Spinal nerves

Spinal nerves arise as roots from the spinal cord. A dorsal and a ventral root attach on each side of the spinal cord, and define each spinal cord segment. For example, the third cervical spinal cord segment has two dorsal roots and two ventral roots attaching to it. The dorsal roots convey primarily sensory nerve fibres into the spinal cord. Each dorsal root contains a spinal ganglion (old name ‘dorsal root ganglion’), housing the nerve cell bodies of these sensory fibres (Fig. 1.1). The ventral roots convey motor nerve fibres away from the spinal cord. Motor fibres may be somatic and innervate striated muscle, or autonomic and innervate smooth or cardiac muscle. The dorsal and ventral roots fuse at the level of the intervertebral foramen to form a spinal nerve. Distal to the intervertebral foramen, the mixed sensory and motor spinal nerve usually splits into a dorsal and ventral branch. The dorsal branch supplies the epaxial muscles and skin, while the ventral branch supplies the hypaxial muscles and skin. A third branch, carrying autonomic fibres, may also arise and pass ventrally towards the midline to supply the viscera.

Spinal nerves may remain as single, discrete nerves all the way out into the periphery, in which case they are named for the number of the spinal cord segment from which they arise, e.g. ‘C3, ventral branch’ is the ventral division of cervical spinal nerve 3; it supplies sensory and motor innervation to the hypaxial tissue of the neck. Alternatively, in a nerve plexus, the ventral branches of two or three adjacent spinal nerves may fuse, giving rise to nerves with specific names, such as radial nerve and femoral nerve (Fig. 1.2).

Fig. 1.2 Anatomical and functional spinal cord regions and the main nerve plexi.

A cranial nerve arises from the brain; it is known by both the number of the nerve and a specific name, e.g. trigeminal nerve or cranial nerve, CN V (Fig. 1.6A).

The PNS comprises afferent (sensory) fibres and their receptors, and efferent (motor) fibres that link to effectors, such as striated, cardiac and smooth muscle.

Afferent nerve fibres originate at different types of sensory receptors and most of their fibres only synapse once they reach the CNS. The sensory nerve cell bodies of spinal nerves are located in spinal ganglia sited at the level of the intervertebral foramen. Cranial nerve ganglia are located just near, or inside, the neurocranium. The area of skin innervated by a spinal nerve is called a dermatome, while the area of skin innervated by a specific named nerve, which originates from two or more spinal nerves (for example, the radial nerve) is called a cutaneous zone. Adjacent dermatomes and cutaneous zones usually overlap. The area of skin innervated purely by one nerve is called an autonomous zone (Fig. 1.3). Autonomous zones are found on the head (Figure 10.13) as well as the body and limbs (Table 1.1).

Fig. 1.3 Sensory innervation of the skin depicting the basis of dermatomes, cutaneous zones and autonomous zones. The zones in this diagram are conceptual and not anatomically accurate to the thoracic limb.

Table 1.1 Autonomous zones in the canine limb

Knowing the position of dermatomes and autonomous zones is useful for assessing sensory function of the PNS. However, there are species differences, for example, the horse does not have an autonomous zone specific to the radial nerve.

Motor neurons can be defined as upper motor neurons (UMNs) or lower motor neurons (LMNs). The UMN is confined to the CNS and its axon influences activity of LMNs. They are the ‘managers’ of the motor system (see Chapter 4). The LMNs are found in cranial nerves, originating from the brainstem, and spinal nerves from the spinal cord. They form synapses at the neuromuscular junction and innervate striated muscle, smooth or cardiac muscle. They are the ‘workers’ of the motor system. The neurochemical that connects the electrical activity of the motor nerve to the striated muscle is acetylcholine (ACh). A nerve impulse arriving at the nerve termination triggers release of ACh (Fig. 1.4). The ACh crosses the synaptic cleft, binds to the receptors on the post-synaptic membrane and may stimulate muscle membrane depolarisation and muscle contraction, depending on stimulus strength and amount of ACh released. The ACh is broken down by acetylcholine esterase and recycled back into the distal end of the LMN. For striated muscle, the motor unit is defined as an axon and the muscle fibres it innervates. Motor units range in size from 3–150 muscle fibres per axon for muscles with fine versus coarse control, respectively. Small motor units are found in extraocular muscles, whereas large motor units are found in the large, postural muscles, e.g. the quadriceps femoris muscle.

Fig. 1.4 The neuromuscular junction.

Cranial nerves

In domestic mammals, there are 12 pairs of cranial nerves. Like the spinal nerves, these may convey sensory, motor and autonomic fibres of the parasympathetic nervous system (see p4). Cranial nerves are abbreviated to CN (singular) or CNN (plural).

Sensory ganglia are located near, or just inside, the neurocranium whereas the somata of motor nerves are located primarily in the brainstem. The exception is the external branch of the accessory nerve (CN XI); these fibres originate in the cervical spinal cord. The cranial nerve fibres generally remain distinct from other cranial nerves, but occasionally they may intermingle. For example, CNN X and XI fibres mix to form the recurrent laryngeal nerve. The nerves and their branches have specific names, e.g. auriculopalpebral branch of the facial nerve, CN VII.

Using Roman numerals, the nerves are numbered in rostro-caudal sequence based on their attachment to the brain. Assessing the function of the cranial nerves is an integral part of the clinical examination. They are covered in more detail in Chapter 10.