1. Neurons
A neuron is, in the simplest sense, the cell which receives and transfers information within the nervous system. The human brain, once it reaches maturity may contain as many as one hundred billion neurons, however this is just an estimate as it would be impossible to count all these cells.
What are the different parts of a neuron?
The neuron is composed of a vast amount of separate parts; these follow below. Differences between sensory and motor neurons will be discussed in greater detail later.
The Cell Membrane All cells in the body are surrounded by a membrane. These membranes are composed of an interconnecting bi-lipid (2 layers of fat) layer. These layers are interconnected, but are still able to move around each other. Embedded in this bi-lipid layer are specialized protein molecules which are known as protein channels. These channels allow exchange of chemicals, wastes, and nutrients necessary for the cells survival.
Cell membranes help to protect the cell from damaging agents or chemicals, and also help to preserve the specific charges within a neuron necessary for proper functioning.
The Cell Body (or Soma) This structure contains many components necessary to the continued survival of the neuron. These include the nucleus, which contain the chromosomes of the neuron; the mitochondria, which is a structure in which the cell performs metabolic functions (discussed later), and finally ribosomes which synthesize new protein molecules for the cell. These ribosomes may be floating unattached in the cell; others are connected to the endoplasmic reticulum which is a series of tubes which transport new proteins to other locations within the cell.
The Axon This structure is long and fibrous and extends from the soma to the presynaptic terminals on nearby neurons. The axon is basically the information sender of the neuron. It may convey impulses to target muscles to invoke a movement or to other neurons to create a response.
Axons contain many branches, at the tip of each of these is a bulbous formation known as the presynaptic terminal. These terminals are the site of neurotransmitter release.
Drugs such as ecstasy work at these sites to cause the pleasurable effects of the drug. These are also the site of 'pumps' which pull released neurotransmitters like dopamine back into the axon.
These dopamine pumps are the pumps which are blocked when cocaine is used. This blockade of the reuptake of dopamine causes increased amounts to act upon the receptor at the next neuron, thus causing the high. Due to the great amount of energy necessary for synthesis and release of the chemicals from the terminal many mitochondria can be found in these cellular areas.
A neuron is limited to only a single axon; however it may have many branches. Axons also vary widely in length. Some are incredibly long, reaching from your spinal chord all the way down to your legs. Others are incredibly short and localized.
The Myelin Sheath This is a fatty substance which covers many axons in the human brain. This substance is created in Scwann cells in the periphery of the body. These sheaths both insulate and increase the speed at which info may be sent through the axon.
Found along myelin sheathes are breaks in the sheath known as Nodes of Ranvier. At these nodes the charge traveling down the axon is strengthened.
The Dendrite Somewhat the opposite of the axon, this structure extends from the soma into the periphery of the nervous system. Its name comes from the Greek word for tree, and dendrites do indeed resemble very small trees. Dendrites are used as the information receiving parts of the neuron, specifically at sites known as synaptic receptors. A dendrite’s ability to process larger or smaller amounts of info is based on its surface area. To take advantage of this dendrites often branch out from their main stalks and develop even smaller outgrowths known as dendritic spines. These spines and branches greatly increase the surface area of the neuron.
A neuron may have any number of dendrites.
The Synaptic Cleft Although not technically part of a neuron, this is the space between presynaptic terminals and synaptic receptors. In this area neurotransmitters are sent from axons to dendrites (where they interact with receptors for those specific neurotransmitters), destruction of neurotransmitters occurs and neurotransmitters are reabsorbed.