The nervous system is one of the most important systems in your body, it allows you to complete any action or movement, and the system can be found in almost any animal.
Our nervous system can be broken down into two main parts; the central nervous system with our brain and spinal cord, and our peripheral nervous system with everything else. The system can be broken down further to the sympathetic and parasympathetic systems as well which we will go through in the upcoming days. The nervous system is made up of nerve cells, the most known ones are neurons, however, the lesser known glial cells play an important and necessary role in supporting the system.
There are two types of neurons; sensory and motor. As the names suggest sensory neurons pick up stimuli and send them to your spinal cord and/or brain and motor neurons control the activity of muscles and send signals away from your brain and spinal cord. Sensory neurons are coupled with receptors to detect and respond to internal and external stimuli; the five main types of receptors are; photoreceptors (light), mechanoreceptors (pressure), thermoreceptors (temperature), chemoreceptors (chemicals/taste) and nociceptors (pain). However, there is another type of neuron called the interneuron (sometimes called relay neurons), this type connects a sensory and motor neuron in the spinal cord and is also the most numerous in the brain.
As mentioned before, glial cells are critical in supporting the nervous system and they make up the majority of cells, but they also make an important contribution to the development of the nervous system and to its function in the adult brain. There are different types of glial cells as well; from oligodendrocytes which wrap produce the myelin sheath to astrocytes which allow for the exchange of nutrients between neurons and anchor them to their blood supply. Along with these two, we have the microglial cells which are immune defences and ependymal cells which line cavities in your brain and spinal cord. All these types of glial cells can be found in your central nervous system, in your peripheral nervous system you can find satellite cells and Schwann cells which are very similar to astrocytes.
Neurons have a structure unlike any other cell in the body. Dendrites are fine branch like objects that extend from the cell body and receive chemical messages from receptor of other neurons. The cell body contains the nucleus of the cell and supplies energy and nutrients to the cell. The axon is a long structure in which nerve signals pass along. There is an axon terminal (sometimes called axon branches) at the end of the axon. The axon is electrically insulated by a sheath or covering made of myelin. The key concept of a neuron is that a signal is received by the dendrite, integrated by the cell body and then transmitted through the axon.
We can categories neurons by the number of processes that extend from the cell body. Most of the neurons in your body are multipolar (along with the one in the diagram below with the axon and many dendrites extending from the cell body), then there are the bipolar and unipolar neurons.
Nerve impulses need to be passed on from one neuron to another, across the synapses (gap or junction between neurons). When an electrical signal reaches the axon terminal, it causes neurotransmitters (chemical compounds) to be released across the synapse. Then the neurotransmitter locks onto receptors in the membrane of the neuron on the other side of the synapse and causes this neuron to fire. This process is called an action potential and will go on and on from one neuron to another until it reaches and effector or muscle. This works because the axonal membrane contains ion channels which open and close to let in or out ions (most commonly Na^+ and K^+).