Neuromotor System

Question 1

The neuromotor system summary

Answer 1

the neuromotor system make direct and indirect connections with motor neurons

Question 2

Upper motor neurons:

Answer 2

• cortical motor neurons within M1
• motor neurons within the brainstem (extrapyramidal neurons)

Question 3

Lower motor neurons:

Answer 3

• motor neurons in the anterior horn of the spinal chord
• motor neurons within the cranial nerve nuclei within the brainstem

Question 4

effector organ of the neuromotor system

Answer 4

skeletal muscle

Question 5

what does the skeletal muscle control

Answer 5

• movement (includes respiration and vocalisation)
• maintains posture
• stabilises joints
• generates heat
• lymphatic drainage
• vision
• reproduction
• digestion
• excretion

Question 6

Structure of muscle - level of organisation

Answer 6

1. Whole muscle (organ)
2. Muscle fibre (multi-nucleated muscle cell - composed of many myofibrils)
3. Cytoskeletal components of myofibrils
4. Protein components of myofibril (myosin = thick filaments, actin = thin filaments, troponin, tropomyosin)

Question 7

what is the neuromuscular synapse

Answer 7

the specialised connection between the motor nerve ending and skeletal muscle

Question 8

neuromuscular junction, how are action potentials generated?

Answer 8

1. action potential arrives at motor nerve terminal
2. AP triggers opening of VGCCs, entry of Ca2+
3. Ca2+ dependent release by exocytosis of ACh from synaptic vesicles - at active zones
4. ACh traverses the synaptic cleft to bind to ligand-gated ion channel
5. binding opens ion channel = lots of Na+ in and small amount of K+ out of muscle cell
6. result is depolarisation = sets up a local current between depolarised membrane and adjacent membrane of muscle
7. local current flow opens voltage-gated Na+ channels in adjacent membrane
8. entry of Na+ causes membrane potnetial to rise from -70 to -60 mV. triggers muscle action potential
9. ACh subsequently destroyed by Acetylcholinesterase

Question 9

Safety factor

Answer 9

the neuromuscular synapse has a high safety factor to ensure that EPPs do not fail to elicit a muscle action potential

Safety Factor
Based on many more AChRs than in needed to get the EPP to Threshold (-85 mV to -65 mV EPP varying amplitudes – due to the number of AChRs that are activated by ACh.