Motor and Sensorimotor Systems Flashcards
What were the Held and Hein experiments and what did they show?
Suggested movement through the environment is necessary to develop visual function (provides feedback)
- Kitten experiments where movement was restricted, though vision intact
- Resulted in impaired vision
What are the three basic motor movement types?
Reflex (cough; knee-jerk):
- Require external stimulus
- Few muscle groups which are graded with stimulus
Rhythmic (e.g. walking):
- Several muscle groups
- Relatively stereotyped
Voluntary (e.g. speech; manipulating objects)
- Goal directed
- Highly modifiable
- No external stimulus required
What is thixotropy?
History of a previous movement:
- Can produce different movement from same command or visa versa
What is Moravec’s Paradox?
What artificial systems find east we find difficult and visa versa
- Recall of information vs. movement through environment
How are motor neurons in the spinal cord organised?
Organised in ventral horn (myelinated):
- Brachial and sacral plexus = enlargement of white matter due to arms/legs
- Flexor-extensor rule: dorsal = flex; ventral = extend
- Proximal-distal rule: medial areas = proximal muscles; lateral = distal
What types of muscle fibres exist? How can these fibres be recruited?
Type 1: Slow twitch
- Low tension
- Hight vascular
Type 2A: fast fatigue resistant:
- Higher tension
- Fast twitch
Type 2B: Fast twitch
- Highest force
- Short bursts
Recruited based on type; force required (rate of neuron firing) and number needed
What are two central competing theories of the organisation of the motor system?
Hierarchical
- Goal oriented
- Come from brain (descending)
- Slower
Reflexive
- In reaction to stimuli
- Ascending inputs
How are locomotive patterns coordinated to achieve walking motion? How does speed change muscle recruitment?
Repetitive cycles of swing (flexor muscles) and stance phases (extensor muscles)
- Coordinated between muscles in one limb and between limbs
Increasing speed requires shortening of stance phase (swing phase fixed):
- Reduces time to produce extensor force so higher power muscle units required
- Change of gait allows different pattern
Trade off between balance and complexity (humans need complex balance system but simpler to coordinate 2 legs)
What is the evidence that the spinal cord is a CPG without brain input?
Spinal interneuron circuit (do not need brain to generate movement)
- Dorsal root transected (supraspinal sensory information removed) – Legs of rat can still move in sequence
- Lamprey spinal cord can be isolated (completely from muscle) and still the spinal cord produces patterns of electrical activity
- Primate spinal cord activity shows CPG when all spinal cord inputs transected
- Muscles connected below lesion can still move (no brain input) – not voluntary but there is movement (not paralysed)
- Mike the headless chicken could walk and survived for >1 year
What is Brown’s half-centre theory for movement generation?
One half centre for flexion; one for extension.
- Excitatory interneurons of a half-centre activate both motor neurons and 1a inhibitory neurons for the opposing half-centre
- Activated muscles provide inhibitory feedback (via Renshaw neurons) to excitatory half-centre
- Allows switching between two half centres
[Think about diagrammatic representation]
What are muscle spindle complexes and what information do they provide?
Intrafusal fibres (receptors) act in parallel to extrafusal fibres (provide muscle force)
Muscle spindle fibres = active on muscle stretch:
- Chain fibres give duration of sustained stretch (innervated by static gamma efferent neurons)
- Bag1 fibres give dynamic (velocity) sensitivity (via type 1a adapting afferents)
- Bag2 fibres give static (length) sensitivity – via type II sensory afferents (non-adapting)
- Contractile section allows length determination (and efferent copy to cerebellum) = proprioceptive function
- Efferent γ fibres adjust length of intrafusal fibres to maintain sensitivity during contraction
[Draw diagram representing an intrafusal muscle spindle complex]
How do Golgi tendon organs prevent tendon damage?
Golgi tendon organs transmit information about muscle contraction (via afferent 1b fibres)
- Firing rate increases with tension (via 1b afferent)
- If force too high (danger of tearing tendon), inhibition of muscle contraction caused
Difference between intra and extrafusal length is perceived as stretch (see angel illusion experiment)
Describe how the Knee-Jerk reflex works?
