72 - Control of Posture and Locomotion

Question 1

List the two types of descending spinal tracts

Answer 1

1 - Lateral pathways - responsible for voluntary control of distal muscles
2 - Ventromedial pathways - involved in control of posture and locomotion, control mainly the axial and proximal muscles

Question 2

What are the five specific tracts of the ventromedial descending spinal tracts

Answer 2

Lateral vestibulospinal tract
Medial vestibulospinal tract
Pontine reticulospinal tract 
Medullary reticulospinal tract
Reticulospinal system

Question 3

Describe the function of the lateral vestibulospinal tract

Answer 3

Involved in control of posture and locomotion, controls mainly axial and proximal muscles

Question 4

Describe the function of the medial vestibulospinal tract

Answer 4

Main function is mediation of reflex head movements in response to vestibular stimuli - it helps adjust the position of the head in response to postural changes

Question 5

Describe the function of the pontine reticulospinal tract

Answer 5

One function is adjustment of posture and body balance by facilitating motoneurons to extensor muscles - Note that both the lateral vestibulospinal tract and the pontine reticulospinal tract BOTH facilitate the extensor muscles

Question 6

Describe the function of the medullary reticulospinal tract (AKA lateral reticulospinal tract)

Answer 6

It doesn’t have one simple function, but in relation to skeletal motor function, it is thought that it facilitates motor neurons to flexors and inhibits those to extensors

Question 7

Describe the function of the reticulospinal system

Answer 7

It can modulate reflex action during movement by integrating vestibular and other sensory inputs with motor commands from the cerebral cortex

Question 8

What is the vestibulocollic reflex?

Answer 8

AKA vestibulo-cervical reflex

A reflex used to control head position

Question 9

What is the sensory input to the vestibulocollic reflex?

Answer 9

Proprioceptive input from the neck muscle spindle and joint receptors

Question 10

Where does the vestibulocollic reflex act?

Answer 10

On the neck

Question 11

What is the vestibulospinal reflex?

Answer 11

A reflex used to control head position

Question 12

What is the sensory input to the vestibulospinal reflex?

Answer 12

Turning of the head activates postural change in the opposing side limb muscles

Question 13

Where does the vestibulospinal reflex act?

Answer 13

Limbs

Question 14

What is the cervcocollic reflex?

Answer 14

A reflex used to control head position

Question 15

What is the sensory input of the cervicocollic reflex?

Answer 15

Neck reflexes reacting on neck muscles contract stretched neck muscles, acting synergistically with vestibulocollic reflexes

Question 16

Where does the cervicocollic reflex act?

Answer 16

On the neck

Question 17

What is the tonic neck reflex?

Answer 17

There are two types of tonic neck reflexes - asymmetrical and symmetrical

Question 18

Describe the asymmetrical tonic neck reflex

Answer 18

• Seen in young infants
• It is a primitive reflex which is present at birth and is inhibited (suppressed to the adult level) by 6 months
  If the head is turned to one side, the limbs on that side extend and the limbs on the other side flex
• This reflex can persist and be seen more prominently in children with cerebral palsy

Question 19

Give an example of the assiymetrical tonic neck reflex

Answer 19

• Head turns to right right arm and leg are extended, left arm and leg are flexed

Question 20

Describe the symmetrical tonic neck reflex

Answer 20

• Seen in infants around 4-6 months and disappears before 1 year
• When the neck is extended, the upper limbs extend and the lower limbs flex
• Seen in neurologically normal infants and can be obligatory in people with cerebral damage

Question 21

What type of patients would you see tonic neck reflexes in?

Answer 21

Tonic Neck Reflexes (TNR) can be seen in patients with vestibular damage. Normally the vestibular postural reflexes (tonic labyrinthine reflex) on the limbs are opposite to those caused by neck proprioceptors. When vestibular reflexes are lost with vestibular damage, the unopposed tonic neck reflexes become visible.

Question 22

What is the labyrinth righting reflex?

Answer 22

This head righting reflex as driven by vestibular input is also called the labyrinth righting reflex

Question 23

Other than vestibular input, what else works in the righting reflex?

