Some neurons have short dendrites, whilst others have longer ones. In the central nervous system, neurons are long and have complex branches that can allow them to receive signals from many other neurons. For instance, cells called Purkinje cells which are found in the cerebellum have highly developed dendrites to receive signals from thousands of other cells.
The soma, or cell body, is essentially the core of the neuron. The soma is enclosed by a membrane which protects it, but also allows it to interact with its immediate surroundings. The soma contains a cell nucleus which produces genetic information and directs the synthesis of proteins. These proteins are vital for other parts of the neuron to function. The axon, also called a nerve fiber, is a tail-like structure of the neuron which joins the cell body at a junction called the axon hillock.
The function of the axon is to carry signals away from the cell body to the terminal buttons, in order to transmit electrical signals to other neurons. Most neurons just have one axon which can range in size from 0.
Some axons are covered in a fatty substance called myelin which insulates the axon and aids in transmitting signals quicker. Axons are long nerve processes that may branch off to transfer signals to many areas, before ending at junctions called synapses. The myelin sheath is a layer of fatty material that covers the axons of neurons. Its purpose is to insulate one nerve cell from another and so to prevent the impulse from one neuron from interfering with the impulse from another.
The second function of the myelin sheath is to speed up the conduction of nerve impulses along the axon. The axons which are wrapped in cells known as glial cells also known as oligodendrocytes and Schwann cells form the myelin sheath. The myelin sheath which surrounds these neurons has a purpose to insulate and protect the axon. Due to this protection, the speed of transmission to other neurons is a lot faster than the neurons that are unmyelinated.
The myelin sheath is made up of broken up gaps called nodes of Ranvier. Electrical signals are able to jump between the nodes of Ranvier which helps in speeding up the transmission of signals. Located at the end of the neuron, the axon terminals terminal buttons are responsible for transmitting signals to other neurons. 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. However, in order for a presynaptic neuron to release a neurotransmitter to the next neuron in the chain, it must go through a series of changes in electric potential.
The action potential is a rapid change in polarity that moves along the nerve fiber from neuron to neuron. In order for a neuron to move from resting potential to action potential—a short-term electrical change that allows an electrical signal to be passed from one neuron to another—the neuron must be stimulated by pressure, electricity, chemicals, or another form of stimuli. The level of stimulation that a neuron must receive to reach action potential is known as the threshold of excitation, and until it reaches that threshold, nothing will happen.
Different neurons are sensitive to different stimuli, although most can register pain. Action potentials : A neuron must reach a certain threshold in order to begin the depolarization step of reaching the action potential.
This process of depolarization, repolarization, and recovery moves along a nerve fiber from neuron to neuron like a very fast wave. While an action potential is in progress, another cannot be generated under the same conditions. In unmyelinated axons axons that are not covered by a myelin sheath , this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. Saltatory conduction is faster than continuous conduction.
The diameter of the axon also makes a difference, as ions diffusing within the cell have less resistance in a wider space. Damage to the myelin sheath from disease can cause severe impairment of nerve-cell function. In addition, some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves. The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials.
Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all. Myelin is created by Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS. There are small gaps in the myelin coating, called nodes of Ranvier. The action potential jumps from gap to gap, allowing the signal to move much quicker. Multiple sclerosis is caused by the slow breakdown of myelin. Neurons are connected to each other and tissues so that they can communicate messages; however, they do not physically touch — there is always a gap between cells, called a synapse.
Synapses can be electrical or chemical. In other words, the signal that is carried from the first nerve fiber presynaptic neuron to the next postsynaptic neuron is transmitted by an electrical signal or a chemical one. Once a signal reaches a synapse, it triggers the release of chemicals neurotransmitters into the gap between the two neurons; this gap is called the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and interacts with receptors on the membrane of the postsynaptic neuron, triggering a response.
Glutamergic — releases glutamine. They are often excitatory, meaning that they are more likely to trigger an action potential. They are often inhibitory, meaning that they reduce the chance that the postsynaptic neuron will fire. Cholinergic — release acetylcholine. These are found between motor neurons and muscle fibers the neuromuscular junction.
Electrical synapses are less common but are found throughout the CNS. Channels called gap junctions attach the presynaptic and postsynaptic membranes. In gap junctions, the post- and presynaptic membranes are brought much closer together than in chemical synapses, meaning that they can pass electric current directly. Electrical synapses work much faster than chemical synapses, so they are found in places where quick actions are necessary, for instance in defensive reflexes.
Chemical synapses can trigger complex reactions, but electrical synapses can only produce simple responses. However, unlike chemical synapses, they are bidirectional — information can flow in either direction. Neurons are one of the most fascinating types of cell in the human body.
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