The Equine Muscular System
The muscles of the body are responsible for creating movement whether it be via the skeletal muscle, smooth muscle, or cardiac muscle. Agonist muscles contract and are the primary mover. While antagonist muscles work by counteracting the agonist muscle therefore creating a balanced and stable movement. When one contracts the other relaxes or elongates enabling movement.
The muscular systems are responsible for:
Expelling waste e.g. urine faeces
Protecting internal structures
The equine mammalian body consists of many cells working together physically and chemically to organize complex systems. Atoms made up of protons, neurons and electrons create elements which bond and create compounds such as carbohydrates, proteins, lipids, and acids.
Carbohydrates in the form of glycogen and glycose provide an energy source for the systems of the body including the muscles.
Protein has many duties and there are more protein compounds found in the body than any other compound, for example they form the structure of muscles, chemical communication as enzymes between structures, they also carry oxygen around the body as haemoglobin and are active with in the immune system as antibodies.
Lipids are involved with the storage of energy, body insulation and as a chemical regulator.
At a cellular level there are four types of specialized cell groups: epithelial, connective, neural and muscular tissue.
Epithelial cells cover the external surface of the horses as skin, protecting the muscles and internal structures from injury, damage, and infection.
Connective tissue is categorized as
Connective tissue proper, e.g. a ligament, tendon or fascia structurally consisting of collagen and reticular fibres.
Fluid connective tissue e.g. blood and lymph fluid.
Supporting connective tissue e.g. bones.
Nerves via neural pathways communicate between the structures enabling communications and instructions to pass throughout the entire body.
Muscular tissue creates movement via the skeleton, the internal function of organs and the beating of the heart. Muscle composition varies according to the action being performed by the muscle.
Muscles tissue has three different types, smooth, cardiac, and skeletal.
Smooth muscle is responsible for internal involuntary movement of the digestive, reproductive, urinary, and respiratory systems. Movements such as swallowing food or bowel movements are examples of smooth muscle activity. The walls of blood vessels are also included.
Cardiac muscle is exclusively located as the heart muscle and is responsible for pumping oxygenated and de-oxygenated blood around the body.
Skeletal muscle is responsible for voluntary bodily movements, such as walking, lowering, and raising the head, trotting, cantering, and galloping to a simply ear twitch.
The structure of skeletal muscle consists of bundles of muscle cells striated in form with blood vessels and nerves running through them. They are held together with connective tissue. The muscle is separated and held from other structures by the epimysium. Where muscle attaches to the periosteum of the bone the epimysium forms a tendon, sheets of fascia or connective tissue.
The contraction and release of muscle occurs due to a neural and chemical reaction within each muscle fibre.
Every muscle fibre contains bundles of myofibrils which can be further divided into groups of myofilaments. Each myofilament contains lots of sarcomeres all lined up along the length of the myofilament. The sarcomere contains two protein enzymes called actin and myosin. Actin when studied under a microscope has two spherical strands twisted together, each of these spheres has a binding site which is attracted to myosin. However, the sites are guarded by two proteins called Troponin and Tropomyosin. The Myosin which is thicker than the actin contains lots of arm like structures with a rounded head which under the right chemical circumstances come into contact with the actin’s binding sites. Nerve impulses through the sarcoplasmic reticulum and T Tubules stimulate the release of calcium which binds to the troponin and causes the tropomyosin to move away and expose the binding sites along the Actin. This allows the Myosin head to bind with the actin cause a ratchet like movement along the sarcomere and therefore contracting or shortening the muscle. This action is taking place many times along the length of each sarcomere within the muscle. The ADP molecule attached to the Myosin head is then released due to cellular energy ATP and the process is repeated causing the contracting and relaxing of the muscle. This process is called the sliding filament theory.
The body creates energy for muscle function both aerobically and anaerobically.
Aerobic metabolism means the muscle utilises oxygen, glycogen, and fatty acid enzymes to produce ATP (Adenosine Triphosphate) as part of muscle contraction. The by-products of aerobic metabolism are carbon dioxide and water, which can easily be carried from the muscle by the respiratory and circulatory systems making aerobic metabolism very efficient. However, this process is quite slow compared to anaerobic metabolism and therefore creates more strength and stamina within a muscle rather than fast bursts of energy. Endurance horses are typical of using aerobic metabolism over long periods and will therefore utilise slower twitch muscle fibres more efficiently. Slow twitch muscle fibres have a particularly good blood supply with mitochondria efficient at using oxygen to convert glycogen and fatty acids into energy.
Anaerobic metabolism is the muscle utilising glycogen without oxygen present to form ATP, a production called glycolysis. Most of the glycogen in the horse’s body is stored in the muscles with around 10% stored in the liver. This allows the muscles to call upon this supply very quickly without the need for the heart and lungs to deliver oxygen. This enables rapid acceleration, speed and muscle function, but ATP production is limited and causes the build-up of lactic acid as a by-product of glycolysis and fatigues the muscles.
Quarter horses, polo and racehorses utilise fast twitch muscles fibres allowing them to perform more effectively.
Skeletal muscles are categorized in the horse as type 1 (slow twitch) or type 2 (fast twitch) muscle groups, with fast twitch muscle being further categorised as FTa, FTb and FTc.
These muscle types are related to metabolism and the production of ATP and the suitability of the horse to his purpose.
Fast twitch muscle fibres categorised as FT a can work both aerobically and anaerobically but FT b working mostly anaerobically. FTc are categorised as having a mixture of the two methods at varying amounts.
Different types and breeds of the horse will utilise differing amounts of muscle fibre types with the thoroughbred and/or hot-blooded horses have a higher percentage of fast twitch fibres compared to a native bred or cold-blooded breed. Certain breeds such as the Arabian has enough slow twitch fibres versus fast twitch to enable it as a very good endurance breed.