Bio 321 Neuroanatomy

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Bio 321 Neuroanatomy Dr. M. Yu
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Nervous System Introduction

• Bio 321 Neuroanatomy

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Medical Significance

• The brain and nervous system control all other
  functions of the body.
• The extreme importance of the nervous system in
  medicine is based on the serious nature of the
  many disorders affecting its structures (more
  than 1000 disorders).
• Causes more hospitalization than any other
  diseases, including heart diseases and cancers.
• Neurological diseases affect 50 million Americans
  and costs us about 400 billions annually

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Introduction

• In this country alone, the numbers are
  overwhelming
• 1. Cerebrovascular Disease - is the 3rd ranking
  cause of death - vascular conditions of brain
  spinal cord annually kill 500,000
• 2. Epileptics seizures 1,500,000
• 3. Movement disorders affect another one million
  people
• 4. There are 2 million totally blind
  individuals over 13 million with visual
  impairments
• 5. There are 17 million totally or partially
  deaf persons

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Introduction

• 6. Over 3 million people are afflicted with
  Alzheimers disease
• 7. At least 700,000 have cerebral palsy
• 8. More than 250,000 have multiple sclerosis
• 9. In addition, there are over 500,000 accidental
  head and spine injuries annually fortunately
  only a minority of which actually injure the
  brain or spinal cord
• 10. Acute head injury is the leading cause of
  death or disability between ages 2 40 (as of
  1995)

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Cellular Components of the Nervous System

• Neurons - the primary functional cells in the
  nervous system (- approx. 100 Billion in CNS)
• 1. responsible for initiating conducting
  electrical signals by which nervous system
  communicates
• 2. size shape varies greatly between regions of
  the nervous system with respective functions
• 3. mature neurons do NOT divide or replicate, do
  NOT regenerate following injury

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Anatomic features (common to all neurons)

• 1. Soma - cell body
• metabolic center of cell
• typical cellular organelles large clear nucleus,
  nucleolus, mitochondria, ER, Golgi Apparatus,
  lysosomes
• 2. endoplasmic reticulum - a characteristic
  feature of neurons - high concentration of RER
  (granules, form Nissl substance, stain basic)

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Neurons

• 3. Cytoplasmic fibrils - another distinguishing
  feature of neurons, make up cytoskeleton
• a. Neurofilaments intermediate cytoskeletal
  filaments - abundant, found throughout soma
  along processes (10 nm in diameter) a principle
  support system component
• b. Neurotubules (Microtubules) especially
  abundant in dendrites - found in cytoplasmic,
  axonal transport, movement of organelles
  throughout cell - (20-30 nm in diameter)
  microtubules, oriented lengthwise
• c. Microfilaments thinnest, associated with
  external membrane dendritic spines - anchor
  membrane constituents, hold Receptors in place -
  (5 nm in diameter) microfilaments in other
  cells Processes of neurons

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Dendrites

• Dendrites - number, length, degree of
  ramification varies between neuronal types
• a. usually many per neuron, typically branch
  extensively
• b. carry incoming signals toward soma
• c. neuron signal receptors (the dendritic field
  of a cell its receptive field)

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Axon

• Axon - one per neuron
• a. conducting process - carries info away from
  soma, toward other neurons or effectors
• b. length varies greatly, can be very long -
  sciatic axons gt 1 meter (spine -gt foot)
• c. no ribosomes, no protein synthesis, depend on
  axonal cytoplasmic transport from soma
• d. collaterals branches of axon, degree of
  branching varies greatly

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Axon

• Axon Hillock Initial segment (of axon)
• a. base of axon as it leaves soma - looks pale
  (no Nissl staining)
• b. specialized segment of membrane for action
  potential initiation due to lower threshold than
  rest of cell
• c. also has an extremely high concentration of
  voltage-gated ion channels
• d. allows action potential conduction to travel
  in ONE direction

