Descending pathways - control of movement Flashcards
How is the motor system in the descending pathway organised ?
Upper motor neuron cell bodies are in the brain or brainstem and do not project outside the CNS. They orchestrate more complex direct movements.
Lower motor neuron cell bodies are in the brainstem or spinal cord and project outside the CNS to muscle. They complete single muscle innervation, cell bodies in the ventral horn of spinal cord or motor nuclei of the brainstem
Interneurons co-ordinate groups of muscles and manages to get multiple LMN to fire at once, like shortcut and contract group of muscles
What are the 2 major systems of descending pathway from the cortex to motor centres?
Lateral pathways;
- Control voluntary movements
- Control distal muscles (fingers, toes, knees)
- Mainly controlled by cerebral cortex (via the corticospinal tracts)
Ventromedial (medial) pathway;
- Mainly control posture and locomotion (responses to change iii centre of gravity)
- Control the axial and proximal muscles
- Mainly controlled by the brainstem
- Mainly uncrossed
What are the characteristics of voluntary movements ?
1). purposeful goal directed
2). Triggered either by imagination or by wilful decision
3). Where the goal could be achieved by different strategies. e.g - writing on a small piece of paper or on a blackboard = motor equivalence - illustrates multiple levels of control movement
4). Often learned - may initially ned much concentration (driving or playing a guitar) but with practice movement can be achieved fluently. Efficiency and accuracy improves - as seen in infants
What 3 steps are involved in making a voluntary movement ?
Example - drinking a glass of wine;
1). Sensory integration: target identification; glass of wine needs to be identified as such and location understood
2). Planning: movements required to bring glass of wine to mouth must be ascertained. Relative positions of wine and mouth ned to be computed so movement can be planned (need spatial appreciation for this)
3). Execution: commands from cortical and brainstem centres need to be ordered and initiated in order to provide the correct strength grip and to smoothy move glass from the table to the mouth
Each phase involves the involvement of distinct areas of the cerebral cortex as well as feedback from basal ganglia and cerebellum
What are some facts about the somatotopically organised body man of the primary motor cortex?
1). the primary motor cortex is somatotopically organised (there is a body map projected onto it)
2). Some areas of the body are represented on thee somatotropin homunculus with a disproportionate size
3). This ‘cortical magnification’ reflects the relative ability for precise movement in that part of the body
4). Ergo hands and lips and tongue have a greater representation than the back
Where have lesions and electrical stimulation studies been mapped to?
Primary motor cortex
What different cortical areas are responsible for generating movement?
Premotor cortex
Supplementary motor cortex
Primary motor cortex
The primary motor area doesnt work alone, these feed each other
What are some features of the premotor cortical areas?
Electrical stimulation of the area in front of th primary motor area also produces movement - known as the pre-motor cortex
However, stronger and more prolonged stimuli are necessary than the primary motor area - implies a less direct path to lower motor neurones
Movements produced are usually more complex;
- Can involve more than one joint
- Can be bilateral
What is the role of the Premotor cortex?
It projects fibres to;
- Brainstem motor nuclei (controls things like facial movements)
- Spinal circuits controlling proximal and axial muscles (mainly postural) - e.g if going to pick up somethings lower back muscles tense so you don’t.fall forward with weight)
The premotor cortex is thought to bee involved in the plasticity of complex sequences of movements based on prior experience and is influenced by memory and the limbic system.
Neurons in this area begin to fire in primates prior to movement but only when a stimulus requiring moment is detected. Shows an intent to move and is also active in planning
It is thought that the area is important in orienting the body in preparation for a voluntary movement (contributing to establishment of an appropriate postural set)
What would happen If the Premotor, Supplementary motor or Primary motor cortex were damaged?
Premotor cortex damage produces more complex deficits:
- Apraxia occurs where you cannot perform tasks that involve a complex sequence of movements like brushing one’s hair or drawing a quick sketch
Supplementary motor cortex - role of this is less clear, one function has been demonstrated by fMRI which is rehearsal if complex movements stimulates this area.
If the primary motor cortex was destroyed then initially electrical stimulation of either premotor areas has no effect - therefore we know their principal actions are mediated through the primary motor cortex
What are the other inputs to the corticospinal output that fine tune movement?
3 major sources of input;
Sensory receptors
- Via the somatic sensory area, the premotor areas r the posterior parietal association cortex
From the cerebellum
- Both planning (feed forward) movement and corrective feedback from proprioception etc allows body to know where it is
From Basal Ganglia (basal nuclei misnamed for years nuclei is correct)
- Both inhibiting complex movement and motor correction plus an overlay of an motional component from the limbic circuits
What is the general organisation of the descending neuronal tracts involved in voluntary movement ?
Signal from cerebral cortex passes down though the thalamus and condenses there. The is taken by the corticospinal tract which is a collection of pathways at this point, down into the brainstem.
In the brainstem corticobulbar tracts split off from it which influence facial muscles and descending postural pathways off the brainstem and they synapse with multiple cranial nuclei outputs and relays.
Some impulse is also received from the multiple cranial nuclei outputs and joins back onto the corticospinal tract in the brainstem.
Most axons in the corticospinal tract decussate at the ventral pyramids in the brainstem (lateral corticospinal tract).
About 10-15% remain ipsilateral until they finally innervated bilaterally at the level of their ventral root (anterior/medial corticospinal). These help to change posture.
What are the multiple cranial nuclei outputs and relays that the corticobulbar tracts synapse with?
Vestibulospinal - balance
Reticulospinal - muscle tone, orientation, breathing
Rubrospinal - cerebellar influenced upper limb movement
Tectospinal pathways - head movements to follow sight (e.g car driving past)
How are the cortical tracts involved in voluntary movement?
The primary motor control route is the corticospinal tract
It contains about 1 million nerves fibres, 2/3 of which come from th primary ,motor cortex, remainder are mainly from pr-motor are and association areas
Most axons (about 85-90%) in the corticospinal tract decussate at the ventral pyramids in the brainstem (lateral corticospinal tract). This pathway feeds control to distal muscles contralaterally
10-15% remain ipsilateral (no decussation) until they finally innervated bilaterally at the level of their ventral root (anterior/medial corticospinal). This pathway feeds control to the axial muscles bilaterally
The axons in the lateral tract are voluntary motor and control distal muscles (for example digits)
How do lesions of the corticospinal neurons occur?
Lesions of corticospinal neurons;
Relatively common as the axons are very long (therefore vulnerable) these neurons are upper motor neurons.
Most commonly caused by infarcts related to cerebrovascular incidents
The middle cerebral artery (supplying the lateral surface of the hemisphere and the internal) is particularly vulnerable
Trauma, tutors and demyelinating diseases can also damage the corticospinal system
