Edward Busuttil DVM, CertAVP, PgCertVPS, MRCVS, gives us an anatomical insight into how stability is maintained in this vital system
Any owner who has held up their horse’s head for an extended period of time when it has been sedated, will know what an energy consuming task this can be. Owners also often recognise that emotional tension may also cause the horse to have abnormal spasm of their neck muscles¹. The horse’s head and neck weigh close to 10% of their total bodyweight². Horses (and many other herbivores) have developed an energy efficient system, which helps them to passively maintain an elevated head position, both when standing still and in motion.
Anatomy of the neck
A basic understanding of the anatomy of the neck is essential. The neck of most animals consists of 7 vertebrae and the horse anatomy is no different. The cervical (neck) vertebral column connects the base of the skull at the occiput, to the first thoracic vertebra, and thus the beginning of the ribcage. The cervical vertebrae are very different to one another, apart from the 3rd to the 7th which may be considered typical.
The first cervical vertebra is called the atlas and together with the occiput, creates an up and down motion of the head, with slight lateral rotation. The second cervical vertebra is called the axis, and together with the atlas, results in rotation of the head.
The 6th and 7th cervical vertebrae are considered to be the lower cervical vertebrae and unlike the rest of vertebrae in this section, have rudimentary dorsal spinous processes as there is a transition from the cervical to thoracic spine. The 6th and 7th cervical vertebrae may also have genetic malformations on their underside where muscles attach³, and this will be discussed later.
Lateral bending of the neck is best achieved at the cervicothoracic (at the 7th cervical and 1st thoracic) junction, whereas axial rotation of the neck (rotation of the neck relative to the rest of the body) is best achieved between the 6th and 7th and cervicothoracic joints.
Figure 1. Horse anatomy – a schematic representation of the 7 cervical vertebrae extending from the base of the skull to the first thoracic vertebrae, where the rib cage begins. Note the difference in shape of the atlas and axis compared with the rest of the vertebrae and rudimentary spinous processes present on the sixth and seventh vertebrae. Also note that the lamellar part of the nuchal ligament does not connect to the first and seventh vertebrae, allowing for more rotation in those regions.
Appropriate movement and positioning of the neck occurs due the elasticity of the nuchal ligament (which is the main part of the energy efficient passive stabilization of the position of the head and neck) and specific muscular control. As the horse’s body is a dynamic and intrinsically connected system, abnormalities within other parts of the musculoskeletal system may also affect the position of the neck, as is evident with forelimb lameness when a characteristic head nod is present. The nuchal ligament consists of two portions, the funicular and lamellar regions. The funicular part of the nuchal ligament is cord-like and extends from the occiput to the top of the dorsal spinous processes of the withers, where it becomes the supraspinous ligament that runs over every dorsal spinous process until the last lumbar vertebra.
The lamellar part is sheet-like and connects the funicular part to the cervical vertebral column only between the 2nd and 6th cervical vertebrae. The nuchal ligament is mainly composed of an elastic-like material. By storing and releasing the elastic strain energy, the nuchal ligament can also contribute (approximately 33%) to the dynamic motion of the head and neck during locomotion⁴. It is therefore clear that the nuchal ligament provides both stability and elasticity to the dorsal part of the cervical column.
Musculature
A number of muscles are also obviously essential in the movement of the head and neck. Muscles above the line of the cervical column are called epaxial muscles, and those below it are called hypaxial muscles. The main function of the epaxial muscles is to lift the neck and head, whereas the hypaxial muscles lower it. Although all the muscles play a key role in the movement of the head and neck, two very important muscles are the multifidus and the Longus colli muscle.
The collection of multifidus muscles are epaxial muscles and connect the dorsal aspect of the cervical vertebrae (each muscle crosses up to 3 vertebrae). The longus colli is a paired muscle (left and right) which lies on the underside of the cervical column, extending from the 1st cervical to the 5th or 6th thoracic vertebra, with segments attaching to each vertebra the muscle transverses. Both of these muscles function to stabilize, fix, flex and rotate the cervical vertebral column. The longus colli is considered to be particularly important in stabilising the cervicothoracic junction⁵. This is important, because this region does not have dorsal stabilisation by the lamellar portion of the nuchal ligament. The muscle is also considered to be cybernetic, meaning that it is extremely densely occupied by nerve endings, which give the brain a lot of information about the posture and locomotion of the neck, resulting in very specific adaptive control and motion.
As discussed earlier, genetic malformation of the 6th cervical vertebrae exists in about 38% of Thoroughbreds⁶, and in these cases, there may be a one, or two-sided absence of the caudal ventral tubercle, which is where the muscle should attach to stabilise the base of the neck. This results in asymmetric use of the muscle, with one side being hypertrophied (larger) than the other³.
