control of movement and chewing

• Motor unit

  
  

  • The connection between the nervous system and muscle, the functional unit of the neuromuscular system.
  • A single motor neuron unit = signle motor neuron and all the muscle fibers that it innervates.

  
• Motor unit

  • Muscle fibers that belong to a single motor unit have relatively similar properties
  • Number of muscle fibers varies among different motor units (innervation ratio)

  
• Innervation ratio


  • 20 for muscles involved in small precise movements (eye, hand)
  • 2000 for muscles involved in powerful movements (arms, legs)
  • Intermediate for jaw closing muscles such as the masseter (~640) and temporalis (~900)

  
• Motor neuron pool

  The population of motor units that activate each muscle

  
• The force exerted by a typical muscle is controlled by about 100 alpha motor neurons whose cell bodies cluster in the ventral horn of the spinal cord

  
• There is a large number of alpha motor neurons for the masseter (~1500) and temporalis (~1300)

  
• Within the motor neuron pool, most motor units exert relatively small forces and only a few motor units are capable of exerting large forces
• Motor unit types

  1. Slow (S, type I) : slow contraction time + low force + fatigue resistant
  2. fast fatigue-resistant (FR, type IIA): fast contraction time + moderate force + fatigue resistant
  3. fast fatigable (FF, type IIB): fast contraction time + powerful force + fatigue easily
• Motor unit types


  • Differ in contraction speed, maximal force, and fatigability
• Different muscles have a different proportion of slow and fast twitch fibres depending on their use - increased slow twitch for endurance and increased fast twitch for power
• Modulation of muscle force

  1. Recruitment of motor units : number of active motor neurons

  2. Rate coding of motor units: rate at which action potential is discharged

  
• Recruitment

  • Orderly and consistent -
  • type S > type FR > type FF
  • small motor units > large, even for rapid high force contractions
  • fatigue resistant > fatigable
• Size principle

  • small motor neuron has small surface area
  • Small motor neurons have fewer ion channels and therefore higher input resistance
  • so their EPSPs reach threshold first according to Ohm's law
• Rate coding


  Increasing the frequency of action potentials allows the twitch force to summate, due to a higher availability of calcium in the cytosol
• Tetanus occurs if the muscle fiber is stimulated so rapidly that it does not have a chance to relax between stimuli

  
• Jaw-closer muscles

  • High proportion of fibres with intermediate characteristics (IIC and IM fibres)
  • Low proportion of type IIA fibres
  • Diameter of type I fibres > type II fibres
  • Diameter of both type I and II fibres smaller than corresponding types in other skeletal muscles
  • Pronounced regional grouping of fibres of the same histochemical type within the muscle

  
• Fibre-type composition of jaw-closer muscles

  • Predominantly type I fibres (fatigue resistant)
  • Low proportion of type IIA fibres
  • High proportion of fibres with intermediate characteristics
  • regional differences in fibre type proportions (related to function demand)

  
• Jaw-opener muscles

  • Lateral pterygoid is similar to jaw closers
  • Digastrics have mixed fibre type composition, similar to limb muscles
  • Digastrics have higher proportion type IIA and IIB fibres, suitable for fast, unloaded contractions (e.g. speech)
• Hierarchy of motor control

  • Planning (cortical association areas, basal ganglia, cerebellum): development of strategies and monitor plans to achieve goals
  • Commanding (motor cortex and brainstem descending pathways): integration of signals from planning areas and formation and delivery of specific commands to subcortical structures (execution)
  • Execution (spinal interneurons and motoneurons): intergration of sensory afferent + descending pathways input onto spinal circuits of interneurons and motorneurons result in activation of msucles required to produce the movement
• Components of the motor system

  • Cerebral cortex and descending pathways
  • Basal ganglia
  • Cerebellum
  • Brainstem
  • Motor neurons and interneurons

  
• • cortical representation of muscles
  • Somatotopic representation of muscles and movements in human motor cortex, with a large area controlling the hand and mouth
  • motor homunculus - stimulation of certain coritcal areas produces movements in different body parts

  
• Corticospinal pathway


  
  

  • cerebral cortex acts on spinal motor neurons directly through the corticospinal tracct
  • Originates from sensorimotor cortex, crosses over in medulla to innervate contralateral motoneurons and interneurons
  • serves fine control of individual muscle groups ( precision grip with finger and thumb)
  • commonly damaged by cerebral vascular accident (stroke)

  
• Corticobulbar pathway


  
  

  • Originates from sensorimotor cortex,
  • innervates motoneuron pools on each side, controls muscles of eye, face, tongue and throat, including the trigeminal nerve which controls muscles of mastication
  • corticobulbaar projection is bilateral to CN V motor nuclei
  • brainstem interneurons controlling the central pattern generator for chewing
  • bilateral innvervation from cortex = mastication is relatively well preserved after unilateral stroke

  
• Mastication


  
  

  • Rhythmic activation of jaw muscles in a highly coordinated manner
  • powerful muscles precisely controlled for efficient breakdown of food
  • coordinated by sensory feedback to optimize and avoid damage to oral strucutre
  • constant conscious effort + monitoring not required
• Chewing cycles and rhythmic activation


  
  

  • shcematic of EMG activation of masticatory muscles during a single chewing cycle

  1. Consistent activation patterns of elevator and depressor muscles during chewing cycle in rhythmic chewing movement
  2. Co-activation of elevator and depressor muscles during power stroke
  3. Pattern influenced by sensory feedback from oral cavity and jaw muscle, driven by central pattern generator in brainstem
• Central pattern generator (CPG)


  Drives the basic pattern of rhythmic chewing movements, modified by sensory feedback
• Control of mastication


  1. Initiated and controlled voluntarily by motor cortex

  2. Details controlled automatically by brainstem CPG

  3. Masticatory rhythm modified by sensory input, swallowing coordinated with chewing
• Cerebral cortex

  • primary motor cortex receives input from all premotor assocaition areas
  • projects to spinal cord through corticospinal tract
• Basal ganglia (nuceli)

  • regulates movement indirectly thorugh cortical association and premotor areas
  • involved in initiating movements and postural adjustments
• Cerebellum

  • projects to thalamus and brain stem pathways
  • probably compares planned action to actual action
  • helps correct disparity to aid coordination
• brain stem

  • intergrating centre for ascending + descending signals
  • origin of descending brainstem pathways to spinal cord (except corticospinal tract)
• motor nuerons and interneurons

  • capable of stereotypes reflex behaviors
  • intergrates afferent and descending inputs on spinal circuits