The Nervous system II

• Induction of the Neural Plate

  1. 2-3 weeks after conception

  2. A patch of tissue on the dorsal surface of the embryo that will become the nervous system

  3. Development induced by chemical signals

  4. “growth factors”: several chemicals produced in developing and mature brain that stimulate neuron development and help neurons respond to injury
• Synaptogenesis

  1. Formation of new synapses

  2. Depends on the presence of chemical cues largely from glial cells – especially astrocytes

  3. Chemical signal exchange between pre- and postsynaptic neurons is needed
• Embryonic brain regions

  • Forebrain
  • Midbrain
  • Hindbrain
  • Telencephalon
  • Diencephalon
  • Mesencephalon
  • Metencephalon
  • Myelencephalon
  • Cerebrum (includes cerebral cortex, white matter, basal nuclei)
  • Thalamus
  • Hypothalamus
  • Epithalamus
  • Pons
  • Medulla oblongata
  • Spinal cord
• Proliferation

  1. Generation of new cells

  2. 3 swellings at the anterior end in humans will become the forebrain, midbrain, and hindbrain

  3. Occurs in ventricular zone

  4. Rate can be 250,000/min

  5. After mitosis “daughter” cells become “fixed” post mitotic
• Migration and Aggregration

  1. Only a soma and immature axon at this point - undifferentiated at start of migration

  2. Differentiation begins as neurons migrate. They develop neurotransmitter making ability, action potential

  3. Radial glial cells act as guide wires for the migration of neurons

  4. Migrating cells are immature, lacking dendrites

  5. Cells that are done migrating align themselves with others cells and form structures (Aggregation)

  6. Bergmann glia perform a similar function in the cerebellum

  7. In this manner – through the sequential migration and differentiation of cells – the many-layered cortex is established

  8. A similar process in the cerebellum ensures that the granule cells migrate to the interior and this happens during the first few weeks after birth
• Cell Death & Synapse Rearrangement

  1. Between 40-75% neurons made, will die after migration – death is normal and necessary

  2. Neurons die due to failure to compete for chemicals provided by targets

  3. Neurotrophins – promote growth and survival, guide axons, stimulate synaptogenesis

  4. Release and uptake of neurotrophic factors

  5. Neurons receiving insufficient neurotropic factor die

  6. Axonal processes complete for limited neurotrophic factor

  7. Synaptic Rearrangement
• Postnatal Cerebral Development

  1. Postnatal growth is a consequence of Synaptogenesis

  2. Increased dendritic branches

  3. Myelination (prefrontal cortex continues into adolescence)

  4. Overproduction of synapses may underlie the greater “plasticity” of the young brain

  5. Young brain more able to recover function af
• Brain Development Phases

  1. Neural plate induction

  2. Neural proliferation

  3. Migration & Aggregation

  4. Axon growth & Synapse formation

  5. Cell death & Synapse rearrangement
• Axon Growth/Synaptogenesis

  1. Once migration is complete and structures have formed (aggregation), axons and dendrites begin to grow to their “mature” size/shape

  2. Axons (with growth cones on end) and dendrites form a synapse with other neurons or tissue (e.g. muscle)

  3. Growth cones and chemo-attractants are critical for this
• The cerebral cortex controls voluntary movement and cognitive functions
• The array of whiskers on the snout sends sensory information to the barrel cortex via the brainstem and thalamus. This then signals to the motor cortex to regulate whisker movement
• Regions of the adult brain viewed from the rear

  • Cerebellum
  • Basal nuclei
  • Cerebrum
  • Left cerebral hemisphere
  • Right cerebral hemisphere
  • Cerebral cortex
  • Corpus callosum
• The frontal lobes have a substantial effect on “executive functions”
• Functions of different brain regions

  • Motor cortex (control of skeletal muscles)
  • Frontal lobe
  • Prefrontal cortex (decision making, planning)
  • Broca’s area (forming speech)
  • Temporal lobe
  • Auditory cortex (hearing)
  • Wernicke’s area (comprehending language)
  • Somatosensory cortex (sense of touch)
  • Parietal lobe
  • Sensory association cortex (integration of sensory information)
  • Visual association cortex (combining images and object recognition)
  • Occipital lobe
  • Cerebellum
  • Visual cortex (processing visual stimuli and pattern recognition)
• Four regions, or lobes (frontal, temporal, occipital, and parietal), are landmarks for particular functions
• Representation of the body in the Cortex

