Lab 4 - Neurophysiology studied in humans

Question 1

Describe the reflex pathway for the quadriceps stretch reflex

Answer 1

Muscle spindles (stretch receptors) are stimulated when the quadriceps muscle is stretched. Signal is sent via afferent fibres to the spinal cord. Information is processed at the level of the spinal cord and via an interneuron, the signal causes the actiavtion of motor neurons to extensor muscle and inhibits motor neurons to the flexor muscles to prevent overstretching

Question 2

Reciprocal innervation meaning?

Answer 2

Describes muscles existing as anatagonistic pairs with contraction of one muscle producing forces opposite to those generated by the contraction of the other (agonist and antagonist)

Question 3

What is the role of the antagonist flexor muscles in the quadriceps stretch reflex

Answer 3

Prevent overstretching by relacing

Semitendinosus

Question 4

What are the functions of the stretch reflex? Why does it need to be rapid?

Answer 4

Protects the inhibited anatgonist muscles from being injured from excessive overstretching (therefore circuit that skips the brain and occurs at the level of the spinal cord)

Question 5

Compound potential

Answer 5

If an electrical impulse is used to stimulate a nerve that innervates a muscle, a compound potential can be recorded from the muscle, which is the result of synchronised depolarisation of a large number of fibres in that muscle, such a response is called a wave

in this lab see two different waves called M and H wave and also see the T-reflex

Question 6

M wave means=

Answer 6

Motor

Question 7

M wave …

Answer 7

Motor - is a compount muscle action potential that results from direct stimulation of the motor nerve innervating that muscle

Question 8

H wave means =

Answer 8

Reflex

Question 9

H wave…

Answer 9

Reflex - is a compound muscle action potential that is a monosynaptic reflex response to sensory nerve stimulation

Question 10

T-reflex …

Answer 10

is a compound muscle action potential that is a monosynaptic reflex response to tendon-jerk i.e. muscle spindle stretch

Question 11

Excitation contraction coupling =

Answer 11

electrical

Question 12

cross bridge cycling =

Answer 12

mechanical (from calcium onwards)

Question 13

When we increase the strength of voluntary movements:

Answer 13

• There is Increased recruitment of motor units occurs…
• Therefore, increased numbers of muscle fibers activated…
• So, increased electrical activity along muscle…
• Resulting in increased EMG amplitude

Question 14

1. What are the physiological processes that generate the electrical EMG waves?

Answer 14

Electromyogram (EMG): recording of electrical activity generated at skeletal muscle (currents flowing along sarcolemma during action potentials)

Question 15

Motor unit

Answer 15

1 alpha motor neuron with all the fibres it innervates

Question 16

2. How are electrical events that generate the EMG waves related to the mechanical events that generate ‘twitches’?

Answer 16

• Electrical events (generate EMG waves)...
       • Local depolarisation of 
         sarcolemma
         (End-plate potential- EPP)
       • Action potential along 
         sarcolemma and
          down T-tubule
• ...lead to release of Ca2+ to initiate cross-
bridge formation
• Mechanical events (physical twitches)
     • Cross-bridge formation & 
      cycling to create tension

Question 17

Was there significant difference in conduction velocity between males and females?

Answer 17

no significant different in conduction velocity between female and male

Question 18

What factors determine conduction velocity in axons?

Answer 18

• Diameter (larger diameter = faster conduction)
• Myelination (more myelination = faster
conduction)

Question 19

Explain why nerve conduction velocity should be determined using two stimulation sites and not one?

Answer 19

• Want to determine fastest velocity along ulnar
nerve only

• At the muscle, the process slows down:
• axons branch/get smaller
• neuromuscular transmission at the NMJ
• generation of end-plate potential on the
sarcolemma

would record NMJ in hand which would show as slower than the electrical energy in the nerve

Question 20

Monosynaptic stretch reflex

Answer 20

stretching a muscle also stretches muscle spindles and activates sensory axons, which synapse directly onto, and stimulate alpha motor neurons in the spinal cord. Action potentials generated in the motor neurons propagate along their axons to the muscle, where contraction is evoked. This is the classic monosynaptic stretch reflex

• Induced with muscle stretch • Reproducible
• Quick

Question 21

What factors determine the latency of the T-reflex in the soleus muscle?