- Hammer stretches the quadricep muscle, activating muscle spindle
- Pathway 1: afferent sensory neurons (1a) synapse with alpha motor neuron in dorsal root ganglion causing contraction of extrafusal fibres in quadriceps
- Pathway 2: sensory (type II) neurons synapse with an inhibitory 1a interneuron to alpha motor neuron, inhibiting hamstring (the antagonistic muscle) contraction
These two pathways prevent further stretch of the muscle which may result in injury.
How are tendon organs and muscle spindles involved in CPG movement?
During programmed motor sequence: tendon organs encourage activity (+ve feedback loop):
- Ankle extensor activity increased during walking
- Overactive tendon organs (+ve feedback) is dangerous (can overextend muscles causing damage) – E.g. grand mal seizure during electroconvulsive therapy
Muscle spindle reflex (keeps tendon organ under control) +ve feedback:
- Inhibit antagonistic muscles once extension reaches threshold.
What evidence is there for modification of stretch reflexes under CPG (rather than brain) influence?
Stim chamber experiment;
- Hoffman reflex evoked by electrical stimulation
- Up or down conditioning trained by giving food reward
Exercise effect in humans:
- Footballers show increased reflex magnitude
- Ballerinas show decreased magnitude (learn to ignore extension reflex)
Treadmill therapy:
- Muscle reflexes can be trained independently of brain (E.g. after lesion)
- Significant increase in motor activity after invoking sensory input (forced walking)
What are the main descending pathways for movement? What are they needed for?
For goal directed movement (not CPG)
Fast pyramidal pathway (mainly ionotropic):
- Corticospinal tracts (both lateral and anterior)
- Conscious
Fast extrapyramidal pathways (mainly ionotropic):
- Reticulospinal: simple motor patterns
- Vestibulospinal: balance
- Tectospinal: eye and head movements
- Rubrospinal: red nucleus connection (blink response?)
Slow pathway (mainly modulation):
- Spinothalamic: pain
- Spinomesencephalic: control and inhibition of pain
- Spinohypothalamic: emotional ties to pain
What are the different corticospinal tracts and what do they control?
Lateral (90% of fibres):
- Contralateral travel from brain level
- Synapse with lower motor neurons in ventral horn
- Innervate limb (distal) muscles
Anterior (10% of fibres):
- Decussate at desired spinal cord level
- Innervate muscles of trunk
What is the function of the reticulospinal tract?
General body orientation/simple patterns:
- Uses CPG and inputs from motor cortex/cerebellum
- Inputs from mesencephalic locomotor region (MLR) in cat: increased stimulation = increasing motor output speed patterns (walking to galloping)
- MLR to reticulospinal formation (RSF) to Spinal cord CPG
What is the evidence for two way communication in descending spinal tracts?
Brainstem spiking caused by fictitious movement:
- CPG activated by NMDA
- NMDA barrier prevents diffusion directly into brainstem
- Therefore activity due to ascending fibres
Efferent copy:
- Necessary for vestibulospinal tract function
- Compare copy with model and correct if necessary (cerebellar activity)
What is the function of the vestibulospinal tract?
Positional information and balance:
- Vestibular system collects positional information
- Vestibulo tract connects to muscles to correct change
- Achieved by efferent copy (feedback) from CNS – comparison of sensory feedback with predicted model from efferent copy (models are subtracted from each other (if the same = no reaction))
What is the function of the tectospinal tract?
Eye and Head movements:
- Coordinates 6 eye muscles in rapid small movements
- Saccade eye movements compensate for adaptation (re-activates areas of retina)
Which modulatory molecules are used by the slow descending pathway?
Predominantly aminergic with some neuropeptides
- Aminergic: 5-HT (Raphe nucleus) slows frequency of output and NA (locus coeruleus)
- Neuropeptides: substance P increases frequency of output, galanin, thyrotropin releasing hormone (TRH), NA, neuropeptide
What is Dale’s principle and how was it disproven?
Dale’s principle: one transmitter type from each neuron
Evidence against:
- Co-localisation of different transmitters from same neuron
- Control of release (amino acids small therefore closer to presynaptic membrane whereas larger may require larger depolarisation and recruitment).
How is cerebral Palsy caused?
Defects in descending system associated with changes to regulation of spinal reflexes:
- In normal infants spinal stretch reflexes occur in both flexor and extensor muscles. Adults only show stretch reflex in stretched muscle.
- Cerebral palsy adults show infant pattern – CPG never learns/matures appropriate response to forced extension
