Descending Neural Pathways Flashcards
what is the definition of a lower motor neurone
there cell bodies are in the brain stem or spinal cord and project outside the CNS to muscle
what is the definition of an upper motor neurone
cell bodies are in the brain or brainstem and DO NOT project outwit the CNS
Role of interneurones
coordinate groups of muscles
what is the role of an upper motor neurone
orchestrate complex directed movements
what is the role of lower motor neurones
single muscle innervation, cell bodies are located in the ventral horn of the spinal cord or the motor nuclei of the brainstem
what are the two major systems of the descending pathway from the cortex to the motor centres
- lateral pathways
2. ventromedial (medial) pathways
what is the role of the lateral pathway
- control voluntary movements
- control distal muscles
- mainly controlled by cerebral cortex (via the corticospinal tracts
what is the role of the medial pathway
- Mainly control posture and locomotion
- Control the axial and proximal muscles
- Mainly controlled by the brainstem
- Mainly uncrossed
what are the 4 characteristics of a voluntary movement
- Purposeful goal directed
- Triggered either by imagination or by a wilful decision
- Where the goal could be achieved by different strategies.
e.g. writing on a small piece of paper or writing on a blackboard (example of motor equivalence - Often learned
may initially need much concentration (Driving or playing a guitar) but
with practice movement can be achieved fluently. Efficiency and accuracy improves
Motor Equivalence
illustrates multiple levels of control of movement
what is the structure of a voluntary movement
- Sensory integration: Target identification
- Planning; movements required
- Execution; commands from cortical and brainstem centres are needed
Each phase involves the involvement of distinct areas of the cerebral cortex as well as feedback from basal ganglia and cerebellum
what is area 4 on Brodmanns cortical map
primary motor cortex
what is immediately posterior to the primary motor cortex
primary somatosensory cortex
on what map is the primary motor cortex somatotopically organised
the somatotopic homunculus
what is immediately anterior to the primary motor cortex
the premotor cortex
what is the role of the pre-motor cortex
this area produces stronger more prolonged stimuli, which implies a less direct route to the lower MN’s
the movements are more complex:
- they can involve more than one joint
- can be bilateral movement
important in orientating the body in preparation for a voluntary movement (contributing to an established postural set)
where does the premotor cortex’s fibres project to ?
- the brainstem motor nuclei
2. spinal circuits controlling proximal and axial muscles (mainly postural)
which area of the brain controls the plasticity of complex sequences of movements based on prior experience, and is influenced by memory and the limbic system
premotor cortex
If primary motor area is destroyed
initially electrical stimulation of either premotor areas has no effect
this means the their principles action are mediated through the primary motor cortex
Damage to the premotor areas
produces more complex deficits
an example of a deficit which has resulted from damage to the premotor cortex
Apraxia -cannot perform tasks that involve a complex sequence of movements like brushing one’s hair or drawing a quick sketch
3 Major sources of input:
• Sensory receptors
via the somatic sensory area, the premotor areas or the posterior parietal association cortex
• From the Cerebellum
-Both planning (feed forward) movement and corrective feedback from proprioception etc.
• From the Basal Ganglia
-Both initiating complex movement and motor correction plus an overlay of an emotional component
from limbic circuits.
when the corticospinal tract enters the brain stem it has an initial branch, what is this called
corticobulbar tracts - these influence facial muscles and descending postural pathways of the brainstem
where fo corticobulbar tracts then go
Multiple cranial nuclei outputs and relays:
Vestibulospinal - balance
Reticulospinal – muscle tone, orientation, breathing Rubrospinal – cerebellar influenced upper limb movement
Tectospinal pathways – head movements to follow sight
there are then the two other ‘main’ branches
the anterior corticospinal and lateral corticospinal tract (this veing around 85% of nerve fibres)
anterior corticospianl tract
this tract descends ipsilaterally and at the level of their central root, innervates bilaterally
lateral corticospinal tract
this tracts descends and then crosses over to innervate, this is the voluntary control of distal muscles
Lesions of corticospinal - upper motor neurone
- common as they are so long
- most commonly caused by infarcts related to cerebrovascular incidents, trauma, tumours and demyelinating diseases can also cause damage.
- middle cerebral artery (this supplys the lateral surface of the hemispheres and the internal capsule) is particularly vulnerable
what is a negative sign
loss of function (weakness or paralysis)
what is a positive sign
this is the appearance of an abnormal response
UMN pattern weakness
- Arm extensors weaker than flexors (flexors are stronger)
* Leg flexors weaker than extensors (extensors are stronger)
Posture is
the position of a body and its parts relative to each other, its vitally important in balance and is adjusted predominantly by involuntary movement
how is posture adjusted
predictively (postural set)
and
Reflexively (compensation)
Compensatory responses are based on
information sent from
- 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)
These nuclei all receive information from the voluntary circuit collaterals which provides feed forward information
postural set is generated where
These sensory inputs converge on nuclei in the brainstem
