2.3 - Cerebral cortex tutorial Flashcards
What is multiple sclerosis (MS)?
Autoimmune disorder which results in loss of myelin from neurons of the CNS (brain and spinal cord)
What are some of the main symptoms of MS? (6)
- blurred vision
- fatigue
- difficulty walking
- numbness/tingling (paraesthesia) in different parts of the body
- muscle stiffness
- spasms
What are the basics of peripheral nerve stimulation?
- stimulus stimulates both sensory and motor neuron
- stimulus –> sensory neuron –> lower motor neuron –> muscle
What is an M-wave?
- 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
What is the H-reflex (wave)?
- 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
What is the F-wave?
- 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
Compare the activation type of M, H and F-waves.
- M-wave: purely motor activation
- H-wave: sensory then motor activation
- F-wave: motor then motor activation
What does orthodromic mean?
Travelling in the normal direction in a nerve fibre
What does antidromic mean?
Travelling in the opposite direction to normal in a nerve fibre
How is cortical motor stimulation done?
Using transcranial magnetic stimulation (TMS)
How does cortical motor stimulation work?
- 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)
What is total motor conduction time (TMCT)?
Time from brain to muscle (MEP latency)
What is peripheral motor conduction time (PMCT)?
- 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
What is central motor conduction time (CMCT)?
- how long it takes from stimulus to brain to travel down upper motor neuron in spinal cord
- TMCT - PMCT
What is the effect of MS on conduction times?
- 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