Terminology and Neuroanatomy Part 1

Fundamental Neuroanatomy and Definitions

• Neuroanatomy Utility: A robust understanding of neuroanatomy significantly enhances the practitioner's ability to perform neurological examinations and accurately interpret the resulting clinical findings.

• Consistency of Terminology: Terminology remains generally consistent across various species. While this material focuses on small animal medicine, the principles and anatomical terms apply broadly.

• Central Nervous System ($CNS$):

  • Components: Comprises the brain and the spinal cord.

  • Primary Functions: Responsible for receiving sensory input (afferent), producing motor output (efferent), and coordinating overall nervous system activity.

• Peripheral Nervous System ($PNS$):

  • Scope: Includes all parts of the nervous system excluding the brain and the spinal cord.

• Upper Motor Neuron ($UMN$):

  • Classification: Identified as the $1^{st}$ neuron in the motor pathway.

  • Location: The cell body is located within the $CNS$, specifically in the cerebral cortex.

  • Synapse: It synapses onto a lower motor neuron located in either the brainstem or the spinal cord.

• Lower Motor Neuron ($LMN$):

  • Classification: Identified as the $2^{nd}$ neuron in the motor pathway.

  • Location of Origin: Begins in the spinal cord or brainstem.

  • Connection: Connects directly to skeletal muscle or a gland.

• Gray Matter:

  • Composition: An unmyelinated region of the $CNS$.

  • Constituents: Contains cell bodies and dendrites.

• White Matter:

  • Composition: A myelinated region of the $CNS$.

  • Constituents: Contains myelinated axons.

  • Note: Within the spinal cord, these represent interconnecting axons.

• Cranial Nerve ($CN$):

  • Function: Provides motor and sensory innervation to the head and neck.

  • Structure: These are paired nerves that emerge directly from the brainstem.

Divisions of the Nervous System

• Somatic Nervous System: A division of the $PNS$ that controls voluntary movement, reflexes, and proprioception.

• Visceral/Autonomic Nervous System: A division of the $PNS$ responsible for controlling internal organ functions.

  • Subdivisions: Includes the sympathetic and parasympathetic nervous systems.

Spinal Cord Anatomy and Segmental Organization

• Spinal Cord Segments: It is critical to remember that these segments differ in large animals compared to small animals. The standard segments include:

  • Cervical: $8$ segments (despite the presence of only $7$ cervical vertebrae).

  • Thoracic: $13$ segments.

  • Lumbar: $7$ segments.

  • Sacral: $3$ segments.

  • Caudal: Approximately $5$ segments (the number of caudal vertebrae is variable).

• Spinal Cord Termini:

  • Canine: The spinal cord ends at the level of the $L6$ to $L7$ vertebrae.

  • Feline: The spinal cord ends between the $L7$ and $S3$ vertebrae.

• Cauda Equina: Refers to the collection of long spinal roots and nerves located caudal to the $L7$ vertebra.

• Internal Distribution of the Spinal Cord:

  • Ventral Horn: Contains motor neurons; facilitates descending (efferent) tracts.

  • Dorsal Horn: Contains sensory neurons; facilitates ascending (afferent) tracts.

Nerve Exit Morphologies and Clinical Significance

• Cervical Nerve Exit Patterns:

  • $C1-7$ Spinal Nerves: These exit the intervertebral foramina cranial to the vertebra of the same number.

  • $C8$ Spinal Nerve: This nerve exits the intervertebral foramen cranial to the $T1$ vertebra.

• General Nerve Exit Patterns: All spinal nerves following $C8$ exit through the foramina caudal to the vertebra of the same number (e.g., the $T1$ nerve emerges between the $T1$ and $T2$ vertebrae).

• PNS/LMN Classification within the Spinal Canal: Spinal nerves are classified as part of the $LMN$ and the $PNS$ once they exit the spinal cord, even if they are still located within the physical spinal canal.

• Clinical Implication of Lesions: A lesion located within the spinal canal will result in $LMN$ signs if the spinal nerve itself is affected.

Neurological Examination Findings and Comparative Medicine

• Postural Reaction Deficit Case Study: Observation of a canine showing a proprioceptive deficit in the left ($L$) pelvic limb. The animal exhibits weight-bearing on the dorsal surface of the paw rather than the pads.

• Hopping Reaction Case Study: Observation of a ewe during a hopping reaction assessment. The ewe demonstrates an abnormal angle of the right forelimb, failing to place the limb underneath her center of gravity as expected.

• Comparative Neurology Principles:

  • Terminology and neuroanatomy are consistent across species, allowing for the application of these principles to large animal neurology.

  • Performance vs. Interpretation: While the neurological exam is performed differently in large animals due to size and handling requirements, the interpretation for lesion localization is similar to that of small animals.

  • Etiology: While anatomy and localization are similar, the specific causes (etiology) of neurologic diseases often differ between small and large animal species.

• Visual Documentation Credits:

  • Dog paw image: JoelMills.

  • Spinal cord cross-section: OpenStax.

  • Hopping sheep: Uwe Gille.

  • Canine spinal regions representation: Przemek Maksim.

  • Spinal nerves schematic: Mysid.

• Spine Region Labeling:

  • A: Cervical.

  • B: Thoracic.

  • C: Lumbar.

  • D: Sacral.

  • E: Caudal.