What is a Neuron?
How are neurons similar to other body-cells?
There are a few similarities between cells in your body and the specialized neuron cell.
- Both are surrounded by a cell membrane. This acts as a wall between the cell's inside world and what's going on outside. Cells now need a way (signals) to transmit information between these 2 worlds.
- Both contain a nucleus which contains genes.
- Contain organelles, such as mitochondria.
- Both carry out basic cell functions and processes such as energy production.
The Basics of a Neuron
Cells: Cells are the basic building blocks of life, making up all organisms. Cells come in a form that is discrete or distinct and therefore distinguishable and recognizable. Cells regulate what messages are transmitted and received across their membrane, maintain their own health and replicate.
Nervous system: Some cells are specialized. The human body itself is made up of trillions of cells. Cells in the nervous system can also be called nerve cells or more commonly, neurons.
The nervous system is a network or nerve cells and fibers which transmit nerve impulses between parts of the body. Nerve impulses is a way to transmit signals across this network- think of an electrical impulse.
Anatomy of a Neuron
Above is a very simple diagram of a neuron. Important features of the anatomy of a neuron include:
- Axon Terminals
Although not included in the diagram, some of the longer axons are covered with something called myelin sheath that is also important to examine.
Soma and the Nucleus:
A typical neuron has a few specialized structures that sets it apart from other cells. The main bulk of the neuron cell is called the soma or the cell body. The cell body or the soma contains the nucleus. The nucleus contains genetic material in the form of chromosomes.
As we can see from the anatomy of a neuron diagram, there are several extensions extending from the soma. These are called dendrites. These often look like branches or spikes extending from the cell body. Dendrites bring electrical impulses to the cell body. You can think of dendrites as signal receivers or relate it to computer science and recognize a dendrite simply as an input. This is how the cell receives information.
Axon and Axon Terminals:
Sometimes indistinguishable from a dendrite, the axon extends outward from the cell body and acts as a way for the cell body to transmit the signal or information away from the cell body. A collection of axons grouped together is called a nerve.
The axon terminals can be thought of as transmitters of the electrical signal or information. Relating this back to computer science, the axon terminals can be thought of as outputs.
Myelin sheath is needed for longer axons. Myelin is made by Shwann's cells (principal glia) and consists of about 70% to 80% fat and with the remaining percentage consisting of protein. This sheath coats and insulates the axon, with periodic breaks where the nodes of Ranvier are located. This allows for the transmission of the signal to be sped up
This is another term that is useful to be familiar with that was introduced in the myelin sheath section. Neurons are a specialized type of cell of the nervous system- but not the sole cell type. Glia are another type of cell of the nervous system. These are non-neuronal cells that maintain homeostasis, produce myelin as mentioned before and importantly provides support and protection for the neurons in the brain and nervous system.
Khan Academy- Neurons
Types of Neurons
There are several types of neurons that are classified by the location of where they transmit information.
- Sensory (Afferent) Neurons: These are the neurons which respond to stimuli. This stimuli includes that which comes in through our sense such as (but not limited to) hearing, seeing, tasting or smelling. This external stimuli is converted from an organism's environment, and reacts by producing an internal stimuli.
- Motor (Efferent) Neurons: These are the neurons located in the central nervous system or CNS. The axons of motoneurons are projected outside of the CNS to control muscles- directly or indirectly. The signals are received from the brain and spinal cord causing muscle contractions.
- Interneurons: These neurons connect neurons together (with other neurons) within the same region in neural networks- so location is important. Interneurons aren't motor or sensory types of neurons.
Neuron Signal Transmission Speed
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Neurons can be classified based on the direction of their output signal, as discussed in the previous section. Neurons can also be classified by the number of extensions that come out of the cell body.
- Bipolar Neurons: These neurons have two processes (extensions) from the cell body. For instance the retinal cells.
- Pseudounipolar Neurons: These neurons have two axons as opposed to an axon and a dendrite. The axons extend toward the spinal cord and the skin/muscles.
- Multipolar Neurons: These neurons have many processes extending from the soma. However, there is only one axon involved.
Firing Neurons with BioTechniques
Ted Talk- Michael Merzenich: Growning evidence of brain plasticity
The word "neuroplasticity" tells us a lot about the meaning of the word. Neuroplasticity deals with how plastic or malleable the brain is. Yes- we are capable of changing our brains! This is a newly accepted idea that used to be widely rejected. Until around the 1970s, the idea that the nervous system was fixed through adulthood was accepted across the neuroscience community.
Although proposed in the late 1700's and again in the late 1800's, it wasn't until Karl Lashley in 1973 demonstrated changes in neuronal pathways that this idea became more accepted in the neuroscience community. This term refers to the changes in neural pathways and synapses. These changes can come from changes in our environments, behavior, neural processes or from injuries to our body. Neuroplasticity occurs on different levels from some cellular changes to large changes that you can obtain from injury. The power of rewiring our brains is in our hands.
- Neurons are specialized cells of the nervous system
- Glia cells are also cells of the nervous system
- Neurons can be classified by direction of the output and and the number of extensions
- Neuroplasticity is a term that describes the brain as malleable or changeable.
For Further Information
Here are some free, useful, content-rich resources that you have access to on neuroscience and neurobiology:
- Fundamental Neuroscience- pdf file by Squire
- Coursera,org -Neuroscience course (this is not the only course provided from the Coursera site)
- Cognitive Science: An Intro - pdf by Friedenber on cognitive science
- Neural Networks for Machine Learning- another awesome Coursera.org course that isn't so generally and is more specific, looking at neuroscience at another level.
- Neuroscience: An Intro for Students- pdf provided by the British Neuroscience Association with very colorful layouts and graphics specially created for the young student or the neuroscience beginner.
There is a plethora of resources available to you online. If you need help finding something specific, please ask and leave some comments below!