Nearly 86 billion neurons work together within the nervous system to communicate with the rest of the body. They are responsible for everything from consciousness and thought to pain and hunger. There are three primary types of neuron: sensory neurons, motor neurons, and interneurons. In addition to having all the normal components of a cell nucleus, organelles, etc.
The structure of a neuron : The above image shows the basic structural components of an average neuron, including the dendrite, cell body, nucleus, Node of Ranvier, myelin sheath, Schwann cell, and axon terminal. Dendrites are branch-like structures extending away from the cell body, and their job is to receive messages from other neurons and allow those messages to travel to the cell body. Although some neurons do not have any dendrites, other types of neurons have multiple dendrites.
Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible connections with other neurons. Like other cells, each neuron has a cell body or soma that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. The cell body contains a specialized structure, the axon hillock, which serves as a junction between the cell body and the axon.
The synapse is the chemical junction between the axon terminals of one neuron and the dendrites of the next. It is a gap where specialized chemical interactions can occur, rather than an actual structure.
The specialized structure and organization of neurons allows them to transmit signals in the form of electric impulses from the brain to the body and back. Individually, neurons can pass a signal all the way from their own dendrites to their own axon terminals; but at a higher level neurons are organized in long chains, allowing them to pass signals very quickly from one to the other.
This is the basic chain of neural signal transmission, which is how the brain sends signals to the muscles to make them move, and how sensory organs send signals to the brain. It is important that these signals can happen quickly, and they do.
Think of how fast you drop a hot potato—before you even realize it is hot. Dendrites, cell bodies, axons, and synapses are the basic parts of a neuron, but other important structures and materials surround neurons to make them more efficient. Some axons are covered with myelin, a fatty material that wraps around the axon to form the myelin sheath. This external coating functions as insulation to minimize dissipation of the electrical signal as it travels down the axon.
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. Periodic gaps in the myelin sheath are called nodes of Ranvier.
The myelin sheath is not actually part of the neuron. Glia function to hold neurons in place hence their Greek name , supply them with nutrients, provide insulation, and remove pathogens and dead neurons.
In the central nervous system, the glial cells that form the myelin sheath are called oligodendrocytes; in the peripheral nervous system, they are called Schwann cells. Neuron in the central nervous system : This neuron diagram also shows the oligodendrocyte, myelin sheath, and nodes of Ranvier.
There are three major types of neurons: sensory neurons, motor neurons, and interneurons. All three have different functions, but the brain needs all of them to communicate effectively with the rest of the body and vice versa. Sensory neurons are neurons responsible for converting external stimuli from the environment into corresponding internal stimuli. They are activated by sensory input, and send projections to other elements of the nervous system, ultimately conveying sensory information to the brain or spinal cord.
Unlike the motor neurons of the central nervous system CNS , whose inputs come from other neurons, sensory neurons are activated by physical modalities such as visible light, sound, heat, physical contact, etc.
Most sensory neurons are pseudounipolar , meaning they 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. Multipolar and pseudounipolar neurons : This diagram shows the difference between: 1 a unipolar neuron; 2 a bipolar neuron; 3 a multipolar neuron; 4 a pseudounipolar neuron.
Motor neurons are neurons located in the central nervous system, and they project their axons outside of the CNS to directly or indirectly control muscles. The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. The structure of motor neurons is multipolar , meaning each cell contains a single axon and multiple dendrites. This is the most common type of neuron.
Located in the CNS, they operate locally, meaning their axons connect only with nearby sensory or motor neurons. Interneurons can save time and therefore prevent injury by sending messages to the spinal cord and back instead of all the way to the brain.
Like motor neurons, they are multipolar in structure. The central nervous system CNS goes through a three-step process when it functions: sensory input, neural processing, and motor output. The sensory input stage is when the neurons or excitable nerve cells of the sensory organs are excited electrically.
Neural impulses from sensory receptors are sent to the brain and spinal cord for processing. After the brain has processed the information, neural impulses are then conducted from the brain and spinal cord to muscles and glands, which is the resulting motor output.
A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic receiving neuron is determined not by the presynaptic sending neuron or by the neurotransmitter itself, but by the type of receptor that is activated.
A neurotransmitter can be thought of as a key, and a receptor as a lock: the key unlocks a certain response in the postsynaptic neuron, communicating a particular signal. Most sensory neurons are pseudounipolar, which means they only have one axon which is split into two branches. Motor neurons of the spinal cord are part of the central nervous system CNS and connect to muscles, glands and organs throughout the body.
These neurons transmit impulses from the spinal cord to skeletal and smooth muscles such as those in your stomach , and so directly control all of our muscle movements. There are in fact two types of motor neurons: those that travel from spinal cord to muscle are called lower motor neurons, whereas those that travel between the brain and spinal cord are called upper motor neurons.
As the name suggests, interneurons are the ones in between - they connect spinal motor and sensory neurons. As well as transferring signals between sensory and motor neurons, interneurons can also communicate with each other, forming circuits of various complexity. They are multipolar, just like motor neurons. In the brain, the distinction between types of neurons is much more complex. Certainly, there are brain neurons involved in sensory processing — like those in visual or auditory cortex — and others involved in motor processing — like those in the cerebellum or motor cortex.
However, within any of these sensory or motor regions, there are tens or even hundreds of different types of neurons. In fact, researchers are still trying to devise a way to neatly classify the huge variety of neurons that exist in the brain. This information is then passed down to the cell body and on to the axon. Once the information has arrived at the axon, it travels down the length of the axon in the form of an electrical signal known as an action potential.
Once an electrical impulse has reached the end of an axon, the information must be transmitted across the synaptic gap to the dendrites of the adjoining neuron. In some cases, the electrical signal can almost instantaneously bridge the gap between the neurons and continue along its path.
In other cases, neurotransmitters are needed to send the information from one neuron to the next. Neurotransmitters are chemical messengers that are released from the axon terminals to cross the synaptic gap and reach the receptor sites of other neurons.
In a process known as reuptake, these neurotransmitters attach to the receptor site and are reabsorbed by the neuron to be reused. Neurotransmitters are an essential part of our everyday functioning. While it is not known exactly how many neurotransmitters exist, scientists have identified more than of these chemical messengers.
The following are just a few of the major neurotransmitters, their known effects, and disorders they are associated with. Acetylcholine: Associated with memory, muscle contractions, and learning. Endorphins: Associated with emotions and pain perception. The body releases endorphins in response to fear or trauma.
These chemical messengers are similar to opiate drugs such as morphine but are significantly stronger. Dopamine: Associated with thought and pleasurable feelings. One category is dopamine agonists, which mimic the effects of dopamine. Another type of agent is levodopa, which is converted into dopamine in the brain. They each carry their own relative benefits and side effects. Researchers also have found strong links between schizophrenia and excessive amounts of dopamine in certain parts of the brain.
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