2.3 - Cerebral cortex tutorial

Question 1

What is multiple sclerosis (MS)?

Answer 1

Autoimmune disorder which results in loss of myelin from neurons of the CNS (brain and spinal cord)

Question 2

What are some of the main symptoms of MS? (6)

Answer 2

• blurred vision
• fatigue
• difficulty walking
• numbness/tingling (paraesthesia) in different parts of the body
• muscle stiffness
• spasms

Question 3

What are the basics of peripheral nerve stimulation?

Answer 3

• stimulus stimulates both sensory and motor neuron
• stimulus –> sensory neuron –> lower motor neuron –> muscle

Question 4

What is an M-wave?

Answer 4

• an electrical stimulus of an appropriate intensity to a peripheral nerve can activate sensory and motor axons
• the activation of motor axons can cause action potentials to travel along the nerve to cause muscle contraction (twitch)
• can be recorded with electromyography (EMG)
• this fast response (8ms) is called the M(motor)-wave

Question 5

What is the H-reflex (wave)?

Answer 5

• the same stimulus can cause activation of the sensory axons (subject feels the stimulus)
• APs can travel along the nerve to the spinal cord = cause the lower motor neurons in the spinal cord to become activated
• APs in the motor axons can travel along the motor neuron to the muscle where they cause muscle contraction (twitch)
• this is reflex activation of the muscle
• this later response is called the H-reflex

Question 6

What is the F-wave?

Answer 6

• a large electrical stimulus can cause activation of the motor axons to conduct antidromically
• these APs travel along the motor nerve to the spinal cord (i.e. the opposite way to normal)
• these can then cause the lower motor neurons in the spinal cord to become activated
• APs in the motor axons can travel along the motor neuron to the muscle where they cause muscle contraction (twitch)
• this later response is called the F-wave and is not a reflex

Question 7

Compare the activation type of M, H and F-waves.

Answer 7

• M-wave: purely motor activation
• H-wave: sensory then motor activation
• F-wave: motor then motor activation

Question 8

What does orthodromic mean?

Answer 8

Travelling in the normal direction in a nerve fibre

Question 9

What does antidromic mean?

Answer 9

Travelling in the opposite direction to normal in a nerve fibre

Question 10

How is cortical motor stimulation done?

Answer 10

Using transcranial magnetic stimulation (TMS)

Question 11

How does cortical motor stimulation work?

Answer 11

• activation of the upper motor neurones causes APs to travel along the entire motor pathway (upper and lower motor neurones) to cause muscle contraction
• this EMG response is known as a motor evoked potential (MEP)

Question 12

What is total motor conduction time (TMCT)?

Answer 12

Time from brain to muscle (MEP latency)

Question 13

What is peripheral motor conduction time (PMCT)?

Answer 13

• time from spinal cord to muscle along motor axon
• PMCT = (M latency + (F latency-1)) / 2
• the -1 the estimated time for the APs arriving at the lower motor neuron cell body to turn around

Question 14

What is central motor conduction time (CMCT)?

Answer 14

• how long it takes from stimulus to brain to travel down upper motor neuron in spinal cord
• TMCT - PMCT

Question 15

What is the effect of MS on conduction times?

Answer 15

• brain stimulation - longer than usual MEP latency
• problem along UMN, LMN or both - we do not know
• TMCT delayed
• peripheral nerve stimulation - normal F-wave latency = no issue with lower motor neurons
• PMCT normal
• therefore problem is in CNS