Neuronal Biology Flashcards
Brain Sizes
newborn = 400g; Adult = 1400g; Bottle Nose Dolphin = 1500g
Neurons General Information
o Number of neurons varies between 10 and 100 billion neurons; more in brain than spinal cord
o Constantly undergoing neuronal growth and death
o Rate at which neurons die vary from age and lifestyle choices
o Do NOT divide after differentiation
o Axonal projections make up 90-99% of their cell surface area
o High metabolic demand; use a lot of oxygen and glucose 15% of cardiac output
o More neurons located in LEFT hemisphere than on the right hemisphere
Neuron Cytoskeleton
– extensive network of cytoskeleton structure
o Different parts of the neuron have different cytoskeletal components
Microtubules
– made up of 13 protofilaments of alternating alpha/beta tubulin
o Development and maintenance of the neuron’s processes
o Tau protein-stabilizes microtubules; phosphorylation leads to destabilization & depolymerization
o Clinical correlation: Alzheimer’s Disease – microtubules in diseased neurons start to aggregate
Tau protein is hyperphosphorylated leading to depolymerization of tubules and form perihelical filaments (clumps of microtubule subunits)
Bielschowsky Silver Stain – used to stain the plaques/tangles
Neurofilament
– abundant in the axon; composed of fibers
o Start out as monomers that form a heterodimer
Dimers come together and form tetramers (protofilaments) 2 tetramers form protofibril 3 protofibrils form neurofilament
o Clinical Correlation: antibody staining against neurofilaments can be used to follow the axonal projections in the brain
Microfilament
– globular actin monomers; main used in the motility of growth cone
Dendrites
– receive/transmit signals from receptors/other neurons at sites of synaptic contact
o Extension of cell body that contains neurofilaments and microfilaments
o Information travels from distal to proximal and converges at the cell body (as ions or NT)
o Dendritic spines (optional) – serve as regions of direct contact with an axon
o Primary dendrite is the extension of the dendrite that is more proximal to the body
Typically thicker and contains more organelles (mitochondria, ER, ribosomes, etc.)
o Secondary dendrite –post bifurcation; more distal from the cell body
o Normally dendrites are NOT myelinated; increase in thickness as they near the cell body
o Unique subsets of neurons with dendritic projections that are able to sense the external/internal environment of the cell and transduce those specific signals to the CNS
Dendritic projections do NOT synapse with other axons but have specific receptors that are used to help detect the external/internal environment
Cell Body (soma, perikaryon)
– support and metabolic center of the cell
o Main Role: integrate/process the information coming in from its dendritic projections
o Consumes a lot of oxygen/ATP to do various things such as protein synthesis
Abundant amount of rough ER and mitochondria to facilitate protein synthesis
o Nucleus – euchromatic nucleus
o Prominent nucleolus – region of the nucleus that’s making a lot of ribosomal RNA
o Basophilic Nissl Stain – stains the cytoplasm of the cell body and nucleolus
Axon
– neuronal projection that sends information to surrounding neurons or other effector cells
o Axon hillock – connection between the axon and the cell body
Histological feature that distinguishes it from dendritic projections; LACK Nissl Staining
Last site in soma where membrane potentials are summated before transmitted to axon
o Axon initial segment (AIS) – earliest site of action potential initiation; unmyelinated; stained with specific antibody
o Cytoplasm/axoplasm – contains dense bundles of microtubules & neurofilaments
NO ribosomes because they do not help make proteins
Key role in transport of metabolites & organelles
Mitochondria because they have ATP dependent ion transporters that help maintain the neuron resting membrane potential
o Axonal length accounts for the majority of surface area of neuron and volume; ends at synapse
o Terminal boutons – ends of the axons
o Large axons heavily myelinated – increases conduction; smaller axons are less myelinated, while the smallest axons are unmyelinated
All axons have intimate contact with oligodendrocytes or Schwann cell
Node of Ranvier
– regions on axon that are unmyelinated; lower membrane resistance
o Contain abundant amount of sodium channels
o Juxtaparanodal region – specific region around node that contains potassium channels
Synthesis and Transport of Axonal Proteins
– primarily in the cell body and proximal dendrites
o (membrane, lumenal, transmembrane) proteins translocated across rough ER during synthesis
Large dense core vesicles and synaptic vesicles
o Maintain function of axon/synapses
o Bidirectional transport of material to and from the cell body of the neuron
Anterograde/orthograde – transporting materials away from the cell body
• Fast – uses kinesin protein units that move vesicles and mitochondria along the length of the microtubule cytoskeleton
• Slow – moves cytoskeletal and cytosolic components
Retrograde – transporting material towards the cell body
