Neuroananatomy - Lecture 1

1. Functional organization of the nervous system

  • sensory input (receptors, peripheral nervoussystem - PNS) -> information processing (integreation, central-nervoussystem - ZNS) -> motor output (muscles, PNS)

2. Development of nervous systems

“Lowest” organism with a neruronal network: Cnidaria (polyps and medusae)

  • the Tripedalia cystophora has a powerful sensorric system
    • total of 24 eyes

Characteristics of Hydra:

  • nerve cells are connected to a network containing aggregates (nerve rings, ganglia)
  • no central nervous systems (CNS)
  • no myelinization
  • symmetrical synapsesm(no distinction between axons and dendrites)
  • peptide neurotransmitters (no low molecular weight transmitters)

→ begin of a functional differentiation

net of multipolar nerve cells below the epithelial cell layer in connection to sensory cells ( “exumbrellar nerve ring”): senesory function

net of nerve cells on top of muscle cells (“subumbrellar nerve…

“Lowest” organism with a central nervous system: Plathelminthes and Nemathelminthes (flatworms and roundworms)

  • contain a body axis
  • contain a spezialized head region (“cephalization”)
  • contain an accumulation of nerve cells in the head region (“brain”)

Important model system: Caenorhabditis elegans (roundworm, nematode)

Control of the development of the nervous system Two principal mechanisms:

  • hard-wired molecular programs (DNA sequence)
  • epigenetic processes (in a broad sense: anything oter than DNA sequence that influences development)
    • example: limiting amount of neurotrophins determines number of surviving neurons
    • concequence of epigenetic processes: you never have a predetermined number of neurons

3. Nervous systems of vertebrates

  • increasedtendency of centralization and cephalization:

    Humans:

    • majority of neurons in brain
    • 99.999% of neurons are interneurons

PNS: groups of neurons (ganglia) and peripheral nerve tracks

  • somatic part
  • autonomous (vegetative) part
    • sympathic (increase of activation)
    • parasympathic (decrease of activation)

Central nervous system (CNS) Dorsally positioned tube-like nerve strand (produced by invagination of the ectoderm)

→ posterior part (spinal cord)
→ anterior part (vesicles, brain (segmental structure, present in both hemispheres of the brain))

  • segmented organization (31 paired spinal nerves in 4 major regions)
  • H-like structure: gray and surrounding white matter
  • divides into dorsal and ventral horn and dorsal, lateral, and ventral columns

Embryonic development and organization of the brain: 5 parts in the adult brain:

  • Forebrain
    • telencephalon (“Endhirn”)
    • diencephalon (“Zwischenhirn”)
  • Brainstem
    • mesencephalon (“Mittelhirn)
    • Hindbrain
      • metencephalon (“Hinterhirn”)
      • myelencephalon (“Nachhirn”)

Principles that govern the organization of the CNS

Seperation of the CNS in gray and white matter.

Functionality of the CNS requires optimization of two competing requirements:

  • high interconnectivity
  • short conduction delays

→ Solution: Segregation between gray and white matter:

  • high connectivity in small regions (“local connections”) (maximal number of cells: about 10 000 neurons with tolerable delay (ms range)) → gray matter
  • fast connections with high conduction speed (“global wiring”) → white matter

.

  • *Serial organization**(“relays”); information is transformed at every step
  • Pathways that link the components (“tracts” oder “Bahnen”)
  • pathways cross to the other part of the body at a certain anatomical level
  • projections occur in an orderly fashion thus producing “neural maps” (“somatotopy”)