Neuron

A neuron, also known as a nerve cell, is an electrically excitable cell that processes and transmits information through electrical and chemical signals. These signals between neurons occur via specialised connections referred to as synapses^[1]. Neurons can connect to each other to form neural networks and they are the primary components of the brain, spinal cord of the central nervous system, and the peripheral nerves in the peripheral nervous system^[2].

Neurons are complex cells. Below are the main anatomical components of a working neuron, along with a description of its function.

1. Soma (Cell Body): The soma is the central part of the neuron. It contains the neuron's nucleus, which houses the cell's genetic material. The soma is also responsible for carrying out the metabolic functions of the neuron, producing the energy that the neuron needs to function. The cell body is also where protein synthesis occurs^[1].

2. Nucleus: The nucleus is located within the soma and contains the neuron's DNA. The DNA carries the genetic information necessary for the cell's structure and function, and it controls the production of proteins within the neuron^[4].

3. Nucleolus: This is a small, dense structure within the nucleus that's responsible for producing ribosomes, the cell's protein-producing factories^[4].

4. Cytoplasm: This is the gel-like substance inside the soma that contains the organelles. The cytoplasm is where many of the cell's metabolic reactions occur^[4].

5. Mitochondria: These organelles are the energy factories of the cell. They use glucose and oxygen to produce ATP (adenosine triphosphate), the cell's main energy currency^[4].

6. Endoplasmic Reticulum (ER): This is a network of tubes that helps process proteins and produce lipids. The rough ER has ribosomes attached to it and is involved in protein synthesis, while the smooth ER is more involved in lipid production^[4].

7. Golgi Apparatus: This organelle packages proteins into vesicles for transport within the cell or for release from the cell^[4].

8. Lysosomes: These organelles contain enzymes that break down waste products and cellular debris^[4].

9. Dendrites: These are branching extensions from the soma. They act like the 'antennae' of the neuron, receiving signals from other neurons and transmitting them towards the soma. The surface of the dendrites is covered in synapses, the points of contact with other neurons^[5].

10. Axon: The axon is a long, slender projection that conducts electrical impulses away from the soma towards other neurons or effector cells. The part of the axon where it emerges from the soma is called the axon hillock. This is where the action potential (electrical impulse) is generated^[5].

11. Axon Hillock: This is the part of the cell body from where the axon originates. It plays a crucial role in the initiation of the neuron's action potentials^[5].

12. Myelin Sheath: This is a layer of fatty, insulating material that covers the axon. It is made up of cells called Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. The myelin sheath serves to speed up the transmission of action potentials^[5].

13. Nodes of Ranvier: These are small gaps in the myelin sheath along the axon. The action potentials jump from node to node in a process called saltatory conduction, which greatly increases the speed of transmission^[5].

14. Axon Terminal (Terminal Buttons): These are small bulbous structures at the end of the axon where it branches out to connect to other neurons. The axon terminals contain vesicles filled with neurotransmitters. When an action potential reaches the axon terminal, it causes these vesicles to release their neurotransmitters into the synaptic cleft^[5].

15. Synapse: The synapse is the junction between neurons where communication occurs. It consists of the axon terminal (presynaptic neuron), the synaptic cleft (gap), and the dendrite or soma of the next neuron (postsynaptic neuron). The signal, often in the form of a neurotransmitter, is transmitted across this junction^[5].

16. Neurotransmitters: These are chemical messengers that transmit signals across the synapse from one neuron to another^[5].

17. Vesicles: These are small membrane-bound sacs in the axon terminals that store neurotransmitters. When an action potential reaches the axon terminal, it triggers the release of neurotransmitters from the vesicles into the synaptic cleft^[5].

These components together facilitate the primary function of a neuron - to receive, conduct, and transmit signals in the form of electrical impulses and chemical messages.

There are many types of neurons, each performing a different function. Some of the most common types of neurons include unipolar, bipolar, psudounipolar, and multipolar neurons.

Some of the neurons which are found in the human nervous system, along with their location and the function they serve, can be found listed below.

1. Sensory Neurons (Afferent Neurons): These neurons transmit signals from sensory receptors to the central nervous system^[2].

2. Motor Neurons (Efferent Neurons): These neurons transmit signals from the central nervous system to effectors in the body like muscles and glands^[2].

3. Interneurons: Found only in the central nervous system, they connect neuron to neuron, and often process signals from sensory neurons and direct the activity of motor neurons^[2].

4. Multipolar Neurons: These neurons have one axon and multiple dendrites. They are the most common type of neuron^[7].

5. Bipolar Neurons: These neurons have one axon and one dendrite. They are often involved in sensory processing, such as vision and smell^[8].

6. Unipolar Neurons (Pseudounipolar Neurons): These neurons have one process that extends from the soma and divides into two branches, acting as both the axon and dendrite. They are often found in the sensory neurons of the peripheral nervous system^[9].

7. Anaxonic Neurons: These neurons have no clear axon and are found in the brain and retina^[10].

8. Pyramidal Neurons: These neurons are found in the cerebral cortex, hippocampus, and the amygdala. They have a pyramid-shaped cell body and a single, long apical dendrite, as well as several shorter, basal dendrites^[11].

9. Purkinje Neurons: These neurons are found in the cerebellum. They have a complex, fan-shaped dendritic tree and are crucial for motor coordination^[12].

10. Granule Cells: These are small, densely packed neurons found in the cerebellum and the hippocampus. They relay information from the periphery to Purkinje cells (in the cerebellum) or pyramidal cells (in the hippocampus)^[13].

11. Renshaw Cells: Found in the spinal cord, they help to regulate motor neuron activity^[14].

12. Basket Cells: These inhibitory interneurons in the cerebellum help shape the firing of Purkinje neurons^[15].

13. Chandelier Cells: These are a type of GABAergic inhibitory interneuron found in the cerebral cortex^[16].

14. Spindle Neurons (Von Economo Neurons): These are a unique class of neurons found in humans and a few other species that may play a role in social cognition^[17].

15. Mirror Neurons: These are neurons that fire both when an individual performs an action and when they observe the same action performed by others, possibly playing a role in empathy and learning^[18].

16. Anterior Horn Cells: These are motor neurons in the anterior horn of the spinal cord^[19].

17. Medium Spiny Neurons: These are found in the basal ganglia of the brain and play a crucial role in the coordination of movement^[20].

Different neurons have different structures that reflect their functions. For instance, sensory neurons often have long dendrites and short axons, while motor neurons have long axons and short or even no visible dendrites. These variations allow neurons to perform their specific tasks in the body.