Neuronal Biology

Question 1

Brain Sizes

Answer 1

newborn = 400g; Adult = 1400g; Bottle Nose Dolphin = 1500g

Question 2

Neurons General Information

Answer 2

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

Question 3

Neuron Cytoskeleton

Answer 3

– extensive network of cytoskeleton structure

o Different parts of the neuron have different cytoskeletal components

Question 4

Microtubules

Answer 4

– 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

Question 5

Neurofilament

Answer 5

– 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

Question 6

Microfilament

Answer 6

– globular actin monomers; main used in the motility of growth cone

Question 7

Dendrites

Answer 7

– 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

Question 8

Cell Body (soma, perikaryon)

Answer 8

– 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

Question 9

Axon

Answer 9

– 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

Question 10

Node of Ranvier

Answer 10

– 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

Question 11

Synthesis and Transport of Axonal Proteins

Answer 11

– 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

Question 12

Rabies Virus and Tetanus Toxin

Answer 12

o Transported in retrograde fashion from site of infection

Question 13

Structural Classification of Neurons

Answer 13

– 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

Question 14

Functional Classification of Neurons

Answer 14

 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

Question 15

Discharge Pattern Classification of Neurons

Answer 15

 Tonic or regular spiking – interneurons in neostriatum
 Fast spiking – cortical inhibitory neurons, retinal ganglion cells
 Phasic or busting – dopaminergic neurons in VTA

Question 16

Anatomical Classification of Neurons

Answer 16

 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

Question 17

Molecular Classification of Neurons

Answer 17

 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

Question 18

Striatum Classification of Neurons

Answer 18

– 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

Question 19

Functional Related Cell Bodies in CNS/PNS

Answer 19

 CNS – Nucleus – Ex: nucleus accumbens, dentate nucleus

 PNS – Ganglion – Ex: dorsal root ganglion, trigeminal ganglion

Question 20

Other Designations for Cell Bodies in CNS

Answer 20

 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

Question 21

Tracts in CNS/PNS

Answer 21

 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

Question 22

Nerve, Neuron, Nerve Fiber, Neuritis, Neuropil

Answer 22

• 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

Question 23

PNS Tissue Layers

Answer 23

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

Question 24

Injury, Repair, and Loss of Neurons

Answer 24

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

Question 25

Selective Vulnerability

Answer 25

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