Essential features of neurons:

• The purpose of neurons is to receive, process, and transfer information. This information is made up of chemical or electrical signals
• They are unidirectional
• It’s an ‘all-in-one’ transfer of information (see Neuronal Signaling section)
• Mature neurons do not regenerate in the CNS if their soma (cell body) is damaged. However, the dendrites can regenerate, and sometimes the axons can generate also. (This allows other neurons to assume the function of damaged ones.) Regeneration in the PNS is more probable and successful.
• Mature neurons do not proliferate after birth, however, progenitor (stem) cells are found in the brain and can divide after birth. Progenitor cells can become neurons or glial cells.
• Histology is the method by which neurons are studied:
• Analysis of the brain is done by first hardening it (e.g., in a formaldehyde solution), then cutting extremely thin slices
• Nissl Stains color the cell body (soma) purple. This makes it possible to separate the cell bodies of neurons from glial cells, count the number of neurons in a particular area, etc.
• Myelin Stains color myelin sheaths, and therefore make it easy to view axons or axonal connections
• Golgi Stains, or Silver Stains, color the entire neurons silver. However, only 2% of the neurons present will be highlighted in this way

There are 3 main types of neurons:

• Sensory neurons (afferent neurons) transfer information from the external environment to the CNS
• Motor neurons (efferent neurons) transfer information from the CNS to external environment
• Interneurons, or association neurons, process information in the CNS and transfers the information from one neuron to the other within the CNS

Neurons are divided into three parts: dendrites, soma, and axon:

• Dendrites
• The ‘input’ of the neuron:
• Receive information from other neurons or the external environment and transfer to the cell body (soma) or axons
• Are numerous, relatively short, and branch extensively in a tree-like fashion
• Dendrite means tree in Greek
• Dendrites have numerous spines on them, and provide a greater surface area for other neurons to synapse on, i.e., attach to.
• Dendrites receive information from other cells at these synapses. This makes dendrites postsynaptic.
• The connection between axons that synapse on dendrites is called axodendritic
• The connection between dendrites that synapse on other dendrites is called dendrodendritic
• Soma
• The cell’s body:
• Has typical cell components used for cell maintenance:
• the membrane (a lipid bilayer), which separates and protects the cell from its environment;
• the nucleus (with a large nucleolus), which contains the genetic information of the cell;
• endoplasmic reticulum and ribosomes, where proteins are produced;
• mitochondria, the energy power houses of the cell;
• golgi apparatus, where proteins are packaged in vesicles for secretion outside the cell;
• and other miscellaneous organelles.
• The soma is usually large
• Soma means body in Greek
• Axon
• The ‘output’ of the neuron:
• Transfer information to other neurons
• Begin at the axon hillock, which is a swelling at the junction of the axon and soma where there are many Na⁺ channels and the action potential starts (see Neuronal Signaling section)
• Are usually long (some reaching several feet)
• Axon means axis in Greek
• Have terminal boutons at the end of the axon where the synapse is located
• This makes axons presynaptic
• This swelling at the terminal bouton is where the neuron synapses with another neuron
• Contains numerous vesicles which hold neurotransmitter
• Has many Ca²⁺ channels in the membrane
• Forms presynaptic membrane in any kind of axonal synapse
• The space between the terminal boutons and the next cell is known as the synaptic cleft, and is approximately 20 nm thick.
• Most are myelinated:
• Have myelin sheaths that are made by Schwann cells or oligodendrocytes (see glia below)
• Myelin acts as insulator to help conduction of action potential
• There are openings between the Schwann cells called Nodes of Ranvier. These help with the conduction of action potentials (see Neuronal Signaling below).
• Some are not myelinated
• Typical of shorter axons
• Axons synapse on other cells in various forms:
• Axoaxonal: Axon is connected to another neuron’s axon
• Axodendritic: Axon is connected to another neuron’s dendrites
• Axosomatic: Axon is connected directly to another neuron’s soma
• In neuromuscular junctions, axons synapse directly on muscles.

Neurons have different shapes:

• Unipolar: Has only one process extending from the soma, branching into dendrites or axon terminals (typical of invertebrate animals)
• Bipolar: The neuron, it has one input process from dendrites and one output process to dendrites (typical of sensory neurons: visual, auditory, olfactory)
• Multipolar: One axon but many dendrites extending directly from the soma (used for motor and sensory processing). This is the prototypical neuron
• Pseudounipolar: Were originally bipolar, but the dendrites and axon extensions have fused (typical of the dorsal root ganglia in the spinal cord)