Motor function Flashcards
Describe the role of the motor cortex
The motor cortex controls the descending pathways that activate motor neurones
However, it is not the site of voluntary initiation of movement; the decision to move comes from elsewhere in the brain
What are the functions of descending motor pathways?
Control of voluntary movement
Modulate reflexes
Gamma bias
Which centres of the brain send descending projections to the spinal cord to inhibit pain transmission? Which neurotransmitters are involved?
PAG and RVM centres
Noradrenaline and Serotonin
Axons from which descending tract form the medullary pyramids?
The corticospinal tract
Name the two major descending tracts
Corticospinal tract
Rubrospinal tract
Which pathways control axial muscles? What movements are these pathways responsible for?
Medial pathways
Posture, balance, coarse control of movement
Where do the corticospinal tracts originate?
In the motor cortex, areas 4 and 6
Describe the course of the corticospinal tracts
Motor cortex
Internal capsule
Cerebral peduncle
Medullary pyramids
Caudal part of the medulla - this is where they decussate
Lateral corticospinal tract (contralateral side)
Synapse in ventral horn onto interneurones or a-motorneurones
Exit spinal cord via ventral root
What forms the ventral corticospinal tract?
Axons from the corticospinal tract that do no decussate in the medulla (about 10%)
- these decussate in the spinal cord at the level at which they terminate
What is the role of nerves in the rubrospinal tract?
Carry voluntary motor commands
Excites flexor muscles and inhibits extensor muscles
Can potentially act as an alternative tracts to the corticospinal tract
Where does the rubrospinal tract originate? Where does it receive most of its input from?
The rubrospinal tract originates in the red nucleus of the midbrain and receives most of its input from the cerebellum (also receives some input from the motor cortex)
Name the three ventromedial motor pathways
Vestibulospinal tracts
Tectospinal tracts
Reticulospinal tracts
What is the main role of the ventromedial pathways?
Maintenance of posture and balance
What is the role of the tectospinal tract?
Ensures that the eye remains stable as the body moves
this means that most fibres terminate in the upper cervical levels of the spinal cord
What are the roles of the vestibulospinal tract?
Stabilise posture and head movements
Primarily controls extension of lower limbs (and upper limbs but to a lesser extent)
What is the role of the reticulospinal tract?
Integration of sensory information into motor output:
- control trunk muscles to maintain balance and body position
- primarily controls flexion of the upper limbs (and lower limbs but to a lesser extent)
What is the role of gamma motoneurones?
innervate intrafusal muscle fibres which helps to maintain the tautness of muscle spindles to keep them sensitive to changes in muscle length
What is the role of alpha motoneurones?
innervate extrafusal muscle fibres, causing muscle contraction
- receive and integrate inputs from various arts of the brain and from sensory receptors
- release acetylcholine at the neuromuscular junction
Name a type of cell that makes up the upper motor neurones
Pyramidal cells
Describe the function(s) of the vestibular nuclei
Receives sensory input from the vestibulocochlear nerve (Cr N VIII) regarding head position.
Is the origin of the vestibulospinal tract, a descending motor pathway that controls extension of the lower (and upper) limbs.
What sort of lesion causes decorticate rigidity? Describe the effects of such a lesion
A lesion in the cortex, above the level of the red nucleus in the midbrain.
Both the rubrospinal and vestibulospinal tracts are still intact, but cortical innervation is disrupted, so the patient tends to have flexed upper limbs and extended lower limbs.
What sort of lesion causes a decerebrate rigidity? Describe the effects of such a lesion
A lesion below the level of the red nucleus but above the level of the vestibular nucleus (i.e. between the midbrain and the pons).
The rubrospinal tract is no longer active, so the upper limbs are no longer flexed.
The vestibulospinal tract however, is still intact, so both the upper and lower limbs are extended.
What is the difference between the cortex and the cerebrum?
The cerebral cortex is the outer layer of gray matter of the cerebrum
