molecules into forms that allow a cell to perform a variety of functions, such as ridding a cell of waste.
Carbohydrates are sugars and starches the cell uses for energy. Lipids are fat molecules in the cell membrane that aid in the storage of energy and relay signals inside and outside cells.
Groups of molecules sometimes form organelles. These structures have an intracellular membrane and perform specific tasks. Mitochondria are one type of organelle. They are responsible for a cell's energy production.
Function of Neurons
The human body has trillions of neurons. Of these, about one hundred billion are located in the brain. A neuron's main purpose is to carry information to gland and muscle cells and to other neurons. Neurons use an electrochemical process to send and receive messages. When sending messages, a neuron transmits an electrical pulse, which is converted into a chemical message. A substance known as a neurotransmitter carries the signals from a neuron to other cells.
Neurons look different from other cells and can range in shape and size. A neuron has most of the same parts that other eukaryotic cells have. Neurons have a cell membrane, a nucleus, cytoplasm, and organelles. However, neurons have several other parts that distinguish them from other cells: a soma, dendrites, and an axon. The soma, or cell body, is the body of the neuron. It contains the nucleus and cytoplasm. Neurons are classified by the size of their soma. The soma of the smallest neurons is only about four microns wide (one micron equals one thousandth of a millimeter); the largest is more than one hundred microns wide.
Neurons have many dendrites, which look like tiny spikes or arms that branch out from the soma. The dendrites receive messages from other cells. An axon is a specialized dendrite. In some neurons, the axon is the same size as the other dendrites, but in others, it is longer. For example, the axon that runs from the spinal cord to the feet can be nearly three feet long. The main purpose of the axon is to transmit messages to other cells.
A layer called the myelin sheath covers the length of long axons. The myelin sheath insulates and protects the axon and helps accelerate communication. At the end of each axon is an axon ending, also called the axon foot, bouton, or synaptic knob. The axon ending is the site where an electrochemical process converts signals that have traveled through the axon into chemical messages, which then are sent to other neurons to communicate with other parts of the body. More than one thousand tiny gaps called synapses are located between the axon ending and dendrite. These are the points of contact between neurons.
Types of Neurons
Scientists classify neurons into three broad categories by function. Sensory, or afferent, neurons send information to the nervous system. They are found in many parts of the body, including the nose, tongue, ear, and eye, and indicate pressure, pain, and temperature. Motor, or efferent, neurons send information away from the nervous system. These activate muscle cells located in the heart, intestines, glands, and other parts of the body. Interneurons send information between sensory and motor neurons. The brain is an example of an interneuron.
Scientists also classify neurons by the number of extensions they have protruding from their soma. A bipolar neuron has two extensions (axon and dendrite). A pseudounipolar neuron has two axons and a dendrite. A multipolar neuron has one axon and more than one dendrite.
Bibliography
Boeree, C. George. "General Psychology: The Neuron." Shippensburg University. Shippensburg U. Web. 22 Dec. 2014. http://webspace.ship.edu/cgboer/theneuron.html
Brain, Marshall. "How Cells Work." HowStuffWorks. InfoSpace. Web. 22 Dec. 2014. http://science.howstuffworks.com/life/cellular-microscopic/cell.htm
"Brain Basics: The Life and Death of a Neuron." National Institute of Neurological Disorders and Stroke, 29 July 2024, www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-life-and-death-neuron. Accessed 20 Nov. 2024. Chudler, Eric H. "Types of Neurons (Nerve Cells)." University of Washington. U of Washington. Web. 22 Dec. 2014. https://faculty.washington.edu/chudler/cells.html
Cooper, Geoffrey M. "Cell Membranes." The Cell: A Molecular Approach. 2nd ed. Sunderland: Sinauer, 2000. Natl. Center for Biotechnology Information. Web. 22 Dec. 2014. http://www.ncbi.nlm.nih.gov/books/NBK9928/
"The Neuron." BrainFacts.org. Soc. for Neuroscience. 1 Apr. 2012. Web. 22 Dec. 2014. http://www.brainfacts.org/brain-basics/neuroanatomy/articles/2012/the-neuron/
"What Is a Cell?" Scitable. Nature Education. Web. 22 Dec. 2014. http://www.nature.com/scitable/topicpage/what-is-a-cell-14023083