Lower Motor Neuron Unit/Neuromuscular Disease Flashcards
where do lower motor neurons originate and where do they go? contrast with upper motor neurons
LMN originate from the spinal cord and innervate muscles in limbs;
UMN originate from the brain, they send the thoughts to move the limb (turn LMN on or off to move limbs)
how does an action potential travel down an axon?
the wave of depolarization travels in big hops (saltatory conduction); axons are myelinated by schwann cells except at nodes of ranvier; the AP jumps from one node to the other, traveling faster than if it ran down the entire length of the axon
what 3 things are required for saltatory conduction? what does this imply?
- sodium
- potassium
- ATP (from glucose or other energy sources)
this means that energy is required to establish an action potential, so an energy source is ESSENTIAL! if not getting neuromuscular transmission, check these 3
how is muscle contraction accomplished? (6)
- an impulse reaches the presynaptic terminal
- this causes an influx of calcium
- calcium binds to presynaptic vesicles that contain ACh
- ACh is exocytosed and released into neuromuscular junction
- ACh binds to the post-synaptic membrane (muscle) and depolarizes the muscle cell
- this depolarization causes actin/myosin interaction and muscle contraction
how is muscle contraction stopped?
acetylcholine esterase degrades ACh and recycles is back to the presynaptic terminal to get ready for the next action potential
what are the 3 ways to assess LMN function?
- reflexes
- muscle tone
- presence of atrophy
THIS IS THE MPST IMPORTANT PART OF THE NEURO EXAM
what are the 2 ways to assess reflexes? describe reliability
- myotactic (tendon reflexes)
- withdrawal reflex (flexor reflexes)
reliability varies by which reflex is being assessed
describe a monosynaptic reflex arc, give an example
there is one synapse involved int he reflex; composed of sensory component (afferent) that synapses with a LMN an efferent component out to cause a response
an example of this is the patellar reflex
compare and contrast extrafusal and intrafusal muscle fibers
muscle spindles are proprioceptive organs located throughout skeletal muscle (size and shape of a grain of rice) contained within a connective tissue capsule; these spindles are stretched when the muscle lengthens, causing the sensory neuron in the spindle to transmit an impulse to the spinal cord, where it will synapse with alpha motor neurons, determining the amount of contraction needed to overcome a certain resistance
muscle fibers within the spindle are called intrafusal; these provide sensory information on muscle length and change in length
muscle fibers outside the spindle are called extrafusal, make up the bulk of the muscle, run tendon to tendon or ligament to ligament, and are controlled by larger alpha motor neurons
what are the 4 parts of the lower motor neuron unit? describe how they are all related
- LMN cell body
- axon
- neuromuscular junction
- muscle
need all 4 components for a reflex, so a lesion in any part will look the same externally (absent or reduced reflex)
what are the 5 functional segments of the spinal cord? what is meant by functional?
- C1-C5: UMN only
- C6-T2: LMN to thoracic limbs
- T3-L3: UMN only to pelvic limb
- L4-S1: LMN to pelvic limb
- S1-3 to S5 +/- caudal segments
function means that a focal or a diffuse lesion in this area will look the same clinically
what are the spinal cord segments for lower motor neuron units?
thoracic limbs: C6-T2
pelvic limbs: L4-S1
what is an intumescence? what are the 2 kinds?
intumescence is a spinal cord swelling of the ventral horn where all cell bodies come together before leaving as a named nerve
cervical intumescence: cell bodies for nerves that innervate the thoracic limb;
this is located at spinal cord segment C6-T2, between vertebrae C5-T1
lumbar intumescence: cell bodies for nerves that innervate the pelvic limb; this is located at spinal cord segment L4-S1, between vertebrae L3-L6
what is found in spinal cord segment C6-T2? (3)
- cervical intumescence
- brachial plexus
- nerves of the thoracic limb
what is found in spinal cord segments L4-S1 and S1-S3 (S5)? (4)
- lumbar intumescence
- lumbar/lumbosacral plexus
- nerves of the pelvic limb
- nerves of the bladder/bowel
give the vertebral location of the end of the spinal cord in dogs, cats, ruminants, swine, and horses
dogs: L6-L7
cats: L7-S3
ruminants: L6-S1
swine: S1-S2
horses: S2
do myotactic and withdrawal reflexes rely on the nervous system cranial or caudal to their reflex arc? how does this help with localization?
no! reflexes just speak to their reflex arc (you can have these reflexes in recently amputated limbs)
this helps you localize, meaning that is the reflex is not normal, then the local LMN is the source of the lesion
how is the thoracic limb assessed? (3)
- withdrawal
- tone
- muscle mass
what reflex is used to assess the thoracic limb? what nerves does it asses?
withdrawal reflex; every animal has it and it is reliable!
assesses all peripheral nerves in thoracic limb
what does atrophy of the suprasinatus and infraspinatus muscles suggest?
damage to suprascapular nerve; located at spinal cord segments C5, C6, C7
what does atrophy of the biceps brachii!!, brachialis, or coracobrachialis suggest?
damage to the musculocutaneous nerve; located at spinal cord segments C6 and C7
what does atrophy of the triceps brachii, extensor carpi radialis, ulnaris lateralis, common digital extensor, and lateral digital extensor suggest?
damage to the radial nerve; located at spinal cord segments C7, C8, T1 and T2
what is the most important nerve of the thoracic limb?
