Musculoskeletal system

The skeleton provides the frame for the dog and consists of a collection of bones connected by joints. These differ depending on the degree of movement allowed between two bones; some of them are immovable, such as those connecting the bones of the skull, whereas others allow movement in three dimensions, such as the joint that connects the skull to the spinal column.
Skeletal muscle, attached to the bones by tendons, gives the skeleton its mobility; when it contracts the bones move in relation to each other, as in flexion and extension.

Muscle contractions are controlled by nerves through the central nervous system: the cerebrum and the cerebellum control voluntary movements and the spinal cord controls reflexes. The neurons involved in controlling movements are called motor neurons compared with sensory neurons which convey information to the brain.
Dogs have three or four different gaits – walk, trot, gallop, amble – which are developed to a greater or lesser extent depending on the breed. Dogs are very good jumpers and moderate swimmers, although again, there are variations between breeds.

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Skeleton and bones

© Diffomédia/Royal Canin
© Diffomédia/Royal Canin

• The axis of the skeleton is the spine, which is formed by various types of vertebrae. Thirteen ribs are attached to the spine through the breastbone (sternum) forming the ribcage. The skull is connected to the first cervical vertebra, a ring-like structure known as the atlas, which is connected to the next vertebra, the axis, in the form of a pivot, to allow the head to move around the axis formed by these two vertebrae.

© Diffomédia/Royal Canin

• The hind limbs are the dog’s propulsion system. They are attached to the pelvis at the hip joints, while the pelvis is connected to the vertebral column by a complex system of ligaments. The forelimbs, which are less involved in propulsion, are simply attached to the vertebral column by the scapula (shoulder blade) and the adjacent muscles.

© Diffomédia/Royal Canin

• Bones are made of a calcified fibrous structure. This calcification occurs gradually during the development of the foetus and in the growth period, which is very long in large breeds. As a result, calcium intake must be carefully regulated in growing puppies to avoid any deficiency or excess. Bone calcium is a reserve available throughout the dog’s life that increases or decreases depending on the blood calcium level, which should be constant. The centre of bones contains bone marrow, a spongy tissue which produces blood corpuscles.

Joints and muscles

© Diffomédia/Royal Canin
© Diffomédia/Royal Canin

• The joints differ depending on what movements they allow. Sutures (as in the skull) do not allow any movement whatsoever, whereas symphyses (cartilaginous joints) allow very slight movements between two bone structures (eg symphysis pubis). True joints have surfaces covered with hyaline cartilage and a capsule shared between two bones, allowing complete movement.

This joint capsule is a cavity filled with viscous synovial fluid, which nourishes and lubricates the cartilage. This is a very fragile tissue which does not regenerate if destroyed, which is why the protective synovial fluid is so important. The joint capsule is often surrounded by a fibrous shell and numerous ligaments which support the individual joint. If the ends of two bones are not an exact match, there may be an additional articular disc between the two, as in the knee for example.

• The musclesare composed of a body of contractile cells interconnected by membranes to form fasciae, which merge at the ends to form fibrous tendons connected to the bony attachments. As their name suggests, the contractile cells are made up of special proteins (actin, myosin) that are able to contract, which in turn contracts the muscle. This demands energy provided by the blood, then stored and metabolised in the cells in the form of the high-energy substances ATP (adenosine triphosphate) and CR (creatine phosphate). Nerves control muscle contraction. The junction between the nerve cell and the muscle cell is called the motor end plate – this is part of a complex system, which enables nerve information to be transformed into a muscle contraction. Because of this, the muscular system is very closely linked to the circulatory and nervous systems, so a change to either has rapid repercussions on the musculoskeletal system.

© Diffomédia/Royal Canin
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