Week 2: Control of movement

Week 2: Control of movement

1. Control of movement

   • Important concept of neuronal pathways: set neurons connected in serial fashion. Ascending and descending pathways
   • Descending pathways, structures responsible for motor control - called motor centers
   • Motor centres control activity of lower motor neurons in spinal cord
   • Motor centers can performed appropriately if they receive continuous input from sensory system.. Sensory-motor system participates in movement

2. Major steps in neuro-muscular transmission

   • Spinal motor neurons have their cell bodies in ventral horn of spinal cord
   • axons leaves the spinal cord to reach appropriate muscle fibres and forming neuro-muscular synapses
   • action potentials of excited motor neurons are conducted to motor nerve endings where ACh is released

3. Motor units and motor neuron pool

   • Single muscle is composed of thousands of individual skeletal muscle fibres
   • Each muscle fibre is innervated by a single motor neuron. However, one motor neuron has many fibres
   • Muscles with large number of small motor units can be finely controlled by upper motor neurons via lower motor neurons
   • Collection of lower motor neurons that innervate all muscle fibres of a muscle, is a motor neuron pool of muscle

4. Synergists and antagonists

   • Flexion bends the limb (decreases the joint angle)
   • Extension straightens the limb (increases join angle)
   • Movements are usually brought about groups of muscles working together (synergists)
   • Muscles which perform the opposite action are there antagonists

5. Proprioceptors: Muscle spindle

   • Group of small modified muscle fibres. Two classes of intrafusal fibres - nuclear bag fibres and nuclear chain fibres
   • Sensory nerves wrapping around the intrafusual fibres
   • Every muscle contains - dozen of muscle spindles
   • Spindles are situated in parallel with extrafusual muscle fibres - signal muscle length

6. Stretch reflex

   • Stretching a muscle spindle of flexor muscle leads to increase in firing of Ia sensory neuron and increase in firing of alpha motor neurons that innervate the same muscle
   • Ia afferents also excite the motor neurons that innervate synergistic muscle and indirectly inhibit the motor neurons that innervate antagonistic muscle
   • Arrangement is example of reciprocal innervation. Results in rapid contraction of stretched muscle and simultaneous relaxation of antogonistic muscle
   • Stretch reflex arc is a negative feedback loop - maintain muscle length
   • Skeletal muscles are always under some degree of stretch. Stretch reflex circuits are responsible for steady level of tension in muscle
   • Stretch reflexes are too rapid for involvement of brain

7. Y motor neurons modulate level of excitability of muscle spindle

   • Flexor contracts muscle spindles is 'unloaded' (Ia afferents should stop firing)
   • Activaition of y motor neurons which terminate on contractile poles of intrafusal fibres, evokes their contraction, thus maintaining tension in middle region where sensory axons terminates
   • Co-activation of alpha and gamma motor neurons allows spindles to function at all length during movement

8. Knee-jerk stretch reflex

   • Tap on patellar tendon in knee stretches the extensory muscles in thigh and monosynaptic reflex makes them contract brieflyy

9. Proprioceptos: Golgi tendon organs

   • Another sensory receptor that is important in reflex regulation of skeletal muscle activity is Golgi tendon organ
   • Golgi tendon organs are encapsulated afferent endings of Ib afferents located at junction of a muscle and a tendon
   • Respond to small length changes of tendon when muscle force is increased
   • Tendon is like a stiff spring and changes in tendon length are proportional to the force produced by the muscle
   • Golgi tendon organs are situated in series with muscle fibres
   • Golgi tendon organs sense force produced by a muscle

10. Golgi tendon organs

    • Information from Golgi tendon organs is used by CNS when a task involves maintaining a steady force
    • Achieve a constant force, the brain sends an excitatory signals to the motor neuron that drives flexor muscle
    • Motor neuron is also inhibited by interneuron that receives input from Golgi tendon organ (no monosynaptic connections from Golgi tendon organ)
    • If force is too high, Golgi tendon organ stimulates the inhibitory interneuron which inhibits the motor neuron and reduces the drive to muscle
    • If force became too lower, this produces less activation of Golgi tendon which results in less inhibition of final motor neuron and thus muscle contraction increases
    • Results in negative feedback loop that keeps muscle force constant
    • Interactions of excitation (EPSP) and inhibition (IPSP) is typical way of CNS operates

11. Spinal flexion-crossed extension reflex

    • Withdrawal reflex is mediated by nociceptors
    • Stimulation of cutaneous noiceptors is the foot leads to activation of spinal cord local circuits that serve to withdraw (flex) the stimulated leg
    • When one leg is withdrawn the other is stiffened to take the extra weight. This requires increased drive to extensor muscles (crossed extensor reflex)
    • Sensory information can be sent to brain where it is further processed
    • Body position will be adjusted to shift the centre of graviy and enable standing on one leg
    • Sensory information will also reach somatosensory cortex and pain will be felt

12. Discriminative pain pathways for somatic pain

    • Nociceptors from injured skin will contact spinal projection neurons in dorsal horn on spinal cord
      • Sends their axons directly to thalamus
      • Thalamic neurons will relay this information to neurons in primary and secondary somatosensory cortex
    • Sensory pathway mediates discriminative aspect of pain: location, intensity and quality of noxious stimulation

13. Motor cortex

    • Organized somatotopically: local groups of muscle are mapped onto local areas of motor cortex, neurons control different groups of muscles
    • Similar to somatosensory cortex, cortical representation of muscles of body is proportional to their motor innervation
      • large number of motor units supplying a muscle is associated with a larger area of motor cortex and greater degree of control

14. Descending neural pathways of somatic motor system

    • Direct and indirect motor pathways to spinal cord
    • Corticospinal tract is longest and one of largest CNS tract. Direct pathway. Originate from motor cortex
    • Fibres of corticospinal tract crossed to opposite side in brain steam, proceeding caudally in lateral white matter of spinal cord

15. Brain regions that involved involtunary movement

    • Conscious decisions to do something that changes position originate in cerebral corrtex but require replay of previously learned sequences - stored in basal ganglia and cortex
    • Cerebral cotex - planning, initiating and directing voluntary movements
    • Basal ganglia - selection, initiaiton and suppression of intentional movements
    • Cerebellum - maintenance of balance, sensory motor coordination of ongoing movement
    • Brain stem - basic movements, regulation of muscle reflex involved with equilibrium and posture