Neurobiology 303 -- Chapter 18 Outline

The brain and motor output

previous lectures focused on elemental types of motor control
        these consist of reflexes, CPGs, and servomechanisms
        they are mediated at lower levels of the brain
        like the spinal cord in vertebrates and
        local ganglia in invertebrates
 

higher levels of the brain are involved in more complex aspects of motor
        control, such as the planning of movements

motor control can be broken down into two levels:
        execution level -- selection and activation of muscles
        executive level -- selection and planning of movements
 

complex animals like mollusks, arthropods, and vertebrates have a more
   fully developed executive function than do simpler animals
    most motor control is organized at the local level in simpler animals
    interesting fact -- lower organisms can do more without their brains
       than higher organisms!
    in more complex animals, motor control is organized around discrete
       behaviors that are controlled by specific brain regions
    for example, electrical stimulation of specific regions of the brain can
       result in production of a discrete behavior

identifiable neurons -- large neurons found in many invertebrate and some
   vertebrate species that can be recognized by their morphology
   as individuals in any member of the same species
    morphology can be studied by injecting neurons with dye
    examples of identifiable neurons include giant interneurons in
       cockroaches and Mauthner cells in goldfish
       these cells play an important role in escape behavior
 

command neuron -- individual neurons that are necessary and sufficient for
   complete and full expression of a behavioral act
    many neurons that are (or were thought to be) command neurons are
        also identifiable neurons

Wiersma and Ikeda introduced the concept of the command neuron
 

the giant interneurons of crayfish, which mediate their escape response,
    may provide the best example of a command neuron
    crayfish actually have two sets of giant interneurons, and they are
       specialized for different functions:
          medial giant -- backward swimming
          lateral giant -- upward swimming
    each set of giants organizes its own movement through activation of
       interneurons that excite some motoneurons and inhibit others
    can the escape response be elicited in the absence of the giants?
       yes! only the response is slower
    so the giants aren't in fact necessary for escape swimming in crayfish
       but they speed it up considerably
 

recommendation neurons -- a population of neurons that determine the
behavior of an animal by some sort of committee vote
    control of initiation of walking in the locust provides an example of
       behavioral control by a committee of recommendation neurons

swimming in leech provides an interesting example of executive control
   it involves coordination of CPGs in each body segment
    swim gating neurons excite the CPG neurons and drive swimming in
       the leech for as long as the swim gating neurons are active
    trigger neurons activate the swim gating neurons
       trigger neurons are themselves activated by sensory input
    swim excitor neurons activate the swim gating neurons
       and excite the CPGs directly
    swim inhibiting neurons inhibit the swim gating neuorns
       and inhibit the CPGs directly
    thus executive control of swimming in the leech involves both parallel
       and serial control
    swim excitors and inhibitors receive descending input from decision
       centers that are poorly understood at present
    in any case, while execution of swimming in the leech may involve
       many neurons, executive control of swimming requires
       relatively few neurons
 

the main difference between vertebrate and invertebrate nervous systems is
    that the former are much more complex than the latter
    the most complex vertebrate brain is the mammalian brain
    motor control in the mammalian brain involves many regions
       including the spinal cord, brainstem, cerebellum, thalamus,
       basal ganglia, and cerebrum

brainstem motor nuclei consist of the:
                pontine reticular nucleus
                medullary reticular nucleus
                vestibular nuclei
                red nucleus

hierarchically, the brainstem motor nuclei are situated between the
    spinal cord and the cerebellum
    they send their outputs to the spinal cord
    they receive inputs from the cerebrum, cerebellum, and basal ganglia
    severing the brainstem has various effects depending upon level and species
    sever the brainstem just above the spinal cord, and any vertebrate will
       lose ability to stand, but some spinal reflexes will be spared
    sever the brainstem above all the brainstem nuclei, and any vertebrate
       will lose ability to make voluntary movements, but some
       vertebrates like cats can still stand and even walk on a treadmill
    sever brainstem between the red nucleus and the other brainstem
       nuclei and vertebrates exhibit decerebrate rigidity due to over
       excitation of all extensors; decerebrate animals can stand
       but can't move their limbs

three regions of the cerebral cortex are devoted to motor control:
                primary motor cortex
                premotor cortex
                supplementary motor area

