ICL 6.3: Upper and Lower Motor Neurons Flashcards
Where does the corticospinal tract originate?
55% in the precentral gyrus = Brodman’s area 4 and the Betz cell
35% from the central gyrus = Brodman’s areas 1, 2, 3
10% from other frontal and parietal areas
The arrangement of control is along a homunculus
What is a homonculus?
The arrangement of the corticospinal tract along the lateral surface of the left cerebral hemisphere
It tells you how many neurons get delegated to each part of the body like your fingers get more than your leg because they have to do finer movements
What is the pathway of the corticospinal tract?
They start in the precentral gyrus of the cerebral cortex and then they travel through the anterior portion of the posterior limb of the internal capsule
Then the fibers travel in the cruz = the most ventral part of the midbrain; this is also where CN 3 also exits the midbrain
The fibers continue down and decussate in the lower ventral medulla in the pyramids —> most fibers continue in the cord as the lateral corticospinal tract while a small percent descend as the anterior corticospinal tract
Then in the spinal cord the fibers synapse on alpha and gamma motor neurons in the Rexed lamina 9 in the anterior horn of the grey matter
Then an anterior horn cell will emerge and go to the target aka a muscle
What is Weber syndrome?
a stroke of the ventral midbrain effecting the cerebral peduncles which contain CN 3 and the corticospinal tract
CN3 comes out of the midbrain at the same place the corticospinal tract passes through so a stroke in this region causes ipsilateral third nerve palsy with contralateral hemiparesis
What is the function of the spinothalamic tract?
Conveys pain, temperature and crude touch
The tract is located in the periphery of the spinal cord
What is the pathway of the spinothalamic tract?
Sensory fibers —> substantia gelatinosa of ipsilateral dorsal horn —> decussate right when they enter the spinal cord after synapsing in rexed lamina II —> synapse in VPL of thalamus after passing through the posterior limb of the internal capsule —> primary and secondary somatosensory cortex which is Brodman’s area 3,1, and 2
In the internal capsule the spinothalamic after EMT fibers travel through the posterior aspect of the posterior limb of the internal capsule
What is the function of the DCML tract?
Tactile discrimination, vibration, joint position sense
Position sense = muscle spindles, golgi tendon organs
Vibration = pancinian corpuscles
Superficial touch - Meissen corpuscles
Large myelinated, fast conducting nerve fibers are what convey this information
What is the pathway of the DCML?
Skin sensation —> afferent sensory nerve —> ipsilateral dorsal column —> dorsal medulla —> synapse in nucleus gracilus and cuneatus —> accurate fibers —> decussate in the medial lemniscus of the medulla —> ascend to VPL of thalamus —> project through posterior limb of the internal capsule to the post central gyrus of the cortex
So this decussate in the lower medulla
What are the negative signs of upper motor neuron injuries?
- Paralysis or paresis or plegia
- Loss of control is more pronounced for distal muscles
- Loss of superficial abdominal and cremasteric reflexes
Negative sign = you lose something you normally have
What are the positive signs of upper motor neuron injuries?
These signs develop over time due to increased abnormal motor function
- Spacticity (usually extension in the legs and flexión of the arms)
- hypertonia = muscle stiffness, increased tone
- Hyperreflexia; Babinski sign
- Clonus
Positive sign = you have a symptom that you don’t normally have
What is clonus?
A sign of upper motor neuron injuries and it’s an oscillatory movement in the foot due to exaggerated reflex
A muscle stretch elicits alternating contractions of agonist and antagonist muscle groups
The hypothesis is that spacticity is due to a loss of inhibitory descending input that keeps the stretch reflex from being overactive
What are the signs of a lower motor neuron problem?
- Weakness
- Hypotonia; flaccidity
- Decreased deep tendon reflexes
- Atrophy
- Fasciculations
What is effected with a lower motor neuron lesion?
The motor unit!
Motor unit = anterior horn cell + nerve + neuromuscular junction + muscle fibers innervated by nerve
Which diseases effect the lower motor neuron?
- ALS (but is also upper motor neuron too!)
- Poliomyelitis
- West Nile virus
Which diseases effect the peripheral nerve?
So this is still related to lower motor neurons
- Guillén Barre
- Tick paralysis
- Diphthiric, porphyric, arsenic polyneuropathy
- Shellfish poisoning
Which diseases effect the neuromuscular junction?
So this is still to do with the lower motor neuron
- Myasthenia gravis = weakness as the day goes on because ACh receptors are targeted
- Botulism
- lambert-Eaton myasthenia syndrome
- Hypermagnesemia
Which diseases effect the muscle?
So this is related to lower motor neurons
- Duchenne muscular dystrophy
What are fasciculations?
A sign of lower motor neuron problems
It’s a spontaneous contraction of motor units
The generator source remains unknown though the spinal cord and peripheral nerve have been implicated
May occur in anterior horn cell disease like ALS, radicals patchy, nerve entrapment, in muscular pain fasciculations syndrome or it may be benign
How can you test the DCML tract through a sensory exam?
DCML
So you test for the joint position sense and vibratory sense
- Joint position sense = with patient’s eyes closed, grasp the sides of the thumb or great toe and flex or extend and ask the patient which direction the digit is pointing
- Vibratory sense = test by using a tuning fork and place it over a bony prominence; most people should be able to perceive vibration for at least 10 seconds
How can you test the spinothalamic tract through a sensory exam?
You’re testing pain and temperature
- Pain = use a sharp object like a broken wooden q-tip or pin to test sensation —> test in dermatome or cutaneous nerve distributions to determine presence or extent of a lesion
- Temperature = use a test tube with either cold or warm water or a cold tuning fork to determine prescience or extent of a lesion
Where would you lose peripheral sensory loss with a C8-T1 radiculopathy?
Down the medial forearm and the pinky and ring finger aka the ulnar nerve!
Where would you have peripheral sensory loss with an axillary mononeuropathy?
The outside shoulder
Where would you have peripheral sensory loss with a lateral femoral cutaneous mononeuropathy?
The lateral right thigh lol
Where would you have peripheral sensory loss with carpal tunnel syndrome?
This is effecting your medial nerve so your first 3 fingers on the palm side
How would you test for a nerve problem?
Electromyography
Nerve conduction studies evaluate the function of a nerve and can diagnose for focal or generalized neuropathy and in combination with EMG it can diagnose nerve root disease or plexopathy
So this can help diagnose carpal tunnel, peripheral neuropathy, ALS and myopathy
What is transverse myelitis?
Inflammatory condition of the cord from infection, inflammation, autoimmune etc.
You’re looking for bilateral paralysis, bilateral sensory loss = cord problem!
It can effect different levels of the spinal cord and you’d see effects from the point of injury and below
It can effect the different tracts running through the spinal cord
What is subacute combined degeneration?
A vitamin B12 deficiency that effects the spinal cord; specifically corticospinal and DCML tracts
So you’ll get paralysis and sensory loss
What is ALS?
Upper and lower motor neuron disease!
So you’ll see paralysis, flacidity and spasticity!
What is brown-Sequard syndrome?
Compression on one side of the cord
So you’ll have ipsilateral motor weakness (corticospinal) and ipsilateral vibration loss (DCML) but contralateral pain/temperature loss (spinothalamic)
