Structures Involved in Epidural Anesthesia | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

1. Spinal Nerve Roots:

   • Each spinal nerve is connected to the spinal cord through two roots: one for movement (ventral or motor root) and one for sensation (dorsal or sensory root).

2. Formation of Spinal Nerve:

   • The ventral and dorsal roots come together just past the spinal ganglion to create the spinal nerve.
   • This nerve exits the spine through small openings called intervertebral foramina.

3. Location of Spinal Ganglion:

   • The spinal ganglion is found on the dorsal root of the spinal nerve.
   • It is typically positioned near or within the intervertebral foramen, just outside the tough covering of the spinal cord (dura mater).

4. Branching of Spinal Nerve:

   • Once out of the intervertebral foramen, each spinal nerve splits into dorsal and ventral primary branches.
   • The dorsal branches go on to supply muscles and skin on the back of the neck and trunk.
   • The ventral branches provide nerves to the front and sides of the trunk, as well as all parts of the limbs.

Fig: Distribution of the spínal nerves of house (schematic)

Functional Components of Spinal Nerves

• Spinal nerves have two main functions: sensory (afferent) and motor (efferent).

Sensory Fibers:

• These fibers can carry sensations from both the body's surface (exteroceptive) and internal organs (visceral).
• Exteroceptive sensations include feelings like pain, temperature, touch, and pressure.
• Proprioceptive sensations arise from muscle movements, joint movements, tendons, and other deep somatic activities.

General Visceral Afferent Fibers:

• These fibers transmit signals from internal organs, glands, blood vessels, and mucous membranes.

Spinal Nerve Fibers:

• Both sensory and visceral fibers are connected to spinal ganglion cells, entering the spinal cord through the dorsal root.

Motor Fibers:

• Motor fibers, responsible for movement, are further divided into somatic motor (related to skeletal muscles) and visceral motor (autonomic, controlling glands and smooth muscles).

Epidural Anaesthesia Effects:

• During epidural anesthesia, when injected in the lower back, it first affects the sensory roots (dorsal), causing numbness.
• Then, sympathetic fibers and eventually motor roots (ventral) are blocked, leading to muscle relaxation.
• This is particularly beneficial for large animals, allowing surgical procedures while the animals are standing, as the sciatic nerve function remains unaffected.

Fig: Caudal spinal cord of the horse