GSE: Spinal

General Somatic Efferents (GSE): Spinal Nerves

Overview
Course Outline

  • General topics covered in the course include:

    • Classification

    • Development

    • Blood Supply

    • Cerebrospinal Fluid (CSF)

    • Meninges

    • Blood-Brain Barrier (BBB)

    • Efferents

    • GSE (General Somatic Efferents) / GVE (General Visceral Efferents)

    • Upper Motor Neurons (UMN)

    • Afferents

    • GSA (General Somatic Afferents) / GVA (General Visceral Afferents) / Proprioception / Vestibular / Cerebellum / Visual / Auditory / Limbic / Thalamus

    • Brains / Spinal Cords – Cross / Sagittal Sections

GSE Definition

  • Definition: General Somatic Efferents (GSE) are nerves that carry orders from the Central Nervous System (CNS) to voluntary striated muscles.

GSE and GVE

  • Both GSE and General Visceral Efferents (GVE), which are nerves carrying orders from the CNS to smooth muscles, are collectively known as Lower Motor Neurons (LMN).

Composition of GSE

  • Components:

    • Comprises all spinal nerves

    • All CNS (Central Nervous System) structures, except cranial nerves I (Olfactory nerve), II (Optic nerve), and VIII (Vestibulocochlear nerve).

Significance of GSE

  • Functionally maintain:

    • Posture

    • Weight

    • Gait

Spinal Nerve Overview

  • Canine Spinal Nerves:

    • Total pairs: 36

    • Cervical (C): 8

    • Thoracic (T): 13

    • Lumbar (L): 7

    • Sacral (S): 3

    • Caudal (Ca): 5

  • Equine Spinal Nerves:

    • Total pairs: 38

    • Cervical (C): 8

    • Thoracic (T): 18

    • Lumbar (L): 6

    • Sacral (S): 5

    • Caudal (Ca): 1

Anatomy of the Spinal Nerve

  1. Spinal Nerve Structure:

    • White matter and gray matter systems

    • Dorsal root: Carries sensory (afferent) information to the CNS

    • Ventral root: Carries motor (efferent) information to muscles and glands

  2. Roots Joining at Spinal Nerve:

    • Dorsal and ventral roots converge to form a short spinal nerve, which then splits into dorsal and ventral rami, and ultimately develops into peripheral nerves that terminate in muscles.

  3. Innervation:

    • Each axon of the spinal nerve innervates approximately:

    • Eyes: 100-150 muscle cells

    • Limbs: 3-4 muscle cells

    • This branching allows for the formation of a motor unit.

The Motor Unit

  • Components of the motor unit:

    • Motor neuron

    • Branches of motor neurons

    • Muscle fibers (myofibrils)

Neuromuscular Junction

  • Structure:

    • Motor end plate features folds in the sarcolemma

    • Key components include:

    • Axon terminal of the motor neuron

    • Synaptic vesicles containing acetylcholine

    • Sarcolemma of the muscle fiber

    • T-tubules

    • Motor end plate area with nicotinic acetylcholine receptors

    • Functionality:

    • Acetylcholine released into the synaptic cleft, activating receptors on the motor end plate, ultimately triggering muscle contraction.

Organization of the Ventral Gray

  • Muscle Groupings:

    • Axial Muscles: Medial Ventral Gray (head/trunk)

    • Appendicular Muscles: Lateral Ventral Gray (limbs)

    • Proximal limb muscles located ventrally, while distal limb muscles are grouped dorsally.

Spinal Reflexes

  • Types of Reflexes:

    • Monosynaptic Reflex

    • Polysynaptic Reflex

Monosynaptic Reflex

  • Involves:

    • Direct communication between afferent and efferent neurons

  • Example:

    • Patellar (knee jerk) reflex

Polysynaptic Reflex

  • Involves:

    • Afferent neurons, efferent neurons, interneurons, and multiple spinal segments

  • Example:

    • Withdrawal reflexes

Reflex Significance

  • Diagnostic Role:

    • Localizes affected nerves and identifies involved spinal segments

Clinical Relevance: Intervertebral Disc Disease (IVDD)

  • Normal Canine Spine:

    • Structure showcases healthy spinal cord, intervertebral discs.

  • IVDD Presentation:

    • Illustrated with prolapsed intervertebral disc affecting spinal cord and spinal nerves.

Reflex Testing of Thoracic Limb

  • Relevant Nerve Roots:

    • C6-T2 involving the brachial plexus

  • Flexor-Withdrawal Reflex:

    • Recognized as the only reliable test, notably with the compression of lateral digit.

Reflex Testing of Pelvic Limb

  • Lumbosacral Plexus:

    • Tests involving:

    • Femoral nerve

    • Sciatic nerve (subdivided into peroneal and tibial branches)

    • Sacral plexus concerning perineal region

Patellar Reflex Testing

  • Identified as a reliable test related to nerve roots L4-L6 with the associated femoral nerve (sensory/motor) facilitating paw tremor and stifle extension.

Flexor/Withdrawal Reflex Among Limbs

  • Motor Inputs for Flexion:

    • Flexion of all joints monitored through sensory input from the base of the claw (5th digit), with nerves that contribute to the reflex including peroneal, tibial, and sciatic branches.

Specific Reflex Testing Responses

  • Gastrocnemius Reflex:

    • Tests tibial branch resulting in a slight extension of the tarsus.

  • Panniculus Reflex:

    • Evaluates skin twitch responses over the flank involving thoracolumbar nerves (T2-L7).

Cutaneous Trunci Reflex

  • Explored under thoracolumbar area (T2-L7) with relevance to the lateral thoracic nerve (C8-T1), also resulting in a skin twitch due to the cutaneous trunci muscle's contraction.

Perineal Reflex Examination

  • Reflected by the contraction of the external anal sphincter and tail flexion, innervated by the sacral plexus (S1, S2) with relevance to pelvic canal and anus.

Conclusion

  • Reinforces the importance of understanding GSE and reflex pathways to diagnose neurological conditions, particularly in veterinary settings.