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The nervous system is complicated because it is in charge of many things. There are three sections and two operating systems. First, there is the brain stem which houses the ancient portion that connects the brain to the body through the cranial and spinal nerves. Here the basics are covered, such as breathing and digestion movements. Next, over the ancient brain stem is the cortex, the more developed portion of the brain and does some of the thinking. Here is where we feel hunger, danger and the need to reproduce. Finally, over this is a very thin layer called the neocortex, which houses all of the memories of our life. All three sections connect with an uncountable number of nerve fibers interacting using neurotransmitters. The two operating systems are the parasympathetic and the sympathetic, which control the non-thinking parts (heartbeat, digestive contractions, etc.) or the voluntary functions (I’m hungry and I’m going eat).

Through shared connections, all sections of the brain use patterns to cause actions. For example, hunger may come from either the action of a hormone secreted by various factors such as an empty stomach, a smell (fresh muffins from the oven), or a visual (see a commercial on TV or see the barn worker arrive = feeding time). Likewise, seeing some foods may cause us not to eat them because there was a violent stomach reaction (memory of a bad event) the last time they consumed that food.

There are groups of cells identified in the brain of all animals that are called islands and are named. Of importance to horses is the amygdala, which stores emotions, including the fight or flight survival reaction. Directly connected to this is the motor cortex, where the movement starts. This connection is responsible for a horse’s explosive response to a stimulus such as a surprise movement outside the window. Only in a long moment does the horse understand the movement and no longer move as his safety is assured, but you might become injured from his reaction. Other times they connect a person (trainer, vet) with an unpleasant emotion in the amygdala and running or rearing occurs. What needs to be changed then is the stimulus and the reaction to that stimulus (training).

The number of problems in the nervous system of horses is limited to central (brain) and peripheral (nerves). Diseases of the central nervous system include rabies and other viral infections, meningitis from a bacterial infection and dummy foal syndrome from lack of oxygen at birth. Other diseases in the brain are cataplexy/narcolepsy, where the horse suddenly collapses asleep (Equine Sleep Disorder), and Cushing’s disease.

Peripheral nerve diseases (neuropathies) include direct trauma to any nerve (Sweeney and other muscle atrophies due to nerve loss), secondary trauma (wobblers, roaring) and inflammatory diseases (head shakers, shivers).

Discoveries in the human brain of diseases could also occur in horses. For example, there is a unique lymph system of the brain called the Glymph system. This fluid bathes the brain after it shrinks up to 40% during deep sleep (delta wave seen in humans), cleansing the brain of waste. Cognitive dysfunction in humans is associated with inflammation due to diet, and dementia in humans is now called diabetes type 3. Something similar in horses is unknown, but could this be happening to horses with metabolic syndrome and insulin resistance (diabetes)? Could a similar reaction of the horse brain cause their sleep disorder or head-shaking? No one is testing for this in horses.

New nerve diseases in horses since I went to vet school include ALS (Lou Gehrig’s Disease) and head-shaker syndrome while other neurologic disorders are appearing with more frequency. Other nerve diseases that have increased are cataplexy and shivers. I’ll look at all of these in the included topics.

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