The horse’s nervous system is a complex array of fibers reaching out into the body from the brain to the spine, and all throughout the body of the horse. The system controls the entire body either automatically or voluntarily through conscious decisions. The nervous system also senses the environment around the body and makes the body react to these senses again either via the autonomic nervous system or voluntarily through a conscious decision.
Breaking the nervous system down to its basic elements, there is the brain, the spinal cord, and then peripheral nerves. The nerve cells within each of these structures are called neurons. The brain is similar to a computer system. This is where all the information comes in, is analyzed and is either automatically or voluntarily acted upon. The brain can be further divided into three parts. The forebrain, midbrain, and hindbrain. The forebrain is where consciousness is thought to reside, with memories and voluntary movements forming from this area. Next is the midbrain. This is where sleep/consciousness, vision, hearing and temperature regulation occurs and is the connection between the forebrain and hindbrain. The hindbrain or also known at the brainstem is the area where essential functions are carried out sub-consciously via reflexes, like breathing, heart activity including rate and contractility, blood pressure regulation, and digestive regulation. The hindbrain also has the cerebellum as a part of this area. The cerebellum is responsible for balance and controlling the movement of the horse taking information from proprioceptive nerves throughout the body and the vestibular system in the ear. The cerebellum collects and analyzes all the incoming information and corrects the movement of feet, head, neck and the whole body to coordinate as the horse walks, trots, lopes or has any other movement according to the world around it. The hindbrain connects the other parts of the brain to the spinal cord. Cranial nerves arise from the brain and brain stem and innervate different parts of the head, neck and body. There are 12 pairs of cranial nerves and they are numbered with Roman numerals from the front of the head back. These nerves contain both sensory and motor divisions and are then divided further into more specific categories. Some of the name of these nerves may sound familiar, like the olfactory nerve that senses smell, and the optic nerve which senses sight, while others you may have never heard about.
The spinal cord is the major connection between the brain and the peripheral nervous system. It is divided into five regions cervical, cervicothoracic, thoracolumbar, lumbosacral, and sacral. A cross section of the cord shows an outer layer of white matter and an inner layer of grey matter in the shape of an “H” on cross section. The major differences between white matter and grey matter is white matter neurons have long axons (axons are long extensions of neuron cells covered in myelin), are less in number and myelinated. In contrast grey matter neurons are smaller without axons, more numerous and almost all are unmyelinated. The myelin sheaths are insulators that increase the speed of electrical nerve signals. Within the white matter are tracts of nerve cells for sending sensory information (pain, touch, temperature and conscious proprioception) up to the brain and motor information back down from the brain to control voluntary movement of the muscles. The grey matter initially receives the sensory information from the body and then transmits it into the white matter neurons to be sent on up to the brain. The grey matter also transmits the information coming from the brain by the white matter neurons for the voluntary muscle movement. The grey matter is also where spinal or tendon reflex arcs are contained. This type of reflex is the same as when you tap your knee and your leg kicks out. The tap on the tendon below the knee cap sends a signal up the nerves in your leg to your spinal cord and back down to your quadriceps muscles to contract and kick your leg out. This type of reflex determines that the peripheral nerve neurons in the lower portion of the leg and the connections to the spinal cord at that level are not damaged. The spinal cord also has two major plexus or webbing of nerves. The brachial plexus and the sacral plexus. (The brachial and sacral plexus are the bundles of nerves that come out from different areas of the spinal cord to exit the thorax and pelvis of the horse innervate the front limb and the hind limb respectively). Nerves of the spinal cord exit the cord by two nerve roots from the top and the bottom of the cord and come together again outside the cord to exit out of the spinal vertebrae or the bone that protects the spinal cord.
The peripheral nerves are the continuation of the spinal nerves as they exit the spinal cord and innervate the rest of the body with either sensory nerves, or motor nerves. Sensory nerves consist of neurons that sense the things in the horses’ environment and sends feedback up to the spinal cord and then on up to the brain. Motor nerves consist of motor neurons that activate voluntary muscle movement that move limbs and other skeletal muscles, or autonomic muscles that control breathing, the heart, or digestion.
When any dysfunction occurs within the nervous system then, a more detailed examination of the horse is made focusing on the nervous system. This type of examination attempts to differentiate the location of the lesion(s) and the neurological deficits that are being caused by the lesion(s). Then more focused tests, such as blood analysis, spinal fluid analysis, radiographs, as well as many other tests can be ordered to further define the nature and cause of the neurological problem. After the results of the tests and analysis are completed, then a treatment protocol can be developed and begun.
