motor systems

Question 1

motor neurons

Answer 1

• supply skeletal muscles
• bring about movement
• axons of motor neurons originate in CNS & end on skeletal muscle
• motor neuron axon terminals release ACh to stimulate muscle contraction
• motor neurons are final common pathway

Question 2

spinal cord

Answer 2

• afferent (sensory) neurons into dorsal horn/root
• efferent (motor) neurons out from ventral horn/root
• each segment contains motor neurons that project to specific ipsilateral skeletal muscles via the ventral root
  
  • ipsilateral: on the same side of the body

Question 3

neuromuscular junction

Answer 3

• motor neurons & skeletal muscle fibers are chemically linked at neuromuscular junctions
• each muscle cell has only 1 neuromuscular junction
• muscle fiber: single long cylindrical muscle cell
• terminal button: enlarged knoblike structure at the end of axon terminal branches
• motor end-plate: shallow depression where axon terminals end
• ACh: neurotransmitter used by motor neurons
  
  • ↑ membrane permeability to Na+ leading to EPSP called end-plate-potential (EPP)
• acetylcholinesterase (AChE): enzyme located in the synaptic cleft that degrades ACh

Question 4

synaptic transmission at NMJ

Answer 4

1. AP propagates down motor neuron
2. local AP triggers opening of voltage-gated Ca2+ channels ➞ Ca2+ enters terminal button
3. Ca2+ entry triggers release of ACh by exocytosis
4. ACh diffuses across synaptic cleft & binds w/ sepcific receptor-channels on motor end plate of muscle cell membrane
5. binding opens chemically-gated cation channels that allow simultaneous passage of Na+ into & K+ out of muscle fiber ➞ more Na+ enters than K+ exits
6. results in local change in membrane potential = end-point potential: local current flow btwn depolarized end plate & adjacent membrane
7. local current flow opens voltage-gated Na+ channels
8. Na+ entry ↓ threshold potential initiating AP that propagates through muscle fiber
9. ACh is destroyed by acetylcholinesteraseminating muscle cell’s response

Question 5

myasthenia gravis

Answer 5

autoimmune disease where body generates antibodies that attack nicotinic ACh receptors ➞ muscle cell never stimulated

• symptoms: muscle weakness, esp during sustained activity ➞ commonly eyelids, facial expression, swallowing
• treatments: ACh-esterase inhibitors: by ↓ rate of ACh degradation ACh lasts longer in NMJ & ↑ chances of binding to nicotinic receptor

Question 6

central motor system

Answer 6

system of neural structures that control skeletal muscles

• spinal cord
• primary motor cortex: voluntary movement
  
  • located on precentral gyrus
  • corticospinal tract: neural pathway from primary motor cortex to muscles
    
    • ALS
• supplementary motor cortex: programming of complex movement
• premotor cortex: coordination of complex movement
• cerebellum: control of motor coordination
  
  • compares intended movement w/ actual movement & makes corrective adjustments
  • MS
• basal ganglia: involved in motor planning & initiation of motor sequences
  
  • parkinson’s disease

Question 7

primary motor cortex

Answer 7

• area of cerebral cortex that controls voluntary movement
• located on precentral gyrus (line above central sulcus)

Question 8

Amynotrophic lateral sclerosis (ALS)

Answer 8

degeneration of corticospinal tract

• still functional sensory pathways but cannot produce motor response/movement
• stephen hawkins

Question 9

supplementary motor & premotor cortices

Answer 9

motor cortical areas that project to & are located anterior to primary motor cortex involved in motor sequences & movement preparation

• supplementary motor cortex: programming of complex movement
• premotor cortex: coordination of complex movement

Question 10

cerebellum

Answer 10

• large neural lobe located on the lower posterior region of the brain
• control of motor coordination
• compares intended movement w/ actual movement & makes corrective adjustments
• cerebellar cortex: outer, highly folded surface of cerebellum
• deep nuclei: nuclear structures that lie below the cortex deep in cerebellum
  
  • analogous to basal ganglia in cerebrum
• multiple sclerosis: autoimmune disease causes demyelination of neurons in cerebellum
  
  • difficulty making precise movements
  • essential tremors (aka action tremors)

Question 11

basal ganglia

Answer 11

• involved in motor planning & initiating motor sequences
• large nuclei interconnected w/ cerebral cortex & thalamus
• parkinson’s disease: autoimmune disease that destroys dopaminergic neurons in basal ganglia ➞ resting tremors

Question 12

multiple sclerosis

Answer 12

autoimmune disease causes demyelination of neurons in cerebellum

• difficulty making precise movements
• essential tremors (aka action tremors)