By adulthood, the brain will have about 100 billion nerve cells, called neurons, to control thoughts and actions by.

Neurons are unique among cells because

  • They have dendrites and axons (defined further on)
  • They work with special chemicals that involves synapses
  • They communicate by electricity and chemicals (electrochemical)
  • They largely stop reproducing after birth

Pictured below are two neurons talking to one another.

Please refer to the image above for this section. Pictured on the far right corner is a distant diagram of two neurons that communicate.The two mushroom-like yellow objects are the specific parts of these neurons involved in communication .

  1. Neurotransmitters are chemicals that act as messages throughout the brain and body
    1. The central nervous system relies heavily on them
    2. They are stored and released by neurons
  2. On the left neuron, we see tiny sacks, called vesicles, which hold neurotransmitters until release
    1. This neuron on the left is presynaptic, before the synapse
    2. The other neuron is postsynaptic, after the synapse
  3. The gap area between the two neurons is the synapse, or, synaptic cleft
    1. A mere 15-50 nanometers wide
  4. Neurotransmitters in the synapse bind to postsynaptic receptors
    1. A good analogy is a key (neurotransmitter) fitting into a lock (receptor)
    2. These receptors are on dendrites
    3. The signal is then passed on to other cells

Let’s move on with a little more detail, referencing the image below.

We already know that neurotransmitters are the chemicals that make neurons communicate. The are released from the vesicles of the presynaptic neuron into the synapse, and make their way over to their receptors on the dendrite of the postsynaptic neuron, to pass on a signal.

But how did the original message hit? What happened before the vesicles were told to release neurotransmitters into the synapse? In essence, what happened from receiving the message, to releasing chemicals to further the message?

The signal had to be strong enough.

  1. A single neurotransmitter binds to a receptor
    1. A single electrical charge or messenger is created in the postsynpatic neuron
    2. The charge or messenger dies off
  2. Several neurotransmitters of the same kind all bind to the same type of receptor at once
    1. Several different electrical charges or messages are combined into one charge or message
    2. The charge or message makes its way past the soma: the cell body, or nucleus, responsible for various genetic actions, the creation of some neurohoromones and neurotransmitters, and integrating neurochemical messages
    3. The charge or message then reaches the region just after the soma, and just before the axon, the axon hillock
    4. A rapid electro-chemical chain-reaction takes place to transmit a message through to the end of the axon, and ultimately to other neurons
      1. If the charge has reached the axon hillock, the charge has also reached one of many (sodium) voltage-gated ion channels that span the axon
      2. So when that charge causes the sodium ion channels to open, causing sodium cations (Na+2) to enter the neuron, and depolarize that intracellular area
      3. This positive charge depolarizes the area around the voltage-gated sodium ion channel, creeping a bit down the axon
      4. A voltag-gated potassium channel around the recently opened sodium ion channel opens up, repolarizing that space by a wave of pottasium cations (K+) exiting the neuron
      5. First the sodium, then the pottassium, ion channel close
      6. However, a positive charge has already made its way down the axon a bit and past the domain of that posttasium ion channel
      7. And when the charge of the initial sodiumion influx reaches a spot along the axon that isn’t covered in myelin sheath (a node of ranvier), another voltage-gated sodium channel opens, repeating the whole process
      8. In this manner, the positive (depolarization) charge “jumps” from node of ranvier, to node of ranvier, until it reaches the end of the axon, the terminal button/axon terminal
    5. When the charge reaches the terminal button, voltage-gated calcium channels on the button open, creating an influx of calcium cations (Ca+2)
    6. This causes the vesicles located in the terminal button to fuse with the cell membrane, releaseing neurotransmitters into the synapse

 

 

Neurotransmitters can be broken down into…

  • Small
    • Minor, modified amino acids
    • Synthesized, packaged, stored by the golgi complex right at the terminal button (end of the axon)
    • Tend to act locally, though the monoamines are an exception.
    • GABA
    • Glutamate
    • Serine
    • Histamine
    • Trace amines
  • Large
    • Peptides (3-30+ amino acids linked into one chain) 
    • Packaged by the golgi complex in the cell body (soma)
    • Influence neurotransmission relatively far from the place that they were released (diffuse effects)
    • Transported to the terminal button by tiny tubes called microtubules
    • Pituitary
    • Opioid
    • Hypothalamuc
    • Brain-gut
    • Miscellaneous

One more important piece of information concerns the myelin sheath: the structure that serves as insulation for neuronal activity. It makes neurotransmission more efficient, and physically protects the neuron. Myelinated axons in the central nervous system, being white, are dubbed white matter. Gray matter is the grayish dendrites and cell bodies (somas) in the central nervous system.

Mental disorders marked by cognitive decline, such as Alzheimer’s Disease and Dementia, may be largely caused by a lack of myelin sheath. Also, inhalant abuse destroys myelin sheath, effectively leaving the brain cells with little protection.

There are various ways that neurotransmitters in the synapse can become deactivated, and in different regions. It depends on the type of neurotransmitter. This will be discussed in other sections.

Pictured below is one type of neuron, the type that comes to mind for most of us when we think of neurons. It is the multipolar neuron. This type of neuron carries motor commands from the central nervous system to various outside sources, controlling movement. Most neurons in the central nervous system are multipolar.

neuron

Source

The unipolar neurons are sensory neurons. They collect and send information of one’s environment to the central nervous system.

Bipolar neurons also play a large roll in communicating information from the senses to the central nervous system.

Interneurons act as an intermediary between unipolar cells of the peripheral nervous system and the multipolar neurons of the central nervous system.

Illustrated below are the permutations (kinds) of neurons.

types_of_neurons1317163869121

 

Apart from neurons, the other type of cell found in the central nervous system is the glial cell. There are about five glial cells for every neuron. Once thought to be unimportant, current research suggests that they do serve an important function, though not much more is known of them. Please refer to the Glial Cells section for more information.

Some fun facts:

  • Cell bodies of neurons can be anywhere from four to 100 microns wide.
  • Axons can be very tiny, or quite big. The largest axon in the human bodies is over three feet long!
  • There are up to 500 trillion neural connections in the human brain. A neuron can have up to 150,000 connections with other cells.
  • Neurons can reach action potential and fire up to 1,000 times per second, reaching a speed of 270 miles per hour.

Sources: Uppers, Downers, All Arounders: Physical and Mental Effects of Psychoactive Drugs, Abnormal Psychology: An Integrative Approach, Stahl’s Essential Psychopharmacology, Dr. Kevin Davis, https://faculty.washington.edu, https://www.verywell.com