  • Genitalia
  • Toes
  • Abdominal organs
  • Tongue
  • Jaw
  • Lips
  • Face
  • Eye
  • Brow
  • Neck
  • Thumb
  • Fingers
  • Hand
  • Wrist
  • Forearm
  • Elbow
  • Shoulder
  • Trunk
  • Hip
  • Knee
  • Tongue
  • Pharynx
  • Jaw
  • Gums
  • Teeth
  • Lips
  • Face
  • Nose
  • Eye
  • Thumb
  • Fingers
  • Hand
  • Forearm
  • Elbow
  • Upper arm
  • Head
  • Neck
  • Trunk
  • Hip
  • Leg
• Lateralization of Cortical Function

  • The two hemispheres make distinct contributions to brain function
  • The left hemisphere is more adept at language, math, logic, and processing of serial sequences
  • The right hemisphere is stronger at pattern recognition, nonverbal thinking, and emotional processing
  • The differences in hemisphere function are called lateralization
  • Lateralization is partly linked to handedness
  • The two hemispheres work together by communicating through the fibers of the corpus callosum
• Information Processing in the Cerebral Cortex

  1. The cerebral cortex receives input from sensory organs and somatosensory receptors

  2. Somatosensory receptors provide information about touch, pain, pressure, temperature, and the position of muscles and limbs

  3. The thalamus directs different inputs to distinct locations

  4. Adjacent areas process features in the sensory input and integrate information from different sensory areas

  5. Integrated sensory information passes to the prefrontal cortex, which helps plan actions and movements

  6. In the somatosensory cortex and motor cortex, neurons are arranged according to the part of the body that generates input or receives commands
• Postnatal Cerebral Development

  1. Postnatal growth is a consequence of Synaptogenesis

  2. Increased dendritic branches

  3. Myelination (prefrontal cortex continues into adolescence)

  4. Overproduction of synapses may underlie the greater “plasticity” of the young brain

  5. Young brain more able to recover function after injury, as compared to older brain

  6. Early visual deprivation → fewer synapses and dendritic spines in visual cortex, deficits in depth and pattern vision

  7. “Enriched” environment → thicker cortices, greater dendritic development, more synapses per neuron
• The cerebrum is essential for awareness, language, cognition, memory, and consciousness
• The vertebrate brain is regionally specialized
• Frontal lobe damage may impair decision making and emotional responses but leave intellect and memory intact
• The somatotopic representation of sensory organs is a common strategy
• The barrel-like organization is preserved throughout the sensory path
• The limbic system also functions in motivation, olfaction, behavior, and memory
• Opium and heroin decrease activity of inhibitory neuron
• Sensory information from the array of whiskers on the snout to the barrel cortex

  Brainstem and thalamus transmit sensory information to the barrel cortex, which then signals to the motor cortex to regulate whisker movement
• Nicotine stimulates dopamine-releasing VTA neuron
• Brain structures involved in the generation and experience of emotions

  • Amygdala
  • Hippocampus
  • Hypothalamus
  • Thalamus
  • Olfactory bulb
• Cells of the Nervous System - Neurones

  • Dendrites
  • Cell body (soma)
  • Synapse
  • Axon hillock and initial segment
  • Cell body (soma)
  • Inhibitory terminal of an axon
  • Basal dendrites
  • Axon hillock
  • Axon (initial segment)
  • Apical dendrites
  • Excitatory terminal of an axon
  • Myelin sheath
  • Presynaptic terminal
• Functional Brain Imaging can Map Activity in the Working Brain
• Representation of the body in the Cortex

  • Fingers
  • Hand
  • Forearm
  • Elbow
  • Upper arm
  • Head
  • Neck
  • Trunk
  • Hip
  • Leg
• Genetic and environmental factors contribute to diseases of the nervous system
• Disorders of the nervous system

  • Schizophrenia
  • Depression
  • Drug addiction
  • Alzheimer’s disease
  • Parkinson’s disease
• Cocaine and amphetamines block removal of dopamine from synaptic cleft
• Areas responsible for language and speech

  • Hearing
  • Speaking
  • Seeing
  • Generating
• Functional Regions of Neurones

  • Secretion
  • Sensory
  • Moto(r)
  • Local interneurone
  • Projection
• Several viruses (e.g. herpes, polio, rabies) exploit retrograde transport to infect neurones, often with devastating effects
• Parts of a Neurone

  • Axon hillock
  • Initial segment
  • Cell body (soma)
  • Inhibitory terminal of an axon
  • Basal dendrites
  • Apical dendrites
  • Excitatory terminal of an axon
  • Myelin sheath
  • Presynaptic terminal
  • Synaptic cleft
  • Node of Ranvier
  • Postsynaptic membrane
  • Synapse
  • Nucleus
• Camillo Golgi's staining method allowed the visualisation of individual neurones