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Terms

• 1. Nucleus - a group of neuronal cell bodies
  within the central nervous system, eg lateral
  geniculate nucleus
• 2. Ganglion (ganglia, pl) - group of cell bodies
  in peripheral nervous system
• 3. Nerve - collection of axons in the PNS (tract,
  fasciculus a bundle of nerve fibers)
• 4. White matter - axons, myelinated
• 5. Gray matter - concentrations of cell bodies
  and unmyelinated dendrites

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Classification of neurons

• A. based on processes
• 1. Unipolar neurons a.
• a single primary process extends from soma, which
  can branch into dendrites axon
• b. the principle neuronal cell type of
  invertebrates
• c. Pseudounipolar neurons, e.g. dorsal root
  ganglion cells, appear unipolar, in embryo are
  actually bipolar axon dendrite extensions fuse
• 2. Bipolar neurons - e.g. bipolar cells in retina
  
• - axon dendrite both extend from soma, from
  opposite ends
• 3. Multipolar neurons
• a. multiple dendritic branches from all parts of
  soma, one axon
• b. many variations due to length, number of
  dendrites, length of axon
• c. e.g. pyramidal neuron, spinal motor neuron,
  purkinje neuron

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Classification of neurons

• B. Based on functions
• Sensory neurons carry sensory information from
• Motor neurons carry motor information to muscle
  sand glands
• Interneurons in between sensory and motor
• Projection neuron / relay neuron has long fibers
  and project information from on region to another
  

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Classification of neurons

• C. Based on orientation
• 1. Afferent - refers to the neuron or process
  extending toward the cell in question
• 2. Efferent - refers to the neuron or process
  extending away from the cell in question

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Neuroglial (Glia)

• supporting cells - Do not conduct action
  potential, body has 10-15 times more glia cells
  than neurons (about one trillion)
• Derived from Neuroectodermal and mesodermal origin

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• Neuroectodermal origin 1-4
• 1. Schwann cells
• - form myelin sheath which insulates an axon in
  peripheral nerves
• - cell winds around axon, inside its own layers,
  piling up layers of lipid/protein cell membranes
• - one Schwann cell associates with and myelinates
  a segment of only one axon
• - Schwann cell, myelin, axon are all surrounded
  by a basement membrane (covers whole unit)
• - help to buffer excess extracellular K (prevent
  rampant depolarization)
• - myelin sheath insulation greatly speeds
  conduction

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• 2. Oligodendroglia
• same function as Schwann cells, but for axons of
  neurons in central nervous system
• have small, round, dense nuclei
• unlike Schwann cells, can myelinate a segment of
  several axons
• no basement membrane surrounds the axon like
  Schwann cells do in PNS
• these features affect ability of CNS cells to
  regenerate following injury (vs. PNS)

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• 3. Astrocytes
• named from star appearance of processes
  radiating out from soma
• oval nuclei (larger much less dense than
  oligodendroglia)

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Function of Astrocytes

• i. nutritive form significant portion of brain
  blood barrier (BBB), surrounding CNS vasculature
• thought to be important communicators between
  neurons and capillaries
• transport nutrients, ions, molecules from
  capillary to neurons
• ii. healing act as scavengers, remove neuronal
  debris, seal off area for healing
• iii. buffers excess extracellular K , preventing
  excess depolarization
• iv. remove chemical neurotransmitters from
  synaptic clefts - high affinity for specific
  neurotransmitters
• v. support provide structure, stability, act
  similarly to connective tissues
• vi. in development act as guides for outgrowth
  migration in the developing NS

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Types of astrocytes

• Distinguished by localization, not on any
  functional difference
• i. fibrous astrocytes - name for those in, around
  white matter (areas rich in axons)
• ii. protoplasmic astrocytes - those in gray
  matter, near synapses, dendrites

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• 4. Ependymal cells
• layer of ciliated columnar epithelial cells with
  tight junctions which line cavities of the neural
  tube (cerebral ventricles, spinal cord central
  canal)
• a. this layer forms a selective barrier between
  nervous tissue ventricular fluid
• b. also forms choroid plexus - produces cerebral
  spinal fluid by filtering its components from
  blood

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Glial cells of mesodermal origin

