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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. Phases of brain development – Neural plate induction – Neural proliferation – Migration & Aggregation – Axon growth & Synapse formation – Cell death & Synapse rearrangement 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 cephalic flexure cervical flexure ~19 days ~23 days Phases of brain development – Neural plate induction – Neural proliferation – Migration & Aggregation – Axon growth & Synapse formation – Cell death & Synapse rearrangement 2. Mitosis/Proliferation 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 Phases of brain development – Neural plate induction – Neural proliferation – Migration & Aggregation – Axon growth & Synapse formation – Cell death & Synapse rearrangement 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 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) 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. 5 Phases of Neurodevelopment – Neural plate induction – Neural proliferation – Migration & Aggregation – Axon growth & Synapse formation – Cell death & Synapse rearrangement 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. Synaptogenesis • Formation of new synapses • Depends on the presence of glial cells – especially astrocytes • Chemical signal exchange between pre- and postsynaptic neurons is needed 5 Phases of Neurodevelopment – Neural plate induction – Neural proliferation – Migration & Aggregation – Axon growth & Synapse formation – Cell death & Synapse rearrangement 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 Synaptic rearrangment Release and uptake of neurotrophic factors Neurons receiving insufficient neurotropic factor die Axonal processes compete for limited neurotrophic factor Synaptic rearrangment, cont’d: Myelination Time after synaptogenesis 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 Neural Tube Defects (NTDs) 1- Spina Bifida Oculta Meningocele Meningomyelocle cystica • • • • Rachischhisis • occulta meningomyelocele meningocele myeloschesis 10 %of normal people L5 or S1 Neural Tube Defects (NTDs) 1- Cranial Bifida Cranial Meningocele Meningoencephalocele Meningohydroencephalocele Anencephaly • • • • • Cranial Bifida Meningoencephalocele Cranial Meningocele Meningohydroencephalocele Histology of the Nervous System 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 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 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 Neuroglia (glial cells) CNS neuroglia: • astrocytes • oligodendrocytes • microglia • ependymal cells PNS neuroglia: • Schwann cells (neurolemmocytes) • satellite cells 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 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 Ependymal cells line ventricles of brain & • central canal of spinal cord produce, monitor & help • circulate CSF (cerebrospinal fluid) Peripheral neuroglia • 1- schwann cell • 2- satellite cell 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 Neuron structure Most axons of the nervous system are • surrounded by a myelin sheath (myelinated axons) of Ranvier 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) Classification of neurons Structural classification based on number of processes coming off of the cell body: Anaxonic neurons • no anatomical clues to determine axons from dendrites • functions unknown Multipolar neuron multiple dendrites & single • axon most common type• Bipolar neuron two processes coming off • cell body – one dendrite & one axon only found in eye, ear & • nose Unipolar (pseudounipolar) neuron single process coming • off cell body, giving rise to dendrites (at one end) & axon (making up rest of process) 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• 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 The Structure of a Typical Synapse 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” Neural Tissue Organization Anatomical structure of Nerves Fig. 14.6