Descending Motor Pathways Flashcards
How are most “voluntary” movements initiated?
By the cerebral cortex achieved when the cortex activates “patterns” of function stored in lower brain areas—the cord, brain stem, basal ganglia, and cerebellum.
How do the lower brain areas aid the cerebral cortex in initiating movement
They send specific control signals to the muscles
How are a rare few movements initiated?
For a few types of movements, the cortex has almost a direct pathway to the anterior motor neurons of the cord, bypassing some motor centers on the way. This is especially true for control of the fine dexterous movements of the fingers and hands.
What is needed for goal directed or purposeful movement?
Goal or purpose, Activation of relevant movement Feedback of movement Refinement/correction of movement Cessation on completion
What are the lower motor neurons (LMNs)?
Lower motor neurons (LMNs)
Located in the anterior grey horn of the spinal cord
Alpha motor neurons activate contraction of extrafusal muscle fibres.
Gamma motor neurons activates contraction of intrafusal muscle fibres, associated with muscle spindles – (monitor muscle stretch)
Both leave spinal cord via ventral roots
What are the upper motor neurons (UMNs)?
Collective term for the neurons located in the cerebrum or brainstem that influence the activity of LMN’s
Where are the motor regions of the cerebrum?
Anterior to central sulcus of the cerebral hemisphere.
Occupies ~1/3 of frontal lobes
What are the sub areas of the motor regions of the cerebrum?
Primary motor cortex
Premotor area
Supplementary motor area
Describe the primary motor cortex
Topographical representations mapped by electrical stimulation of motor cortex areas.
> ½ primary motor cortex controls hands and muscles of speech.
Excitation of a single UMN usually excites a specific movement – not one specific muscle.
-A pattern of separate muscles are excited.
Found directly anterior and parallel to somatic area 1
Describe he premotor area
On posteriolateral aspect of motor region closest to the lateral sulcus
Similar topographical representation to primary motor cortex.
Nerve signals give more complex “patterns” of movement.
Planning of movements
Describe the supplementary motor area
On superior aspect of motor region
Contractions elicited are often bilateral (e.g., grasping movements of both hands).
Usually functions together with premotor area.
Gives background movement onto which premotor and primary motor cortex add finer control.
What are the specialised areas of motor control?
Identified by electrical stimulation or observation of loss of motor function with destructive lesions.
Examples: Broca’s area (motor speech area) “Voluntary” eye movement field Head rotation area Hand skills area
Where is the brainstem?
A cranial continuation of the spinal cord.
What role does the brainstem have in control of motor functions?
Brainstem nuclei are involved in motor + sensory functions of face and head regions.
Also, provides many special control functions:
Control of respiration, CV system
Partial control of GI function
Control of many stereotyped body movements, equilibrium, eye movements
Also serves as a way station for “command signals” from higher neural centers.
What is the corticospinal tract?
AKA pyramidal tract
Carries signals direct from cortex to spinal cord.
Concerned with voluntary, discrete, skilled movements.
Especially distal parts of the limbs.
Where do the inputs of the corticospinal tract originate from?
30% from primary motor cortex;
30% premotor/supplementary motor;
40% other areas (primary somatosensory cortex).
What tract is also known as the pyramidal tract?
Corticospinal tract
What is the pathway of the corticospinal tract?
Primary motor cortex -> pyramids of medulla -> (majority/~90%) decussate in lower medulla -> lateral corticospinal tracts -> mainly terminate on interneurons.
Those axons that do not decussate (~10%) pass ipsilaterally in ventral corticospinal tracts.
Where os the decussation of the pyramids in the corticospinal tract?
The medulla
What structure of the corticospinal tract is present in the medulla?
The decussation of the pyramids
What is the lateral corticospinal tract?
Lateral corticospinal tract innervates mainly distal muscles groups of the extremities via direct communication with the LMN in contact with a specific muscle.
But does influence muscles of the entire limb.
Influences LMN circuits to integrate sensory and motor actions to achieve the desired movement pattern.
E.g., gait cycle
Where is the lateral corticospinal tract found?
In the motor region between the dorsal and ventral horns of the grey matter of the spinal cord
What is the ventral corticospinal tract?
Postural adjustments to stabilize trunk during limb movements led by LSCp tract.
Ventral corticospinal tract not cross at the pyramidal decussation BUT majority do cross the midline at the relevant spinal cord level.
Bilaterally innervates LMN’s controlling the trunk and proximal musculature.
What is posture?
Posture = position of body and its parts relative to each other. Is a compromise between balance and movement.
How does the body maintain balance?
Posture is adjusted predominantly by involuntary movement driven both predictively (postural set) and reactively (compensation).
Is controlled and driven by the brainstem and is where the postural set is governed.
Movement changes balance and so one’s posture must compensate.
There are mechanisms for both unexpected and expected movement.
Most important interplay therefore is between sensory information and movements.
Where is sensory information integrated from in posture control?
Muscle proprioceptors (detect changes in muscle length and or tension).
Sense of balance derived from movements of the head relative to the earths gravitational field (vestibular apparatus).
Visual inputs (detecting movements in visual field representing movement of the body).
This sensory information is predominantly integrated in the brainstem and acted upon via extra pyramidal tracts.
What are the extra pyramidal tracts to know other than the corticospinal tracts*****?
Reticulospinal tracts
Vestibulospinal tracts
Rubrospinal tract
Tectospinal tract
