Theme 2 Flashcards
What is motor control
A dynamically changing mix of conscious and unconscious regulation of muscle force, informed by continuous and complex sensory feedback operating in a framework sculpted by evolutionary pressures
What are the different types of motor control?
- Voluntary (running, talking)
- Goal-directed (conscious, explicit, controlled)
- Habit (unconscious, implicit, automatic)
- Involuntary (eye movements, cardiac, intercostals)
Describe briefly how we escape pain?
- Pain
- Spinal cord
- Escape
- Motor, autonomic, endocrine defense related ouput
Describe briefly how we avoid looming threat?
- Loom
- Sensorimotor midbrain
- Avoidance
- Motor, autonomic, endocrine defense related ouput
Describe briefly how we avoid a learned threat?
- Threat
- Cortex and limbic system
- Avoidance / Solution
- Motor, autonomic, endocrine defense related ouput
What does more complex, sophisticated threat detection and avoidance require?
Additional or more complex neural processing
What is the overview of the basal ganglia and cerebellum
- Basal ganglia (WHAT to do)
- Cerebellum (HOW to do it)
Where do upper motor neurons have their cell bodies?
Higher centres (cortex and brain stem), project down to lower motor neurons
Where do lower motor neuron have their cell bodies?
Brainstem or grey matter of spinal cord, projecting to muscle
Describe the descending control system
- Association cortex
- Motor cortex
- Brainstem circuits
- Spinal circuits
- Motor units
- Affect on the world
Feedback on all levels
How do individual muscle fibres act
All or none, contracted or not
So control relays on how lower motor neurons activate different fibres
What are the three types of muscle
- Skeletal
- Smooth
- Cardiac
How do we achieve a wide range of movements with all or none muscle contractility?
- Antagonistic arrangement
- Recruitment of a range of muscle fibres (fast / slow twitch, small and large motor units)
What is antagonistic arrangement?
Combined co-ordinated action between opposing muscles
How does training affect muscle fibres
Number of muscle fibres remains unchanged
They change type (fast / slow twitch), and diameter
Describe the structure of muscle fibres in skeletal muscle
- Attached to bone by tendon
- Muscle fasciculi (groups of muscle fibres)
- Muscle fibres (groups of muscle cells)
- Made of myofibrils (actin and myosin)
- When depolarised, actin and myosin slide against each other - contraction
Describe the cellular mechanism of muscle movement
- ACh causes cascade resulting in release of calcium from inside muscle cell
- Myosin head changes shape and binds with actin
- ATP required to break bond between actin and myosin
(rigor mortis, no more ATP, stiff muscles)
What is a motor unit?
Single alpha motor neuron + all the muscle fibres it innervates
How does motor unit size affect movement
Fewer fibres per neuron means greater movement resolution (eg, fingers and tongue)
What is the size principle?
Motor units are recruited in order of size (smallest first)
Fine control typically required at lower forces
What are slow muscle fibres for?
Type 1
Stuff you can do all day eg. maintaining posture
What are fast fatigue resistant muscle fibres for?
Type 2a
Bursts of force, they will fatigue but not for a while (running, swimming)
What are fast fatigueable muscles?
Type 2b
Very quick and powerful contractions, but tire easily
Where do all the alpha motor neurons from the motor units go to?
The motor pool for each muscle in the spinal cord (ventral horn)
How are motor pools arranged in spinal cord?
Proximal muscles medially
Distal muscles laterally
What can activate cell bodies in the ventral horn / motor pools?
- Sensory information from muscle
- Descending information from brain
(inter-neurons between dorsal column and ventral column for communication)
What does the CNS need to know to control movement?
- Muscle tension (golgi tendon organ)
- Stretch of muscle (muscle spindles)
What does the golgi tendon organ do?
Send ascending sensory information to the brain about how much force is in the muscle
Critical for proprioception
Can inhibit muscle fibres via spinal cord circuit to prevent damage
What does the muscle spindle do?
Sense the length of muscles, ie amount of stretch
Describe the basic reflex circuit of muscle spindles
- Muscle spindle detects stretch change
- Sends signal into spinal cord
- Activates alpha motor neurons
- Affects muscle and returns it to original position
- Important for maintaining steady state
What is the role of intrafusal muscle fibres
Detect changes in muscle length
Why are intrafusal muscle fibres supplied by a different motor neuron than extrafusal?
If it was the same, when the muscle was slack or taught the system would not be sensitive to slight changes
What innervates intrafusal muscle fibres?
Gamma motor neurons
What coils around the intrafusal fibres of muscle spindles?
Sensory fibres
Describe the withdrawal reflex
- Painful stimulus
- Signal sent to spinal cord by cutaneous afferent fiber from nociceptor
- Interneuron relays signal to motor neuron (from dorsal column to ventral)
- Signal travels down alpha motor neuron - muscle withdrawn
Describe the vestibular righting reflex?
- Vestibular system detects that body is not upright, as well as any acceleration due to gravity
- Information from vestibular system combined with visual somatosensory and proprioceptive sensory input
- Motor plan specified by this input to restore uprightness
- Cerebellum compares intended motor plan with the actual situation (critical for computing the desired activity)
Why is it hard to build moving robots?
Control of gross movement patterns can be devolved to simple spinal circuity
BUT
Constant modulation based on feedback is required - higher CNS centres constantly adjust ongoing activity to resolve conflicting demands on the motor system
How does the brainstem control movement?
Balance and postural control - integrating eye muscles to vestibular system
What is the role of the motor cortex?
exerts direct, top down control over muscle activity (with as few as one synapse in the spine between a cortical neuron and innervation of muscles)
Describe the pathway of the motor cortex
- Motor command originates in pyramidal cells (layer 5-6 grey matter)
- These are the upper motor neurons
- Pyramidal cell axons project directly or indirectly (eg. via brainstem) to spinal cord
- Synapse with lower motor neurons
What do axons of upper motor neurons descending to lower motor neurons form?
The pyramidal tract
What is the issue with the homunculus?
Reasonable representation, but oversimplified
In reality, overlap and plasticity make it more complex
Few motor commands require isolated activation of a single motor unit
Where does the dorsolateral path travel?
(lateral corticospinal tract)
- Direct pathway is from motor cortex
- Decussate in medulla
- Down dorsolateral tract to contralteral distant limbs
Indirect via red nucleus
Where does the ventromedial path travel?
(anterior corticospinal tract)
- From motor cortex
- Down to spinal cord
- Down ventromedial tract to both sides proximal limbs and trunk
Indirect route via tectum, vestibular nuclei, reticular formation and cranial nerve nuclei
Similarities between dorsolateral and ventromedial tracts
- Both have direct corticospinal route
- Both contain an indirect route via brainstem nuclei
Differences between dorsolateral and ventromedial tracts
- Lateral innervates contralteral side of one segment of spinal cord
- Lateral sometimes projects directly to alpha motor neuron
- Medial has diffuse innervation projecting to both sides and multiple segments of spinal cord
What is the basal ganglia?
Group of nuclei lying deep within cerebral hemispheres
How does the basal ganglia modulate motor control, briefly?
- Receives excitatory input from many areas of cortex
- Output goes back to cortex via the thalamus (GABA)
Why is the basal ganglia important
Acting to ensure you are not doing more than 1 thing at 1 time, balancing our behaviour to get control of the motor system
Reducing interference
What is the cerebellum?
Large brain structure that acts as a parallel processor, enabling smooth, co-ordinated movements
Also involved in a range of cognitive, emotional, and reward processes
What % of CNS neurons are found in the cerebellum?
Around 50%