• Microglia
• - cells from mesoderm which migrate into CNS
• - normally in small numbers, proliferate as
  needed
• - become macrophages in response to injury or
  damage,
• - act as scavengers, have a phagocytic role in
  removing debris, damaged cells

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Synapse

• A. Chemical synapse
• Presynaptic membrane, postsynaptic membrane, and
  synaptic cleft
• can also be classified as axodendritic,
  axosomatic, axoaxonal, and dendrodendritic
• neurotransmitter (adrenaline, acetylcholine,
  dopamine, serotonin, glutamic acid)
• neuro-modulators (excitable or inhibitive
  neurotransmitters)
• B. Electrical synapse
• neuron close together, rare in mammalian nervous
  system

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Meninges

• There is very little connective tissue in brain
  tissue, except in linings of vasculature
• A. CNS meninges Connective tissue membranes
  surrounding the NS
• 1. pia mater - immediately next to the nervous
  tissue, thin delicate
• 2. arachnoid - middle layer
• 3. dura mater - outermost meninges, thicker
  very tough

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PNS

• 1. endoneurium thin collagenous layer,
  immediately surrounds a myelinated nerve fiber
• 2. perineurium Connective tissue covering
  surrounding a fascicle of nerve fibers
• 3. epineurium thick connective tissue layer
  surrounding many fascicles which make up a nerve
  trunk
• the 3 CNS meningial layers are continuous with
  the connective tissue layers around PNS nerves

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Axonal Transport

• Fast Axonal Transport - involves microtubules as
  a track -some organelles move along in stepwise
  fashion - especially synaptic vesicles,
  precursors, etc - requires oxidative metabolism,
  energy utilization

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• 1. Anterograde orthograde, from cell body to
  terminal
• a. mechanism by which synaptic vesicles move from
  soma to terminal along microtubules
• b. speed 410 mm/day
• 2. Retrograde from terminal to soma
• a. mechanism for returning worn out materials to
  cell body for degradation or recycling
• b. returns to cell body information from, about
  the axon - communicates with neighbors
• c. nerve growth factor (NGF) travels from source
  to soma, stimulates neuronal growth
• d. toxin transport, e.g. neurotropic viruses get
  to CNS herpes simplex, rabies, polio
• e. speed approx 1/2 - 2/3 that of anterograde
  transport

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Axonal Transport

• Slow Axoplasmic Flow
• 1. predominant means of organelle movement within
  the neuron
• 2. movement is through cytoplasm of axon, not
  along microtubules
• 3. speed approx. 10 mm/day

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Main regions of the central nervous system

• Cerebrum
• Frontal, parietal, occipital, temporal and insula
• Interpretation
• initiating voluntary movements
• storing memory
• retrieving memory
• Reasoning
• center for intelligence and personality
• Cerebellum
• Large in human, receiving information from
  sensory systems and the cerebral cortex, main
  function to maintain balance, posture, mostly not
  initiated from the cerebral cortex.

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Brain stem

• Medullla, pons and midbrain are generally
  referred as brain stem
• Medulla Oblongata
• Continuation from spinal cord into the brain,
  part of the brain stem.
• Pons
• Dorsal part consists of sensory and motor tracts
• Ventral part contains connection between two
  hemispheres, contribute to motor efficiency
• Midbrain
• Involved with visual and auditory system, red
  nucleus and substantia nigra are also located
  here (more later)

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• Diencephalon
• Forms the central core of the cerebrum, including
  thalamus, hypothalamus, epithalamus, and
  subthalamus, controls automatic nervous system,
  endocrine function via hormones and nervous
  impulses.
• Telencephalon
• Includes cerebral cortex, corpus striatum, and
  medullary center. Areas of cerebral cortex
  (paleocortex) receive primitive function from
  olfactory system, which is common to lower
  vertebrates. Other areas are called archicortex,
  includes limbic system (emotions, and some
  memories, early vertebrates). Most areas of the
  cerebral cortex (90) in human are referred as
  neocortex, which controls all sensations (except
  smell), involves emotions, memories and
  intellectual activities.