4. Spinal Cord, Reflexes & Muscle Tone Flashcards
Overview of motor control hierarchy Spinal segments, dorsal & ventral roots & arrangement of grey & white matter. Types of motor control, includingspinal reflex arcs, central pattern generators & controlof voluntary movement (focusing on the corticospinal tract)
What part of the motor system is at the apex of the hierarchy and what does it do?
- Cerebral cortex
- responsible for spontaneous, novel and adaptive voluntary movements
What is at the bottom of the hierarchy and what does it do?
- Monosynaptic spinal reflex arc
- most basic type of involuntary movement
What are the 4 types of movement?
- reflexive
- postural
- rhythmic
- voluntary
What are basal ganglia loops and what neurotransmitter are they controlled by?
- They are a set of loops connecting motor cortex and basal ganglia
- arise from a particular region of the frontal lobe -> passes through basal ganglia -> projects back to the cortical region of origin (via relay in the thalamus)
- controlled by neurotransmitter Dopamine
What does the basal ganglia do?
- Initiation of voluntary actions
- Selection of a particular action amongst a range possible/potential behaviours
- learning and performance of various repetitive, semi automatic behaviours (or ‘habits’)
What disease can affects basal ganglia?
- damage to basal ganglia commonly associated with movement disorders e.g. Parkinson’s
- however, basal ganglia also involved in cognition, behaviour and emotion -> due to presence of non motor loops that arise and terminate in other parts of the cerebral cortex
What does the cerebellum do? What does damage to it do?
- thought of as motor structure (even though it doesn’t initiate movement and damage to it doesn’t cause weakness and paralysis) -> but it gathers vast amount of sensory and other info and uses this to “advise” the motor areas of the frontal lobe
- this helps actions to be performed in a smooth, precise and coordinated manner
- this is why damage to cerebellum causes clumsiness and poor coordination (called cerebellar ataxia)
Describe the pathway of how cerebellum received info and what it does to correct movements to make them more accurate and coordinate?
- Constant stream of info about intended movements that motor cortex aiming to make is coming from the FRONTAL LOBE to the CONTRALATERAL cerebellum via the PONS
- constant stream of sensory information is also coming to the cerebellum from range of sources (propioreceptors, inner ear, visual system etc.) which continually updates information on precise relative positions of the head, limbs and trunk
- so now with both these info (info about intended movements from frontal lobe and info of precise relative positions of the body) the intended movements (planned) are compared with the actual movements (performed) and any discrepancies are detected and sent as an “error signal” to the frontal lobe via efferent from cerebellum so it can correct the current on going action to make it more accurate)
- this process is only suitable for slow enough movements for movement-by-movement feedback to cerebellum and for correction to take place while action is being performed -> this of movement called NON-BALLISTIC (slow)
- ballistic -> rapid/explosion of movement e.g. fast, single joint movement like throwing dart -> not enough time for cerebellum to make “on time” correction once the movement has already started
How can you divide up white matter?
- divided by lines of attachment of dorsal and ventral roots into 3 longitudinal columns (known as funniculi): anterior, posterior and lateral
- the columns contain ascending and descending pathways
What are the ascending and descending pathways?
- Ascending:
- Dorsal Column: propioception, vibration, discriminative touch
- Spinothalamic: pain and temperature
- Spinocerebellar: feeds info to cerebellum about what lower limbs are doing so it can compare that info with intended actions and any mismatch sent to motor cortex as error signal so the movement can be corrected
- Descending:
- Only have to know about corticospinal tract
So the corticospinal tract can be divided into lateral and anterior, where are they, do their fibres cross over and what are they responsible for?
Lateral corticospinal tract:
- occupies lateral column of spinal cord
- contains 90% of fibres, these cross over at level of foramen magnum
- deals with distal flexors only
Anterior corticospinal tract:
- occupies anterior column of spinal cord
- don’t cross over
- they are a direct continuation of the pyramids of medulla
- deals with proximal extensors
In cross sectional images of the spinal cord at each level, what contains the most white matter and the most grey matter and why?
White matter -> Cervical region -> as all the ascending and descending pathways at this level (lowermost levels have least as most descending pathways has been terminated and ascending yet to join spinal cord)
Grey Matter -> amount is proportional to amount of muscle at each level -> so C7 and Lumbersacral has both as they need to supply limbs (thoracic has least)
We mentioned 4 types of movements earlier: voluntary, rhythmic, reflexive and postural.
Describe Voluntary
- most complex and least automatic (writing, speaking, playing piano)
- original from frontal lobe
- goal oriented actions that requires participation of cerebral cortex
- direct connection between cerebral cortex and spinal cord (lateral corticospinal tract)
Describe Rhythmic movement
What are they, where are they initiated, what co ordinates them
- semi automatic, rhythmic patterns of activity (basic motor patterns of walking and breathing)
- initiated by subcortical regions like brain stem and spinal cord
- little to no contribution from cerebral hemispheres or cortex
- coordinated by network of neurons called Central Pattern generator
Describe Reflexive
- automatic (and in their most basic form consists of only 2 cells: sensory and motor plus a synapse)
- example is stretch relax triggered by proprioceptors, these are reflex arcs
- rapid and automatic responses to sensory stimuli
- some reflexes can be more complex (can have numerous cells, synapses and interneurons extending over numerous spinal cord segments/brain stem levels or even in some cases both side of the body (e.g. limb withdrawal reflex)
