Previous Chapter	Electronic Syllabi Home	Table of contents	Next chapter

  

---

Chapter 8. LOWER MOTOR NEURONS

---

8.1 OBJECTIVES:

1. Define the lower motor neuron (LMN).

2. Enumerate common neurological signs of LMN lesions.

3. Identify nuclei of the GSE LMN and state the muscle(s) which is/are innervated by them.

4. Define strabismus and be able to differentiate clinical signs caused by injury to CNN III, IV and VI.

5. Describe muscle innervation and neurological signs after lesion on CNN V, VII, IX and X.

6. Identify GSE and SVE nuclei from slides, photos or illustrations.

 

---

8.2 READING ASSIGNMENT:

deLahunta, Chapters 4 and 5

Jenkins, pp 172-173; 208-209; 224-226; 234-235; 313-322.

 

---

8.3 GENERAL CHARACTERS OF GSE AND SVE LOWER MOTOR NEURONS:

1. They are large or medium size multipolar neurons with myelinated axons.

2. Their neural inputs come from:

Long descending motor tract (pyramidal and extrapyramidal) fibers of upper motor neuron origin. Or from local sensory fibers (forming the reflex arc).

3. Their axons innervate the target skeletal muscles directly without interruption, and are termed the final common path. Each LMN may innervate more than one muscle fiber. A LMN with all the muscle fibers it innervates is called a motor unit.

4. LMN lesions result in what is clinically termed lower motor neuron signs:

hypotonia/atonia - muscle weakness

paresis/paralysis

hyporeflexia/areflexia - weaken or abolished reflexes

atrophy of the innervated muscle follows quickly

 

 

---

 8.4 NUCLEI OF GSE LOWER MOTOR NEURONS

1. Spinal Cord LMNs are in the ventral part of the ventral gray column (lamina IX of Rexed). In the cervical and lumbar intumescences, these LMNs form two major subgroups:

medial nuclear group - innervates trunk muscles

lateral nuclear group - innervates limb muscles

2. Hypoglossal Nucleus LMNs innervate somatic skeletal muscles of the tongue.

3. Abducens Nucleus LMNs go to extrinsic muscles of the eye - lateral rectus and retractor bulbi.

4. Trochlear Nucleus LMNs have their fibers decusate in the rostral medullary velum. They then innervate the dorsal oblique.

5. Oculomotor Nucleus LMNs and neurons of Edinger-Westphal nucleus form the oculomotor nuclear complex. GSE fibers innervate most of the extra-ocular muscles and the levator palpebra of the eyelid.

 

---

8.5 STRABISMUS

Deviation of the eyeball from its normal position. Mainly caused by lesions in LMN innervating the extra-ocular muscles.

CN VI lesion - medial strabismus (squint)

CN IV lesion - difficult to detect in the round pupils' animals, but can be diagnosed by visual inspection of the eye.

CN III lesion - lateral and ventral strabismus, but other signs may be seen: dropping of the upper eyelid (ptosis) and pupillary dilation (GVE defect)

 

---

8.6 NUCLEI OF SVE LOWER MOTOR NEURON

Their neurons innervate skeletal muscles of the head and neck which differentiated from the branchial arches.

8.6.1 1MOTOR NUCLEUS OF THE TRIGEMINAL:

1. Axons innervate muscle of mastication, rostral belly of digastric, myelohoid and tensor tympany.

2. Injury - bilateral injury results in a dropped jaw; unilateral lesions are difficult to detect. Atony of involved muscles may be seen.

 

 

8.6.2 MOTOR NUCLEUS OF THE FACIAL NERVE:

1. Nerve fibers innervate muscles of face, stylohyoid, caudal belly of digstric and stapedius.

2. Unilateral lesions result in facial paresis/paralysis, drooping lip or dropping ear on the affected side. Or deviation of nasal philtrum (horse) to the normal side.

 

8.6.3 NUCLEUS AMBIGUS:

1. CN IX innervates stylopharyngeal and other pharyngeal muscles. CN X innervates muscles of pharynx, larynx and upper esophagus.

2. Lesions may result in dysphagia, laryngeal paralysis or gag reflex deficit.

8.6.4 SPINAL ACCESSORY NUCLEUS:

Lesions result in lower motor neuron signs of some neck muscles (sternocephalicus, trapezius etc.)

 

 

---