Koper Equine

A myotome and a dermatome are both regions of the body that correspond to specific spinal nerve roots, but they differ in terms of the types of structures they innervate.

1. Myotome:

Definition: A myotome is a group of muscles that are innervated by the motor fibers of a single spinal nerve root. The exact boundaries of these regions can vary, but the general pattern follows the spinal nerve roots as they emerge from the vertebrae and spread to different parts of the body.

Function: Myotomes are primarily involved in motor control, meaning they govern the contraction and movement of specific muscle groups.

Clinical Relevance: Testing the strength and function of muscles within a myotome can help identify and locate the level of spinal nerve compression or spinal cord damage. For example, if a certain muscle group shows discomfort, weakness or paralysis, it can indicate an issue with the corresponding spinal nerve root.

2. Dermatome:

Definition: A dermatome is an area of skin that is supplied by the sensory fibers of a single spinal nerve root. Each dermatome innervates a specific band of skin that wraps around the horse’s body, extending from the spine toward the belly and limbs. The bands overlap slightly, ensuring that the entire surface of the body receives sensory input from specific spinal nerves, creating a comprehensive map of skin innervation.

Function: Dermatomes are involved in sensory perception, meaning they provide the brain with information about touch, temperature, pain, and other sensations from specific skin areas.

Clinical Relevance: Sensory testing within dermatomes can help diagnose nerve root compression or damage. For example, a reaction to a specific dermatome could indicate a problem with the corresponding spinal nerve root.

The spinal nerve roots of a myotome and the corresponding dermatome typically come from the same section of the spine. In other words, the muscles (myotome) and the skin area (dermatome) that are innervated by a specific spinal nerve root are both supplied by nerve fibers originating from the same spinal segment.

Spinal Nerve Roots: Each spinal nerve root has two primary components:

– Ventral (anterior) root: Contains motor (efferent) fibers that innervate specific muscles (myotomes).

– Dorsal (posterior) root: Contains sensory (afferent) fibers that innervate specific areas of skin (dermatomes).

Correspondence: The motor fibers that form a myotome and the sensory fibers that form a dermatome are bundled together in the same spinal nerve before they split into their respective roots:

– For example, the spinal nerve root at the C5 segment innervates the C5 dermatome (a specific area of skin) and also innervates the C5 myotome (a specific group of muscles).

Clinical Relevance:

Diagnosis: When diagnosing nerve root problems, Veterinarians often look for patterns of both motor and sensory deficits that correspond to a particular spinal nerve root. For example:

– If a horse has muscle weakness or reactivity in a specific myotome and sensory loss or hyperactivity in the corresponding dermatome, it strongly suggests an issue with the same spinal nerve root.

– This correlation helps your Veterinarian or therapist accurately identify the affected spinal segment, enabling targeted treatment options such as massage therapy, chiropractic adjustments, or other interventions to address the underlying issue.

Cervical Region – C1 to C7

The cervical spinal nerves (C1–C7) primarily innervate muscles of the neck and forelimbs.

• C1 (Atlas):
  • Innervates muscles such as the rectus capitis and obliquus capitis (which move the head and upper neck).
  • Controls head extension, flexion, and rotation at the atlanto-occipital joint.
• C2 (Axis):
  • Innervates the splenius and longus capitis, contributing to head and neck extension and lateral flexion.
  • Also affects the sternocephalicus, involved in neck flexion.
• C3–C5:
  • Innervate the brachiocephalicus and omotransversarius muscles, allowing neck flexion and forelimb protraction.
  • Also control the scalenus and longus colli, which help in neck flexion and stabilization.
• C6–C7:
  • Innervate the trapezius, rhomboideus, and serratus ventralis, controlling movements of the shoulder girdle and forelimb elevation.

Thoracic Region – T1 to T18

The thoracic spinal nerves (T1–T18) mainly innervate muscles of the upper trunk and forelimbs.

• T1–T2:
  • These myotomes contribute to the brachial plexus and innervate the pectoralis major and minor, which aid in forelimb adduction.
  • Also control parts of the latissimus dorsi for forelimb retraction.
• T3–T9:
  • Innervate the thoracic portion of the longissimus dorsi, which supports spinal extension along the back.
  • They also innervate the intercostal muscles (for rib movement during respiration) and external obliques.
  • These myotomes also influence trunk stabilization during movement.
• T10–T18:
  • Innervate the dorsal muscles of the back, particularly the epaxial muscles (longissimus dorsi and spinalis), which help in spinal extension.
  • Involved in trunk extension and stabilization of the horse’s back during motion.

Lumbar Region – L1 to L

The lumbar spinal nerves (L1–L6) innervate muscles of the lower back, flanks, and hindquarters.

L1–L2 Myotomes:

• L1-2:
  • Innervates the iliopsoas muscle, which contributes to hip flexion.
  • Also controls the quadratus lumborum muscle, assisting in spinal stabilization and lateral flexion of the lumbar spine.

