Page 72 - Canine Lameness

44 4 The Neurologic Examination

nervous system result from conditions of the PNS although some can involve the CNS (i.e. spinal
cord). As such, this chapter focuses on these two components and the aspects relevant to canine
lameness. More complete description of the entire neuroanatomy and neurologic conditions are
available (De Lahunta et al. 2015; Dewey and Da Costa 2016).
Nerve cells (neurons) are composed of a cell body and the nerve fibers (axons). The axons are
bundled into either sensory tracts conveying afferent information from peripheral regions to higher
processing centers or motor tracts relaying efferent information from the motor planning centers to
the periphery.
The spinal cord is functionally divided into the following regions, defined by the normal spinal
enlargements (intumescences) in the cervical and lumbar areas: cervical (C1–C5), cervical intu-
mescence (C6–T2), thoracolumbar (T3–L3), lumbar intumescence (L4–S3), and caudal regions
(segments caudal to S3; Figure 4.2). In dogs, the spinal cord ends around the sixth or seventh lum-
bar vertebrae in medium–sized individuals and further caudal (e.g. L7, S1) in dogs of small size.
The PNS consists of the sensory and motor nerves, muscles, and neuromuscular junctions. The
cell bodies of motor neurons are located in the ventral gray matter of the spinal cord, while the cell
bodies of the sensory neurons are located outside the spinal cord in the spinal ganglia. The respec-
tive fibers arise as ventral and dorsal nerve roots on each side of the spinal cord. Thus, the dorsal
roots convey primarily sensory information, while the ventral roots carry motor nerve fibers. The
origin of these nerve roots from the spinal cord defines each spinal cord segment (Figure 4.2).
Motor fibers can be somatic, innervating skeletal (striated) muscle in the PNS, or autonomic,
innervating smooth and cardiac muscle in the ANS. The dorsal and ventral nerve roots fuse at the
level of the intervertebral foramen and form a spinal nerve. Distal to the intervertebral foramen, the
nerve usually splits into a dorsal branch innervating the epaxial muscles and skin, as well as a ven-
tral branch supplying limb muscles and skin.

4.2.2 Functional Components of the Nervous System

Functionally, the nervous system can be divided into the sensory and motor systems. The sensory
system conveys sensory information toward the CNS, while the motor system activates muscles in
the PNS and thereby initiates and controls motion.

4.2.2.1 Motor Systems
Motor neurons are distributed within two motor systems, the UMN and LMN systems, and are
named accordingly. The UMNs are confined entirely within the CNS, while the LMNs originate in
the CNS (ventral grey matter of spinal cord) but the distal portions are located in the periphery.
Hence, the spinal cord connects the UMNs in the CNS to the LMNs in the PNS. The UMNs greatly
influence activity of LMNs so accordingly, the entire function of the CNS is manifested through the
LMN. Thus, the LMNs are the motor effector cells of the PNS. Using the five divisions of the CNS,
the C1–C5 and T3–L3 spinal cord segments are components of the UMN system, and C6–T2 and
L4–S3 are components of the LMN system (Figure 4.2).
Based on the location of a lesion (whether within the LMN or UMN system), specific deficits are
observed which are referred to as UMN or LMN signs (Table 4.1). For example, when the term
UMN disease is used, this refers to the UMN signs produced by diseases of the CNS, specifically
spinal cord white matter and/or brain. The term LMN disease is sometimes used when referring to
diseases of the PNS. However, this can create confusion since the LMNs are the true motor compo-
nent of the PNS while the term PNS also includes the sensory portions (sensory nerves). Thus,
this text will use LMN disease when referring to conditions affecting the motor arm of the PNS
specifically and PNS disease when referring to dysfunction of the entire PNS. The nomenclature