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Spinal motor systems
Organisation, muscle fibres, reflexes
| Term | Definition |
|---|---|
| What is contained in the dorsal horn of the spinal cord? | Sensory inputs -> A delta and C fibres |
| What is contained in the intermediate zone grey matter of the spinal cord? | Interneurons |
| What is contained in the ventral horn of the spinal cord? | Motor outputs -> A alpha motor neurons |
| What is a motoneuron pool? | 100-500 alpha motoneurons innervating given muscle (motoneuron pool) located together in ventral horn |
| What is a motor unit? | Each alpha motoneuron nnervates multipe muscle fibres distribted throughout muscle belly -> 1 motoneuron stimulated -> all muscle fibres of motor unit contract -> all muscle fibres in one motor unit are of the same type |
| What is the somatotopic organisation of the ventral horn? | Flexor (dorsal), extensor (ventral), proximal (medial), distal (lateral), hand (dorsal) -> wrist -> elbow -> shoulder (medial) |
| What are examples of motoneuron degeneration? | ALS (amyotrophic lateral sclerosis - motoneuron disease) -> death of presynaptic neuron leads to xcs postsynaptic receptors (hypersensitive) -> endogenous ACh produces involuntary twitches (denervation fasciculations), poliomyelitis |
| What are examples of peripheral nerve damage? | Autoimmune demyelinating disease (Guillain-Barre Syndrome) -> hands/feet muscle weakness -> can spread to cranial nerves/breathing muscles -> can change ANS function (dangerous HR/BP) |
| What are examples of spinal cord damage? | Vertebral column dislocation -> weak spinal joints from injury to muscles/ligaments -> abnormal bone mvmt |
| What are the 3 histochemical profiles of muscle fibres? | Slow, fast fatigue resistant, fast fatiguable |
| What is the anatomy/biochemistry of slow muscle fibres? | Small and few fibres / motor unit, highly vascular, rely on oxidative respiration, high myoglobin content -> red meat |
| What is the physiology/function of slow muscle fibres? | Slow twitch fibres, low tension, fatigue resistant, slow axons -> continuous generation of small forces |
| What is the anatomy/biochemistry of fast fatiguable muscle fibres? | Large and many fibres / motor unit, few capillaries, anaerobic (glycolytic) respiration, low myoglobin content -> white meat |
| What is the physiology/function of fast fatiguable muscle fibres? | Fast twitch fibres, high tension, fatiguable, fast axons -> brief generation of large force |
| What is rate coding? | Vary firing rate of motoneurons -> high AP frequency = high force output, limited by tetany -> amount of vesicular neurotransmitter available/exocytosis rate |
| What is size principle? | Vary motoneuron recruitment -> if more force is needed, more motoneurons are recruited from low force (slow twitch) -> next lowest force generating motor unit -> high force (fast twitch) -> allows for smooth muscle mvmt |
| How do motoneurons show developmental plasticity? | Motoneurons w/ low thresholds innervate few muscle fibres -> induced as slow, low force, fatigue resistant fibres, vice versa -> random inputs automatically recruits motoneurons from low to high force -> no need for brain control of motoneurons |
| What gives inputs to motoneurons? | Muscle spindle afferents (muscle spindle), descending fibres (direct pathways from brainstem/cerebral cortical structures in primates), spinal interneurons (sensory/descending pathways from brainstem/cerebral cortex) |
| What types of reflexes are there? | Simple reflex actions -> specific small group of muscles -> stretch reflexes w/ associated reciprocal/recurrent inhibition, complex reflexes -> functional mvmts w/ multiple muscles (nociceptive withdrawal reflex) |
| What do proprioceptors do? | Provide info on body state (kinaesthesia) -> joint position/mvmt, muscle force |
| What are proprioceptive afferent inputs? | Proprioceptors, cutaneous sensation, visual information, vestiblar systems |
| What types of proprioceptors are there? | Muscle spindle afferents (muscle stretch receptors), Golgi tendon organ (muscle tension receptors), joint receptors (Pacinian corpuscles/joint position/mvmt extremes before snapping) |
| How many spindles are in a muscle? | Typical muscle has 20-100 spindles |
| What is the structure of a muscle spindle? | Encapsulated purple bundle of small specialised intrafusal muscle fibres -> 6-12 intrafusal muscle fibres in each spindle, 3 axons to 1 spindle, striated ends (small compared to extrafusal force-generating fibres) |
| What is the purpose of muscle spindles? | Intrafusal contractions don't generate tendon tension but affect spindle sensory nerve ending sensitivity -> contribute to proprioception rather than mvmt |
| What are the types of intrafusal fibre? | Bag fibres -> swollen central region w/ multi-layered nuclei, contractile viscoelastic end regions and non-contractile elastic bag, chain fibres -> uniform diameter w/ single chain of nuclei, uniformly contractile, several of each type in one spindle |
| What are the sensory afferents/locations from muscle spindles? | Primary -> Ia spindle afferents -> large/fast conducting axons (A-alpha) -> terminal branches coil around intrafusal central nuclei, secondary -> II spindle afferents -> small/slow conducting axons (A-beta) -> terminal branches coil adj to central nuclei |
| What happens to bag fibres when the spindle is stretched? | Central bag elongates -> Ia afferent coil tightens/activates -> viscous contractile ends relax to even out length change -> central bag no longer elongated -> Ia afferent adaptation -> large dynamic response (change), small static response (length) |
| What happens to chain fibres when the spindle is stretched? | Uniformly contractile -> uniform stretch along fibre -> small dynamic response (change), large static response (length) -> no adaptation of II afferents |
