Descending Tracts Flashcards
What are the two main sets of descending pathways?
Pyramidal
Extra pyramidal
What motor functions do descending tracts mediate?
Voluntary movement Involuntary movement Regulation of muscle tone Modulation of spinal segmental reflexes Regulation of visceral functions
What are the two pyramidal pathways?
Corticospinal
Corticobulbar
What are the 5 extra pyramidal tracts?
Reticulospinal Tectospinal Rubrospinal Vestibulospinal Olivospinal
Define an upper motor neurone
An upper motor neurone is a neurone of the brain that innovates lower motor neurones of the spinal cord and brain stem either directly or through an interneurone
What are corticospinal neurones?
Upper motor neurones that project to the spinal cord
What are corticobulbar neurones?
An upper motor neurone that projects to lower motor neurones of the brainstem
What is the function of the basal ganglia?
To control movement by receiving inputs from the cerebral cortex and feedback signals to different regions of the frontal cortex involved in the initiation of movement
What is the function of the cerebellum?
To receive inputs from most parts of the central nervous system that contribute to motor function
What is the function of the corticospinal tract?
Precise voluntary movements
Where is the corticospinal tract originate?
30% primary motor cortex
30% premotor cortex
40% somato-sensory cortex
What proportion of the motor cortex produces movement of the lower limb?
Dorsal and medial aspects of the precentral gyrus
What proportion of the motor cortex produces movement of the upper limb?
Lateral aspect of the precentral gyrus
What proportion of the motor cortex produces movement of the face and tongue?
Far lateral aspect of the precentral gyrus
What is the significance of the fibres that arise from the primary motor cortex of the corticospinal tract?
They are responsible for voluntary control over precise movements that affect the distal musculature
The actions are upon the central horn cells via interneurons
They are heavily dependent on feedback signals
What is the significance of the fibres that arise from the primary somatosensory cortex that projects to the dorsal column?
They serve as a sensory filtering mechanism and allows specific sensory signals such as positioning of a limb and force of contraction to reach the relevant areas of the primary motor cortex while preventing sensory signals relevant to the movement from reaching cortical neurones
What is the significance of area six and the premotor cortices of the corticospinal tract?
Provide a programming mechanism for the sequencing of response patterns that are essential for producing movements such as walking and lacing up shoes
Project directly to spinal cord and send signals to the primary motor cortex
Where does the corticobulbar tract originate?
Lateral aspect of the primary motor cortex
What does the corticobulbar tract function as?
It serves as an UMN to the cranial nerve motor nuclei onto which they make synaptic contact
They are responsible for voluntary control over muscles of facial expression, eye movements and opening and closing of the jaw and movements of the tongue
Which corticobulbar tract are not bilateral?
CN VII below the eyes
Hypoglossal nucleus
What tracts are responsible for axial musculature?
Reticulospinal and vestibulospinal
Where does the lateral reticulospinal tract arise from and where does it go to?
The medulla and goes to all levels of the spinal cord
Where it is the medial reticulospinal tract arise from and where does it go?
It arises from the pons and goes to all levels of the spinal-cord
What is the medial reticulospinal tract responsible for?
Voluntary or cortically induced movements and increased muscle tone
What is the lateral reticulospinal tract responsible for?
Inhibits voluntary movements and cortically induced movement and decreases muscle tone
What side does the reticulospinal tract go to?
It remains ipsilateral
Where does the vestibulospinal tract arise from?
The medial and lateral vestibular nuclei
Where does the medial vestibulospinal tract project?
The cervical levels
What does the medial vestibulospinal tract do?
It activates then UMN of the spinal accessory nerve
It is responsible for rotation and lifting of the head and the shoulder blade Important for posture and balance
What does the lateral vestibulospinal tract do and where does it project?
Its projects to all spinal levels Important in the innovation of the extensor muscles
Important for antigravity muscles of the lower limbs in posture and balance
Where does the tectospinal tract arise and where does it project to?
The superior colliculus of the midbrain
All levels of spinal cord
Does the tectospinal tract desiccate?
Yes it crosses to the contralateral side in the dorsal tegmental ganglion
What is the tectospinal tract responsible for?
Coordination of eye-head movements Responses to visual and auditory stimuli
Where does the rubrospinal tract originate and why does it descend to?
Red nucleus and descends to the cervical level
Does the rubrospinal tract desiccate?
Yes it crosses over immediately to the contralateral side in the ventral tegmental deccussation
What is the rubrospinal tract responsible for?
It facilitates motor neurones that innovate flexor muscles and provides tone in facial muscles
Why do you get clonic jerks during sleep?
There is massive inhibition when falling asleep but so much conductance that you excite them so there is a reverse potential
It is still inhibition but it’s so deep it appears as excitement
In babies clonic movement is due to descending pathways not being fully formed yet
Explain how gamma motor neurones regulate tone?
They innovate intrafusal muscle fibres and splits to make 2 terminal points
It produces local contraction at the terminal points and a stretch in the centre which activates alpha motor neurones
The muscle is always under tonic activity from gamma motor neurones
Explain muscle recruitment order
The smallest neurones are innovated first and therefore you can decide how much force you generate with increasing power
Explain why descending inhibition does not affect the monosynaptic reflex
This is because there is only one synapse so it is still hit by inhibition but it doesn’t inhibit it effectively due to decreased synapses.
