• Blog 3 or paper due next Tuesday, April 12, 2022.

• The neuroscience of action, continued

• Smooth
  • Arteries, hair follicles, uterus, intestines
  • Regulated by ANS (involuntary)
• Striated (striped)
  • Skeletal muscles
  • Voluntary control, mostly connected to tendons and bones
  • Regulated by somatic branch of PNS
• Cardiac

• Motoneuron (in ventral horn of spinal cord) ->
• Neuromuscular junction
  • Releases ACh

• Motor endplate
  • Contains nicotinic ACh receptor
  • Generates excitatory endplate potential (EPP)
    • Muscle fiber depolarizes
    • Depolarization spreads along fiber
    • Causes release of Ca++ from stores inside muscle

• Muscle fibers segmented in to sarcomeres
• Myofibrils (w/in sarcomere)
  • Paired actin & mysosin proteins
  • “Molecular gears”
• Bind, move, unbind in presence of Ca++ plus energy source (ATP)

• Fast twitch/fatiguing
  • Type II
  • White meat
• Slow twitch/fatiguing
  • Type I
  • Red meat

• Intrafusal fibers
  • Sense length/tension
  • Contain muscle spindles linked to Ia afferents
  • ennervated by gamma (\(\gamma\)) motor neurons
• Extrafusal fibers
  • Generate force
  • ennervated by alpha (\(\alpha\)) motor neurons

• Muscle stretched (length increases)
• Muscle spindle in intrafusal fiber activates
• Ia afferent sends signal to spinal cord
  • Activates alpha (\(\alpha\)) motor neuron
• Muscle contracts, shortens length

• Gamma (\(\gamma\)) motor neuron fires to take up intrafusal fiber slack

https://www.nytimes.com/2020/11/09/sports/emily-harrington-free-climb-yosemite.html

• Polysynaptic inhibition of antagonist muscle
• Prevents/dampens tremor

• Pyramidal system
  • Pyramidal cells (from Cerebral Cortex Layer 5) in primary motor cortex (M1)
  • Corticobulbar (cortex -> brainstem) tract
  • Corticospinal (cortex -> spinal cord) tract
• Crossover (decussate) in medulla
  • L side of brain ennervates R side of body

• Extrapyramidal system
  • Tectospinal tract
  • Vestibulospinal tract
  • Reticulospinal tract
• Involuntary movements
  • Posture, balance, arousal

• Parkinson’s
• Huntington’s

• Symptoms
  • Slow, absent movement, resting tremor
  • Cognitive deficits, depression
• Biological correlates
  • DA neurons in substantia nigra degenerate
  • Autoimmune disorder? (Garretti, Agalliu, Lindestam Arlehamn, Sette, & Sulzer, 2019)

• Treatments
  • DA agonists
  • DA agonists linked to impulse control disorders in ~1/7 patients (Ramirez-Zamora, Gee, Boyd, & Biller, 2016)
  • Levodopa (L-Dopa), DA precursor
  • Deep brain stimulation

• Formerly Huntington’s Chorea
  • “Chorea” from Greek for “dance”
  • “Dance-like” pattern of involuntary movements
  • Cognitive decline

• Genetic + environmental influences
  • Gene fragment (CAG) duplication
  • High levels of huntingtin protein accumulate in basal ganglia
  • People with familial risk may want to consider genetic testing
• No effective treatment
  • Promising gene therapy trial halted last year (Kwon, 2021)

• Control of movement determined by multiple sources
• Cerebral cortex + basal ganglia + cerebellum + spinal circuits

• Vision