“Understanding that hypertrophy can lead to overuse injuries, and overuse injuries can lead to arthritic changes⁷, proper conditioning of the neck is an essential component in maintaining the athletic capability of your horse and preventing arthritic changes.”
Figure 2. Horse Anatomy –some of the deep muscles of the neck which are involved in stabilising and rotating the neck.
- Blue – Dorsal capitis muscle
- Green – Oblique capitis muscle (cranial and caudal portions)
- Red – Multifidus cervicis Purple – Longus colli
- Yellow – Spinalis cervicis
At the cranial-most part of the nuchal ligament, two other structures are also commonly found; the cranial and caudal nuchal bursae. The simplified role of each of these bursae is to help the glide of the funicular part of the nuchal ligament over the atlas (cranial nuchal bursa) and axis (caudal). They are not always present and may develop after birth.
Hyperflexion of the head causes an excessive build up of pressure at both of the sites where these bursae may be present⁸, and the horse may adapt a posture with an extended head to decrease the pressure at this location. As inflammation develops, the horse may develop pain at their poll, and resentment to pressure, as well as lesions within the insertion of the nuchal ligament onto the skull ⁹. Horses ridden in a rollkur posture (particularly when competing in dressage) are more predisposed to this, as are horses ridden by less experienced riders. These injuries can also be caused by trauma.
There is a long list of pathological changes, which can occur within the cervical vertebrae of the horse, some of which may be out of our control, such as genetics, the development of nuchal bursae, trauma, and the horse’s use before it was acquired. Correct strengthening and conditioning exercises, together with preventative care, can help to reduce the development of clinical signs.
Neck strengthening exercises and stretches
Before initiating strengthening techniques, it is important to ensure that the exercises are not going to be detrimental to other underlying conditions. Consulting with your vet, chiropractor, physiotherapist or riding instructor can be extremely beneficial, especially initially when putting together a treatment plan and learning how to properly carry out the exercises.
During stretching exercises, the muscles are momentarily lengthened – the origin and insertion of the muscle do not change, therefore the length returns to normal when the muscle is returned to a neutral position. The lengthening, which happens during stretching of the neck, especially during warming up, can help to enhance the horse’s balance when moving at speed, the tone and spring of muscles and proprioception (spatial awareness).
Stretching of capsules and ligaments, including the nuchal bursae, can help to increase tolerance to pressure and suppleness of the unit being stretched ⁵.
References
- Denoix, J. and Pailloux, J., 2011b. Treatment by Anatomical area. In: J. Denoix and J. Pailloux, ed., Physical Therapy and Massage of the Horse, 2nd ed. Boca Raton (FL): CRC Press, pp.175-180.
- Buchner, H., Savelberg, H., Schamhardt, H. and Barneveld, A., 1997. Inertial properties of Dutch Warmblood horses. Journal of Biomechanics, 30(6), pp.653-658.
- May-Davis, S. and Walker, C., 2015. Variations and Implications of the Gross Morphology in the Longus colli Muscle in Thoroughbred and Thoroughbred Derivative Horses Presenting With a Congenital Malformation of the Sixth and Seventh Cervical Vertebrae. Journal of Equine Veterinary Science, 35, pp.560-568.
- Gellman, K. and Bertram, A., 2002. The equine nuchal ligament 2: passive dynamic energy exchange in locomotion. Veterinary and Comparative Orthopaedics and Traumatology, 15(01), pp.07-14.
- Denoix, J. and Pailloux, J., 2011a. Mobilization and Stretching. In: J. Denoix and J. Pailloux, ed., Physical Therapy and Massage of the Horse, 2nd ed. Boca Raton (FL): CRC Press, pp.101-130.
- May-Davis, S., 2014. The occurrence of a congenital malformation in the sixth and seventh cervical vertebrae predominantly observed in Thoroughbred horses. Journal of Equine Veterinary Science, 34, pp.1313-1317.
- Shaw, H. and Benjamin, M., 2007. Structure-function relationships of entheses in relation to mechanical load and exercise. Scandinavian Journal of Medicine and Science in Sports, 17, pp.303-315.
- Dippel, M., Zsoldos, R. and Licka, T., 2019. An equine cadaver study investigating the relationship between cervical flexion, nuchal ligament elongation and pressure at the first and second cervical vertebra. The Veterinary Journal, 252.
- Hinchcliff KW, Kaneps A, Geor R. Equine sport medicine & surgery. 2nd ed. Philadelphia, Pennsylvania: Sounders Elsevier; 2013. p.451-2.
