Introduction to the Neural Motor System

August 27, 2004

So far as our present knowledge goes we are led to think that the tip of a twig of the [axon's] arborescence is not continuous with but merely in contact with the substance of the dendrite or cell body on which it impinges. Such a special connection of one nerve cell with another might be called a synapsis.

- C.S. Sherrington (1897)

Individual neurons can encode complex information and concepts into simple electrical signals; the meaning behind these signals is derived from the specific interconnections of neurons.

- S.W. Kuffler & J.G. Nicholls (1976) From Neuron to Brain

  1. Neuron structure and function
    1. Structural features
      1. Soma
      2. Dendrites
      3. Axon
    2. Functional properties
      1. Cell membrane
        1. Channels
          1. Allow passage of certain ions
        2. Receptors
          1. Channel
          2. Second-messenger
      2. Resting membrane potential
        1. Inside of cell -70 mV with respect to outside
      3. Action potential
        1. Transient change in voltage across membrane, from -70 mV to +55 mV that travels down axon at speeds ranging from less than 1 to 100 meters/second
    3. Synaptic transmission
      1. Excitatory
        1. Results in depolarization, positive shift of membrane potential
      2. Inhibitory
        1. Results in hyperpolarization, negative shift of membrane potential
      3. Electrotonic conduction of EPSPs and IPSPs in along dendrites
      4. Summation at axon hillock where action potential is generated
      5. Synthesis and transport of neurotransmitter to synaptic cleft
    4. Diversity of neuronal types
      1. Excitatory
      2. Inhibitory
      3. Long-range connections
      4. Local connections
    5. Types of neural circuits
      1. Excitatory loops
      2. Inhibitory loops
      3. Lateral inhibition
  2. Overview of the Central Nervous System
    1. Basic principles
      1. Hierarchical processing
        1. e.g., corticospinal tract
      2. Parallel processing
        1. e.g., frontal eye field and superior colliculus
      3. Recurrent Loops
        1. e.g., basal ganglia
      4. Functional areas
        1. e.g., Primary motor cortex
      5. Topographic maps
        1. e.g., Map of body in motor cortex
      6. Compartmentalization
        1. e.g., striosomes and matrix in striatum
    2. Spinal cord
      1. motor neurons - final common pathway (Sherrington)
    3. Brainstem
      1. Medulla
      2. Pons
      3. Tectum
    4. Cerebellum
    5. Basal ganglia
    6. Limbic system
    7. Thalamus
    8. Cerebral cortex
  3. Muscle
    1. The length-tension relation
    2. motor units and recruitment
    3. Proprioception
      1. Muscle spindles
      2. Golgi tendon organs
      3. Joint receptors
      4. Cutaneous receptors
  4. Spinal cord
    1. Final common pathway
    2. Spinal reflexes
      1. Servo theory
      2. - coactivation
      3. recurrent inhibition
      4. reciprocal inhibition
  5. Cerebellum
    1. Part of extrapyramidal motor system
    2. Evolved with vestibular system
    3. Structure
      1. Three main divisions
        1. vestibulocerebellum
          1. inputs from vestibular nuclei
          2. phylogenetically oldest, maintain upright floating
        2. paleocerebellum, spinocerebellum
          1. vermis & medial hemispheres
          2. input from spinal cord
          3. next oldest, maintain upright posture
        3. neocerebellum, cerebrocerebellum
          1. lateral hemispheres
          2. input from pontine nuclei (cerebrum)
          3. most recently evolved, refine movement
      2. Comprised of just 5 types of neurons in a neurocrystal organization
    4. Function
      1. Compares intention with performance and makes appropriate adjustments
      2. Circuits modified by experience; thus it plays a role in motor learning
      3. Each division has different role
        1. Vestibulocerebellum
          1. balance & eye movements
        2. Spinocerebellum
          1. control execution of movement
          2. regulate muscle tone
        3. Neocerebellum
          1. ablation causes delays in initiation, decreased muscle tone & ataxia
  6. Basal Ganglia
    1. Part of extrapyramidal motor system
    2. Structure
      1. Caudate & putamen
      2. Globus pallidus
      3. Substantia nigra
        1. pars reticulata
        2. pars compacta
    3. Connectivity
      1. Loop: cortex basal ganglia thalamus cortex
      2. Four circuits
        1. Skeletal Motor
        2. Ocular Motor
        3. Dorsolateral prefrontal
        4. Lateral orbitofrontal
      3. Neurotransmitters
  7. Primary motor cortex (area 4)
      1. Topographic map of the musculature
      2. Thalamic input
        1. Ventrolateral & ventroanterior nuclei
          1. Receive inputs from basal ganglia & cerebellum
      3. Cell properties
        1. Muscle force
        2. Directional tuning  (Population vector, Presumed mental rotation)
        3. Somatosensory input
      4. Corticospinal tract
      5. Effects of lesions - fractionation.
  8. Posterior parietal cortex (areas 5, 7)
    1. Cell properties
      1. "Reach" neurons
      2. Directional tuning
      3. Response begins 60 msec after activation of MI. Could be corollary discharge.
    2. Effects of lesions - apraxia.
  9. Premotor cortex (area 6 - lateral)
    1. Anatomy
      1. Input through thalamus from cerebellum
      2. Major source of input to MI
      3. Output to brainstem reticular formation
    2. Cell properties
      1. Sensory (visual, tactile, auditory). Anticipatory activity
      2. Movement related (trajectory dependence)
      3. Preparatory set cells
      4. Conditional motor learning
  10. Supplementary motor area (area 6 - medial)
    1. Anatomy
      1. Input through thalamus from basal ganglia
      2. Major source of input to MI
      3. Contributes to corticospinal tract
    2. Stimulation effects
      1. Stimulation evokes more organized movement patterns
      2. Map
    3. Effects of lesions
    4. Cell properties
      1. Sensory
      2. Movement
      3. Cognitive
        1. Sequential movements
        2. Response to instructions
        3. Self-generated versus externally-triggered movements
    5. PET studies
    6. Readiness potential