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Neurons

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Parts of the neuron | Types of neuron | Neuron shapes | See also

Neurons are the basic cells of the nervous system. They act to process and transmit information around the system.

Neurons make up about half the cells of the CNS. The rest, in particular glia, are provide additional supporting roles.

Parts of the neuron

Soma

The soma is the cell body of the neuron that contains the nucleus and much of the other 'life' of the neuron.

Dendrites and axons reach out from the soma to connect with other neurons.

Dendrite

Dendrites stretch out from the soma much like branches from a tree, breaking down into smaller branches and twigs (dendrite is the Greek word for 'tree'). These branches are sometimes called an arborization.

Dendrites may have small growths that stud the surface. These are called dendritic spines.

Dendrites are the 'receivers' of the neuron, connecting with axons from other neurons.

Axon

There is usually one long axon per neuron that reaches out to connect with other neurons. This may divide into sub-branches, typically towards the end of the axon, thus allowing it to connect with multiple other neurons in a given vicinity.

Axons often connect with dendrites, although they can also connect with the soma or even other axons.

The basic messages transmitted down the axon are electrical/chemical events, are called action potentials.

The substance in the axon is called axoplasm. Transport of items towards terminal button is called axoplasmic transport. Movement towards the terminal buttons is called anterograde axoplasmic transport and is achieved with a walking protein called kinesin. Movement away from the terminal buttons is called retrograde axoplasmic transport and is achieved with a protein called dynein.

Axons are structured with microtubules, long filaments down which chemicals are send to terminal buttons.

Most axons are insulated by a myelin sheath that acts in a similar way to the plastic sheath around an electrical wire. This is created in segments by oligodendrocytes or Schwann cells, with small gaps between these, called the nodes of Ranvier.

Projection fibres are sets of axons that start from neurons in one region of the brain and reach across to another, thus bridging and connecting the brain into a single organ.

Terminal buttons

At the end of each axon is a set of 'buttons' that connect with dendrites on other neurons. Sometimes the French bouton or terminals are used instead.

The terminal button contains mitochondria and synaptic vesicules suspended in cytoplasm. The vesicules contain neurotransmitters.

The connection between terminal buttons and dendrites is not through direct touch. There is a small gap sustained called the synapse or synaptic gap, across which neurotransmitters travel to create the connecting signal. This is caused when an action potential reaches the button.

Types of neuron

Sensory neuron

Sensory neurons receive information from the outside world through our five senses. They run from stimulus receptors to the central nervous system.

The cell bodies of sensory neurons leading to the spinal cluster in ganglia, next to the cord. They then connect to interneurons in the cord.

Interneuron

Interneurons receive information from one or more other neurons, integrates this and transmits information to other neurons. A lot of the brain is made up of inter-neurons.

Motor neuron

Motor neurons send signals to muscles to move the body. This can range from moving limbs to turning eyes to making the heart keep pumping. They also connect to glands.

Most motor neurons are stimulated by interneurons, although they can also be stimulated directly by sensory neurons.

Neuron shapes

Multipolar neurons

This is the most common type of neuron, with one axon and many dendrites.

Bipolar neurons

Bipolar neurons have one axon and one dendrite tree, each extending from opposite ends of the soma. The dendrite connects with cilia which are

Bipolar neurons are usually connected with sensing, detecting environmental effects and transmitting information about these.

Unipolar neurons

Unipolar neurons have a single axon connecting the dendrite tree and terminal buttons. The soma sticks out to the side of this, connected by a small stalk.

Unipolar neurons, like bipolar neurons, transmit sensory information, in particular touch, temperature change and other skin sensation. They also transmit information about events in muscles, joints and organs.

Pyramidal neurons

Pyramidal neurons are large and multipolar, with triangular somas and are largely found in the corticospinal tract, the cerebral cortex, the hippocampus, and in the amygdala. In the cortex they are associated with cognitive ability. in the corticospinal tract, bundles of axons connect the cortex with the spinal cord.

See also

Neuron internals