Lecture 5

Neuronal Circuits and Behavior

Neuronal Circuits Control Behavior

  • Neuronal circuits are the neuroanatomical bases by which neural circuits mediate behavior.
  • The circuit consists of:
    • Sensory neuron
    • Interneuron
    • Motor neuron
    • Most variety neurons

Local Circuits

  • Local circuits carry out specific neural computations coordinated to mediate complex behaviors.
  • Distinction between local networks versus neural pathways.

Major Somatosensory Afferent Pathway (Upper Limbs)

  • Ascending Dorsal Column – Medial Lemniscal Pathway
    • Ipsilateral dorsal column nuclei
    • Contralateral
    • VPN
  • Pathway steps:
    1. Detected by a specialized neuron, generating an action potential (AP) in the dendrite that propagates.
    2. The pathway bifurcates into ascending and descending pathways.
    3. Synapses onto a second-order neuron. This is the 1st synapse in the pathway within the cuneate nucleus in the medulla (lateral from the midline).
    4. Axon from the cuneate nucleus projects into the sensory decussation, crossing the midline at the sensory decussation point.
    5. Ascends the medial lemniscus until it reaches the VPN (ventral posterior nucleus).
    6. VPN sends the signal to the appropriate location in the primary sensory cortex.
      • This process primarily determines the location of touch sensation, not the nature of what touched the finger.

Neuronal Circuits and Simple Behavior

  • A simple behavior is mediated by many parts of the brain:
    • Premotor cortex
    • Amygdala
    • Hypothalamus
    • Basal ganglia
    • Motor cortex
    • Hippocampus
    • Brain stem nuclei
    • Visual cortices
    • Posterior parietal cortex
    • Cerebellum
    • Spinal cord

Somatosensory Circuits

  • Information from the trunk and limbs is conveyed to the spinal cord.
    • Axons enter the dorsal root.
    • Dorsal horn, white matter, gray matter are present.
  • Cell Stain:
    • Somal cell bodies are located in the gray matter
    • Larger ventral motor neurons project into muscles.
    • Information from the body's surface is sent to the brain stem.
  • Fiber Stain:
    • Dorsal roots run through axons/dendrites.
    • White matter sends/receives messages via ventral roots through the ventral sulcus.
  • Gracile fascicle and cuneate fascicle are ascending axons.
    • Nuclei are groups of cells with axon projections.
  • Spinal cord handles reflexes and senses.
    • Dorsal region: Entering (dorsal root).
    • Ventral region: Exiting (ventral root).
    • Motor neurons: Innervation of skeletal muscle.
    • Sensory neurons.
    • Lateral and ventral columns: Ascending and descending axons.

Spinal Cord

  • The spinal cord is not uniform; its internal and external appearances vary at different levels but same organization.
  • Changes:
    • Size
      • Cervical (head, neck, arms) - Cervical enlargement
      • Thoracic (upper trunk) - not much to do/little sensation
      • Lumbar (lower torso and legs) - Lumbar enlargement
      • Sacral (lower torso and legs)
      • Filum terminale
    • White to grey matter ratio
      • Enlargement is due to axon amount (cumulative white matter).
      • Increasing proportion of ascending and descending axons.
      • Increasing proportion of gray matter.

Primary Sensory Neurons

  • The primary sensory neurons of the trunk and limbs are clustered in the dorsal root ganglia.

Somatosensory Information Pathways

  • Somatosensory information from the limbs and trunk is conveyed to the thalamus and cerebral cortex by two ascending pathways.

Pain Pathways

  • Pain involves a process and reaction.
  • Pathway:
    • Crossing over at the spinal cord.
    • Ascending through the spinoreticular tract.
    • Multiple branch points for feedback.
    • Decussation at the spinal cord.
    • Goes from the spine to the thalamus without synapses in between.

Thalamus

  • The thalamus is an essential link between almost all sensory receptors and the cerebral cortex (except olfaction).

  • Thalamic Nuclei:

    • Anterior nuclei
    • Reticular nucleus
    • Ventrolateral nuclei:
      1. Lateral dorsal
      2. Lateral posterior
      3. Ventral anterior
      4. Ventral lateral
      5. Ventral posterior medial
      6. Ventral posterior lateral (VPN)
    • Midline nuclei
    • Medial nuclei
    • Intralaminar nuclei
    • Posterior nuclei
    • Medial geniculate nucleus (auditory cortex)
    • Lateral geniculate nucleus (LGN) - Visual cortex
    • Pulvinar

  • Inputs:

    • Dorsal column nuclei
    • Dorsal root ganglion
    • Retina

  • Outputs:

    • Primary somatosensory cortex
    • Visual cortex

  • Everything arriving at the thalamus will go to multiple areas of the brain.

Sensory Information Processing in the Cerebral Cortex

  • Sensory information processing culminates in the cerebral cortex, organized based on the body's representation (sensory and motor homunculus).
  • Sensory Homunculus
    • Illustrates the somatotopic map of the body within the sensory cortex.
  • Motor Homunculus
    *Shows the motor map.
  • Areas Shown:
    • Foot, Toes, Genitals, Leg, Hip, Trunk, Neck, Head, Shoulder, Arm, Elbow, Forearm, Wrist, Hand, Ring, Little, Index, Thumb, Eye, Fingers Middle, Nose, Face, Lips, Jaw, Tongue
  • Sensory information diverges and then converges to an output.