Answer 23

Visual input can also keep the head upright (even in the absence of vestibular or neck proprioceptor input). This is a type of optical righting reflex

Question 24

What are two mechanisms the body uses for controlling posture?

Answer 24

Feed-forward and feedback mechanisms

Question 25

What is the difference between feed-forward and feedback mechanisms?

Answer 25

Feed-forward is an anticipatory where feedback is a compensatory

Question 26

Describe feed-forward

Answer 26

i. Anticipate or predict the effect an environmental disturbance (or motor command) will have on variable to be controlled (i.e. postural stability)
ii. Apply corrective action before there is error (i.e. postural adjustment)

Question 27

Describe feedback (compensatory)

Answer 27

i. Appropriate correction is applied after error (ex. loss of balance) is detected
ii. Automatic adjustments can be very rapid & improve with practice & learning
iii. Anticipatory mechanisms can sometimes predict postural disturbance and preprogram what stabilizing response will occur

Question 28

What are stepping pattern generators?

Answer 28

Humans have stepping pattern generators (SPGs) in the spinal cord which control stepping movements at the hip and knee. The SPGs are networks of spinal interneurons which can elicit alternating patterns of flexion and extension at the knee and hip.

Question 29

How do SPGs accomplish this?

Answer 29

They accomplish this by activating the lower motor neurons for hip and knee muscles. Each limb has its own SPG but the actions of both legs are coordinated through commissural fibers. The SPGs are activated by descending input.

Question 30

What is the swinging room experiment?

Answer 30

Walls move, ground stays still, see if you can still stand up or if you fall down

Question 31

How do toddlers do on the swinging room test compared to adults?

Answer 31

Toddlers can have difficulty standing with their eyes closed
In the swinging room experiment, toddlers can even fall since their posture is so dependent on vision
Adults just increase their normal body sway

Question 32

Which sensory systems are involved in control of posture?

Answer 32

a. Vestibular system – senses body sway through head motion
b. Proprioceptive (muscle spindles, GTO, joint receptors)
c. Cutaneous receptors (touch and pressure)
d. Visual

Question 33

Define posture

Answer 33

“Posture - overall position of the body and limbs relative to each other and
orientation of body and limbs in space.
“

Question 34

Describe three behavioral functions of postural adjustments

Answer 34

1. Support head and body against gravity and other external forces.
2. Keep center of gravity of body aligned and balanced over base of support on the ground. System needs to maintain a steady stance.
3. Stabilize supporting parts of the body during movement of other parts.
   Postural system needs to generate responses that anticipate how voluntary movements would perturb posture.

Question 35

What happened in the cat experiment in terms of feedback and feed-forward following a thoracic lesion?

Answer 35

Following transection of the spinal cord at the thoracic level, a cat’s hindlimbs will still make coordinated locomotor movements if the animal is supported and placed on a moving treadmill.

Question 36

What is a decerebrate posture?

Answer 36

Decerebrate posture includes extension of all four limbs and the neck. In humans, the posture can be intermittent. It can be seen in response to noxious peripheral stimuli

Question 37

When would you see decerebrate posturing?

Answer 37

• Decerebrate posture includes extension of all four limbs and the neck.
• In humans, the posture can be intermittent. It can be seen in response to noxious peripheral stimuli.

Question 38

Describe the lesions that cause decerebrate posturing

Answer 38

If there is a lesion below the red nucleus but above the vestibular nuclei, the lateral vestibulospinal tracts (and pontine reticulospinal tracts if the lesion is high enough) increase their facilitation of extensor motor neurons. This extensor drive is responsible for the decerebrate posturing.

Question 39

What is decoricate posture?

Answer 39

Arms are flexed and legs are extended when patient is lying on his/her back with head facing straight forward.

Question 40

What causes decoricate posture?

Answer 40

This posture can result from lesions of the internal capsule that release both vestibulospinal and tonic neck reflexes from cortical control. Can also be caused by lesions in the upper midbrain/diencephalon.

Question 41

What will this condition resemble?

Answer 41

If patient’s head is turned, you will see what looks like asymmetrical tonic neck reflexes in arms.