L3–L4 Myotomes:

• L3–4:
  • Innervates the tensor fasciae latae (TFL), involved in hip flexion and stifle extension.
  • Controls the rectus abdominis muscle, contributing to abdominal compression and spinal support.

L5–L6 Myotomes:

• L5–6:
  • Innervates the gluteal muscles, including the gluteus medius, which contribute to hip extension and hind limb abduction.
  • Also controls the gluteus superficialis, assisting in hip extension and hind limb abduction.

Sacral Region – S1 to S5

The sacral spinal nerves (S1–S3) innervate muscles involved in hind limb movement and pelvic support.

The myotomes of the sacral region (S1–S5) in horses innervate several important muscle groups in the hindquarters and pelvic region:

• S1:
  • Gluteal Muscles: Includes the gluteus medius and gluteus superficialis, which are involved in hip extension and hind limb abduction.
  • Piriformis Muscle: Contributes to hip extension and rotation.
• S2:
  • Gluteal Muscles: Continues to provide innervation for hip extension and hind limb abduction.
  • Semitendinosus and Semimembranosus Muscles: Involved in hip extension and stifle flexion.
• S3:
  • Gluteal Muscles: Continues to affect hip extension and hind limb abduction.
  • Adductor Muscles: Including the adductor and gracilis muscles, which are involved in adduction of the hind limbs.
• S4:
  • Perineal Muscles: Including the muscles of the pelvic floor, which support the pelvic organs and contribute to pelvic stability.
  • Anal Sphincter Muscle: Involved in the control of the anal opening.
• S5:
  • Tail Muscles: Contributes to the movement and control of the tail.
  • Coccygeus and Levator Ani Muscles: Help in tail movement and support of the pelvic floor.

Caudal Region – Ca1 to Ca5+

The caudal spinal nerves (Ca1–Ca5 and beyond) innervate the muscles of the tail.

• Ca1–Ca5+:
  • These myotomes innervate the coccygeal muscles, which control tail movement for swishing, communication, and balance.
  • These muscles also help in minor adjustments to body posture during locomotion.

Dermatomes generally correspond to the same regions that their associated myotomes innervate. Each dermatome is a region of skin that receives sensory input from a specific spinal nerve, matching the motor innervation provided by the myotomes in that area.

Cervical Region (C1 to C7):

Dermatomes in this region provide sensory input to the skin of the head, neck, and forelimbs.

• C1: Does not have a dermatome (it only provides motor innervation).
• C2: Sensory innervation to the back of the head and the upper neck.
• C3–C5: These dermatomes cover the neck, including the area around the throat, and extend into the shoulder area.
• C6–C7: Provide sensory input to the base of the neck, shoulders, and the upper forelimbs.

Thoracic Region (T1 to T18):

Thoracic dermatomes correspond to the upper trunk, chest, and back.

• T1–T2: Sensory input to the shoulder girdle and upper forelimbs (similar to their myotomal innervation).
• T3–T9: These dermatomes provide sensory input along the back, from the withers to the mid-back.
• T10–T18: Sensory regions extend along the lower back, including the flanks, and move ventrally toward the abdomen. These dermatomes wrap around from the spine to the belly.

Lumbar Region (L1 to L6):

Lumbar dermatomes provide sensory input to the lower back, hindquarters, and upper hind limbs.

• L1–L2: These dermatomes innervate the skin of the lower back and extend ventrally to the flank and abdomen.
• L3–L6: These dermatomes cover the hindquarters, including the pelvic region, gluteals, and the upper hind limbs.

Sacral Region (S1 to S5):

The dermatomes of the sacral region (S1–S5) in horses provide sensory innervation to specific areas of the hindquarters, pelvis, and tail. Here’s a breakdown of what each sacral dermatome covers:

• S1:
  • Provides sensory input to the gluteal region and the upper part of the hindquarters.
  • Covers the lateral aspect of the pelvis and the lower part of the back near the sacral area.
• S2:
  • Sensory innervation extends to the middle part of the hindquarters and the posterior part of the flank.
  • Includes the inner thigh area, near the base of the tail.
• S3:
  • Covers the gluteal region, inner thigh, and extends into the perineum area.
  • Sensory input includes the region around the tail base and parts of the pelvic floor.
• S4:
  • Provides sensory input to the pelvic floor, including the perineal area.
  • Includes the genital region and the area around the anus.
• S5:
  • Covers the tail and tail base, providing sensory input to the dorsal and lateral aspects of the tail.
  • Includes the region around the tail tip and the area adjacent to the sacral vertebrae.

Sacral dermatomes innervate the skin of the hindquarters, groin, and tail base.

• S1–S3: Provide sensory input to the gluteal region, the inner thighs, and the perineal area (the region under the tail, including the groin).

These dermatomes also extend to the ventral pelvic region and portions of the hind limbs.

Caudal Region (Ca1 to Ca5):

Caudal dermatomes correspond to the tail and the skin around the tail base.

• Ca1–Ca5: These dermatomes provide sensory input to the tail, which is involved in tactile sensation and response to stimuli. They also cover the dorsal tail base and adjacent skin areas.