Neocortex

  • The neurons of the neocortex are arranged in distinctive layers.
  • Layers (from outermost to innermost):
    • I: Molecular layer
      • Hallmarks: Mostly acellular; purely dendrites/glia; involved in communication
    • II: External granule cell layer
      • Hallmarks: Small cells with very short processes; numerous
    • III: External pyramidal cell layer
      • Hallmarks: Large pyramidal neurons with long axons
    • IV: Internal granule cell layer
      • Hallmarks: Very big neurons; not much connectivity; commonly absent
    • V: Internal pyramidal cell layer
      • Hallmarks: Slightly larger processes; doesn't exist in all parts of the brain; lots of connectivity
    • VI: Multiform layer
      • Hallmarks: Even larger neurons with more complex dendritic arbors; most diverse layer with more branches the deeper you get

Stains

  • Golgi stain: stains the whole neuron (black).
    • Random; stains a very low percentage of neurons.
  • Nissl stain: stains just the soma (blue/purple).
    • Reveals no connectivity.
  • Weigert stain: stains only dendrites/axons.

Neocortex Projections

  • Cortical Projecting Neurons:
    • Cortex to cortex (primarily)
      Leaving cortex
  • Labeled as 2, 3
  • Subcortical Projecting Neurons.
    • Leaving cortex
  • Labeled as 5, 6

Sensory Information Processing

  • Primary sensory areas → Unimodal association areas → Multimodal association areas
  • Example:
    • Somatosensory cortex projects to multiple regions.
    • Primary somatic sensory cortex → Unimodal association cortex (Anterior parietal cortex)
    • Anterior parietal cortex (Remember stimulus & regulate behavior to respond.) → Multimodal association cortex (Posterior parietal cortex, Cingulate cortex, Temporal association cortex, Parahippocampal cortex)
      • Posterior parietal cortex: More specific info.
  • Everything happens at the same time.
  • Regulatory centers for emotions memory.
  • Ignoring:
    • Cerebellum
    • Pons
    • Midbrain

Voluntary Movement

  • Voluntary movement is mediated by direct connections between the cortex (V) and the spinal cord.
  • Descending lateral corticospinal pathway
    • Cerebral cortex → Primary motor cortex → Internal capsule (posterior limb) → Midbrain (Cerebral peduncle) → Pons → Medulla (Pyramid) → Medulla-spinal cord juncture (Pyramidal decussation crosses over at pyramid) → Lateral corticospinal tract (Lateral column) → Synapse in ventral horn of spinal cord → Bifurcation of motor neurons

Motor System Coordination

  • Voluntary movement requires coordination of all components of the motor system.
  • Pathway is complex, although the tract is simple
  • Involves motor cortex, premotor cortex, ventrolateral nucleus of the thalamus, somatosensory cortex, pontine nuclei, pyramidal tract, putamen, basal ganglia, thalamus, midbrain, globus pallidus, red nucleus, deep cerebellar nuclei, cerebellar cortex, medial corticospinal tract, and lateral corticospinal tract, spinal cord.

Motorcortical Signal

  • Motor cortex sends signal (Neuron sends signal soma axon).
  • Bypasses thalamus.
  • Axoaxonic synapses & branching into thalamus (Sends/receives signals).
  • Cerebellum gets info from pons which sends back to thalamus.
    • Modification of signal - fine tuning - for a smooth movement.
  • Basal ganglia projects mostly to caudate nucleus & putamen.
    • Hard copy plans based on experience.
  • Midbrain is responsible for autonomic behaviors (voluntary eye movements)
  • Activation / modification of active/inactive neurons participating to a certain behavior.

Peripheral Nervous System

  • The peripheral nervous system is anatomically distinct from the central nervous system.
  • Much more spread out, localized, simpler
  • Peripheral Nervous System:
    • Somatic
    • Visceral
    • Autonomic

Memory

  • Memory is a complex behavior mediated by structures distinct from those that carry out sensation or movement.
  • Memory Circuit:
    • Hippocampus
      • Dentate gyrus (DG) - Start of "arrow"; goes to CA3, then CA1.
      • CA3
      • CA1
      • Subiculum
      • Entorhinal Cortex (EC)

Hippocampal System

  • The hippocampal system is interconnected with the highest-level polysensory cortical regions.
  • Memory formation/recall is not a loop circuit but a unidirectional pathway
  • Unimodal association areas -> Polymodal association areas -> Hippocampal formation (Perirhinal cortex, Dentate gyrus, Entorhinal cortex, Parahippocampal cortex, Retrosplenial cortex, Subiculum, Hippocampus).

Hippocampal System and Neocortex

  • Hippocampal formation and Neocortex interconnected through:
    • Neocortex
    • Entorhinal cortex
    • Dentate gyrus
    • CA3
    • CA1
    • Subiculum