| What is the dynamic response? | Receptor informs changes in length -> increase/decrease in AP frequency -> fast type Ia afferents for moving stimulus |
| What is the static response? | Receptor informs absolute length -> slow type II afferents |
| What are the motor efferents to muscle spindles? | A gamma motoneuron axons -> cause intrafusal muscle fibre contraction -> dynamic gamma motoneurons to chain fibres, static gamma motoneurons to bag fibres |
| What motor efferent signals are sent when the muscle contracts? | Spindle centre contracts -> afferent fibre coil too slack to signal -> no Ia/II afferent AP -> A gamma signals -> spindle ends contract to compensate -> centre returns to original length -> afferent coil tightens to normal -> restore sensitivity |
| What motor efferent signals are sent when the muscle is stretched? | Spindle centre stretched -> afferent fibre coil too tight -> saturated AP -> A gamma signals -> spindle ends relax to compensate -> centre returns to original length -> afferent coil relaxes to normal -> restore sensitivity |
| What is the purpose of the motor efferents to muscle spindles? | Adjust sensory afferent sensitivity by changing tension of coil around intrafusal muscle fibre centre compensating for changes in length -> prevents signal saturation when muscle too/not stretched |
| What causes muscle spindle afferents to fire? | Changes in muscle stretch, altered gamma motoneuron activity |
| How does the brain interpret muscle spindle afferent signals? | Efference copy from gamma motoneuron to remove spindle afferent ambiguity -> knows that it was due to gamma motoneuron rather than changes in muscle stretch |
| When are Golgi tendon organs activated? | Active tendon tension in collagen fibres due to extrafusal muscle fibre contraction generating force -> passive muscle (no AP) -> passive muscle under load (no AP) -> stimulated alpha motoneurons contract muscle (Golgi AP frequency proportional to load) |
| What is special about the muscle stretch reflex? | Monosynaptic -> muscle spindle afferents directly synapse w/ alpha motoneurons (no interneurons) -> all muscles xcp extraocular (constant eyeball rotation w/out changes in load) |
| What are the targets for muscle spindles? | Muscles of spindle afferents (adjust length), muscles of synergist spindle afferents (adjust length around joint) |
| What is the advantage of the stretch reflex? | Simplifies mvmt control for unknown/unpredictable loads -> brain determines initial changes in muscle length, spinal cord determines force required |
| What is the disadvantage of the stretch reflex? | Gain is less than 1 (less force of contraction than extent of muscle stretch), feedback system will have delays and generate oscillations |
| What can result from damage to descending systems? | Motor cortex damage (stroke, cerebral palsy) -> spasticity -> hypertonia (continuous contraction), high gain (exaggerated stretch reflex), myoclonus (oscillating muscle contractions) |
| How is the stretch reflex regulated? | Gamma motoneurons reset muscle spindle afferent sensitivity, low mvmt/precision required -> brain sets high gain/spindle sensitivity -> strong stretch reflex -> ballet dancers w/ overlearned mvmts/predictable environment -> dampened stretch reflexes |
| What is the role of proprioceptors in supraspinal motor systems? | Sensory info used for predictive feedforward mvmt (cerebellum/motor cortex) using model systems, prioprioceptor -> spinocerebellar pathway -> cerebellum |
| What is reciprocal inhibition? | Muscle spindle afferents excite glycinergic group Ia inhibitory interneurons associated w/ each motoneuron pool -> synapse w/ antagonist muscles -> prevent further stretch |
| What is recurrent inhibition? | Motoneuron axons have recurrent collateral branches in spinal cord -> innervate inhibitory Renshaw cells -> synapse back on motoneuron cell body in spinal cord ventral horn -> prevent all motoneurons from firing at once (smooth/precise mvmt) |
| What are the characteristics of spinal reflexes? | Activate specific mvmt patterns (specific muscle groups), all have interneurons (xcp stretch), context dependent (same stimuli gives different efferent), multijoint, multisensory (different afferents), controlled by descending motor pathways |
| What is the pathway of the nocicpetive withdrawal reflex? | Noxious stimuli -> nociceptor -> sensory afferent -> substantia gelatinosa -> interneuron -> motoneuron efferent -> excite flexors/inhibit extensors |
| What are the neonatal reflexes? | Grasp reflex, Babinski's sign (plantar reflex) -> stick from heel to toe -> toes extend/leg flexes (adult toes flex to support body weight), reflex stepping (dragging baby has flexed legs like stepping but can't actually support body weight) |
| What are the characteristics of pathological reflexes? | Response to loss of motor cortex control -> normally repressed brainstem takes over spinal reflex control, exaggerated reflexes (spasticity -> stiff uncontrolled muscles w/ strong reflex clonus -ve feedback oscillation), adult -> child Babinski reversion |
| What happens to locomotion when spinal cord is separated from brain? | Intact stepping motor pattern via central pattern generators -> don't need sensory inputs -> intrinsic spinal mechanism responsible |
| What happens to locomotion when the spinal cord is transected? | Loss of functional locomotion -> different infant stepping patterns -> larger cerebral cortex as compensation (encephalisation) -> weaker ability to generate locomotion in spinal cord -> CPG outputs must be adjusted by sensory systems/brain |
Created by:
vykleung
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