Think for a minute about a cat ready to pounce on a toy. Or a dog getting ready to chase a ball, then bounding after it. In each case, thousands of nerves – starting with sensory nerves in the eye, to the brain, to the nerves going to muscles, are activated to coordinate sensation, thought and action.
Then think about digesting food or taking a breath – activities that require no conscious thought but happen automatically.
The nervous system is the hub of almost all activities, conscious and unconscious, that occur in our pets (and our) bodies. However, for many of us, it’s a bit of a mystery when it comes to how this important body system works.
In this article, we’ll take a look at some of the basics of how the nervous system functions. In the next part of this series, we’ll use this baseline knowledge to discuss one of the most common, but frightening, diseases affecting the nervous system – seizures.
In the third part of our series, we’ll cover spinal cord diseases, another common problem affecting our pets.
The Nervous System Has Two Main Parts
To begin, it’s easiest to think about the anatomy of the nervous system by dividing it into two main parts:
- Central nervous system – consists of the brain and spinal cord
- Peripheral nervous system – all the nerves branching out from these main structures
Although a bit simplistic, we can divide the nervous system further:
- Somatic nervous system – voluntary functions
- Autonomic nervous system – involuntary functions
The autonomic nervous system is further divided into two parts:
- Sympathetic
- Parasympathetic
The sympathetic system is responsible for the “fight or flight” response and the parasympathetic system as “rest and digest.” The actual responses are quite complex but these descriptions still work well for thinking about these division of the autonomic nervous system.
Let’s move on to discuss the neuron, the nerve cell that makes up all parts of the nervous system.
The Neuron Is the Specialized Cell Type of the Nervous System
There are millions of neurons in the body. These amazing cells can be long (the sciatic nerve is the longest neuron in human bodies) or incredibly small. But regardless of size, all neurons work the same. Their purpose is simply to receive, process and transmit electrical signals to and from various organs.
The body of a neuron looks a bit like a star with several points. Just like all cells, a neuron also has a nucleus.
Coming from the body is a tail-like structure called an axon. Dendrites are other structures that extend from the body of the cells. Dendrites tend to receive signals, and axons relay signals.
Lastly, at the end of the axon is a space between the end of the axon and the dendrite of the next neuron called the synapse.
Neurons are further specialized by function. Some neurons are called motor neurons and their function is to send a signal to generate movement. Other neurons are sensory neurons and, as their name implies, they’re responsible for detecting light, heat, sound, smell, taste and pressure, and sending those inputs to the brain for processing.
And, as mentioned earlier, neurons of the autonomic nervous system are busy helping regulate involuntary processes like heartbeat and digestion.
Now that we know more about how the neurons are structured, we can move on to the “business” part of the nervous system – how signals are transmitted from one part of the body to another.
How Information is Transmitted Through the Nervous System
Signals in the nervous system are transmitted from one cell to another by a combination of electrical and chemical signals.
An electrical signal begins in the neuron, which sends the electrical signal down its axon. At the tip of the axon, this signal is converted from an electrical signal to a chemical signal. The chemical is then released into the synapse, where it crosses the space and is picked up by the dendrite of another neuron, which converts the chemical signal back to an electrical signal and the process begins again.
Sounds complicated, right?
As pet parents, we don’t need to know all the intricacies of this process but there are a couple of points that are important:
- Electrical signals are generated through the movement of sodium and potassium in and out of cells.
This means that imbalances in blood sodium or potassium levels can have profound effects on nervous system function.
- Chemical transmitters released at the synapse include:
- GABA
- Glutamate
- Glycine
- Endorphins
- Serotonin
- Histamine
- Dopamine
- Epinephrine (adrenaline)
- Norepinephrine
- And more
Chemical neurotransmitters are the target of lots of medications, including antidepressants, sedatives, and illicit drugs such as cocaine and amphetamines.
The take-home message for pet parents when it comes to nerve transmission is that many toxins exert their effects by interfering with this process. And other commonly prescribed medications, such as gabapentin and the anticonvulsant levetiracetam (Keppra) all work at the level of the synapse.
Glia – Important Supporter Cells
Lastly, non-neuron cells known as glia perform a variety of functions in the nervous system. These include:
- Providing physical support to neurons
- Creating myelin, which is an insulating material
- Repair damaged neurons and eliminate dead neurons
Science to Save Animals
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Stay tuned for the next two blogs in this series on seizures and spinal cord diseases!
