Local motor control

Local motor control

1. Motor neurones: many muscle fibres. Smallest neurones innervating the tonic and fatigable muscle fibres, recruited first.

   • Renshaw cell: lateral inhibition, discourage synchronization.
   • Somatotopic organisation: Medial neurones innervate trunk. Lateral n innervate distal limbs. Flexors and extensors governed by dorsal & ventral groups.
   • =Mechanism: withdrawal or flexion reflex: rapid flexion when skin touched by hot object/ noxious stimulus.

2. Descending Pathways

   • Reticular formation: extrapyramidal tract, direct contact with MN. Located in Sc. Stretches from: superior cervical SC intralaminar thalamic nuclei. Dendritic fields perpendicular to axis of brainstem: isodendritic core receive collaterals from sensory fibres. 3 zones: raphe, medial & lateral. Level activity & gen rhythm.
     • Clinical aspects: reduced conscious states, cushing reflex & abnormal respiratory patterns.
     • Project cerebellum & BG.
   • Vestibular nuclei: movement head & gravity, lateral vestibulospina- uncrossed, end interneurons, excite monosynaptically.
     • Medial tract exists: bilaterally from medial VNcervical and upper thoracic regions.
     • Maintain posture, control extensors.
   • Red nucleus: rubrospinal. Flexion.
   • Superior colliculus: Tectospinal, vision & hearing, spier reflex.
   • Corticospinal: cerebral cortex, 80% cross in the medulla, descend as lateral CST, uncrossed become anterior. 30% white matter. Internal capsule- site of ischaemic stroke= paralysis. Fine, skilled voluntary movements.
   • SC isolated from brain=
     • flaccid paralysis: loss voluntary movements and muscle tone.
     • below cerebral cortex: spasticity: resistance to passive stretch.
     • level of colliculi- rigidity: hyperactive= rigid

3. Sensory feedback from muscles

   • Proprioception: sense of oneself in space muscles & joints. Exteroception: sense of external world. >Proprioceptive feedback: Primary 1a from spindle (direct input onto MN), Golgi 1b detect stretch & contraction.
   • Sensory receptors in muscle
     • Golgi tendon: Force transducer, clasp-knife reflex- transduce loading of muscle. 1b afferents. –ve feedback to reg tension.
     • Muscle spindles: Muscle length, modified y-efferent fibres to intrafusal fibres. parallel to muscle fibres= regulation of muscle contraction and matches F gen to motor task.
     • Proprioceptive afferents (in joint capsules): protect & respond extreme flexion & extension- 1 limb extended other positioned similarly.= if disturbed= reposition to inappropriate location.
     • Sensory input: 1a sense stretch.
   • Spindle reflex: 1a afferents monosynaptically excite MN= reflex arc. Normal people response feeble, in patients the contraction is ^.
   • Muscle Tone: Spindles maintain the attitude of the body against forces of gravity. A innervate extrafusal and y innervate intrafusal.
   • Simple Servo hypothesis: Sensory endings: difference between amount of y activity & shortness of muscle- spindle is comparator generator
     • Load: Controlling manipulation: feedforward to predict and regulate output. If load increases the cerebellum would need to constantly monitor and review the situation- instead decided position.
       • Servoassistance: 1. Positon command- tells muscle via y & spindles what the length is meant to be. 2. Force command: estimate of load. Spindles= stretch reflex to correct the response & modify ballistic programs for future.

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