Motor Systems Flashcards
Which tracts deal with motor commands?
The descending tracts of the spinal cord
What are the two systems of the descending tracts?
Upper motor neurons (UMN) - brains command and control
Lower motor neurons (LMN) - spinal cords command and control
What are UMN?
Supraspinal neurons that arise above the decussation of the pyramids
They innervate LMNs
What are LMNs?
Neurons located in the motor nuclei of the brainstem and Ant horn of the spinal cord
What motor functions do the descending tracts mediate?
Voluntary / involuntary movement
Muscle tone
Spinal reflexes
Regulation of visceral functions
Name the pyramidal tracts
Corticospinal - arise in the cerebral cortex and project to LMNs in the spinal cord
Corticobulbar (nuclear) - arise in the cerebral cortex and project to LMNs in the brainstem (cranial nerve nuclei)
Name the extrapyramidal tracts
Vestibulospinal tract
Reticulospinal tract
Rubrispinal tract
Tectospinal tract
Describe the vestibulospinal tract
Arises in the vestibular nuclei of the pons and medulla
Projects to the spinal cord
Describe the reticulospinal tract
Arises in the reticular formation of the pons and medulla
Projects to the spinal cord
Describe the rubrospinal tract
Arises in the red nucleus of the midbrain
Projects to the spinal cord
Describe the tectospinal tracts
Arise in the superior colliculus
Project to cervical levels of the spinal cord
Where do the Corticospinal tracts originate?
The primary and premotor areas of the frontal lobes and central gyri of the parietal lobes
Where do the corticospinal tract fibres cross?
85% of fibres will cross at the decussation of the pyramids to form the lateral Corticospinal tract
Descend in the contralateral white column
What about the Corticospinal fibres that do not cross over?
10-15% of the fibres will travel ipsilaterally from the anterior corticospinal tract and descend in the Ant white column
Where do the Corticospinal fibres travel before any of the cross over?
They travel with corticonuclear fibres through the corona radiate and converge in the Post limb of the internal capsule
Describe the lateral Corticospinal tract
Fibres terminate in the Ant grey column of all spinal cord segments
Target neurons in the lateral portion of the Ant horn
Innervate distal muscle groups
Allow precise movements and synergistic movement of limbs
Describe the Ant corticospinal tract
Fibres descend ipsilaterally
Cross the midline at the segmental level which they terminate
Terminate on LMNs controlling trunk and proximal musculature
Has no effect below L2
Important for bilateral postural adjustments
Where does the corticonuclear tract originate?
The lateral aspect of the primary motor cortex
Then UMNs to cranial nerve nuclei
Describe the corticonuclear tract
Descends through the corona radiata and converge in the internal capsule
Synapse directly or indirectly with cranial nerves
Most fibres Innervate neurons bilaterally (except the lower face)
Some fibres terminate in the reticular formation, superior colliculus and red nucleus
What are corticonuclear tracts responsible for?
Motor control of the face
Help coordinate cortical and brainstem motor systems
Where does the rubrospinal tract originate?
The red nucleus in the tegmentum of the midbrain
Where does the rubrospinal tract cross?
The midline in the ventral tegmentum decussation
Descends to enter the lateral white column of the spinal cord
What provides input to the rubrospinal tract?
Primary and premotor cortex and cerebellum
What is the rubrospinal tract responsible for?
Facilitates neurons that Innervate flexor muscles of the upper limbs and inhibits extensor antigravity muscles
Back up role of integrator and relay nucleus in cerebellar circuits
Where does the rubrospinal tract terminate?
Along with corticospinal it lies in the lateral columns of the spinal cord and terminates on the interneurons and motor neurons that control the distal musculature of limbs
Where does the tectospinal tract originate?
The superior colliculus of the midbrain
Where does the tectospinal tract cross over?
Crosses to the contralateral side in the dorsal tegmental decussation
How does the tectospinal tract descend?
Through the Ant white column close to the Ant median fissure to cervical levels of the spinal cord
What is the tectospinal tract responsible for?
