• Wrap-up on somatosensation
• The neuroscience of action

• Dorsal column/medial leminiscal pathway
  • Touch, proprioception
• Spinothalamic tract
  • Pain, temperature

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• Perceiving Where
  • Somatotopic maps – where on skin
  • Kinesthesia – configuration of limbs
• Perceiving What
  • Patterns of smoothness, roughness, shape, temperature

• Nociceptors (Latin nocere to harm or hurt) detect harmful or potentially harmful stimuli of varied types:
  • chemical
  • mechanical
  • thermal

• External
  • Skin, cornea (eye), mucosa
• Internal
  • Muscles, joints, bladder, gut

• Receptors for
  • metabolism (acidic pH, hypoxia, …)
  • cell rupture (ATP and glutamate)
  • cutaneous parasite penetration (histamine)
  • mast cell (white blood cell) activation (serotonin, bradykinin, …)
  • immune and hormonal activity (cytokines and somatostatin)

• Periaqueductal grey (PAG) in midbrain
• Insular cortex (insula)
• Hypothalamus
• Amygdala

• Thalamus
  • Ventroposterior lateral nucleus
  • Ventroposterior medial nucleus
  • Ventromedial nucleus

• Somatosensation
  • Exteroception via
    • Cutaneous receptors + proprioception
  • Interoception via
    • Widely distributed receptors
    • Specific and non-specific

• Pain
  • Multiple receptor channels
  • Highly interconnected CNS network
  • Multiple targets for modulation

• What types of actions are there?
• How are they produced?
  • By the muscles
  • By the nervous system

• Nervous system "output" includes
  • Movements
  • Autonomic responses
  • Endocrine responses

• Reflexes
  • Simple, highly stereotyped, unlearned, rapid
• vs. Planned or voluntary actions
  • Complex, flexible, acquired, slower
• Discrete (reaching) vs. rhythmic (walking)
• Ballistic (no feedback) vs. controlled (feedback)

• Primary motor cortex (M1)
• Non-primary motor cortex
• Basal ganglia
• Brain stem
• Cerebellum
• Spinal cord

• Axial
  • Trunk, neck, hips
• Proximal
  • Shoulder/elbow, pelvis/knee
• Distal
  • Hands/fingers, feet/toes

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

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

• Motor endplate
  • Nicotinic ACh receptor
• Excitatory endplate potential
  • Muscle fibers depolarize
  • Depolarization spreads along fibers like an action potential
  • Sarcomeres are segments of fibers
  • Intramuscular stores release Ca++

• Myofibrils (w/in sarcomere)
  • Actin & mysosin proteins
  • “Molecular gears”
• Bind, move, unbind in presence of Ca++, 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

• Polysynaptic inhibition of antagonist muscle
• Prevents/dampens tremor

• Pyramidal tracts
  • Pyramidal cells (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

• Slow, absent movement, resting tremor
• Cognitive deficits, depression
• DA Neurons in substantia nigra degenerate
• Treatments
  • DA agonists
  • DA agonists linked to impulse control disorders in ~1/7 patients (Ramirez-Zamora et al. 2016)
  • Levodopa (L-Dopa), DA precursor

• Formerly Huntington’s Chorea
  • "Chorea" from Greek for "dance"
  • “Dance-like” pattern of involuntary movements
• Cognitive decline
• Genetic + environmental influences
• Disturbance in striatum
• No effective treatment

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

• Exam 3