Neurons are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. Each part of the neuron plays a role in communicating information throughout the body.
Neurons carry messages throughout the body, including sensory information from external stimuli and signals from the brain to different muscle groups in the body. In order to understand exactly how a neuron works, it is important to look at each individual part of the neuron. The unique structures of the neuron allow it to receive and transmit signals to other neurons as well as other types of cells.
There are many types of specialized neurons. Sensory neurons respond to one particular type of stimuli such as touch, sound, or light and all other stimuli affecting the cells of the sensory organs , and converts it into an electrical signal via transduction, which is then sent to the spinal cord or brain. Motor neurons receive signals from the brain and spinal cord to cause everything from
muscle contractions and affect glandular outputs.
Interneurons connect neurons to other neurons within the same region of the brain or spinal cord in neural networks.
A typical neuron consists of a cell body (soma),
dendrites , and an axon . The term neurite is used to describe either a dendrite or an axon, particularly in its
undifferentiated stage. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometers and branching multiple times, giving rise to a complex “dendritic tree”. An axon (also called a nerve fiber when myelinated) is a special cellular extension (process) that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 meter in humans or even more in other species. Most neurons receive signals via the dendrites and send out signals down the axon. Numerous axons, also known as nerve fibers, are often bundled into fascicles , and in the
peripheral nervous system, bundles of fascicles make up what we refer to as nerves (like strands of wire make up cables). The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: for example, neurons can lack dendrites, or have no axon, and synapses can connect an axon to another axon or a dendrite to another dendrite.
All neurons are electrically excitable, due to maintenance of voltage gradients across their membranes by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium , potassium , chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels . If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated and this change in cross-membrane potential travels rapidly along the cell’s axon, and activates synaptic connections with other cells when it arrives.
In most cases, neurons are generated by special types of stem cells during brain development and childhood. Neurons in the adult brain generally do not undergo cell division. Astrocytes are star-shaped glial cells that have also been observed to turn into neurons by virtue of the stem cell characteristic pluripotency . Neurogenesis largely ceases during adulthood in most areas of the brain. However, there is strong evidence for generation of substantial numbers of new neurons in two brain areas, the hippocampus and olfactory bulb.
Neuron cell body
A neuron is a specialized type of cell found in the bodies of all eumetozoans . Only sponges and a few other simpler animals lack neurons. The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions that transmit signals to other cells. The body’s neurons, plus the glial cells that give them structural and metabolic support, together constitute the nervous system. In vertebrates, the majority of neurons belong to the central nervous system , but some reside in peripheral ganglia , and many sensory neurons are situated in sensory organs such as the retina and cochlea .
A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the
axon hillock, and can extend for great distances, giving rise to hundreds of branches. Unlike dendrites, an axon usually maintains the same diameter as it extends. The soma may give rise to numerous dendrites, but never to more than one axon. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon. A typical synapse, then, is a contact between the axon of one neuron and a dendrite or soma of another. Synaptic signals may be
excitatory or inhibitory . If the net excitation received by a neuron over a short period of time is large enough, the neuron generates a brief pulse called an action potential, which originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes.
Many neurons fit the foregoing schema in every respect, but there are also exceptions to most parts of it. There are no neurons that lack a soma, but there are neurons that lack dendrites, and others that lack an axon. Furthermore, in addition to the typical axodendritic and axosomatic synapses, there are axoaxonic (axon-to-axon) and dendrodendritic (dendrite-to-dendrite) synapses.
The key to neural function is the synaptic signaling process, which is partly electrical and partly chemical. The electrical aspect depends on properties of the neuron’s membrane. Like all animal cells, the cell body of every neuron is enclosed by a plasma membrane , a bilayer of
lipid molecules with many types of protein structures embedded in it. A lipid bilayer is a powerful electrical
insulator , but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane and ion pumps that actively transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage gated, meaning that they can be switched between open and closed states by altering the voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane.
Neurons communicate by chemical and electrical synapses in a process known as neurotransmission , also called synaptic transmission. The fundamental process that triggers the release of neurotransmitters is the action potential , a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.
Dendrites are treelike extensions at the beginning of a neuron that help increase the surface area of the cell body. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma. Dendrites are also covered with synapses .
Most neurons possess these branch-like extensions that extend outward away from the cell body. These dendrites then receive chemical signals from other neurons, which are then converted into electrical impulses that are transmitted toward the cell body.
Some neurons have very small, short dendrites, while other cells possess very long ones. The neurons of the central nervous systems have very long and complex dendrites that then receive signals from as many as a thousand other neurons.
If the electrical impulses transmitted inward toward the cell body are large enough, they will generate an action potential. This results in the signal being transmitted down the axon.
The soma, or cell body, is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional.
Characteristics of the soma:
Think of the cell body as a small factory that fuels the neuron. The soma produces the proteins that the other parts of the neuron, including the dendrites, axons, and synapses, need to function properly.
The support structures of the cell include mitochondria , which provide energy for the cell, and the Golgi apparatus, which packages products created by the cell and dispatches them to various locations inside and outside the cell.
The axon hillock is located at the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal (known as an action potential ) down the axon.
The axon hillock acts as something of a manager, summing the total inhibitory and excitatory signals. If the sum of these signals exceeds a certain threshold, the action potential will be triggered and an electrical signal will then be transmitted down the axon away from the cell body. This action potential is caused by changes in ion channels which are affected by changes in polarization.
In a normal resting state, the neuron possesses an internal polarization of approximately -70mV. When a signal is received by the cell, it causes sodium ions to enter the cell and reduce the polarization.
If the axon hillock is depolarized to a certain threshold, an action potential will fire and transmit the electrical signal down the axon to the synapses. It is important to note that the action potential is an all-or-nothing process and that signals are not partially transmitted. The neurons either fire or they do not.
The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.
Axons can range dramatically in size. Some are as short as 0.1 millimeters, while others can over 3 feet long.
The myelin surrounds the neurons protects the axon and aids in the speed of transmission. The myelin sheath is broken up by points known as the nodes of Ranvier or myelin sheath gaps. Electrical impulses are able to jump from one node to the next, which plays a role in speeding up the transmission of the signal.
Axons connect with other cells in the body including other neurons, muscle cells, and organs. These connections occur at junctions known as synapses. The synapses allow electrical and chemical messages to be transmitted from the neuron to the other cells in the body.
Terminal Buttons and Synapses
The terminal buttons are located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.
The terminal buttons contain vesicles holding the neurotransmitters. When an electrical signal reaches the terminal buttons, neurotransmitters are then released into the synaptic gap. The terminal buttons essentially convert the electrical impulses into chemical signals. The neurotransmitters than cross the synapse where they are then received by other nerve cells.
The terminal buttons are also responsible for the reuptake of any excessive neurotransmitters released during this process.
A Word From Verywell
Neurons serve as basic building blocks of the nervous system and are responsible for communicating messages throughout the body. Knowing more about the different parts of the neuron can help you to better understand how these important structures function as well as how different problems, such as diseases that impact axon myelination, might impact how messages are communicated throughout the body..