Motor Systems - Descending Tracts

Question 1

Spinal cord codes certain motor programmes. …..,….. and ….. by ……

Answer 1

Accesses, executed and modified by descending tracts front he brain

Question 2

What are the 2 systems used

Answer 2

Upper motor neurons – brain’s command & control
• Supraspinal neurons that arise above the decussation of the pyramids, they innervate the LMNs of the brainstem and spinal cord.
Lower motor neurons – spinal cord command & control
• Located in the motor nuclei of the brainstem & anterior horn of spinal cord
Upper motor neurons descend and influence lower motor neurons

Question 3

Descending tracts from higher centres mediate

Answer 3

Motor functions
Voluntary and involuntary movement
Muscle tone
Spinal reflexes
Regulation of visceral functions

Question 4

Describe pyramidal tracts

Answer 4

Corticospinal tracts – arise in cerebral cortex project to LMNs in the spinal cord
Corticobulbar tracts – arise in cerebral cortex project to LMNs in the brainstem (cranial nerve nuclei)

Question 5

Describe extrapyramidal tracts

Answer 5

Vestibulospinal tracts – arise in vestibular nuclei of pons & medulla project to spinal cord
Reticulospinal tracts – arise in reticular formation of pons and medulla project to spinal cord
Rubrospinal tract – arise in red nucleus of midbrain project to spinal cord
Tectospinal tract – arise in superior colliculus project to cervical levels of spinal cord

Question 6

Describe the 3 neurons that make up descending pathways

Answer 6

1st order neuron which has its cell body in the cerebral cortex whose axon descends to synapse with a
2nd order neuron an internuncial neuron situated in the grey column of the spinal cord. The axon of this neuron is short and synapses with a
3rd order neuron, the lower motor neuron in the anterior grey column.

Question 7

Describe corticospinal tracts

Answer 7

Arise in the 1o & premotor areas of frontal lobes & post central gyri of parietal lobes
Travel with corticobulbar fibres through corona radiate and converge in posterior limb of internal capsule
85% of fibres cross at decussation of pyramids forming lateral corticospinal tract
Descend in contralateral lateral white column
10-15 % of fibres travel ipsilaterally form the anterior corticospinal tract & descend in anterior white column

Question 8

Describe the lateral corticospinal tract

Answer 8

Fibres terminate in anterior grey column of all spinal cord segments
Target neurons in the lateral portion of anterior horn
Responsible for innervation of distal muscle groups
Allows precise skilled movements and synergistic movement of limbs
Same LMNs contacted by the lateral corticospinal tract also innervated by tracts that activate gross & strength related movements

Question 9

Describe the anterior corticospinal tract

Answer 9

Fibres descend ipsilaterally and cross the midline at the segmental level at which they terminate
Terminate on lower motor neurons controlling trunk and proximal musculature
Below L2 the only innervations to lower motor neurons is from the lateral corticospinal tract
The anterior corticospinal tract is important for bilateral postural adjustments

Question 10

Branches of the corticospinal tract in the cerebral cortex

Answer 10

Giant betz cells
Caudete nucleus and putamen
Red nuclei then on to rubrospinal tract
Pontine nuclei and olivary nuclei
Reticular formation then onto reticulospinal tract
Cerebellum via reticulocerebellar & vestibulocerebellar tracts
Vestibular nuclei the onto vestiulospinal tract

Question 11

Describe the corticobulbar/nuclear tract

Answer 11

Arises from the lateral aspect of the 1o motor cortex
UMNs to cranial nerve nuclei
Descend through the corona radiata and converge in the internal capsule
Synapse directly or indirectly with cranial nerves
Responsible for motor control of face
Most corticobulbar fibres innervate neurons bilaterally (except muscles of lower face)
A subset of fibres terminate in reticular formation, superior colliculus and red nucleus
Help co-ordinate cortical & brainstem motor systems

Question 12

Rubrospinal tract

Answer 12

Arises from the red nucleus in the tegmentum of midbrain
Cross the midline in the ventral tegmentum decussation and descend to enter lateral white column of spinal cord
Receive afferent input from 1O and premotor cortex and cerebellum
Humans rubrospinal tract facilitates neurons that innervate flexor muscles of upper limb and inhibit extensor antigravity muscles
Both rubrospinal and corticospinal tracts activate muscles that act against gravity are involved in the control of fine movement
In humans it plays back up more of an integrator and relay nucleus in cerebellar circuits
The corticospinal and rubrospinal tracts lie in the lateral columns of the spinal cord and terminate on the interneurons and motor neurones thatcontrol the distal musculature of the limbs.
Both tracts are referred to as the lateral motor system of the spinal cord.

Question 13

Describe tectospinal tract

Answer 13

Arises in the superior colliculus of the midbrain
Crosses over to the contralateral side in the dorsal tegmental decussation
Descends through the anterior white column close to the anterior median fissure to cervical levels of the spinal cord
Innervate motor neurons responsible for neck movements
It is thought to orientate the head and neck during eye movements in response to visual stimuli that reach the superior colliculus

Question 14

Resticular formation gives rise to ….

