Functional hierarchy of the Motor system Flashcards
How does the brain voluntary control muscles?
via alpha motoneurones in the spinal cord - circuits at each segmental spinal level
What division of the NS does reflex control of muscles come under
PNS - autonomic
Descending inputs from which tracts regulate trunk and limb muscle reflexes?
vestibulospinal and reticulospinal
Brainstem nuclei recieve control inputs about voluntary movements from higher centres too. Give examples
the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia and the cerebellum
What are the 4 systems that control movement?
descending control pathways
basal ganglia (found deep within cerebral hemispheres)
cerebellum
local spinal cord/brain stem circuits
More proximal muscles like the shoulders and arms lie where in the spinal cord?
more medially
More distal muscles like the legs lie where in the spinal cord?
more laterally
The spinal cord receives descending input via what? and direct cortical input via what?
the brainstem and direct cortical input via the corticospinal 9pyramidal) tract
Where does sensory input enter
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 is the result of damage to sensory inputs (at spinal level)
paralysis as if the motoneurones themselves had been damaged (as sensory info isn’t being sent to brain so brain doesn’t know what muscles to move)
what is the simplest reflex found in all muscles?
the stretch reflex
best example of the stretch reflex?
knee-jerk reflex when you hit the patellar tendon
uses info from muscle spindles which monitor muscle length
Describe the knee-jerk reflex
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 which increases the number of APs in afferent nerves (towards CNS) projecting through the dorsal horn into the spinal cord.
spindle sensory afferents divide and make 3 types of connections
what are the 3 types of connections that spindle sensory afferents divide into?
- Many directly activate the alpha-motoneurones to the stretched muscle
- Sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles.
- Spindle afferent information also ascends in the dorsal columns - somatosensory cortex - tells brain about length of muscles
directly activating the alpha-motoneurones to the stretched muscle cause what
rapid contraction of the agonist muscle - monosynaptic reflex, no interneurons involved
When the agonist muscle contracts what happens to the antagonist muscle?
it relaxes (stretches)
this is called reciprocal inhibition - it happens because spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha-motoneurones to the antagonist muscle - which then relaxes it
How is the stretch reflex a classic negative feedback loop?
Antagonist muscle stretch (relaxation) - stimulates muscle spindles which causes reflex muscle contraction – muscle shortens to previous length
what is the inverse stretch reflex called?
golgi tendon organ or clasp-knife reflex
Describe the golgi tendon organ
caused by afferent nerves from the Golgi Tendon Organs (GTO) – which monitor muscle tension
muscle contracts and shortens – this pulls strongly on the tendon and 1b sensory nerves from the GTOs increases firing of APs
this causes -
1. 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.
Why do we have a clasp-knife reflex?
This reflex is polysynaptic and protective
It prevents muscles contracting so hard that the tendon insertion is torn away from the bone
What is the flexor (withdrawal) reflex and what does it do?
Flexor or withdrawal reflexes use information from pain receptors (nociceptors) in skin, muscles and joints.
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.
How does the flexor reflex happen?
Increased sensory APs from pain receptors causes:
Increased 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
FLEXOR REFLEX - ipsilateral (same side) flexion in response to pain
however if you withdraw limb from something and do nothing else you would fall over - need contralateral limb extension
Describe how contralateral limb extension works in order to stop you from falling over when you do the flexor 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 - tells the brain what is going on
True or false: the flexor and crossed extensor reflex is far slower than the stretch reflex
true
What happens if you put an excessive load on a muscle?
the GTO reflex is activated and the load is dropped rapidly however depending on what the load is you can control it via voluntary input from the CNS
ie if it was rocks then you would drop it but if it was a baby then you wouldn’t
