Descending pathways Flashcards
where are lower motor neuron cell bodies found
in brainstem or spinal and the axon leaves cns
where are upper motor neuron cell bodies found
in brain or brainstem but do not project outside the cns
why are there interneurons
to act as shortcuts, they attached to a number of lower motor neurons and synchronise their firing to contract a group of muscles
also have direct contact with upper motor neurons
what are the two major systems of the descending pathway
lateral pathways
ventromedial pathways
what do lateral pathways do
control voluntary movements
control distal muscles
mainly controlled by cerebral cortex (via corticospinal tracts)
ARMS ON LATERAL SIDE OF BODY (way to remember)
what do ventromedial pathways do
mainly control postural set and locomotion
control axial and proximal muscles
controlled by brainstem
mainly uncrossed
what are the characteristics of voluntary movements
purposeful goal directed
triggered by wilful decision
can be achieved by DIFFERENT STRATEGIES
often learned- with practice movement can be achieved fluently (eg learning instrument)
what is motor equivalence
illustrates multiple levels of control of movement
get same goal but different muscles used
has to be voluntary
v useful for plasticity
describe the structure of drinking wine (voluntary movement)
SENSORY INTERGRATION- identify target and location
PLANNING- movements must be ascertained, relative positions of wine and mouth need to be computed so movement can be planned
EXECUTION- commands from cortical and brainstem centres need to be ordered and initiated to give correct strength grip and to smoothly move glass
each phases involves distinct areas of cerebral cortex plus feedback from basal nuclei and cerebellum
what is cytoarchitecture
study of the cellular composition of the central nervous system’s tissues under the microscope
how brodmann’s cortical map was derived
what is the parietal lobe used for
association area
visual understanding
what is somatotopic homunculus
map along the cerebral cortex of where each part of the body is processed.
the projection of the body surface onto a brain area that is responsible for our sense of touch
bigger when fine motor output is larger- cortical magnification
eg. hands and lips> back
what are the two main targets for descending motor outputs
corticobulbar tract- take things that are remote from the head (hand, arm, feet etc)
corticospinal tract take things associated with head
what are the three motor cortices
primary motor cortex
supplementary motor cortex
premotor cortex
last two are anterior to motor cortex
does the primary motor cortex work alone
no
if stimulate promotor cortical areas you would get more complicated movements eg. more than one joint and bilateral
what do the brainstem nuclei control
facial movements
have fibres from premotor cortices
what do spinal circuits controlling proximal and axial muscles do
postural set
is the premotor cortex involved in plasticity of complex sequences of movements
yes
also important in orienting the body in preparation for a voluntary movement (establishing an appropriate postural set)
what happens to the premotor area if the primary motor area
primary movement initially stops and premotor areas electrical stimulation has no effect
therefore, principal actions are mediated through primary motor cortex
what happens when there is damage to the premotor areas
more complex deficits
eg. apraxia
what is apraxia
difficulty doing more complex tasks like writing, brushing teeth
what is a special function of the supplementary motor cortex
mental rehearsal of movements
therefore possibly involved in motor planning
in fMRI supplementary area lights up with mental rehearsal
good for prosthetic limbs
what other inputs to corticospinal out that fine tune movement are there
sensory receptors- via somatic sensory area, premotor areas or posterior parietal association cortex
cerebellum- planning and corrective feedback
basal nuclei- initiating complex movement and motor correction plus and overlay of emotional component from limbic circuits
describe the descending pathways
motor signal down cortex through thalamus as a mass that contains many pathways.
in brainstem corticobulbar tracts synapse with a number of cranial nuclei outputs and relays
85-95% of corticospinal tract decussates to contralateral side to pyramids–> LATERAL corticospinal tract
10-15%small amount does not decussate, ipsilateral- innervates bilaterally for pre-emptive movement to change posture voluntarily–> ANTERIOR corticospinal tract
VOLUNTARY pathways
what is the primary spinal motor control route
corticospinal tract
contains 1000000 nerve fibres (2/3 come from primary motor cortex)
why is it common to get corticospinal lesions
the axons are long
what is the most common cause of upper motor neuron lesions
infarcts related to cerebrovascular incidents
the middle cerebral artery is particularly vulnerable
other causes include trauma, tumours, demyelinating diseases
give an example of a positive sign following corticospinal lesions
babinski sign
what is a negative sign
what is a positive sign
negative is a loss of function (weakness or paralysis)
positive is an abnormal response
why are there differences in umn and lmn lesions
umn synapse with lower motor neurons and do not leave the cns
lmn synapse with muscle
what happens with upper motor neuron lesions
pyramidal muscle weakness
no muscle atrophy (lmn) still contacting muscle and tension is still there
no fasciculation (damage stops signal before lmn-lmn axon can only cause it)
Increased muscle tone- spasticity (if cut descending inhibition , increase tension)
Increased stretch reflexes
Abnormal reflexes
LMN are neurotrophic
yes
if they are cut you lose tension created by this
LMN lesions signs
muscle weakness
muscle atrophy
fasciculation
reduced muscle tone
reduced stretch reflexes (need tone to get stretch reflex)
what are the UMN pattern weakness in the limbs
ARMS- flexors> extensors
LEGS- extensors> flexors
what is the ventromedial pathways
central ones that control POSTURE
imporrtant in balance
adjusted predominately by involuntary movement
driven
PREDICTIVELY (postural set)
REFLEXIVELY (compensation)
good test is standing with heels to the wall and touching toes– cant stick bum to counterbalance and maintain postural set
is there interplay between voluntary and involuntary movement
yes due to feedforward compensating for upcoming postural instability and feedback for unexpected postural instability
what are the principal compensatory reflexes
vestibular- body
collicular - head/body
reticular- body
tectal- head and trunk
what are three maim impacts
muscle proprioceptors (detect change in muscle length or tension)
sense of balance (mov of the head relative to the Earths gravitational field)
visual inputs (detecting movements in visual field representing mov of the body)
these sensory inputs converge on nuclei in brainstem and so the postural set is generated there
the nuclei receive info from the voluntary circuit collaterals which provide feed forward info
