Spinal Column - Anatomy & Physiology

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Divisions and Landmarks

The common pattern of canine landmarks, according to Dyce, Sack, et al, can be quantified as such:

Vertebrae and Joints

• Vertebrae consist of a body which encloses the vertebral foramen (through which the spinal cord and meninges run), a spinous process, and a transverse process, as well as articular processes by which they join together
• The form of the spinous process varies with respect to species and region

Cervical Vertebrae

• The first two cervical vertebrae are known as the atlas and the axis respectively, and are modified to allow movement of the head
  • The atlas has no conventional body: instead it is complsed of two lateral masses joined by dorsal and ventral arches
  • The atlas and axis are fused in embryonic life
  • The wing of the atlas is the transverse process of this vertebra and allows the spinal column to articulate with the skull by providing a resting place for the occipital condyles
  • The axis is the longest vertebra
  • The nuchal ligament connects the spinous process of the axis to the spinous process of the first thoracic vertebra (T1)
• The last (C7) cervical vertebra has a taller spinous process than those proceeding it, and articulates with the first pair of ribs

Thoracic Vertebrae

• Thoracic vertebrae articulate with the ribs
• They are distingushed by short bodies with flattened extremities, costal facets, short transverse processes, and prominent spinous processes
• They reach a maximum height a few vertebrae behind the cervicothoracic junction (constituting the withers of the horse) and then decline
• The orientation of spinous processes shifts from caudo- to craniodorsal

Lumbar Vertebrae

• Longer and more uniform in shape than thoracic vertebrae
• Shorter in height, with long, flattened transverse processes that project laterally

Sacral Vertebrae

• Sacrum: a single bone formed by the fusion of several vertebrae that articulates with the pelvic girdle
• Allows the thrust of the hindlimbs to be transmitted to the trunk
• Narrows caudally and is curved to present a concave surface to the pelvic cavity

Caudal Vertebrae

• Number varies greatly even within species
• Progressive simplification of form

Joints of the Spinal Column

• Two types of joints:
  • Cartilaginous: direct connections between vertebral bodies
    • Bodies of adjacent vertebrae connected by thick, flexible intervertebral discs, consisting of two parts:
      • Nucleus pulposus: slightly eccentric, notochord derivative, contained under pressure and prone to escape
      • Annulus fibrosis: encircling bundles of fibrous tissue that pass obliquely from one vertebra to another, with changing orientation
  • Synovial: between facets on vertebral arches
• Modified in the regions of the head and pelvis
  • Joints of the atlas:
    • Atlanto-occipital joint: between condyles of the skull and corresponding cavities of the atlas
      • Functions as a ginglymus: movement is restricted to flexion/extension in the sagittal plane (eg nodding)
    • Atlantoaxial joint: ventral arch of atlas and the body of the axis face into a single synovial cavity with limited areas of contact
      • Movement is rotational about a longitudinal axis (eg. head shaking)

Hypaxial and Epaxial Muscles

• Epaxial muscles: extensors of the vertebral column
  • Dorsal to the line of the transverse processes of the vertebrae
  • Innervation comes from the dorsal branches of the spinal nerves
  • Rarely of clinical importance
  • Arranged in three parallel columns
    • Lateral column: Iliocostalis arises from the ilium and transverse processes of the lumbar vertebrae to insert on cranial lumbar vertebrae and ribs, spanning about 4 vertebrae
    • Middle column: Longissimus; strongest, extending from the ilium and sacrum to the head and neck
    • Medial column: Transversospinalis; most complex, lying between medial vertebral arches and spinous processes
• Hypaxial Muscles: flexors of the neck and tail
  • Longus colli: from cranial thoracic region to the atlas, covering ventral vertebral bodies
  • Rectus capitis ventralis: atlas to ventral skull
  • Longus capitis: midcervical vertebrae to skull
  • Scalenus muscles: caudal cervica vertebrae to first few ribs, which they stabilize on inspiration

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