3. Photographs of Human Fetal Brain Development
Lateral view of the human brain shown at one-third size at several stages of fetal development. Note the gradual emergence of gyri and sulci.

4. Phases of brain development
Neural plate induction
Neural proliferation
Migration & Aggregation
Axon growth & Synapse formation
Cell death & Synapse rearrangement

5. Induction of the Neural Plate
2-3 weeks after conception
A patch of tissue on the dorsal surface of the embryo that will become the nervous system
Development induced by chemical signals
“growth factors”: several chemicals produced
in developing and mature brain that stimulate
neuron development and help neurons respond
to injury

11. cephalic flexure
cervical flexure

18. ~19 days
~23 days

21. Phases of brain development
Neural plate induction
Neural proliferation
Migration & Aggregation
Axon growth & Synapse formation
Cell death & Synapse rearrangement

22. 2. Mitosis/Proliferation
Generation of new cells
3 swellings at the anterior end in humans will become the forebrain, midbrain, and hindbrain
Occurs in ventricular zone
Rate can be 250,000/min
After mitosis “daughter” cells become “fixed” post mitotic

23. Phases of brain development
Neural plate induction
Neural proliferation
Migration & Aggregation
Axon growth & Synapse formation
Cell death & Synapse rearrangement

24. 3. Migration: slow movement to the “right place”
Only a soma and immature axon at this point
-undifferentiated at
start of migration.
But, differentiation begins as neurons migrate.
They develop neurotransmitter making ability, action potential

25. 3. Migration
Radial Glia
Radial glial cells act as guide wires for the migration of neurons
Migrating cells are immature, lacking dendrites
Cells that are done migrating align themselves with others cells and form structures (Aggregation)

26. Growth Cones: tips of axons on migrating, immature neurons
Growth cones crawl forward as they elaborate the axons training behind them. Their extension is controlled by chemical cues in their outside environment that ultimately direct them toward their appropriate targets.

27. 5 Phases of Neurodevelopment
Neural plate induction
Neural proliferation
Migration & Aggregation
Axon growth & Synapse formation
Cell death & Synapse rearrangement

28. 4. Axon Growth/Synaptogenesis
Once migration is complete and structures have formed (aggregation), axons and dendrites begin to grow to their “mature” size/shape.
Axons (with growth cones on end)
and dendrites form a synapse with
other neurons or tissue (e.g. muscle)
Growth cones and chemo-attractants
are critical for this.

29. Synaptogenesis Formation of new synapses
Depends on the presence of glial cells – especially astrocytes
Chemical signal exchange between pre- and postsynaptic neurons is needed

30. 5 Phases of Neurodevelopment
Neural plate induction
Neural proliferation
Migration & Aggregation
Axon growth & Synapse formation
Cell death & Synapse rearrangement

31. 5. Neuronal Death Between 40-75% neurons made, will die after
migration – death is normal and necessary !!
Neurons die due to failure to compete for chemicals provided by targets
Neurotrophins –
promote growth and survival
guide axons
stimulate synaptogenesis

32. Synaptic rearrangment
Release and uptake of neurotrophic factors
Neurons receiving insufficient neurotropic factor die
Axonal processes compete for limited neurotrophic factor

33. Synaptic rearrangment, cont’d: Myelination
Time after synaptogenesis

34. Postnatal Cerebral Development Human Infants
Postnatal growth is a consequence of
Synaptogenesis
Increased dendritic branches
Myelination (prefrontal cortex continues into adolescence)
Overproduction of synapses may underlie the greater “plasticity” of the young brain
Young brain more able to recover function after injury, as compared to older brain

35. Neural Tube Defects (NTDs)
1- Spina Bifida
Oculta
Meningocele
Meningomyelocle cystica
Rachischhisis

37. meningocele
occulta
meningomyelocele
myeloschesis

38. 10 %of normal people
L5 or S1

40. Neural Tube Defects (NTDs)
1- Cranial Bifida
Cranial Meningocele
Meningoencephalocele
Meningohydroencephalocele
Anencephaly

41. Cranial Bifida Meningoencephalocele Cranial Meningocele
Meningohydroencephalocele
Cranial Meningocele

44. Histology of the Nervous System

45. The Nervous system has three major functions:
Sensory – monitors internal & external environment through presence of receptors
Integration – interpretation of sensory information (information processing); complex (higher order) functions
Motor – response to information processed through stimulation of effectors
muscle contraction
glandular secretion

46. General Organization of the nervous system
Two Anatomical Divisions
Central nervous system (CNS)
Brain
Spinal cord
Peripheral nervous system (PNS)
All the neural tissue outside CNS
Afferent division (sensory input)
Efferent division (motor output)
Somatic nervous system
Autonomic nervous system

47. Histology of neural tissue
Two types of neural cells in the nervous system:
Neurons - For processing, transfer, and storage of information
Neuroglia – For support, regulation & protection of neurons

48. Neuroglia (glial cells)
CNS neuroglia:
astrocytes
oligodendrocytes
microglia
ependymal cells
PNS neuroglia:
Schwann cells (neurolemmocytes)
satellite cells

50. Astrocytes
create supportive framework for neurons
create “blood-brain barrier”
monitor & regulate interstitial fluid surrounding neurons
secrete chemicals for embryological neuron formation
stimulate the formation of scar tissue secondary to CNS injury

51. Oligodendrocytes
create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons
Microglia
“brain macrophages”
phagocytize cellular wastes & pathogens

52. Ependymal cells
line ventricles of brain & central canal of spinal cord
produce, monitor & help circulate CSF (cerebrospinal fluid)

53. Peripheral neuroglia
1- schwann cell
2- satellite cell

54. Schwann cells
surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons
creates myelin sheath around most axons of PNS
Satellite cells
support groups of cell bodies of neurons within ganglia of the PNS

59. Neuron structure

60. Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons)
The presence of myelin speeds up the transmission of action potentials along the axon
Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “nodes of Ranvier”
Regions of the nervous system containing groupings of myelinated axons make up the “white matter”
“gray matter” is mainly comprised of groups of neuron cell bodies, dendrites & synapses (connections between neurons)
of Ranvier

61. Classification of neurons
Structural classification based on number of processes coming off of the cell body:

62. Anaxonic neurons
no anatomical clues to determine axons from dendrites
functions unknown

63. Multipolar neuron
multiple dendrites & single axon
most common type

64. Bipolar neuron
two processes coming off cell body – one dendrite & one axon
only found in eye, ear & nose

65. Unipolar (pseudounipolar) neuron
single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

66. Classification of neurons
Functional classification based on type of information & direction of information transmission:
Sensory (afferent) neurons –
transmit sensory information from receptors of PNS towards the CNS
most sensory neurons are unipolar, a few are bipolar
Motor (efferent) neurons –
transmit motor information from the CNS to effectors (muscles/glands/adipose tissue) in the periphery of the body
all are multipolar
Association (interneurons) –
transmit information between neurons within the CNS; analyze inputs, coordinate outputs
are the most common type of neuron (20 billion)
are all multipolar

68. Conduction across synapses
In order for neural control to occur, “information” must not only be conducted along nerve cells, but must also be transferred from one nerve cell to another across a synapse
Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter

69. The Structure of a Typical Synapse

70. Anatomical organization of neurons
Neurons of the nervous system tend to group together into organized bundles
The axons of neurons are bundled together to form nerves in the PNS & tracts/pathways in the CNS. Most axons are myelinated so these structures will be part of “white matter”
The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS. These are unmyelinated structures and will be part of “gray matter”

71. Neural Tissue Organization

72. Anatomical structure of Nerves
Fig. 14.6