Neuron

Neuron

Schematic of an anatomically accurate single pyramidal neuron, the primary excitatory neuron of cerebral cortex, with a synaptic connection from an incoming axon onto a dendritic spine.

Diagram of the human nervous system. The relationship between the brain, spinal cord, and rest of the nerves in the body is demonstrated.

Neuron cell body

Diagram of a typical myelinated vertebrate motor neuron

Golgi-stained neurons in human hippocampal tissue

Actin filaments in a mouse Cortical Neuron in culture

A neuron, traditionally referred to as a neurone in British English, is an electrically excitable cell that constitutes the foundational unit of the nervous system. Its primary function is to relay information across the body using electrical and chemical processes. These cells are intricate in design, with complex mechanisms allowing them to communicate with other cells through unique structures named synapses.

Overview

The significance of neurons is evident in almost all animals, with the notable exception of sponges and placozoans. Interestingly, while animals possess these intricate cells, other multicellular organisms, like plants, do not have neurons. A collection of interconnected neurons forms what is commonly referred to as a neural circuit.

Anatomy and Histology

The neuron's structure consists of several vital components:

• Cell Body (Soma): This is the central part of the neuron, typically compact in nature.
• Dendrites: Resembling tree branches, these protrude from the soma and can spread out extensively. They play a crucial role in receiving signals from other neurons.
• Axon: Extending from a region of the soma known as the axon hillock, this long, filamentous structure can span distances as vast as a meter in humans. The axon is instrumental in transmitting signals to other cells.
• Axon terminals: These are situated at the furthest tips of the axon branches, acting as transmission points where signals are sent across the synapse to another cell.

Though most neurons possess both dendrites and an axon, some may lack one or the other. The term neurite is employed to describe either dendrites or axons, especially in cases where differentiation is challenging.

Membrane

The neuron membrane is responsible for maintaining voltage gradients, rendering the neuron electrically excitable. This excitability paves the way for the neuron to produce a rapid, all-or-nothing electrochemical pulse termed an action potential.

Histology and Internal Structure

Neurons have a specialized internal structure, allowing them to effectively process and transmit signals. Synapses are critical junctures that facilitate communication between neurons or between a neuron and another type of cell. They can be both chemical and electrical, dictating whether a neuron gets excited or inhibited.

Classification

Neurons are typically categorized based on function and structure:

Structural Classification

This categorization is based on the number of protruding structures from the neuron's cell body.

Functional Classification

Functionally, neurons fall into three main categories:

• Sensory Neurons: These neurons react to external stimuli, such as light, sound, or touch, and relay this information to the brain or spinal cord.
• Motor Neurons: They are involved in receiving signals from the brain or spinal cord, subsequently controlling functions ranging from muscle movements to gland activity.
• Interneurons: These facilitate communication between neurons within the same area of the brain or spinal cord.

Connectivity and Neural Circuits

A network of interconnected neurons is called a neural circuit. Most neurons receive signals through their dendrites and soma and dispatch signals via their axon. Although the majority of synapses involve signals transitioning from one neuron's axon to another's dendrite, axon-to-axon and dendrite-to-dendrite synapses also exist.

Nervous System: The Grand Network

The nervous system, comprising both the central nervous system (CNS) and the peripheral nervous system (PNS), is the primary residence of neurons. The CNS encompasses the brain and spinal cord, while the PNS consists of autonomic and somatic systems.

Neurons, along with glial cells, are the main components of the nervous system. In vertebrates, the majority of neurons are housed within the CNS, though some are located in peripheral ganglia. Numerous sensory neurons are also embedded in organs like the retina and cochlea. Axons in the PNS group together to form nerves, while those in the CNS cluster into tracts.

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