CNS Motor control; Spinal Reflexes

Question 1

Describe components of a reflex arc?

Answer 1

• sense organ
• afferent neuron
• dorsal horn of spinal cord
• effector neurone
• ventral root
• relevant muscle

Question 2

What are the levels of integration?

Answer 2

Spinal cord level 
- homeostasis and reflex actions 
- simplest level 
Subcortical level 
- primitive actions 
- securing food & reproduction
Cortical level 
- highest level of neural activity 
- science & philosophy

Question 3

What fibres sense changes in length of a muscle?

Answer 3

intrafusal fibres - lost contractile proteins and are wrapped in nerves

Question 4

What organ is the length transducer in muscles and what fibres come from it?

Answer 4

spindle

• afferent fibres type 1a and II both myelinated but 1a faster
• efferent but fewer in order to maintain sensitivity of spindle in face of muscle shortening

Question 5

What fibres are found within the spindle?

Answer 5

dynamic nucleus bag
Static nuclear bag fibres
Nuclear chain fibres

Question 6

What are the properties and function of Dynamic nuclear bag fibres?

Answer 6

fibres to sense velocity

• contained in dynamic nucleus bag
• fast adapting (when stimulus changes firing rate changes quickly)
• innervated by Ia (fast)
• phasic response (senses acceleration)

Question 7

What are the properties and function of Static

nuclear bag fibres?

Answer 7

• slowly adapting (reacts slowly to stimulus changes)

- innervated by type II (slow)

Question 8

What are the properties and function of Nuclear chain fibres?

Answer 8

• signal absolute muscle length

- provide static response of muscle spindle

Question 9

What is the function of the efferent nerves coming off the spindle?

Answer 9

• efferent gamma nerves are co-activated when the main alpha nerve of the muscle is activated
• when activated they help shorten the spindle too
• preventing spindle unloading and becoming neurally silent
• can also sensitise the spindle

Question 10

Describe the stretch reflex?

Answer 10

• afferent nerves from bag and chain fibres innervate muscle when stretched
• causes contraction against stretch, damping rapid changes in muscle length
• reciprocal innervation of antagonistic muscle via inhibitory interneuron means inhibited to aiding damping

Question 11

What organ can monitor force of the muscle?

Answer 11

Golgi tendon organ

Question 12

Describe the structure/location of the Golgi tendon organ?

Answer 12

• Within the fascicles of a tendon (therefore in series with the muscle)
• constitutes of net of nerve endings

Question 13

What will illicit firing from a Golgi tendon?

Answer 13

• contraction of a muscle (force)
• but not stretch
• too much force will illicit response which reduces muscle contraction

Question 14

Through what pathway does the Golgi tendon feedback information and to where?

Answer 14

• innervates main Alpha nerve via an inhibitory interneurone
• supra-spinal system

Question 15

What is clonus (symptom) and what causes it?

Answer 15

• Exaggerated tremor in response to small stimuli

- upper motor neurone lesion when descending inhibitory input to gamma fibres absent

Question 16

What is a characteristic symptom of patients with lower motor neurone lesion and concomitant spastic paralysis?

Answer 16

• Inverse response to the stretch reflex

- resulting in resistance to movements to suddenly and rapidly give way

Question 17

What is the Flexor reflex?

Answer 17

• Stimulation of flexor muscle inhibits antagonistic extensor muscle on same limb
• opposing limb (opposite side of body) will reflexively act the opposite way (extensors contract and flexors inhibited)

Question 18

What is the positive supporting reflex?

Answer 18

• leg extends to push down on a finger touching sole of the foot

Question 19

When is the positive supportive reflex useful to test?

Answer 19

• upper motor neurone lesions

- more evident in patients with lesions

Question 20

Give examples of spinal level integrated reflexes?

Answer 20

• Positive supportive reflex
• Cord righting reflex
• walking

Question 21

What is required to keep us standing (reflex)?

Answer 21

• proprioception
• sensation from soles of feet
• vestibular function
• leg muscles pressing against floor
• trunk muscles keeping gravity above base
• neck muscles hold head up

Question 22

What reflexive processes are required for walking?

Answer 22

• pattern generators within spinal cord set alternate contractions of flexors and extensors
• precisely timed and rhythmically