Answer 21

• Path length (height) e.g. longer legs vs shorter legs
• Axon myelination (demyelination leads to slower reflexes which is what happens for diabetics )
• Axon diameter

Question 22

Astronauts and soleus muscle experiment

Answer 22

done on returning astronauts - reflex arc becomes weaker and nerves change
used for presercation of muscles and reflex arcs, to prepare them for coming back to earth (stimulate them artificially)

Question 23

stimulation and recording sites and the direction that the action potentials travel on the following diagram.

Answer 23

APs propagate in BOTH directions in recruited axons

At threshold voltage:
• 1a sensory axons tend to be recruited first
• Motor neurons are activated via Hoffmann
Reflex which is similar to T-reflex but induced by direct stimulation of sensory axons, not muscle spindle stretch
• H-waves on EMG (muscle innervation via reflex path)

As voltage increases:
• M-waves begin to appear when some motor axons are directly stimulated.
• H-wave amplitude increases when more sensory axons are recruited, which will stimulate more motor neurons via reflex arc
Note: directly stimulated motor neurons are in refractory period so won’t fire again via reflex arc

At highest voltage (100V):
• M-wave amplitude at maximum when all site motor axons are directly stimulated
• H-wave amplitude disappears when all motor axons are refractory

Question 24

H wave disappears when

Answer 24

all motor neurons are refractory

Question 25

AP propagates in both directions in recruited axons… elaborate….

Answer 25

axons cannot control direction so when stimulated fire both ways
The reason why the H wave disappears first is due to refractory periods - so the acon sending H waves cannot activate the synapse since the other axon sending the M wave is in its absolute (or relative) refractory period
Can get both waves because some axons will be in refractory
think of it like a race, voltage increases, M wave faster than H wave

Question 26

M wave vs H wave and propagation of APs

Answer 26

M wave has a shorter distance
M direct stimulation of muscle so shorter latency, M as max when all recruited and all motor in refractory

H wave - sensory neurons signal goes to synapse and activates the motor neuron to get a response, initially these at lower voltage due to large diameter meaning they can send the signal

increasing voltage, M wave because more become activated - more and more axons are going into refractory

Question 27

Diabetes mellitus

Answer 27

A group of metabolic diseases resulting in high blood glucose
(hyperglycemia)

Question 28

Diabetic Peripheral Neuropathy: nerve damage due to diabetes

Answer 28

Burning, tingling, numbness, pain in periphery

Weakness/swelling in peripheral joints

Question 29

Suspected causes of nerve damage in diabetes mellitus are chronic…

Answer 29

• high blood glucose (hyperglycemia)
• damage to blood vessels (reduced blood flow to nerves)
• inflammation
• low insulin levels/insulin insensitivity
• high adipose levels

Question 30

Would the conduction velocity in Mr DPN’s tibial nerve be affected by his diabetes? Explain.

Answer 30

Yes, conduction velocity will be reduced, possibly due to consequences of hyperglycemia…
• Metabolic factors
• E.g. Altered Na+/K+ ATPase activity

• Nerve damage
• E.g. Inflammation from pro-inflammatory immune cells causes damage
to nerve tissue

• Myelin abnormalities
• E.g. Demyelination – insufficient current to depolarise at next node of Ranvier (action potential fails)

can affect autonomic which innervates enteric nervous system therefore a range of gut problems can arise

Question 31

Will the strength of Mr DPN’s stretch reflex, activated by tapping the Achilles tendon, be affected by his diabetes? Explain.

Answer 31

• Yes, weaker response because fewer sensory axons and motor units would be recruited (lower reflex amplitude) and longer latency due to…
  
  • Nerve damage
  • Myelin abnormalities

yes weaker and slower due to loss of myelin and damaged axons

Question 32

What would you expect to see if you recorded M and H-waves from Mr DPN’s soleus muscle?

Answer 32

• Lower amplitude of M-waves and H-waves and longer latency, compared with a healthy individual (or pre-diabetic recordings taken from Mr DPN).
• Same reasons as above