Descending Motor Pathways Flashcards
What does the ventromedial pathway do?
Controls axial and proximal limb muscles
Whole body movement
What are the three types of ventromedial pathway?
- Reticulospinal
- Vestibulospinal
- Tectospinal
What are the dorsolateral systems for?
Goal-directed movements of limbs, esp. hands, feet and face
What is the rubrospinal pathway?
Arises in red nucleus in midbrain
Type of dorsolateral system
Vestigial in man
What is the corticospinal tract?
Arises in motor cortex
Largest descending pathway in mammals
Dominant descending control system in man
Give three systems that contribute to postural stability
- Somatosensory
- Vestibular
- Visual
What is the vestibular system?
Proprioceptive system located in the inner ear labyrinth
Particular important for postural control
What are the vestibular system receptors?
Sensory hair cells
Located in the labyrinth in the semicircular canals and otolith organs (utricle and saccule)
Hair cells have directional sensitivity
What is the structure of the utricle and saccule?
Hair cells project into a jelly-like mass on which gravity acts
Different hair cells arranged to have different directional sensitivity
Afferents give head position signal
What is the structure of the semicircular canals?
Hair cells embedded in a cupula, which almost closes off the canal
Cupula is neutrally buoyant in the endolymph so is stationary when the head is stationary
Hair cells activated when cupula is deflected
What is the fluid in the semicircular canals?
Endolymph
What kind of signal do the utricle and saccule send?
Head position
Static signal
What kind of signal do the semicircular canals send?
When head movement begins or stops
Dynamic signal
Provide signals for feedforward control
What is the vestibular reflex?
- Postural instability detected by vestibular receptors
- Generates powerful vestibular correcting responses through connections in vestibulospinal pathway
- Principally influences extensor anti-gravity muscles
When might the vestibular system be damaged?
- Labyrinthitis (peripheral damage)
- Brainstem stroke (central)
- Cerebellar damage (central)
Plasticity reduces symptoms with time
What are neck reflexes?
Exactly equal and opposite to vestibular reflexes
Generated by neck proprioceptors
Permits differentiation between vestibular signals generated by body sway or neck movement
What mediates adjustment of neck reflexes?
The cerebellum
Where is the control system for consensual eye movement?
Brainstem
What is the vestibulo-ocular reflex?
Vestibular system detects head movement and drives equal and opposite movement in the eyes
Feedforward control mechanism
Calibration performed by cerebellum
What is the pathway of the vestibulo-ocular reflex?
- Movement detected in semicircular canals
- Afferents to vestibular nuclei
- Efferents from oculomotor nuclei
via MLF (medial longitudinal fasciculus) in brainstem
What is the optokinetic system?
Moves eyes to follow slow smooth movements in visual field
Movements driven by visual cortex
What is nystagmus?
Rapid resetting of eye in orbit by saccade once it has reached its limit of movement
When does nystagmus occur?
- In response to optokinetic or vestibular stimuli (physiological)
- Following cerebellar or vestibular damage (pathological)
What are saccades?
Rapid movements of the eyes to foveate visual stimuli
Gaze-shifting mechanism
Minimise time of blurring