Innervating motor neurons responsible for neck movement
Thought to orientate the head and neck during eye movements in response to visual stimuli that reach the superior colliculus
Describe the 2 reticulospinal tracts
Most primitive descending motor system
Reticular formation gives rise to 2 tracts
Medial / Pontine
Lateral / Medullary
Describe the medial reticulospinal tract
Arises from large cells in the medial part of the reticular formation
Descends through Ant white column to all levels of the spinal cord
Facilitates voluntary or cortically controlled movements
Increases motor tone
Excites antigravity muscles
Receive input from vestibular apparatus and cerebellum
Describe the lateral reticulospinal tract
Arises from cells in the medial 2/3 of the medulla oblongata
Descends through lateral white column of the spinal cord
Opposes the pontine reticulospinal tract
Inhibits voluntary movement and decreases motor tone
Receives input from rubrospinal and corticospinal tracts
How do the reticulospinal tracts work?
Facilitate or inhibit alpha and gamma motor neurons
Influence voluntary and reflex activity
How do the reticulospinal tracts affect breathing?
Neurons in the reticular formation of the medulla oblongata project to the respiratory muscles
Influence the motor control of breathing
What else can the reticulospinal tract influence?
Input from the limbic system can influence emotional motor systems
Where are the vesitibular nuclei found?
In the pons and medulla oblongata
How many tracts are included in the vestibulospinal tracts?
Medial
Lateral
Describe the medial vestibulospinal tract
The descending medial longnitudinal fasciculus
Descends bilaterally through the brainstem
Travels in the Ant white column of the spinal cord to cervical and upper thoracic regions
Activate LMNs associated with the spinal accessory nerve
Influences rotation of the shoulder
Important in changes to maintain posture and balance, keeping the head stable while walking
Describe the lateral vestibulospinal tract
Arises from the lateral vestibular nucleus
Descends ipsilaterally
Projects to all levels of spinal cord
Facilitates alpha and gamma neurons that Innervate extensor muscles
Inhibits activity of flexor muscles
Modulated the vestibular apparatus and cerebellum
Mediates postural adjustments to compensate for changes in body positions
Orientates the head and body
What is Brodmann’s area 4?
The primary motor cortex
What is Brodmann’s area 6?
The Premotor and supplementary area
What is Brodmann’s area 5
The Post parietal cortex
Receives input from primary somatosensory cortical areas 3,2,1
What is Brodmann’s area 7?
A target for higher order visual cortical areas
What is important to know about Brodmann’s area 6?
Prefrontal cortex and parietal cortex both send axons that converge on area 6
This makes it a junction where signals encoding what actions are converted into signals that specify how the actions will be carried out
What do LMNs receive synaptic inputs from?
Dorsal root ganglion cells
UMNs in the cortex
Interneurones in spinal cord
What are the two types of spinal cord LMNs?
Alpha - directly trigger the generation of force by muscles
Gamma
What are the two ways the CNS controls and grades muscle contractions?
1 - Via ACh and summation at the neuromuscular junction
2 - Recruiting additional synergistic motor units with the extra tension being provided by how many muscles are in that unit
How are fine control muscles innervated?
With much smaller innervation ratios
E.G three muscle fibres per alpha motor neuron
Muscles with higher numbers of small motor units can be more finely controlled by the CNS
What is a muscle spindle?
A stretch receptor consisting of several specialised muscle fibres in a fibrous capsule deep within skeletal muscle
What are Ia axons?
Sensory axons that wrap around the muscle fibres of the spindle
They enter the dorsal roots of the spinal cord, reach extensively and form excitatory synapses with interneurons and alpha neurons of the neural horn
What do alpha motor neurons innervate?
Extrafusal fibres
What do gamma motor neurons innervate?
Intrafusal fibres
What is the difference between muscle spindles and Golgi tendon organs?
Muscle spindles - skeletal muscle stretching, situated in parallel with muscle fibres
GTO - tendon stretching, situated in series
Where are Golgi tendon organs found?
At the junction of the muscle and the tendon
What Innervates Golgi tendon organs?
Ib sensory axons that entwine among collagen fibrils
What other function do Golgi tendon organs have?
Provide prorioceptive inputs from the muscles as they act like sensitive strain gauges as they monitor tension and force of contraction
What do Ib axons do?
Enter the spinal cord
Synapse with Ib inhibitory interneurons in the ventral horn
What do Ib interneurons do?
Form inhibitory connections with the alpha motor neurons innervating the same muscle forming a spinal reflex to prevent the muscle becoming overloaded
Normal function is to regulate muscle tension with an optimal range
What happens when muscle tension falls/ increases?
Increase - the inhibition from the alpha motor neuron slows muscle contraction
Falls - inhibition from alpha motor neurons is reduced and muscle contraction increases
