Neuromuscular control

Question 1

what are the lower motor neurons of brainstem and spinal cord?

Answer 1

alpha motor neuron (antra/anterior horn cells)

Question 2

what do alpha motor neurons do?

Answer 2

innervate extrafusal muscles fibres of skeletal muscles

Question 3

what does activation of alpha motor neurons do?

Answer 3

causes muscle contraction

Question 4

what is a motor neuron pool?

Answer 4

all alpha motor neurons that innervate a single muscle

Question 5

what is a motor unit?

Answer 5

single motor neuron together with all the muscle fibers that it innervates

smallest functional unit with which to produce a force

Question 6

how many fibres does a motor neuron supply?

Answer 6

600 muscle fibres

Question 7

what does stimulation of one motor unit cause?

Answer 7

contraction of all muscle fibres in that unit

Question 8

can there be cross motor neuron stimulation?

Answer 8

no- none of the muscle fibres innervated by 1 motor neurone will be innervated by a different motor neuron also

Question 9

what are the types of motor unit?

Answer 9

Slow, (S, type 1)

Fast, fatigue resistant (FR, type II)

fast, fatigable (FF, type III)

Question 10

what are the differences in diameter cell bodies for each type motor unit?

Answer 10

Slow= smallest

FF= largest

Question 11

what are the differences in dendritic tree sizes between motor unit?

Answer 11

S= small

FF= largest

Question 12

what are the differences in axon thickness between difference types motor unit?

Answer 12

S= thinnest

FF= thickest

Question 13

what is the difference in conduction velocity in the different types of motor unit?

Answer 13

S= slowest

FF= fastest

Question 14

what is the response to repeated stimulation at level that evokes maximum tension for different types of motor unit?

Answer 14

Question 15

what muscle fibres are used for postural stability fibres?

Answer 15

slow muscle fibres

Question 16

what are the mechanisms of regulation of muscle force?

Answer 16

recruitment

rate coding

Question 17

what is recruitment of muscle fibres?

Answer 17

1. Motor units can be recruited (not random)
2. Depends on ‘size principle’
   
   1. Smaller units recruited first (slow twitch units)
   2. As more force required more units recruited
   3. Allows fine control where low force required

Question 18

what is the response of each motor unit type for a single motor neuron action potential?

Answer 18

Question 19

what is rate coding for regulation of muscle force?

Answer 19

1. Motor units can fire at range of frequencies
2. Slow units = lower frequency
3. As firing rate increases force produced by unit increases
4. Summation occurs when units fire at freq too fast to allow muscle to relax between action potentials
5. During voluntary contraction all motor units start at firing rate of 8

Question 20

what growth factor prevent neuronal death?

Answer 20

neurotrophic factors

Question 21

what is neurotrophic factor?

Answer 21

• Type of growth factor
• Prevent neuronal death
• Promote growth of neurons after injury
• Motor unit and fibre characteristics dependant on nerve which innervates them
• If fast twitch muscle and slow muscle are cross innervated, the slow becomes fast and the fast becomes slow.
• Motor neuron has effect on properties of muscle fibre it innervates

Question 22

what can happen to fibre types under specific conditions?

Answer 22

can change properties

• Type IIB to IIA most common following training
• Type I to II possible in severe deconditioning or spinal cord injury
  
  • Microgravity during spaceflights results in shift from slow to fast muscle fibre types
• Aging associated with loss type I and II
  
  • Faster loss type II
  • Larger proportion type I fibres left in ages muscle (slower contraction times)

Question 23

what is a reflex?

Answer 23

• Automatic response to stimulus that involves nerve impulse passing from receptor to nerve centre to effector without reaching level consciousness
• Involuntary coordinated pattern of contraction and relaxation elicited by peripheral stimuli
• usually one muscle contract, one inhibited

Question 24

what is the magnitude and timing of a reflex determined by?

Answer 24

intensity and onset of stimulus

Question 25

how do reflexes differ from voluntary contraction?

Answer 25

cannot be stopped once released

Question 26

what can reflexes be influenced by?

Answer 26

• Jendrassik manoeuvre (Reduce inhibition brain or upper CNS exert on reflexes)
• Larger patellar tendon reflex
• Clenching teeth or pulling against locked fingers

Question 27

what is the level of supraspinal control of reflexes?

Answer 27

higher centres of CNS exert inhibitory and excitatory regulation upon stretch reflex

• Decerebration (separate cortex from lower brainstem and spinal cord regions) reveals the excitatory control from supraspinal areas (greater reflex response and elevated level of tonic contraction before relaxation

Question 28

what can occur to reflexes during brain stem

Answer 28

rigidity and spasticity from brain damage giving over-active or tonic stretch reflex

Question 29

what is hyper-reflexia?

Answer 29

overactive reflexes due to loss of descending inhibition

Question 30

what is hyper-reflexia associated with?

Answer 30

upper motor neuron lesions (stroke)- loss voluntary movement in UMNL

Question 31

what is clonus?

Answer 31

involuntary and rhythmic muscle contraction from manual stretch

Question 32

what is babinski sign?

Answer 32

• Particularly evident in corticospinal tract lesions
• Stimulate sole foot with blunt instrument- the big toe movement:
  
  • Curl downwards= normal
  • Curl upwards= abnormal in adults (+ve Babinski sign) (note normal in infants as corticospinal area not yet developed) and toes flex

Question 33

what is hyporeflexia?

Answer 33

below normal/ ansent reflexes

Question 34

what is hyporeflexia associated with?

Answer 34

lower motor neuron lesions