Components of the motor system

1. Cerebral cortex
   • Pre-central gyrus (Brodmann area 4)
   • Controls contralateral musculuar activity (pyramidal decussation)
2. Subcortical areas
   • Basal ganglia
   • Brainste
   • Cerebellum
3. Spinal cord
4. Motor neurones

   

   • Alpha motorneurone: large diameter fibre innverating majority of worker fibre (extrafusal - not encased within connective tissue sheaths)
   • Gamma motoroneurone: small fibre innervate intrafusal fibres of muscle spindle - alters initial lenght of muscle spindle and sensitivity of spindle to the stretching
5. Motor units
   • Consists of motorneurone and muscle fibres it innervates
   • Large muscles, large units; small delicate muscles, small units
6. Receptors / afferent pathways

Reflex

Automatic response to a stimulus

Spinal reflex

1. Withdrawal reflex
   • Cutaneous nocioception connect to afferent pathway to stimulate alpha neurones
   • Automatic contraction of muscle in response + polysynaptic inhibition of antagonist muscles
2. Stretch reflex
   • Reflex contraction following stretch of fibres
   • Mediated by muscle spindle receptors
   • Nuclear bag fibres (Group Ia) -
   • Nuclear chain fibres (Group II)
   • Patellar tendon stretch reflex:

     

     (1) patellar tendon stretched (2) stretch of quadriceps muscle (3) spindle fibre stretch (4) afferents discharge back to alpha-motorneurone in ventral horn of spinal cord

Muscle Types

1. Skeletal - striated / voluntary
   • Type I: Slow twitch, slow fatigue (high concentration of myoglobin) - eg. soleus
   • Type II: Fast twitch, fast fatigue (large reserves of glycogen)
   • Calcium binding protein = Troponin
2. Cardiac - striated / involuntary
3. Smooth - voluntary
   • Actin/myosin filaments irregularly arranged throughout cell
   • Shows spontaneous 
   • Calcium binding protein = Calmodulin

Skeletal Muscle Contraction

1. Action potential spreads from motor endplate to T-tubule system
2. Leads to release of Calcium from Sarcoplasmic Reticulum
3. Calcium binds troponin C on light chains
4. Leads to displacement of tropomyosin (removes steric hinderance)
   
5. Actin and myosin can cross link
6. Filaments slide (energy generated from hydrolysis of ATP to ADP)

Cardiac Muscle Contraction

• Cardiac cells are mononuclear (multi in skeletal)
• Nuclei centrally located
• Cardiac muscle fibres are branched
• Cardiac cells connected by intercalated disks - contract as syncitium
• Larger T-tubule system 

1. Rapid depolarisation - Influx of Na
2. Partial repolarisation - closure of VSCC
3. Plateau phase - Slow inward current of Ca
   • Myocytes cannot be stimulated to produce tetanic contractions
   • Myocytes are non-fatiguable
4. Repolarisation - closure of Ca channels
5. Placemaker potential
   • Unstable membrane potentials
   • Decay spontaenously to produce AP
   • Caused by progressive reduction in membrane's permeability to K