|The Equine Nervous System|
By Stephen Champion, BSc (Hons) BVM&S MRCVS,
Throughout evolution, the ancestors of the modern horse have had to react appropriately to changes in their environment in order to survive. The regulation of these reactions is the responsibility of its nervous system. This is the most complicated body system of the horse that we shall discuss in this series. It is fundamental however to the functioning of the horse as a higher mammal, an athlete and a valuable companion to the human race. This article shall cover briefly the anatomy and function of the nervous system.The horse needs to be able to perceive changes in its environment. These may be weather conditions, the approach of its owner or an oncoming fence! These changes are recognised by a receptor organ within the structure affected by the change, such as the nose, the skin or a joint. The response that is provoked in answer to the stimulus is performed by an effector organ. These two types of organ are connected by a chain of specialised cells called neurons, which conduct high-speed electrical impulses to convey their messages around the body. The neurons are the building-blocks of the nervous system and are connected together at junctions called synapses. Certain anaesthetics used in veterinary medicine work to prevent transmission across these synapses, and hence prevent pain impulses reaching the brain. The structures described above make up the stimulus-response apparatus or reflexes. This is the innate ability of the animal to produce a response instantly to a particular stimulus, such as well aimed kick in response to a unfortunately-timed slap on the rump! It’s a wee bit more complicated than that, but you get the basic idea. The example mentioned involves the brain, to adjust the balance and recruit other muscles to take the strain. It also requires inhibitory reflexes, to prevent the kick from damaging the horse itself. Simpler reflexes, such as the patellar or knee-jerk reflex (synonymous with the doctor’s surgery) do not involve the brain, but remain local to the area of the body affected. The nervous system forms in reality an integrated whole, but is easier to understand if we divide it into parts. The most fundamental division is into the central nervous system or CNS, which comprises the brain and spinal cord, and the peripheral nervous system, which comprises all the nerve trunks stretching out around the horse’s body. Of these nerve trunks, those leading away from receptor organs towards the brain are called sensory nerves and those taking information from the brain to affector organs are called motor nerves. When your veterinary surgeon performs “nerve blocks” on your horse’s leg to diagnose lameness, he blocks the sensory nerves so that the horse does not feel pain from the lower leg. Large motor nerves remain unblocked however, so the horse can still move his leg, and does not (hopefully) fall over. The nature of the information that is conveyed, and of the activities that are directed, allows the further distinction of somatic and visceral nervous systems. The somatic system is concerned with the “animal” functions - locomotion and so forth - which control the relationship of the horse with the outside world. The visceral or autonomic nervous system is concerned with the “vegetable” functions that relate to his internal environment, such as heart rate and digestive function. Generally speaking, the horse has more control over his somatic than his visceral functions. No horse would choose to have colic! The brain and spinal cord are continuous anatomically, without any clear demarcation. The brain is responsible for the higher functions of the nervous system, which are more diverse than you may first suppose. Ranging from thinking, problem solving, and memory to the control of movement, balance, breathing, reflexes and instinctive behaviour patterns, the brain is fundamental to almost every aspect of the horse’s existence. It is safely protected within the hard cranium of the horse’s head and impulses generated from, and arriving at, the brain, do so through the spinal cord, which is protected within the vertebral column in the horse’s back. The spinal cord is packed with neurons arranged in pathways, which connect with peripheral nerves stretching right to the extremities of the body, as any laminitic pony will tell you. The size of the brain bears no constant relationship to that of the animal from which it came, but is relatively smaller in larger species. Generally speaking, the larger the brain in relation to bodyweight, the more sophisticated functions the animal is able to perform. The ratio of brain weight to body weight for different species are of the order of 1:48 in man, 1:100 in the dog and 1:800 in the horse. Make of that what you will!