TUTORIAL: Cerebral cortex

Question 1

What is multiple sclerosis (MS)?

Answer 1

MS is an autoimmune disorder which results in the loss of myelin from neurons of the central nervous system, i.e. the brain and spinal cord.

Question 2

What are some of the main symptoms of MS?

Answer 2

Blurred vision, fatigue, difficulty walking, numbness or tingling (paraesthesia) in different parts of the body, muscle stiffness and spasms

Question 3

What are the general effects of peripheral nerve stimulation?

Answer 3

An electrical stimulus to a peripheral nerve (specifically to the middle of the nerve) can activate, either:
a) the motor axons alone
b) both sensory and motor axons

Question 4

When/ How is an M wave response caused?

Answer 4

When a motor axon alone is activated when the peripheral nerve is stimulated by an electrical impulse:
- The activation of the motor axons can cause action potentials to travel along the nerve
- From the stimulus to the muscle (usual direction)
- to cause muscle contraction, a twitch.
- This FAST response is called the M wave
(a.k.a direct motor response)

Question 5

When/ How is an H-reflex (wave) response caused?

Answer 5

When both a sensory and motor axon have been activated by the peripheral nerve stimulation via an electrical impulse: (when the stimuli is small)

At the same time that the AP’s travel in the motor axons producing M waves, the same stimulus can cause activation of the sensory axons (the subject feels the stimulus):
- The action potentials can travel along the nerve to the spinal cord.
- Sensory neuron synapses with the motor neuron
- These can then cause the lower motor neurons in the spinal cord to become activated
- Action potentials in the motor axons can travel along the motor neuron to the muscle where they cause muscle contraction, a twitch.
- This is a reflex activation of the muscle.
- This later response is called the H-reflex (or Hoffmann reflex)

Question 6

Does the H reflex start directly after the M wave when both the sensory and motor axons are activated?

Answer 6

NO;
there is a short delay because sensory axons have sent signals back to the spinal cord which have activated motor neurons

Question 7

Where/ How are F wave responses caused?

Answer 7

When the electrical stimulus is much larger (so it becomes painful) it causes activation of the motor axons to conduct antidromically. (sets up AP’s to travel in the motor axons but the wrong way):

• These action potentials travel along the motor nerve back towards the spinal cord instead of the muscle (in the opposite way to normal)
• These can then cause the lower motor neurons in the spinal cord to become activated.
• Action potentials in the motor axons can travel along the motor neuron to the muscle where they cause muscle contraction, a twitch.
• This later response is called the F-wave, it is not a reflex.

Question 8

When a very large painful stimulus is applied to a peripheral nerve, causing F wave response, what would you see on an EMG?

Answer 8

1. Stimuli artefact (the interference b/t the stimulus electrodes and the recording electrodes)
2. VERY LARGE M wave (we’ve activated many, many motor axons in the nerve & they sent impulses towards the muscle)
3. F wave: a bit later, the AP’s that had traveled the wrong way (that went to the spinal cord then turned around) are now producing the F wave

Question 9

What is orthodromic conduction?

Answer 9

Orthodromic - travelling in the normal direction in a nerve fibre

Question 10

In which direction would Orthodromic conduction of the sensory axon occur?

Answer 10

“Orthodromic”: travelling in the normal direction in a nerve fibre

sensory: from periphery -> spinal cord

Question 11

What is Antidromic conduction?

Answer 11

travelling in the opposite direction to that normal in a nerve fibre

Question 12

In which direction would Orthodromic conduction of the motor axon occur?

Answer 12

“Orthodromic”: travelling in the normal direction in a nerve fibre

motor: spinal cord -> muscle

Question 13

How is brain stimulation used to stimulate the entire motor pathway?

Answer 13

Cortical motor stimulation using transcranial magnetic stimulation (TMS):
- Use a magnetic stimulating coil over the motor cortex
- The activation of the upper motor neurons causes action potentials to travel along the entire motor pathway (upper and lower motor neurons) to cause muscle contraction.
- This EMG response is known as a motor evoked potential (MEP).

Question 14

How can we use both brain and peripheral nerve stimulation to assess for MS and confirm the central nature of MS?

Answer 14

We can calculate the central and peripheral motor conduction times and compare the normal with MS patients to see where the issue is (in the PNS or CNS)

Question 15

What is meant by TMCT?

Answer 15

“Total motor conduction time”
total time to conduct pulses from the motor cortex all the way to the muscle

Question 16

What is meant by the MEP Latency?

Answer 16

time taken for the pulses to go from brain to muscle

Question 17

What is meant by PMCT? How can you calculate PMCT?

Answer 17

“Peripheral motor conduction time”
Time from spinal cord to muscle along motor axon
- Can be calculated using the formula
PMCT = (M latency + F latency-1) /2
(The -1 is the estimated time for the action potentials arriving at the lower motor neuron cell body to turn around)

Question 18

How do you calculate central motor conduction time?

Answer 18

(CMCT) = TMCT (Total motor conduction time) - PMCT (Peripheral motor conduction time)

Question 19

What effect does MS have on the central and peripheral motor conduction time?

Answer 19

1. Brain stimulation - longer than usual MEP latency.
   Problem along upper motor neurons, lower motor neurons or both – we don’t know!
   - [Total motor conduction time - delayed]
   BUT if there is also:
2. Peripheral nerve stimulation – normal F wave latency. We can assume there is no issue with lower motor neurons
   [Peripheral motor conduction time - normal]

THEREFORE
MS is a problem in the CNS