Functional Hierarchy of the Motor System Flashcards
How does voluntary brain control of msucles happen?
voluntary brain control of muscles is via alpha motoneurones in the spinal cord
What is in control of the reflex contorl of muscles?
reflex control of muscles is autonomous
It is hard-wired into motoneurone circuits at each segmental spinal level
brainstem nuclei themselves receive control inputs about voluntary movements from what higher centres?
the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia and the cerebellum
What 4 systems contorl movements?
descending control pathways
basal ganglia
cerebellum
local spinal cord/brain stem circuits
What motor neurons from the spinal cord control medial and lateral muscles?
- proximal shoulder muscle are mapped to medial motoneurones
- distal finger muscles map to more lateral motoneurones
How does the spinal cord recieve descending input?
Higher brain centres are functionally interdependent and control different aspects of voluntary movements
the spinal cord receives descending input via the brainstem AND direct cortical input via the Corticospinal (Pyramidal) tract
Sensory inpput is cruical, what levels does it enter at?
- at the spinal cord in the form of proprioceptors, touch, pain etc
- at the brainstem the vestibular system informs about balance
- at cortical level, we make movements in response to visual, olfactory, auditory, emotional, intellectual cues
What can damage to sensory inputs at the spinal level elad to?
paralysis if the motorneurones themself have been damaged
Apart form the proprioceptive system, what else can give information on where the parts of the body are?
the visual system
What is a spinal reflex?
those which act through the spinal cord
building blocks for movements
What is the simplest reflex that is found in all muscles?
strech reflex
What is an example of the strech reflex?
patellar tendon or knee-jerk reflex
What is the process of the strech reflex being initiated?
follows a sharp tap to an inelastic tendon
force is transmitted to the muscle fibres – they are more elastic than tendons and so are more able to stretch
stretch activates the sensory nerves in the muscle spindle
this increases the number of APs in afferent nerves projecting through the dorsal horn into the spinal cord
in the strech reflex, the spindle sensory afferents divide and make 3 types of connection, what are they?
- Many directly activate the a-motoneurones to the stretched muscle causing rapid contraction of the AGONIST muscle - this is a MONOSYNAPTIC REFLEX, no interneurones are involved, it is the only such connection known
- the sensory fibres connect indirectly with and influence the antagonist muscles
So, when the AGONIST muscle contracts, the ANTAGONIST muscle relaxes (stretches)
This happens because spindle afferents connect with and activate inhibitory interneurones which decreases activation of a-motoneurones to the antagonist muscle - which then relaxes. This is called RECIPROCAL INHIBITION
- Spindle afferent information also ascends in the dorsal columns and makes connections in the somatosensory cortex to tell the brain about length of muscles
What type of feedback loop is the strech reflex?
Negative feedback loop
Muscle stretch stimulates muscle spindles which causes reflex muscle contraction - muscle shortens to previous length
What causes the inverse strech reflex?
caused by afferent nerves from the Golgi Tendon Organs (GTO) – which monitor muscle tension
How are APs fired from the GTO?
muscle contracts and shortens – this pulls strongly on the tendon and 1b sensory nerves from the GTOs increase firing of APs
What do APs from the GTOs cause?
- Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength
- Activation of excitatory interneurones to antagonist muscles
- Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex
What happens in the golgi-tendon organ reflex and why?
agonist muscle is inhibited and relaxes rapidly
antagonist muscle is activated leading to it contracting
this reflex is polysynaptic and protective
it prevents the muscle contracting so hard that the tendon insertion is torn away form the bone
Why is the GTo reflex sometimes called the clasp-knife reflex?
because greatly increasing tension in tendon leads to a collapse of resistance
like a spring-loaded knife opening up
it is a protective mechanism to prevent muscle damage
Flexor (Withdrawal) reflex - with ___________
crossed extension
What does the flexor reflex use information from?
Flexor or withdrawal reflexes use information from pain receptors (nociceptors) in skin, muscles and joints
What is the flexor reflex?
they are polysynaptic and protective
they withdraw part of the body away from the painful stimulus and in towards the body - so they flex the affected part
ipsilateral flexion in response to pain
How does the flexor reflex occur?
Increased sensory APs from pain receptors cause:
- activity in the flexor muscles of the affected part via a number of excitatory interneurones
- At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited
What is the cuircity of the flexor withdrawl reflex?
Small diameter Aδ nociceptive fibres triggering pain enter cord
they branch and activate interneurons in several spinal segments
which activate α motoneurones controlling all the flexor muscles of the affected limb
If you withdraw the limb form the stimulu and do nothing else what would happen and therefore what is done about this?
if you withdraw limb from a tack and do nothing else, you would fall over – which is NOT protective!
to prevent this the contralateral limb extends
How does the contralateral limb extend during the flexor withdrawl reflex?
- Several excitatory interneurones which cross the spinal cord excite the contralateral extensors
- At the same time, via several interneurones, there is inhibition of the contralateral flexors - this helps to maintain an upright posture by extending the limb to bear the body weight.
- Sensory information ascends to the brain in the contralateral spinothalamic tract
What is the Spinal cord circuitry for flexor and crossed extensor reflex?
one leg flexes to withdraw from the painful stimulus (on left)
the other extends to maintain balance because of shifted weight
there are several interneurons in the pathway - each with a small synaptic delay
nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly
So: the flexor and crossed extensor reflex is far slower than the stretch reflex
Can reflexes be over-ridden consciously?
Ask someone to hold an increasing load. The load stretches the muscle and its spindles, causing reflex contraction to restore the arm position (stretch reflex)
When load is excessive, GTO reflex is activated and load is dropped rapidly but if the load was not blocks, but a child, would you inevitably drop it because of the Golgi tendon reflex?
NO!
The GTO reflex can be over-ridden by voluntary input from the CNS
How can the inhibition from the GTOs be over ridden and contraction maintained?
one α motoneurone receives >10,000 synapses (in red below)
many are from the brain: descending cortical excitatory and inhibitory signals so continual integration of EPSPs and IPSPs
So when holding something heavy but very important, descending voluntary excitation of a motoneurones can override
Can the strech reflex also be overridden?
strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not be evoked
so when testing reflexes, subjects may need to be distracted to prevent voluntary effects on the reflex responses
So the absence of stretch reflex responses in an uncooperative patient may not be evidence of peripheral nerve damage
