Motor cortex and reflexes Flashcards
What is the function of the cerebral cortex?
Voluntary movement and motor learning.
Sensory input provides constant feedback for adjustments. Involves the precentral gyrus / primary motor cortex
What type of control is in the two types of descending pathways?
Lateral = Voluntary Ventromedial = Unconscious brainstem control
What are the features of the motor system?
Hierarchy organisation
Feedback loops from sensory input
Somatotopic representation
What is somatotopic representation?
Allows the cortex to detect where the information is coming from so that the correct muscles contract.
Why is the cortex not needed for every movement?
The brainstem allows unconscious control due to motor memories e.g. walking
Where is a protective reflex produced?
Spinal cord as part of a closed loop e.g. limb withdrawal
How is a rhythmic motor pattern produced?
Uses combination of reflexes and voluntary e.g. chewing, breathing. Usually involuntary overrides
Where does the command for a voluntary purposeful movement originate?
Higher centres in an open loop for conscious control
Motorneuron activity is regulated by which types of inputs?
Sensory input to provide local feedback via dorsal roots.
Spinal interneurons to generate contraction or relaxation in the ventral horn.
UMN for initiation and control and input to interneurons.
Why can coordinated movements still occur even if the descending influences are severed?
Spinal cord circuits generate movement in isolation but is dependent on sensory input to induce the reflex. = central pattern generators
Which type of motor neurons do interneurons influence?
alpha motor neurons
What is the purpose of UMN descending input?
Sophisticated, adaptable patterns of voluntary movement superimposed on intrinsic circuit of the spinal cord
What are medial motorneurons used for?
POSTURE, locomotion and larger movements to control axial/trunk and proximal muscles (upto elbow and knee)
EXTENSORS
Involuntary and unconscious
What are the lateral motorneurons used for?
Fine motor and voluntary control over distal muscles (hands, feet, digits)
FLEXORS
Which descending tracts use the lateral motorneurons?
Corticospinal tract
Rubrospinal tract
Which descending tracts use the ventromedial motorneurons?
Reticulospinal tract
Vestibulospinal tract
Tectospinal tract
What is the pathway taken by the corticospinal tract?
Projects directly from motor cortex in areas 4 + 6 to later spinal motor neurons. Forms synapse with alpha motor neurons in ventral horn for control over distal muscles, particularly flexors.
What is the pathway taken by the rubrospinal tract?
Originates in the red nucleus of midbrain. The tract receives input from the motor cortex. For voluntary movement of distal muscles. The contralateral projections from the red nucleus run down the lateral column of the spinal cord.
What does a lesion in the lateral pathway result in?
Motor deficits in the voluntary movements causing them to be SLOWER. Posture unaffected.
What is severing of the lateral ascending pathways used for? What is its risk
To treat intractable pain, but it may damage the lateral descending pathway at the same time.
What compensates for a loss of corticospinal tract?
Rubrospinal can compensate for all but fine digit control. Shows plasticity as recruits cortical output via rubrospinal tract.
What are ventromedial pathways also known as?
Extra pyramidal tracts
Where do ventromedial pathways originate from and terminate?
Brain stem nuclei with contra and ipsilateral pathways. Terminate at medial alpha MNs.
What is the function of modulating spinal reflexes?
Allows for small adjustments to body position, balance and visual input
How are the ventromedial pathways paired?
Pontine reticulo-spinal + medullary reticulo-spinal
Vestibulospinal + tectospinal
Which nuclei does the pontine reticulo and medullary reticulo spinal tract pass through?
Reticular nuclei
Which nuclei does the vestibulospinal and tectospinal tracts pass through?
Superior colliculus and vestibular nuclei
What is the function of the vestibulospinal tract?
Relays GRAVITATIONAL sensory information form vestibular labyrinth of the inner ear and stretch receptors in axial muscles to maintain head, neck and leg position.
What is the function of the tectospinal tract?
Relays VISUAL sensory information from retina and visual cortex to orientate head and eyes towards the visual and auditory stimuli
What is the function of the pontine reticulo-spinal tract?
Originates in the reticular nuclei of the pons to enhance ANTIGRAVITATIONAL reflexes of the spinal cord to facilitate leg extensors to maintain standing POSTURE.
What is the function of the reticulo-spinal tract?
Originates from the reticular nuclei of the medulla. Has opposing effects to pontine to free antigravitational muscles from reflex control for VOLUNTARY override e.g. sitting from standind.
What is involved in the process of movement other than the execution?
Sensory input
Planning and thinking
Deciding appropriate action
Holding plan in memory
What structures form the sensorimotor system?
Pre and post central gyrus
Which areas form the cortical motor areas?
Area 4 = primary motor cortex
Area 6 = Premotor area + supplementary motor area
What are the differences between area 4 and area 6?
Area 4 has a lower stimulus threshold to from strong synaptic links.
Area 6 is for more complex movement and involved in underlying thought processes.
What does the primary motor cortex do?
Control of DISTAL musculature for FINE motor control
What does the premotor cortex do?
Control of PROXIMAL musculature for POSTURE and BALANCE and movement sequencing, preparation and initiation
What does the supplementary motor area do?
Role in planning and initiating for bimanual coordination.
Which tract do most UMN contribute to?
From the primary motor cortex, 50% pass to corticospinal tract.
What is the somatotopic organisation of the motor cortex?
Organised into motor pools which activate a small group of muscles each rather than single muscles. But individually encode the force or direction of movement.
What type of neurons make up the cortical column outputs of the UMN?
Betz cell neurons and interneurons
What runs with the UMN columns?
Sensory inputs from muscles and joints to provide feedback on the circuit to control betz cells.
What may cause UMN damage?
Stroke or tumour
What are the signs of UMN damage?
Initial muscle weakness
Spasticity (continuously contracted state causing stiffness and increased resistance to passive movement)
Hypertonia
Hyper-reflexia as no inhibiting cortical influence
Affects the CONTRALATERAL side
