unit one

• although many anatomists in the early days of neuroscience believed that neurons were continuous with one another, forming a system of tubes, Santiago Ramón y Cajal argued convincingly that they are contiguous, that is close together but not joined. We now know that the human brain contains about 80 to 90 billion discrete neurons
• the brain is composed of neurons and other cells that are independent from one another, structurally, metabolically, and functionally
• information tis transmitted between cells across tiny gaps called gap junctions
• most neurons possess three major parts: the dendrites (input), the axon (output); and the cell body (integration zone, containing the nucleus)
• components of the synapse - 1. axon: this is the output zone of the presynaptic cell
• components of the synapse - 2. axon terminal (or synaptic bouton). this is located at the start of the axon
• components of a synapse - 3. synaptic vesicles. these contain molecules of neurotransmitters
• components of the synapse - 4. neurotransmitter substance. a chemical that produces changes in the postsynaptic membrane
• components of a synapse - 5. synaptic cleft. this is approximately 20 to 40 nanometers wide
• components of the synapse - 6. postsynaptic membrane. this contains the receptors for neurotransmitters
• components of a synapse - 7. dendrite. this is the input zone of the postsynaptic cell
• an individual neuron may have as many as 100000 synapses, particularly cells with elaborate dendrites. a prominent feature of many dendrites is dendritic spines, which may be modified by experience
• the electrical impulse that a neuron conveys to other neurons begins at the cone-shaped axon hillock and travels along the axon toward the axon terminal at high speed. a single-cell axon may innervate more than one postsynaptic cell because of the division of the axon into axon branches
• a radioactive form of glucose labels metabolically active neurons that are autoradiographic
• tract tracers label cell bodies and projections and locates the source of projection
• immunohistochemistry labels intracellular proteins and employs antibodies
• golgi stains label the entire neuron, including processes but labels only a few cells
• nissl stain labels cell bodies and labels all cells
• neurons can be classified based on function. neurons whose axons innervate muscles or glands are called motor neurons. neurons that respond to environmental stimuli are called sensory neurons. neurons that communicate strictly with other neurons are called interneurons; these are by far the most common functional type of neuron
• unlike neurons, new glial cells have been shown to be produced throughout life. one form of glial cell, the astrocyte, is found in close proximity to the blood vessels of the brain. microglia are very small and mobile, and they serve to remove debris from sites of injury, disease, and cell death
• axons frequently are sheathed in myelin, which contributes to the conduction of electrical nerve impulses along the axon. this process of myelination is performed by oligodendrocytes in the brain and spinal cord, and by schwann cells outside the brain and spinal cord. the gaps that are found between adjacent segments of myelin on an axon are called nodes of Ranvier
• three medical problems associated with glial cells are the demyelinating diseases multiple sclerosis, edema (the swelling that follows brain injury), and the growth of tumors that are formed from glial cells,
• similarities between neurons and other cells: nucleus and organelles, plasma membrane
• differences between neurons and other cells: neurons are post-mitotic, communicate messages through action potentials (for long or short distances), and have a very distinct morphology
• Dendrites are branching extensions of the cell body that receive information from other neurons.
• The cell body, or soma, contains the genetic information, maintains neuron's structure, and provides energy to drive activities, and contains the nucleus and specialized organelles.
• The axon hillock is the integration center of the neuron, deciding whether an action potential will occur.
• The axon is the long portion of a neuron that conducts impulses.
• Myelin is the fatty insulation around an axon, formed by glial cells, that speeds the conduction of action potentials.
• The axon terminal, or synaptic bouton, is the end of an axon, which forms a synapse on a neuron or other target cell.
• A node of Ranvier is a gap between successive segments of the myelin sheath where the axon membrane is exposed.
• the cell membrane is a phospholipid bilayer that separates extracellular fluid (outside the cell) from intracellular fluid (inside the cell)
• what can pass through the cell membrane and what can't?

  oxygen and carbon dioxide pass through membrane; water passes less easily; ions cannot cross membrane
• how are things moved through the cell membrane that can't go through on their own?

  proteins channels and pumps in the cell ions pass through
• synapse: the tiny gap between neurons where information is passed from one to the other
• presynaptic referring to the region of a synapse that releases neurotransmitters
• postsynaptic: referring to the region of a synapse that receives and responds to neurotransmitters
• synaptic cleft (gap): the space between the presynaptic and postsynaptic membranes
• different types of synapses:

  axodendritic: referring to a synapse in which a presynaptic axon terminal synapses onto a dendrite of the postsynaptic neuron, either via a dendritic spine or directly onto the dendrite itself

  axo-axonic: referring to a synapse in which a presynaptic axon terminal synapses onto another axon's terminal

  axo-somatic: referring to a synapse in which a presynaptic axon terminal synapses onto the cell body of the postsynaptic neuron

  dendrodendritic: referring to a synapse in which a synaptic connection forms between the dendrites of two neurons
• multipolar neuron: a nerve cell that has many dendrites and a single axon