primary motor cortex illustrates the principle that the motor system has a
    parallel as well as a serial organization
    primary motor cortex projects to spinal cord motoneurons indirectly
       via the brainstem motor nuclei and directly via pyramidal tract
    pyramidal tract is so called because its fibers criss-cross at the
       junction of the spinal cord and brainstem at the pyramids
    pyramidal control is limited to distal muscles of the limbs, such as:
       forearms, hands, fingers, feet, and toes

motor control must be studied in awake, behaving animals
    Evarts developed a method of painlessly recording the activity of
       neurons during voluntary movement
    a hole is made in the skull beforehand during surgery under deep
       anesthesia; later on electrodes can be inserted into the brain
       painlessly because the brain contains no pain sensing neurons
    Evarts experiments showed that neurons in the cerebral cortex could
       encode the direction and force of arm movements

neurons in the motor cortex begin to fire before the movement begins,
    indicating that they initiate the movement
    motor cortex neurons fire in relation to one of four parameters:
              dynamic -- rate of force development
              static -- steady-state level of force
              intermediate -- between static and dynamic
              direction -- in which limb is meant to travel

direction selective cortical neurons each have a preferred direction of
    limb movement for which they discharge most vigorously
    they discharge less vigorously for limb movements progressively
       further from their preferred direction according to their
       directional tuning curve

population coding indicates limb movement direction
    every cortical neuron is active to some extent for each limb
       movement, and the direction of movement is the population
       average of the preferred directions of all the active neurons,
       weighted by their rate of discharge

premotor and supplementary motor areas are involved in planning
    movements in three ways:
          control visually guided movement
          coordinate postural adjustments
          plan complex movements

lesions of premotor or supplementary motor areas produce deficits in motor
    planning and anticipation of events, for example:
    normal monkey reaching for food will move its hand around a
       transparent barrier once contact is made with it
    lesion premotor area --monkey cannot avoid the barrier
    normal monkey can push food through a hole with one hand and
       catch it in the other hand
    lesion supplementary motor area -- monkey cannot coordinate hands

brain imaging sheds light on motor control by cerebral cortex
    perform simple action -- primary motor cortex alone is active
    perform complex action -- primary and supplementary both active
    imagine complex action -- supplementary cortex alone is active

the basal ganglia are an interconnected group of nuclei that include:
         caudate nucleus
         putamen
         globus pallidus
         substantia nigra
         subthalamic nucleus

function of the basal ganglia isn't clear but damage to them causes disease
    Parkinson's disease -- loss of ability to perform preplanned movement
       in absence of sensory cues, ultimately leads to paralysis; due to
       disruption of connection between the substantia nigra and the
       subthalamic nucleus, associated with loss of dopamine secreting
       neurons in substantia nigra
    Huntington's disease -- production of exaggertated, uncontrollable, writhing,
       dance-like movements, ultimately leads to dementia; due to disruption
       of connection between basal ganglia and thalamus, associated with
       loss of GABAergic neurons in the striatum

the cerebellum is divided structurally into cerebellar cortex and deep nuclei
    cerebellar cortex can be further subdivided structurally into:
       anterior, posterior, and flocculonodular lobes
    cerebellar cortex can be further subdivided functionally into:
       spinocerebellum, cerebrocerebellum, and vestibulocerebellum
    deep nuclei can be subdivided structurally into:
       fastigial, interpositus, and dentate nuclei
    vestibular nuclei can also be considered cerebellar deep nuclei

cerebellar input and output
    principle neurons in the cerebellar cortex are the Purkinje cells
       input to Purkinje cells comes from cerebellar granule cells
       input to granule cells comes from mossy fibers
       mossy fibers come from many regions throughout the brain directly,
       and from the pontine nulcei which relay cerebral cortex input
    Purkinje cells also receive climbing fiber input from inferior olive
    Purkinje cells are inhibitory
    output of Purkinje cells goes to deep cerebellar and vestibular nuclei
    output of deep nuclei goes to cortex via thalamus, and to motoneurons
       via red nucleus and reticular nuclei
    output of vestibular nuclei goes to motoneurons directly

the function of the cerebellum appears to involve smooth coordination of
    posture and of multi-joint movements, in learning conditioned
    responses, and maybe even in cognitive functions
        schizophrenia is associated with disruption of circuits that
        interconnect cerebellum, forebrain, and thalamus