Answer 14

2 reticulospinal tracts
Medial/pontine reticulospinal tract
Lateral/medullary reticulospinal tract

Question 15

Describe the medial/pontine reticulospinal tract

Answer 15

Arises from large cells in the medial part of the reticular formation within the pons
Descends through anterior white column to all levels of spinal cord
Faciliatates voluntary or cortically controlled movements
Increases motor tone
Excites anti-gravity muscles
Receives input from vestibular apparatus and cerebellum

Question 16

Describe the lateral/medullary reticulospinal tract

Answer 16

Arises from cells in he medial 2/3 of the medulla
Descends through lateral white column of the spinal cord
Opposes the pontine reticulospinal tract
Inhibits voluntary movement and decreases motor tone
Receives input from corticospinal tract, rubrospinal tract etc
Counterbalances signals from the pontine reticulospinal tract

Question 17

Both sets of fibres enter ….

Answer 17

Anterior grey columns of the SC and facilitate or inhibit active of a or y motor neurons
May influence voluntary movements and reflex activity
Through to provide a pathway for hypothalamus to control sympathetic and sacral parasympathetic outflow

Question 18

Neurons in reticular formation of medulla aoblongata …

Answer 18

Project to respiratory muscles
Influence motor control of breathing
Receive input from limbic system an influence emotional motor system

Question 19

Describe vestibulospinal tracts

Answer 19

Vestibular nuclei found in pons and medulla oblongata
Give rise to 2 tracts;
• Medial & lateral
Functions with the pontine reticular nuclei to control anti gravity muscles
Convey signals from the inner ear and cerebellum
Vestibular nuclei controls the excitatory signals to different anti gravity muscles to maintain balance and equilibrium

Question 20

Describe the medial vestibulospinal tracts

Answer 20

Also known as descending medial longitudinal fasciculus
Descends bilaterally through the brainstem and travels in the anterior white column of spinal cord to cervical and upper thoracic regions
Activate LMNs associated with spinal accessory nerve
Influences rotation and lifting of the head and rotation of the shoulder
Important in changes to maintain posture and balance, keeping the head stable while walking

Question 21

Describe the lateral vestibulospinal tract

Answer 21

Arises from lateral vestibular nucleus
Descends ipsilaterally in brainstem and projects to all levels of spinal cord
Facilitates α and γ motor neurons that innervate extensor muscles and inhibits activity of flexor muscles
Modulated by activation of vestibular apparatus (nuclei) and cerebellum
Mediates postural adjustments to compensate for movements and changes in position of the body and co-ordinate the orientation of the head And body

Question 22

The posterior parietal cortex area receives input from

Answer 22

The primary somatosensory cortical areas 3,2,1

Question 23

Area 7 is a target of

Answer 23

Higher order visual cortical areas

Question 24

Prefrontal cortex and parietal cortex……

Answer 24

Both send axons that converge in area 6

Question 25

area 6

Answer 25

area 6 lies at the junction where signals encoding what actions are converted into signals that specify how the actions will be carried out

Question 26

Lower motor neurons controlled by …. From …..

Answer 26

Synaptic inputs from dorsal rot ganglion cells, UMNs in cortex, Interneurons in spinal cord

Question 27

What are the 2 types of LMN of the spinal cord

Answer 27

Alpha - directly trigger the generation of force by muscles
Gamma

Question 28

What 2 ways does the CNS control and grade muscle contraction

Answer 28

1. Via ACh and summation at the NMJ
2. Recruiting additional synergistic motor units with the extra tension being provided by how many muscles are in that muscle unit

Question 29

what type of muscles can be more finely controlled by the CNS

Answer 29

Large number of small motor units

Question 30

Smaller muscles that control fine movement have….

Answer 30

Much smaller innervating ratios, as few as 3 muscle fibres per alpha motor neuron

Question 31

What is the muscle spindle

Answer 31

A stretch receptor consisting of several specialised muscle fibres in a fibrous capsule deep within skeletal muscle
Is sensory axons wrap around the muscle fibres of the spindle

Question 32

Describe the path of muscle spindle innervating

Answer 32

Group Ia axons enter the dorsal roots of the spinal cord, branch extensively and form excitatory synapses with both Interneurons and alpha motor neurons of the ventral horn
Alpha motor neurons innervate extrafusal fibres, gamma intramural fibres

Question 33

Alpha motor neurons innervate

Answer 33

extrafusal fibres, gamma intramural fibres

Question 34

Activation of alpha and gamma motor neurons has….

Answer 34

Opposite effects on Ia output
Alpha alone decreases Ia activity while Gamma alone increases Ia activity

Question 35

Golgi tendon organs provide…

Answer 35

Proprioceptive inputs from the muscles as they act like sensitive strain gauges as they motor muscle tension or force of contraction

Question 36

Where are Golgi tendon organs located

Answer 36

At the junction of the muscle and the tendon and are innervated by Ib sensory axons that entwine among collagen fibrils

Question 37

Golgi tendons organs are situated in…..

Answer 37

Series

Question 38

Ib axons enter the ……. And synapse with …….. in the ……

Answer 38

Spinal cord
Ib inhibitory Interneurons
Ventral horn

Question 39

Describe muscle tension in regards to Ib Interneurons

Answer 39

Ib interneurons form inhibitory connections with the alpha motor neurons innervating the same muscle forming a spinal reflex preventing the muscle becoming overloaded, however its normal function is to regulate muscle tension within an optimal range
As muscle tension increases, the inhibition of the alpha motor neuron slows muscle contraction; as muscle tension falls the inhibition of alpha motor neurons is reduced and muscle contraction increases