Lower Motor Neurons (LMN) Flashcards
What is lower motor neuron
Anterior/ventral horn cells (efferent neuron) of PNS
What does lower motor neuron do
Connects the CNS with innervated striated muscles
- the entire CNS function is expressed through LMN
CNS EXERTS ITS CONTROL OVER PNS THROUGH LMN
What are the two components of lower motor neuron
General somatic efferent (GSE)
General visceral efferent (GVE)
Function of lower motor neuron GSE component
Final motor innervation to the muscles
Contraction of these muscles maintains posture, supports weight and provides gait
What is included in GSE (general somatic efferent)
Neuron that innervates striated voluntary skeletal muscles so –> all neurons of spinal nerves and all cranial nerves except CN1, 2, and 8
Why is the shape and size of the ventral grey column important
It correlates with the number of neurons present
It is determined by the volume of muscles innervated
What do the GSE neurons in the medial portion of the ventral grey horn innervate
Axial muscles
What do the GSE neurons of lateral portion of the grey column innervate
Appendicular muscles
Specifically, what do the GSE neurons of the ventral lateral grey column innervate
Proximal limb muscles
Specifically, what do the GSE neurons of the dorsal lateral grey column innervate
Distal limb muscles
Where are axons in the white matter
Between lateral and ventral funiculi
What is the spinal cord portion of the axon myelinated by
oligodendrocytes
What is the peripheral portion of the axon myelinated by
Schwann cells
A motor unit is
Muscle cells innervated by one neuron
What does the strength of muscle contraction depend on
Motor unit activation in muscle
How does the axon progress from spinal nerve to peripheral nerve
- leave through ventral root, spinal nerve to motor end-plate of neuromuscular ending of the limb muscles as peripheral nerve
What is the synaptic cleft
Space formed by axolemma in sarcoplasmic trough (presynaptic membrane) and folds of the postsynaptic membrane (sarcolemma membrane)
What is contained in the synaptic vesicles at the presynaptic ending
Acetylcholine
What occurs upon arrival of an action potential
Opening of Ca channels and in flow
- formation of substructure which directs ACH vesicle to move down and fuse with membrane
- release ACH to activate receptor and depolarize membrane
What is excess ACH broken down by
Acetylcholinesterase
What is LMNL
Lower motor neuron lesion
Where can damage occur on LMN and do the signs differ
cell body, along axon and at innervated muscle
- regardless of where it occurs, signs are the same
What are the signs of an LMNL
Muscle paresis or paralysis
Hyporeflexia or areflexia (LMN is motor component)
Hypotonia or atonia (muscle tone depends on stimulation from LMN)
Neurogenic atrophy
What is flaccid paresis
Weakness and loss of muscle strength, difficulty supporting weight and deficiency in generation of gait (short strides, lameness)
What is paralysis
Severe paresis with complete absence of any movement
What is hypoflexia
Weakness of spinal reflexes
What is areflexia
Loss of spinal reflexes
What occurs if the sacrocaudal LMN is damaged
Anal, perineal and tail reflexes are affected
What is hypotonia
Weakness in muscle tone
Atonia is
Loss of muscle tone
Neurogenic atrophy
Loss of muscle function and atrophy because of complete loss of innervation
What happens if multiple limbs are affected by LMNL
Animals tend to collapse with difficulty standing up from recumbent position
What happens if both pelvic limbs are affected
Animal tends to flex both hips to advance limbs –> bunny-hopping
Is ataxia a sign of LMNL
No, it is a sensory system abnormality
What is the significance of LMNL of the oculomotor nucleus or nerve
- Dysfunction of dorsal, ventral and medial rectus and ventral oblique muscles
- Lateral strabismus/deviation will occur do to lateral rectus pulling with now resistance
- Paralysis of lavatory muscle of eye lid –> drooping eyelid
What is the significance of a lesion of parasympathetic nucleus of oculomotor nerve
Paralysis of constrictor muscle of pupil
Leads to constantly dilate pupil (Mydiasis)
Animal shows no pupillary light reflex
What are the clinical signs of an LMNL of trochlear nucleus
Paralysis of dorsal oblique leads to ventral rotary deviation of lateral part of the eye distorting line of vision dorsomedially
What is the clinical significance of LMNL in trigeminal nerve (motor)
Bilateral: difficult chewing and dropped jaw
Unilateral: mandibular deviation to unaffected side
What is the clinical significance of LMNL in trigeminal nerve (sensory)
Loss of sensation to head
Where is oculomotor nucleus located
midbrain
Where is the trochlear nucleus located
Decussate in rostral medullary velum, emerges caudal to caudal colliculus
Where is the trigeminal nucleus located
In Tegmentum
Where is the abducen nucleus located
In pontine medullary boundary, fiber emerges ventrally behind pons
What is the clinical significance to LMNL in abducens nucleus
Medial deviation (strabismus) and protrusion of eye due to lack of innervation to lateral rectus and retractor bulbus
Where is facial nucleus located
Rostral medulla
What is the clinical significance of LMNL in facial nucleus (motor)
paralysis of mouth and inability to close eyelid
- lack of innervation of orbicularis oculi
What is the clinical significance of LMNL in facial nucleus (sensory)
Malfunction of lacrimal gland resulting in dry cornea
due to sensory innervation of lacrimal and salivary glands
Where is the hypoglossal nucleus located and what does it innervate
Caudal medulla
Intrinsic muscle of tongue
What does LMNL in hypoglossal nucleus result in
Paralysis of tongue shown by difficulty in swallowing
What is nucleus ambiguus
“Small nucleus”
Supplies fibers to vagus and glossopharyngeal nerves so innervates larynx and pharynx
What is the clinical significance of LMNL in nucleus ambiguus
Pharyngeal paralysis and laryngeal paralysis