the radial nerve! provides innervation to the extensors; cannot support weight without radial innervation
what reflex is used to assess the pelvic limb? what peripheral nerve does it assess?
patellar reflex! assesses femoral nerve
how is the pelvis limb assessed? (4)
- patellar reflex
- withdrawal
- tone
- muscle mass
what does atrophy in the iliopsoas, quadriceps, and sartorius suggest?
damage to femoral nerve, located L4-L6
what does atrophy of the biceps femoris, semimembranosus, and semitendinosus suggest?
damage to sciatic nerve, located L6-L7, S1-S2
what does atrophy of the peroneus longus, lateral digital extensor, long digital extensor, and cranial tibial suggest?
damage to the common fibular nerve
what does atrophy of the gastrocnemius, popliteus, superficial digital flexor, and deep digital flexor suggest?
damage to the tibial nerve
what is seen clinically if the femoral nerve is damaged? what reflex assesses this nerve?
animal can’t support weight on the stifle; patellar reflex assesses sensory and motor function of femoral nerve
what does the withdrawal reflex assess in the pelvic limb?
the sciatic nerve
if an animal is walking plantigrade, what nerve is damaged?
tibial nerve
what is the cutaneous trunci reflex?
assesses the segmental sensory neurons of the lateral thoracic nerve that ascend via interneurons from C8-T1
how are the sacral segments assessed? (3)
- perineal reflex: assesses pudendal nerve, located S1-S3
- anal sphincter tone: assesses pudendal nerve, located S1-S3
- tail tone: assesses caudal segments
define cutaneous area, overlap zone, and autonomous zone
cutaneous area: where a nerve innervates the skin
over lap zone: regions that multiple nerves innervate
autonomous zone: a region innervated by only one nerve
describe the autonomous zones of the thoracic limb and paw (3)
- dorsal part of limb and paw: radial nerve
- ventral part of limb and digit 5 : ulnar nerve
- ventral paw and digits 2-4: median nerve
describe the autonomous zones of the pelvic limb
- dorsal limb and paw: fibular nerve (branch of sciatic nerve)
- ventral limb and paw (digits 2-5): tibial nerve (branch of sciatic nerve)
- digit 1: femoral nerve, saphenous branch, but less reliable so stay away from the first digit!
what is plegia?
paralysis; lack of voluntary movement
what is paresis?
weakness; decreased voluntary movement
describe tetra, para, and mono paresis
tetraparesis: all 4 limbs
paraparesis: pelvic limbs
monoparesis: single limb
weakness
what are the 3 aspects of gait seen with LMN deficits?
- short-strided, choppy: can look like lameness from musculoskeletal disease or LMN deficit
- crouched stance: overflexion of joints; posture with limbs under the body
- exercise intolerance
what is something to be careful of when assessing postural reactions?
if the patient’s weight is supported by a vet or assistant, this can compensate for a LMN deficit; allowing the patient to walk or replace their limb even though they are actually abnormal
the LMN tipoff!! when supporting weight during assessment is exercise intolerance
describe the flaccid paresis/paralysis observed with LMN deficits (3)
- hyporeflexia or areflexia
- decreased muscle tone
- muscle atrophy
how does tick paralysis work? (mechanism)
the neurotoxin binds to presynaptic vesicles and inhibits ACh release
describe the progression of tick paralysis
- clinical signs begin 5-7 days after tick infestation
- once clinical signs occur, there is rapid progression of the disease (24-48 hours)
- ascending weakness; from pelvic to thoracic limbs
- cranial nerves are rarely affected
how can tick paralysis be distinguished from LMN disease?
there is no short and choppy stride or ataxia
describe acute canine polyridiculoneuritis
is an idiopathic immune reaction to infection (potentially campylobacter, definitely found in raccoon saliva) as antibodies cross react with axons and cause inflammation of the spinal roots (both ventral and dorsal); can affect cranial nerve VUU and cause facial muscle weakness
what are the clinical signs (3) and treatment of polyrediculoneuritis?
clinical signs:
1. generalized weakness
2. hyperesthesia!!
3. muscle atrophy
treatment: supportive care, recovery can be days to weeks to months
what is the pathology of myasthenia gravis?
- autoantibodies are directed against the AChR; bind to and block the AChR on the muscle
- can be primary (idiopathic) or secondary (due to treating a thymoma with methimazole in cats)
what are the clinical signs of myasthenia gravis? (5)
- episodic exercise intolerance that worsens with exercise and improves with rest
- preserved reflexes (palpebral reflex)
- NO muscle atrophy
- cranial nerves may be affected: facial or pharyngeal/laryngeal
- regurgitation: due to megaesophagus
how is myasthenia gravis diagnosed?
a tensilon test: give animal an ultrashort acting, nonspecific acetylcholine esterase blocker; if animal improves that means has myasthenia gravis
describe the pathology of botulism
the clostridium botulinum is absorbed from the GI tract, reaches the NMJ and binds to presynaptic nerve terminal to inhibit the release of ACh
what are the clinical signs of botulism?
NOSE TO TAIL
generalized weakness:
1. facial weakness
2. pharyngeal/laryngeal weakness
3. appendicular
4. anal sphincter
5. tail
describe treatment of botulism
- supportive care
- antitoxin: is ineffective when clinical signs are evident as the toxin is already bound and within the nerve terminal
