Functional hierarchy of the motor system

Question 1

What is innervated by visceral motor neurons?

Answer 1

Involuntary structures: head, neck, cardiac muscle, smooth muscle etc.

Question 2

What is the final common pathway?

Answer 2

The alpha-motorneurons by which nerve impulses from many central sources pass to a muscle or gland in the periphery.

Question 3

How are posture and balance controlled?

Answer 3

By higher order reflexes - brainstem nuclei that control spinal reflexes, vestibulospinal and reticulospinal tracts (both extrapyramidal).

Question 4

What is the extrapyramidal motor system?

Answer 4

The part of the motor system that controls involuntary movements.

Question 5

Which structures control brainstem nuclei?

Answer 5

Cerebral cortex (motor cortex, premotor cortex and supplementary motor areas), basal ganglia and cerebellum.

Question 6

What are the 4 systems that control movement?

Answer 6

1) Descending control pathways
2) Basal ganglia
3) Cerebellum
4) Local spinal cord and brain stem circuits

Question 7

What would a LMN lesion cause?

Answer 7

Flaccid paralysis and muscle atrophy.

Question 8

What would an UMN lesion cause?

Answer 8

Spasticity and some paralysis.

Question 9

What would occur due to a corticospinal lesion?

Answer 9

Weakness (paresis) rather than paralysis.

Question 10

Describe the spatial map body musculature of the spinal cord.

Answer 10

This applies to arms and legs:

• proximal shoulder muscles are mapped to medial motorneurones
• finger muscles are mapped to lateral motorneurones

Question 11

What are the pyramidal tracts?

Answer 11

• Corticospinal (terminates in the spinal cord)

- Corticobulbar (terminates in the brainstem)

Question 12

Which system informs about balance?

Answer 12

Vestibular system - in the brainstem.

Question 13

What happens if sensory inputs are damaged at spinal level?

Answer 13

Paralysis - as if the alpha-motorneurones have been damaged themselves.

Eg. viral infection that caused the loss of all proprioception by attacking the DRG sensory neurones (sensory neuronopathy) - these do not regenerate. Lose sense of body in space and motor control.

Question 14

What is the simplest segmental reflex?

Answer 14

The stretch reflex - patella tendon.

This reflex is found in every muscle.

Muscle is stretched (eg with a tendon hammer), afferent inputs to spinal cord, efferent alpha-MN’s cause contraction of that muscle and relaxation of antagonist muscle (reciprocal inhibition).

Question 15

Which spinal cord levels are spinal reflexes found?

Answer 15

1) Biceps jerk: C6
2) Triceps jerk: C7
3) Patellar tendon: L4
4) Achilles tendon: S1

Question 16

What is the significance of testing reflexes?

Answer 16

• Can detect the level of spinal cord damage. Reflexes can be elicited above but not below the level of damage.
• Impaired reflexes can indicate areas of nerve damage eg Diabetic patients can lose their ankle jerk reflexes

(Patients need to be distracted as these can be overridden)

Question 17

What is the flexor and crossed extensor reflex?

Answer 17

Polysynaptic reflex - eg standing on a nail:

• A-delta nocioceptive fibres detect pain and this enters the spinal cord
• Activation of alpha-MN’s controlling the flexor muscles which causes the affected limb to flex towards the body - ipselateral flexion in response to pain (and extensors are inhibited)
• Excitatory interneurones on the contralateral side cause extension of this limb

–> slower than the stretch reflex as the noiceceptive sensory fibres conduct more slowly than muscle spindle afferents

Question 18

What is the golgi tendon reflex?

Answer 18

This occurs when there is excess load on a muscle eg holding heavy blocks:

• neurons from the GTO fire
• MN is inhibited
• Muscle relaxes
• Load is released

Question 19

What is the golgi tendon organ?

Answer 19

Proprioceptive sensory receptor organ that senses changes in muscle tension.

Question 20

How can the golgi tendon reflex be overridden?

Answer 20

If the load is eg a child:

• descending voluntary excitation of alpha-MN’s can override the inhibition from GTO’s and maintain muscle contraction
• this hyperpolarises alpha-MN’s so that they cannot be evoked

(Integration of EPSP and IPSP)

Question 21

What would happen if there was high gamma-MN activation of muscle spindles?

Answer 21

This causes muscles to become extremely resistant to stretch –> spasticity.

Many neurological disorders are associated with this.

Question 22

In the withdrawal reflex, how many spinal segments are involved?

Answer 22

The more powerful the pain stimulus, the greater the spinal spread and larger the response.

–> Facilitation: increases the effects of sensory inputs (eg. finger touches something hot, hand moves; hand touches something hot, whole arm moves)

Alpha-MN’s are maintained in a more depolarised state.

Question 23

In the stretch reflex, how many spinal segments are involved?

Answer 23

Only 1 or 2 spinal segments, so useful clinically for localising a problem of the spinal cord.

Question 24

Describe Babinski’s sign.

Answer 24

Extension when the lateral aspect of the sole of the foot is stroked (this should be flexion) - damage to the corticospinal tract or in young children (<1) who don’t have a fully developed motor system. Can occur transiently after an epileptic seizure.

Question 25

What happens in spinal cord transection?

Answer 25

Spinal shock:

• immediate sensory and autonomic effects (loss of bowel, bladder and sexual regulation)
• loss of reflexes for 2-6 weeks then regain exaggerated reflexes

Question 26

What is clonus?

Answer 26

Involuntary, rhythmic muscle contraction and relaxation.