BIOL 455 Final

• Reciprocal Innervation

  excitatory and inhibitory signals project to inhibitory spinal inter-neurons, which in turn project onto the motor neurons
• Reflexes


  no subject to volitional control, they are automatic in response to sensory stimuli, usually involve multiple neurons that form a reflex arc
• Reflexes are...


  For the most part, they do learn, they emerge without prior experience. It is possible however, to acquire new reflexes through learning
• How are Reflexes Strengthened?


  The strength of many reflexes can be adjusted through trial and error learning to compensate for the changes in the body or the external environment
• Central Patten Generator 


  vegetative processes and bodily movements that are subject to sensory modulation but do not require sensory inputs to generate the rhythmic activity
• Locomotor rhythms are created within the central nervous system independently of sensory inputs
• Half Centered Oscillators:

  consist of two neurons that reciprocally inhibit each other. Some CPGs contain half-center oscillators, but most contain more than two sets of inhibitory neurons as well as several types of excitatory interneurons

  ·      Tonic excitation is required to keep the network going
• CPG for walking in mammals


  Can be modulated by external inputs
• Modulation of CPG for walking


  • Entrained by leg movements when decerebrate cats are placed on a moving treadmill where they can adjust their speed based on that of the treadmill
• Walking CPG


  Can be modulated by descending input from the midbrain locomotor area
• Stimulation of midbrain locomotor area


  1. Walking CPG becomes active

  2. With increasing stimulation intensity, its rhythm accelerates
• Human spinal cords do contain CPG but this CPG is more difficult to activate in humans than in other vertebrates
• Motor Programs

  movements that we have to think consciously about like picking up a water bottle

  Emotional or actional movements

  Primary motor cortex activity codes for movement which means it does not react to it
• Motor Cortex


  Located in the posterior portion of the frontal lobe, anterior to the central sulcus
• Primary motor cortex (M1)


  • Thin strip of tissue in the most posterior part of the Motor Cortex
  • Corticospinal neurons are most concentrated here
  • Most project to the intermediate zone and spinal cord interneurons
• Corticobulbar projections


  • Project to the medulla where they target control respiration, urination, and other vegetative functions as well as regions
• Premotor cortex


  • Most anteriorly in the Motor Cortex
  • Involved in muscle recruitment and body positioning (goal oriented actions)
• Supplementary motor area


  • Most medially in the Motor Cortex
  • (33%) goal oriented that fire no matter how the joystick was held (monkeys)
  • (50%) were muscle contraction oriented which only fired when the joystick was held in the way that it was trained
• Motor Cortex


  Most voluntary movements, many unconscious and involuntary reflexes including the muscle stretch reflexes, involve long loop circuits through the motor cortex
• Motor Cortex Lesions in Humans
• Homunculus


  A way to show that adjacent regions in the motor cortex tend to control adjacent body parts
• Motor cortex


  • The head and hand can be evoked from disproportionately large regions (face and hand overrepresented)
  • Controls movements rather than muscles
• Population code


  Movements are encoded in the pattern of activity across a population of neurons
• Neurons of M1 have broad tuning curves which is why population code is needed
• Cortical movement control involves the concerted activity of many different neurons and individual neurons are activated during many different movements
• Premotor cortex


  • Contains multiple subdivisions and its neurons exhibit remarkably complex activity patterns
  • Most neurons increase their firing rates shortly before and during arm, hand, lip, or tongue movements
  • Codes for the intention of others and goals of motor acts
• Mirror neurons


  • Increase their firing rate when the organism observes the movement that they select for when they perform the movement themselves
  • Code for the intention of others
• Constraint-induced movement theory


  • Requires patients who have lost the function of one arm to put their good arm in a sling for several hours at a time and use the impaired arm to perform various tasks
  • The idea is that forcing subjects to use their impaired arm will increase plasticity within the brain regions controlling that impaired arm
• Cerebellar granule cells


  • Lie just beneath the Purkinje cell layer, 1st major source of input to Purkinje cells
  • Axons are called parallel fibers because they split into left and right branches that extend in straight lines parallel to one another and to the cerebellar surface
  • Purkinje cells respond to granule cell activity only when thousands of those cells fire simultaneously. Even then, granule cell activity elicits only simple spikes in the Purkinje cells
• Inferior Olive


  • Lies in the medulla and is a second major source of input to Purkinje cells
  • Each neuron sends an axon to just a single Purkinje cell but forms thousands of synapses in that cell (climbing fibers)
  • Whenever an inferior olive neuron fires an action potential the Purkinje cell to which that neuron projects also fires an unusually large and long action potential called a complex spike
• All cerebellar Purkinje cells use GABA as their main neurotransmitter and therefore inhibit their target cells
• Cerebellum functions using adaptive feedforward control


  • Adjusted through learning, feedforward means before learning adjusts the commands so that the target state is reached with only minimal error
  • Adaptive Purkinje cells adapt through trial-and-error learning allowing a controller to predict the error that would occur if there had been no previous learning and to correct for it before the error arises
• Cerebellar circuits


  Help organisms discriminate sensations that are caused by their own movements from sensations that are caused by external factors
• Frontostriatal system


  Overarching function includes the selection of goals, of actions to attain those goals, and of specific movements to perform the actions
• Frontostriatal system


  • Includes parts of the neocortex as well as several subcortical structures which are collectively referred to as the basil ganglia
• Dorsal Striatum


  • Involved in learning and control of stereotyped movements
  • Receives inputs from the posterior regions of the frontal lobe, including the motor and premotor cortices
  • Outputs to the dorsal pallidum which projects to the thalamus
• Ventral Striatum


  • Generally involved in generating cravings for food, water, sex, and drugs
  • Receives inputs from the orbital prefrontal cortex which occupies the inferior surface of the frontal love
  • Outputs to the ventral pallidum which projects to the thalamus
• Thalamus


  • Areas project to the frontal lobe: the parts receiving input from the ventral pallidum project to the anterior prefrontal cortex and the parts receiving input from the dorsal pallidum project to more posterior portions of the frontal lobe
• Ventral Membrane


  • Made up of the substantia nigra, compact division (SNc) and the ventral tegmental area (VTA)
• Direct pathway through the striatum


  Contains medium spiny neurons that project directly to the pallidum