Descending pathways

Question 1

where are lower motor neuron cell bodies found

Answer 1

in brainstem or spinal and the axon leaves cns

Question 2

where are upper motor neuron cell bodies found

Answer 2

in brain or brainstem but do not project outside the cns

Question 3

why are there interneurons

Answer 3

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

Question 4

what are the two major systems of the descending pathway

Answer 4

lateral pathways
ventromedial pathways

Question 5

what do lateral pathways do

Answer 5

control voluntary movements
control distal muscles
mainly controlled by cerebral cortex (via corticospinal tracts)

ARMS ON LATERAL SIDE OF BODY (way to remember)

Question 6

what do ventromedial pathways do

Answer 6

mainly control postural set and locomotion
control axial and proximal muscles
controlled by brainstem
mainly uncrossed

Question 7

what are the characteristics of voluntary movements

Answer 7

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)

Question 8

what is motor equivalence

Answer 8

illustrates multiple levels of control of movement
get same goal but different muscles used
has to be voluntary
v useful for plasticity

Question 9

describe the structure of drinking wine (voluntary movement)

Answer 9

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

Question 10

what is cytoarchitecture

Answer 10

study of the cellular composition of the central nervous system’s tissues under the microscope
how brodmann’s cortical map was derived

Question 11

what is the parietal lobe used for

Answer 11

association area
visual understanding

Question 12

what is somatotopic homunculus

Answer 12

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

Question 13

what are the two main targets for descending motor outputs

Answer 13

corticobulbar tract- take things that are remote from the head (hand, arm, feet etc)
corticospinal tract take things associated with head

Question 14

what are the three motor cortices

Answer 14

primary motor cortex
supplementary motor cortex
premotor cortex

last two are anterior to motor cortex

Question 15

does the primary motor cortex work alone

Answer 15

no
if stimulate promotor cortical areas you would get more complicated movements eg. more than one joint and bilateral

Question 16

what do the brainstem nuclei control

Answer 16

facial movements
have fibres from premotor cortices

Question 17

what do spinal circuits controlling proximal and axial muscles do

Answer 17

postural set

Question 18

is the premotor cortex involved in plasticity of complex sequences of movements

Answer 18

yes
also important in orienting the body in preparation for a voluntary movement (establishing an appropriate postural set)

Question 19

what happens to the premotor area if the primary motor area

Answer 19

primary movement initially stops and premotor areas electrical stimulation has no effect
therefore, principal actions are mediated through primary motor cortex

Question 20

what happens when there is damage to the premotor areas

Answer 20

more complex deficits
eg. apraxia

Question 21

what is apraxia

Answer 21

difficulty doing more complex tasks like writing, brushing teeth

Question 22

what is a special function of the supplementary motor cortex

Answer 22

mental rehearsal of movements
therefore possibly involved in motor planning
in fMRI supplementary area lights up with mental rehearsal
good for prosthetic limbs

Question 23

what other inputs to corticospinal out that fine tune movement are there

Answer 23

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

Question 24

describe the descending pathways

Answer 24

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

Question 25

what is the primary spinal motor control route

Answer 25

corticospinal tract
contains 1000000 nerve fibres (2/3 come from primary motor cortex)

Question 26

why is it common to get corticospinal lesions

Answer 26

the axons are long

Question 27

what is the most common cause of upper motor neuron lesions

Answer 27

infarcts related to cerebrovascular incidents
the middle cerebral artery is particularly vulnerable

other causes include trauma, tumours, demyelinating diseases

Question 28

give an example of a positive sign following corticospinal lesions

Answer 28

babinski sign

Question 29

what is a negative sign
what is a positive sign

Answer 29

negative is a loss of function (weakness or paralysis)
positive is an abnormal response

Question 30

why are there differences in umn and lmn lesions

Answer 30

umn synapse with lower motor neurons and do not leave the cns
lmn synapse with muscle

Question 31

what happens with upper motor neuron lesions

Answer 31

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

Question 32

LMN are neurotrophic

Answer 32

yes
if they are cut you lose tension created by this

Question 33

LMN lesions signs

Answer 33

muscle weakness
muscle atrophy
fasciculation
reduced muscle tone
reduced stretch reflexes (need tone to get stretch reflex)

Question 34

what are the UMN pattern weakness in the limbs

Answer 34

ARMS- flexors> extensors

LEGS- extensors> flexors

Question 35

what is the ventromedial pathways

Answer 35

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

Question 36

is there interplay between voluntary and involuntary movement

Answer 36

yes due to feedforward compensating for upcoming postural instability and feedback for unexpected postural instability

Question 37

what are the principal compensatory reflexes

Answer 37

vestibular- body
collicular - head/body
reticular- body
tectal- head and trunk

Question 38

what are three maim impacts

Answer 38

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