• Fast – uses dynein proteins
o Ex: neurotropic and growth factors taken up by synaptic cleft
Rabies Virus and Tetanus Toxin
o Transported in retrograde fashion from site of infection
Structural Classification of Neurons
– by polarity
Unipolar – simplest nerve cell with one process (axon) that gives rise to several branches (dendrites)
Bipolar – two single dendrites and axons emerging from an elongated cell body
• Ex: many sensory cells
Pseudo-unipolar – single process bifurcates
• One branch travels to periphery to sensory receptors
• One branch travels to spinal cord
• Cell body is in the dorsal root ganglion
Multipolar – one axon with several dendrites emerging from the cell body
• Vary in cell body shape, axonal length, and number
• Majority of neurons (99%)
• Number and extensiveness of dendritic branches are related to their number of synaptic relationships
Amacrine – special, axonless interneuron in retina with many dendritic branches
• Classify based on location in the general organization of nerve tissue & by shape
Functional Classification of Neurons
Sensory (afferent) - deliver information from target tissues/organs to CNS
Motor (efferent) – send information from the CNS to target tissues
Interneurons – serve as connections between neurons; neither sensory or motor
Discharge Pattern Classification of Neurons
Tonic or regular spiking – interneurons in neostriatum
Fast spiking – cortical inhibitory neurons, retinal ganglion cells
Phasic or busting – dopaminergic neurons in VTA
Anatomical Classification of Neurons
Shape – Pyramidal neuron – cell body looks like a pyramid
Location & Projection – Nigrostraital neuron – cell body located in the substantia nigra with axonal projections into the striatum
Historical – cerebellar Purkinje neuron – named after person who discovered it
Molecular Classification of Neurons
Activity on target cell – excitatory, inhibitory or modulatory
• Modulatory – long lasting effects not directly related to firing rate
Type of Neurotransmitter – do NOT dictate whether it is excitatory or inhibitory
• Postsynaptic membrane and postsynaptic receptor that determine whether the NT from presynaptic neuron is either inhibitory or excitatory
Striatum Classification of Neurons
– in the caudate under the layers of the cortex; receives info for motor movement
Medium spiny neurons – make up 90% of the neurons – use GABA, neuropeptides
Fast spiking interneurons – use GABA
Low-threshold spiking interneurons – use GABA
tonically active neurons – use ACh
Functional Related Cell Bodies in CNS/PNS
CNS – Nucleus – Ex: nucleus accumbens, dentate nucleus
PNS – Ganglion – Ex: dorsal root ganglion, trigeminal ganglion
Other Designations for Cell Bodies in CNS
Column
• Cortex – group of nerve cell bodies related in function and in the location of the stimulus that drives them (perpendicular to the cortical plane)
o Ex: ocular dominance columns
• Spinal Cord-group of functionally related nerve cell bodies parallel to its long axis
o Ex: Clarke’s column
Layer, Lamina, Stratum – functionally related cells that form a layer parallel to the larger structure
• Ex: Layer V of the cerebral cortex, various strata of hippocampal CA fields
Tracts in CNS/PNS
CNS – Tract, Fasciculus, Lemniscus – bundle of parallel axons within the CNS
o Ex: optical tract, medial longitudinal fasciculus, medial lemniscus
• Funiculus – group of several parallel tracts, fasciculi
o Ex: lateral funiculus of spinal cord
PNS – root, ramus, nerve, plexus – bundle of parallel axons and associated cells; a plexus is a complex network of nerves
• Ex: ventral roots of spinal cord, gray ramus, facial nerve, brachial plexus
Nerve, Neuron, Nerve Fiber, Neuritis, Neuropil
- Nerve – macrostructure – axons/supporting cells
- Neuron – nerve cell
- Nerve fiber – axon
- Neuritis – neuron projections (development, in vitro)
- Neuropil – collection of projections, support cells within CNS grey matter
PNS Tissue Layers
o Epineurium – outermost connective tissue of nerve
Vasculature blends with surrounding tissue
o Perineurium – connective tissue around fascicles
o Endoneurium – innermost connective tissue (ex: Schwanna cells, fibroblasts, etc.)
o Fascicle (Fasciculus) – bundle of parallel axons
Injury, Repair, and Loss of Neurons
o If you destroy the cell body of a neuron, that neuron will die
o Possibility of repair if damage to cell body of neuron doesn’t kill it
o Wallerian (anterograde) degeneration – destruction of nerve distal to cell body
Central Chromatolysis - less severe damage to neuron or injury to axon
• Results in swelling in the cell body of neuron nucleus can get displaced from the center and there is a dispersion of the Nissl substance
Selective Vulnerability
o Neurons are post-mitotic – limited stem cell pools
o High energy demands – 20% O2, 15% cardiac output
o Complexity of Nervous System – anatomically very complex
Many different cells, phenotypes
Very integrated between cells and systems
o Limited repair mechanisms – difficult for axons and dendrites to re-grow
o Variety of insults – trauma, toxins, cardiovascular, metabolic, age-related, genes, developmental
