Upper & Lower Motor Neurone Disorders

Question 1

What are the main roles of the brain and spinal cord?

Answer 1

Brain:

• initiation
• integration
• co-ordination

spinal cord:

• simple reflexes
• pattern generation

Question 2

What are the 2 descending systems (upper motor neurones) and what feeds into them?

Answer 2

Motor cortex:

• planning, initiating and directing voluntary movements
• the basal ganglia feed into this - gating proper initiation of movement

brainstem centres:

• basic movements and postural control
• the cerebellum feeds into this - sensory motor coordination

Question 3

What are the 3 different types of spinal reflexes?

Answer 3

• Myotatic stretch reflex
• inverse myotatic reflex
• crossed extensor / flexor withdrawal reflex

Question 4

What is the definition of a reflex?

Answer 4

A fast, predictable automatic response to a change in the environment or a stimulus

Question 5

What are the 5 functional components of the reflex arc?

Answer 5

1. Sensory receptor
2. sensory neurone (afferent)
3. integration centre
4. motor neurone
5. effector organ (muscle or gland)

Question 6

What are examples of sensory receptors?

What is their role in the reflex arc?

Answer 6

• Golgi tendon organ
• ruffini endings
• meisseners corpuscles
• muscle spindle
• free nerve endings

once activated (upon stimulus) they fire action potentials via a sensory afferent

Question 7

What is the role of the sensory neurone in the reflex arc?

Answer 7

It carries the action potential to the spinal cord

it has cell bodies in the dorsal root ganglion and enters the spinal cord via the dorsal root / dorsal horn

Question 8

What is the role of the integration centre in the reflex arc?

What are the different types of integration?

Answer 8

It relays sensory information into motor information

monosynaptic:

• sensory afferent neurone synapses directly with the motor neurone
• this allows for a rapid response

polysynaptic:

• 2 or more synapses
• complexity is added in the form of interneurones
• slower response, but more complex

Question 9

What is the role of the interneurones?

Answer 9

They can be inhibitory or excitatory and will alter the activity of the lower motor neurone

Question 11

What is the myotatic stretch reflex?

Which receptor is involved?

Answer 11

Muscle spindle detects stretching (change of length) within the muscle

this leads to contraction of the same muscle

Question 12

What are the functions of the myotatic stretch reflex?

Answer 12

• Antigravity
• posture
• movement

antigravity - muscles maintain the upright posture of the body against the force exerted on them by gravity

Question 13

What is the sequence of events involved in the agonist response in the patellar tap reflex?

Answer 13

• Tap quadriceps tendon and stretch of quadriceps muscle
• activation of the muscle spindle and increased firing of 1a afferent
• afferent terminals synapse directly with and excite the alpha motor neurone (monosynaptic reflex)
• increased alpha motor neurone efferent axon activity
• contraction of the agonist homonymous muscle (i.e. the muscle from which the afferent arose)

Question 16

What happens to the antagonistic muscle in the myotatic stretch reflex?

Answer 16

Reciprocal inhibition of the antagonist muscle groups during the myotatic reflex

(antagonist muscle must do the opposite all the time)

there is indirect inhibition via the Ia inhibitory interneurone of motor neurones innervating the antagonist (flexor) muscle groups

Question 17

What does tapping the patellar tendon and activating the Ia afferent lead to?

Answer 17

• Direct excitation of motor neurones innervating agonist (extensor) muscle groups
• Indirect inhibition (via interneurone) via the Ia inhibitory interneurone of motor neurones innervating antagonist (flexor) muscle groups

Question 18

What happens in the myotatic stretch reflex when there is addition of a load?

(e.g. arm muscle groups)

Answer 18

• Stretch of the flexor biceps brachii and activation of the Ia afferent
• Direct excitation of agonist (flexor) muscle groups (biceps)
• Indirect inhibition via the Ia inhibitory interneurone of antagonist (extensor) muscle groups (triceps brachii)

Question 19

What is the receptor in the inverse myotatic stretch reflex?

What is the role of this reflex?

Answer 19

Once a muscle has contracted, there is tension in the muscle

The Golgi tendon organ detects the tension and elicits a response

controlling the tension of an active muscle in response to muscle contraction to avoid tendon damage

Question 20

What are the functions of the inverse myotatic (Golgi tendon) stretch reflex?

Answer 20

• Tension feedback
• overload protection

Question 21

What are the 5 stages in the inverse myotatic reflex (Golgi tendon reflex)?

Answer 21

Increased muscle tension leads to:

• Golgi tendon organ excitation
• increased firing in Ib afferent
• Indirect inhibition via inhibitory interneurone (disynaptic link) of the a motor neurone
• decreased a motor neurone activity to the homonymous muscle
• relaxation / inhibition of the homonymous muscle

Question 22

What happens in the inverse myotatic reflex (e.g. leg muscle groups) when there is increased muscle tension?

Answer 22

• Increased muscle tension activates the Golgi tendon organ and excites Ib afferents
• this leads to indirect inhibition via an inhibitory interneurone of motor neurones innervating the homonymous muscle and muscle relaxation
• simulatenously, there is indirect excitation of motor neurones innervating antagonist muscle groups

Question 23

What is the crossed extensor / flexor withdrawal reflex?

Answer 23

Noxious (harmful / unpleasant) cutaneous stimulation causes flexion withdrawal from the offending stimulus

simultaneous extension of the contralateral limb (crossed extensor) may occur for weight bearing

Question 24

What are the functions of the crossed extensor / flexor withdrawal reflex?

Answer 24

• Damage limitation of avoidance
• maintained balance on limb withdrawal

Question 25

In general, what are the 2 actions which occur in the crossed extensor / flexor withdrawal reflex?

Answer 25

Inputs from nociceptor afferents lead to:

contralateral inhibition** of flexors and **excitation of extensors

ipsilateral excitation** of flexors and **inhibition of extensors

Question 26

What are the sequence of events on activation of the cutaneous nociceptor in the lower limb withdrawal reflex?

Answer 26

• Increased activity in A-delta and C afferents
• polysynaptic activation of ipsilateral flexors
• polysynaptic inhibition of ipsilateral extensors
• polysynaptic inhibition of contralateral flexors
• polysynaptic excitation of contralateral extensors

this enables automatic maintenance of balance during the reflex

Question 27

Complete the table

Answer 27