ICL 6.3: Upper and Lower Motor Neurons

Question 1

Where does the corticospinal tract originate?

Answer 1

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

Question 2

What is a homonculus?

Answer 2

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

Question 3

What is the pathway of the corticospinal tract?

Answer 3

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

Question 4

What is Weber syndrome?

Answer 4

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

Question 5

What is the function of the spinothalamic tract?

Answer 5

Conveys pain, temperature and crude touch

The tract is located in the periphery of the spinal cord

Question 6

What is the pathway of the spinothalamic tract?

Answer 6

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

Question 7

What is the function of the DCML tract?

Answer 7

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

Question 8

What is the pathway of the DCML?

Answer 8

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

Question 9

What are the negative signs of upper motor neuron injuries?

Answer 9

1. Paralysis or paresis or plegia
2. Loss of control is more pronounced for distal muscles
3. Loss of superficial abdominal and cremasteric reflexes

Negative sign = you lose something you normally have

Question 10

What are the positive signs of upper motor neuron injuries?

Answer 10

These signs develop over time due to increased abnormal motor function

1. Spacticity (usually extension in the legs and flexión of the arms)
2. hypertonia = muscle stiffness, increased tone
3. Hyperreflexia; Babinski sign
4. Clonus

Positive sign = you have a symptom that you don’t normally have

Question 11

What is clonus?

Answer 11

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

Question 12

What are the signs of a lower motor neuron problem?

Answer 12

1. Weakness
2. Hypotonia; flaccidity
3. Decreased deep tendon reflexes
4. Atrophy
5. Fasciculations

Question 13

What is effected with a lower motor neuron lesion?

Answer 13

The motor unit!

Motor unit = anterior horn cell + nerve + neuromuscular junction + muscle fibers innervated by nerve

Question 14

Which diseases effect the lower motor neuron?

Answer 14

1. ALS (but is also upper motor neuron too!)
2. Poliomyelitis
3. West Nile virus

Question 15

Which diseases effect the peripheral nerve?

Answer 15

So this is still related to lower motor neurons

1. Guillén Barre
2. Tick paralysis
3. Diphthiric, porphyric, arsenic polyneuropathy
4. Shellfish poisoning

Question 16

Which diseases effect the neuromuscular junction?

Answer 16

So this is still to do with the lower motor neuron

1. Myasthenia gravis = weakness as the day goes on because ACh receptors are targeted
2. Botulism
3. lambert-Eaton myasthenia syndrome
4. Hypermagnesemia

Question 17

Which diseases effect the muscle?

Answer 17

So this is related to lower motor neurons

1. Duchenne muscular dystrophy

Question 18

What are fasciculations?

Answer 18

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

Question 19

How can you test the DCML tract through a sensory exam?

Answer 19

DCML

So you test for the joint position sense and vibratory sense

1. 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
2. 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

Question 20

How can you test the spinothalamic tract through a sensory exam?

Answer 20

You’re testing pain and temperature

1. 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
2. Temperature = use a test tube with either cold or warm water or a cold tuning fork to determine prescience or extent of a lesion

Question 21

Where would you lose peripheral sensory loss with a C8-T1 radiculopathy?

Answer 21

Down the medial forearm and the pinky and ring finger aka the ulnar nerve!

Question 22

Where would you have peripheral sensory loss with an axillary mononeuropathy?

Answer 22

The outside shoulder

Question 23

Where would you have peripheral sensory loss with a lateral femoral cutaneous mononeuropathy?

Answer 23

The lateral right thigh lol

Question 24

Where would you have peripheral sensory loss with carpal tunnel syndrome?

Answer 24

This is effecting your medial nerve so your first 3 fingers on the palm side

Question 25

How would you test for a nerve problem?

Answer 25

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

Question 26

What is transverse myelitis?

Answer 26

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

Question 27

What is subacute combined degeneration?

Answer 27

A vitamin B12 deficiency that effects the spinal cord; specifically corticospinal and DCML tracts

So you’ll get paralysis and sensory loss

Question 28

What is ALS?

Answer 28

Upper and lower motor neuron disease!

So you’ll see paralysis, flacidity and spasticity!

Question 29

What is brown-Sequard syndrome?

Answer 29

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)